Joints are vital to allow movement and function to an otherwise rigid skeletal frame. The more free the movement, the less stable the joint. For example, both the hip and shoulder are ball-and-socket joints. Since the hip has a deeper socket, it is more stable but this comes at the cost of restricted movement. The shoulder has a shallower joint and allows more freedom of movement, but is less stable.
When a joint dislocates (such as after a fall or a sports injury), contact between the surfaces of the bones forming that joint is lost. Shoulders, fingers, toes and elbows are commonly dislocated, with knees, ankles and vertebrae less frequently. The bones must be put back in place; after some sedation, they are manipulated until they move back into the right position. Sometimes, an operation is needed, but in some cases, nothing is needed at all (such as for dislocations of the clavicle, or collarbone).