RESULTS AND FINDINGS :

• The musculoskeletal system's cartilaginous neoplasms are a diverse group of lesions with a range of clinicopathologic characteristics. Osteochondroma and periosteal chondroma share remarkable clinical characteristics with BPOP, a benign but locally aggressive fibro-osseous tumour. There are unanswered questions regarding the aetiology, diagnosis, and therapy of this uncommon disease.

• It takes more than just a physical exam and clinical history to make a diagnosis. Osteochondromas and periosteal chondroma are more frequent in the second and third decades, whereas BPOP is most frequent in the fourth decade. Since it might be present or absent, pain is not a reliable indicator of presence. These tumours can have comparable radiographic and histologic features in addition to having a similar clinical presentation.
• All lesions exhibit a well-demarcated ossified mass in a juxtacortical position, with or without sclerotic borders. The key radiographic feature is the cortical aspect of the affected bone and its continuity (or lack of) between the lesion and the underlying medullary cavity. BPOP and periosteal chondromas normally do not have continuity with the medullary cavity, in contrast to osteochondromas. BPOP originates from the periosteal aspect of an intact cortex, whereas the periosteal chondroma exhibits a characteristic saucerization of the underlying cortex.

RADIOLOGY FINDINGS OF BPOP: 

X-rays alone are sufficient to diagnose BPOP in typical radiographic appearance and typical clinical findings.

Radiographically, Nora's lesion is a well-defined cortical growth (Figure 1). It does not have the characteristic orientation distal from the physis as in cases of osteochondromas. There may or not be a periosteal reaction or lysis. Absent medullary involvement is seen. But radiographic features alone cannot diagnose the lesion. On MRI, the lesion is T1 hypointense and hyperintense on T2 weighted and STIR sequences. The medullary cavity and adjacent soft tissues are uninvolved.

DIFFERENTIAL DIAGNOSIS :

• Given its parosteal location, BPOP must be distinguished from parosteal osteosarcoma and chondrosarcoma, which are extremely rare in the hands and feet. Features of such malignancies such as cortical flaring, cortical destruction, periosteal reaction, and soft tissue invasion are not found with BPOP. Histologically, malignant features of sarcomas are absent in BPOP.
• Because of its surface location and pattern of ossification, BPOP may be mistaken for an osteochondroma (Figure 2). Osteochondromas are also rare in the phalanges of the distal extremities.  A related entity to osteochondromas, the subungual exostosis, is a distal extremity lesion, but unlike BPOP it typically involves the dorsal aspect of the distal phalanx and direct continuity to bone is present in these lesions. The cartilaginous cap that is a feature in the majority of osteochondromas is an inconsistent feature in BPOP(figure 3).

-Algorithm for the classification of radiodense bone lesions is shown in figure 4.

PATHOLOGICAL ANALYSIS :

Grossly, BPOP appears as an exophytic mass without continuity with the underlying medullary cavity, containing bone and cartilage. Histologically, disorganized architecture is present with aggregates of cartilage, new bone, and fibrous tissue. 

The appearance of "blue bone" on H and E staining is a typical histologic finding in BPOP cases. This appearance was first described by Meneses as having a "distinct blue tinctorial characteristic," and this has been used as a useful characteristic for the diagnosis of this entity (figure 5). The cartilaginous components of the lesion demonstrate hypercellularity with groups of binucleated and bizarre chondrocytes. Hyperchromasia and cytological atypia are not present in BPOP.