FORCE RESPONSE OF THE FINGERTIP PULP TO REPEATED COMPRESSION - EFFECTS OF LOADING RATE, LOADING ANGLE AND ANTHROPOMETRY

Repeated loading of the fingertips has been postulated to contribute t o tendon and nerve disorders at the wrist during activities associated with prolonged fingertip loading such as typing. To fully understand the pathomechanics of these soft tissue disorders, the role of the fin gertip pulp in attenuating the applied dynamic forces must be known. A n experiment was conducted to characterize the response of the in vivo fingertip pulp under repeated, dynamic, compressive loadings, to iden tify factors that influence pulp dynamics, and to better understand th e force modulation by the pulp. Twenty subjects tapped repeatedly on a flat plate with their left index finger, while the contact force and pulp displacement were measured simultaneously. Tapping trials were co nducted at three fingertip contact angles from the horizontal plane (0 degrees, 45 degrees, and 90 degrees) and five tapping rates (0.25, 0. 5, 1, 2, and 3 Hz). The fingertip pulp responds as a viscoelastic mate rial, exhibiting rate-dependence, hysteresis, and a nonlinear force-di splacement relationship. The pulp was relatively compliant al forces l ess than 1 N, but stiffened rapidly with displacement at higher forces for all loading conditions. This suggests that high-frequency fords o f a small magnitude (< 1 N) are attenuated by the nonlinearly stiffeni ng pulp while these forces of larger magnitude are transmitted to the bone. Pulp response was significantly influenced by the angle of loadi ng. Fingertip dimensions, gender, and subject age had little to no inf luence on pulp parameters. (C) 1997 Elsevier Science Ltd.