Imaging features of osteoid osteoma of the phalanges

Osteoid osteoma (OO) of the phalanges is regarded as a difficult diagnosis to make, both clinically and radiologically. Our aim was to assess the imaging features of this relatively rare entity to assess for consistent imaging features. Retrospective analysis of databases at our institutions was performed to identify patients with histologically confirmed OO of the phalanges. Radiographs were assessed for a nidus, surrounding sclerosis, periosteal reaction, cortical erosion and generalised soft tissue swelling. MRI scans were reviewed for visibility of the nidus; reactive marrow oedema; periostitis; and a soft tissue mass associated with the lesion. Ten patients with OO occurring in a phalanx were identified. Nine out of ten patients (90 %) were male. The mean age was 20 years (range 11-45). At radiography, a nidus was visible in 100 % (10 out of 10) cases. Internal matrix within the nidus was seen in 60 % (6 out of 10). Sclerosis surrounding the nidus was present in 90 % (9 out of 10). Cortical erosion was identified in 10 % (1 out of 10). Generalised swelling of the affected digit was noted in 100 % (10 out of 10). Periosteal/cortical sclerosis was seen in 20 % (2 out of 10). Of the 8 patients who had an MRI scan, the nidus was visible in 100 % (8 out of 8). Reactive marrow oedema within the affected phalanx was present in 100 % (8 out of 8). There was a soft tissue mass surrounding the affected phalanx in 75 % (6 out of 8). Periosteal oedema was noted in 13 % (1 out of 8). Phalangeal OO demonstrates similar imaging appearances to OO at other locations in the majority of cases. An inflammatory soft tissue mass often accompanies the nidus at MRI.