Frozen shoulder, also known as adhesive capsulitis, is a common condition in which the articular shoulder capsule (a sac of ligaments surrounding the joint) swells and stiffens, restricting its mobility. It typically affects only one shoulder, but one in five cases affect both.
The term “frozen shoulder” is often used incorrectly for arthritis, even though the two conditions are unrelated. Frozen shoulder refers specifically to the shoulder joint, while arthritis may refer to other/multiple joints.
The shoulder has a spheroidal joint (ball – and – socket joint), in which the round part of one bone fits into the concavity of another. The proximal humerus (round head of the upper arm bone) fits into socket of the scapula (shoulder blade). Frozen shoulder is thought to cause the formation of scar tissue in the shoulder, which makes the shoulder joint’s capsule (not to be confused with the rotator cuff) thicken and tighten, leaving less room for movement. Therefore, movement may be stiff and even painful.