Measurement of mechanical properties of rectal wall

Mechanical experimentation of the gastrointestinal tract: a systematic review

The gastrointestinal (GI) organs of the human body are responsible for transporting and extracting nutrients from food and drink, as well as excreting solid waste. Biomechanical experimentation of the GI organs provides insight into the mechanisms involved in their normal physiological functions, as well as understanding of how diseases can cause disruption to these. Additionally, experimental findings form the basis of all finite element (FE) modelling of these organs, which have a wide array of applications within medicine and engineering. This systematic review summarises the experimental studies that are currently in the literature (n = 247) and outlines the areas in which experimentation is lacking, highlighting what is still required in order to more fully understand the mechanical behaviour of the GI organs. These include (i) more human data, allowing for more accurate modelling for applications within medicine, (ii) an increase in time-dependent studies, and (iii) more sophisticated in vivo testing methods which allow for both the layer- and direction-dependent characterisation of the GI organs. The findings of this review can also be used to identify experimental data for the readers' own constitutive or FE modelling as the experimental studies have been grouped in terms of organ (oesophagus, stomach, small intestine, large intestine or rectum), test condition (ex vivo or in vivo), number of directions studied (isotropic or anisotropic), species family (human, porcine, feline etc.), tissue condition (intact wall or layer-dependent) and the type of test performed (biaxial tension, inflation-extension, distension (pressure-diameter), etc.). Furthermore, the studies that investigated the time-dependent (viscoelastic) behaviour of the tissues have been presented.

Mechanobiological considerations in colorectal stapling: Implications for technology development

Technological advancements in minimally invasive surgery have led to significant improvements in patient outcomes. One such technology is surgical stapling, which has evolved into a key component of many operating rooms by facilitating ease and efficacy in resection and repair of diseased or otherwise compromised tissue. Despite such advancements, adverse post-operative outcomes such as anastomotic leak remain a persistent problem in surgical stapling and its correlates (i.e., hand-sewing), most notably in low colorectal or coloanal procedures. Many factors may drive anastomotic leaks, including tissue perfusion, microbiome composition, and patient factors such as pre-existing disease. Surgical intervention induces complex acute and chronic changes to the mechanical environment of the tissue; however, roles of mechanical forces in post-operative healing remain poorly characterized. It is well known that cells sense and respond to their local mechanical environment and that dysfunction of this "mechanosensing" phenomenon contributes to a myriad of diseases. Mechanosensing has been investigated in wound healing contexts such as dermal incisional and excisional wounds and development of pressure ulcers; however, reports investigating roles of mechanical forces in adverse post-operative gastrointestinal wound healing are lacking. To understand this relationship well, it is critical to understand: 1) the intraoperative material responses of tissue to surgical intervention, and 2) the post-operative mechanobiological response of the tissue to surgically imposed forces. In this review, we summarize the state of the field in each of these contexts while highlighting areas of opportunity for discovery and innovation which can positively impact patient outcomes in minimally invasive surgery.

Predicting the micromechanics of embedded nerve fibers using a novel three-layered model of mouse distal colon and rectum

Mechanotransduction plays a central role in evoking pain from the distal colon and rectum (colorectum) where embedded sensory nerve endings convert micromechanical stresses and strains into neural action potentials. The colorectum displays strong through-thickness and longitudinal heterogeneity with collagen concentrated in the submucosa thus indicating the significant load-bearing role of this layer. The density of sensory nerve endings is also significantly the greatest in the submucosa, suggesting a nociceptive function. Thus biomechanical heterogeneity in the colorectum influences the micromechanical stresses and strains surrounding afferent endings embedded within different layers of the colorectum which is critical for the mechanotransduction of various mechanical stimuli. In this study we aimed to: (1) calibrate and validate a three-layered computational model of the colorectum; (2) predict intra-tissue distributions of stresses and strains during mechanical stimulation of the colorectum ex vivo (i.e. circumferential stretching, punctuate probing, and mucosal shearing); and (3) establish a methodology to calculate local micromechanical stresses and strains surrounding afferent nerve endings embedded in the colorectum. We established three-layered FE models that include mucosa, submucosa, and muscular layers, and incorporated residual stretches, to calculate intra-tissue stresses and strains when the colorectum undergoes the mechanical stimuli used to characterize afferent neural encoding ex vivo. Finally, we established a methodology for detailed calculations of the local micromechanical stresses and strains surrounding afferent endings embedded in the colorectum and demonstrated this with a representative example. Our novel methodologies will bridge the existing neurophysiological and biomechanical evidence from experiments to advance our mechanistic understanding of colorectal mechanotransduction.

A literature review on large intestinal hyperelastic constitutive modeling

Impacts, traumas and strokes are spontaneously life-threatening, but chronic symptoms strangle patient every day. Colorectal tissue mechanics in such chronic situations not only regulates the physio-psychological well-being of the patient, but also confirms the level of comfort and post-operative clinical outcomes. Numerous uniaxial and multiaxial tensile experiments on healthy and affected samples have evidenced significant differences in tissue mechanical behavior and strong colorectal anisotropy across each layer in thickness direction and along the length. Furthermore, this study reviewed various forms of passive constitutive models for the highly fibrous colorectal tissue ranging from the simplest linearly elastic and the conventional isotropic hyperelastic to the most sophisticated second harmonic generation image based anisotropic mathematical formulation. Under large deformation, the isotropic description of tissue mechanics is unequivocally ineffective which demands a microstructural based tissue definition. Therefore, the information collected in this review paper would present the current state-of-the-art in colorectal biomechanics and profoundly serve as updated computational resources to develop a sophisticated characterization of colorectal tissues.

Computational Modeling of Mouse Colorectum Capturing Longitudinal and Through-thickness Biomechanical Heterogeneity

Mechanotransduction, the encoding of local mechanical stresses and strains at sensory endings into neural action potentials at the viscera, plays a critical role in evoking visceral pain, e.g., in the distal colon and rectum (colorectum). The wall of the colorectum is structurally heterogeneous, including two major composites: the inner consists of muscular and submucosal layers, and the outer consists of circular muscular, intermuscular, longitudinal muscular, and serosal layers. In fact the colorectum presents biomechanical heterogenity across both the longitudinal and through-thickness directions thus highlighting the differential roles of sensory nerve endings within different regions of the colorectum in visceral mechanotransduction. We determined constitutive models and model parameters for individual layers of the colorectum from three longitudinal locations (colonic, intermediate, and distal) using nonlinear optimization to fit our experimental results from biaxial extension tests on layer-separated colorectal tissues (mouse model, 7×7 mm2, Siri et al., Am. J. Physiol. Gastrointest. Liver Physiol. 316, G473-G481 and 317, G349-G358), and quantified the thicknesses of the layers. In this study we also quantified the residual stretches stemming from separating colorectal specimens into inner and outer composites and we completed new pressure-diameter mechanical testing to provide an additional validation case. We implemented the constitutive equations and created two-layered, 3-D finite element models using FEBio (University of Utah), and incorporated the residual stretches. We validated the modeling framework by comparing FE-predicted results for both biaxial extension testing of bulk specimens of colorectum and pressure-diameter testing of bulk segments against corresponding experimental results independent of those used in our model fitting. We present the first theoretical framework to simulate the biomechanics of distal colorectum, including both longitudinal and through-thickness heterogeneity, based on constitutive modeling of biaxial extension tests of colon tissues from mice. Our constitutive models and modeling framework facilitate analyses of both fundamental questions (e.g., the impact of organ/tissue biomechanics on mechanotransduction of the sensory nerve endings, structure-function relationships, and growth and remodeling in health and disease) and specific applications (e.g., device design, minimally invasive surgery, and biomedical research).

The Macro- and Micro-Mechanics of the Colon and Rectum I: Experimental Evidence

Many lower gastrointestinal diseases are associated with altered mechanical movement and deformation of the large intestine, i.e., the colon and rectum. The leading reason for patients' visits to gastrointestinal clinics is visceral pain, which is reliably evoked by mechanical distension rather than non-mechanical stimuli such as inflammation or heating. The macroscopic biomechanics of the large intestine were characterized by mechanical tests and the microscopic by imaging the load-bearing constituents, i.e., intestinal collagen and muscle fibers. Regions with high mechanical stresses in the large intestine (submucosa and muscularis propria) coincide with locations of submucosal and myenteric neural plexuses, indicating a functional interaction between intestinal structural biomechanics and enteric neurons. In this review, we systematically summarized experimental evidence on the macro- and micro-scale biomechanics of the colon and rectum in both health and disease. We reviewed the heterogeneous mechanical properties of the colon and rectum and surveyed the imaging methods applied to characterize collagen fibers in the intestinal wall. We also discussed the presence of extrinsic and intrinsic neural tissues within different layers of the colon and rectum. This review provides a foundation for further advancements in intestinal biomechanics by synergistically studying the interplay between tissue biomechanics and enteric neurons.

EXPERIMENTAL ASSESSMENT OF INTACT COLON DEFORMATION UNDER LOCAL FORCES APPLIED BY MAGNETIC CAPSULE ENDOSCOPES

Magnetically guided capsule endoscopy is a promising technology for clinical application. A platform that simulates the magnetic capsule endoscope system is built to study the deformation process of the colon when its lumen suffers local forces. Force-displacement curves of the porcine large intestine under various experiment conditions, including different loading positions (haustra or taeniae coli), loading directions, colon inner pressures and specimen lengths, were measured to analyze the mechanical behavior of the intact large intestine during interactions with magnetic capsule endoscopes. In the practical application of the magnetic capsule endoscope, these data are imperative to optimize the control scheme and reduce operation risks. Based on our experiments, the taeniae coli of the intact large intestine show higher linear stiffness than the haustra, and inflation reduces the linear stiffness of the colon. Magnetic capsule with small edge radii can more easily damage or even perforate the colon. Based on our test results, we suggest that the force applied to the colon should be limited to below 17[Formula: see text]N when the capsule is actuated forward along the colon and limited to below 10[Formula: see text]N when the capsule is vertical to the colon during lesion screening.

Quantitative assessment of intestinal stiffness and associations with fibrosis in human inflammatory bowel disease

Inflammatory bowel disease (IBD) continues to increase in prevalence in industrialized countries. Major complications of IBD include formation of fibrotic strictures, fistulas, reduced absorptive function, cancer risk, and the need for surgery. In other chronic gastrointestinal disease models, stiffness has been shown to precede fibrosis; therefore, stiffness may be a reasonable indicator of progression toward stricture formation in IBD patients. Herein, we seek to quantify tissue stiffness and characterize fibrosis in patients with IBD and to compare mechanical properties of unaffected human tissue to common animal species used for IBD studies. Inflamed and unaffected tissue from IBD patients and unaffected tissue from mice, pigs, and cows were indented using a custom device to determine the effective stiffness. Histology was performed on matched tissues, and total RNA was isolated from IBD tissue samples and used for gene expression analysis of pro-fibrotic genes. We observed an increase in the effective stiffness (steady-state modulus, SSM) (p < 0.0001) and increased expression of the collagen type I gene (COL1A1, p = 0.01) in inflamed tissue compared to unaffected areas in our IBD patient cohort. We also found that increased staining of collagen fibers in submucosa positively correlated with SSM (p = 0.093). We determined that unaffected animal bowel stiffness is significantly greater than similar human tissues, suggesting additional limitations on animal models for translational investigations regarding stiffness-related hypotheses. Taken together, our data support development of tools for evaluation of bowel stiffness in IBD patients for prognostic applications that may enable more accurate prediction of those who will develop fibrosis and more precise prescription of aggressive therapies.

Tensile properties of the rectal and sigmoid colon: a comparative analysis of human and porcine tissue

For many patients, rectal catheters are an effective means to manage bowel incontinence. Unfortunately, the incidence of catheter leakage in these patients remains troublingly high. Matching the mechanical properties of the catheter and the surrounding tissue may improve the catheter seal and reduce leakage. However, little data is available on the mechanical properties of colorectal tissue. Therefore, our group examined the mechanical properties of colorectal tissue obtained from both a common animal model and humans. Uniaxial tension tests were performed to determine the effects of location, orientation, and species (porcine and human) on bowel tissue tensile mechanical properties. Bowel tissue ultimate strength, elongation at failure, and elastic modulus were derived from these tests and statistically analyzed. Ultimate tensile strength (0.58 MPa, 0.87 MPa), elongation at failure (113.19%, 62.81%), and elastic modulus (1.83 MPa, 5.18 MPa) for porcine and human samples respectively exhibited significant differences based on species. Generally, human tissues were stronger and less compliant than their porcine counterparts. Furthermore, harvest site location and testing orientation significantly affected several mechanical properties in porcine derived tissues, but very few in human tissues. The data suggests that porcine colorectal tissue does not accurately model human colorectal tissue mechanical properties. Ultimately, the tensile properties reported herein may be used to help guide the design of next generation rectal catheters with tissue mimetic properties, as well as aid in the development of physical and computer based bowel models.

Biomechanical characterisation of fresh and cadaverous human small intestine: applications for abdominal trauma

Intestinal injuries are responsible for significant morbidity and mortality arising from trauma to the abdomen. The biomechanical characterisation of the small intestine allows for the understanding of the pathophysiological mechanisms responsible for these injuries. Studies reported in the literature focus principally on quasi-static tests, which do not take into account the stresses experienced during high kinetic trauma. In addition, the use of embalmed human tissue can alter the recorded response. The stress-strain curves from 43 tensile tests performed at 1 m/s were analysed. Samples were prepared from four fresh human intestines and from four embalmed cadaveric intestines. The data indicated a two-phase response, with each response consisting of a quasi-linear increase in the stress followed by an inflection in the curve before a peak preceding the loss of stress. The fresh tissue was more deformable than the embalmed tissue, and its first peak stress was lower (P = 0.034). A complementary histological analysis was performed. The results of the analysis enable an investigation of the response of the intestinal wall layers to stress as a two-layer structure and highlight the high sensitivity of the structure's mechanical behaviour to the speed of loading and the method of preservation.

Compliance and capacity of the normal human rectum--physical considerations and measurement pitfalls

The assessment of parameters which adequately represent rectal and neorectal compliance is complex. Biological properties of the rectum during distension and relaxation show significant departures from in vitro physical compliance measurements; as much dependent upon the viscoelastic charateristics of hollow organ deformation as upon the technique of compliance calculation. This review discusses the pressure/volume characteristics of importance in the rectum during distension from a bioengineering perspective and outlines the disparities of such measurements in living biological systems. Techniques and pitfalls of newer methods to assess rectal wall stiffness (impedance planimetry and barostat measurement) are discussed.

Passive mechanical properties of large intestine under in vivo and in vitro compression

This paper presents experimental data obtained from both in vivo and in vitro compression of the large intestine of goat. In vivo experimental data were obtained from compression tests on the large intestine of an anesthetized goat using force-displacement acquisition equipment. In vitro experimental data were also obtained from tissue excised after the in vivo experiments, and two types of data were then compared. The results demonstrated that the stress values had a strong dependence on the compressive rate in the in vivo experiments, although such effect was not distinct in the in vitro experiments. Additionally, at a lower compression rate, the intestinal tissues were found to be stiffer in the in vitro experiments than in the in vivo ones. This paper is a preliminary report on the mechanical properties of the large intestine based on in vivo and in vitro experimental data.