Bizarre Parosteal Osteochondromatous Proliferation

25-year-old man with 2 years of progressively worsening pain at the small finger with recent loss of range of motion.  

Bizarre parosteal osteochondromatous proliferation (BPOP), also known as Nora lesion, is an osteochondromatous proliferation that arises from the periosteal aspect of an intact cortex. The lesion occurs most commonly in the hands and feet (proximal and middle phalanges, and metacarpals/metatarsals) of young adults. About 25% of cases occur in the long bones.

It is thought to be part of a spectrum of reactive changes in response to trauma, initially presenting as soft-tissue swelling.

The stage after soft-tissue swelling is florid reactive periostitis, where there is a laminated or mature periosteal reaction and often minimal periosteal elevation. There is juxtacortical soft tissue calcification, without disruption of the underlying cortex or medullary changes.

The next stage is BPOP, where there is a well-marginated mass of heterotopic mineralization arising from an intact cortex without periosteal reaction or medullary changes.

The final stage is acquired osteochondroma or turret exostosis, where the focus of ossification matures and a bony base is formed with a cartilage cap.

Histologically, BPOP is composed of variable amounts of cartilage (usually predominant), bone, and fibroblastic spindle cells and is similar to fracture callus. The cartilagenous component can have areas of ossification, and large immature chondrocytes can raise concern for malignancy. The rest of the lesion consists of trabeculae of woven bone mixed with fibroblastic spindle cells.

Differential Considerations

• Osteochondroma: There is continuity of the intramedullary cavity with the lesion in osteochondroma. BPOP, as described above, is not continuous with the medullary cavity. In addition, there is absence of the characteristic orientation away from the physis in BPOP.
• Myositis ossificans: There is ossification along the periphery of the lesion, which is located in the peripheral soft tissues.
• Parosteal osteosarcoma: There is cortical and soft-tissue infiltration and periosteal elevation, which are not seen in BPOP.

References

• Bush JB, Reith JD, Meyer MS. Bizarre parosteal osteochondromatous proliferation of the proximal humerus: case report. Skeletal Radiol. 2007 Jun;36(6):535-40.
• Dhondt E, et al. Nora's lesion, a distinct radiological entity? Skeletal Radiol. 2006 Jul;35(7):497-502.
• Hoch B, Montag A. Reactive bone lesions mimicking neoplasms. Semin Diagn Pathol. 2011 Feb;28(1):102-12.