Histogenetic and taxonomic considerations on a case of post-traumatic bizarre parosteal osteochondromatous proliferation (BPOP).

A case of bizarre parosteal osteochondromatous proliferation (BPOP) arising in the head of the fibula of a young man following injury on the lateral aspect of the knee is reported. Microscopically, four zones could be recognized proceeding from the periphery to the deep portion: 1) an outer thick fibrous cap showing an abundant cellular component, composed of thin spindled elements of fibroblastic appearance, haphazardly arranged and exhibiting intense and diffuse immunohistochemical expression of S100 protein (a previously unreported observation); 2) an irregular and atypical cartilaginous cap beneath the fibrous cap; 3) a layer of blue bone in which islands of atypical chondrocytes are still present but gradually decreasing, moving towards the deeper areas; 4) a layer of mature bone, pink bone, which is implanted in the skeletal segment (the deep margin is devoid of periosteum). The intratrabecular spaces of blue or pink bone did not contain haematopoietic marrow, but rather a loose myxoid stroma. This case confirms the fact that BPOP can occur in skeletal segments other than those indicated in the original report of Nora, and that trauma, even if minor, is an important factor in the development of the lesion. The latter begins at the periosteal level, where fibroblasts may acquire a chondroformative function (becoming chondrofibroblasts), as witnessed by the strong and widespread expression of P S100, first documented in this report.