The purpose of this communication was to discuss a new mechanism of osteoc hondral injury of the glenoid fossa observed in a baseball player. A nineteen-year-old man was admitted to our hospital complaining of pain in his right shoulder when throwing since seventeen years of age. The ROM was not limited. While the Crank test and resistance test for rotator cuff muscles were positive, no instability was demonstrated. Radiographic examinations showed sclerosis in the osteochondral bone of the glenoid fossa. Computed tomography and magnetic resonance arthrography (MRA) revealed an osteochondral defect in the postero-inferior glenoid fossa. Oblique axial MRimages with the arm in abduction and external rotation (ABER) position showed a thick and tight AIGHL(anterior inferior glenohumeral ligament ) and a posterior shift of the humeral head relative to the glenoid fossa. Arthroscopic examination revealed a step in the glenoid fossa and loosening of the posterior joint capsule. The labrum, rotator cuff, and glenoid cartilage were intact. Based on these observations we carried out thermal shrinkage of the posterior capsule in arthroscopically. We considered that the thick and tight AIGHL caused posterior subluxation of the humeral head when the shoulder joint was in ABER position. This mechanism yielded a sharing and compression force to the glenoid fossa and resulted in a stress fracture in the postero-inferior osteochondral bone of the glenoid fossa.
