A frozen shoulder occurs when there is thickening and swelling of the flexible tissue that surrounds your shoulder joint. This tissue is known as a capsule.
Your shoulder is a ball and socket joint, in which the end of your upper arm bone (humerus) sits in the socket of your shoulder blade (scapula). The shoulder capsule is fully stretched when your arm is raised above your head, and hangs down as a small pouch when your arm is lowered.
In cases of frozen shoulder, it is thought that bands of scar tissue form inside the shoulder capsule, causing it to thicken, swell and tighten. This leaves less space for your upper arm bone in the joint and makes any movement stiff and painful.
Risk factors
The reason why a frozen shoulder occurs is not fully understood. In some cases, it may not be possible to identify a cause. However, there are several risk factors that make developing a frozen shoulder more likely.
Shoulder injury or surgery
It is possible to develop a frozen shoulder following a shoulder or arm injury, such as a broken bone (fracture) or after having surgery to your shoulder area.
This may be because keeping your arm and shoulder immobile (still) for long periods of time during your recovery may cause your shoulder capsule to tighten up from lack of use. For this reason, it is important not to ignore a painful injury to your shoulder area and to always visit your GP.
Diabetes
If you have diabetes (a long-term condition caused by too much glucose in the blood), you are more likely to develop a frozen shoulder. The exact reason for this is unknown. If you have diabetes, you are also more likely to:
- develop the condition in both your shoulders
- have more severe symptoms
It is estimated that around a third of people with diabetes have a frozen shoulder.
Other health conditions
Your risk of developing a frozen shoulder may also be increased by having other health conditions including:
- heart disease, where the heart’s blood supply is blocked or interrupted
- lung disease, which affects your airways
- an overactive thyroid gland (hyperthyroidism), where the thyroid gland, which is found in your neck, produces too many hormones (powerful chemicals)
- Parkinson's disease, a chronic (long-term) condition that affects the way the brain co-ordinates body movements, including walking, talking and writing
- a stroke, when the blood supply to the brain is disturbed or interrupted
- Dupuytren's contracture, a condition that causes your fingers to bend into the palm of your hand due to the thickening and shortening of the tissue in your palm
Immobility
Being immobile (not moving) for a long period of time is also a risk factor for frozen shoulder. This can occur if you are in hospital, for example after having a stroke or car accident.
Glucose
Glucose (or dextrose) is a type of sugar that is used by the body to produce energy.
Joint
Joints are the connection point between two bones that allow movement.
Thyroid gland
The thyroid gland in the throat makes hormones to help control growth and metabolism (the process that turns the food we eat into energy).
Tissue
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.
