Osteochondroma of the coronoid process (Jacob's disease).

Osteochondroma is the most common benign tumor of the skeleton. The region is a rare location for osteochondroma. In 1899, Jacob was the first to describe osteochondroma of the coronoid process, forming a pseudoarthrosis joint between the coronoid process and the zygomatic arch. The name “Jacob’s disease” is given to this condition.1,2 Osteochondromas of the coronoid process can be bilateral or unilateral. Most of these lesions grow like a mushroom on the coronoid process and do not destroy the coronoid process. 3 These masses sometimes cause expansion of the zygoma or the zygomatic arch. These benign tumors grow insidiously, 4 and the most common symptom is limited mouth opening and facial deformity. Seventy percent of patients are men who are usually younger than 40. 3,5 To image the osteochondroma, panoramic radiography and paranasal sinus radiographs are useful. However, to visualize the relationship between the coronoid process and zygomatic arch, computed tomography (CT) is essential.4,6 Surgical excision is the only treatment, and recurrence is rare.7 As the disease develops slowly, patients with complaints of pain and limited mouth opening may be treated for a misdiagnosis of temporomandibular joint (TMJ) disorder. Histopathologic examinations of the tumor have shown that the center of the tumor includes endochondral ossification regions enclosed by hyaline cartilage. Growth of an osteochondroma is similar to that which occurs at the epiphysis, with the cap of cartilage acting as the epiphyseal plate. Chondrocytes migrate to the center to form cancellous bone. 8 Solitary osteochondroma may have a low incidence of sarcomatous change (1%). If the osteochondroma is a part of an autosomal dominant syndrome known as osteochondromatosis, the risk of sarcoma becomes higher (11%). 7