Tibial Slope and Its Effect on Graft Force in Posterior Cruciate Ligament Reconstructions

Background: A flattened posterior tibial slope may cause excessive unwanted stress on the posterior cruciate ligament (PCL) reconstruction graft and place patients at risk for PCL reconstruction graft failure. To date, there is a paucity of biomechanical studies evaluating the effect of posterior tibial slope on the loading properties of single-bundle (SB) and double-bundle (DB) PCL grafts. Purpose/Hypothesis: The purpose of this study was to quantify the effect of sagittal plane tibial slope on PCL reconstruction graft force at varying slopes and knee flexion angles for SB and DB PCL reconstructions. The null hypothesis was that there would be no differences in SB or DB PCL graft forces with changes in posterior tibial slope or knee flexion angle. Study Design: Controlled laboratory study. Methods: Ten male fresh-frozen cadaveric knees had a proximal posterior tibial osteotomy performed and an external fixator placed for tibial slope adjustment. SB (anterolateral bundle [ALB] only) and DB PCL reconstruction procedures were performed and tested consecutively for each specimen. The ALB and posteromedial bundle graft forces were recorded before (unloaded force) and after (loaded force) compression with a 300-N axial load. Unloaded and loaded graft forces were tested at flexion angles of 45 degrees, 60 degrees, 75 degrees, and 90 degrees. Tibial slope was varied between -2 degrees and 16 degrees of posterior slope at 2 degrees increments under these test conditions. Results: Modeling for unloaded testing revealed that tibial slope had an independently significant and linear decreasing effect on the force of all PCL grafts regardless of flexion angle (coefficient = -1.0, SE = 0.08, P < .001). Higher knee flexion angles were significantly associated with higher unloaded graft force for all PCL grafts (P < .001). After the graft was subjected to loading, tibial slope also had an independently significant and linear decreasing effect on the loaded force of all PCL grafts regardless of flexion angle (coefficient = 20.70, SE = 0.11, P < .001). The ALB graft of DB reconstructions had a significantly lower loaded graft force than the ALB graft of the SB PCL reconstruction (coefficient = 14.8, SE = 1.62, P < .001). The posteromedial bundle graft had a significantly lower loaded graft force than the ALB graft in both reconstruction states across all flexion angles (both P < .001). Higher knee flexion angles were also significantly associated with higher loaded graft force for all graft constructs (P < .001). Conclusion: PCL graft forces increased as tibial slope decreased (flattened) in the loaded and unloaded states. An increased posterior tibial slope was protective of PCL reconstruction grafts. The findings of this study support the effect of tibial slope on PCL grafts that has been noted clinically, and a flat tibial slope should be considered a factor when evaluating the cause of failed PCL reconstructions. Clinical Relevance: The authors validated that decreased tibial slope increased the loads on PCL reconstruction grafts. Patients with flat tibial slopes in chronic tears or revision PCL reconstruction cases should be evaluated closely for the possible need of a first-stage or concurrent slope-increasing tibial osteotomy.