Poster 317: How Steep is Too Steep? Assessing the Limits of Lateral Extra-Articular Tenodesis and Slope-Reducing Osteotomy on ACL Graft Force and Knee Stability

Abstract

Objectives:

There is growing recognition of the adverse impact of excess posterior tibial slope (PTS) on anterior cruciate ligament (ACL) graft forces, contributing to an increased failure rate in patients with excessive PTS. In patients with elevated PTS and a history of ACLR failure, a slope-reducing osteotomy has been offered as a surgical option. Lateral extra-articular tenodesis (LET) has also been proposed to augment knee stability, thereby diminishing ACL graft forces in patients burdened with excess PTS. Therefore, the primary aim of this biomechanical study was to evaluate whether LET could counteract the effects of excess PTS on knee kinematics and ACLR graft forces, to an extent akin to a slope-reducing osteotomy.

Methods:

Anatomic single-bundle ACL reconstruction (ACLR) was performed on seven fresh-frozen cadaveric specimens. To facilitate desired adjustments to the PTS, a posterior osteotomy was created, with direct measurements allowing for PTS’s of 0°, 10°, and 20°, as confirmed via lateral standard X-ray. Subsequent to ACLR, baseline biomechanical testing was conducted utilizing a six-degrees-of-freedom robot, outfitted with a custom load cell affixed to the ACL. Testing encompassed three sequential specimen states: ACLR only, ACLR with sectioning of the ALC, and finally, the incorporation of ACLR + LET. Assessments of anterior tibial translation (ATT), internal rotation (IR), and ACL graft forces were performed across the specified tibial slopes for each successive specimen state, following a simulated pivot shift maneuver (comprising 100 N compression, 10 Nm valgus torque, 5 Nm IR rotation torque). Within the ALC-sectioned state, the sectioning involved both the Anterolateral Ligament and Kaplan fibers at their respective attachment sites, while the LET state was attained via the modified Lemaire technique. The statistical methodology employed mixed models, incorporating classical sandwich estimation, with the application of Holm’s test for multiple comparisons. A p-value threshold of <0.05 was established as the criterion for statistical significance

Results:

At 10 degrees of PTS, LET reduced graft forces during pivot shift loading more than the 10-degree slope-reducing osteotomy (104.9 N vs 125.7 N, p=0.03). However, at 20 degrees of PTS, no statistically significant differences in graft loads were found between LET and 10-degree slope-reducing osteotomy (109.5 N vs 111.3 N, p=0.71) (Figure 1). Analysis of tibiofemoral kinematics revealed an inverse linear relationship between knee stability and increasing posterior tibial slope (PTS) (Figures 2 &3). Additionally, our data demonstrated that ALC sectioning potentiates the negative ramifications of excess posterior tibial slope on knee stability, and such stability was restored following LET at all degrees of tibial slope (p<0.001). In evaluating LET vs. 10-degree slope-reducing osteotomy, no significant differences in ATT (2.50 vs. 1.73 p=0.25; 2.70 vs. 3.25, p=0.25) and IR (23.93 vs. 21.91 p=0.17; 25.40 vs. 27.03 p=0.23) were noted.

Conclusions:

Excess PTS significantly increases anterolateral knee instability and ACL graft forces under pivot shift loading. At 10° PTS, LET outperformed a 10° slope-reducing osteotomy in reducing ACL graft forces but not at 20° PTS. Though less effective than osteotomy in graft force reduction at 20° PTS, LET matched osteotomy in improving knee stability, decreasing IR and ATT at both 10° and 20° PTS.