52 - nora's lesion

Bizarre Parosteal Osteochondromatous Proliferation (Nora's Lesion)

Bizarre parosteal osteochondromatous proliferation (BPOP) is a rare lesion that occurs most commonly in the hands and feet, usually of adults in their 20's and 30's, average age 30 -33 years, range, 2 to 73. Males and females are equally affected. The lesion is most common in the hands followed by the feet, long bones (commonly of the upper extremity), skull, jaw, and other sites. It most commonly occurs on the proximal and middle phalanges, metacarpals, and metatarsals, and it is more rare in long tubular bones. This lesion grows rapidly and has aggressive features on imaging studies as well as confusion findings on histopathology, leading to many errors in diagnosis and potentially inappropriate treatments. The lesion is thought to be related to myositis ossificans, reactive periostitis and subungual exostosis. It was described in 1983 by Dr. Nora, and is sometimes called Nora's disease or Nora's lesion.

There is a history of a mildly painful mass that seems to increase in size over many weeks or a few months. There is usually no history of trauma. There may be stiffness of a nearby joint, or other mechanical symptoms.

On plain radiographs, a bony mass with well defined margins is seen to be applied to the surface of the bone, and it may project into the soft tissues, but lacks the characteristic orientation away from the nearby physis that is seen in osteochondromas. There is a wide base, and the lesion may appear pedunculated. The lesion in this example started on the dorsolateral surface of the bone and then grew towards the palmar surface, wrapping itself extensively around the distal metadiaphysis. Other lesions in the leterature seem more pedunculated. The matrix of the lesion can bee seen to contain mature bone with trabeculae.CT scan is helpful to distinguish these lesions from osteochondromas. In the latter, carefully aligned images should be able to show that the cortex and medullary cavity of the bone is continuous with the cortex and medullary cavity of the lesion. In BPOP, there is no such finding. The well formed cartilage cap that is characteristic of an osteochondroma is also absent, but may not be well demonstrated on CT. MRI images reveal that the tissues within the lesion have signal characteristics that differ sharply from normal bone and cartilage. The lesion is dark on T1 and bright on T2, which is a typical pattern for neoplastic lesions. MRI will further differentiate the lesion from an osteochondroma. In the latter, the cartilage cap is well seen, and the signal characteristics of the bony parts of the lesion are typical of normal bone. The differential diagnosis includes a number of surface lesions such as osteochondroma, exostosis, myositis ossificans, low or high grade surface osteosarcoma, and periosteal chondroma.

On gross pathology, the lesion has a nodular surface covered with glistening cartilage. The bone in the lesion is said to have a distinct blue tint. The cut surface of the lesion bleeds freely. Histologically, the lesion contains very cellular cartilage, a proliferation of bizarre fibroblasts, and disorganized bone with spindle shaped fibroblasts in the intertrabecular spaces, leading to confusion with chondrosarcoma, fibrosarcoma, low grade parosteal osteosarcoma, and conventional osteosarcoma. The periphery of the lesion has lobulated hypercellular cartilage and fibrous tissue with occasional large chondrocytes. The chonrrocytes may be binucleate but lack atypia or hypochromasia. There is no "columnation" of the cartilage as in osteochondroma, rather the cartilage is disorganized and irregular, with patchy areas of ossification. The osteoblasts seen on the bony trabeculae are uniform and do not exhibit cellular atypia.

The lesion is benign but may recur locally in as many as 50% of cases. Recurrence is usually seen from 2 months to 2 years after surgery. Complete excision with the best possible margin is the treatment of choice. Due to the difficulty in establishing the correct diagnosis, referral to an orthopaedic oncologist is warranted. The lesion does not metastasize. A recent report from Korea (J. H. Choi et al Skel Rad 2001 30:44-47) describes a intermediate grade fibrosarcoma that occurred adjacent to a fibular Nora's lesion.