In vivo evaluation of visco-elasticity in a biological tube. Part 2. Application of a mechanical model.

Based on a theoretical model, the urethral stress-relaxation response following a sudden forced dilatation was studied in ten healthy female volunteers. The energy dissipation following a sudden urethral dilatation proved to follow the function Y = Z + C alpha e-t/tau alpha + C beta e-t/tau beta. Hence, a mechanical model was chosen, consisting of two Maxwell elements and one Hooke element coupled in parallel. The decay in force following deformation may be described as F = F(o) exp (-t.E/eta) for each Maxwell element. Thus, it was possible to determine the elastic E and viscous eta coefficients for the participating mechanical equivalents in the model, and thereby quantitatively describe the visco-elastic properties in the urethra. The reproducibility of the elastic and the viscous coefficients proved to be fairly high, and they seemed to be unaffected by the size of dilatation. On the other hand, the rate of dilatation clearly influenced the computed parameters. However, this phenomenon was easily explained by the duration of the deformation, which allowed the viscous elements to move before any measurements were performed. The present method permits in vivo evaluation of the elastic and viscous properties of the urethra, as well as other accessible biological tubes, and may be of value in the description of normal physiological and pathophysiological behaviour of the structures studied.