Skeletal System – Part 6

Bones of the Pelvic Girdle

Pelvic Girdle - Formed by two coxal bones, or ossa coxae, commonly called the hip bones.

Bony Pelvis – Hip bones + coccyx + sacrum

Bones of the Pelvic Girdle

Large and heavy bones, and they are attached to the axial skeleton.

The sockets, which receive the thigh bones, are deep and heavily reinforced by ligaments that attach the limbs firmly to the girdle.

Functions of the Girdle:1. Bearing weight is the most important function.2. Protect the reproductive organs, urinary bladder, and

part of he large intestine.

Structure of the Hip Bones

Each hip bone is formed by the fusion of three bones:

1. Ilium2. Ischium3. Pubis

Ilium

Ilium – Large, flaring bone that forms most of the hip bone. Connects posteriorly

with the sacrum at the sacroiliac joint.

When you put your hands on your hips, they are resting over the winglike portion of the ilia.

Iliac Crest – The upper edge of the winglike portion of the ilium.

Ischium

Ischium – “Sitdown bone”; Forms the the most inferior part of the coxal bone.

Pubis – Most anterior part of a coxal bone.

Acetabulum

Acetabulum – Deep socket that receives the head of the thigh bone. Formed by the fusion of the ilium,

ischium, and pubis.

Regions of the Bony Pelvis

False Pelvis – Superior to the true pelvis; The area medial to the flaring portions of the ilia.

True Pelvis - Surrounded by bone; Lies inferior to the flaring parts of the ilia and the pelvic brim. Dimensions of the true pelvis

of a woman are very important because they must be large enough to allow the infant’s head to pass during childbirth.

Differences Between a Male and Female Pelvis

The pelvis of a female tends to be:

1. Inlet is larger and more circular.

2. As a whole is shallower, and the bones are lighter and thinner.

3. Ilia flare more laterally.4. Sacrum is shorter and

less curved.5. Ischial spines are shorter

and farther apart; thus the outlet is larger.

6. Pubic arch is more rounded because the angle of the pubic arch is greater.

Bones of the Lower Limbs

The lower limbs carry our total body weight when we are erect.

Hence, it is not surprising that the bones of the lower limbs are much thicker and stronger than the comparable bones of the upper limb.

The 3 segments of the lower limbs:1. Thigh2. Leg3. Foot

Thigh

Femur – Thigh bone. Only bone in the thigh. The heaviest, strongest

bone in the body. It slants medially as it runs

downward to join with the leg bones. This brings the knees in

line with the body’s center of gravity.

The medial course of the femur is more noticeable in females because of the wider female pelvis.

Structure of the Femur Bone

Its proximal end has a:1. Ball-like head The head of the femur

articulates with the acetabulum of the hip bone in a deep, secure socket.

2. A neck Common site of fractures,

especially in old age.

The Distal Femur

Anteriorly on the distal femur is the smooth patellar surface, which forms a joint with the patella (kneecap).

Leg

Two bones form the skeleton of the leg:

1. Tibia2. Fibula

The tibia and fibula are connected along their length by an interosseous membrane.

Tibia and Fibula

Tibia – Shinbone; Larger and more medial. At the proximal end, the tibia

articulates with the distal end of the femur to form the knee joint.

Fibula – Lies alongside the tibia; Thin and sticklike. Forms joints with the tibia both

proximally and distally. Has no part in forming the

knee joint. The distal end of the fibula

forms the outer part of the ankle.

Foot

The foot is composed of the:

1. Tarsals2. Metatarsals3. Phalanges

Two important functions of the foot:

1. Supports our body weight

2. Serves as a lever that allows us to propel our bodies forward when we walk or run.

Foot: Tarsals

Tarsus – The posterior half of the foot. Composed of 7 tarsal

bones. Body weight is mostly

carried by the two largest tarsals:

1. Calcaneus - Heelbone2. Talus – Tarsal that lies

between the tibia and the calcaneus.

Foot: Metatarsals and Phalanges

The sole of the foot: Composed of 5

metatarsals.

The toes of the foot: Composed of 14

phalanges. Like the fingers of the

hand, each toe has three phalanges, except the great toe which has two.

Arches of the Foot

The bones in the foot are arranged to form three strong arches:

1. Two longitudinal (medial and lateral)

2. One Transverse

Ligaments (bind the foot bones together) and tendons:

Help to hold the bones firmly in the arched position but still allow a certain amount of give or springiness.

Joints

Every bone in the body (except the hyoid bone of the neck) forms a joint with at least one other bone.

Joints (Articulations) – Sites where two or more bones meet.

Functions of Joints

Joints have two functions:1. Hold the bones together securely 2. Give the rigid skeleton mobility

Joints are classified in two ways:1. Functionally (focuses on the amount of movement

allowed by the joint)2. Structurally

Functional Classification of Joints

1. Synarthroses: Immoveable joints;Allows no movement. - Example: Bones in the skull

2. Amphiarthroses – Slightly moveable joints; Allow a small amount of restricted movement. - Example: Vertebrae and the joints

between the two bones of the lower leg.

3. Diarthroses - Freely Moveable; Permit movement in one or more directions- Example: Shoulder, neck, and knee

Structural Classification of Joints

Structurally, there are three types of joints:

1. Fibrous Joints2. Cartilagenous

Joints3. Synovial Joints

Fibrous Joints

Fibrous Joints – The bones are united by fibrous tissue; As a general rule, these are immoveable. Bones are bound tightly together by

CT fibers, allowing essentially no movement. Example: Sutures of the skull.

Cartilagenous Joints

Cartilagenous Joints – Bone ends are connected by cartilage. Slightly Moveable Examples:

Pubic symphysis of the pelvis Intervertebral joints of the

spinal column (connected by discs of fibrocartilage)

Immoveable Examples: The Epiphyseal plates of

growing long bones Joints between the first ribs

and the sternum.Most cartilagenous joints

are slightly moveable.

Synovial Joints

Synovial Joints – Joints in which the articulating bone ends are separated by a joint cavity containing synovial fluid.

All synovial joints have four distinguishing characteristics:

1. Articular cartilage Covers the ends of the

bones forming the joint.

2. Fibrous articular capsule

Joint surfaces are enclosed by a sleeve or capsule of fibrous CT

The capsule is lined with a smooth synovial membrane (the reason these joints are called synovial joints).

All synovial joints have four distinguishing characteristics (continued):

3. Joint cavity The articular capsule

encloses a cavity, called the joint cavity, which contains lubricating synovial fluid.

4. Reinforcing ligaments

The fibrous capsule is usually reinforced with ligaments.

Bursae and Tendon Sheaths

Not strictly part of synovial joints, but they are often found closely associated with them. Bursae – Flattened fibrous

sacs lined with synovial membrane and containing a thin film of synovial fluid. Common where ligaments,

muscles, skin, tendons, or bones rub together.

Tendon Sheath – An elongated bursae that wraps completely around a tendon subjected to friction. Like a bun around a hot

dog.

Types of Synovial Joints: Based on Shape

1. Plane2. Hinge3. Pivot4. Condyloid5. Saddle6. Ball-and-socket

Plane Joint

Plane Joint – The articular surfaces are essentially flat, and only short slipping or gliding movements are allowed. Movements are nonaxial (does not involve rotation

around any axis). Examples: Intercarpal joints of the wrist.

Hinge Joint

Hinge Joint – The cylindrical end of one bone fits into a trough-shaped surface on another bone. Angular movement is allowed in

just one plane, like a mechanical hinge. Classified as uniaxial (they allow

movement around one axis only). Examples: elbow joint, ankle

joint, and the joints between the phalanges of the fingers.

Pivot Joint

Pivot Joint – The rounded end of one bone fits into a sleeve or ring of bone. Because the rotating bone can

turn only around its long axis, pivot joints are also uniaxial joints.

Examples: Proximal radioulnar joint Joint between the atlas and the

dens of the axis

Condyloid Joint

Condyloid Joint – The egg-shaped articular surface of one bone fits into an oval concavity in another. Both of these articular surfaces

are oval. Allow the moving joint to travel

(1) from side to side and (2) back and forth. But the bone cannot rotate

around its long axis. Movement occurs around two

axes, hence these joints are biaxial.

Saddle Joint

Saddle Joint – Each articular surface has both convex and concave areas, like a saddle. These biaxial joints allow

essentially the same movements as condyloid joints.

Example: Carpometacarpal joints in the thumbs

Ball-and-Socket Joint

Ball-and-Socket Joint - the spherical head of one bone fits into a round socket in another.

These multiaxial joints allow movement in all axes, including rotation and are the most freely moving synovial joints.

Examples: Shoulder and hip

Dislocations

A dislocation happens when a bone is forced out of its normal position in the joint cavity. Reduction – The process of

returning the bone to its proper position. Should be done only by a

physician. Attempts by an untrained person

to “snap the bone back into its socket” are usually more harmful than helpful.

Sprains

Sprains – The ligaments or tendons reinforcing a joint are damaged by excessive stretching, or they are torn away from the bone. Since tendons and ligaments get poor blood supply,

sprains heal slowly and are extremely painful.