Onlay Reconstruction of the Posterior Cruciate Ligament: Biomechanical Comparison of Unicortical and Bicortical Tibial Fixation

Abstract

AbstractPosterior cruciate ligament (PCL) injuries are generally associated with high-energy trauma. There are many controversies regarding optimal surgical technique in regard to graft selection and fixation methods. The recently described onlay technique allows for direct fixation of a hamstring autograft to the posterior aspect of the tibia with cancellous screw and spiked washer, while protecting the neurovascular structures and avoiding the so-called “killer turn.” The objective of this study was to compare immediate postimplantation biomechanics of unicortical versus bicortical tibial fixation of onlay PCL grafts. Eight knees were randomly assigned to one of two onlay PCL techniques (n = 4 knees/technique), performed by a single experienced surgeon. Testing consisted of a posterior-directed force at four knee flexion angles, 10, 30, 60, and 90 degrees, to measure load to 5 mm of posterior displacement, maximum displacement (at 100 N load), and stiffness. For statistical analyses, data for each knee were normalized to the native PCL-intact knee and were then grouped into unicortical or bicortical groups accordingly. Data for load to 5 mm (strength), displacement at 100 N, and stiffness were compared among PCL-intact, PCL-deficient, unicortical fixation, and bicortical fixation categories using one-way analysis of variance to assess for statistically significant (p < 0.05) differences. When compared with PCL-deficient knees, both fixation techniques had less laxity. When compared with PCL-intact knees, unicortical had more laxity at all angles, and bicortical had more laxity only at 90 degrees (p < 0.001). For relative graft strength, intact knees required significantly higher loads than both treatment groups. Bicortical, however, outperformed unicortical at all angles (p < 0.001) for relative strength. Regarding stiffness, there were no significant differences between unicortical and bicortical, and both were superior to PCL-deficient and inferior to PCL-intact knees. Based on cadaveric biomechanical testing, none of the reconstructed PCL knees was able to replicate the intact native PCL, but both techniques were superior to PCL-deficient knees. The bicortical tibial fixation technique appears to have biomechanical advantages when opting for onlay PCL reconstruction.