Zone I flexor digitorum profundus repair: an ex vivo biomechanical analysis of tendon to bone repair in cadavera

PURPOSE: Biomechanical studies of standard flexor digitorum profundus (FDP) tendon to bone repairs show ultimate strengths greater than the applied loads of early motion rehabilitation protocols. Strain data, however, indicate the potential for significant repair site gapping under these physiologic loads. Gaps in excess of 3 mm have been shown to prevent the time accrual of strength in midsubstance tendon repairs and may prevent the restoration of the normal architecture of the tendon-bone interface. Improving the time-zero tensile properties of FDP insertion site repairs may help obviate these issues and improve clinical outcomes. The purpose of this study was to evaluate the ex vivo biomechanical properties of 2 new repair techniques in comparison with the standard FDP tendon to distal phalanx cortical surface repair. METHODS: Thirty human cadaver FDP tendons were released from their insertion sites by sharp dissection and repaired to bone using 1 of 3 repair techniques. Load to failure testing was performed with a servohydraulic materials-testing system (model 8500R; Instron, Canton, MA) analyzing ultimate force, strain at 20 N, rigidity, force to 2-mm gap formation, and displacement at failure. RESULTS: The results of the failure tests indicate that repairs performed with the addition of a peripheral suture had a greater ultimate force, had increased resistance to gap formation, and had increased rigidity and decreased strain at 20 N compared with the tunnel-only and volar cortical surface to the distal phalanx repairs. Although there were no statistically significant differences in ultimate force or rigidity between the tunnel-only and volar cortical surface repairs, the tunnel-only repairs showed lower strain values and increased values for resistance to 2-mm gap formation when compared with the volar cortical surface repairs. There were no differences among any of the repair groups with regard to the magnitude of tendon displacement from the repair site at failure. CONCLUSIONS: The addition of a peripheral suture to the FDP tendon to bone tunnel repair construct improves the time-zero tensile properties as evidenced by statistically significant increases in ultimate force, rigidity, and resistance to gap formations of 2 mm. In comparison with a volar cortical surface repair, the bone tunnel-only repairs were effective at decreasing the amount of repair site strain during applied loads of 20 N. If these improved time-zero tensile properties persist during the early stages of healing, they may help decrease the incidence of repair-site gap formation associated with the forces of early motion rehabilitation protocols.