Background

High tibial osteotomy in the medial opening wedge technique is commonly associated with an unintentional increase in posterior tibial slope (PTS). An increased PTS following medial open wedge high tibial osteotomy has been shown to result in degeneration of the anterior cruciate ligament (ACL), which might increase the risk of subsequent ACL injury. However, causes of ACL fiber degeneration after PTS increase are unknown.

Purpose

To quantify the effect of an isolated increase in PTS on knee kinematics and in situ forces in the ACL. It was hypothesized that an isolated PTS increase would increase anterior tibial translation and decrease the in situ-forces in the ACL.

Methods

A 6 degrees of freedom robotic testing system (MJT Model FRS2010) was used to apply external loads to 9 fresh-frozen cadaveric knee specimens (mean age 53.4 ± 17.8 years). The following loads were continuously applied from full extension (defined as 1-Nm extension moment) to 90° flexion: (1) 200-N axial compression, (2) 5-Nm internal tibial + 10-Nm valgus torque, and (3) 5-Nm external tibial + 10-Nm varus torque. Kinematic data and in situ forces in the ACL and PCL were acquired for two PTS states: (1) native PTS, and (2) increased PTS. An anterior opening wedge high tibial osteotomy was performed to increase the PTS by 10°. In situ-forces in the ACL were calculated using the principle of superposition. A two-way ANOVA was conducted to determine the effects of PTS and knee flexion on knee kinematics and in-situ forces in the ACL. Level of significance was set at p < 0.05.

Results

Increasing the PTS resulted in increased anterior (1.9 mm; p < 0.05) and proximal (0.6 mm; p > 0.05) tibial translation at 60° flexion and reduced in-situ force in the ACL at 30° (59.5%), 60° (71%), and 90° (74.3%; p < 0.05) flexion in response to 200-N axial compressive load. In response to 5-Nm internal tibial + 10-Nm valgus torque, there was significantly more valgus rotation at 60° (2.8°, p < 0.05) and 90° (2.1°, p < 0.05) flexion and 41.1% (p < 0.05) less in-situ force in the ACL at 90° flexion in the increased compared to the native PTS state. Significantly more lateral tibial translation at 30° (1.8 mm; p < 0.05) and 90° (2.4 mm; p < 0.05) flexion, more varus rotation at 30° (1.6°, p < 0.05) flexion, and less in-situ force in the ACL at 60° (74.8%) and 90° (76.8%; p < 0.05) flexion were observed in the increased compared to the native PTS state in response to 5-Nm external tibial + 10-Nm varus torque.

Conclusion

An isolated increase in PTS affected both translational and rotatory knee kinematics. In addition, increasing the PTS caused a clinically relevant reduction of the in-situ forces in the ACL by up to 76.8% in response to isolated compressive and combined rotatory loads. Surgeons should be aware that intentionally or unintentionally increasing the PTS during high tibial osteotomy causes stress deprivation of the native ACL, which may promote ACL fiber degeneration and subsequent ACL injury.