Finite element analysis of the penetrations of shear and normal vibrations into the soft tissues in a fingertip.

It is well accepted that the effects of mechanical vibration on the finger-hand-arm system are strongly frequency-dependent: low frequency vibration can transmit from hand to arm, while high frequency vibration is absorbed in the local tissue of fingers. This assertion has not been validated directly. The purpose of the present study is to analyze the frequency- and deformation-dependent dynamic strains in the soft tissues in a fingertip that is subjected to vibration normal or tangential to the contact surface. The dynamic responses of the fingertip were analyzed using a multi-layered two-dimensional finite element model. The major anatomical substructures, i.e., skin, subcutaneous tissue, bone, and nail, are included in the model. The fingertip was found to have a major resonance around 100-125 Hz and a second resonance around 250 Hz. The resonances of the fingertip are found to be independent of the direction of exposure (in normal or shear direction). The simulations further indicated that the dynamic strains induced by the vibration at low frequencies will penetrate deeper into the tissue (> 3 mm) while that at high frequencies will be concentrated in the superficial skin layer (< 0.8 mm). The model predictions are consistent with the published experimental observations.