LAUSR

Bizarre parosteal osteochondromatous proliferation (also known as Nora’s lesion) was first described by Nora et al. in 1983 [1]. It is a rare exophytic osteochondral lesion that typically affects the hands or feet, with cases also documented in the skull and long bones. A recent case report from 2014 stated that there had only been 200 published cases up to that point [2]. We performed a subsequent literature search, which only yielded a further 13 cases published to date. It is a particularly rare occurrence in children, with most phalangeal lesions being described in the adult population in the third and fourth decades. A benign lesion, the main differentials include os t eochond roma , pa ro s t e a l o s t eo sa r coma , and chondrosarcoma. Although ultimately a histological diagnosis, there are radiological features that distinguish it from its main differential, the osteochondroma. Both have a cartilaginous cap; however, BPOP demonstrates more irregular matrix calcification and unlike osteochondromata, there is no communication with the medullary canal [3]. Osteochondromata also typically orientate away from the physis, which is not a defining characteristic of BPOP. A further differential includes turret exostosis, otherwise known as an acquired osteochondroma. Predominantly affecting the middle and proximal phalanges of the hand, turret exostoses are usually secondary to minor injury, resulting in the formation of a subperiosteal haematoma that subsequently ossifies [4, 5]. Radiologically very similar to BPOP, both have been described as belonging to the same lesional spectrum [6]. However, turret exostoses are characterised by pain and softtissue swelling, as opposed to the non-tender and nontraumatic characteristics of the BPOP in this case. The initial radiograph at presentation demonstrated a broad-based cortical exophytic lesion arising from the diametaphysis of the middle phalanx of the right ring finger. There was no periosteal reaction or soft-tissue component (Fig. 1). On subsequent follow-up imaging 3 months later, the lesion had continued to grow, but failed to demonstrate any aggressive features (Fig. 2a and b). BPOP was considered the most likely diagnosis with a plan made for confirmation by excisional biopsy. Excision is recommended in symptomatic cases of BPOP [6], and in this case, the patient had lost volar flexion at the distal interphalangeal joint because of the mass effect of the lesion on the adjacent tendons. However, BPOP lesions should be allowed to mature before excision, due to the otherwise high rates of recurrence [7]. Before The case presentation can be found at https://doi.org/10.1007/s00256017-2804-5