Causes / typical injury mechanism: Internal impingement of the shoulder occurs when the arm is maximally abducted with external rotation, such as in the late cocking phase of a baseball player's throwing motion. Repetitive motion like this can accumulate microinjuries, resulting in fraying and partial tears of the undersurface of the rotator cuff.

Classic history and presentation: Internal impingement syndrome is typically observed in pitching athletes, or any profession in which there is repetitive maximal external rotation and abduction of the shoulder.

Patients typically present with diffuse shoulder pain that is most prominent in the posterior aspect of the shoulder, exacerbated by maximal abduction and external rotation (cocking position for a pitcher). The pain is worsened with repetition of these motions and resolves with rest and NSAIDs, but it will resume after activities are commenced again. Patients may also demonstrate reduced internal rotation of the shoulder compared to the contralateral side.

Prevalence:

• Age – Middle-aged adults, or patients younger than 40 engaged in activities with repetitive abduction and external rotation of the arm (eg, baseball pitching).
• Sex / gender – No meaningful difference.

Risk factors:

• Repetitive abduction and external rotation of the shoulder
• Throwing sports
• Overhead activities
• Smoking
• Fluoroquinolone antibiotics

Pathophysiology: Maximal abduction with external rotation can pinch the posterior undersurface of the supraspinatus tendon between the posterior-superior glenoid and the humeral head, especially if the rotator cuff is weak and the posterior glenohumeral capsule is stiff.

Related topic: shoulder impingement syndrome