Strain Measurement of Thumbnail in Response to Compressive Force

Abstract

The fingernail plays an important role when an object such as a switch is pressed with human fingertip or when an object such as a stick is grasped by human hand. The patients who lost nails by an injury would have much difficulty to pinch objects with the fingers. In this study, we investigated the deformation of the nail under compressive force. The strain distribution of a human thumbnail when the finger-pulp was pressed on a flat plate was analyzed using three 0°/90° biaxial strain gauges affixed on the proximal-radial, distal-central, and proximal-ulnar parts of the thumbnail. Each axis of the strain gauge corresponded to the axial and transverse directions of the thumb. Twelve normal men (aged 21-24 years, mean 22) participated in this study. All subjects were right-handed. The relationships between the compressive force on a flat plate and the strains in the nail were investigated as a parameter of the contact angle from the flat plate, which was set at 30, 45, and 60°. The compressive strain in the nail increased according to the increase in the compressive force. No significant differences were found among each contact angle. From the comparisons among the strains to each direction on all measurement parts, the strain to transverse direction on the proximal-ulnar part was significantly larger than that on the proximal-radial part. So, an asymmetry strain distribution of the thumbnail surface to the transverse direction when the finger-pulp was pressed on a flat plate was showed.