A 31-year-old male presented to the emergency room with a one-day history of right hand pain after relatively minor trauma. The pain was exacerbated with movement. After further questioning, the patient reported an insidious onset of increase in size of the fifth digit, along with decreased range of motion. Physical examination revealed visible enlargement of the right fifth digit without erythema or hematoma. Conventional radiographic evaluation was performed in the emergency department.
Right hand small finger enlargement
Calcified/osseous exophytic mass
Bizarre parosteal osteochondromatous proliferation (BPOP)
Osteochondroma
Parosteal osteosarcoma
Florid reactive periostitis
Osseous exostosis is a relatively common radiologic finding seen in many different pathologies. Evaluation begins clinically with special attention given to the time course of symptoms. Radiologic examination becomes critical to distinguish between the many underlying causes. A key radiologic discriminating factor when evaluating exostosis is the presence or absence of disrupted osseous cortex; followed by structural changes involving the underlying medullary cavity. These diagnostic findings are essential in arriving at the appropriate diagnosis.
BPOP (otherwise known as Nora’s lesion) is a rare, benign, exostotic osteochondromatous tumor of the hands and feet which pathologically is seen as part of a spectrum of reactive lesions.1 The cause is unknown, but is thought to be related to trauma. Nora’s lesion usually presents as a minimally painful pedunculated or sessile mass which grows slowly over months or years.2 Its size typically ranges from 0.4 to 3 cm in diameter.3 It most commonly affects patients in their thirties or forties without gender predilection.2 The tumor primarily affects bones of the hands and feet but can also be found in the mandible and long bones. 4 Lesions originate from the periosteum of an intact underlying cortex.1 Pain is an infrequent symptom, and in rare instances, erythema or discoloration is seen in the overlying skin. Joint motions may be limited, depending on the location of the lesion.5
Conventional radiographs show parosteal calcification or bony masses with a typical mushroom-shape arising from the cortical surface of the underlying bone, usually involving the metaphysis. The lesion may be calcified or ossified with well-defined margins and broad-based attachment to the underlying bone without cortical disruption. There may be decreased mineralization of the cortex of the host bone, but no periosteal new bone formation. Cortical flaring at the junction with the lesion is not a feature of BPOP. The absence of continuity between the lesion and medullary cavity of the bone is a key radiographic finding that differentiates BPOP from osteochondromas. Evolution of the lesion can be seen radiographically with a first stage consisting of periosteal soft tissue swelling or mass, sometimes with tiny calcification; further along in the disease course, calcification becomes more prominent leading to complete ossification of the lesion.
Osteochondroma is the most common cartilage containing tumor. Characteristically solitary, metaphyseal, and usually pointing away from the adjacent joint, it is found most commonly around the knee. While osteochrondroma is one of the most common benign bone tumors, they are uncommon to arise in the distal extremities, a feature which distinguishes it from BPOP. The majority of cases occur in young patients less than 20 years old. Imaging of osteochondromas is fairly characteristic with normal marrow, cortex, and periosteum extending from parent bone into the exophytic lesion which has a cartilaginous cap. Histologically, osteochondromas do not display cytological atypia and show more regular alignment of chondrocytes, as opposed to the ‘bizarre’ appearance in BPOP. Patients with osteochondromas commonly suffer mechanical complications related to the exostosis. Occasionally, osteochondromas can degenerate into chondrosarcoma.
Parosteal osteosarcoma is low grade osteosarcoma, arising along the surface of bone. The majority of cases are seen between the ages of 20-50 and commonly present with pain and swelling, along with a mass. Parosteal osteosarcoma is most commonly seen along the posterior distal femoral metaphysis, but can involve the tibia and humerus as well. Parosteal osteosarcoma involving the hands and feet is rare.3 On imaging, parosteal osteosarcoma is observed to arise juxtacortically from the bone. The bulk of the mass extends into the soft tissues with smooth, lobulated margins and a characteristic cleavage plane between the tumor and the underlying bone with a “stuck-on” appearance. Parosteal osteosarcoma can dedifferentiate into a higher grade osteosarcoma.
Florid reactive periostitis is an entity that falls within the spectrum of diseases that includes BPOP and myositis ossificans. Like BPOP, there is usually a history of antecedent trauma. Florid reactive periostitis most commonly involves the hands, usually affecting a proximal or middle phalanx. On imaging, early florid reactive periostitis appears as an ossified or calcified soft tissue mass without underlying bony abnormality; later, its relationship with periosteum and cortex becomes more conspicuous. Histologically, islands of bone and hyaline cartilage are separated by a fibrous stroma, and focal osteoclastic remodeling of the bone is evident.6 Local excision is usually the definitive therapy; recurrence is uncommon.
Bizarre parosteal osteochondromatous proliferation
Osseous exostosis is a relatively common finding which may be seen with a variety of conditions. A combination of clinical and imaging findings aid in narrowing the differential diagnosis and may even lead to a single diagnosis. In this case, the presence of a well-marginated mass arising from an intact underlying cortex was helpful in establishing the diagnosis of BPOP over the more common differentials discussed above. Although a rare entity, BPOP should be considered in the differential diagnosis of an exophytic osseous or chrondromatous growth found in the hand.
Shulman DR, Petrey WB. Exophytic Osseous Growth. J Am Osteopath Coll Radiol. 2012;1(2):35-37.
