Adventitious Bursitis Overlying an Osteochondroma of the Humerus Facing the Thoracic Wall

One of the complications of osteochondromas is the development of a bursa over the cartilaginous cap. We report a 15-year-old boy with a rapidly expanded adventitious bursitis overlying an osteochondroma of the humerus facing the thoracic wall, a location not previously reported for such bursa formation. Magnetic resonance imaging readily showed adventitious bursitis overlying the osteochondroma, thereby dispelling concerns for malignant transformation. 1. Introduction One of the complications of osteochondromas is the development of a bursa over the cartilaginous cap. Differential diagnosis of a rapid enlargement of the bursa overlying an osteochondroma and malignant transformation of the cartilaginous cap of osteochondroma is important because a rapid enlargement of the bursa may be interpreted as a malignant transformation [1]. We describe a large adventitious bursa formation related with an osteochondroma of the humerus. Although adventitious bursa formation on osteochondromas was reported in the upper extremities, a bursa formation in the humerus was not reported before. 2. Case Report A right-hand-dominant 15-year-old boy was followed since five years of age for a slow-growing solitary osteochondroma in the right humerus. He presented with a rapidly increased swelling at the site of the lesion over the course of a month. There was no pain or neurovascular symptoms. He had a history of recent trauma to the region of concern while playing with his older brother. Physical examination revealed a nontender, painless, fluctuating soft tissue mass at the anteromedial side of the proximal right arm. The right shoulder of the patient had no limitations during the active range of motions. Laboratory findings were normal. Anteroposterior oblique plain radiographs demonstrated a broad-based osteochondroma originating from the proximal diaphysis of the right humerus and an increased density of the overlying soft tissue. The expansion of the osteochondroma was towards the chest wall (Figure 1). A 3.5 × 3 × 2？cm, broad-based osteochondroma of the right humerus associated with an overlying fluid-containing sac showing peripheral enhancement on the postcontrast images suggestive of an adventitious bursitis was visualized on MRI (Figure 2). Cartilage cap of the osteochondroma was 3？mm in thickness and could be differentiated from the overlying cystic sac. Following nonsteroidal anti-inflammatory drug therapy, soft tissue swelling at the site of osteochondroma subsided. Figure 1: Oblique radiograph of the right arm shows soft tissue swelling overlying a broad-based