“Frozen shoulder” remains an enigma. The cells which form the collagen in the ligaments and capsule lining the shoulder joint are called fibroblasts. In the frozen shoulder these cells appear to make more collagen than usual and the capsule thickens greatly, so preventing movement. It is not at all clear why this happens, but the condition is extremely painful. Stiffness tends to follow the onset of pain.
The only other condition causing so much pain in such a short time is infection inside the joint, so here the history of onset of the painful stiffness is very important. We prefer to call the condition 'primary shoulder contracture' (PSC) to distinguish it from the other causes of contracture, and because the shoulder doesn’t “freeze” - in fact there are thousands more small blood vessels lining the joint during the early phase (which would tend to make it a warm joint!). An X-ray of the shoulder is nearly always normal in PSC.
Related treatment information: