Summary

This study found that increasing posterior tibial slope (PTS) in cadaveric knees significantly elevated in-situ forces in the ACL at 30° of flexion under axial and anterior tibial loading, supporting the hypothesis that higher PTS increases the risk of ACL injury.

Abstract

Introduction

An increased posterior tibial slope (PTS) is a recognized risk factor for anterior cruciate ligament (ACL) injury and failure of ACL reconstruction (ACL-R) procedures. To reduce ACL-R failure, PTS-reducing osteotomies have been introduced with the rationale that decreasing PTS reduces anterior translation, thereby lowering the force on the ACL. This procedure has been shown to have a protective effect on articular cartilage and the ACL graft in the context of ACL reconstruction. Therefore, the objective of this study was to quantify the effect of increasing PTS on the in-situ force in the ACL. It was hypothesized that an increased PTS would result in increased in-situ force on the ACL.

Methods

A 6-degrees-of-freedom robotic testing system (MJT Model FRS2010) was used to apply external loads to seven fresh-frozen human cadaveric knees (mean age, 50±13.4 years). The following loads were continuously applied from full extension to 90° flexion: 1) 200N axial compression + 134N anterior tibial load, and 2) 5Nm internal tibial torque + 10Nm valgus torque. In-situ forces in the ACL were measured for two PTS states: 1) Native PTS, and 2) Increased PTS (Mean 7.4±0.7˚). To simulate the effects of PTS increase, an anterior infra-tuberosity level opening wedge osteotomy was performed. The osteotomy was rigidly fixated using an external fixator, allowing for multiple PTS adjustments, i.e., changing the native PTS to an increased PTS and back. Thus, the kinematics obtained in response to the loading conditions could be replayed before and after PTS change, and the resultant forces in the ACL in both PTS states could be determined using the principle of superposition. The Mann-Whitney U test was used to compare the in-situ forces in the ACL between the two PTS states (native PTS, increased PTS) at seven flexion angles (full extension, 15°, 30°, 45°, 60°, 75°, 90°). Significance was set at p < 0.05.

Results

PTS was increased from 9.1±3.1° to 16±3.1°. In response to a 200N axial compression combined with a 134N anterior tibial load, the in-situ force in the ACL at 30˚ of flexion increased by 13% after PTS was increased (p<0.05). No significant differences were found at other flexion angles. Under a combined rotatory load of 10Nm valgus and 5Nm internal rotation torques, no significant differences were found for the in-situ force in the ACL after the PTS was increased (p>0.05).

Conclusion

This study demonstrated that an increase in PTS in a native knee resulted in an increase in the in-situ forces in the ACL at 30˚ of flexion under a 200N axial compression and a 134N anterior tibial load. This finding supports the clinical notion that increased PTS leads to higher forces in the ACL at low flexion angles, thereby increasing the risk of ACL injury in patients.