This presentation was made at CAASE18, The Conference on Advancing Analysis & Simulation in Engineering. CAASE18 brought together the leading visionaries, developers, and practitioners of CAE-related technologies in an open forum, to share experiences, discuss relevant trends, discover common themes, and explore future issues.

Resource Abstract

Intestinal failure (IF) is a rare multifactorial clinical condition that results in patients’ inability to sustain normal growth and nutritional and hydration status. Short bowel syndrome (SBS) is the most common cause of IF which is a devastating condition owing to loss of significant intestinal length thereby affecting the organ’s ability to absorb nutrients. Current treatment strategies for SBS involve parenteral nutrition and small bowel transplantation; various transit slowing and bowel lengthening procedures have been employed in highly selected subpopulations [1]. However, these therapies have shown limited success and are associated with high rates of sepsis, intestinal failure-associated liver disease, and mortality [1].

Intestinal lengthening by distraction enterogenesis from use of mechanical forces has been studied as a potential treatment for SBS. [2]. Previous studies in our lab have shown successful lengthening of jejunum using Self expanding springs intestines in rats and pigs for both metal and biodegradable springs [3,4]. However, mechanisms that generate lengthening remains poorly understood from a biomechanical perspective. Main aim of this study was to determine the mechanism behind intestine lengthening as well as optimizing spring design and characterization to have maximum intestine lengthening using finite element modeling. The mechanics of lengthening as well as correlation of stress experienced by tissue and tissue growth were tested with finite element models.

Three-dimensional models were created using ABAQUS FEA (D. S. Simulia). Initial geometry was matched to real pictures which was a hollow cylinder (including several tissue layers), while nonlinear material properties were taken. At the first step by applying same spring force to the geometry, model could predict consistent values for tissue lengthening and other geometrical measurements compare to experimental data. Then using UMAT subroutine as described by Young et al. [5], stress-dependent growth was implemented in the models to investigate role of stress in intestinal tissue behavior (lengthening, tube expansion and thickening of tissue) where stress experienced by tissue because of spring force triggers tissue response to the mechanical force. At the tissue level growth can be simulated using theory of Rodriguez et al [6], The total deformation is described by the deformation gradient tensor F, which can be decomposed as F=F^*.G where G is the growth tensor and F* is the elastic deformation gradient tensor. The growth tensor defines the stress-free configuration for each material element after it grows.
With computational models we saw how the geometry of the intestine change over time as well as the model supports this hypothesis that mechanical force can play an important role in intestinal lengthening two steps. First by stretching the tissue directly and in second step by turning on the tissue response to the stress and triggering growth. However future work will include additional complications for the further investigations.

References
[1]. Thompson JS, et al. Current management of the short bowel syndrome. Surg Clin North Am. 2011; 91(3):493-510.
[2]. Sullins VF, et al. Function of mechanically lengthened jejunum after restoration into continuity. J Pediatr Surg 2014; 49:971–5.
[3]. Scott A, et al. Repeated mechanical lengthening of intestinal segments in a novel model. J Pediatr Surg 2015; 50:954–7.
[4]. Sullins VF, et al. A novel biodegradable device for intestinal lengthening. J Pediatr Surg 2014; 49:109–13.
[5]. Young J. M., et al. Automatic generation of user material subroutines for biomechanical growth analysis. ASME J. Biomech. Eng. 2010; 132: 104505.
[6]. Rodriguez, E. K., et al. Stress-dependent finite growth in soft elastic tissues. J. Biomech. 1994; 27: 455-467.