Introduction: Transient posterolateral subluxation of the lateral femoral condyle has been postulated as a non-contact mechanism of ACL rupture. The consistent location of MRI bone bruises on the anterior half of the lateral femoral condyle and the posterior rim of the lateral tibial plateau in ACL injuries suggest this mechanism of injury. Previous studies utilizing radiographs have investigated and eliminated the possibility of an increase in posterior tibial slope angle as a risk factor for ACL injury. None, however, have specifically measured the slope angle of the lateral tibial plateau. This study was designed to utilize MRI images for specific measurement of the lateral tibial slope angle and investigate this as a risk factor for ACL injuries.

Materials & methods: The MRI images of 30 knees with complete ACL ruptures (Group I) and those of 30 knees with an intact ACL were studied. Both groups were age-matched (range 15 to 50 years) and predominantly male. The slope angles of the medial plateau and the lateral tibial plateau were measured separately. Statistical comparison was made between the slope angles of the medial and lateral tibial plateaus within both groups, and between the lateral tibial slope angles of Group I and Group II., using a Student’s t-Test.

Results: The mean slope angle of the medial plateau was 3.43 degrees in Group I and 3.67 in Group II. The mean slope angle of the lateral tibial plateau was 6.40 degrees in Group I, and 5.43 in Group II. The difference between medial and lateral slope angles was statistically significant, as well as that between the lateral tibial slopes of Group I and Group II. There was no statistical difference between the mean slope angles of the medial plateau of both groups.

Conclusion: There is a significant difference in slope angles of the medial and lateral tibial plateaus, and thus the need to specify the lateral tibial slope angle in determining slope angles as a risk for ACL injuries. While there is a statistically significant difference in lateral tibial slope angles of patients with ACL tears, the actual measured difference of one degree is not clinically useful as a predictor of ACL injury risk.