Increased Lateral Tibial Slope is a Risk Factor for Pediatric ACL Injury: An MRI-based Case-Control Study of 152 Patients

OBJECTIVES: Recent research has asserted that increased posterior tibial slope measured on plain radiographs is associated with elevated risk of ACL rupture in adults. It is unclear, however, if tibial slope is also of significance in children and adolescents. The purpose of this case-control study was to 1) determine if alterations in posterior tibial slope are associated with ACL rupture in pediatric and adolescent athletes, and 2) to quantify changes in tibial slope by age. METHODS: T2-weighted sagittal MRI scans of the knee were reviewed by three raters in a 1:1 sample of cases and controls, totaling 152 patients. Seventy-six skeletally immature patients with a complete non-contact ACL rupture were compared to an age- and gender-matched control group of 76 knees without ACL injury. The mean age of both groups was 14.8 +/- 1.3 years old and 46% of both groups were male. The medial and lateral compartments were each measured separately. All measurements were made using PACS software, employing a method similar to that which has been standardized in the adult literature. Our technique differed from previous studies in adults, though, in that the slope was measured on the cartilage surface, not the subchondral bone. RESULTS: The mean posterior slope of the medial tibial plateau (MTS) was 5.4° ± 2.2° and 5.1° ± 2.3° (t-test; P=0.42) in the ACL-injured and control knees, respectively. The mean posterior slope of the lateral plateau (LTS) was 5.7° ± 2.4° in the ACL-injured knees and 3.4° ± 1.7° in the control knees (t-test; P<0.001) (Table 1). There was no difference in LTS between males and females (4.46° and 4.58°, P = 0.75). An interaction variable created by combining gender with LTS in a logistic regression model was not significant (P=0.12). Receiver operating characteristic (ROC) analysis of lateral tibial slope was used to determine a threshold value that might predict elevated risk of ACL injury. A cutoff of lateral posterior tibial slope >4° resulted in a sensitivity of 76% and a specificity of 75% for predicting ACL rupture in this cohort. Interrater reliability of tibial slope measurements was excellent (ICC=0.62 and 0.77 for the medial and lateral tibial plateaus, respectively). Spearman correlation analysis revealed that tibial slope decreased, or flattened, by 0.18° (P=0.028) and 0.21° (P=0.009) degrees per year as adolescents age. CONCLUSION: The biomechanical basis for an association between posterior tibial slope and ACL injury is well established in adults. To our knowledge, this is the first study that evaluates the association between tibial slope (as measured along the cartilage surface on MRI rather than subchondral bone on plain radiographs) and the risk of ACL rupture in a large adolescent cohort using a case-control design. Employing standard measurement techniques, the lateral tibial slope averaged 5.7° in ACL-injured knees and 3.4° in ACL-intact knees. A cutoff of >4° for the lateral posterior slope is 76% sensitive and 75% specific for predicting ACL rupture in this cohort of skeletally immature athletes. LTS did not influence the incidence of ACL injury differently between genders.