Q: What Causes Subacromial Impingement Syndrome?

A: Subacromial Impingement Syndrome occurs when the subacromial space is narrowed and weak structures in that space get compressed. There are several causes for subacromial impingment such as:

•Anatomical variations: The shape of the acromion plays an important role: There are 3 types of acromial shapes can be distinguished: (4)

•Type I: flat shape

•Type II: curved shape

•Type III: hooked shape (most likely to contribute to impingement and irritation)

Also Bony spurs also known as osteophytes at the bottom aspect of the acromion can also be involved.

•Rotator cuff weakness, causing the humeral head to drift superior or higher.

•Chronic rotator cuff irritation due to overuse.

•Posterior GH capsule tightness

•Poor posture (forward shoulder posture can cause functional narrowing of Subacromial space)

•Abnormal muscle activation

Definition: Subacromial impingement Syndrome is defined as "the mechanical compression of subacromial structures between the coraco-acromial arch and the humerus during active elevation of the arm above shoulder height."(2)

Clinically Relevant Anatomy:

Structures involved in subacromial impingment:

•the coracoacromial arch composed of acromion, processus coracoideus and ligamentum coracoacromiale

•the humerus

•the tendons of the Rotator Cuff 

•the long head of biceps brachii

•the subacromial bursa

•shoulder capsule

The subacromial space is the margin between the superior portion of the humeral head and the inferior portion of the acromion. It contains: (3)

•belly and tendon of the supraspinatus muscle

•long head of the biceps muscle

•subacromial bursa

1. Kachingwe AF, Phillips B, Sletten E, Plunkett SW. Comparison of Manual Therapy Techniques with Therapeutic Exercise in the Treatment of Shoulder Impingement: A Randomized Controlled Pilot Clinical Trial. The Journal of Manual fckLRManipulative Therapy 2008;16(4):238-­‐247

2. TATE A.R., MCCLURE P.W., YOUNG I.A., SALVATOR R., MICHENER L.A. Comprehensive impairment-based exercise and manual therapy intervention for patients with subacromial impingement syndrome: a case series. The Journal of orthopaedic and sports physical therapy. 2010 Aug; 40(8): 474-93

3. BIRRER R.B., O’CONNOR F.G. Sports medicine for the primary care physician. 3rd edition, Boca Raton: RCR PRESS, 2004.p507- 10

4. Bigliani LU, Morrison DS, April EW. The morphology of the acromion and its relationship to rotator cuff tears. Orthop Trans 1986;10:228.