The human knee is of particular interest because of its importance in mobility. Pain and stability can be directly related to the motion, or kinematics, of the knee. Many studies have been conducted to quantify human knee kinematics, both in vitro and in vivo. One of the inherent issues with in vivo, skin mounted measurement systems is that they do not account for soft tissue artifact. Compensation for soft tissue artifact has been a difficult challenge for skin mounted tracking systems and has not yet been achieved. Therefore, bone mounted skeletal pins were chosen as the method of gathering kinematic data for this study. Mounting bone pins is not the quintessential method to study motion due to its invasive nature; nevertheless, it provides a great amount of trustworthy, useful insight. Murphy conducted an in vivo experiment to capture the 3D kinematics of the normal human knee. The kinematic data were used to find the Instantaneous Screw Axis or Instantaneous Helical Axes (IHA). If progressive IHA’s are plotted on the same plot, the surface that is created is called the moving axode of the motion. Several degrees of freedom are needed to accurately describe the kinematics of the human knee during normal movement. The current study further analyzes the data that Murphy reported in 1990. The goal is to find an effective way to express kinematic information in a coordinate system-independent manner so that comparison is meaningful and feasible between gait/ROM trials, subjects, and knee repair/replacement methods. Axodes were used to compare knee kinematics, trial to trial, for gait, range of motion (ROM), and pivot step. It was established that 6 independent screws are required to fully describe the motion during gait. Thus, the knee behaves like a 6 DOF mechanism during gait and, therefore, two-, three-, four-, or five-screw system models are insufficient to adequately and uniquely define the screw system. Screw invariants were found to be a viable option of understanding knee kinematics. Axodes were plotted with pre-stance, stance phase, and post-stance phase indicated. Screw invariants, pitch and moment, were plotted as a function of flexion angle
