Ball and socket joints allow side-to-side, back-and-forth and rotational 360 degree movements. The rounded head of one bone fits snuggly into a cavity in another. Ball-and-socket joints have the greatest range of movement of all the joints. In the shoulder joint, the head of the humerus (upper arm bone) fits into the shallow socket of the upper shoulder blade. Because the socket is shallow and the joint loose (relying on muscles and ligaments to keep it in place), the shoulder is the body’s most mobile joint. It is also most prone to injuries/problems.
It would be more anatomically correct for it to be called the ‘shoulder complex’ as it comprises a number of linked bone/muscle groups which work together.
Watch the shoulder joint in action.
Hip joints - the hip joint is a synovial joint formed by the rounded head of the thigh bone (femur) and the deep cup (acetabulum) of the pelvis. Both joint surfaces are covered with a strong but lubricated (greased) layer called articular hyaline cartilage. Its primary function is to support the weight of the body.
Condyloid joints (plane joint) – an ovoid articular surface, or condyle, is received into an elliptical cavity in such a manner which allows only a small amount of gliding (back-and-forth and side-to-side) movement, but no axial rotation. Examples are the small wrist joints and cuneiform/bones of the foot.
Pivot joints allow rotation – usually in a half-circle. This movement is facilitated by the rotation of one bone around another (longitudinal axis). Examples are; between the 1st and 2nd cervical vertebrae (atlas and axis) Radio-ulnar (turning your palm facing upwards and downwards). The neck is a very important pivot joint as it allows the second bone in the vertebral column to move so that we can turn our heads to look over our shoulders (the first vertebra – atlas – is joined to the base of the skull). The only other true pivot joint in the human body is at the elbow/forearm where the radius rotates around the ulna.