Imaging features of osteoid osteoma of the phalanges.

Objectives: 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.
Materials and Methods: 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.
Results: 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).
Conclusion: 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.