Summary

LET decreased forces on the ACL graft at all tibial slopes, with its protective effect increasing as the PTS increased in pivot shift, suggesting that orthopaedic surgeons should consider addition of a LET in patients with increased posterior tibial slope.

Abstract

Objective

The addition of a lateral extra-articular tenodesis (LET) at the time of anterior cruciate ligament reconstruction (ACLR) is known to reduce ACL graft forces. Accordingly, LET has been shown to lower the risk of ACL graft rupture in patients who are high risk due to young age, elevated tibial slope, high grade laxity, and participation in pivoting sports. However, it is unknown whether the effectiveness of LET in patients with increasing posterior tibial slope (PTS) may plateau or drop-off at a threshold slope value. Although ACL graft force increases with increasing slope without an LET, the relationship between ACL graft force and tibial slope with an LET is less well understood. The objective of this study was to evaluate how increasing posterior tibial slope alters the protective effect of a lateral extra-articular tenodesis on ACL graft force during pivot and anterior drawer.

Methods

Eighteen finite element knee models representing the intact ACL, ACLR, and ACLR with LET conditions at six levels of PTS ranging from 0 º to 20º were derived from a single cadaveric specimen. Computed tomography scans, experimentally measured envelope of motion, and manually digitized soft-tissue attachments were collected to personalize the models. Rotational knee laxity was equalized across all slope-adjusted intact ACL knee models utilizing a computational soft-tissue calibration procedure which best matched simulated and experimental internal/external and varus/valgus laxities. All ligament and graft materials were modeled as either single or multi-bundle nonlinear springs. A simulated pivot shift was performed on 1) ACLR and 2) ACLR with LET models by applying 5 N·m valgus and 3 N·m internal rotation moments to the proximal tibia at 30° flexion. Additionally, a modified anterior drawer was simulated with a 710 N compressive force and an anterior force on the tibia, loading the ACL graft to 75% of its tensile failure limit. ACLR and LET forces were recorded for each slope condition.

Results

During pivot shift, the addition of a LET decreased forces on the ACL graft at all tibial slopes, with the protective effect increasing as the PTS increased. Increasing PTS from 0° to 20° resulted in an increase in ACL graft forces from 101.3N to 166.1N while LET force increased from 18.0% to 60.4% of ACLR graft force. Comparing ACLR with and without LET, the protective effect of LET increased with tibial slope, from 11.2% to 41.3%, averaging 24.1 ± 10.0%. The protective effect during anterior drawer was approximately constant at 3.0 ± 0.6% across all tibial slopes.

Conclusions

LET decreased forces on the ACL graft at all tibial slopes, with its protective effect increasing as the PTS increased in pivot shift, suggesting that orthopaedic surgeons should consider addition of a LET in patients with increased posterior tibial slope.