REVISITING FINGER FLEXOR EXCURSIONS WITH CURRENT MODELING TECHNIQUES Aaron M. Kociolek and Peter J. Keir Occupational Biomechanics Laboratory, McMaster University, Hamilton, ON, Canada Email: kociolam@mcmaster.ca INTRODUCTION The pathophysiologies of many distal upper extremity work-related musculoskeletal disorders are uncertain. Differential motion between the extrinsic finger flexor tendons is thought to contribute in the development of hand/wrist tenosynovitis and carpal tunnel syndrome due to increased frictional work. Armstrong and Chaffin [1] developed anthropometric-based regression equations to estimate flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendon excursions with wrist and finger joint displacements from four cadaveric specimens. Treaster and Marras [2] used the equations to predict FDP and FDS cumulative tendon excursions during keyboarding. The equations have also been used in a two-dimensional model to estimate frictional work at the tendon sheath [3]. However, there is a need to evaluate this approach with empirical data and current modeling techniques. Holzbaur et al. [4] recently developed a three- dimensional biomechanical model of the upper extremity with 15 degrees of freedom and 50 muscle actuators to simulate movement at the shoulder, elbow, forearm, wrist, thumb and index finger. However, several additions are needed to assess FDP and FDS tendon excursions. Efforts to model joint displacements of the third, fourth and fifth digits with anatomically correct extrinsic finger flexors are needed to calculate differential tendon motion (and to develop an analytical model for frictional work). The purpose of this study was to develop a biomechanical hand model with accurate extrinsic finger flexor tendon excursions. METHODS Software for Interactive Musculoskeletal Modeling (SIMM) by MusculoGraphics Inc. (Santa Rosa, CA) was used to further refine an existing hand model (Figure 1). The Holzbaur et al. [4] upper extremity model served as a starting point, which has representations of the carpals, metacarpals and phalanges scaled as a 50 th percentile male. Metacarpophalangeal (MCP) joints of the third, fourth and fifth digits were modeled as universal joints to simulate flexion/extension and abduction/adduction. Proximal and distal interphalangeal (DIP and PIP) joints were modeled as hinges to simulate flexion/extension. Since detailed descriptions of the MCP and IP joint rotation axes did not exist for the third, fourth and fifth digits, cylinders were fit at the bone surfaces to characterize movement. This method was also used for the MCP and IP joints of the second digit [4]. Also, range of motion parameters were set equivalent to those existing for the second digit [4]. Eight extrinsic finger flexors were included in the model. The muscle–tendon paths were described with a series of points in the bone local coordinate systems. Points that constrain the paths of the FDP and FDS (such as annular pulleys) were added to improve the anatomical accuracy of the hand model. Wrapping surfaces were also added at the MCP and IP joints to simulate accurate tendon excursions. Muscle-tendon excursions of the FDP and FDS were calculated with MCP and IP flexion/extension Figure 1: Hand model in (a) palmar and (b) dorsal views illustrating muscle-tendon geometry (red cords) and wrapping surfaces (blue shapes). (a) (b)