Knee joint motion: description and measurement.

Knee joint motion has been described in various ways in the literature. These are explained and commented on. Two methods for describing knee joint motion with 6 degrees of freedom (DOF)--Euler angle and the helical axis of motion--are discussed. Techniques to measure joint motion which can either approximate the motion to less than 6 DOF or fully measure the spatial motion are identified. These include electrical linkage methods, radiographic and video techniques, fluoroscopic techniques and electromagnetic devices. In those cases where the full spatial motion is measured, the data are available to describe the motion in simpler terms (or with less DOF) than three rotations with three translations. This is necessary for clinical application and to facilitate communication between the clinician and the engineer.