Biomechanical model for the determination of the forces acting on the finger pulley system.

A mathematical model proposed by Hume et al., 1991. Journal of Hand Surgery-American Volume 16, 722-730 for the determination of the forces acting on the A2 and A4 pulley was used. The parameters necessary for this determination include the angle of flexion, the positioning of the pulley with respect to the centre of rotation in the proximal interphalangeal joint (PIP), the relative mismatch between bone and tendon width at the location of the respective pulleys as well as the tendon height at this position. This model was further developed to include the stiffness of the respective pulley, as well as the fact, that there are two flexor tendons of which only one passes through both pulleys. Each parameter was then evaluated using a sensitivity analysis proposed by Fasham et al., 1990. Journal of Marine Research 48, 591-639 in order to determine their relative importance for the outcome of the model. The most important parameter proofed to be the positioning of the pulley with respect to the centre of rotation in the PIP joint. This observation enabled us to give the best possible placement for a pulley graft after pulley rupture.