Summary

Posterior tibial slope affects cruciate ligament reconstruction graft loading in a linear fashion where flat or decreased slope increases force on PCL grafts when and increased posterior tibial slope increases ACL graft force independent of knee flexion angle when an axial load is applied.

Abstract

Background

Sagittal plane posterior tibial slope may influence the risk for cruciate ligament injury of either the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) and can lead to reconstruction failure of a cruciate ligament depending on the degree of slope. There is a paucity of biomechanical studies evaluating the effect of tibial slope on the loading properties of cruciate ligament reconstructions.

Purpose/Hypothesis: The purpose of this study was to quantify the effect of sagittal plane tibial slope on ACL graft force, single bundle (SB) PCL graft force, and double bundle (DB) PCL graft force at varying slopes and knee flexion angles.

Study Design: Controlled laboratory study.

Methods

For ACL reconstructed specimens, ten male, fresh-frozen, cadaveric knees had a proximal posterior tibial osteotomy performed and an external fixator placed for tibial slope adjustment. Following ACLR, specimens were compressed with a 200 N axial load at flexion angles of 0°, 15°, 30°, 45°, and 60°, and loading was recorded through a force transducer clamped to the graft. Slope was varied between -2° and 20° at 2° increments. For the PCL specimens, a different set of ten cadaveric knees was used with similar osteotomy and external fixator. Following SB (anterolateral bundle (ALB) only) and DB PCLR, specimens were compressed with a 300 N axial load at flexion angles of 45°, 60°, 75°, and 90°. The ALB and posteromedial bundle (PMB) graft loads were measured with a force transducer clamped to the graft. . Slope was varied between -2° and 16° at 2° increments for each test state.

Results

ACL graft force, in the loaded testing state, increased in a linear fashion as slope increased. This was independent of flexion angle. The final models utilized 2-factor linear mixed-effects regression models overall noting a significant highly positive, linear relationship between tibial slope and ACL graft force in axially loaded knees at all flexion angles tested (slope coefficient = 0.92, SE = 0.08, p<0.001). For PCL specimens, increased tibial slope had an independently significant, linear decreasing effect on the force on all PCL grafts regardless of flexion angle (coefficient = -0.70, SE = 0.11, p < 0.001). The ALB graft of DB reconstructions had significantly lower graft force than the ALB graft of the SB reconstruction (coefficient = 14.8, SE = 1.62, p < 0.001). Higher knee flexion angles were also significantly associated with higher graft force for all grafts (p < 0.001).

Conclusion

For ACL reconstructed knees, a flatter tibial slope was protective of ACL grafts, while steeper slope increased ACL graft loading supporting the role of slope decreasing tibial osteotomies in patients with slopes >12°. Increased posterior tibial slope was protective of PCL reconstruction grafts, and decreased slope (flattened) increased PCL graft force. In addition, DB PCL reconstruction grafts had lower forces than SB PCL reconstructions. Overall, these biomechanical findings support recent clinical evidence of increased ACL graft failure with increased tibial slope secondary to increased graft loading and support the noted negative impact of decreased tibial slope on PCL grafts.