Development of an active elbow flexion simulator to evaluate joint kinematics with the humerus in the horizontal position

In-vitro simulation of active joint motion is useful to evaluate rehabilitation protocols and surgical procedures in the laboratory prior to their application in patients. To date, simulated active elbow flexion has been reliably achieved and well established only in the dependent position (humerus vertical with hand down). We have developed and evaluated the performance of a new elbow motion simulator capable of active flexion in the dependent, varus, valgus and horizontal positions. Muscle loading and motion control were achieved via a combination of motors and actuators attached to relevant tendons. Simulated active flexion was compared to passive flexion in terms of repeatability, motion pathways and joint laxity. The joint kinematics of active flexion were significantly more repeatable than passive flexion (p<0.05). Active flexion reduced varus–valgus joint laxity by 29% (supinated p<0.05) and 26% (pronated p<0.05) compared to passive flexion. Greater repeatability of simulated active flexion suggests that this mode of in-vitro testing should increase statistical power and decrease required sample sizes.