Recognizing Posterior Accessory Muscles During Posterior Ankle Arthroscopy: A Case Report.

The flexor hallucis longus (FHL) tendon is a key anatomical landmark in posterior ankle arthroscopy and a commonly used graft for chronic Achilles tendon rupture. Peroneocalcaneus internus (PCI) is a rare accessory muscle that is frequently overlooked due to its close anatomical relationship to FHL and its asymptomatic presence. Failure to recognize this variation may result in tendon misidentification, ineffective reconstruction, or neurovascular injury. We report a 72-year-old woman with a chronic Achilles tendon rupture treated with endoscopic FHL transfer. Preoperative MRI demonstrated an accessory muscle running parallel to the FHL that was initially unrecognized. Intraoperatively, a tendon resembling the FHL was identified but showed no excursion with passive hallux motion, prompting further dissection. Distal tendon tracing confirmed the presence of a PCI inserting at the sustentaculum tali, allowing correct identification and safe harvesting of the true FHL. Retrospective MRI review highlighted distinguishing features of the PCI, including a separate muscle belly isointense to skeletal muscle, a posterolateral position relative to the FHL, and a calcaneal insertion. This case emphasizes the importance of recognizing accessory muscles on preoperative MRI and applying strict intraoperative identification criteria. Awareness of the PCI and its distinguishing features can improve surgical planning, prevent reconstruction failure, and enhance safety during posterior ankle arthroscopy.