The relationship between the posterior tibial slope and the sagittal femoral condylar shape: Two circles and ellipses.

Abstract

Whether the posterior tibial slope (PTS) is associated with the femoral condylar shape has remained unclear. Classical anatomical studies considered the sagittal profiles of femoral condyles to consist of two circles. Recently, an elliptical method was described to simplify the "two circles" model. Our purpose was to analyze the relationship between the PTS and the shape of the sagittal femoral condyle, that is, two circles and two ellipses, using magnetic resonance imaging (MRI) scans. Eighty right knees of healthy subjects were scanned by MRI at full extension. The medial and lateral PTS were measured (mPTS and lPTS). On the distal-most medial and lateral slices, the femoral condylar articular surfaces were best fitted by circles or ellipses, respectively. The radii of the medial and lateral posterior circles and inferior circles (rm and rl , Rm and Rl , respectively), the semi-major axes and the semi-minor axes of the medial and lateral ellipses (am and al , bm and bl , respectively), and the rm /Rm , bm /am , rl /Rl , bl /al ratios were measured and calculated. mPTS correlated significantly with the parameters of the medial condyle (all p < .05) and the ratios bm /am and rm /Rm (p = .017 and p = .027, respectively). However, the lPTS did not correlate with the parameters of the lateral condyle (all p > .05) or the ratios bl /al and rl /Rl (p = .461 and p = .241, respectively). The mPTS is associated with the sagittal shape of the medial femoral condyle but the lPTS is not. Both two circles and two ellipses are feasible ways to represent the sagittal femoral condylar shape.