Steep Tibial Slope Increases ACL Graft Failure Despite Lateral Augmentation

Navigating the Biomechanical Limits of ACL Augmentation

Surgeons increasingly utilize lateral extra-articular tenodesis (a procedure that anchors a portion of the iliotibial band to the lateral femur to provide rotational reinforcement) as an adjunct to primary anterior cruciate ligament reconstruction to address residual rotatory laxity and reduce graft failure rates [1, 2, 3]. Meta-analyses of randomized trials demonstrate that adding this procedure reduces the graft failure rate (odds ratio 0.34; 95% confidence interval, 0.20 to 0.55; p < 0.0001) compared to reconstruction alone [3]. While this combined approach effectively improves pivot-shift grades, it may increase the risk of surgical site infections and long-term lateral compartment osteoarthritis [4, 5, 6]. A 2025 international consensus statement strongly recommends lateral augmentation for high-risk populations, such as young athletes and patients with grade III pivot-shift findings [7, 8]. Anatomic variations, particularly an increased posterior tibial slope (the posterior inclination of the tibial plateau relative to the longitudinal axis of the bone), are known to increase strain on the graft, but the threshold at which these forces bypass the protective effects of a tenodesis is not well defined [7, 9]. A new retrospective cohort study now offers fresh insights into how specific slope measurements influence graft survival across different risk profiles.

Long-Term Outcomes in High-Risk Hamstring Autografts

To evaluate the durability of lateral augmentation, researchers conducted a retrospective cohort study providing Level 3 evidence (clinical evidence derived from non-randomized, controlled studies) on 585 patients who underwent primary anterior cruciate ligament reconstruction. Every patient in the study received a hamstring tendon autograft between 2014 and 2017. The investigators maintained a rigorous longitudinal perspective, ensuring a minimum follow-up period of 6 years to capture late-stage graft failures that might be missed in shorter clinical windows. This duration is particularly relevant for clinicians assessing the long-term viability of biological grafts in active populations, as many failures occur well beyond the initial two-year postoperative mark. The study population was nonrandomly allocated into two distinct cohorts based on predefined clinical risk criteria to reflect real-world surgical decision-making. The low-risk group consisted of patients receiving isolated anterior cruciate ligament reconstruction, while the high-risk group underwent the reconstruction combined with lateral extra-articular tenodesis. To ensure diagnostic accuracy, the researchers defined graft rerupture through a combination of clinical examination findings and confirmation via imaging, such as MRI or subsequent arthroscopy. This dual-verification method ensures that the reported failure rates reflect objective structural loss rather than subjective instability alone, providing a more reliable metric for surgical success.

Quantifying the Impact of Posterior Tibial Slope

To isolate the specific contribution of anatomy to surgical outcomes, the researchers employed multivariable logistic regression (a statistical method that allows investigators to determine the independent effect of a single variable while controlling for other potential confounding factors). This analysis was used to assess the association between posterior tibial slope and graft failure. The results demonstrated that posterior tibial slope independently predicted graft rerupture in both the low-risk and high-risk groups, confirming that this anatomic feature remains a critical determinant of stability regardless of the patient's initial clinical risk profile. The magnitude of this risk, however, varied significantly between the two cohorts. In the high-risk group, which received lateral extra-articular tenodesis as an adjunct to reconstruction, the effect of posterior tibial slope on rerupture was notably stronger, yielding an adjusted odds ratio of 1.63 (P = .005). This indicates that for every one-degree increase in slope, the odds of graft failure rose by 63 percent in these patients. In comparison, the effect of posterior tibial slope on rerupture in the low-risk group was less pronounced but still statistically significant, with an adjusted odds ratio of 1.21 (P = .029). These findings suggest that while lateral augmentation provides rotational stability, it may not sufficiently counteract the increased sagittal plane stress imposed by a steep posterior tibial slope, particularly in patients already predisposed to failure due to high-demand activities or ligamentous laxity.

The Nonlinear Threshold for Graft Rerupture

To better understand the complex interaction between anatomy and surgical outcomes, the researchers utilized restricted cubic spline regression (a statistical method used to model nonlinear relationships between variables by creating a smooth curve that can change direction at specific points). This analysis revealed that the relationship between posterior tibial slope and graft failure was nonlinear, meaning the risk of rerupture does not increase at a constant rate across all measurements. In the high-risk group, the researchers identified a specific safe zone where the lowest failure probabilities were observed within a posterior tibial slope range of 5 to 9 degrees. For patients in this high-risk cohort with even flatter anatomy, the risk of failure plateaued at posterior tibial slope values below 5 degrees, suggesting that further reductions in slope do not provide additional protective benefits against graft rupture. The data also highlighted a critical upper limit for tibial anatomy that challenges the efficacy of current surgical techniques. In both the low-risk and high-risk groups, posterior tibial slope values exceeding 11 to 12 degrees were associated with high failure probabilities, indicating a threshold where the mechanical strain on the graft becomes difficult to overcome even with lateral reinforcement. Throughout the study population, increasing posterior tibial slope values were linked to a progressively higher risk of reruptures in both groups. These findings carry significant clinical implications for surgical planning, as the researchers concluded that an elevated posterior tibial slope may attenuate the relative protective effect of lateral extra-articular tenodesis in high-risk patients. For clinicians, this suggests that while lateral augmentation is a valuable tool, its ability to prevent graft failure is significantly diminished when the patient's posterior tibial slope exceeds 12 degrees, potentially necessitating a slope-reducing osteotomy in extreme cases.

References

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