An Approach to Robotic Testing of the Wrist Using 3-D Imaging and a Hybrid Control Strategy

Robotic technology is increasingly used for sophisticated in-vitro testing designed to understand the subtleties of joint biomechanics. Typically, the joint coordinate systems in these studies are established via palpation and digitization of anatomic landmarks. However, overlying soft tissues and indistinct bony features can introduce considerable variation in landmark localization, leading to descriptions of kinematics and kinetics that may not appropriately align with the bony anatomy. Numerous studies have measured in-vitro properties employing either load or displacement control with standard material testers. However, these control modes either do not consider all six degrees of freedom (DOF) or reflect non-linear properties of wrist. The development of an appropriate protocol to investigate complexities of wrist mechanics would potentially advance our understanding of the normal, pathological, and artificial wrist. In this study, we report a novel methodology for using CT imaging to generate anatomically aligned coordinate systems and a new control strategy for robotic testing of wrist. The methodology is demonstrated with the testing of 9 intact cadaver specimens in 24 unique directions of wrist motion to a resultant torque of 2Nm. The mean orientation of the major principal axis of ROM envelope was oriented 12.1 degrees +/- 2.7 degrees towards ulnar flexion, which was significantly different (p<0.001) from the anatomical flexion/extension axis. The largest wrist ROM was 98 degrees +/- 9.3 degrees in the direction of ulnar flexion, 15 degrees from pure flexion. Radial and ulnar components of the resultant torque were the most dominant across all directions of wrist motion.