Mechanical Integrity of a Flexor Hallucis Longus Tendon Transfer Construct Using an All-Inside Endoscopic Approach

Research Type: Level 5 - Case report, Expert opinion, Personal observation Introduction/Purpose: Flexor hallucis longus (FHL) tendon transfer remains a popular treatment course for chronic Achilles tendon ruptures. However, surgical approach with a percutaneous plantar incision have reported potential for damaging neurovascular structures. Previously, we have reported the development of a novel all-inside endoscopic FHL tendon zone 1 approach that exhibited no insult to the neurovascular structures. The purpose of this study was to assess the mechanical integrity of the novel surgical FHL tendon construct. Methods: An all-inside endoscopic FHL tendon transfer was performed on 10 cadaveric specimens without fluoroscopic guidance. Following anatomic evaluation that documented medial / lateral position of the FHL tendon relative to the interference screw, a tendon stripper was used to separate the transferred FHL tendon from the attached muscle belly. Mesh wire clamps were used to encase each FHL tendon and securely fastened with a cable clamp to prevent slippage. Each construct was then affixed to a custom tendon clamp end effector on a uniaxial hydraulic loading platform (MTS). Each specimen underwent 50 cycles of viscoelastic conditioning (20-60 N at 1 Hz) [1,2]. Ultimate failure uniaxial loading was then applied at a rate of 1.25 mm/s until failure was achieved [1]. Results: One specimen was damaged during tendon stripping and a second failed during preconditioning, which left 8 specimens for evaluation. Mean failure strength for the FHL tendon transfer construct was 222 (75) N. Failure strength was not influenced by whether the tendon was seeded on the medial or lateral side of the interference screw (222 (58) N vs. 222 (99) N, respectively). FHL tendon peak strain at failure was 26.2 (11.8) % with a Young’s Modulus of 58.3 (22.0) MPa. Causes of failure included interference screw loosening (N = 2), FHL tendon pullout (N = 5), and FHL tendon rupture (N = 1). Conclusion: The ultimate failure strength of our FHL tendon transfer construct was slightly stronger than those reported in the literature for screw (172 (40) N), anchor (189 (26) N), or suture (110 (46) N) fixation [1,2]. This strength was irrespective of tendon location within the screw tunnel. Thus, the all-inside endoscopic FHL zone 1 approach exhibits lower propensity for neurovascular damage and equivalent or greater construct strength versus existing FHL tendon transfer techniques, making it an ideal surgical approach. Further, ultimate failure strength consistency indicates that performing surgeons do not need to consider tendon location within the screw hole during surgery.