Ball and Socket Joint
Ball and socket joints are multiaxial, synovial joints. They are lubricated by a clear, sticky fluid called synovia.
Also called spheroidal joints, the ball and socket joints are formed by the rounded or "ball-shaped" head of one bone fitting into the cup-like cavity of another bone. The articulating bone fits into the cavity and allows the distal bone to move around. The hip and shoulder joints are examples of the ball and socket joint.
The purpose of joints is to provide movement for the body. Different types of joints move in different ways. The ball and socket joint is fully mobile under the control of muscles, ligaments, and tendons. The ends of the bones are covered with tough cartilage and are lined with the synovial membrane.
Each joint contains a small amount of synovial fluid which lubricates it. Synovial fluid provides protection for the ball and socket joint and allows for stress-free movement.
The ball and socket joint provides swinging and rotating movements. The articulating bone is received into the cavity of another bone, allowing the distal bone to move around three main axes with a common center. The joint has stabilizing ligaments that limit the directions and extent to which the bones can be moved. However, the ball and socket joint is the most mobile in the body.
Role in human health
Ball and socket joints are the most mobile and intricate of all the joints. They are also the most prone to disease and prone to require medical intervention. Hip or shoulder replacements are common forms of surgical intervention that restore a patient's quality of life by replacing worn ball and socket joints with prosthetic ones.
Common diseases and disorders
There are many disorders and diseases that can afflict the synovial joints, making the ball and socket joint vulnerable to pain and discomfort. Degenerative or inflammatory diseases, conditions involving the membranes around the joints, generalized and congenital disorders, and dislocations and fractures can all cause damage to ball and socket joints.
Arthritis is one of the conditions that causes pain and dysfunction in the ball and socket joint. There are several types of arthritis, but osteoarthritis and rheumatoid arthritis are the most common.
Osteoarthritis is a degenerative disease that affects the cartilage in the joints, and it can cause inflammation in the tissues surrounding the affected joint or joints. Degeneration is commonly thought to be caused by stress
on the joints or by injury to the joint lining. Osteoarthritis can affect all joints, but it is usually found in the fingers, feet, hips, spine, and knees. It causes joint stiffness and pain. Symptoms of osteoarthritis can be treated, but the disease is irreversible.
Rheumatoid arthritis is an inflammatory disease that involves the muscles and the membrane linings of cartilage and joints. The areas commonly affected are the hands, hips, knees, legs, and joints. The symptoms include low-grade fever, stiffness in the morning, and redness, pain, warmth, and tenderness in the affected joints. Rheumatoid arthritis can cause crippling pain and deformities of the hands and causes painful swelling of the joints.
Hip dysplasia is a dislocation of the hip joint that can be caused either by a congenital condition or by an accident. Dysplasia occurs when the thighbone (femur) does not fit correctly into or pops out of the cup-shaped socket at the hipbone (acetabulum). If hip dysplasia is caused by a congenital condition, the acetabulum is too shallow to hold the head of the femur. Physical therapy can help remedy the problem by deepening the cavity of the socket. Accidental hip dislocation is usually caused by a hip fracture.
Axis/axes—A central or principal structure about which something turns or is arranged. The cup-like structure of the ball and socket joint is the axis where the distal bone rotates.
Dysplasia—Dislocation of a bone from its proper place. Hip or shoulder displacement are two of the best-known types of dysplasia.
Multiaxial—Having more than one axis. The ball and socket joint has at least three axes on which it rotates.
Synovial fluid—A transparent, viscous fluid found in the synovial joints. It lubricates the ball and socket joint for easier movement.
Irons-George, Tracy. Magill's Medical Guide. Vols. 1, 2, 3. Pasadena, CA: Salem Press, 1998.
"Ball-and-Socket Joints." <http://www.innerbody.com/text/skel34.html>.
"UCI Medicine for the New Millennium: Hip/Joint Replacements at UCI Medical Center." <http://www.ucihealth.com/News/milestones/replacehip.htm>.
Peggy E. Browning