joint anatomy and basic biomechanics - joint anatomy... · his chapter provides an academic picture...

FUNDAMENTAL CONCEPTS, PRINCIPLES, AND TERMS

LeversBody PlanesAxes of MovementJoint MotionSynovial JointsBony ElementsArticular CartilageLigamentous ElementsSynovial FluidArticular Neurology

JOINT FUNCTIONMECHANICAL FORCES ACTING ON

CONNECTIVE TISSUETension ForcesCompression ForcesShear ForcesTorque Forces

PROPERTIES OF CONNECTIVE TISSUEMuscleLigamentsFacet JointsIntervertebral Discs

MODELS OF SPINE FUNCTION

This chapter provides an academic picture of the appliedanatomy and biomechanics of the musculoskeletal system.The human body may be viewed as a machine formed ofmany different parts that allow motion. These motionsoccur at the many joints formed by the specific parts thatcompose the body’s musculoskeletal system. Althoughthere is some controversy and speculation among thosewho study these complex activities, the information pre-sented here is considered essential for understanding clin-ical correlations and applications. Biomechanical discus-sions require specific nomenclature, which enables people

working in a wide variety of disciplines to communicate(see Appendix 3). Biomechanics is often overwhelmingbecause of its mathematical and engineering emphasis.This chapter will present a nonmathematical approach todefining clinically useful biomechanical concepts neces-sary for the ability to describe and interpret changes injoint function. Thorough explanations of biomechanicalconcepts are discussed in other works.1-3

FUNDAMENTAL CONCEPTS, PRINCIPLES, AND TERMS

Mechanics is the study of forces and their effects. Biomechanicsis the application of mechanical laws to living structures,specifically to the locomotor system of the human body.Therefore biomechanics concerns the interrelations of theskeleton, muscles, and joints. The bones form the levers, theligaments surrounding the joints form hinges, and the mus-cles provide the forces for moving the levers about the joints.

Kinematics is a branch of mechanics that deals with thegeometry of the motion of objects, including displacement,velocity, and acceleration, without taking into account theforces that produce the motion. Kinetics, however, is thestudy of the relationships between the force system actingon a body and the changes it produces in body motion.

Knowledge of joint mechanics and structure, as wellas the effects that forces produce on the body, has impor-tant implications for the use of manipulative proceduresand, specifically, chiropractic adjustments. Forces havevector characteristics whereby specific directions are de-lineated at the points of application. Moreover, forces canvary in magnitude, which will affect the acceleration ofthe object to which the force is applied.

Levers

A lever is a rigid bar that pivots about a fixed point, calledthe axis or fulcrum, when a force is applied to it. Force is

C H A P T E R 2Joint Anatomy and Basic Biomechanics

O U T L I N E

applied by muscles at some point along the lever to movethe body part (resistance). The lever is one of the sim-plest of all mechanical devices that can be called a ma-chine. The relationship of fulcrum to force to resistancedistinguishes the different classes of levers.

In a first-class lever, the axis (fulcrum) is located be-tween the force and the resistance; in a second-class lever,the resistance is between the axis and the force; and in athird-class lever, the force is between the axis and the re-sistance (Figure 2-1). Every movable bone in the body

12 Chiropractic Technique

Force

Resistance

Fulcrum

F

F

FR

F

F

F F RR

R

R

R

R

A

A

A

A

Figure 2-1 A, Lever system showing components. B, First-class lever system. C, Second-class lever sys-tem. D, Third-class lever system. A, Axis (fulcrum); F, force; R, resistance.

A

B

C

D

acts alone or in combination, forcing a network of leversystems characteristic of the first- and third-class levers.There are virtually no second-class levers in the body, although opening the mouth against resistance is an example.

With a first-class lever, the longer the lever arm is,the less force is required to overcome the resistance. Theforce arm may be longer, shorter, or equal to the resis-tance arm, but the axis will always be between these twopoints. An example of a first-class lever in the humanbody is the forearm moving from a position of flexioninto extension at the elbow through contraction of thetriceps muscle.

Third-class levers are the most common types in thebody because they allow the muscle to be inserted nearthe joint and can thereby produce increased speed ofmovement, although at a sacrifice of force. The force armmust be smaller than the resistance arm, and the appliedforce lies closer to the axis than the resistance force. Anexample of a third-class lever is flexion of the elbow jointthrough contraction of the biceps muscle.

Body Planes

It is also necessary to delineate the specific body planes ofreference, since they will be used to describe structural po-sition and directions of functional movement. The stan-

dard position of reference, or anatomic position, has thebody facing forward, the hands at the sides of the body,with the palms facing forward, and the feet pointingstraight ahead. The body planes are derived from dimen-sions in space and are oriented at right angles to one an-other. The sagittal plane is vertical and extends from frontto back, or from anterior to posterior. Its name is derivedfrom the direction of the human sagittal suture in the cra-nium. The median sagittal plane, also called the midsagittalplane, divides the body into right and left halves (Figure 2-2, A, Table 2-1). The coronal plane is vertical and extendsfrom side to side. Its name is derived from the orientationof the human coronal suture of the cranium. It may also bereferred to as the frontal plane, and it divides the body intoanterior and posterior components (Figure 2-2, B). Thetransverse plane is a horizontal plane and divides a struc-ture into upper and lower components (Figure 2-2, C).

Axes of Movement

An axis is a line around which motion occurs. Axes are re-lated to planes of reference, and the cardinal axes are ori-ented at right angles to one another. This is expressed asa three-dimensional coordinate system with x, y, and zused to mark the axes (Figure 2-3). The significance ofthis coordinate system is in defining or locating the ex-tent of the types of movement possible at each joint—rotation, translation, and curvilinear motion. All move-ments that occur about an axis are considered rotational,whereas linear movements along an axis and through aplane are called translational. Curvilinear motion occurswhen a translational movement accompanies rotationalmovements. The load that produces a rotational move-ment is called torsion; a force that produces a translationalmovement is called an axial or shear force.

Joint Motion

Motion can be defined as a continuous change in positionof an object. The axis around which movement takes

Chapter 2 Joint Anatomy and Basic Biomechanics 13

Figure 2-2A, Midsagittal plane. Movements of flexion and extension takeplace in the sagittal plane. B, Coronal plane. Movements ofabduction and adduction (lateral flexion) take place in thecoronal plane. C, Transverse plane. Movements of medial andlateral rotation take place in the transverse plane.

A B C

TABLE 2-1

Body Planes of Movement

Plane of Movement Axis Joint Movement

Sagittal Coronal [x] Flexion and extensionCoronal Sagittal Abduction and

(antero- adduction posterior) [z] (lateral flexion)

Transverse Longitudinal Medial and lateral (vertical) [y] rotation (axial

rotation)

place and the plane through which movement occurs de-fine specific motions or resultant positions. The coronalaxis (x-axis) lies in the coronal plane and extends fromone side of the body to the other. The motions of flexionand extension occur about this axis and through the sagit-tal plane. Flexion is motion in the anterior direction forjoints of the head, neck, trunk, upper extremity, and hips(Figure 2-4, A). Flexion of the knee, ankle, foot, and toesis movement in the posterior direction. Extension is themotion opposite of flexion.

The sagittal axis (z-axis) lies in the sagittal planeand extends horizontally from anterior to posterior.Movements of abduction and adduction of the extrem-ities, as well as lateral flexion of the spine, occur aroundthis axis and through the coronal plane. Lateral flexionis a rotational movement and is used to denote lateralmovements of the head, neck, and trunk in the coronalplane (Figure 2-4, B). In the human, lateral flexion isusually combined with some element of rotation.Abduction and adduction are also motions in a coronalplane. Abduction is movement away from the body, andadduction is movement toward the body; the referencehere is to the midsagittal plane of the body. This wouldbe true for all parts of the extremities, excluding thethumb, fingers, and toes. For these structures, refer-ence points are to be found within that particular extremity.

The longitudinal axis (y-axis) is vertical, extending ina head-to-toe direction. Movements of medial (internal)and lateral (external) rotation in the extremities, as well asaxial rotation in the spine, occur around it and throughthe transverse plane (Figure 2-4, C). Axial rotation is usedto describe this type of movement for all areas of thebody except the scapula and clavicle. Rotation occursabout an anatomic axis, except in the case of the femur,which rotates around a mechanical axis.4 In the humanextremity, the anterior surface of the extremity is used asa reference area. Rotation of the anterior surface towardthe midsagittal plane of the body is medial (internal) ro-tation, and rotation away from the midsagittal plane islateral (external) rotation. Supination and pronation arerotation movements of the forearm.

Because the head, neck, thorax, and pelvis rotateabout longitudinal axes in the midsagittal area, rotationcannot be named in reference to the midsagittal plane.Rotation of the head, spine, and pelvis is described as ro-tation of the anterior surface posteriorly toward the rightor left. Rotation of the scapula is movement about asagittal axis, rather than about a longitudinal axis. Theterms clockwise or counterclockwise are used.

Translational movements are linear movements or,simply, movements in a straight line. Gliding movementsof the joint are translational in character. The term slidehas also been used in referring to translational move-ments between joint surfaces. Posterior-to-anterior (P-A)glide (anterolisthesis) and anterior-to-posterior (A-P)glide (retrolisthesis) are translational movements alongthe z axis. Lateral-to-medial (L-M) glide and medial-to-lateral (M-L) glide (laterolisthesis) translate along the xaxis. Distraction and compression (altered interosseousspacing) translate along the y axis. Curvilinear motioncombines both rotational and translational movementsand is the most common motion produced by the jointsof the body (Figure 2-5).

Moreover, the potential exists for each joint to exhibitthree translational movements and three rotationalmovements, constituting 6 degrees of freedom. The ex-


Figure 2-4A, Sagittal plane movement of flexion. B, Coronal planemovement of lateral flexion. C, Transverse plane movement ofaxial rotation.

A B C

Y

Z

X

Translation

Rotation

Figure 2-3Three-dimensional coordinate system identifying the transla-tional and rotational movements along or around the threeaxes to produce 6 degrees of freedom.

tent of each movement is based more or less on the jointanatomy and, specifically, the plane of the joint surface.Each articulation in the body has the potential to exhibit,to some degree, flexion, extension, right and left lateralflexion, right and left axial rotation, A-P glide, P-A glide,L-M glide, M-L glide, compression, and distraction.

Joints are classified first by their functional capabili-ties and then are subdivided by their structural character-istics. Synarthroses allow very little, if any, movement; di-arthroses, or true synovial joints, allow significantmovement. The structural characteristics of these jointsare detailed in Table 2-2.

Synovial Joints

Synovial joints are the most common joints of the hu-man appendicular skeleton, representing highly evolved,movable joints. Although these joints are consideredfreely movable, the degree of possible motion varies ac-

cording to the individual structural design, facet planes,and primary function (motion vs. stability). The compo-nents of a typical synovial joint include the bony ele-ments, subchondral bone, articular cartilage, synovialmembrane, fibroligamentous joint capsule, and articularjoint receptors. An understanding of the basic anatomyof a synovial joint forms the foundation for appreciationof clinically significant changes in the joint that lead tojoint dysfunction.

Bony Elements

The bony elements provide the supporting structure thatgives the joint its capabilities and individual characteris-tics by forming lever arms to which intrinsic and extrin-sic forces are applied. Bone is actually a form of connec-tive tissue that has an inorganic constituent (lime salts). Ahard outer shell of cortical bone provides structural sup-port and surrounds the cancellous bone, which containsmarrow and blood vessels that provide nutrition.Trabecular patterns develop in the cancellous bone, cor-responding to mechanical stress applied to and requiredby the bone (Figure 2-6). Bone also has the importantrole of hemopoiesis (formation of blood cells). Further-more, bone stores calcium and phosphorus, which it ex-changes with blood and tissue fluids. Finally, bone has theunique characteristic of repairing itself with its own tis-sue as opposed to fibrous scar tissue, which all other bodytissues use.

Articular Cartilage

Articular cartilage covers the articulating bones in syn-ovial joints and helps to transmit loads and reduce fric-tion. It is bonded tightly to the subchondral bonethrough the zone of calcification, which is the end of bone visible on x-ray film. The joint space visible on x-ray film is composed of the synovial cavity and noncalci-fied articular cartilage. In its normal composition, articu-lar cartilage has four histologic areas or zones (Figure 2-7). These zones have been further studied and refinedso that a wealth of newer information regarding cartilagehas developed.

The outermost layer of cartilage is known as the glid-ing zone, which itself contains a superficial layer (outer)and a tangential layer (inner). The outer segment is madeup solely of collagen randomly oriented into flat bundles.The tangential layer consists of densely packed layers ofcollagen, which are oriented parallel to the surface of thejoint.5 This orientation is along the lines of the joint mo-tion, which implies that the outer layers of collagen arestronger when forces are applied parallel to the joint mo-tion rather than perpendicular to it.6 This particular ori-entation of fibers provides a great deal of strength to thejoint in normal motion. The gliding zone also has a rolein protecting the deeper elastic cartilage.


A

B

A�

B�

A

B

A�

B�

Instantaneousaxis of rotation

Figure 2-5A, Translational movement. B, Curvilinear movement: a com-bination of translation and rotation movements.

A

B

The transitional zone lies beneath the gliding zone. Itrepresents an area where the orientation of the fibers be-gins to change from the parallel orientation of the glid-ing zone to the more perpendicular orientation of the ra-dial zone. Therefore fiber orientation is more or lessoblique and, in varying angles, formed from glucuronicacid and N-acetylgalactosamine with a sulfate on eitherthe fourth or sixth position. The keratin compound isformed with galactose and N-acetylgalactosamine. All ofthis occurs in linked, repeating units (Figure 2-8).

Articular cartilage is considered mostly avascular.Articular cartilage must rely on other sources for nutrition,

removal of waste products, and the process of repair.Therefore intermittent compression (loading) and distrac-tion (unloading) are necessary for adequate exchange ofnutrients and waste products. The highly vascularized syn-ovium is believed to be a critical source of nutrition for thearticular cartilage it covers. The avascular nature of artic-ular cartilage limits the potential for cartilage repair bylimiting the availability of the repair products on whichhealing depends. Chondrocytes, the basic cells of cartilagethat maintain and synthesize the matrix, are containedwithin a mesh of collagen and proteoglycan that does notallow them to migrate to the injury site from adjacenthealthy cartilage.7 Moreover, the articular cartilage matrixmay contain substances that inhibit vascular andmacrophage invasion and clot formation that are also nec-essary for healing.8 After an injury to the articular cartilage,the joint can return to an asymptomatic state after thetransient synovitis subsides. Degeneration of the articularcartilage depends on the size and depth of the lesion, theintegrity of the surrounding articular surface, the age andweight of the patient, associated meniscal and ligamentouslesions, and a variety of other biomechanical factors.7Continuous passive motion has increased the ability offull-thickness defects in articular cartilage to heal, produc-ing tissue that closely resembles hyaline cartilage.9

Ligamentous Elements

The primary ligamentous structure of a synovial joint isthe joint capsule. Throughout the vertebral column, thejoint capsules are thin and loose. The capsules are at-tached to the opposed superior and inferior articular


TABLE 2-2

Joint Classification

Joint Type Structure Example

SynarthroticFibrous Suture—nearly no movement Cranial sutures

Syndesmosis—some movement Distal tibia-fibulaCartilaginous Synchondrosis—temporary Epiphyseal plates

Symphysis—fibrocartilage PubesIntervertebral discs

DiarthroticUniaxial Ginglymus (hinge) Elbow

Trochoid (pivot) Atlantoaxial jointCondylar Metacarpophalangeal joint

Biaxial Ellipsoid Radiocarpal jointSellar (saddle) Carpometacarpal joint of the thumb

Multiaxial Triaxial ShoulderSpheroid (ball and socket) Hip

Plane (nonaxial) Intercarpal jointsPosterior facet joints in the spine

Medial “compression”trabecular system

Verticaltrabeculae

Lateral “tension”trabecular system

Horizontaland obliquetrabeculae

Figure 2-6Trabecular patterns corresponding to mechanical stresses in thehip joint and vertebra. (Modified from Hertling D, Kessler RM:Management of common musculoskeletal disorders: Physical therapyprinciples and methods, ed 2, Philadelphia, 1990, JB Lippincott.)

facets of adjacent vertebrae. Joint capsules in the spinehave three layers.10 The outer layer is composed of densefibroelastic connective tissue made up of parallel bundlesof collagen fibers. The middle layer is composed of looseconnective tissue and areolar tissue containing vascularstructures. The inner layer consists of the synovial mem-brane. This joint capsules covers the posterior and lateralaspects of the zygapophyseal joint. The ligamentumflavum covers the joint capsules anteriorly and medially.

Synovial Fluid

Although the exact role of synovial fluid is still unknown,it is thought to serve as a joint lubricant or at least to in-teract with the articular cartilage to decrease friction be-tween joint surfaces. This is of clinical relevance becauseimmobilized joints have been shown to undergo degen-eration of the articular cartilage.11 Synovial fluid is simi-lar in composition to plasma, with the addition of mucin


Glidingzone

Tangentialzone

Transitionallayer

Radialzone

Zone ofcalcifiedcartilage

Subchondralplate

Figure 2-7Microscopic anatomy of articular cartilage. (Modified fromAlbright JA, Brand RA: The scientific basis of orthopaedics, EastNorwalk, Conn, 1979, Appleton-Century-Crofts.)

Chondroitin–4 sulfate

Glucuronic acid

COO�

CH2OH

NHCOCH3

OSO3

OH

OH

O

O

O

O O

COO�

CH2OSO3�

NHCOCH3

?

OH

OH

O

O

O O

O

CH2OH

CH2OSO3�

NHCOCH3

OH

OH

OH

O

O

O O

N-acetylgalactosamine

Glucuronic acid N-acetylgalactosamine

Chondroitin–6 sulfate

Keratan sulfate

Figure 2-8Structure of chondroitin and keratin compounds.

(hyaluronic acid), which gives it a high molecular weightand its characteristic viscosity. Three models of joint lu-brication exist. The controversy lies in the fact that noone model of joint lubrication applies to all joints underall circumstances.

According to the hydrodynamic model, synovial fluidfills in spaces left by the incongruent joint surfaces.During joint movement, synovial fluid is attracted to thearea of contact between the joint surfaces, resulting in themaintenance of a fluid film between moving surfaces.This model was the first to be described and works wellwith quick movement, but it would not provide adequatelubrication for slow movements and movement under in-creased loads.

The elastohydrodynamic model is a modification ofthe hydrodynamic model that considers the viscoelasticproperties of articular cartilage whereby deformation ofjoint surfaces occurs with loading, creating increasedcontact between surfaces. This would effectively reducethe compression stress to the lubrication fluid. Althoughthis model allows for loading forces, it does not explainlubrication at the initiation of movement or the period ofrelative zero velocity during reciprocating movements.12

In the boundary lubrication model, the lubricant isadsorbed on the joint surface, which would reduce theroughness of the surface by filling the irregularities andeffectively coating the joint surface. This model allowsfor initial movement and zero velocity movements.Moreover, boundary lubrication combined with the elas-tohydrodynamic model, creating a mixed model, meetsthe demands of the human synovial joint (Figure 2-9).

Articular Neurology

Articular neurology gives invaluable information on thenature of joint pain, the relationship of joint pain to jointdysfunction, and the role of manipulative procedures inaffecting joint pain. Synovial joints are innervated bythree or four varieties of neuroreceptors, each with a widevariety of parent neurons. The axons differ in diameterand conduction velocity, representing a continuum fromthe largest heavily myelinated A �-fibers to the smallestunmyelinated C fibers. All are derived from the dorsal andventral rami, as well as the recurrent meningeal nerve ofeach segmental spinal nerve (Figure 2-10). Informationfrom these receptors spreads among many segmental lev-els because of multilevel ascending and descending pri-mary afferents. The receptors are divided into the fourgroups according to their neurohistologic properties,which include three corpuscular mechanoreceptors andone nociceptor.13

Type I receptors are confined to the outer layers ofthe joint capsule and are stimulated by active or passivejoint motions. Their firing rate is inhibited with joint ap-proximation, and they have a low threshold, makingthem very sensitive to movement. Some are consideredstatic receptors because they fire continually, even withno joint movement. Because they are slow adapting, theeffects of movement are long lasting. Stimulation of typeI receptors is involved with the following:1. Reflex modulation of posture, as well as movement

(kinesthetic sensations), through constant monitoringof outer joint tension

2. Perception of posture and movement3. Inhibition of flow from pain receptors via an enkeph-

alin synaptic interneuron transmitter4. Tonic effects on lower motor neuron pools involved

in the neck, limbs, jaw, and eye musclesType II mechanoreceptors are found within the

deeper layers of the joint capsule. They are also lowthreshold and again are stimulated with even minorchanges in tension within the inner joint. Unlike type Ireceptors, however, type II receptors adapt very rapidlyand quickly cease firing when the joint stops moving.Type II receptors are completely inactive in immobilizedjoints. Functions of the type II receptors are likely to in-clude the following:1. Movement monitoring for reflex actions and perhaps

perceptual sensations2. Inhibition of flow from pain receptors via an enkeph-

alin synaptic interneuron neural transmitter3. Phasic effects on lower motor neuron pools involved

in the neck, limbs, jaw, and eye musclesType III mechanoreceptors are found in the intrinsic

and extrinsic ligaments of the peripheral joints, but theyhad been previously thought to be absent from all of thesynovial spinal joints. However, McLain14 examined 21cervical facet capsules from three normal human subjects


Hydrodynamic Elastohydrodynamic

Boundary

Figure 2-9Lubrication models for synovial joints. (Modified fromHertling D, Kessler RM: Management of common musculoskele-tal disorders: Physical therapy principles and methods, ed 2,Philadelphia, 1990, JB Lippincott.)

and found type III receptors, although they were lessabundant than either type I or type II. These receptorsare very slow adapters with a very high threshold becausethey are innervated by large myelinated fibers. Theyseem to be the joint version of the Golgi tendon organ inthat they impose an inhibitory effect on motoneurons.Although the functions of type III receptors are notcompletely understood, it is likely that they achieve thefollowing:1. Monitor direction of movement2. Create a reflex effect on segmental muscle tone, pro-

viding a “braking mechanism” against movement thatoverdisplaces the joint

3. Recognize potentially harmful movementsType IV receptors are composed of a network of free

nerve endings, as well as unmyelinated fibers. They are as-sociated with pain perception and include many differentvarieties with large ranges of sensations, including itch andtickle. They possess an intimate physical relationship to themechanoreceptors and are present throughout the fibrousportions of the joint capsule and ligaments. They are ab-sent from articular cartilage and synovial linings, althoughthey have been found in synovial folds.15,16 They are veryhigh-threshold receptors and are completely inactive in thephysiologic joint. Joint capsule pressure, narrowing of theintervertebral disc, fracture of a vertebral body, dislocationof the zygapophyseal joints, chemical irritation, and inter-stitial edema associated with acute or chronic inflammationmay all activate the nociceptive system. The basic functionsof the nociceptors include the following:1. Evocation of pain2. Tonic effects on neck, limb, jaw, and eye muscles3. Central reflex connections for pain inhibition

4. Central reflex connections for a myriad of autonomiceffectsA relationship exists between mechanoreceptors and

nociceptors such that when the mechanoreceptors func-tion correctly, an inhibition of nociceptor activity oc-curs.13 The converse also holds true; when the mechano-receptors fail to function correctly, inhibition ofnociceptors will occur less, and pain will be perceived.13

Discharges from the articular mechanoreceptors arepolysynaptic and produce coordinated facilitory and in-hibitory reflex changes in the spinal musculature. Thisprovides a significant contribution to the reflex control ofthese muscles.13 Gillette15 suggests that a chiropractic ad-justment produces sufficient force to coactivate a wide variety of mechanically sensitive receptor types in the paraspinal tissues. The A-�-mechanoreceptors and C-polymodal nociceptors, which can generate impulsesduring and after stimulation, may well be the most physi-ologically interesting component of the afferent bombard-ment initiated by high-velocity, low-amplitude manipula-tions. For normal function of the joint structures, anintegration of proprioception, kinesthetic perception, andreflex regulation is absolutely essential.

Pain-sensitive fibers also exist within the annulus fi-brosus of the disc. Malinsky16 demonstrated the presenceof a variety of free and complex nerve endings in theouter one third of the annulus. The disc is innervatedposteriorly by the recurrent meningeal nerve (sinuverte-bral nerve) and laterally by branches of the gray ramicommunicantes. During evaluation of disc material sur-gically removed before spinal fusion, Bogduk17 foundabundant nerve endings with various morphologies. Thevarieties of nerve endings included free terminals,


Spinal nerve root

Spinal nerve ganglion

Sinovertebral nerve toannulus fibrosus

Articular facet innervation

Sinovertebral nerve toposterior longitudinal ligament

Anterior primary ramus

Nerve to joint capsule

Interspinous andsupraspinous ligaments

Nerves to spinous processand interspinous ligament

Nerve to articular capsuleNerves to yellowligament

Posterior primary rami

Posterior longitudinalligament

Nerve to vertebral body

Anterior longitudinalligament and nerve

Sinovertebral nerveto vertebral body

Figure 2-10Innervation of the outer fibers of the disc and facet joint capsule by the sinuvertebral nerve.A, Oblique posterior view. B, Top view. (Modified from White AA, Panjabi MM: Clinical bio-mechanics of the spine, Philadelphia, 1978, JB Lippincott.)

A B


complex sprays, and convoluted tangles. Furthermore,many of these endings contained substance P, a putativetransmitter substance involved in nociception.

Shinohara18 reported the presence of such nerve fibersaccompanying granulation tissue as deep as the nucleus indegenerated discs. Freemont et al19 examined discs fromindividuals free of back pain and from those with backpain. They identified nerve fibers in the outer one third ofthe annulus in pain-free disc samples, but they found nervefibers extending into the inner one third of the annulusand into the nucleus pulposus of the discs from the painsample. They suggest that their findings of isolated nervefibers that express substance P deep within diseased inter-vertebral discs may play an important role in the patho-genesis of chronic low back pain. Abundant evidenceshows that the disc can be painful, supporting the ascribednociceptive function of the free nerve endings.16-27

Because structure and function are interdependent,the study of joint characteristics should not isolate struc-ture from function. The structural attributes of a joint aredefined as the anatomic joint, consisting of the articularsurfaces with the surrounding joint capsule and ligaments,as well as any intraarticular structures. The functional attributes are defined as the physiologic joint, consisting of

the anatomic joint plus the surrounding soft tissues, in-cluding the muscles, connective tissue, nerves, and bloodvessels (Figure 2-11).

JOINT FUNCTION

The physiologic movement possible at each joint occurswhen muscles contract or when gravity acts on bone tomove it. This motion is termed osteokinematic movement.Osteokinematic movement describes how each bony jointpartner moves relative to the other. The specific move-ments that occur at the articulating joint surfaces are re-ferred to as arthrokinematic movement. Consideration ofthe motion between bones alone or osteokinematic move-ment is insufficient, because no concern is given to whatoccurs at the joint and because movement commonly in-volves coupling of motion around different axes.Furthermore, arthrokinematic movements consider theforces applied to the joint and include the accessory mo-tion present in a particular articulation.

It is therefore important to relate osteokinematicmovement to arthrokinematic movement when evaluat-ing joint motion (Figure 2-12). This involves determiningthe movement of the mechanical axis of the moving bone

NerveRectusfemoristendon

Gastrocnemiusmuscle

Articularcartilage

Joint capsuleand ligaments

Synovium

Bone

Periosteum

Menisci

Ana

tom

ic jo

intP

hysi

olog

ic jo

int

Blood vessel

Figure 2-11Structures that make up the anatomic joint and the physio-logic joint in the knee.

Figure 2-12A, Osteokinematic movement of knee and trunk flexion. B, Arthrokinematic movements of tibiofemoral and T6-T7joint flexion.

A B

relative to the stationary joint surface. The mechanical axisof a joint is defined as a line passing through the movingbone, oriented perpendicular to the center of the station-ary joint surface (Figure 2-13).

When one joint surface moves relative to the other,spin, roll, slide, or combinations occur. MacConnail andBasmajian28 use the term spin to describe rotationalmovement around the mechanical axis, which is possibleas a pure movement only in the hip, shoulder, and prox-imal radius. Roll occurs when points on the surface of onebone contact points at the same interval of the otherbone. Slide occurs when only one point on the movingjoint surface contacts various points on the opposingjoint surface (Figure 2-14).

In most joints of the human body, these motions occursimultaneously. The concave-convex rule relates to this ex-pected coupling of rotational (roll) and translational (slide)movements. When a concave surface moves on a convexsurface, roll and slide movements should occur in the samedirection. When a convex surface moves on a concave sur-face, however, roll and slide should occur in opposite di-rections (Figure 2-15). Pure roll movement tends to resultin joint dislocation, whereas pure slide movement causesjoint surface impingement (Figure 2-16). Moreover, cou-pling of roll and slide is important anatomically becauseless articular cartilage is necessary in a joint to allow formovement and may decrease wear on the joint.

These concepts are instrumental in clinical decision-making regarding the restoration of restricted joint mo-tion. Roll and spin can be restored with passive range-


Path followed bymechanical axis

Mechanicalaxis

Spin

Swing

Figure 2-13Mechanical axis of a joint and MacConnail and Basmajian’s con-cept of spin and swing. (Modified from Hertling D, Kessler RM:Management of common musculoskeletal disorders: Physical therapyprinciples and methods, ed 2, Philadelphia, 1990, JB Lippincott.)

b�

Roll Slide

b�b�

bb

aa

a�

a�a�

Figure 2-14Arthrokinematic movements of roll and slide. (Modified fromHertling D, Kessler RM: Management of common musculoskele-tal disorders: Physical therapy principles and methods, ed 2,Philadelphia, 1990, JB Lippincott.)

Roll

Roll

Slide

Slide

Figure 2-15Concave-convex rule. A, Movement of concave surface on aconvex surface. B, Movement of a convex surface on a concavesurface.

A

B

of-motion procedures that induce the arthrokinematicmovements of the dysfunctional joint. Manipulative(thrust) techniques are needed to restore slide move-ments and can also be used for roll and spin problems.29

In addition, when an object moves, the axis aroundwhich the movement occurs can vary in placement fromone instant to another. The term instantaneous axis of ro-tation (IAR) is used to denote this location point.Asymmetric forces applied to the joint can cause a shift inthe normal IAR. Furthermore, vertebral movement maybe more easily analyzed as the IAR becomes more com-pletely understood (Figure 2-17). White and Panjabi1

point out that the value of this concept is that any kind ofplane motion can be described relative to the IAR.Complex motions are simply regarded as many very smallmovements with many changing IARs.1 This concept isdesigned to describe plane movement, or movement intwo dimensions.

When three-dimensional motion occurs between ob-jects, a unique axis in space is defined called the helicalaxis of motion (HAM), or screw axis of motion (Figure 2-18). HAM is the most precise way to describe motionoccurring between irregularly shaped objects, such asanatomic structures, because it is difficult to consistentlyand accurately identify reference points for such objects.

Clearly, most movements occur around and throughseveral axes simultaneously, so pure movements in the


Pure slide Pure roll

Impingement

Dislocation

Figure 2-16Consequences of pure roll or pure slide movements.(Modified from Hertling D, Kessler RM: Management of com-mon musculoskeletal disorders: Physical therapy principles and meth-ods, ed 2, Philadelphia, 1990, JB Lippincott.)

Position 1

B1

A1

A2

B2

Position 2

Instantaneousaxis of rotation

Figure 2-17Instantaneous axis of rotation. (Modified from White AA,Panjabi MM: Clinical biomechanics of the spine, Philadelphia,1978, JB Lippincott.)

Z

X

Y

Figure 2-18Helical axis of motion. (Modified from White AA, PanjabiMM: Clinical biomechanics of the spine, Philadelphia, 1978, JBLippincott.)

human frame rarely occur. The nature and extent of in-dividual joint motion are determined by the joint structureand, specifically, by the shape and direction of the jointsurfaces. No two opposing joint surfaces are perfectlymatched, nor are they perfectly geometric. All joint sur-faces have some degree of curvature that is not constantbut changing from point to point. Because of the incon-gruence between joint surfaces, some joint space and“play” must be present to allow free movement. Thisjoint play is an accessory movement of the joint that is es-sential for normal functioning of the joint.

The resting position of a joint, or its neutral position,occurs when the joint capsule is most relaxed and thegreatest amount of play is possible. When injured, a jointoften seeks this maximum loose-packed position to allowfor swelling.

The close-packed position occurs when the jointcapsule and ligaments are maximally tightened. More-over, there is maximal contact between the articularsurfaces, making the joint very stable and difficult tomove or separate.

Joint surfaces will approximate or separate as the jointgoes through a range of motion. This is the motion ofcompression and distraction. A joint moving toward itsclose-packed position is undergoing compression, and ajoint moving toward its open-packed position is under-going distraction28 (Table 2-3).

Joint motion consists of five qualities of movementthat must be present for normal joint function. Thesefive qualities are joint play, active range of motion, pas-

sive range of motion, end feel or play, and paraphysio-logic movement. From the neutral close-packed posi-tion, joint play should be present. This is followed by arange of active movement under the control of the mus-culature. The passive range of motion is produced bythe examiner and includes the active range, plus a smalldegree of movement into the elastic range. The elasticbarrier of resistance is then encountered, which exhibitsthe characteristic movement of end feel. The smallamount of movement available past the elastic barriertypically occurs postcavitation and has been classified asparaphysiologic movement. Movement of the joint be-yond the paraphysiologic barrier takes the joint beyondits limit of anatomic integrity and into a pathologiczone of movement. Should a joint enter the pathologiczone, there will be damage to the joint structures, in-cluding the osseous and soft tissue components (seeFigures 3-22 and 3-23).

Both joint play and end-feel movements are thoughtto be necessary for the normal functioning of the joint. Aloss of either movement can result in a restriction of mo-tion, pain, and most likely, both. Active movements canbe influenced by exercise and mobilization, and passivemovements can be influenced by traction and some formsof mobilization, but end-feel movements are affectedwhen the joint is taken through the elastic barrier, creat-ing a sudden yielding of the joint and a characteristiccracking noise (cavitation). This action can be accom-plished with deep mobilization and a high-velocity, low-amplitude manipulative thrust.


TABLE 2-3

Close-Packed Positions for Each Joint

Region Specific Joint Close-Packed Position

Fingers Distal interphalangeal joints Maximal extensionProximal interphalangeal joints Maximal extensionMetacarpophalangeal joints Maximal flexion

Hand Intermetacarpal joints Maximal oppositionWrist Intercarpal joints Maximal dorsiflexionForearm Radioulnar joints 5 degrees of supinationElbow Ulnohumeral joint Extension in supination

Radiohumeral joint Flexion in supinationShoulder Glenohumeral joint Abduction and external rotation

Acromioclavicular joint 90 degrees of abductionSternoclavicular joint Maximal elevation

Toes Distal interphalangeal joints Maximal extensionProximal interphalangeal joints Maximal extensionMetatarsophalangeal joints Maximal extension

Foot Intermetatarsal joints Maximal oppositionAnkle Tarsometatarsal joints Maximal inversion

Tibiotalar joint Maximal dorsiflexionKnee Tibiofemoral joint Maximal extension and external rotationHip Coxofemoral joint Maximal extension, internal rotation, and abductionSpine Three-joint complex Maximal extension

MECHANICAL FORCES ACTING ON CONNECTIVE TISSUE

Whereas an understanding of structure is needed toform a foundation, an understanding of the dynamicsof the various forces affecting joints will aid in the ex-planation of joint injury and repair. Functionally, themost important properties of bone are its strength andstiffness, which become significant qualities when loadsare applied (Figure 2-19). Living tissue is subjected tomany different combinations of loading force through-out the requirements of daily living. Although eachtype of loading force is described individually, most ac-tivities produce varying amounts and combinations ofall of them.

Tension Forces

The force known as tension occurs when a structure isstretched longitudinally. Tensile loading is a stretchingaction that creates equal and opposite loads outwardfrom the surface and tensile stress and strain inward.Therefore a tension force tends to pull a structure apart,

causing the cross-sectional area of the structure to de-crease. When a material is stretched in the direction ofthe pull, it contracts in the other two directions. If theprimary stress is tensile, there will be secondary stressesthat are compressive and vice versa.

The tension elements of the body are the soft tissues(fascia, muscles, ligaments, and connective tissue) andhave largely been ignored as construction members ofthe body frame. The tension elements are an integralpart of the construction and not just a secondary support.In the spine, the ligaments are loaded in tension.30

Tensile forces also occur in the intervertebral disc duringthe rotational movements of flexion, extension, axial ro-tation, and lateral flexion. The nucleus tends to bear thecompressive load, and the annular fibers tend to bear thetensile loads.

Compression Forces

Compression occurs when a load produces forces thatpush the material together, creating a deforming stress.The behavior of a structure in compression depends agreat deal on its length and how far or long the load isapplied.

Compressive forces are transmitted to the vertebralbody and intervertebral disc in the spine. The nucleuspulposus is a semiliquid or gel that has the characteristicsof a fluid or hydraulic structure. It is incompressible andmust therefore distort under compressive loads. The nu-cleus pulposus dissipates the compressive force by redi-recting it radially.

It is important clinically to note that mechanical fail-ure occurs first in the cartilaginous endplate when com-pressive forces applied alone are too great. The result isnuclear herniation into the vertebral body, called aSchmorl’s node. However, failure may be modified whenthe spine is loaded in either flexion or extension.Compressive loads applied in flexion tend to cause ante-rior collapse of the endplate or vertebral body, where thebony structure is weaker. With compressive loads appliedin extension, a significant percentage of the compressiveload is transmitted through the facets, leading to capsu-lar injuries.

Compressive loads applied with torque around thelong axis can produce circumferential tears in the disc an-nulus. Compression loading (axial loading) on bone cre-ates equal and opposite loads toward the surface andcompressive stress and strain inward, causing the struc-ture to become shorter and wider. Compression fracturesof the vertebral bodies are examples of failure to with-stand compressive forces.

Bending loads are a combination of tensile and com-pressive loads. The magnitude depends on the distance ofthe forces from the neutral axis. Fractures to long bonesfrequently occur through this mechanism.


BendingCompressionTensionUnloaded

Shear Torsion Combined loading

Figure 2-19Loads to which bone may be subject. (Modified fromSoderberg GL: Kinesiology: Application to pathological motion,Baltimore, 1986, Williams & Wilkins.)

Shear Forces

The biomechanical effects on living things would be agreat deal easier to understand if the loads, stresses, andstrains were all either tensile or compressive ones. How-ever, living things are also subjected to shear forces. Ashear force creates sliding or, more specifically, resistanceto sliding. Shear loading causes the structure to deforminternally in an angular manner as a result of loads ap-plied parallel to the surface of the structure.

Primarily, the facet joints and the fibers of the annu-lus fibrosus resist shear forces in the spinal motion seg-ment. Under normal physiologic conditions, the facetscan resist shear forces when they are in contact. If, how-ever, the disc space is narrowed by degeneration withsubsequent thinning of the disc, abnormally high stressesmay be placed on the facet joints, and the limit of resis-tance to such forces is not well documented.31,32

Since there is no significant provision for resistingshear stress, the risk of disc failure is greater with tensileloading than with compression loading.1 However, thestudies available demonstrating the effects of shear forceshave been performed mostly on cadavers in which theposterior elements have been removed. The lumbarfacets are aligned mostly in the sagittal plane with an in-terlocking mechanism that only allows a few degrees ofrotation. Therefore, at least in the lower lumbar seg-ments, the facet joints do provide resistance to shearstress. Cancellous bone is most prone to fracture fromshear loading, with the femoral condyles and tibialplateaus often falling victim.

Torque Forces

Torsion occurs when an object twists, and the force thatcauses the twisting is referred to as torque. Torque is aload produced by parallel forces in opposite directionsabout the long axis of a structure. In a curved structure,such as the spine, bending also occurs when a torque loadis applied.

Farfan et al33 estimates that about 90% of the resis-tance to torque of a motion segment is provided by itsdisc. They further state that the annulus provides the ma-jority of the torsional resistance in the lumbar spine andspeculate that with torsional injury, annular layers willtear, leading to disc degeneration.33 This concept is de-veloped around the idea that when torsional forces arecreated in the spine, the annular fibers oriented in one di-rection will stretch while those oriented in the other di-rection will relax. The result is that only half of the fibersare available to resist the force.

However, Adams and Hutton34 disagreed with Farfanet al and demonstrated that primarily the facets resist thetorsion of the lumbar spine and that the compressed facetwas the first structure to yield at the limit of torsion.Others have performed experiments that further suggest

and support that the posterior elements of the spine, in-cluding the facet joints and ligaments, play a significantrole in resisting torsion.35,36 In deference to Farfan et al’sconclusions, these authors suggest that torsion alone isunimportant in the etiology of disc degeneration and pro-lapse, since rotation is produced by voluntary muscle ac-tivity and the intervertebral disc experiences relativelysmall stresses and strains. Bogduk and Twomey37 state thataxial rotation can strain the annulus in torsion, but ordi-narily the zygapophyseal joints protect it. Normal rota-tion in the lumbar spine produces impaction of the facetjoints, preventing no more than 3% strain to the annulus.With further rotation force, the impacted facet joint canserve as a new axis of rotation, allowing some additionallateral shear exerted on the annulus. Excessive rotationalforce can result in failure of any of the elements that re-sist rotation.37 Fracture can occur in the impacted facetjoint; the pars interarticularis can also fracture; capsulartears can occur in the nonimpacted facet joint; and cir-cumferential tears can occur in the annulus (Figure 2-20).Spiral fractures are another example of the results of tor-sional loads applied to long bones.


Axis

Tearfracture

Fracture Annulartear

Axis

Figure 2-20Effects of rotation on lumbar segments. A, Rotation is limitedby impaction of facet joint. B, Further rotation causes a shift inthe axis of rotation. C, The impacted facet is exposed to frac-ture, and the distracted facet is exposed to avulsion or capsulartear. D, The disc is exposed to lateral shear that can lead tocircumferential tears in the annulus. (Modified from BogdukN, Twomey LT: Clinical anatomy of the lumbar spine, ed 2,Melbourne, 1991, Churchill Livingstone.)

A B

C D

PROPERTIES OF CONNECTIVE TISSUE

The response of connective tissue to various stress loadscontributes significantly to the soft tissue component ofjoint dysfunction. Within the past several decades, a greatdeal of scientific investigation has been directed to defin-ing the physical properties of connective tissue.

Connective tissue contributes to kinetic joint stabilityand integrity by resisting rotatory moments of force.When these rotatory moments of force are large, consid-erable connective tissue power is required to produce theneeded joint stability and integrity. Connective tissue ismade up of various densities and spatial arrangements ofcollagen fibers embedded in a protein-polysaccharidematrix, which is commonly called ground substance.Collagen is a fibrous protein that has a very high tensilestrength. Collagenous tissue is organized into many dif-ferent higher-order structures, including tendons, liga-ments, joint capsules, aponeuroses, and fascial sheaths.The principal sources of passive resistance at the normalextremes of joint motion include ligaments, tendons, andmuscles. Therefore, under normal and pathologic condi-tions, the range of motion in most body joints is pre-dominantly limited by one or more connective tissuestructures. The relative contribution of each to the totalresistance varies with the specific area of the body.

All connective tissue has a combination of two quali-ties—elastic stretch and plastic (viscous) stretch (Figure2-21). The term stretch refers to elongation in a linear di-rection and increase in length. Stretching, then, is theprocess of elongation. Elastic stretch represents springlikebehavior, with the elongation produced by tensile load-ing being recovered after the load is removed. It is there-fore also described as temporary, or recoverable, elonga-tion. Plastic (viscous) stretch refers to putty-like behavior;the linear deformation produced by tensile stress remainseven after the stress is removed. This is described as per-manent, or nonrecoverable, elongation.

The term viscoelastic is used to describe tissue thatrepresents both viscous and elastic properties. The vis-cous properties permit time-dependent plastic or perma-nent deformation. Elastic properties, on the other hand,result in elastic or recoverable deformation. This allowsit to rebound to the previous size, shape, and length.

Different factors influence whether the plastic or elas-tic component of connective tissue is predominantly af-fected. These include the amount of applied force and theduration of the applied force. Therefore the major factorsaffecting connective tissue deformation are force and time.When a force great enough to overcome joint resistance isapplied over a short period of time, elastic deformation oc-curs. However when the same force is applied over a longperiod of time, plastic deformation occurs.

When connective tissue is stretched, the relative pro-portion of elastic and plastic deformation can vary widely,depending on how and under what conditions thestretching is performed. When tensile forces are contin-uously applied to connective tissue, the time required tostretch the tissue a specific amount varies inversely withthe force used. Therefore a low-force stretching methodrequires more time to produce the same amount of elon-gation as a higher-force method. However, the propor-tion of tissue lengthening that remains after the tensilestress is removed is greater for the low-force, long-duration method. Of course, high force and long dura-tion will also cause stretch and possibly rupture of theconnective tissue.

When connective tissue structures are permanentlyelongated, some degree of mechanical weakening occurs,even though outright rupture has not occurred. Theamount of weakening depends on the way the tissue isstretched, as well as how much it is stretched. For thesame amount of tissue elongation, however, a high-forcestretching method produces more structural weakeningthan a slower, lower-force method.

Because plastic deformation involves permanentchanges in connective tissue, it is important to knowwhen plastic deformity is most likely to occur. The great-est impact will occur when positions of stress are main-tained for long periods. Awkward sleep postures, poorseated posture, and stationary standing for extended peri-ods can create plastic changes that have the potential forskeletal misalignment, joint dysfunction, and instability.

After trauma or surgery, the connective tissue involvedin the body’s reparative process frequently impedes func-tion; it may abnormally limit the joint’s range of motionas a result of fibrotic tissue replacing elastic tissue. Scartissue, adhesions, and fibrotic contractures are commontypes of pathologic connective tissue that must be dealtwith during chiropractic manipulative procedures.

Connective tissue elements can lose their extensibilitywhen their related joints are immobilized.38 With immobi-lization, water is released from the proteoglycan molecule,allowing connective tissue fibers to contact one another,encouraging abnormal cross-linking, and resulting in aloss of extensibility.39 It is hypothesized that manual ther-apy can break the cross-linking and any intraarticular cap-sular fiber fatty adhesions, thereby providing free motionand allowing water inbibition to occur. Furthermore, pro-


Elastic qualities Viscous qualities

Tensileforce

Figure 2-21Model of connective tissue properties.

cedures can stretch segmental muscles, stimulating spindlereflexes that may decrease the state of hypertonicity.40

Muscle

The role of muscles is to move bone and allow the humanbody to perform work. In the normal man, muscle ac-counts for about 40% to 50% of body weight. For thewoman, this falls to approximately 30% of total bodyweight. Three types of muscle are found in the body: stri-ated skeletal muscle, nonstriated smooth involuntarymuscle, and cardiac muscle. Only the skeletal muscle isunder voluntary control.

There are three gross morphologic muscle types instriated muscle (Figure 2-22). Parallel muscles have fibersthat run parallel throughout the length of the muscle andend in a tendon. This type of muscle is essentially de-signed to rapidly contract, although it typically cannotgenerate a great deal of power. Pennate muscles are thosein which the fibers converge onto a central tendon. Amuscle of this type is unipennate if the fibers attach toonly one side of a central tendon, and it is bipennate ifthe muscle attaches to both sides of a central tendon.Finally, there is a multipennate muscle in which the mus-cle fibers insert on the tendon from a variety of differingdirections. This form of muscle can generate largeamounts of power, although it will perform work moreslowly than a parallel muscle.

Muscle has three layers of connective tissue (Figure 2-23). An epimysium formed of connective tissue sur-rounds the muscle; a perimysium separates the muscle

cells into various bundles; and an endomysium surroundsthe individual muscle cells. The muscle fibers also havethree layers. The outermost layer is formed of collagenfibers. A basement membrane layer comprises polysaccha-rides and protein and is approximately 500 Å thick. Theinnermost layer, the sarcolemma, forms the excitablemembrane of a muscle.

Muscle fibers contain columns of filaments of con-tractile proteins. In striated muscle, these molecules areinterrelated layers of actin and myosin molecules. Thesemyofibrils are suspended in a matrix called sarcoplasm,composed of the usual intracellular components. Thefluid of the sarcoplasm is rich with potassium, magne-sium, phosphate, and protein enzymes. Numerous mito-chondria lie close to the actin filaments of the I bands,suggesting that the actin filaments play a major role inusing adenosine triphosphate (ATP) formed by the mito-chondria.41 The sarcoplasmic reticulum functions in acalcium ion equilibrium. A transverse tubular systemtransmits membrane depolarization from the muscle cellto the protein. Also located within the sarcoplasm is theprotein myoglobin that is necessary for oxygen bindingand oxygen transfer.

Skeletal muscle occurs in two forms, originally knownas white and red muscle. The white muscle is a fast-twitch,or phasic, muscle. It has a rapid contraction time andcontains a large amount of glycolytic enzyme. Essentially,this muscle allows for rapid function necessary for quickcontractions for short periods. Red muscle is a slow-twitch, or tonic, muscle. It contracts much more slowlythan does white muscle and contains a great deal moremyoglobin and oxidative enzymes. Red muscle is moreimportant in static activities that require sustained effortover longer periods. Standing is a good example of this.


Figure 2-22Morphologic muscle types. A, Unipennate. B, Bipennate.C, Multipennate.

A B C

Perimysium

Endomysium Epimysium

Figure 2-23Connective tissue layers.

In the human body, each individual muscle is composedof a mix of both types of muscle.

When a stimulus is delivered to a muscle from a mo-tor nerve, all fibers in the muscle contract at once.42 Twotypes of muscle contractions have been defined. Duringan isotonic contraction, a muscle shortens its fibers undera constant load. This allows work to occur. During anisometric contraction, the length of the muscle does notchange. This produces tension, but no work. No musclecan perform a purely isotonic contraction, because eachisotonic contraction must be initiated by an isometriccontraction.

Muscle contraction refers to the development of tensionwithin the muscle, not necessarily creating a shortening ofthe muscle. When a muscle develops enough tension toovercome a resistance so that the muscle visibly shortensand moves the body part, concentric contraction is said to oc-cur. Acceleration is thus the ability of a muscle to exert aforce (concentric contraction) on the bony lever to producemovement around the fulcrum to the extent intended.

When a given resistance overcomes the muscle ten-sion so that the muscle actually lengthens, the movementis termed an eccentric contraction. Deceleration is the prop-erty of a muscle being able to relax (eccentric contrac-tion) at a controlled rate. There are numerous clinical ap-plications of the eccentric contraction of muscles,particularly in posture.

Muscles can perform various functions because oftheir ability to contract and relax. One property is that ofshock absorption, another is acceleration, and a third isdeceleration. Each is very important to the overall un-derstanding of the biomechanics of the body and will bediscussed separately. The predominant responsibility forthe dissipation of axial compression shocks rests with themusculotendon system. As a result, shock causes manymusculoskeletal complaints. Shin splints, plantar fasciitis,Achilles tendinitis, lateral epicondylitis, as well as someforms of back pain, can result from the body’s inability toabsorb and dissipate shock adequately.

Although the muscular system is the primary stabilizerof the joint, if the muscle breaks down, the ligaments takeup the stress. This is often seen in an ankle sprain, whenthe muscles cannot respond quickly enough to protect thejoint and the ligaments become sprained or torn. If theligaments are stretched but not torn completely through,this can lead to a chronic instability of the joint, especiallyif the surrounding musculature is not adequately rehabil-itated. When the muscles fail and the ligaments do notmaintain adequate joint stability, the stress cannot be fullyabsorbed by those tissues, and the bone and its architec-ture take up the stress.

Forces applied to joints in any position may causedamage to the bony structure, ligaments, and muscles.Tensile forces generated by muscle contractions can pullapart the cement from the osteons, resulting in fractures

(the most common of which is at the base of the fifthmetatarsal from the pull of the peroneus brevis).Calcaneal fractures from the pull of the Achilles tendonalso occur through this mechanism. Because the closed-packed position has the joint surfaces approximated andcapsular structures tight, an improperly applied forcemay cause fracture of the bone, dislocation of the joint,or tearing of the ligaments. Kaltenborn43 states that it isimportant to know the closed-packed position for eachjoint because testing of joint movements and manipula-tive procedures should not be done to the joint in itsclosed-packed position (Table 2-3). When an improperlyapplied force is used in the open-packed position, thejoint laxity and loss of stability may allow damage to theligaments and supporting musculature.

One of the signs of segmental dysfunction is the pres-ence of muscle hypertonicity. Localized increasedparaspinal muscle tone can be detected with palpation, andin some cases with electromyography. Janda44 recognizesfive different types of increased muscle tone: limbic dys-function, segmental spasm, reflex spasm, trigger points,and muscle tightness. Liebenson45 has discussed the treat-ment of these five types using active muscle contractionand relaxation procedures.

Acute traumatic injury to muscle is generally consid-ered to result from a large force of short duration, influ-encing primarily the elastic deformation of the connec-tive tissue. If the force is beyond the elastic range of theconnective tissue, it enters the plastic range. If the forceis beyond the plastic range, tissue rupture occurs. Morecommonly encountered by the chiropractor is the micro-trauma seen in postural distortions, repetitive minortrauma occurring in occupational and daily living activi-ties, and joint dysfunction as a result of low gravitationalforces occurring over a long period, thus creating plasticdeformation.

Muscle immobilized in a shortened position developsless force with contraction and will tear at a shorter lengththan nonimmobilized muscle with a normal restinglength.46 For this reason, vigorous muscle stretching hasbeen recommended for muscle tightness.44 However, forthe stretch to be effective, the underlying joints should befreely mobile. Patients therefore often require manipula-tion that specifically moves associated joints before musclestretching.

Ligaments

Ligaments are usually cordlike or bandlike structures madeof dense collagenous connective tissue similar to that of atendon. Ligaments are composed of type I and type III col-lagen, with intervening rows of fibrocytes. Also interwovenwith the collagen bundles are elastin fibers that provide ex-tensibility. The amount of elastin varies from ligament toligament. Ligaments exhibit a mechanical property called


crimping that provides a shock-absorbing mechanism andcontributes to the flexibility of the ligament.

Large loads are capable of overcoming the tensile re-sistance of ligaments, resulting in complete or partial tearinjuries. Ligament healing occurs through the basicmechanisms of inflammation, repair, and remodeling.Immobilization of ligamentous tissue results in a dimin-ished number of small diameter fibers47 that presumablylead to joint stiffness. However, the precise mechanismby which immobilization leads to joint stiffness has notbeen determined. It likely results from a combination ofintraarticular adhesion formation and contracture of lig-aments by fibroblasts.48-50

Facet Joints

The common factor in all of the spinal segments from theatlantooccipital joint to the pelvis is the fact that each hastwo posterior spinal articulations. These paired compo-nents have been referred to as the zygapophyseal (meaningan “oval offshoot”) joints and are enveloped in a somewhatbaggy capsule, which has some degree of elasticity. Eachof the facet facings is lined with articular cartilage, as isthe case with all contact-bearing joint surfaces, with theexception of the temporomandibular joint and the sterno-clavicular joint. These joints have intracapsular fibrocarti-laginous discs that separate the joint surfaces.

Compared to intervertebral discs, facet joints havebeen the focus of very little biomechanical research. Yet,these structures must control patterns of motion, protectdiscs from shear forces, and provide support of the spinalcolumn. The orientation of the joint surface varies witheach spinal region, largely governing the degree of free-dom each region can accomplish (Figure 2-24).

Because these joints are true diarthrodial (synovial)articulations, each has a synovial membrane that suppliesthe joint surfaces with synovial fluid. The exact role ofsynovial fluid is still unknown, although it is thought toserve as a joint lubricant or, at least, to interact with thearticular cartilage to decrease friction between joint sur-faces. In addition, the synovium may be a source of nu-trition for the avascular articular cartilage. Intermittentcompression and distraction of the joint surfaces mustoccur for an adequate exchange of nutrients and wasteproducts to occur.2 Furthermore, as mentioned, immobi-lized joints have been shown to undergo degeneration ofthe articular cartilage.11 Certainly, the nature of synovialjoint function and lubrication is of interest because thereis evidence that the facet joints sustain considerable stressand undergo degenerative changes.

The capsule is richly innervated with nociceptors(pain) and mechanoreceptors (proprioception), allowingthe supporting structures to react to many combinations oftension and compression moments imposed by differentpostures and physical activity. Each movement of the joint

must first overcome the resistance of the capsule but mustthen be able to return to its original position maintainingjoint apposition. The lateral portions of the capsule aremuch more lax and contain fewer elastic fibers.51

Although the posterior joints were not designed to bearmuch weight, they can share up to about one third of thisfunction with the intervertebral disc. Moreover, as a part ofthe three-joint complex, if the disc undergoes degenerationand loses height, more weight-bearing function will fall onthe facets. During long periods of axial loading, the discloses height through fluid loss, thereby creating moreweight bearing on the facets on a daily basis.

The posterior joints also have been found to containfibroadipose meniscoids that apparently function to adaptto the incongruity of the articular surfaces but whose clin-


45°

0°

60°

90°

90°

120°

Figure 2-24Facet planes in each spinal region viewed from the side and above. A, Cervical (C3-C7). B, Thoracic. C, Lumbar.(Modified from White AA, Panjabi MM: Clinical biomechanicsof the spine, Philadelphia, 1978, JB Lippincott.)

A

B

C

ical significance remains controversial. Bogduk andEngel52 provide an excellent review of the meniscoids ofthe lumbar zygapophyseal joints. Although the genesis oftheir article was as a literature review to support the con-tention that the meniscoids could be a cause of an acutelocking of the low back because of entrapment, the articlealso provided a comprehensive review of the anatomicconsideration of lumbar meniscoids.

The meniscoids appear to be synovial folds continu-ous with the periarticular tissues and with both intracap-sular and extracapsular components. Microscopically, thetissue consisted of loose connective and adipose tissue,mixed with many blood vessels (Figure 2-25). Themeniscoids could present in various shapes, including an-nular menisci found in the thoracic region, with lin-guiform menisci and filiform menisci commonly found inthe lumbar region.53

These meniscoid structures can project into the jointspace when the joint surfaces of articular cartilage are notin contact. Bogduk and Engel52 noted two groups ofmenisci: one located along the dorsal and ventral marginsof the joint and one located at the superior and inferioraspects of the joint. In their view, only the ones locatedalong the dorsal and ventral borders of the joint repre-sented true meniscoids. Functionally, Bogduk and Engelfeel these structures may help to provide greater stabilityto a lumbar zygapophyseal joint by helping to distributethe load over a wider area. In their words, meniscoidsplay a space-filling role.52

Clinically and theoretically, these meniscoids may be-come entrapped or extrapped.54 Entrapment of the menis-coid between the joint surfaces itself is not believed to bepainful, although pain can be created by traction on thejoint capsule through the base of the meniscoid. This could,

through a cascade of events, lead to more pain and reflexmuscle spasm, known as acute locked low back, which isamenable to manipulative therapy. Extrapment of themeniscoid may occur when the joint is in flexed position andthe meniscoid is drawn out of the joint but fails to reenterthe joint space on attempted extension. It gets stuck againstthe edge of the bony lip or articular cartilage, causing abuckling of the capsule that serves as a space-occupying lesion. Pain is produced through capsular distention.55

Giles and Taylor51,56 examined the innervation ofmeniscoids (synovial folds) in the lumbar zygapophysealjoints, using both light microscopy and transmissionelectron microscopy. The authors removed part of theposteromedial joint capsule along with the adjacent liga-mentum flavum and synovial folds after a laminectomy,fixed these specimens in various solutions, and preparedthem for microscopy. They demonstrated that neuro-logic structures were located in the areas studied. Nervesseen in the synovial fold were 0.6 to 12 �m in diameter.These neurologic structures may give rise to pain.

Taylor and Twomey57 suggest that because of theirrich blood supply, spinal joint meniscoids do not undergodegeneration with age as do the intervertebral disc andarticular cartilage. However, with degenerative changesto disc and especially articular cartilage, the meniscoidinclusions are exposed to abnormal biomechanical forcesthat may result in their demise.

Adams and Hutton58 examined the mechanical func-tion of the lumbar apophyseal joints on spines taken fromcadavers. The authors wanted to examine various loadingregimens on the function of these joints. They found thatthe lumbar zygapophyseal joints can resist most of the in-tervertebral shear force only when the spine is in a lor-dotic posture. These joints also can aid in resisting theintervertebral compressive force and can prevent exces-sive movement from damaging the intervertebral discs.The facet surfaces protect the posterior annulus, whereasthe capsular ligament helps to resist the motion of flex-ion. The authors noted that in full flexion the capsularligaments provide nearly 40% of the joint’s resistance.They conclude that “the function of the lumbar apophy-seal joints is to allow limited movement between verte-brae and to protect the discs from shear forces, excessiveflexion and axial rotation.”58

Taylor and Twomey57 studied how age affected thestructure and function of the zygapophyseal joints. Theytook transverse sections of the lumbar spine from cadav-ers ranging in age from fetus to 84 years and preparedthem in staining media. They noted that fetal and infantlumbar zygapophyseal joints are coronally oriented, andonly later (in early childhood) become curved or biplanarjoints. In the adult, the joint has a coronal component inthe anterior third of the joint and a sagittal component inthe posterior two thirds of the joint. The joint is gener-ally hemicylindrical.


Superiorarticular

facet

Articularcapsule

Adipose tissuecells of the base of the meniscoid

Articularcartilage

Fibrous capof meniscoid

Inferiorarticularfacet

Figure 2-25Fibroadipose meniscoid in a lumbar facet joint. (Modifiedfrom Bogduk N, Engel R: Spine 9:454, 1984.)

The structures located in the anterior third of thejoint, primarily articular cartilage and subchondral bone,tend to show changes that are related to loading the jointin flexion. The posterior part of the joint shows a varietyof different changes related to age. There may bechanges from shearing forces. The subchondral bone willthicken as it ages and is wedge shaped. These changes oc-cur because of loading stresses from flexion.57

Taylor and Twomey57 are careful to note that theycould make no clinical correlation with their findings,which is one of the problems with cadaveric studies ofthis sort. They believe that this work has biomechanicalimplications; they feel that the lumbar zygapophysealjoints limit the forward translational component of flex-ion to only a very small displacement. Indeed, they feelthis fact may be the most important component limitingforward flexion. Although the lumbar facet joints are ori-ented in the sagittal plane, they are not purely sagittal,and flexion with anterior translation will result in im-paction of the facets limiting this movement.37

Intervertebral Discs

The intervertebral discs are fibrocartilaginous mu-copolysaccharide structures that lie between adjoiningvertebral bodies. In the adult, there are 23 discs, eachgiven a numeric name based on the segment above. Thusthe L5 disc lies between the fifth lumbar segment and thesacrum, and the L4 disc lies between the fourth and fifthlumbar segments. In the early years of life, the discs be-tween the sacral segments are replaced with osseous tis-sue but remain as rudimentary structures, generally re-garded as having no clinical significance.

The unique and resilient structure of the disc allowsfor its function in weight bearing and motion. The ante-rior junction of two vertebrae is an amphiarthrodial sym-physis articulation formed by the two vertebral endplatesand the intervertebral disc. The discs are responsible forapproximately one fourth of the entire height of the ver-tebral column. The greater the height of the interverte-bral disc as compared to the height of the vertebral body,the greater the disc to vertebral body ratio and thegreater the spinal segmental mobility. The ratio is great-est in the cervical spine (2/5) and least in the thoracicspine (1/5), with the lumbar region (1/3) in between. Adisc has three distinct components: the annulus fibrosus,the nucleus pulposus, and the cartilaginous endplates.

The cartilaginous endplates are composed of hyalinecartilage that separates but also helps attach the disc tothe vertebral bodies. There is no closure of cortical bonebetween the hyaline cartilage and the vascular cancellousbone of the vertebral body. The functions of the end-plates are to anchor the disc, to form a growth zone forthe immature vertebral body, and to provide a permeablebarrier between the disc and the body. This role allows

the avascular disc material to receive nutrients and repairproducts.

The annulus fibrosus is a fibrocartilaginous ring thatencloses and retains the nucleus pulposus, although thetransition is gradual, with no clear distinction betweenthe innermost layers of the annulus and the outer aspectof the nucleus. The fibrous tissue of the annulus isarranged in concentric, laminated bands, which appear tocross one another obliquely, each forming an angle ofabout 30 degrees to the vertebral body (Figure 2-26).The annular fibers of the inner layers are attached to thecartilaginous endplates, and the outer layers are attacheddirectly to the osseous tissue of the vertebral body bymeans of Sharpey’s fibers.59

Superficially, the anterior longitudinal ligament andthe posterior longitudinal ligament (PLL) reinforce thefibers. The PLL is clinically significant in that as itcourses caudally, its width narrows until covering onlyabout 50% of the central portion of the lower lumbardiscs. The weakest area of the annulus, and hence thearea most likely to be injured, is the posterolateral aspect.This is the most likely spot for a disc herniation in thelumbar spine.60


Nucleus Annularlaminates

30°30°

Figure 2-26Intervertebral disc. A, Nucleus pulposus and annulus fibrosus.B, Orientation of annular fibers. (Modified from White AA,Panjabi MM: Clinical biomechanics of the spine, Philadelphia,1978, JB Lippincott.)

A

B

The annulus fibrosus contains little elastic tissue,and the amount of stretch is limited to only 1.04 timesits original length, with further stretch resulting in atearing of fibers. The functions of the annulus fibrosusinclude enclosing and retaining the nucleus pulposus,absorbing compressive shocks, forming a structuralunit between vertebral bodies, and allowing and re-stricting motion.

The nucleus pulposus is the central portion of the discand is the embryologic derivative of the notochord. It ac-counts for about 40% of the disc and is a semifluid gel thatwill deform easily but is considered incompressible. Thenucleus is composed of a loose network of fine fibrousstrands that lie in a mucoprotein matrix containing mu-copolysaccharides, chondroitin sulphate, hyaluronic acid,and keratan sulfate. These large molecules are stronglyhydrophilic, capable of binding nearly nine times theirvolume of water, and are therefore responsible for thehigh water content of the disc. In young adults, the watercontent of a disc approaches 90% and maintains an inter-nal pressure of about 30 pounds per square inch.1 The wa-ter content, however, steadily decreases with age. Thecomposition of the nucleus produces a resilient spacerthat allows motion between segments, and although itdoes not truly function as a shock absorber, it does serveas a means to distribute compressive forces.

The image of the nucleus as a round ball between twohard surfaces must be abandoned. This gives the impres-sion that the nucleus can roll around between the twoendplates. The only means for significant nuclear migra-tion is through a tear in the annular fibers, allowing thenucleus to change shape but not actually shift position.The result of nuclear migration will be a potential changein the instantaneous axis of movement and potentialaberrant segmental motion.

The intervertebral disc is a vital component for theoptimal, efficient functioning of the spinal column. Inconjunction with the vertebral bodies, the discs form theanterior portion of the functional unit responsible forbearing weight and dissipating shock. In so doing, it dis-tributes loads, acts as a flexible buffer between the rigidvertebrae, and permits adequate motion at low loadswhile providing stability at higher loads.

The simple compression test of the disc has been oneof the most popular experiments because of the impor-tance of the disc as a major load-carrying element of thespine. Axial compression forces continually affect the discduring upright posture. The nucleus bears 75% of thisforce initially but redistributes some to the annulus.

Furthermore, the ability of the disc to imbibe watercauses it to “swell” within its inextensible casing. Thusthe pressure in the nucleus is never zero in a healthy disc.This is termed a preloaded state. The preloaded state givesthe disc a greater resistance to forces of compression.

With age and exposure to biomechanical stresses, thechemical nature of the disc changes and becomes more

fibrous. This reduces the imbibition effect and, in turn,the preloaded state. As a result, flexibility is diminished,and more pressure is exerted on the annulus and periph-eral areas of the endplate. A disc that has been injuredwill deform more than a healthy one.

The preloaded state also explains the elastic proper-ties of the disc. When the disc is subjected to a force, thedisc exhibits dampened oscillations over time. If the forceis too great, however, the intensity of the oscillations candestroy the annulus, thus accounting for the deteriora-tion of intervertebral discs that have been exposed to re-peated stresses.

Compressive forces are transmitted from endplate toendplate by both the annulus and the nucleus. Whencompressed, the disc bulges in the horizontal plane. Adiseased disc will compress more, and as this occurs,stress is distributed differently to other parts of the func-tional unit, notably the apophyseal articulations. Becausethe disc is prepared for axial compression, it should benoted that under large loads, the endplate will fracture(Schmorl’s node) (Figure 2-27) or the anterior vertebralbody will collapse.

Axial tensile stresses are also produced in the annulusduring the movements of flexion, extension, and lateralflexion. The motions create compression stresses ipsilat-erally and tensile stresses contralaterally. This causes abulging (buckling) on the concave side and a contractionon the convex side of the disc (Figure 2-28).

Axial rotation of the spine also produces tensilestresses in the disc. Studies have shown that the greatesttensile capabilities of the disc are in the anterior and pos-terior regions; the center portion of the disc is the weak-est. When the disc is subjected to torsion, shear stressesare produced in the horizontal and axial planes. Shearstresses act in the horizontal plane, perpendicular to thelong axis of the spine. It has been found that torsionalforces, and hence shear forces, can be the injury-causingload factors. During normal movements, the disc is pro-


Figure 2-27Effects of axial loads on vertebral body and disc. A, Normaldisc height. B, Normal disc under mild to moderate axial load,showing slight approximation of bodies. C, Diseased disc un-der same axial load, showing significant loss of disc height.D, Endplate fracture from significant axial load causing aSchmorl’s node.

A B C D

tected from excessive torsion and shear forces by thelumbar facet joints.

All viscoelastic structures, which include the disc, ex-hibit hysteresis and creep. Cadaveric studies allowedTwomey and Taylor61 to study creep and hysteresis in thelumbar spine. Hysteresis refers to the loss of energy whenthe disc or other viscoelastic structures are subjected torepetitive cycles of loading and unloading. It is the ab-sorption or dissipation of energy by a distorted structure.For example, when a person jumps up and down, the shockenergy is absorbed by the discs on its way from the feet tothe head. The larger the load is, the greater the hysteresiswill be.1 When the load is applied a second time, the hys-teresis decreases, meaning there is less capacity to absorbthe shock energy (load). This implies that the discs are lessprotected against repetitive loads.

Creep is the progressive deformation of a structureunder constant load. When a load is applied to a vis-coelastic structure, it immediately deforms under the

load. If the load is maintained, there will be continueddeformation over time. As might be expected, the creepand hysteresis created in differing types of load forces(e.g., flexion loading vs. extension loading) may differ,but this has not been quantified for the lumbar spine.

Because the disc is under the influence of the pre-loaded state of the nucleus, movements will have specificeffects on the behavior of the nucleus and annular fibers.When a distraction force is applied, the tension on theannular fibers increases, and the internal pressure of thenucleus decreases. When an axial compression force isapplied symmetrically, the internal pressure of the nu-cleus increases and transmits this force to the annularfibers. The vertical force is transformed into a lateralforce, applying pressure outward.

During the asymmetric movements of flexion, exten-sion, and lateral flexion, a compressive force is applied tothe side of movement, and a tensile force occurs on theopposite side. The tension transmitted from the nucleusto the annular fibers helps to restore the functional unitto its original position by producing a “bowstringlike”tension on the annular fibers.

During axial rotation, some layers of the annulus arestretched while others are compressed (slackened).Tension forces reach a maximum within the internal lay-ers of the annulus. This has a strong compressive forceon the nucleus and causes an increased internal pressureproportional to the degree of rotation.

Kurowski and Kubo62 investigated how degenerationof the intervertebral disc influences the loading condi-tions on the lumbar spine. Because disc degeneration iscommon, almost inevitably it will contribute to low backdysfunction by influencing motion and load bearing ateach individual level. Kurowski and Kubo62 examinedload transmission through the lumbar spine with differ-ing amounts of disc degeneration and used fine elementanalysis to study stress transmission. In a healthy disc,they found the highest effective stresses in the center ofthe endplate of the vertebra, but in an unhealthy and de-generated disc, they found these stresses in the lateral as-pects of the endplates, as well as in the cortical wall andvertebral body rims.

MODELS OF SPINE FUNCTION

Understanding the overall function of the human spinehas proved to be difficult and frustrating. It is importantto view the spine as an integrated functioning unit. Itmust be remembered, however, that the spine is also apart of the larger locomotor system. If consideration isnot given to the whole locomotor system, the potentialfor clinical failures results.

The vertebral column is a flexible axis composed ofthe articulated vertebrae. The spine must be rigid for itto maintain upright bipedal posture, yet it has to deformits shape to allow for mobility. In addition, it houses and


Compression

Instantaneous axis of rotation

Tension

Instantaneous axis of rotation

Tensilestress

Compressivestress

Figure 2-28Disc stresses with bending movements of flexion, extension,and lateral flexion. Tension is produced on the convex side,whereas compression and buckling occur on the concave side.

protects the spinal cord while providing a means for neu-rologic transmission to and from the periphery.

Many models of spine function have been developed,each attempting to define spine function according to newand different parameters. Gracovetsky63 proposed a modelof the spine based on the concept that spinal joints con-tain stress sensors that drive a feedback mechanism. Hebelieves that this mechanism creates an arrangement thatcan react to loads by modifying muscular action to de-crease or minimize stress at those joints and cut the risk ofinjury. This model depicts the spine in terms of stresses,forces, and moments acting at the intervertebral joints.

As Gracovetsky63 notes, mathematical models of erec-tor spinae muscles demonstrate that these muscles cannotsupport more than 50 kg of weight, so other or additionalmechanisms must explain the human ability to carry loadsgreater than that. To explain this feature of the humanspine, Gracovetsky63 theorizes that the interaction be-tween the erector spinae group and the abdominal mus-cles is of “fundamental importance” in understandingspinal function. He later uses this theory to show howposture and behavior may produce spinal injury.

One of the major problems with this model is that nosuch monitoring system of stress sensors has been delin-eated by neurophysiologic research. Gracovetsky showshow the system would have to work if such sensors didexist, and thus this model of the spine is based on the de-marcation of stress, loads, and moments. With such amathematical model in place, it is possible to determinespinal disability.

Aspden64 notes that many theories of the spine tend tofall into two broad categories: those that treat the spine asa cantilever and those that perform an elastic analysis ofthe system. When the spine is treated as a cantilever system that is connected to a series of free bodies, themovement created by the spinal muscles acting about thesacrum balances forward-bending moments. However, inusing such a model to make mathematical calculations,the forces generated are extremely high and may be dan-gerous. Furthermore, they probably do not exist.

Aspden’s model to explain the static behavior of thehuman spine looks at the spine as an arch rather than asthe more accepted cantilever system. According toAspden,64 if the spine is considered an arch, its mechani-cal stability can be described and the forces developedalong the spinal axis for any given posture or load can becalculated.

Aspden64 notes that the human spine shares manycharacteristics with a masonry arch and that a masonryarch can be analyzed using plasticity theory. The plastic-ity theory describes the behavior of a structure once it hasbeen loaded beyond its limits, and it describes how thesematerials flow in response to stress. To reduce stress, thestructure deforms. Also, the theory helps provide thelimits of elastic behavior.

In normal erect posture, the lumbar lordosis forms anarch, which is convex anteriorly, and this brings the ver-tebral bodies almost directly into the center of the body.With spinal flexion, this arch will flatten and even reverseso that it is concave anteriorly, and a single arch is formedwith the thoracic and lower cervical vertebrae. Bodyweight is transmitted along the spinal axis. The forcesgenerated can then be calculated. Muscle forces can beoverlaid on this, and then forces can be recalculated.

Aspden shows how this can be calculated for a spineplaced in certain configurations.64 When a practitionerhas this information, it is then possible to predict failurewhen these criteria are not met. Using these procedures,Aspden demonstrates that compressive forces developedin the spine may not be as high as was previously be-lieved. He also demonstrated that normal spinal curva-tures are necessary for proper load-bearing function. Thepresence of the normal lumbar lordosis, coupled with in-traabdominal pressure, helps to provide the spine withstrength and to protect the spine from injury duringheavy load lifting.

The human spine, with its musculature removed,cannot carry normal physiologic loads. This fact ledPanjabi et al65 to devise a model of spinal stability and in-tersegmental muscle force. They note that muscles arenecessary to stabilize the spine and to allow the spine tocarry out its other physiologic functions.

This stabilization feature is in addition to the obviousneed for a muscle to move body parts. Their experimentsimulated intersegmental muscle forces on spinal insta-bility, subjecting cadaveric lumbar functional spinal units(FSUs) to biomechanical tests of increasing muscleforces. Compressive preload and six physiologic move-ments were applied to a series of FSUs to determinethree-dimensional motion of the spine. The FSUs werealso then given a series of injuries, and incremental inter-segmental muscle forces were applied to the upper verte-bra of the FSUs. The same tests were then repeated onthe injured segments. The injuries included division ofthe supraspinous and interspinous ligaments,1 left medialfacetectomy,2 and bilateral medial facetectomy.3 Somebiomechanical parameters, including range of motionand neutral zone, were then calculated.

When the forces produced by muscle were applied,range of motion and neutral zone increased in flexionloading, although both decreased in extension loading.65

With lateral bending, neither of these parameters was af-fected by applying the muscle forces. With rotation, therange of motion was significantly decreased. Panjabi etal65 concluded that an action of the intersegmental mus-cle forces is to maintain or decrease intervertebral mo-tions after injury.

Louis66 examined spinal stability from an entirely dif-ferent perspective—that of the three-column spine. Henotes an axial stability and a transverse stability in the


spine. The axial stability is maintained along a verticalcolumn system consisting of two columns at the C1 toC2 level and three columns from C2 to the sacrum(Figure 2-29). These three columns consist of one ante-rior column (formed by the vertebral bodies and the disc)and two posterior columns (formed by the posteriorjoints). The transverse stability is the result of the cou-pling of bony buttresses and ligamentous brakes.

Louis sees the C1 vertebra as two lateral masses joinedby two arches. He sees the C2 vertebra as three pillars: “avertical conical pillar lying medially and anteriorly (densand body) and two lateral oblique pillars.”66 These threepillars are fused above in the body of C2 and then divergebelow that area. The three resultant pillars run down to thesacrum, where three points of contact support the pillars atthe sacral base and at the two sacral facets. Of the three, theanterior pillar (as compared to the two posterior pillars) isby far the largest. It takes on the characteristics of a quad-rangular pyramid that is formed by alternating vertebralbodies and intervertebral discs down to the sacral base. Inthis model, the spinous processes and transverse processesdo not contribute to spinal stability. Louis66 believes thisthree-column model of the spine provides the simplest andmost efficient system of stability (Figure 2-29).

Gracovetsky and Farfan67 use system theory to de-scribe a model of the human spine. After a great deal ofdiscussion of the evolutionary considerations of the hu-man spine, they make the point that intervertebral jointsare essential for our survival as a species. They describethe mechanical behavior of the intervertebral joint andthen use that information to calculate spinal motion andmuscular action. This allows the authors to ultimatelydevise a new theory of human locomotion, which also al-lows for the calculation of safe loads for the spine. Theirsis one of the most detailed and important papers con-cerning mathematical modeling of the human spine.

Another model of the spine considers the structuralintegrity of the spine as a whole, providing an interestinglook at how adaptation to upright biped posture placesspecific demands on the spinal components. A structure isdefined as any assemblage of materials that is intended tosustain loads. Each life form needs to be contained by astructure. Even the most primitive unicellular organismhas to be enclosed and protected by cell membranes thatare both flexible and strong, yet capable of accommodat-ing cell division during reproduction. With advancementof and competition in evolving life forms, the structurerequirements need to become more sophisticated. Themajority of living tissues have to carry mechanical loadsof one kind or another. Muscles also have to apply loads,changing shape as they do so. By making use of contrac-tile muscles as tension members and strong bones ascompression members, highly developed vertebrate ani-mals have been able to withstand necessary loads and stillallow for mobility, growth, and evolution.

Parallels have been drawn between the spine and themast of a ship. Compressive loads are concentrated inthe vertebrae of the spine and the wooden mast of theship. Tension loads are diffused into tendons, skin, andother soft tissues of the body and into the ropes andsails of the ship to maintain an upright position. How-ever, a ship mast is immobile, rigidly hinged, verticallyoriented, and dependent on gravity. These rigidcolumns require a heavy base to support the incumbentload. The biologic structure of the spine, however, must


Figure 2-29Three-column spine model, representing vertebral bodies andarticular pillars. (Modified from Louis R: Anat Clin 7:33, 1985.)

be a mobile, flexibly hinged, low-energy-consuming,omnidirectional structure that can function in a gravity-free environment.68

Comparisons have also been made between the spineand a bridge (or truss). The musculoskeletal configura-tion of a large four-legged animal (e.g., horse) has a largebody, is capable of bearing a substantial load in additionto it own weight, rests on four slender compressionmembers (leg bones), and is supported efficiently by anassortment of tension members (tendons, muscles, andskin). Trusses have flexible, even, frictionless hinges, withno bending moments about the joint. The support ele-ments are either in tension or compression only. Loadsapplied at any point are distributed about the truss as ten-sion or compression.68

Although this model sounds quite plausible for thespine, it is not a complete explanation. Most trusses areconstructed with tension members oriented in one direc-tion. This means that they will function in only one di-rection and can therefore not function as the mobile, om-nidirectional structure necessary for describing the spinalfunctions. Moreover, bridges do not have to move,whereas vertebrate animals do. Furthermore, the com-parison cannot be directly applied to the human skeleton,since it is upright and the forces are applied in the longaxis rather than along it.

Levin68 identifies another class of truss called tenseg-rity structures that are omnidirectional so that the ten-sion elements always function in tension regardless of

the direction of the applied force. The structure thatfits the requirements of an integrated tensegrity modelhas been described and constructed as the tensegrityicosahedron. In this structure, the outer shell is undertension, and the vertices are held apart by internalcompression struts that seem to float in the tensionnetwork (Figure 2-30). In architecture, stable form isgenerated through an equilibrium between many inter-dependent structures, each of which is independentlyin a state of disequilibrium. Complex architecture can-not be broken up into isolated pieces without losingqualities that are inherent to the structural whole. Thisis extremely important in biologic systems in whichevery functional unit is literally more than the sum ofits constituent parts.69

Many architectural structures are dependent on compressive forces for structural integrity. Compression-dependent structures are inherently rigid and poorlyadapted for a rapidly changing environment. Most natu-rally occurring structures depend on natural forces fortheir integrity.70 The human body can be described as atensile structure in which tensional integrity (tensegrity)is maintained by muscles suspended across compression-resistant bones.

Fuller71 spoke for many years of a universal system ofstructural organization of the highest efficiency based ona continuum of tensegrity. Fuller’s theory of tensegrity de-veloped out of the discovery of the geodesic dome, themost efficient of architectural forms, and through study ofthe distribution of stress forces over its structural ele-ments. A tensegrity system is defined as an architecturalconstruction that is composed of an array of compression-resistant struts (bones) that do not physically touch oneanother but are interconnected by a continuous series oftension elements (muscles and ligaments).69 Because ac-tion and reaction are equal and opposite, the tensionforces have to be compensated by equal and oppositecompressive forces and vice versa.

Gravitational force is a constant and greatly underes-timated stressor to the somatic system. The most obviouseffect of gravitational stress can be evaluated by carefulobservation of posture, which is both static and dynamic.The static alignment of body mass with respect to grav-ity is constantly adjusted by dynamic neuromuscular co-ordination as the individual changes position. Over time,individual static postural alignment conforms to inherentconnective tissue structure, as well as the cumulativefunctional demands of both static and dynamic posturalconditions.

Musculoligamentous function is also significantly in-fluenced by, as well as responsible for, static and dynamicpostural alignment.72 The development of asymmetricfunctional barriers in the spine likely has more than onecause. A unifying factor, however, is the transfer of forceswithin the soft tissues, creating altered and asymmetrictension, namely the tensegrity mechanism.


Compressionmember

Tensionmember

Figure 2-30Tensegrity icosahedron with rigid compression members andelastic tension members. Multiple units sharing a compressionmember form a structural model of the spine. (Modified fromBergmann TF, Davis PT: Mechanically assisted manual tech-niques: Distraction procedures, St Louis, 1998, Mosby.)

When the various principles and research noted hereare combined, a more complete picture of spinal biome-chanics is developed, one in which pathologic changesmay ultimately be better studied as well.

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