Frozen shoulder is caused by inflammation of the tissue that surrounds the shoulder joint. This tissue, which is usually flexible, starts to thicken and limit the range of movement in the shoulder. In more severe cases, movement in the shoulder may be restricted altogether. Medical terms for frozen shoulder are “adhesive capsulitis” and “shoulder contracture”.
The flexible tissue that surrounds the shoulder joint is called the joint capsule. The shoulder joint is comprised of the ball-like structure at the top of the upper arm bone (humerus) and the socket of the scapula (shoulder blade) that it sits in. The capsule is fully stretched when an arm is raised above the head and forms a relaxed pouch when the arm is lowered. In the case of frozen shoulder, scar tissue forms on the inside of the capsule, causing it to become thick, swollen and tight. This limits the space inside the joint where the upper arm bone sits and its movement is restricted.
Although the exact cause of frozen shoulder is unclear, there are several factors that raise the risk of frozen shoulder developing. Some of these factors include:
- An injury involving bones in the shoulder may mean the arm is kept immobile for long periods. Not using the shoulder for long periods can cause the shoulder to tighten.
- People with diabetes are twice as likely to develop frozen shoulder than people without diabetes. Those with diabetes are also more likely to develop severe symptoms that are more difficult to treat.
- Other conditions that increase the risk of frozen shoulder include heart disease, lung disease, thyroid disease and breast cancer.
- People with Dupuytren’s contracture (a tissue disorder in the hands that causes fingers to bend) are at an increased risk of frozen shoulder.
- Other shoulder conditions such as calcific tendonitis and rotator cuff tear can also increase the risk of frozen shoulder developing.