Summary

Posterior positioning of the graft on the tibia caused increasing of the articular contact forces at the medial and lateral compartments of the knee after anterior cruciate ligament reconstruction.

Abstract

Introduction

Numerous in-vivo and in-vitro studies have shown that even a small change in the femoral graft position could significantly alter the graft length and tensioning patterns during flexion. However, few studies have investigated the effect of tibial tunnel positioning on the cartilage contact forces after ACL reconstruction and it is reported that a more anterior graft position on the tibia could result in better kinematics after ACL reconstruction. The objective of this study is to quantitatively investigate the effect of tibial graft positions on the knee joint biomechanics after ACL reconstruction when the knee is subject to a simulated muscle force.

Methods

A previously validated 3D computational model was used to simulate single-bundle ACL reconstruction using a BPTB graft. 10mm BPTB graft was modeled using a single nonlinear spring element. The graft center was simulated the anatomic center of the ACL on the femur. On the tibial side, the graft insertion center was placed at three different positions within the ACL footprint: the center of the anteromedial (AM) bundle, the center of the ACL insertions (Center) and the center of the posterolateral (PL) bundle. Grafts were simulated to be fixed at 30° of flexion to successfully restore the anterior stability of the knee under a 134N anterior load. Biomechanics of the knee was investigated in intact, ACL deficient and reconstructed conditions when the knee was subjected to an 800N quadriceps load at 0°, 30°, 60° and 90° of flexion. The anterior tibial translation (ATT), internal tibial rotation (ITR), graft forces, anteroposterior (AP) component and proximal-distal (PD) component of graft forces, medial and lateral contact forces were calculated.

Results

Under the anterior load, the ATT and ITR for AM, Center and PL groups were similar at all flexion angles. The quadriceps load did not affect the ACL function at 60 and 90° in this model. Under quadriceps load, the ATT and ITR for all reconstructed groups were similar to those of the intact knee. The AP component of graft force decreased slightly while the PD components of graft force were increased when the tibial position moving posteriorly. The medial and lateral contact forces followed the same trend as PD component of the graft forces at 0 and 30 degrees. For example, AM, Center and PL groups insertions in 1% lower, 1% larger and 4% larger medial contact forces than those of the intact knee and 1% larger, 2% larger and 5% larger lateral contact forces at 0° of flexion, respectively.

Discussion

The main finding of this study is that the tibial graft positioning had little effect on the anteroposterior stability, but had significant effect on the proximal-distal component of graft forces under the quadriceps load. Posterior positioning of the graft caused increasing of the medial and lateral articular contact forces. Anteromedial graft placement within tibial footprint could be a better choice when the graft is placed at the center of femoral footprint. However, Roof impingement should be considered when placing the tibial tunnel more anteriorly.