Development of an Anisotropic Hyperelastic Material Model for Porcine Colorectal Tissues

Abstract

Many colonic surgeries include colorectal anastomoses whose leaks may be life-threatening, affecting thousands of patients annually. Various studies propose that mechanical interaction between the staples and neighboring tissues may play an important role in anastomotic leakage. Therefore, understanding the mechanical behavior of colorectal tissue is essential to characterizing the reasons for this type of failure. So far, experimental data characterizing the mechanical properties of colorectal tissue have been few and inconsistent, which has significantly limited understanding their behavior. This research proposes an approach to developing an anisotropic hyperelastic material model for colorectal tissues based on uniaxial testing of freshly harvested porcine specimens, which were collected from several age- and weight-matched pigs. The specimens were extracted from the same colon tract of each pig along their circumferential and longitudinal orientations. We propose a constitutive model combining Yeoh isotropic hyperelastic material with fibers oriented in two directions to account for the hyperelastic and anisotropic nature of colorectal tissues. Experimental data were used to accurately determine the model's coefficients (circumferential, R² = 0.9968; longitudinal, R² = 0.9675). The results show that the proposed model can be incorporated into a finite element model that can simulate procedures such as colorectal anastomoses reliably.

Keywords: colorectal tissues; experimental characterization; soft tissue modeling.

Figure A1

Colon physiological structure.

Figure 1

Specimen preparation: (a) colon extraction and (b) colon tract slicing and flattening.

Figure 2

Specimen preparation. (a) Die cut to prepare specimens. (b) Colorectal tissue specimen glued to two sacrificial clamps. A jig was used to ensure consistent specimens. Table 1 includes the specimens’ dimensions.

Figure 3

Uniaxial testing machine. (a) Components: A. stepper motor; B. fixed frame; C. load cell; D. moving frame; E. optical table; F. black curtain; G. environmental monitor. (b) A specimen under loading with universal joints connected to the clamps. (c) View from the camera. Table 1 includes the specimens’ dimensions.

Figure 4

Uniaxial testing of colorectal tissues. (a) Initial instant of a specimen in the post-conditioning phase. (b) Final instant of a specimen in the post-conditioning phase immediately before the tissue tearing. (c) A typical colorectal tissue load time history.

Figure 5

Monitoring the changes in elongation and width of the specimens during uniaxial testing. (a) Original image. (b) Black and white equivalent image; boundaries were identified. (c) Black and white equivalent image with the four boundaries separated into four regions.

Figure 6

Engineering stress–strain curves: (a) circumferential direction and (b) longitudinal direction.

Figure 7

Average and standard deviations of the engineering stress–strain curves: (a) circumferential direction and (b) longitudinal direction.

Figure 8

Pareto front for the multi-objective minimization of the anisotropic hyperelastic constitutive porcine colorectal tissue model.

Figure 9

Various examples of the multi-objective minimization of the anisotropic hyperelastic constitutive porcine colorectal tissue model. (a) No consideration of the longitudinal stress–strain data. (b) No consideration of the circumferential stress–strain data. (c) An example of an acceptable solution.

Figure 10

A meshed FEM model of a post-conditioning specimen.

Figure 11

Comparison of stress normal to the direction of loading at the end of the FEM simulations: (a) circumferential and (b) longitudinal.

Figure 12

Comparison of the circumferential and longitudinal results of the experiments, proposed model, and finite element analysis: (a) true stress and strain and (b) reaction force.