Joint motion studies are essential for understanding injury mechanisms, and developing and refining reconstruction and rehabilitation procedures. Traditional kinematic descriptors are frequently dependent on a specific joint coordinate system and on an ordered sequence of rotations that often vary between joints and throughout the research community. Consequently, it is difficult to compare scientific and clinical results. Additionally, motion descriptors have generally not provided both a visual and mathematical aspect to bridge the understanding between engineers and medical professional. In light of the foregoing, the aim of this thesis was to establish the Screw Displacement Axis (SDA) technique for describing motion characteristics of the elbow. Long-term, we hope to implement this approach for use in computer-assisted surgeries.

A computer program was coded that conducted an SDA analysis on joint motion data collected in six degrees-of-freedom. Methods for quantifying the SDA patterns were devised, with requirements that were simple to employ, and provided practical and informative information to the researcher and/or clinician. Numerical simulations and experimental methods employed in an accuracy study of the SDA technique determined small position, orientation, translation and rotation errors.

A series of in-vitro studies employing passive and simulated loading and motion of the normal, injured and surgically reconstructed elbow, showed SDAs to be an effective method for visualizing and evaluating the integrity of the elbow joint. SDAs predicted similar rotational stability results obtained employing traditional analyses. Furthermore, SDAs were able to detect translational differences, undetectable in traditional rotation analyses.

In the final study, SDAs were employed for defining a joint coordinate system for the elbow that relied on its natural motion and external palpable bony landmarks rather than invasive joint sectioning employed in traditional methods. This new joint coordinate system would progress testing from in-vitro to in-vivo case studies. Both an Euler rotation and SDA analysis showed near similar trends and magnitudes when comparing this system to an anatomically-derived coordinate system. Since the anatomical method has long been established for describing elbow joint kinematics, the motion based system gives acceptable and meaningful results for describing upper extremity motion. We propose that an elbow joint coordinate system should be standardized across laboratories, which is based on kinematic data and anatomic landmarks, and can be employed in both in-vitro and in-vivo investigations.

The greatest strength of the SDA approach is its ability to visually depict three-dimensional joint motion. This technique should be useful for prosthetic design and implantation, and optimizing surgical reconstruction techniques. Additionally, SDAs should be a useful tool for computer-guided surgeries, increasing the effectiveness of surgical treatments and therefore improving the quality of life for patients.