INTRAOPERATIVE SARCOMERE LENGTH MEASURES AND MODELING OF SURGICAL TRANSFER OF THE WRIST FLEXOR 157

Abstract

Sarcomere length (SL) changes that occur during normal movement depend both on muscle fiber length and moment arm. Since surgical transfer alters this relationship, strength may be compromised postsurgically due to altered SL operating range. The purpose of this study, was to measure SL in humans (n=5) who were undergoing surgical tendon transfer of the flexor carpi ulnaris (FCU) to extensor carpi radialis longus (ECRL) muscle for radial nerve palsy. After anesthesia induction, a helium-neon laser beam was inserted into the muscle and FCU SL was measured. The FCU was then transferred into the ECRL and FCU SL again measured. Prior to tendon transfer, FCU sarcomere length ranged from 2.84±. 12 μm (mean±SEM) with the wrist flexed to 4.16±. 15 μm with the wrist extended. After transfer into the ECRL tendon, sarcomere length ranged from 4.82±. 11 μm with the wrist flexed (the new longest position of the FCU) to 3.20±.09 μm with the wrist extended, resulting in a shift in the sarcomere length operating range to significantly longer lengths (p<0.001). At these longer lengths, the FCU muscle would develop high active tension only when the wrist was highly extended. Biomechanical modeling revealed that changing FCU muscle length from 200-260 mm (the range observed intraoperatively) resulted in very large changes in absolute peak moment produced as well as the angular dependence of peak moment due to the change in the region of FCU operation on its sarcomere length-tension curve.