Ball-and-socket joints are a special class of synovial joints that enjoy the highest freedom of motion in the body thanks to their unique structure. The shoulder and hip joints are the only ball-and-socket joints in the human body due to the need for great motion at the end of the body’s limbs and the vast amount of musculature needed to move and support such flexible joints.
Two main components make up a ball-and-socket joint: a bone with a spherical head and a bone with a cup-like socket. In the shoulder joint, the spherical head of the humerus (upper arm bone) fits into the glenoid cavity of the scapula (shoulder blade). The glenoid cavity is a small and shallow cavity that permits the shoulder joint the greatest range of motion in the human body. A hyaline cartilage ring called the labrum surrounds the glenoid cavity to provide a flexible reinforcement to the joint, while muscles of the rotator cuff hold the humerus in place within the cavity.
The hip joint is somewhat less mobile than the shoulder, but is an overall stronger and more stable joint. The added stability of the hip joint is necessary to bear the weight of the body resting on the legs while performing actions such as standing, walking, and running. In the hip joint the rounded, almost spherical head of the femur (thigh bone) fits tightly into the acetabulum, a deep socket in the os coxa (hip bone). Many tough ligaments and the powerful hip muscles hold the head of the femur in place and resist some of the most powerful strains in the body. The depth of the acetabulum also prevents dislocations of the hip by limiting the movement of the femur within its socket.
Ball-and-socket joints are classified functionally as multiaxial joints because they can move bones along several axes. The muscles that surround the joints permit the humerus and femur to move away from the body’s midline (abduction), toward the body’s midline (adduction), forward (flexion), and backwards (extension). The humerus and femur can also move around the joint in a full circle (circumduction) as well as rotate both medially and laterally around their axis. Other parts of the body, such as the wrist and ankles, require at least two separate joints working together to achieve all of the movements of the ball-and-socket joints.