Sub-Acromial Shoulder Impingement Explained

Functional Anatomy of the Shoulder

The shoulder complex is made up of the gleno-humeral (GH), acromio-clavicular (AC) and sterno-clavicular (SC) joints.  In addition your shoulder has an articulation between the scapula and the thorax (chest). Your GH joint is enclosed by a loose and shallow joint capsule which promotes a large range of movement at the cost of less joint stability. This lack of static joint stability means that the shoulder complex relies heavily on the rotator cuff muscles for dynamic joint stability.  Good shoulder stability  also requires control of scapula on the chest wall, for optimal shoulder biomechanics.  A change in this control is a major factor leading to sub-acromial shoulder impingement.

What is the Sub-Acromial Space?

The sub-acromial space refers to the space underneath the acromion of the scapula and the head of the humerus. The tendons of the supraspinatus and long head of biceps muscle pass through this joint space as well as the sub-acromial bursa.

What is Sub-Acromial Shoulder Impingement?

Sub-acromial shoulder impingement refers to the tendons of either or both of supraspinatus and long head of biceps getting compressed in the sub-acromial space. This compression causes irritation of the tendons and the sub-acromial bursa, causing inflammation and a reduction in the sub-acromial space.  Shoulder impingement occurs with overhead movements of the arm, resulting in pain around the tip of the shoulder.

What Causes Sub-Acromial Shoulder Impingement?

Sub-acromial shoulder impingement can be caused by any one of the following factors including

  • Rotator cuff tears
  • GH joint instability
  • Poor scapula-humeral rhythm
  • Tight posterior shoulder capsule
  • AC joint or labral injuries
  • Bone spurs and deficits in GH joint external rotation.

These factors in isolation or in combination with each other cause a reduction in the sub-acromial space resulting in sub-acromial shoulder impingement.

Assessment and Treatment of Sub-Acromial Shoulder impingement

People with sub-acromial shoulder impingement will usually present with a positive “empty can” test. However it is important to differentiate between sub-acromial impingement and supraspinatus tears by then performing the empty can test with shoulder distraction. With distraction in the empty can test, pain should be reduced with sub-acromial impingement as the sub-acromial space is increased where as a supraspinatus tear will remain just as painful. It is crucial when assessing the shoulder to determine the cause of the reduction in the sub-acromial space leading to shoulder impingement. Once these factors have been determined, rehabilitation should focus on correcting any mechanical deficiencies that may be contributing to sub-acromial impingement such as poor scapula-humeral rhythm of lack of rotator cuff control.

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