Causes of Frozen shoulder

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A lining of fibrous tissue, known as a capsule, surrounds the shoulder joint. The capsule is fully stretched when the arm is raised above the head, and it hangs down as a small pouch when the arm is lowered. Frozen shoulder occurs when this lining becomes thickened and swollen.

The exact cause of frozen shoulder is not known, but it is thought that scar tissue forms in the capsule, causing it to thicken and contract. This leaves less space for the upper arm bone (humerus) to move around. Scar tissue may form for a number of reasons, such as after a minor shoulder injury or from heavy exercise, but sometimes there is no obvious cause.

Frozen shoulder is five times more common in people with diabetes, although the exact reason for this is unknown. It may be linked to a substance called collagen that helps to hold the bones together in a joint. Large numbers of glucose (sugar) molecules attach to collagen in people with diabetes. This build-up in the joint can cause the shoulder to stiffen.

Frozen shoulder is more common in women than men. This may be linked to hormonal changes, as it is particularly common around the time of the menopause.

Posture has also been linked to frozen shoulder. Some studies have shown that a consistently round-shouldered posture may cause a shortening in one of the shoulder ligaments (the band of tissue that connects the bones). Long periods of immobility, such as following surgery or an arm injury, may cause the condition to develop.

People with other health conditions, including heart disease, lung disease, and chronic bronchitis may have an increased risk of developing frozen shoulder. Cardiovascular disease, Parkinson's disease, and an overactive thyroid gland (hyperthyroidism) are also linked to this condition.