Joints (Articulations)
Bones articulate at the joints, which are often classified according to the amount of movement they allow.
Classification
Some bones, such as those that make up the cranium, are sutured together by a thin layer of fibrous connective tissue and are immovable. Such joints are called synarthroses. Note the following immovable sutures:
coronal suture occurs between the parietal bones and the frontal bone;
lambdoidal suture occurs between the parietal bones and the occipital bone;
squamosal suture occurs between each parietal bone and each temporal bone;
sagittal suture occurs between the parietal bones (not shown).
Other joints are slightly movable, connected by hyaline cartilage or fibrocartilage, and are referred to as amphiarthroses. For example, the vertebrae are separated by intervertebral disks that increase vertebrae flexibility. Also, the ribs are joined to the sternum by costal cartilage, and the pubic symphysis occurs between the pubic bones. Due to hormonal changes, this joint becomes more flexible during late pregnancy, which allows the pelvis to expand during childbirth.
Most joints are freely movable synovial joints, known as diarthroses, in which the two bones are separated by a cavity. Ligaments composed of fibrous connective tissue bind the two bones to one another, holding them in place as they form a capsule. In a double-jointed individual, the ligaments are unusually loose. Tendons are cords of fibrous connective tissue that connect muscles to bones and help stabilize joints. The joint capsule is lined by a synovial (sl-no've-al) membrane, which produces synovial fluid, a lubricant for the joint.
The knee is an example of a synovial joint.
In the knee, as in other freely movable joints, the bones are capped by cartilage. In addition, the knee contains menisci (sing., meniscus), crescent-shaped pieces of cartilage between the bones. These give added stability, helping to support the weight placed on the knee joint. Unfortunately, athletes often suffer an injury to the menisci, known as torn cartilage. The knee joint contains 13 fluid-filled sacs called bursae (bur'se), which ease friction between tendons and ligaments, and between tendons and bones. Inflammation of the bursae is called bursitis. Tennis elbow is a form of bursitis.
Different types of freely movable synovial joints are:
Saddle joint, in which each bone is saddle-shaped and fits into the complementary regions of the other. A variety of movements is possible. Example: the joint between the carpal and metacarpal bones of the thumb.
Ball-and-socket joint, in which the ball-shaped head of one bone fits into the cup-shaped socket of another. Movement in all planes and rotation is possible. Example: the shoulder and hip joint.
Pivot joint, in which a small, cylindrical projection of one bone pivots within a ring formed of bone and ligament of another. Only rotation is possible. Examples: the joint between the proximal ends of the radius and ulna, and the joint between the atlas and axis.
Hinge joint, in which the convex surface of one bone articulates with the concave surface of another. Up-anddown motion in one plane is possible. Example: the elbow and knee joint. Gliding joint, in which flat or slightly curved surfaces of bones articulate. Sliding or twisting in various planes is possible. Examples: the joints ben.veen the bones of the wrist and between the bones of the ankle.
Condyloid joint, in which the oval-shaped condyle of one bone fits into the elliptical cavity of another. Movement in different planes is possible, but rotation is not. Examples: the joints between the metacarpals and phalanges.
Synovial joints are subject to various disorders. A sudden movement that twists or wrenches a joint can stretch or tear a ligament, which is called a sprain. Arthritis is a more serious disorder. In rheumatoid arthritis, the synovial membrane becomes inflamed and grows thicker. Degenerative changes take place that make the joint almost immovable and painful to use. Evidence indicates that these effects are brought on by an autoimmune reaction. In osteoarthritis, the cartilage at the ends of the bones disintegrates so that the two bones become rough and irregular.
Gout, or gouty arthritis, is caused by an excessive buildup of uric acid (a metabolic waste) in the blood. Rather than being excreted in the urine, it becomes deposited as crystals in the joints, where it causes inflammation and pain.
.Joints are regions of articulations betvveen bones. They are classified according to their degree of movement. Some joints are immovable, some are slightly movable, and some are freely movable.
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