Single Biotenodesis Screw vs Biotenodesis Screw with Cortical Button Tension Slide Technique for Flexor Hallucis Longus Transfer: A Biomechanical Analysis

CATEGORY: Ankle; Hindfoot; Sports; Other INTRODUCTION/PURPOSE: Restoration of functional plantarflexion of the ankle joint following chronic Achilles injury is achieved through transfer of the flexor hallucis longus (FHL) tendon. Fixation of the FHL tendon transfer with a cortical button tension slide technique is an advancement that has not been measured against traditional methods. The purpose of this study is to evaluate the biomechanical differences between a single biotenodesis screw versus biotenodesis screw with cortical button tension slide technique for the fixation of FHL transfer. The authors hypothesize a biomechanical advantage for the tension slide technique compared to the biotenodesis screw alone for fixation of the flexor hallucis longus in chronic Achilles pathology. METHODS: 12 pairs (n=24) of fresh-frozen cadaveric tibia-to-toe samples (6 male, 6 female) were utilized to assess the biomechanics of FHL tendon transfer fixation. All specimens underwent bone density analysis. Each pair of cadaveric samples were randomized to receive either the biotenodesis screw or the tension slide technique. FHL tendon transfer was performed via standard technique using a 6.25x15mm biotenodesis screw with the foot in 20 degrees plantarflexion. A 2.6x12mm cortical button was inserted and FHL tendon was introduced with a tension-slide technique. Biomechanical loading was applied between 20N and 60N at a rate of 1Hz for 100 cycles. Post-cyclic load to failure occurred at 1.25mm/sec. Cyclic displacement, structural stiffness, and ultimate load were derived from resultant load-displacement curves. Student t-tests were used to evaluate significant effects between both FHL tendon transfer techniques. Linear regression analysis was performed to assess interactions between bone density and strength of FHL tendon transfer. RESULTS: Average tendon diameter of the 24 FHL samples was 5.44+-0.46 mm. Average bone density for all 24 calcaneus samples was determined to be 1.06+-0.08 g/cm(2). Loading data was normalized with respect to bone density and tendon diameter to account for inherent anatomical differences between cadaveric specimens. The addition of a cortical button for FHL transfer to the calcaneus did not significantly affect cyclic displacement (0.78+-0.52mm vs 0.87+-0.80mm) or structural stiffness (162.11+- 43.34N/mm vs. 167.57+-49.19N/mm). Addition of the cortical button to the FHL tendon transfer construct resulted in a 34% significant increase in ultimate load (343.72+-68.93 N) compared to biotenodesis screw alone (255.62+-77.17 N) (p=0.0002). Linear regression analyses did not reveal any significant interactions between bone density and FHL tendon transfer technique. CONCLUSION: Enhanced strength can be achieved for Achilles pathology repaired with FHL tendon transfer utilizing a biotenodesis screw and cortical button tension slide technique demonstrated by a statistically significant 34% increase in ultimate load, subsequent to cyclic loading. Cortical buttons in the setting of FHL tendon transfer to the calcaneus offers an additional level of support in the case of extreme tendon transfer failure.