Poster 268: The Relation Between Posterior Tibial Slope Angle and Bilateral ACL Tear

OBJECTIVES: Increased posterior tibial slope (PTS) is a risk factor for primary anterior cruciate ligament (ACL) tear, as well as subsequent ACL graft tear. Prior literature has shown conflicting relationships between an increased PTS and bilateral ACL tears. The purpose of this study is to compare PTS angles for patients who had bilateral ACL tears to those who had only sustained a unilateral ACL tear. METHODS: Retrospectively we identified patients who had bilateral and unilateral ACL tears at our institution over the last 10 years. Those who had bilateral surgery were matched to those who had unilateral surgery on a ratio of 1:1 based on age, sex, and BMI. Patients with unilateral ACL reconstruction were excluded if they had subsequent revisions or were less than 5 years post-operative. Lateral knee radiographs were used to measure the posterior tibial slope. The tibial axis was found using the circles measurement method which consisted of drawing circles along the tibial shaft and placing a line through the center of each to estimate the anatomical tibial axis. The PTS was then defined as the angle between the perpendicular line to this axis and the tibial plateau tangent line. Three blinded orthopedic residents performed these measurements. In addition to comparison of mean PTS between cohorts, both cohorts were stratified into “high slope” and “low slope” using 10-degress of PTS as a cutoff. An interclass correlation coefficient (ICC) was then calculated. RESULTS: Fifty-three patients with bilateral ACL tears were successfully matched to 53 patients with unilateral tears. The average PTS in patients with bilateral ACL tears was found to be significantly higher than patients with unilateral ACL tears (11.17 vs. 9.72, p = 0.005). An intraclass correlation coefficient was calculated to 0.80 (95% confidence interval: 0.75 – 0.85). These cohorts were then stratified by the aforementioned sub-groups, finding that 69.8% of the bilateral cohort had more than 10-degrees of PTS, while this was true for only 46.4% of the unilateral cohort (p = 0.006). CONCLUSIONS: These results demonstrate that patients with bilateral ACL tears have higher PTS angles than those who only had unilateral ACL tear. Clinicians should consider routinely assessing PTS in a newly-presenting patient with an initial ACL tear, as marked deviations from the general population could be a predisposing risk factor for subsequent contralateral tear. Further prospective research is necessary to determine if patients with unilateral ACL tears who present with this radiographic finding are at greater risk for contralateral tear.