Abstract
A three-dimensional analysis of the patello-femoral resultant force vector (
) is presented. Results of the analysis proved to be largely independent on the valgus angle of the knee. Therefore Q angle measurements will suffice for clinical analysis. The lateral direction of 
decreases rapidly with knee flexion when the Q angle is small, less so for larger Q angles. Consequently, the steepness of the lateral femoral trochlea required for passive stability decreases with increasing knee flexion. In the light of these findings the dynamic stabilizing action of the vastus medialis obliquus muscle is discussed. Calculations of the magnitude of 
showed that the traditional analysis of the sagittal projection of the joint (two-dimensional analysis) is justified.
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