Load dependence in carpal kinematics during wrist flexion in vivo

OBJECTIVE: The hypothesis tested was that generation of torque at the wrist affects joint kinematics. DESIGN: An in vivo study of normal wrist kinematics during plantar flexion motion against a constant load was undertaken, using a custom-designed instrumented apparatus to track the motion of the hand during the task. BACKGROUND: Despite clinical observations of a relationship between motion-loading and pain in wrists affected by rheumatoid arthritis, there is little published literature on thein vivo kinematics of the normal human wrist under load. METHODS: Ten volunteers with no wrist pathology were tested while generating torques of zero, 1.1 and 2.2 N m in a planar, unidirectional flexion motion. Hand kinematics were computed using the Planar Rigid Body Method algorithm and an 8 degrees angular step size. The finite radius of motion and the range and standard deviation of the residuals to a fitted second-order curve were used as indices of changes in the kinematics. RESULTS: The magnitude of both the range and standard deviation of the residuals were found to increase significantly with torque at the 95% confidence level. CONCLUSIONS: The wrist does not behave like a smooth mechanism when generating torque. Load affects carpal kinematics. RELEVANCE: We propose that fluctuations in the finite radius of motion are the natural kinematic consequence of intercarpal motion known to occur during wrist flexion. Wrist kinematics may be particularly sensitive to load and joint integrity because orchestrated intercarpal motion depends on the soundness of articular and ligamentous structures, the first to be affected in joint degenerating conditions such as rheumatoid arthritis. Thus, wrist kinematics under load may be a key to characterizing joint integrity. In wrist pathologies, simple planar testing of carpal kinematics under reproducible and controlled joint torque conditions may be a useful way to assess joint involvement before the onset of gross dysfunction, and to evaluate treatment outcomes.