Biomechanical alterations in the carpal arch and hand muscles after carpal tunnel release: a further approach toward understanding the function of the flexor retinaculum and the cause of postoperative grip weakness.

The difference between maximal and minimal distance covered (the distance between the trapezium ridge and hamate hook; moment exerted on structures: 1 Nm) by an intact flexor retinaculum (FR; minimum, 3.3 +/- 0.1 cm; maximum, 3.7 +/- 0.2 cm) and the increase in the maximal distance on carpal tunnel release (CTR; increase, 1.6 +/- 0.2 mm) were significant. Under an external supination moment, the distance between the attachments of the trapeziopisiform band increased after CTR. Under external pronation and ulnar abduction moments, the distance between the attachments of the scaphoideohamate band increased after CTR. The CTR resulted in an anatomic attachment loss for the following muscles: the superficial head of the flexor pollicis brevis (shortening by approximately 25%, relative to rest length), the ulnar part of the abductor pollicis brevis (with opposition and adductory functions, approximately 20%), the opponens pollicis (approximately 20%), the middle part of the abductor pollicis brevis (approximately 7%), and the opponens digiti minimi (approximately 10%). Preoperative and postoperative (2-7 weeks after surgery) measurements of the reaction force of the distal phalanx (under isometric thumb opposition and finger II-IV flexion with extended carpal joint) led to differentiation of three groups: (1) significant strength loss--the patients showed difficulties with grasping, lifting, twisting off lids and caps, screwing, pulling ropes, and pinching; (2) no significant change in force values; and (3) a significant increase in strength (patients who could grip more firmly).