Subacromial impingement is one of the most common causes of pain in the shoulder. It occurs when there is a reduction in the space between the rotator cuff tendons and the overlying bone, the acromion.
The rotator cuff is a group of four muscles which envelope the ‘ball’ of the shoulder joint (the humeral head). The muscles help lift the arm but are also instrumental in keeping the humeral head down and away from the overlying bone in the shoulder, the acromion. Between the acromion and the rotator cuff muscles is the subacromial space in which there is a fluid filled sac called the bursa. The bursa helps to provide a smooth, low friction environment for the shoulder to move.
There are many causes of impingement but if the rotator cuff is injured, or if there is a build up of bone on the under surface of the acromion (sometimes referred to as a bone spur), inflammation can occur within the subacromial bursa. This is known as subacromial bursitis and can result in swelling with associated pain and restricted movement. If there is contact between the rotator cuff and the under surface of the acromion, then the two surfaces can rub together. This is referred to as subacromial impingement. This can be most apparent during shoulder movement, and in particular when the arm is elevated to shoulder height or when the arm is rotated either in elevation or when taking the arm behind ones back. This can result in pain and can also cause clicking.
If the bursitis and inflammation continues, then progressive thickening and scarring of the bursa can occur which will further restrict movement. If the impingement continues unchecked, then the rotator cuff tendons may undergo degeneration and eventually result in a tear of the tendon. This may result in more pain and weakness
- Initially minor shoulder pain that can be present both at rest and with activity.
- Referred pain to the front and also down the side of the arm / shoulder. There can also sometimes be some swelling and localised tenderness.
- Pain is usually made worse when the arm is taken through an arc of movement to, and above, shoulder height. Pain can also be present when the arm is lowered back to your side.
- Sudden severe pain may occur on lifting or reaching movements.
- Night pain and difficulty lying on the affected side may occur.
- Stiffness and restricted movement
- Undertaking activities, especially those involving taking your hand behind your back, may become difficult and painful.
Causes & Risk factors
There are numerous causes of subacromial impingement, some of the more common ones include:
- The rotator cuff tendons; if the tendons become weak and painful due to an injury (tendonitis), through degeneration (tendinosis) or following a tendon tear, then the rotator cuff tendons/muscles may not be strong enough to control the humeral head. This weakness, or imbalance, can result in the tendons rubbing against the acromion.
- A bone spur; a prominent thickening of the edge of the acromion can result in a bony spur forming. This can impinge on the underlying tendons of the rotator cuff and cause pain.
- Capsular restrictions; if the capsule of the joint should become inflamed (capsulitis) such as in a Frozen Shoulder, the abnormal movement which follows can result in muscular imbalances which results in impingement between the tendons of the rotator cuff and the overlying acromion.
- Shoulder instability; in cases of subtle instability, usually secondary to a weak rotator cuff, the humeral head (ball) can move inappropriately against the glenoid (shoulder socket) thereby resulting in impingement type symptoms.
The diagnosis of subacromial impingement is made clinically from the history and examination.
Imaging can also be used to help determine if there is any damage to the shoulder which may have predisposed the shoulder to the impingement symptoms. X-rays can be used to confirm the presence of an acromial bone spur but often ultrasound scans are the most effective investigation to look for inflammation of the bursa and the possible presence of rotator cuff tears which might have caused the impingement. MRI scans (Magnetic resonance imaging) can also be used to assess the rotator cuff tendons. These scans are particularly helpful if the shoulder is stiff or if a rotator cuff tear is also suspected.
When there is some doubt in the diagnosis, local anaesthetic can be injected into the subacromial space. This is called a Neer or Impingement injection test. By bathing the inflamed tendons in local anaesthetic the pain caused by the impingement should improve thereby supporting the diagnosis.
There are a number of treatments available for subacromial impingement. However, often more than one treatment modality is used to decrease the symptoms. The options include;
- Conservative treatment comprising of rest, activity modification and the use of anti-inflammatory medications.
- Physiotherapy; a stretching and eventually a strengthening physiotherapy regime can be used to improve the range of shoulder movement and also to balance the stabilising muscles of the shoulder so that the shoulder moves more favourably and therefore less painfully. A strong rotator cuff also keeps the humeral head low against the socket thereby minimising the risk of impingement like symptoms.
- Injection treatments; the space between the rotator cuff and the acromion (the subacromial space) can be injected with local anaesthetic and Cortisol. These injections can be used for both diagnostic and therapeutic reasons. The anti-inflammatory component of the injection can reduce swelling and pain and can be used in conjunction with physiotherapy to aid recovery.
- Arthroscopic subacromial decompression; If the pain is resistant to non-operative treatments then surgery may be necessary. The aim of the surgery is initially to arthroscopically examine the shoulder to confirm the diagnosis and make sure that there are no other possible causes for the pain and then to increase the size of the subacromial space so that the rotator cuff tendons are not aggravated during shoulder movement. A subacromial decompression (acromioplasty) can be performed as a ‘key-hole’ procedure through 2 or 3 small incisions. Any inflamed tissue can be removed and a some of the bony of the acromion (or a spur) can be shaved away to create more space for the rotator cuff tendons. Where appropriate, other procedures such as a rotator cuff repairs or ACJ excisions can also be performed at the same time.
- Open acromioplasty; This allows direct visualisation of the acromion and may be used if a large open operation is being undertaken which might be needed when more complicated surgery is necessary which is not amenable to arthroscopic techniques.