Bizarre Parosteal Osteochondromatous Proliferation (Nora Lesion)

• Age: 2nd and 3rd decades most common, but ranges from childhood to the elderly
• Sex: No strong sex predilection; slight female predominance in some series
• Incidence: Rare; exact incidence unknown, but account for a minority of surface bone lesions
• Sites: Phalanges of hands and feet (most common), followed by metacarpals and metatarsals; long bones less frequently

• Mass: Painless or mildly tender swelling on the surface of a bone in the hand or foot
• Growth: Slowly enlarging over months to years
• Functional impact: May cause mechanical irritation, deformity of adjacent digit, or limit motion
• History: Often no preceding trauma identified, though trauma has been proposed as a trigger

• Rare worldwide: No population-based incidence data; reported across all continents
• No ethnic predilection: Cases reported in diverse populations
• Age distribution: Most common in 2nd and 3rd decades but ranges from adolescents to the elderly
• Acral predilection: The hand and foot predominance is consistent across all reported series globally

• High-resource: CT or MRI for characterization, musculoskeletal pathology review, hand/foot surgeon excision
• Limited-resource: Plain radiographs often sufficient for diagnosis; excision by any trained orthopaedic surgeon
• Universal principle: The diagnosis is primarily clinical-radiologic; pathology review confirms but should not be interpreted in isolation
• Key message: In any resource setting, avoid misdiagnosis as sarcoma by correlating clinical, radiographic, and histologic findings

BPOP has historically been considered a reactive process, but molecular studies showing clonal gene rearrangements (COL1A1, COL1A2) suggest it may be a benign neoplasm. The distinction has no practical impact on management but is of academic interest.

Some authors recommend biopsy before excision if there is diagnostic uncertainty; others argue that classic radiologic features are sufficient to proceed directly to excision. No consensus exists, and practice varies by institution and surgeon experience.

Marginal excision is universally recommended, but the extent of cortical curettage or smoothing at the base is variable. Some surgeons advocate more aggressive local treatment of the cortical bed to reduce recurrence, but evidence is anecdotal.

Most recurrences occur within 2 years, but late recurrences have been reported. There is no consensus on the optimal duration of follow-up. Most authors recommend clinical and radiographic review for at least 2-3 years post-operatively.