A magnetic resonance imaging safe distance beyond the flexor hallucis longus tendon and the application of the safe arthroscopic approach to prevent neurovascular injury in posterior ankle arthroscopy

Abstract

Background: The flexor hallucis longus (FHL) tendon is used as a medial landmark for posterior ankle arthroscopy. However, some posterior ankle pathologies require working medial to the FHL. The safe distance beyond the FHL to the neurovascular bundle has never been established. Purpose: To report the safe distances beyond the FHL and neurovascular bundle and the incidence of neurovascular injury after using “safe arthroscopic approach”. Methods: The 63 MRIs with posterior ankle pathologies were included. Measurements were the FHL-tibial nerve (TN) and FHL-posterior tibial artery (PTA) distances at the level of fibular tip, 1.2 cm above and 1.2 cm below. Twenty FHL tenosynovitis patients underwent arthroscopic debridement utilizing a safe arthroscopic approach developed by the authors based on the MRI findings. The incidence of neurovascular injury and clinical outcomes were assessed. Results: The closest mean (standard deviation, SD) distances of FHL-TN and FHL-PTA were 2.14 (0.81) mm and 5.23 (2.11) mm. The incidence of neurovascular injury after using “safe arthroscopic approach” was 1/20. The NRS for pain during activity daily living (ADL) and FAAM for ADL were statistically improved at 6-month follow-up from 6.36 (1.50) to 1.14 (1.29) and 72.4 (5.78) to 89.4 (7.96), p<0.001. Conclusion: This study demonstrated close margins between the FHL and the TN and PTA of 2.14 mm and 5.23 mm. The “safe arthroscopic approach” for arthroscopic FHL debridement had a TN injury incidence of 1/20 which was equivalent to previous reported incidences of the arthroscopic surgery which did not go beyond FHL.