Anatomical variations of flexor hallucis longus tendon increase safety in hindfoot endoscopy.

Purpose: The flexor hallucis longus (FHL) tendon is the main anatomical landmark during hindfoot endoscopy, and anatomical variations related to the FHL can pose a risk to the tibial nerve and posterior tibial vessels during hindfoot endoscopy. The aim of this study was to determine the distance between the FHL tendon and the tibial neurovascular bundle in the posterior ankle joint when an anatomical variant of the FHL is present. The hypothesis was that the shortest distance between the tibial neurovascular bundle and the FHL tendon in the working area of the hindfoot endoscopy is increased when an anatomical variant of the FHL is present.

Methods: A retrospective review was performed using consecutive ankle magnetic resonance imaging (MRI) scans obtained during 1 year. All scans with anatomical variations related to the FHL were included in the study. A control group including scans without anatomical variations was obtained for comparison. The shortest distance between the FHL tendon and the neurovascular tibial bundle was measured in both groups.

Results: Three-hundred and fifty-five ankle MRIs were reviewed. 35 scans with anatomical variants of the FHL (9.8%) were found and comprised the study group that was compared to 35 scans without variants (control group). The mean distance from FHL to the neurovascular tibial bundle in the control group was 0.9 mm. The study group consisted of 18 cases with distal muscle belly insertion (5.1%), and 17 cases with an accessory tendon corresponding to a flexor digitorum accessorius longus (4.5%). In these subgroups, the mean distance from FHL to the neurovascular tibial bundle was 1.1 and 1.5 mm respectively. Overall this distance was found to be higher in the group with anatomical variants (1.3 mm) when compared to the control group (0.9 mm) (p < 0.05).

Conclusion: During hindfoot endoscopy, the presence of an anatomical variant related to the FHL tendon has proven safer anatomically than in its absence, due to the increased distance between the FHL tendon and the tibial neurovascular bundle in the working area. However, the minimal difference observed in safety distances still poses a major risk of injury during hindfoot endoscopic procedures in all cases.