Flexor Hallucis Longus Hypertrophy Secondary to Achilles Tendon Tendinopathy: An MRI-based Case-Control Study

Abstract

Background: The purpose of this study was to outline indirect signs of advanced Achilles tendinopathy on magnetic resonance imaging (MRI) and develop a potential tool that could aid in surgical decision-making.Methods: Magnetic resonance imaging (MRI) scans of Achilles tendon were analyzed retrospectively in two consecutive cohorts. Control group consisted of patients that had an MRI due to other reasons and no signs of tendinopathy. Two parameters from two muscle bellies were measured and compared on axial MRI scans 4 to 5 cm above the ankle joint line at the level of greatest thickness: area and diameter of the triceps surae (TS) and of the flexor hallucis longus (FHL). Interobserver agreement was analyzed. A receiver operating characteristic (ROC) curve was created for both quotients to assess potential cut-off points.Results: A total of 60 patients for each study group were included. The ratios for area and for diameter showed significant higher values for FHL in the tendinopathy group (p<0.001). There was strong to very strong interobserver agreement (rho=0.744). A diameter ratio FHL/TS of 2.0 or higher had a sensitivity of 49% and specificity of 90% for concomitant Achilles tendinopathy.Conclusion: In our patient cohort, flexor hallucis longus hypertrophy was observed even before a tendon transfer was employed as a possible compensatory mechanism for Achilles tendon tendinopathy. Using the tool described in this study, measuring a value of 2.0 or higher on an axial MRI, may be indicative as an indirect sign of functional deterioration of the Achilles tendon.