Introduction

Lateral extra-articulair tenodesis (LET) is frequently performed in combination with intra-articular ACL-reconstruction. The main goal for LET is to aid the intra-articular ACL reconstruction to control the pivot shift phenomenon. However, no biomechanically optimal LET has been described. It is important to describe a biomechanically optimal LET, because small changes in position around the rotational axis of the femur can easily affect the knee kinematics and tunnel malpositioning could lead to unwanted graft behavior, graft failure or even osteoarthritis. The purpose of this study was to determine the optimal tibiofemoral tunnel positions for LET using length changes of various simulated grafts on the anterolateral side of the knee in healthy subjects during in-vivo full weightbearing motions.

Methods

Nineteen healthy subjects were studied using a combined computed tomography and biplane fluoroscopic imaging technique during a lunge (full extension to ~110° flexion). In detail, 3D bony models of the knee, created with the use of the CT scanner, were manipulated in six degrees-of-freedom until the projections of the models matched the outlines of the fluoroscopic images of the lunge and box squat. Using the true lateral-medial view, a grid described by Stephen et al. was modified and applied to the lateral femoral condyle. Using this grid, 158 femoral attachments were projected to the lateral femoral condyle. The length changes of 158 simulated grafts on the lateral femoral epicondyle connected to either Gerdy’s tubercle (GT) or the anatomic tibial attachment of the anterolateral ligament (TA-ALL) were analyzed. To define the optimal tibiofemoral tunnel positions for a LET the following three conditions were formulated: (1) <10% length change between 20-40° of flexion, (2) the graft is most taut between 20-40° flexion, and (3) the graft slackens at >40° of flexion. The area that met all three conditions was defined as the optimal tunnel position for LET.

Results

The optimal location for LET was found to be posterior-proximal to the lateral femoral epicondyle when connected to both GT or the TA-ALL attachment. The center of the location was on average 42.6 ± 0.4% and 45.5 ± 0.5% proximal to the inferior border of the lateral femoral condyle and 18.0 ± 0.2% and 23.9 ± 0.3% anterior to the posterior border of the femoral condyle for the GT and TA-ALL, respectively. Moving the femoral attachments anteriorly affected the length changes as graft lengths increased with increased flexion angles. Moving the femoral attachments posterior-distally resulted in slackening of the grafts at flexion angles >20° of flexion.

Discussion

The optimal tunnel position for LET would be posterior-proximal lateral femoral epicondyle. Care must be taken when moving the femoral tunnel position of the LET from posterior-proximal to positions anterior to the lateral femoral epicondyle, as the length of a graft will increase during deeper flexion angles which may cause overconstraint of the lateral knee compartment.

CLINICAL RELEVANCE: This in vivo biomechanical study presents the optimal position for LET using two tibial attachments and may be used as a guideline for surgical procedures