Summary

This study found that posterior tibial slope (PTS) consistently increased over a mean 9-year follow-up in a revision ACL-R cohort, with greater PTS changes (>2˚) significantly associated with prior posterior medial meniscus resection and a PTS ≥12˚ potentially indicating a progressive deformity.

Abstract

Introduction

Increased posterior tibial slope (PTS) is a recognized risk factor for failed anterior cruciate ligament reconstruction (ACL-R). This study aims to determine PTS changes over time in a skeletally mature revision ACL-R cohort. Furthermore, this study aims to identify factors associated with changes in PTS over time.

Methods

A ten-year chart review of patients who underwent revision ACL-R was performed.  Inclusion criteria included having index knee radiographs spanning at least five years, with the first radiograph taken before primary ACL-R and any subsequent radiograph taken at least five years later, and age >14 years. Exclusion criteria included inadequate radiographs, multi-ligament injuries, concomitant meniscus transplantation, prior history of infection, and age ≤14 years. Standard error of measurement method was used to calculate minimum detectable change (MDC) using radiographic PTS measurements. As determined by three raters, mean MDC was 1.0˚ (95% confidence interval (CI)). Patients were categorized into two groups based on changes in PTS >2˚ (Group 1) and ≤2˚ (Group 2), ensuring changes >2˚ were considered free of measurement error. Two blinded researchers collected demographic data, clinical and operative histories including meniscal injuries and procedures by side and location, presence of cartilage procedures, and clinical outcomes such as the International Knee Documentation Committee Subjective Knee Form (IKDC sKF) and Marx Activity Scores, as well as return-to-sports status. The reliability of PTS measurements was assessed by two blinded raters measuring 25 randomly selected radiographs at three separate time points.

Results

A total of 76 patients with a mean age of 25.3±10.3 years met inclusion/exclusion criteria and were included in this study. The mean radiographic follow-up duration was 8.9±3.6 years. There was a significant increase in PTS between the most recent and oldest available radiographs for the entire cohort, 12.6±3.3˚ vs. 11.5±3.2˚ (p<.001), respectively, with a mean increase of 1.1±1.5˚. This change was greater than the mean MDC. Radiographic follow-up length did not correlate with the magnitude of PTS change. Group 1 (n=19) did not differ in baseline demographics and other investigated parameters compared to Group 2 (n=57), including radiographic follow-up length, number of previous ACL-R, and duration of ACL deficiency. Additionally, Groups 1 and 2 showed no significant differences in latest IKDC SKF scores, Marx Activity scores, or return to sports status. Having a PTS increase of >2˚ was associated with prior posterior medial meniscal resection (p=.003) compared to ≤2˚ PTS increase. No other surgical history showed significant associations with greater PTS change. A significant association was observed between having a PTS ≥12˚ on the latest radiograph and a greater change in PTS over time (p <.001). Interobserver reliability was excellent, with an intraclass correlation coefficient (ICC) of 0.916. Intraobserver reliability also demonstrated excellent consistency, with an ICC of 0.968 for Rater 1 and 0.985 for Rater 2, respectively.

Conclusion

In this radiographic follow up study, PTS consistently increased in a revision ACL-R cohort over a mean follow-up of 9 years. Posterior medial meniscus resection was significantly associated with a greater (>2˚) increase in PTS over time. The findings suggest that increasing PTS over time may be the fate of the failed ACL. Additionally, PTS ≥12˚ may be considered a deformity that may continue to progress over time unless corrected.