Myositis Ossificans Affecting the Rotator Cuff Muscles and Proximal Humerus: A Rare Clinical Entity.

Introduction: Myositis Ossificans (MO) is a benign condition characterized by the formation of non-neoplastic heterotropic ossification in extra-skeletal soft tissues. It is generally associated with preceding trauma; however, cases of atraumatic MO have also been reported in the literature. While the elbow is the most common site, MO can occur anywhere in the body and is particularly common in young male athletes in the second to third decade of life. MO involving the rotator cuff muscles is an extremely rare entity with its pathology poorly understood till date. In such cases where it originates in atypical locations, a definite diagnosis is difficult without histopathological examination. Case Report: We report a rare case of a 23-years-old man who presented with complaints of progressive pain in the right shoulder associated with decreased mobility and joint stiffness of 3 years' duration following innocuous local trauma. The patient was reviewed at multiple centers, underwent a number of radiological and histopathological investigations, but was diagnosed incorrectly as synovial chondromatosis. On reporting to our center, the patient underwent a battery of non-invasive and invasive tests and was found to have MO in the rotator cuff muscles and around the proximal humerus. Conclusion: This report concerns a single patient seen in the mature stage of disease. Long-standing pain and stiffness of the shoulder after modest trauma, without destruction of bone, was finally shown to be MO of the subscapularis and infraspinatus with change around the proximal humerus. Synovial chondromatosis and later a possible malignant change in osteochondroma had been suggested earlier. The case only reminds that this diagnosis still belongs in the differential at an uncommon site. It does not map the early course of the lesion, and it should be understood that needle biopsy may come back non-diagnostic. Open biopsy and the zonal pattern on computed tomography were what settled the matter here. As the ossification was diffuse, he was managed without excision.