Sarcomatous transformation of osteochondroma of the coronoid process forming pseudoarthrosis with zygomatic arch mistaken for Jacob disease.

Osteochondromas are the most common benign osseous neoplasms, with a propensity to involve long bones. The involvement of the coronoid process is extremely rare, and osteochondromas of the coronoid process may form pseudoarticulations with the zygomatic arch when large, an entity that is extremely rare and termed as Jacob disease. Sarcomatous degeneration of osteochondroma of the coronoid process is an extremely rare entity and has not been described to date. Only 1 case of primary mesenchymal chondrosarcoma has been described. We present a case of a 24-year-old man with a chondrosarcoma of the coronoid process, who was wrongly diagnosed with Jacob disease on the basis of radiologic findings.