Single- versus double-bundle posterior cruciate ligament reconstruction: effects of femoral tunnel separation.

BACKGROUND: Double-bundle posterior cruciate ligament reconstructions are performed to more closely replicate the anatomy of the native posterior cruciate ligament and to better restore normal knee biomechanics and kinematics than a single graft. The femoral tunnel for the anterolateral graft is normally located near the anterior margin of the posterior cruciate ligament footprint. However, there is considerable variability with regard to placement of the posteromedial tunnel within the footprint margins. HYPOTHESIS: A double-bundle posterior cruciate ligament reconstruction will better replicate normal knee biomechanics and kinematics than a single anterolateral graft, and the separation distance between femoral tunnels will significantly affect the recorded measurements. STUDY
DESIGN: Controlled laboratory study.
METHODS: The posterior cruciate ligament's femoral origin was mechanically isolated using a cylindrical coring cutter, and a cap of bone containing the ligament fibers was attached to a load cell that recorded resultant force in the posterior cruciate ligament as the knee was loaded. Cast acrylic replicas of the femoral bone cap, with 9-mm and 6-mm holes for the anterolateral and posteromedial grafts, respectively, were attached to the load cell. Graft isometries, anterior-posterior laxities, graft forces, and tibial rotations were measured for an anterolateral graft alone, and for anterolateral and posteromedial grafts with narrow (0-mm) and wide (3-mm) bridges between tunnels.
RESULTS: Mean laxities with an anterolateral graft alone were within 1.2 mm of normal, between 0 degrees and 90 degrees; means with double-bundle grafts were 1.7 mm to 2.4 mm less than normal, between 10 degrees and 45 degrees. Relative length change of the anterolateral graft between 0 degrees and 90 degrees was within +1.3 mm, while the posteromedial graft, placed in either tunnel, tightened approximately 6 mm with knee extension from 90 degrees to 0 degrees. At 0 degrees, mean forces with a single anterolateral graft were not significantly different from posterior cruciate ligament forces for any loading mode tested; mean forces with double-bundle grafts were 74 N to 154 N higher than posterior cruciate ligament forces at 0 degrees . During passive knee extension, the double-bundle reconstruction externally rotated the tibia (relative to intact) between 0 degrees and 50 degrees. There were no significant differences in mean knee laxities, graft forces, or tibial rotations between narrow and wide tunnel separations.
CONCLUSION: In contrast to the anterolateral graft, which experienced minimal length changes, the posteromedial graft tightened 3.1 mm to 4.3 mm from 30 degrees to 0 degrees. When the posteromedial graft was tensioned and fixed at 30 degrees, it developed relatively high graft forces as the knee was extended to 0 degrees ; this tended to reduce knee laxity and increase graft forces. With double-bundle grafts, tunnel separation distance was not an important variable with respect to the biomechanical and kinematic measurements recorded in this study. CLINICAL RELEVANCE: The need for a posteromedial graft during posterior cruciate ligament reconstruction is questioned, especially in view of the relatively high graft forces at full extension that could cause it to permanently elongate with time. If a double-bundle reconstruction is performed, there is no biomechanical advantage in making the bone bridge between tunnels less than 3 mm.