Peroneus Tertius Syndrome: A Rare Cause of Anterolateral Ankle and Rearfoot Pain.

There is limited literature describing anterolateral ankle or rearfoot pain associated with the peroneus tertius tendon. The purpose of this study is to define peroneus tertius syndrome, in which the peroneus tertius tendon causes catching or locking over the anterolateral ankle or rearfoot with accompanying pain. We present a retrospective case series involving 4 patients diagnosed with peroneus tertius syndrome, discuss symptoms for clinical diagnosis, review radiographic imaging, and outline a minimally invasive operative technique for resection of the symptomatic tendon. Furthermore, we sought to evaluate postoperative outcomes and satisfaction scores after surgical management. All patients experienced resolution of symptoms by the fourth postoperative week. There was a statistically significant improvement in visual analog scale scores (average ± SD, 7.75 ± 0.96 preoperative to 0.75 ± 1.19 postoperative) (p = .002). Overall patient satisfaction was 100%, with improved functional status and no evidence of recurrent symptoms at final follow-up (average 18.88 ± 8.68 months). Our results demonstrate that excision of a symptomatic peroneus tertius provides resolution of symptoms, facilitates a quick return to activity, and has excellent patient outcomes. We suggest that during the evaluation of anterolateral ankle or rearfoot pain, peroneus tertius syndrome be considered as part of the differential diagnosis.