Mechanical Behaviour of Human and Porcine Urethra: Experimental Results, Numerical Simulation and Qualitative Analysis

Abstract

Low urinary tract dysfunctions and symptoms (LUTS) affect both men and woman, with the incidence increasing with age. Among the LUTS, urinary incontinence (UI) is a common dysfunction, characterised by the involuntary loss of urine. These medical conditions become debilitating, with a severe impact on patients’ routines and overall well-being. To mitigate LUTS-associated symptoms, the mechanical behaviour of both normal and LUTS-affected urethrae can be an important tool. The current work approaches the porcine urethra as a mechanical replacement candidate for the human urethra. It aims to provide a framework based on in silico (numerical) simulations and experimental data, to compare the candidate’s mechanical behaviour against the human urethra. Porcine urethral samples were mechanically evaluated through low-cycle fatigue tests in both circumferential and longitudinal orientations. The specimens were collected from porcine urethrae from crossbred pigs raised for human consumption. The experimental results were compared with human references found in the literature, with similar experimental conditions. The experimental data were used as the input for the mechanical properties estimation (nonlinear fitting to hyperelastic constitutive models) and for the simulation of the urethral tensile behaviour, using those models. In the longitudinal orientation, the results for the porcine and human urethra were in good agreement, while in the circumferential direction, the differences increased with deformation. Previous data on the mechanical behaviour of the equine urethra is in line with these findings. The nonlinear mechanical behaviour of a porcine urethra was modelled using the finite element method (FEM) and hyperelastic constitutive models. For the longitudinal urethra, the simulation results approximate experimental data for stretches up to λ≈1.5 (50% deformations), whereas for the circumferential urethra, the same was true for stretches up to λ≈1.35 (35% deformations). The hyperelastic models with a higher number of parameters performed better with the third-order Ogden model (six parameters), displaying the best performance among the studied models. The pig urethra is a suitable candidate for an implant targeted at human urethra replacement or as a model to study the human urinary system. Nevertheless, the data available on the circumferential mechanical behaviour need to be consolidated with additional mechanical tests. The tensile behaviour of the porcine urethra over large deformations can be modelled using the third-order Ogden model; however, to extend the modelling capabilities to larger deformations requires the use of hyperelastic models more adequate to soft tissue behaviour.