Increased posterior tibial slope (PTS) has been identified as a risk factor for anterior cruciate ligament (ACL) injuries and ACL reconstruction failure. The relationship between PTS and sagittal plane knee biomechanics (such as knee angular velocity) has not been evaluated. Prior work has identified that higher knee angular velocities are associated with faster peak ACL strains, which may have implications on risk of ACL graft rupture. The purpose of this study was to evaluate the relationship between PTS and sagittal plane mechanics during jump landing in a cohort of patients after ACL reconstruction. It was hypothesized that sagittal plane landing mechanics would differ between those with PTS >= 12 degrees compared to those with PTS < 12 degrees.
A cohort of patients who underwent primary, unilateral ACL reconstruction at a single institution were identified in a prospectively collected ACL database. At 2-years post-ACL reconstruction a follow-up visit was conducted to collect biomechanics data and patient reported outcome scores, including Marx Activity Score, International Knee Documentation Committee (IKDC) Score, and Knee Injury and Osteoarthritis Outcome Score (KOOS). Three-dimensional motion analysis data were collected during the initial landing phase of a bilateral drop vertical jump task. Lateral posterior tibial slope (LPTS) was measured on lateral knee radiographs of the injured knee. Biomechanical variables of interest included ground reaction forces and sagittal plane knee kinematic and kinetic variables on the reconstructed limb. Pearson correlations were calculated to evaluate the association between LPTS and biomechanical variables of interest (a=0.05). The cohort was categorized based on LPTS into low slope (PTS < 12 degrees) and high slope (PTS >= 12 degrees) groups. Demographic, outcomes, and biomechanics data were compared between the groups with independent samples t-tests (a=0.05).
A total 29 patients with lateral radiographs for LPTS measurements and complete biomechanics data were included (17 females) with a mean age of 22.3 years (range 17 to 35 years). There were no significant differences in age, body mass index, Marx activity score, IKDC Score, or KOOS subscales between the high slope and low slope groups. The mean LPTS was 13.4 +/- 0.9 degrees and 8.7 +/- 1.2 degrees in the high slope and low slope groups respectively. During landing, higher involved knee angular velocity was associated with higher LPTS (r=0.43, p=0.02). Individuals with high LPTS had significantly higher involved knee angular velocity compared to patients with low LPTS.
At 2 years post-ACL reconstruction, there were no differences in activity or patient reported outcome scores in those with high or low LPTS. During a double-leg landing task, high LPTS is associated with higher involved knee angular velocity. Further investigation of the associations among anatomic considerations (such as PTS), knee mechanics, and ACL graft strain are needed to identify patients at higher risk of graft rupture and inform post-operative rehabilitation and discussions regarding graft failure risk.