Patients with Bilateral ACL Reconstruction (ACLR) have 3x the Rate of Posterior Tibial Slope Greater than 12-Degrees Compared to Unilateral ACLR: MRI and Radiographic Evaluation

Abstract

Objectives: Increased posterior tibial slope (PTS) is a risk factor for primary anterior cruciate ligament (ACL) tear and graft failure, but PTS has not been well-defined in those who have experienced bilateral ACL injuries. We aimed to compare PTS angles, as well as the rate of elevated PTS (>12° on lateral radiograph; >7° on MRI), between patients who underwent bilateral ACL reconstruction (ACLR) vs unilateral ACLR. A secondary purpose was to examine whether these associations remained consistent on both plain radiographs and MRIs. Methods: We retrospectively identified patients who underwent primary ACLR at our institution from the years 2012 to 2020. Patients with non-simultaneous bilateral ACLR (n=53) were propensity matched to those with unilateral ACLR (n=53) by age, sex, and BMI. Exclusion criteria were: rotated lateral radiographs, inadequate quality MRI, concomitant ligament injury or fracture. Unilateral ACLRs with less than 5-year follow-up were further excluded. Three blinded readers measured PTS on lateral radiographs, while medial (MPTS) and lateral PTS (LPTS) were measured on MRI. Bivariate regression was performed to determine correlation between radiograph and MRI measurements. Results: A final cohort of 106 patients was analyzed (mean age: 27.4 ± 10.5 years, 49.1% male, BMI: 25.5 ± 4.3). PTS in the bilateral ACLR cohort was significantly greater than the unilateral ACLR cohort on radiographs (11.26° vs 10.13°, p=0.044) and LPTS (7.32° vs 6.08°, p=0.012), but not MPTS (4.55° vs 4.17°, p=0.467). The percentage of bilateral patients with radiograph PTS >12° was 41.0%, compared to 13.2% in unilateral cohort (p=0.012). The bilateral cohort had a significantly greater rate of LPTS >7° compared to unilateral patients (53.8% vs 32.1%, p=0.016), but not MPTS (p=0.467). On MRI, LPTS (5.93° ± 3.20°) was significantly greater than MPTS (5.12° ± 2.78°, p<0.001). There was weak correlation between MPTS-radiograph ( R=0.24, p=0.02) and LPTS-radiograph was not significantly correlated ( R=0.03, p=0.810). Conclusions: Patients who underwent bilateral ACLR had significantly greater PTS on radiographs and LPTS on MRI compared to those with unilateral ACLR. The rate of PTS >12° was 3.1x greater among bilateral ACLR compared to unilateral ACLR. PTS measurements on radiographs demonstrated weak-to-negligible correlation with MRI measurements, suggesting future normative PTS values should be reported specific to the imaging modality.