A comparison of different two-dimensional approaches for the determination of the patellar tendon moment arm length

The purpose of this study was to estimate and compare the moment arm length of the patellar tendon (d) during passive knee extension using three different reference landmarks; instant centre of rotation (ICR), tibiofemoral contact point (TFCP) and geometrical centre of the posterior femoral condyles (GCFC). Measurements were taken on the right leg on seven healthy males during passive knee rotation performed by the motor of a Cybex Norm isokinetic dynamometer. Moment arms lengths were obtained by analysing lateral X-ray images recorded using a GE FlexiView 8800 C-arm videofluoroscopy system. The d–knee joint angle relations with respect to GCFC and ICR were similar, with decreasing values from full knee extension (~5.8 cm for d _GCFC and ~5.9 cm for d _ICR) to 90° of knee flexion (~4.8 cm for both d _GCFC and d _ICR). However, the d _TFCP–knee joint angle relation had an ascending–descending shape, with the highest d _TFCP value (~5 cm) at 60° of knee flexion. There was no significant difference between the GCFC and ICR methods at any knee joint angle. In contrast, there were significant differences (P < 0.01) between d _ICR and d _TFCP at 0°, 15°, 30° and 45° of knee flexion and between d _GCFC and d _TFCP at 0°, 15° and 30° of knee flexion (P < 0.01). This study shows that when using different knee joint rotation centre definitions, there are significant differences in the estimates of the patellar tendon moment arm length, especially in more extended knee joint positions. These differences can have serious implications for joint modelling and loading applications.