Joints (articulations) are functional junctions between bones. Although they vary considerably in structure, they can be classified according to the amount of movement they make possible. On this basis, three general groups can be identified-- immovable joints, slightly movable joints, and freely movable joints.
Immovable joints (synarthroses) occur between bones that come into close contact with another. The bones at such joints are separated by a thin layer of fibrous connective tissue or cartilage, as in the case of a suture between a pair of flat bones of the cranium. No active movement takes place at an immovable joint.
SLIGHTLY MOVABLE JOINTS
The bones of slightly movable joints (amphiarthroses) are connected by disks of fibrocartilage or by ligaments. The vertebrae of the vertebral column, for instance, are separated by joints of this type. The articulating surfaces of the vertebrae are covered by thin layers of hyaline cartilage. This cartilage, in turn, is attached to the intervertebral disk, which separates adjacent vertebral bodies.
Each intervertebral disk is composed of a band of fibrous fibrocartilage (annulus fibrosus) surrounding a pulpy of gelatinous core (nucleus pulposus). The disk acts as a shock absorber and helps to equalize pressures between adjacent vertebral bodies during body movements.
Due to the slight flexibility of the disks, these joints allow a limited amount of movement, as when the back is bent forward or to the side or is twisted. Other examples of slightly movable joints include the symphysis pubis, the sacroiliac joint, and the joint in the lower leg between the distal ends of the tibia and fibula.
FREELY MOVABLE JOINTS
Most joints within the skeletal system are freely movable, and they have more complex structures than immovable or slightly movable ones.
The ends of bones at freely movable joints (diarthroses) are covered with hyaline cartilage (articular cartilage) and held together by surrounding tube like capsule of dense fibrous tissue. This joint capsule is composed of an outer layer of ligaments and an inner lining of synovial membrane, which secretes synovial fluid. For this reason, freely movable joints are often called synovial joints. Synovial fluid has a consistency somewhat
Bursae are commonly located between the skin and underlying bony prominences, as in the case of the patella of the knee or the olecranon process of the elbow. They aid in the movement of tendons that pass over these bony part or over other tendons.
Based on the shapes of their parts and the movements they permit, freely movable joints can be classified as follows:
1. Ball-and-socket joints: A ball-and-socket joint consists of a bone with a ball-shaped head that articulates with a cup-shaped socket of another bone. Such a joint allows for a wider range of motion than does any other kind. Movements in all planes, as well as rotational movement around a central axis, are possible. The shoulder and hip contain joints of this type.
2. Condyloid joints: In a condyloid joint, an oval-shaped condyle of one bone fits into an elliptical cavity of another bone, as in the case of the joints between the metacarpals and phalanges. This type of joint allows a variety of movements in different planes; rotational movements, however, are not possible.
3. Gliding joints: The articulating surfaces of gliding joints are nearly flat or only slightly curved. Such joints are found between some wrist bones, some ankle bones, and between the articular processes of adjacent vertebrae. They allow sliding and twisting movements.
4. Hinge joints: In a hinge joint, the convex surface of one bone fits into the concave surface of another, as in the case of the elbow and the joints of the phalanges. This type of joint allows movement in one plane only, like the motion of a single-hinged door.
5. Pivot joints: In a pivot joint, a cylindrical surface of one bone rotates within a ring formed of bone and fibrous tissue. The movement at such a joint is limited to rotation about a central axis. The joint between the proximal ends of the radius and the ulna is of this type.
6. Saddle joints: A saddle joint is formed between bones whose articulating surfaces have both concave and convex regions. The surfaces of one bone fit the complementary surfaces of the other. This arrangement allows a variety of movements, as in the case of the joint between a carpal (trapezium) and the metacarpal of the thumb.
TYPES OF JOINT MOVEMENTS
Movements at synovial joints are produced by the actions of skeletal muscles. Typically, one end of a muscle is attached to a relatively immovable or fixed part on one side of a joint, and the other end of the muscle is fastened to a movable part on the other side. When the muscle contracts, fibers within the muscle pull its movable end (insertion) towards its fixed end (origin), and a movement occurs at the joint.
The following terms are used to describe various movements at joints.
flexion- bending a joint so that the angle between its parts is decreased and the parts come closer together (bending the leg at the knee).
extension- straightening a joint so that the angle between its parts is increased and the parts move father apart (straightening the leg at the knee).
dorsiflexion- flexing the foot at the ankle (bending the foot upward).
plantar flexion- extending the foot at the ankle (bending the foot downward).
hyperextension- excessive extension of the parts at a joint, beyond the anatomical position (bending the head back beyond the upright position).
abduction- moving a part away from the midline (lifting the arm horizontally to form a right angle with the side of the body).
adduction- moving a part towards the midline (bringing the arm horizontally downward to the side of the body).
rotation- moving a part around an axis (twisting the head from side to side).
circumduction- moving a part so that its end follows a circular path (moving the finger in a circular motion without moving the hand).
pronation- turning the hand so the palm is downward or turning the foot so that the medial margin is lowered.
supination- turning the hand so the palm is upward or turning the foot so that the medial margin is raised.
eversion- turning the foot so that the sole is inward.
retraction- moving a part backward (pulling the chin backward).
protraction- moving a part forward (thrusting the chin forward).
elevation- raising a part (shrugging the shoulders).
depression- lowering a part (drooping the shoulders).