Frozen shoulder occurs when the sleeve that surrounds the shoulder joint, known as the capsule, becomes swollen and thickened. It's unclear why this happens.

The shoulder is a ball and socket joint. The end of your upper arm bone (humerus) sits in contact with the socket of your shoulder blade (scapula).

The shoulder capsule is fully stretched when you raise your arm above your head, and hangs down as a small pouch when your arm is lowered.

In frozen shoulder, bands of scar tissue form inside the shoulder capsule, causing it to thicken, swell and tighten. This means there is less space for your upper arm bone in the joint, which limits movements.

Who's most at risk?

It is not fully understood why frozen shoulder occurs, and it's not always possible to identify a cause. However, a number of factors can increase your risk of developing it. These are outlined below.

Age and gender

Most people affected by frozen shoulder are aged between 40 and 60. The condition is more common in women than men.

Previous shoulder injury or surgery

Frozen shoulder can sometimes develop after a shoulder or arm injury, such as a fracture, or after having surgery to your shoulder area.

This may partly be a result of keeping your arm and shoulder still for long periods of time during your recovery. Your shoulder capsule may tighten up from lack of use.

Because of this, it is very important not to ignore a painful shoulder injury and to always seek medical advice.

Diabetes

If you have diabetes, you have a greater risk of developing a frozen shoulder. The exact reason for this is unknown.

It is estimated that people with diabetes are twice as likely to develop a frozen shoulder.

If you have diabetes, the symptoms of frozen shoulder are likely to be more severe and harder to treat. You are also more likely to develop the condition in both shoulders.

This means it's important to have your diabetes checked regularly to make sure it is controlled with the right medication.

Other health conditions

You may have a greater risk of developing a frozen shoulder if you have other health conditions, such as:

• heart disease
• stroke
• lung disease
• an overactive thyroid (hyperthyroidism) or an underactive thyroid (hypothyroidism)
• breast cancer
• Dupuytren's contracture (where small lumps of thickened tissue form in the hand, causing the fingers to bend into the palm)

Other shoulder conditions

Frozen shoulder can sometimes develop alongside other shoulder conditions, such as:

• calcific tendonitis – where small amounts of calcium are deposited in the tendons of the shoulder
• rotator cuff tear – the rotator cuff is a group of muscles that control shoulder movements

Immobility

Not moving for long periods of time can also increase your risk of a frozen shoulder. This can sometimes happen if you spend time in hospital.