This paper deals with two fundamental questions with regard to total knee replacement kinematics. First, it provides quantitative information about the effect of knee prosthesis size on restored knee rotation by the so-called performance function. Second, the paper introduces a hypothesis which considers the effect of slide-roll on the performance function. By means of statistical methods, a strong linear correlation between slide-roll and performance-function of the examined total knee replacements was deduced. This result can be interpreted as follows: alteration of slide-roll ratio may enhance the overall performance of total knee replacements with regard to the restored kinematics, or in this specific case, the rotation.