The internal impingement of the shoulder (athlete's shoulder)

Among the various forms of impingement, internal impingement is a special form. In all other forms of impingement, the tendon or bursa is trapped between the humeral head and the bony processes of the shoulder blade, the acromion or coracoid process.

Internal impingement occurs when extreme movement in the shoulder joint causes the tendon insertion of the supraspinatus and infraspinatus tendons to become crushed by the posterior acetabular rim. The most common movement that can trigger internal impingement is extreme external rotation in the shoulder joint with simultaneous strong abduction of the arm. This rarely occurs in normal everyday life. It primarily affects throwing athletes in sports such as handball, javelin throw, and baseball. Volleyball players can also be affected if they excessively rotate the ball externally when smashing. Athletes who have practiced their sport for years often experience increased external rotation combined with reduced internal rotation. This phenomenon is also known as GIRD syndrome and is a classic trigger for internal impingement.