Summary

Varus-Alignment significantly increases the Forces acting on an ACL reconstrucion, further intensified by a posterolateral corner instability, PLC reconstruction neutralizes the effect

Abstract

Introduction

Both a varus alignment of the knee as well as an insufficiency of the posterolateral corner (PLC) has been suspected to increase forces on anterior cruciate ligament (ACL) reconstructions.

Purpose

This study aimed to investigate (1) the influence of a varus alignment on the forces acting on an ACL reconstruction, and (2) how an insufficiency / reconstruction of the PLC influences these forces.

Methodology

Eight human cadaveric knee joints were tested in a materials testing machine (Zwick Roell) at five degrees of flexion (0°, 10°, 20°, 30°, 40°). The experimental setup has six degrees of freedom, allowing a dynamic varus thrust under axial compression, with a force of 200N. The ACL was detached at its tibial insertion, sutured with high strength suture material, and refixated transosseously using a custom-made ligament tensioner equipped with a load cell (± 0.1 N), that allowed tensioning of the construct with 80N. To achieve 0°, 3°, 5°, and 8° of varus alignment, a stepwise lateral-opening wedge distal femoral osteotomy was performed and stabilized with an external fixator. The osteotomy gap was filled using 3D-printed wedges. Three conditions of the knee joint were tested: (1) the ACL-reconstructed state; (2) after transection of the PLC (lateral collateral ligament and popliteus tendon); and (3) after reconstruction of the PLC in Arciero technique. Statistical analysis was performed using mixed linear models.

Results

Axial loading of the knee joint led to forces on the ACL reconstruction ranging from 86.3 ± 5 N with a neutral axis in full extension to a maximum of 109.9 ± 46.8 N with 8° of varus in 40° knee flexion. A significant increase was observed in 30° (p=0.006) and 40° (p=0.001) of flexion in the ACL force between neutral alignment and 8° varus. No further significant differences between the flexion angles, and among the different varus alignments (0°, 3°, 5°, and 8°) were found in the simulated ACL-reconstructed state (P>0.05). After transection of the PLC, a significant increase of forces acting on the ACL reconstruction at varus angles of 5° and higher, and with increasing flexion angle was found. Exemplary, in 40° of flexion an increase from neutral to 5° of varus led to an increase of ACL forces from 80 ± 12.3 N to 105.7 ± 28 N (P < 0.001). 8° of varus alignment further increased the load to 153.18 ± 58.4 N (P < 0.001). After reconstruction, this effect disappeared, with no significant effects of the varus alignment or flexion angle observable, including 8° of varus-alignment (P > 0.05).

Conclusion

In this biomechanical model, a varus alignment from 5° upwards, led to an increased load on a simulated ACL reconstruction, with further loading caused by a posterolateral deficiency. This may lead to a higher ACL graft rupture rate in a combined varus knee with posterolateral deficiency.