Double-bundle ACL reconstruction: influence of femoral tunnel orientation in knee laxity analysed with a navigation system - an in-vitro biomechanical study

Background: This paper reports an in-vitro study for evaluating the influence of the femoral tunnel orientation in anterior cruciate ligament (ACL) double-bundle reconstructions. Methods: This work describes the experimental protocol and results obtained for six cadaver knees using the FlashPoint optical system (Image Guided, Boulder, Colorado, USA) and a computer-assisted technique for the elaboration of anatomical and kinematic data. Each specimen was examined by the same surgeon in the following steps: (1) intact knee stability was evaluated by performing antero-posterior displacement and internal-external rotation test at 90°; (2) the ACL was resected and the knee evaluated again; (3) the ACL was reconstructed using the gracilis semitendinous tendon (through horizontal tunnels in femur), and the new kinematics recorded; (4) the ACL was reconstructed again with the same tendon, but with a more vertical orientation of the femoral tunnel (vertical tunnel) and kinematics was once more recorded; (5) finally the knee was dissected to digitise the anatomical structures. Results: Off-line computer analysis of the acquired anatomical and kinematic data showed that there was a significant statistical difference (Wilcoxon test with the Montecarlo method for small samples – p = 0.035) between horizontal tunnel (HT) and vertical tunnel (VT) reconstruction both in the antero-posterior test (median antero-posterior displacement in horizontal tunnel was 0.8 mm less than in vertical tunnel reconstruction) and in the internal-external (IE) rotation test (median internal-external rotation in horizontal tunnel reconstruction was 5° less than in vertical tunnel reconstruction). Conclusion: The analysis of graft behavior in reconstructed knees compared with normal and ACL-deficient knees suggests that the most horizontal tunnel performed better than the vertical tunnel, thus constraining optimally both antero-posterior and internal-external rotations. This finding suggests that femoral tunnel direction may be an important issue in ACL surgery.