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THE FUNDAMENTAL THUMB-TIP FORCE VECTORS PRODUCED BY THE MUSCLES OF THE THUMB Jonathan L. Pearlman1, Michal Weisman1, Francisco J. Valero-Cuevas1,2 and Stephanie Roach1 1 Neuromuscular Biomechanics Laboratory, Cornell University, Ithaca, NY, U.S.A. 2 The Hospital for Special Surgery, New York, NY, U.S.A. E-mail: [email protected] Web: http://www.mae.cornell.edu/nmbl METHODS Figure 1: Experimental Apparatus. INTRODUCTION A rigorous description of the magnitude and direction of the 3D force vector each thumb muscle produces at the thumb-tip is necessary to understand the biomechanical consequences to pinch of a variety of paralyses and surgical procedures. Available descriptions of the biomechanical function of thumb musculature include muscle architectural parameters and moment arms; and graphical descriptions of the relative lines of action of intact and transferred tendons at each joint (Brand and Hollister, 1999). These descriptions, however, do not translate directly or unequivocally into descriptions of the 3D force vector each muscle produces at the thumb-tip—where pinch forces occur and where force vectors need to be restored by tendon transfers, for example. In this study, we characterized the 3D force vector each muscle produces at the thumb-tip, and investigated if these thumbtip force vectors scaled linearly with tendon tension. We measured the output 3D thumb-tip force vector produced by each tendon acting on the thumb, plus two common tendon transfers, as a function of input tendon tension (n = 13). After fixing the hand to frame (Fig 1), we mounted the thumb by configuring it in standardized key or opposition pinch posture and coupling the thumb-tip to a rigidly-fixed 3D force/torque sensor. Computer-controlled linear actuators applied tension to the distal tendons of the four extrinsic thumb muscles, and to six Nylon cords reproducing the lines of action of i) the four intrinsic thumb muscles and ii) two tendon transfers commonly used to restore thumb opposition following low median nerve palsy. Transfer A (TRa), is performed by routing the extensor indicis proprius muscle to the insertion of the failed abductor pollicis brevis (AbPB ) via the pisiform bone. Transfer B (TRb) is done by routing the flexor digitorum superficialis of the ring finger to the insertion of the failed AbPB via a slip in the flexor carpi ulnaris (Hentz and Leclercq, 2002). We measured the 3D force vector at the thumb-tip while each actuator ramped tendon tension from 0 to 1/3 of predicted maximal muscle force expected at each tendon, and back to zero. RESULTS AND DISCUSSION Many thumb-tip force vectors act in unexpected directions (e.g., the opponens force vector is parallel to the distal phalanx, Fig 2). This underscores how standard anatomical nomenclature can be of little value in describing fingertip force production for the purposes of the clinical restoration of pinch function. The two tendon transfers produced patently different force vectors, demonstrating how alternative (but presumably equivalent) tendon transfers to restore thumb opposition can be compared and contrasted by analyzing the 3D thumb-tip output force they produce. TRb better reproduces the radial component of force lost after low median nerve palsy (i.e., previously provided by AbPB). For most muscles, the increase of the thumb-tip force vectors with tendon tension is best described by a quadratic function (p<0.05)—likely due to load-dependent viscoelastic tendon paths and joint seating. Thumb-tip force vectors were sensitive to mounting procedure, as the moderate sensitivity to joint seating was compounded by inaccuracies in thumb posture. We are now studying the effect of these nonlinearities and sensitivities on the production of thumb-tip forces by the simultaneous action of multiple muscles. REFERENCES Brand, P. W. and Hollister, A. (1999) Clinical mechanics of the hand (3rdEdn). Mosby, St. Louis, Mo. Hentz, V. R. and Leclercq, C. (2002) Surgical rehabilitation of the upper limb in tetraplegia. W B Saunders. ACKNOWLEDGEMENTS This material is based upon work supported under a National Science Foundation Graduate Research Fellowship (JLP) and a Whitaker Foundation Grant (FVC). Figure 2. Average thumb-tip output vectors (N) for each functional posture in anatomical projections. Flexor Pollicis Longus (FPL), Extensor Pollicis Longus (EPL), Extensor Pollicis Brevis (EPB), Abductor Pollicis Longus (AbPL), Abductor Pollicis Brevis (AbPB), Flexor Pollicis Brevis (FPB), Opponens Pollicis (OPP), Adductor Pollicis oblique and transverse heads (ADDo and ADDt), and first dorsal interosseous (DIO). Transfers A and B (TRa and TRb, respectively) are also shown.