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Anatomy and Pathophysiology

In the normal shoulder, the coordinated muscle tension within the rotator cuff compresses the humeral head, keeping it centered within the glenoid fossa. By coupling with the force of the deltoid, a fulcrum is created that allows strength through a wide arc of motion.

The overlying subacromial bursae reduces friction between the tendonous cuff and the coracoacromial arch. With normal overhead movement, the bursae facilitates smooth gliding of the tendons within this limited space.

In most individuals, the space between the greater tuberosity and the undersurface of the acromion is approximately 7 to 14 mm while standing with the arm at the side. There is little room for clearance, and during normal overhead shoulder function light contact between the rotator cuff and coracoacromial arch may occur.

Any process that interferes with the rotator cuff’s capability to keep the humeral head centered or compromises the normal coracoacromial arch can lead to impingement of the rotator cuff. This can include calcium deposits, thickened bursae, and an unfused os acromiale.

Specific acromial morphologies affect the size of the outlet. Bigliani described three types of acromia. Type I is flat, Type II is curved, and Type III is hooked. A curved or hooked acromion decreases the space available for tendons to glide and they have been associated with impingements symptoms and rotator cuff tears.

Outlet impingement may also occur from hypertrophy or calcification of the coracoacromial ligament. Osteophytes at the acromioclavicular joint or at the lateral acromion may cause impingement in these areas

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