Differences in Medial and Lateral Posterior Tibial Slope: An Osteological Review of 1090 Tibiae Comparing Age, Sex, and Race

Background: Injuries to the anterior cruciate ligament (ACL) are common, and a number of knee morphological variables have been identified as risk factors for an ACL injury, including the posterior tibial slope (TS). However, limited data exist regarding innate population differences in the TS. Purpose: To (1) establish normative values for the medial and lateral posterior TS; (2) determine what differences exist between ages, sexes, and races; and (3) determine how internal or external tibial rotation (as occurs during sagittal knee motion) influences the stereotactic perception of the TS. Study Design: Cross-sectional study; Level of evidence, 3. Methods: A total of 545 cadaveric specimens (1090 tibiae) were obtained from the Hamann-Todd osteological collection. Specimens were leveled in the coronal, sagittal, and axial planes using a digital laser. Virtual representations of each bone were created with a 3-dimensional digitizer apparatus. The TS of the medial and lateral tibial plateaus were measured using techniques adapted from previous radiographic protocols. Medial and lateral TS were then again measured on 200 tibiae that were internally and externally rotated by 10 degrees (axially). Results: The mean (SD) medial TS was 6.9 degrees +/- 3.7 degrees posterior, which was greater than the mean lateral TS of 4.7 degrees +/- 3.6 degrees posterior (P < .001). Neither the medial nor lateral TS changed with age. Women had a greater mean TS compared with men on both the medial (7.5 degrees +/- 3.8 degrees vs 6.8 degrees +/- 3.7 degrees, respectively; P = .03) and lateral (5.2 degrees +/- 3.5 degrees vs 4.6 degrees +/- 3.5 degrees, respectively; P = .04) sides. Black specimens had a greater mean medial TS (8.7 degrees +/- 3.6 degrees vs 5.8 degrees +/- 3.3 degrees, respectively; P < .001) and lateral TS (5.9 degrees +/- 3.3 degrees vs 3.8 degrees +/- 3.5 degrees, respectively; P < .001) compared with white specimens. Axial rotation was shown to increase the perception of the medial and lateral TS (P < .001). Conclusion: The medial TS was shown to be greater than the lateral TS. Important sex- and race-based differences exist in the TS. This study also highlights the role of axial rotation in measuring the TS.