The shoulder is actually a team of joints that control the movement of your arm. The main joint of the shoulder is the glenohumeral joint, which is a ball and socket joint, similar to the hip. A ball and socket design allows circular movement of the arm in all directions. The acromio-clavicular joint connects the collar bone to the shoulder blade and the sterno-clavicular joint connects the collar bone to the breast bone. Small gliding movements from these two joints, which you don’t even have to think about, increases the range of motion of the shoulder.
The socket is shallow, to give you great mobility, but offers little stability. Fibrous tissue that attach to the edge of the socket, the glenoid labrum, increases the depth of the socket. Articular cartilage lines the inside of the ball and socket joint. It absorbs shock and compressive forces, like catch and throw of a medicine ball. The cartilage allows the joint to move freely without friction. The cartilage in the shoulder is thinner than in weight bearing joints like the hip.
Injury to the joint, glenoid or cartilage causes painful shoulder joint movements. When a “simple joint sprain” remains inflamed, the bone or cartilage is likely injured.
A synovial membrane lines the joint, almost like insulation. This membrane secretes a fluid (synovial fluid) that lubricates the joint like oil in a car’s engine. The joint capsule (ligaments that surround the joint) covers the synovial membrane.
The shoulder joint is the most mobile, but least stable joint in the body, therefor dislocations happen easier than with other joints. Because the shoulder joint can move in all directions it has limited passive stability (provided by ligaments). When dynamic stability (provided by muscles) is poor, extra stress is placed on the shoulder joint.