TENSIONS OF THE FLEXOR DIGITORUM SUPERFICIALIS ARE HIGHER THAN A CURRENT MODEL PREDICTS

Existing isometric force models can be used to predict tension in the finger flexor tendon, however, they assume a specific distribution of forces across the tendons of the fingers. These assumptions have not b een validated or explored by experimental methods. To determine if the force distributions repeatably follow one pattern the in vivo tension of the flexor digitorum superficialis (FDS) tendon of the long finger was measured in nine patients undergoing open carpal tunnel release s urgery. Following the release, a tendon force transducer (Dennerlein e t al. 1997 J. Biomechanics 30(4), 395-397) was mounted onto the FDS of the long finger. Tension in the tendon, contact force at the fingerti p, and finger posture were recorded while the patient gradually increa sed the force applied by the fingertip from 0 to 10 N and then monoton ically reduced it to 0 N. The average ratio of the tendon tension to t he fingertip contact force ranged from 1.7 to 5.8 (mean = 3.3, s.d. = 1.4) for the nine subjects. These ratios are larger than ratios predic ted by current isometric tendon force models (mean = 1.2, s. d. = 0.4) . Subjects who used a pulp pinch posture (hyper-extended distal interp halangeal joint (DIP)) showed a significantly (p = 0.02) larger ratio (mean = 4.4, s.d. = 1.5) than the five subjects who flexed the DIP joi nt in a tip pinch posture (mean = 2.4, s.d. = 0.6). A new DIP constrai nt model, which selects different force distribution based on DIP join t posture, predicts force ratios that correlate well with the measured ratios (r(2) = 0.85). (C) 1998 Elsevier Science Ltd. All rights reser ved.