Bizarre Parosteal Osteochondromatous Proliferation (Nora Lesion)

The lesion. Bizarre parosteal osteochondromatous proliferation, BPOP, is a benign surface lesion of bone that produces a well-circumscribed mass of bone and cartilage attached to the cortex. Nora and colleagues described it in 1983, and the name that stuck is theirs.

Why it matters. The lesion is rare, but it has a striking tendency to be misdiagnosed as chondrosarcoma or parosteal osteosarcoma on histology alone. The cartilage is bizarre and hypercellular and looks malignant, yet the lesion is entirely benign; knowing it exists is what prevents an unnecessary wide resection or amputation, and the hands and feet are exactly where preserving function matters most.

Who. The peak is usually given as the second and third decades, though the pooled review in the evidence section places it in the third and fourth; the range runs from childhood to the elderly (8 to 73 in Meneses's series, 14 to 74 in Nora's). There is no strong sex predilection, with a slight female predominance in some series. The exact incidence is unknown; BPOP accounts for a minority of surface bone lesions.

Where. Hands and feet predominate, but not overwhelmingly:

• Hands 55%: phalanges most often, then metacarpals
• Feet 15%: metatarsals and phalanges
• Long bones 25%: one BPOP in four is not acral

That last figure is the one to remember. An acral site is the strongest clinical clue and raises the prior for BPOP; it does not define the lesion, and the long-bone BPOP is the one that gets resected as a sarcoma.

How it presents. A painless or mildly tender swelling on the surface of a hand or foot bone, enlarging slowly over months to years. It may cause mechanical irritation, deformity of the adjacent digit or limited motion. A preceding injury is often not identified, although trauma has been proposed as a trigger.

Reactive or neoplastic. BPOP has been classified as a reactive or pseudoneoplastic proliferation rather than a true neoplasm, within the spectrum of florid reactive periostitis. The aetiology is unknown; the proposed mechanisms are reactive change after trauma or chronic irritation. Molecular studies have since demonstrated rearrangements of the type I collagen genes COL1A1 and COL1A2, often as fusions with the non-coding RNA genes LINC-PINT and MIR29B2CHG, which supports a clonal but benign process and suggests it may be a benign neoplasm. The distinction has no practical impact on management.

What it makes. Mature bone and cartilage growing on the cortical surface, the cartilage cap hypercellular with enlarged chondrocytes and bizarre nuclei that mimic chondrosarcoma. There is no evidence of true malignant transformation. The mass sits on the cortex without any connection to the medullary canal, and that is the feature everything else on this page comes back to.

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Histopathology. The bone component is mature, well-formed trabecular bone with osteoblastic rimming, capped by cartilage. The cartilage is hypercellular with enlarged, bizarre chondrocytes and may show binucleated cells and myxoid change. Meneses drew attention to a distinctive blue (basophilic) tinctorial quality of the lesional bone, a helpful diagnostic clue.

What is absent. Despite the alarming appearance there are no mitoses, no true cytologic atypia and no infiltrative growth. The border is pushing and well circumscribed, and unlike chondrosarcoma there is no permeation of pre-existing bone trabeculae. Those absences, together with the clinical and radiological setting, are what separate BPOP from its malignant mimics:

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The molecular story. Older texts teach two recurrent chromosomal abnormalities in BPOP, t(1;17)(q32-42;q21-23) and inv(7)(q21.1-22q31.3-32), described long before the genes at the breakpoints were known. They are still correct, and they are the same finding as the fusions: COL1A1 sits at 17q21.33 and MIR29B2CHG at 1q32.2, giving the t(1;17); COL1A2 sits at 7q21.3 and LINC-PINT at 7q32.3, giving the inv(7). Karyotype and fusion are one event seen at two resolutions.

What that means in practice. BPOP is not associated with hereditary multiple exostoses (EXT1/EXT2) or any other hereditary syndrome. Molecular testing can help separate BPOP from osteochondroma and parosteal osteosarcoma in difficult cases, though it is not routinely available and changes practice only in selected referral cases.

BPOP sits within a family of reactive surface lesions of the small bones. One classic member, turret exostosis, is worth knowing because it shares the same reactive pathogenesis and acral site. The others have pages of their own: subungual exostosis, myositis ossificans and, on the neoplastic side of the differential, osteochondroma, the lesion BPOP is separated from by the absence of cortico-medullary continuity.

Turret exostosis. A smooth, dome- or turret-shaped subperiosteal ossification on the dorsum (extensor surface) of a proximal or middle phalanx, arising from a post-traumatic subperiosteal haematoma that ossifies. Compared with BPOP it is a more mature, organised excrescence, and it often limits extensor tendon gliding and finger extension. Excise it once it has matured, since early excision raises recurrence.

One process or several. Some authors propose that these are stages of a single reactive process, florid reactive periostitis evolving through turret exostosis to BPOP, which would explain their shared site, trauma association and overlapping histology. Whatever the precise relationship, the practical message is the same: an acral surface lesion in this spectrum is benign and treated by excision once mature, not as a sarcoma.

Telling the members apart on imaging. Subungual exostosis may lack medullary continuity just as BPOP does; its subungual location, fibrocartilaginous cap and distal-phalanx origin place it in a different pattern. Florid reactive periostitis starts as a soft-tissue mass with peripheral calcification and mild periosteal thickening over an intact cortex, evolves toward more organised mineralisation, and can mimic an aggressive surface tumour while it matures. Myositis ossificans is intramuscular and separate from the parent cortex, with mature peripheral mineralisation around a central lucency, the reverse zonation of many malignant ossifying lesions.

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History. A slowly growing mass over months to years, usually painless or mildly tender. The patient may or may not recall a preceding injury, and may have noticed progressive deformity or a limited range of motion in the digit.

Examination. A firm, non-mobile mass on the surface of a phalanx, metacarpal or metatarsal: bony hard, fixed to the underlying bone, non-tender or mildly tender. The overlying skin is typically normal, without erythema or warmth. Confirm the digit is neurovascularly intact distally and, with a large lesion, check for nerve compression.

Radiographs. PA, lateral and oblique views of the affected digit or foot. The hallmark is a well-circumscribed surface mass of mature bone on the cortex of a short tubular bone with no medullary continuity, and well-defined borders suggest benignity. The rest of the picture:

• The underlying cortex is intact, though it may show slight thinning or scalloping
• The cartilage cap may be visible as a radiolucent zone at the periphery, and the cartilage is typically more prominent than expected for the size of the lesion
• There is no soft-tissue mass and no aggressive feature

Cortical destruction would raise concern for malignancy. Read that with one caveat from the evidence section: Gitto's review records cortical interruption and medullary involvement in BPOP, rarely, so their presence does not by itself exclude the diagnosis.

CT. The best modality for confirming the absence of medullary continuity and for defining the interface between lesion and cortex. It shows the mature bone mass on the cortical surface, no connection to the medullary canal and an intact underlying cortex, and it defines the extent of the lesion for planning the excision.

MRI. Requested when there is concern about soft-tissue extension or nerve or vessel involvement, or to help separate the lesion from a sarcoma. BPOP is a well-defined surface lesion with the signal characteristics of a cartilage cap, without marrow oedema or a soft-tissue mass; MRI helps exclude the aggressive features expected in parosteal osteosarcoma.

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The differential is the other surface lesions of bone, and the first question for every one of them is whether the medullary canal runs into the mass.

Osteochondroma. The continuity-positive mimic: it flares out of the parent bone, whose cortex and medullary canal run on into it, and that continuity is diagnostic. It arises from the metaphysis of a long bone (knee, proximal humerus) in adolescents with an active growth plate, sessile or pedunculated, under a normal-appearing cartilage cap that is thin and uniform on MRI. Both lesions are benign, but BPOP recurs far more often after excision; osteochondroma is excised if symptomatic.

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The cartilage lesions of long bones. Periosteal chondroma is purely cartilaginous, with no bone formation within the mass, on the surface of the humerus or femur. Periosteal chondrosarcoma is the malignant surface cartilage lesion: a long-bone site, cortical destruction and chondroid MRI signal are warning features against BPOP, and biopsy planning belongs in a sarcoma unit.

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Parosteal osteosarcoma is the malignant surface lesion BPOP must never be confused with. It is the commonest surface (juxtacortical) osteosarcoma, a low-grade tumour of slightly older patients (third to fourth decade), arising classically on the posterior metaphysis of the distal femur and other long bones rather than the hands and feet. Both it and BPOP are surface lesions without medullary continuity, so continuity does not separate them; site, the radiological signs and the histology do. Full detail is on parosteal osteosarcoma.

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The operation. Marginal excision: complete removal of the surface mass with a narrow margin of normal tissue, preserving the adjacent neurovascular structures and joint function. Wide resection or amputation is never indicated for a Nora lesion; the high recurrence rate is expected and is managed by re-excision, and over-treatment after a misdiagnosis of sarcoma is the greatest iatrogenic harm this lesion causes.

Before surgery. Review the CT to confirm the surface location and plan the approach. A pre-operative core needle biopsy is considered only if the diagnosis is uncertain; when the radiological appearance is classic for BPOP in a characteristic location, excision may proceed without a prior biopsy.

Consent. Roughly half of these lesions recur, typically within 2 years: 18 of 35 (51%) in Nora's original series and about 55% of the 40 patients Meneses could follow. A further excision may then be needed, and it makes recurrence much less likely rather than certainly ending it, because at least 8 of Nora's lesions recurred more than once. No distant metastasis and no malignant transformation has been reported in either series.

The one modifiable factor. Michelsen's hand series had 1 recurrence in 10 cases, which he attributed to excising the pseudocapsule and the periosteum beneath the lesion and decorticating any abnormal host cortex. Ten patients and no control group, so treat that as a reason to operate meticulously rather than as the rate to quote at consent.

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When it comes back. Recurrence does not indicate malignancy or a need for more aggressive surgery; each recurrence is treated by repeat marginal excision, with the diagnosis confirmed again on pathology review.

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The greatest risk is misdiagnosis, not the lesion itself. Multiple case reports document unnecessary wide resection or amputation for a BPOP misread as chondrosarcoma or parosteal osteosarcoma, which is why the clinical-radiologic-pathologic correlation above is mandatory before any aggressive plan for a surface lesion of the hand or foot. The rest of the complications come from recurrence and from the surgery itself.

Nora lesion is universally benign. No case of malignant transformation has been reported in either of the two large series, and the prognosis for function is excellent when it is managed correctly. After a single marginal excision without recurrence the patient keeps full hand or digital function; after a re-excision function is good, with minor stiffness possible; after multiple excisions the lesion is eventually cured, though there may be cumulative stiffness from the repeated surgery. Unnecessary functional loss follows a wide resection or amputation for a lesion misdiagnosed as sarcoma, and that is preventable with expert pathology review.

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