SARCOMERE-LENGTH CHANGES AFTER FLEXOR CARPI ULNARIS TO EXTENSOR DIGITORUM COMMUNIS TENDON TRANSFER

Sarcomere length was measured intraoperatively on five patients underg oing tendon transfer of the flexor carpi ulnaris (FCU) to the extensor digitorum communis (EDC) for radial nerve palsy. The most significant result was that the absolute sarcomere length and sarcomere length op erating range of the FCU increased after transfer into the EDC (p <.00 1). Preoperatively, with the wrist fully extended and fingers flexed, FCU sarcomere length was 4.22 +/- .24 mu m and decreased to 3.19 +/- . 05 mu m as the wrist was fully flexed. This represented an overall sar comere length range of 1.03 mu m After the tendon transfer using stand ard recommended techniques, all sarcomere lengths were significantly l onger (p <.001). Specifically, sarcomeres were 0.74 +/- .14 mu m longe r with the muscle in its fully lengthened position (4.96 +/- .43 mu m with the wrist and digits flexed) and 0.31 +/- .16 mu m longer with th e FCU in the fully shortened position (3.50 +/- .06 mu m with the wris t and digits extended). At these sarcomere lengths, the FCU muscle was predicted to develop relatively high force only during movement invol ving synergistic wrist flexion and finger extension. Under the conditi ons of the procedures performed, the transferred FCU muscle was predic ted to produce maximum force over the range of about 30 degrees of wri st flexion and 0 degrees of finger flexion to 70 degrees of wrist exte nsion and 90 degrees of finger flexion. While this is acceptable, a mo re desirable result was predicted to occur if the muscle was transferr ed at a longer length. In this latter case, greater stretch of the FCU during transfer (increasing sarcomere length to about 5 mu m) was pre dicted to improve the transfer. The more highly stretched FCU was pred icted to result in maximum force as the wrist and fingers progressed f rom about 60 degrees of wrist extension and 0 degrees of finger flexio n to 80 degrees of wrist extension and 70 degrees of finger flexion. T hese results quantify the relationship between the passive tension cho sen for transfer, sarcomere length, acid the estimated active tension that can be generated by the muscle. The results also demonstrate the feasibility of using intraoperative laser diffraction during tendon tr ansfer as a guide for optimal placement of the transferred muscle.