The Effect of Femoral Tunnel Position and Graft Tension on Patellar Contact Mechanics and Kinematics After Medial Patellofemoral Ligament Reconstruction

Abstract

Background: An incorrect femoral tunnel position or inappropriate graft tensioning during medial patellofemoral ligament (MPFL) reconstruction may cause altered patellofemoral joint kinematics and contact mechanics, potentially resulting in pain and joint degeneration. Hypothesis: Nonanatomic positioning of the tunnel or graft overtensioning during MPFL reconstruction will have an adverse effect on patellar tracking and patellofemoral joint contact mechanics. Study Design: Controlled laboratory study. Methods: Eight fresh-frozen cadaveric knees were placed on a customized testing rig, with the femur fixed and the tibia mobile through 90° of flexion. Individual heads of the quadriceps muscle and the iliotibial band were separated and loaded with 205 N in anatomic directions using a system of cables and weights. Patellofemoral contact pressures and patellar tracking were measured through the flexion range at 10° intervals using Tekscan pressure-sensitive film inserted between the patella and trochlea and an optical tracking system. The MPFL was transected and then reconstructed using a double-strand gracilis tendon graft. Pressures and kinematics were recorded for reconstructions with the graft positioned in anatomic, proximal, and distal tunnel positions. Measurements were then repeated with an anatomic tunnel and graft tension of 2 N, 10 N, and 30 N, fixed at 3 different flexion angles of 0°, 30°, and 60°. Statistical analysis was undertaken using repeated-measures analysis of variance, Bonferroni post hoc analysis, and paired t tests. Results: For a graft tensioned to 2 N, anatomically positioned MPFL reconstruction restored intact medial and lateral joint contact pressures and patellar tracking ( P > .05), but femoral tunnels positioned proximal or distal to the anatomic origin resulted in significant increases in peak and mean medial pressures and medial patellar tilt during knee flexion or extension, respectively ( P < .05). Grafts tensioned with 10 N or 30 N also caused significant increases in medial pressure and tilt. Graft fixation at 30° or 60° restored all measures to intact values ( P > .05), but fixation at 0° caused significant increases ( P < .05) in medial joint contact pressures compared with intact knees. Conclusion: Anatomically positioned reconstruction with 2-N tension fixed at 30° or 60° of knee flexion restored joint contact pressures and tracking. However, graft overtensioning or femoral tunnels positioned too proximal or distal caused significantly elevated medial joint contact pressures and increased medial patellar tilting. The importance of a correct femoral tunnel position and graft tensioning in restoring normal patellofemoral joint kinematics and articular cartilage contact stresses is therefore evident. Clinical Relevance: A malpositioned femoral tunnel or overtensioned graft during MPFL reconstruction resulted in increased medial contact pressures and patellar tilting. This may lead to adverse outcomes such as early degenerative joint changes or pain if occurring in a clinical population.