Type II sciatic nerve variant: an unexpected interventional hazard.

A 12-year-old girl presented with a 1-week history of right hip pain. MRI revealed a lesion in the incompletely fused posterior triradiate cartilage of the right acetabulum with imaging characteristics of an osteoid osteoma. The surgeon requested guidewire placement under CT guidance for lesion drilling. The CT approach was planned to avoid the usual course of the sciatic nerve, and the lesion was successfully removed and confirmed at histology to be an osteoid osteoma. At a follow-up visit, MRI showed no recurrence but demonstrated an abnormal common peroneal nerve from the sciatic notch to the popliteal fossa with denervation oedema in the tibias anterior and peroneal musculature. Review of the imaging showed a bifid piriformis muscle and type II sciatic nerve variant, with the common peroneal nerve component exiting the sciatic notch between the muscle bellies. The nerve's unexpected superolateral position placed it adjacent to the course of the guidewire, resulting in a presumed thermal injury at the time of drilling. This unusual case highlights the importance of the pre-procedural documentation of sciatic nerve variants in the planning of pelvic intervention.