Deformation and pressure propagation in deep tissue during mechanical painful pressure stimulation

Manual palpation or pressure stimulation is often used for pain sensitivity assessment. The aim of the current study was to define a method for investigating the relation between pressure pain sensitivity and pressure propagation in soft or harder muscles. Three-dimensional finite-element computer-models were developed to simulate the tissue stress and strain distribution during pressure stimulation on the tibialis anterior and gastrocnemius muscles. Four cases were modelled representing females and males who were trained and untrained. The model geometry was based on MR images of the lower leg during pressure stimulation. Stress and strain were extracted from the models at pressure intensity levels equivalent to the pressure pain threshold. The principal strain peaked in the adipose tissue at 0.30 and 0.14 for stimulation on the gastrocnemius and tibialis anterior muscle, respectively. The principal strain in the muscle was higher for four models of the stimulation on the gastrocnemius muscle (0.22-0.30) compared with the four models of stimulation on the tibialis anterior muscle (0.11-0.14). Average pressure pain thresholds were significantly lower for the tibialis anterior compared with the gastrocnemius muscle (319 vs. 432 kPa) These data show different pressure propagation profiles in soft and hard muscle at the same pressure pain sensation level. This new approach is relevant as the clinical routine assesses all muscles equally. This results in a different exposure to pressure in relation to the muscle evaluated which may affect the outcome of the examination.