INTRAOPERATIVE MEASUREMENT AND BIOMECHANICAL MODELING OF THE FLEXOR CARPI ULNARIS TO EXTENSOR CARPI RADIALIS LONGUS TENDON TRANSFER

Sarcomere length was measured intraoperatively in five patients underg oing tendon transfer of the flexor carpi ulnaris (FCU) to the extensor carpi radialis longus (ECRL) for radial nerve palsy. All measurements were made with the elbow in 20 deg of flexion. Prior to tendon transf er, FCU sarcomere length ranged from 2.84 +/- .12 mu m (mean +/- SEM) with the wrist flexed to 4.16 +/- .15 mu m with the wrist extended. Af ter transfer into the ECRL tendon, sarcomere length ranged from 4.82 /- .11 mu m with the wrist flexed (the new longest position of the FCU ) to 3.20 +/- .09 mu m with the wrist extended, resulting in a shift i n the sarcomere length operating range to significantly longer sarcome re lengths (p < 0.001). At these longer sarcomere lengths, the FCU mus cle was predicted to develop high active tension only when the wrist w as highly extended. A biomechanical model of this tendon transfer was generated using normative values obtained from previous studies of mus cle architectural properties, tendon compliance, and joint moment arms . Predicted sarcomere lengths pre-and post-tendon transfer agreed well with intraoperative experimental measurements. The theoretical wrist extension moment-wrist joint angle relationship was also calculated fo r a variety of values of FCU muscle length. These different lengths re presented the different conditions under which the FCU could be suture d into the ECRL tendon. Variation in FCU muscle length over the range 200 mm to 260 mm resulted in large changes in absolute peak moment pro duced as well as the angular dependence of peak moment. This was due t o the change in the region of FCU operation on its sarcomere length-te nsion curve relative to the magnitude of the ECRL moment arm. These da ta demonstrate the sensitivity of a short-fibered muscle such as the F CU to affect the functional outcome of surgery. In addition, we demons trated that intraoperative sarcomere length measurements, combined wit h biomechanical modeling provide the surgeon with a powerful method fo r predicting the functional effect of tendon transfer surgery.