Extrusion properties of porcine intestines and surrogate materials for ventral hernia modelling

Regional differences in stomach stretch during organ filling and their implications on the mechanical stress response

As part of the digestive system, the stomach plays a crucial role in the health and well-being of an organism. It produces acids and performs contractions that initiate the digestive process and begin the break-up of ingested food. Therefore, its mechanical properties are of interest. This study includes a detailed investigation of strains in the porcine stomach wall during passive organ filling. In addition, the observed strains were applied to tissue samples subjected to biaxial tensile tests. The results show inhomogeneous strains during filling, which tend to be higher in the circumferential direction (antrum: 13.2%, corpus: 22.0%, fundus: 67.8%), compared to the longitudinal direction (antrum: 4.8%, corpus: 24.7%, fundus: 50.0%) at a maximum filling of 3500 ml. Consequently, the fundus region experienced the greatest strain. In the biaxial tensile experiments, the corpus region appeared to be the stiffest, reaching nominal stress values above 400 kPa in the circumferential direction, whereas the other regions only reached stress levels of below 50 kPa in both directions for the investigated stretch range. Our findings gain new insight into stomach mechanics and provide valuable data for the development and validation of computational stomach models.

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.

The suture pullout characteristics of human and porcine linea alba

There is a substantial prevalence of post-operative incisional hernia for both laparoscopic and laparotomy procedures, but there have been few attempts at quantifying abdominal wound closure methodology in the literature. One method to ascertain a more robust method of wound closure is the identification of the influence of suture placement parameters on suture pullout force. Current surgical practice involves a recommended bite depth and bite separation of 10mm, but the evidence base for this is not clear. In this paper, the suture pullout characteristics of both porcine and human linea alba were investigated to ascertain a suture placement protocol for surgical wound closure. Uniaxial suture pullout force testing on fresh frozen porcine and human linea alba samples was performed using standard materials testing machines. The influence of the number of suture loops, the bite depth and the bite separation of the sutures and the orientation of the sutures with respect to the principal fibre direction in the linea alba were assessed. Results showed a clearly identifiable relationship between pullout force of the suture, bite separation and bite depth, with low suture separation and high suture depth as optimal parameters for increasing pullout force. Resistance to pullout could be improved by as much as 290% when optimizing test conditions. Both human and porcine tissue were observed to exhibit very similar pullout force characteristics, corroborating the use of a porcine model for investigations into wound closure methodology. Orientation of suture application was also found to significantly affect the magnitude of suture pullout, with suturing applied longitudinally across a transverse defect resulting in higher pullout forces for small suture bite separations. Although further assessment in an environment more representative of in vivo conditions is required, these findings indicate that increasing the bite depth and reducing the bite separation with respect to the current surgical recommendations may reduce the risk of post-operative incisional hernia.

Biomechanical abdominal wall model applied to hernia repair

BACKGROUND

Most surgical innovations require extensive preclinical testing before employment in the operative environment. There is currently no way to develop and test innovations for abdominal wall surgery that is cheap, repeatable and easy to use. In hernia repair, the required mesh overlap relative to defect size is not established. The aims of this study were to develop a biomechanical model of the abdominal wall based on in vivo pressure measurements, and to apply this to study mesh overlap in hernia repair.

METHODS

An observational study of intra-abdominal pressure (IAP) levels throughout abdominal surgery was conducted to identify the peak perioperative IAP in vivo. This was then applied in the development of a surrogate abdominal wall model. An in vitro study of mesh overlap for various defect sizes was then conducted using this clinically relevant surrogate abdomen model.

RESULTS

The mean peak perioperative IAP recorded in the clinical study was 1740 Pa, and occurred during awakening from anaesthesia. This was reproduced in the surrogate abdomen model, which was also able to replicate incisional hernia formation. Using this model, the mesh overlap necessary to prevent hernia formation up to 20 kPa was found, independent of anatomical variations, to be 2 × (defect diameter) + 25 mm.

CONCLUSION

This study demonstrated that a surgically relevant surrogate abdominal wall model is a useful translational tool in the study of hernia repair. Surgical relevance This study examined the mesh overlap requirements for hernia repair, evaluated in a biomechanical model of the abdomen. Currently, mesh size is selected based on empirical evidence and may underpredict the requirement for large meshes. The study proposes a relationship between the defect size and mesh size to select the appropriate mesh size. Following further trials and investigations, this could be used in clinical practice to reduce the incidence of hernia recurrence.

Mechanical characterization of stomach tissue under uniaxial tensile action

In this article, the tensile properties of gastric wall were investigated by using biomechanical test and theoretical analysis. The samples of porcine stomach strips from smaller and greater curvature of the stomach were cut in longitudinal and circumferential direction, respectively. The loading-unloading, stress relaxation, strain creep, tensile fracture tests were performed at mucosa-submucosa, serosa-muscle and intact layer, respectively. Results showed that the biomechanical properties of the porcine stomach depended on the layers, orientations and locations of the gastric wall and presented typical viscoelastic, nonlinear and anisotropic mechanical properties. During loading-unloading test, the stress of serosa-muscle layer in the longitudinal direction was 15-20% more than that in the circumferential direction at 12% stretch ratio, while it could reach about 40% for the intact layer and 50% for the mucosa-submucosa layer. The results of stress relaxation and strain creep showed that the variation degree was obviously faster in the circumferential direction than that in the longitudinal direction, and the ultimate residual values were also different for the different layers, orientations and locations. In the process of fracture test, the serosa-muscle layer fractured firstly followed by the mucosa-submucosa layer when the intact layer was tested, the longitudinal strips firstly began to fracture and the required stress value was about twice as much as that in the circumferential strips. The anisotropy and heterogeneity of mechanical characterization of the porcine stomach were related to its complicated geometry, structure and functions. The results would help us to understand the biomechanics of soft organ tissue.