In vitro study of inter-individual variation in posterior slope in the knee joint

BackgroundThe posterior tibial slope of the knee is an issue in total knee arthroplasty. It has also been suggested to be a determinant for the difference in the incidence of anterior cruciate ligament rupture between males and females. The current literature features discrepancies in inter-individual variation in posterior slope and the effect of gender. This in vitro study was aimed at quantifying the inter-individual variations of the posterior tibial slope and the gender difference.MethodsThe slope of the medial tibial plateau of 61 male and 44 female, bare human tibias was measured in three sagittal planes, separated by 5 mm, with the use of a custom-made device. The effect of measurement location, gender and ethnicity was determined with analysis of variance.FindingsThe difference between the measurement locations was statistically significant but smaller than the variation between specimens. The mean posterior slope was 8.4° (SD 3.7°), 95% confidence interval was 1.0–5.8°. Ethnicity did affect posterior slope (p = 0.001), but gender did not reach significance (p = 0.091).InterpretationThe results support the existence of a substantial inter-individual variation in posterior slope. For total knee arthroplasty, the pre-operative posterior slope is apparently not compatible with the suggestion not to cut a slope greater than 8°, as more than half of the tibias had a greater slope. Secondly, part of the variance over subjects is related to ethnicity and possibly also to gender, which might have implications for the risk of sustaining an anterior cruciate ligament rupture and for the success of total knee arthroplasty.