Simulated active control produces repeatable motion pathways of the elbow in an in vitro testing system

The purpose of this study was to determine if the repeatability and pattern of elbow kinematics are affected by changing the relative magnitudes of lo ads applied to muscles around the elbow in vitro. In eight cadaveric upper extremities, passive and three methods of simulated active elbow flexion we re tested with the forearm maintained in both pronation and supination. Pas sive flexion involved moving the elbow manually through a full are of motio n. Simulated active flexion used a custom designed loading system to genera te elbow motion by applying loads to various tendons via pneumatic actuator s. Three different simulated active loading protocols, with loading ratios based on muscle activity and physiologic cross-sectional area, were tested. Testing was performed initially on an intact elbow, and then an unstable e lbow model created by transection of the lateral collateral ligament (i.e. the radial and lateral ulnar collateral ligaments). An electromagnetic trac king device was used to measure rotation of the ulna relative to the humeru s. Varus-valgus angulation and internal-external rotation were less repeata ble during passive flexion than simulated active flexion, regardless of the loading ratio used, in both the intact (p <0.05) and unstable (p <0.05) el bows. Throughout the are of flexion, the motion pathways were similar for t he three simulated active motion protocols employed in this study (p >0.05) . The pathways followed during passive motion were different from those gen erated with simulated active motion, especially in the unstable elbow with the forearm supinated (p <0.001). These results suggest that using simulate d active motion rather than manual passive motion can improve the repeatabi lity of elbow kinematics generated in the laboratory, and that a wide range of muscle loading ratios may produce similar kinematic output. (C) 2001 El sevier Science Ltd. All rights reserved.