Shoulder Impingement Syndrome Causes

Shoulder impingement is a symptom of something else and not a specific diagnosis in itself.

The tendons of the rotator cuff muscles become impinged as they pass through a small gap called the subacromial space in the shoulder. This can be due to a number of possible causes.

What causes shoulder impingement?

It is thought shoulder impingement syndrome begins as an over use injury of the supraspinatus tendon which runs along the top of the shoulder blade. Pain then causes dysfunction of the rotator cuff muscles which causes the upper arm bone to shift slightly and possibly also result in inflammation of the bursa or small sack of fluid (subacromial bursitis).

Over time the pain causes more dysfunction and impingement in a vicious circle which may eventually lead to ossification or bony spurs growing and causing injury to the rotator cuff tendons and so on. So it is vitally important that impingement syndrome is rested and treated as soon as possible to avoid longer term damage.


The subacromial space is so called because it is under the arch of the acromion. It is a small gap where the rotator cuff tendons pass through and subacromial bursa is located.

There are at least nine different diagnoses which can cause impingement syndrome which include bone spurs, rotator cuff injury, labral injury, shoulder instability, biceps tendinopathy and scapula dysfunction. If left untreated, shoulder impingement can develop into a rotator cuff tear.

The supraspinatus muscle is probably the most commonly involved in impingement syndrome of the shoulder.

Types of shoulder impingement

External impingement

External shoulder impingement is classified as primary or secondary.

  • Primary external impingement is usually due to bony abnormalities in the shape of the acromial arch in the shoulder joint. It can sometimes be due to congenital abnormalities known as os acromial, or due to degenerative changes, where small spurs of bone grow out from the arch with age, and impinge on the tendons.
  • Secondary external impingement is usually due to poor scapular or shoulder blade stabilization which alters the physical position of the acromion, hence causing impingement on the tendons. Is often due to a weak serratus anterior muscle and a tight pectoralis minor muscles. Other causes can include weakening of the rotator cuff tendons due to overuse for example in throwing and swimming, or muscular imbalance with the deltoid muscle and rotator cuff muscles.

Internal impingement

This occurs predominantly in athletes where throwing is the main part of the sport, e.g. pitches in baseball or javelin throwers. The under side of the rotator cuff tendons are impinged against the glenoid labrum. This tends to cause pain at the back of the shoulder joint as well as sometimes at the front.

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