LAUSR

Abstract The relative rigid body motions between the femur and the tibia (termed tibiofemoral kinematics) during flexion activities provide an objective measure of knee function. However, to obtain physiologic 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. Thus the primary purpose of this paper was to explain these requirements. The only valid method for obtaining physiologic tibiofemoral kinematics is to insure that coordinate systems meet four necessary requirements which are 1) the joint coordinate system must follow the convention of Grood and Suntay ( Grood and Suntay, 1983 ), 2) the body-fixed axes must correspond to the functional FE axis of the femur and the functional IE axis of the tibia, 3) the origins of the Cartesian coordinate systems, which serve as a stepping stone for computing translations, must lie on the functional body-fixed axes, and 4) the reference position between the tibia and femur must correspond to neutral with the knee in extension and the change in the components of the vector locating the origins of the Cartesian coordinate systems from the neutral position must be used to determine translations by projecting changes in vector components along the joint coordinate system 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 may not be possible following the ISB recommendation thus indicating the need for revision to this recommendation.