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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

From: Harmony Medical3

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


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