Background

Lateral extra-articular tenodesis (LET) reduces ACL graft failure rates two years after surgery when performed as an adjunct to ACL reconstruction (ACLR). Interestingly, previous biomechanical studies have shown that LET may reduce tibial rotation beyond that of the intact knee, while others found no such kinematic overconstraint. Parameters of ligament engagement have proven useful in characterizing the biomechanical function of the ACL and the anterolateral ligament; however, they have not been used to describe the biomechanics of LET.

Purpose

To compare engagement parameters (engagement point, in-situ stiffness, and tissue force at peak applied load) of an LET-reconstructed knee compared to the native lateral tissues in response to an internal rotation torque at 0°, 30°, 60°, and 90° of knee flexion.

Methods

Seven cadaveric knees (mean age: 39 ± 12; range: 28-54; 4 male) were mounted to a robotic manipulator. The robot applied an internal rotation torque of 5 Nm while monitoring the resulting internal tibial rotation (ITR) (in degrees). Each knee was tested following a bone-patellar tendon-bone ACL reconstruction with intact lateral tissues (consisting of the anterolateral ligament and Kaplan fibers) and after sectioning these tissues and performing LET (modified Lemaire technique). Resultant forces carried by the native lateral tissues and the LET were determined via superposition. The parameters of engagement were determined for both the native lateral tissues and the LET and compared via two-way repeated measures ANOVA (p < 0.05).

Results

At full extension, both the LET-reconstructed and native lateral tissues did not engage. At 30°, 60°, and 90° knee flexion, the native lateral tissues engaged with greater ITR than the LET-reconstructed lateral tissues. Specifically, the native lateral tissues engaged with 8° (p < 0.001), 13° (p < 0.001), and 14° (p < 0.001) greater ITR than the LET-reconstructed lateral tissues at 30°, 60°, and 90° knee flexion, respectively. At 30° of flexion, the LET-reconstructed lateral tissues engaged with 9° (p < 0.001) and 10° (p < 0.001) greater ITR than at 60° and 90° knee flexion. Across all three tested knee flexion angles (30°, 60°, and 90°), the LET-reconstructed lateral tissues had greater in situ stiffness than the native lateral tissues. Compared to the native lateral tissues, the LET carried 29 N greater force on average at the peak applied internal rotation torque at 30° of flexion (p = 0.006).

Discussion And Conclusion

LET creates a supraphysiologic restraint to ITR compared to the native lateral tissues. Specifically, LET engaged with less internal rotation at all flexion angles tested but full extension. The LET also carried greater force at the peak applied load and had a greater in situ stiffness at 30° of flexion than the native lateral tissues. Discrepancies between previous biomechanical studies may arise from variations in one or more of these modifiable surgical parameters.

Clinical Relevance: The engagement point of an LET is surgically modifiable by altering the flexion angle or degree of tibial rotation at which the tenodesis is fixed. Parameters of engagement may be used to inform clinical decision-making to tune LET to achieve the desired level of restraint to internal tibial rotation towards improved clinical outcomes.