Coordinate system requirements to determine motions of the tibiofemoral joint free from kinematic crosstalk errors.
The relative rigid body motions between the femur and the tibia (termed tibiofemoral kinematics) during flexion activities can provide an objective measure of knee function. Clinically meaningful tibiofemoral kinematics are defined as the six relative rigid body motions expressed in a joint coordinate system where the motions about and along the axes conform to clinical definitions and are free from kinematic crosstalk errors. To obtain clinically meaningful tibiofemoral kinematics, coordinate systems must meet certain requirements which neither have been explicitly stated nor in fact satisfied in any previous publication known to the author. Starting with the joint coordinate system of Grood and Suntay (1983) where motions conform to clinical definitions, the body-fixed axes must correspond to the functional (i.e. actual) axes in flexion-extension and internal-external axial rotation to avoid kinematic crosstalk errors in rotations and both functional axes must be body-fixed throughout knee flexion. To avoid kinematic crosstalk errors in translations, the origins of the femoral and tibial Cartesian coordinate systems, which serve as stepping stones for computing translations, must lie on the functional body-fixed axes. Neither the paper by Grood and Suntay nor the ISB recommendation (Wu et al., 2002) which adopted the joint coordinate system of Grood and Suntay explains these requirements. Indeed meeting these requirements conflicts with the ISB recommendation thus indicating the need for revision to this recommendation. Future studies where clinically meaningful tibiofemoral kinematics are of interest should be guided by the requirements described herein.