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Chapter 8: Joints of the Skeletal System

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Chapter Summary

Introduction (page 2 71)

A joint forms wherever two or more bones meet. Joints are the functional junctions between bones.

Classification of Joints (page 2 71)

Joints are classified according to the type of tissue that binds the bones together.

1. Fibrous joints a. Bones at fibrous joints are fastened tightly together by a layer of dense connective tissue with many collagenous fibers.

b. There are three types of fibrous joints.

(1) A syndesmosis is characterized by bones bound by relatively long fibers of connective tissue.

(2) A suture occurs where flat bones are united by a thin layer of connective tissue and become interlocked by a set of bony processes.

(3) A gomphosis is formed by the union of a cone-shaped bony process to a bony socket.

2. Cartilaginous joints a. A layer of cartilage holds together bones of cartilaginous joints.

b. There are two types of cartilaginous joints.

(1) A synchondrosis is characterized by bones united by hyaline cartilage that may disappear as a result of growth.

(2) A symphysis is a joint whose articular surfaces are covered by hyaline cartilage and attached to a pad of fibrocartilage.

3. Synovial joints a. Synovial joints have a more complex structure than other types of joints.

b. These joints include articular cartilage, a joint capsule, and a synovial membrane.

General Structure of a Synovial Joint (page 2 74)

Articular cartilage covers articular ends of bones.

1. A joint capsule strengthened by ligaments holds bones together.

2. A synovial membrane that secretes synovial fluid lines the inner layer of a joint capsule.

3. Synovial fluid moistens, provides nutrients, and lubricates the articular surfaces.

4. Menisci divide some synovial joints into compartments.

5. Some synovial joints have fluid-filled bursae.

a. Bursae are usually located between the skin and underlying bony prominences.

b. Bursae cushion and aid movement of tendons over bony parts.

c. Bursae are named according to their locations.

Types of Synovial Joints (page 2 76)

1. Ball-and-socket joints a. In a ball-and-socket joint, the globular head of a bone fits into the cup-shaped cavity of another bone.

b. These joints permit a wide variety of movements.

c. The hip and shoulder are ball-and-socket joints.

2. Condyloid joints a. A condyloid joint consists of an ovoid condyle of one bone fitting into an elliptical cavity of another bone.

b. This joint permits a variety of movements.

c. The joints between the metacarpals and phalanges are condyloid.

3. Gliding joints a. Articular surfaces of gliding joints are nearly flat.

b. These joints permit the articular surfaces to slide back and forth.

c. Most of the joints of the wrist and ankle are gliding joints.

4. Hinge joints a. In a hinge joint, the convex surface of one bone fits into the concave surface of another bone.

b. This joint permits movement in one plane only.

c. The elbow and the joints of the phalanges are the hinge type.

5. Pivot joints a. In a pivot joint, a cylindrical surface of one bone rotates within a ring of bone and fibrous tissue.

b. This joint permits rotational movement.

c. The articulation between the proximal ends of the radius and the ulna is a pivot joint.

6. Saddle joints a. A saddle joint forms between bones that have complementary surfaces with both concave and convex regions.

b. This joint permits a variety of movements.

c. The articulation between the carpal and metacarpal of the thumb is a saddle joint.

Types of Joint Movements (page 2 76)

1. Muscles acting at synovial joints produce movements in different directions and in different planes.

2. Joint movements include flexion, extension, hyperextension, dorsiflexion, plantar flexion, abduction, adduction, rotation, circumduction, supination, pronation, eversion, inversion, elevation, depression, protraction, and retraction.

Examples of Synovial Joints (page 2 79)

1. Shoulder joint a. The shoulder joint is a ball-and-socket joint that consists of the head of the humerus and the glenoid cavity of the scapula.

b. A cylindrical joint capsule envelops the joint.

(1) The capsule is loose and by itself cannot keep the articular surfaces together.

(2) It is reinforced by surrounding muscles and tendons.

c. Several ligaments help prevent displacement of the bones.

d. Several bursae are associated with the shoulder joint.

e. Because its parts are loosely attached, the shoulder joint permits a wide range of movements.

2. Elbow joint a. The elbow has a hinge joint between the humerus and the ulna and a gliding joint between the humerus and the radius.

b. The joint capsule is reinforced by collateral ligaments.

c. A synovial membrane partially divides the joint cavity into two portions.

d. The joint between the humerus and the ulna permits flexion and extension only.

3. Hip joint a. The hip joint is a ball-and-socket joint between the femur and the coxa.

b. A ring of fibrocartilage deepens the cavity of the acetabulum.

c. The articular surfaces are held together by a heavy joint capsule that is reinforced by ligaments.

d. The hip joint permits a wide variety of movements.

4. Knee joint a. The knee joint includes two condyloid joints between the femur and the tibia and a gliding joint between the femur and the patella.

Critical Thinking Questions b. Ligaments and tendons strengthen the relatively thin joint capsule.

c. Several ligaments, some of which are within the joint capsule, bind articular surfaces.

d. Two menisci separate the articulating surfaces of the femur and the tibia.

e. Several bursae are associated with the knee joint.

f. The knee joint permits flexion and extension; when the lower limb is flexed at the knee, some rotation is possible.

Life-Span Changes (page 289)

1. Joint stiffness is often the earliest sign of aging.

a. Collagen changes cause the feeling of stiffness.

b. Regular exercise can lessen the effects.

2. Fibrous joints are the first to begin to change and strengthen over a lifetime.

3. Synchondroses of the long bones disappear with growth and development.

4. Changes in symphysis joints of the vertebral column diminish flexibility and decrease height.

5. Over time, synovial joints lose elasticity.

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