Mechanical behaviour of synthetic surgical meshes: finite element simulation of the herniated abdominal wall

Numerical investigation of a finite element abdominal wall model during breathing and muscular contraction

BACKGROUND AND OBJECTIVE

Ventral hernia repair is faced with high recurrence rates. The personalization of the diagnosis, the surgical approach and the choice of the prosthetic implant seem relevant axes to improve the current results. Numerical models have the potential to allow this patient-specific approach, yet currently existing models lack validation. This work extensively investigated a realistic finite element abdominal wall model including the implementation of muscle activation.

METHODS

A parametric 3D finite element model composed of bone, muscle and aponeurotic structures was introduced. Hyperelastic anisotropic materials were implemented. Two loading scenarios were simulated: passive inflation of the abdominal cavity to represent, e.g., breathing, and passive inflation followed by muscular activation to simulate other daily activities such as cough. The impact of the inter-individual variability (e.g., BMI, tissue thickness, material properties, intra-abdominal pressure (IAP) and muscle contractility) on the model outputs was studied through a sensitivity analysis.

RESULTS

The overall model predictions were in good agreement with the experimental data in terms of shape variation, muscles displacements, strains and midline forces. A total of 34 and 41 runs were computed for the passive and active sensitivity analysis respectively. The regression model fits rendered high R-squared in both passive (84.0 ± 6.7 %) and active conditions (82.0 ± 8.3 %). IAP and muscle thickness were the most influential factors for the selected outputs during passive (breathing) activities. Maximum isometric stress, muscle thickness and pre-activation IAP were found to drive the response of the simulations involving muscular contraction. The material properties of the connective tissue were essential contributors to the behaviour of the medial part of the abdominal wall.

CONCLUSIONS

This work extensively investigated a realistic abdominal wall model and evaluated its robustness using experimental data from literature. Such a model could improve patient-specific simulation for ventral hernia surgical planning, prevention, and repair or implant evaluation. Further investigations will be conducted to evaluate the impact of the surgical technique and the mechanical characteristic of prosthetic meshes on the model outputs.

Patient-specific computational simulations of wound healing following midline laparotomy closure

In the current study, we developed a new computational methodology to simulate wound healing in soft tissues. We assumed that the injured tissue recovers partially its mechanical strength and stiffness by gradually increasing the volume fraction of collagen fibers. Following the principles of the constrained mixture theory, we assumed that new collagen fibers are deposited at homeostatic tension while the already existing tissue undergoes a permanent deformation due to the effects of remodeling. The model was implemented in the finite-element software Abaqus® through a VUMAT subroutine and applied to a complex and realistic case: simulating wound healing following midline laparotomy closure. The incidence of incisional hernia is still quite significant clinically, and our goal was to investigate different conditions hampering the success of these procedures. We simulated wound healing over periods of 6 months on a patient-specific geometry. One of the outcomes of the finite-element simulations was the width of the wound tissue, which was found to be clinically correlated with the development of incisional hernia after midline laparotomy closure. We studied the impact of different suturing modalities and the effects of situations inducing increased intra-abdominal pressure or its intermittent variations such as coughing. Eventually, the results showed that the main risks of developing an incisional hernia mostly depend on the elastic strains reached in the wound tissue after degradation of the suturing wires. Despite the need for clinical validation, these results are promising for establishing a digital twin of wound healing in midline laparotomy incision.

Numerical analysis of biothermal-fluids and cardiac thermal pulse of abdominal aortic aneurysm

Abdominal aortic aneurysms are serious and difficult to detect, conditions can be deadly if they rupture. In this study, the heat transfer and flow physics of Abdominal Aortic Aneurysm (AAA) were discussed and associated with cardiac cycle to illustrate the cardiac thermal pulse (CTP) of AAA. A CTP and infrared thermography (IRT) evaluation-based on AAA and abdomen skin surface detection method was proposed, respectively. Infrared thermography (IRT) is a promising imaging technique that may detect AAA quicker and cheaper than other imaging techniques (as biomarker). From CFD rigid-wall and FSI Analysis, the transient bioheat transfer effect resulted in a distinct thermal signature (circular thermal elevation) on the temperature profile of midriff skin surface, at both regular body temperature and supine position, under normal clinical temperature. However, it is important to note that thermography is not a perfect technology, and it does have some limitations, such as lack of clinical trials. There is still work to be done to improve this imaging technique and make it a more viable and accurate method for detecting abdominal aortic aneurysms. However, thermography is currently one of the most convenient technologies in this field, and it has the potential to detect abdominal aortic aneurysms earlier than other techniques. CTP, on the other hand, was used to examine the thermal physics of AAA. In CFD rigid-wall Analysis, AAA had a CTP that only responded to systolic phase at regular body temperature. In contrast, a healthy abdominal aorta displayed a CTP that responded to the full cardiac cycle, including diastolic phase at all simulated cases. Besides, the findings from FSI Analysis suggest the influence of numerical simulation techniques on the prediction of thermal physics behaviours of AAA and abdominal skin surface. Lastly, this study correlated the relationship between natural convective heat transfer coefficient with AAA and provided reference for potential clinical diagnostic using IRT in clinical implications.

THE USE OF SURGICAL ADHESIVE AND SUTURE FIXING MESHES TO THE ABDOMINAL WALL: AN EXPERIMENTAL STUDY IN RATS

AIM

Proper fixation of the surgical mesh determines the success of a herniorrhaphy. Understanding the inflammatory response and the mechanical properties of the mesh helps to define whether a fixation method is superior. This study aimed to evaluate the healing of defects in the abdominal wall of rats, comparing the repair of macroporous polypropylene meshes fixed with surgical glue and polypropylene thread.

METHODS

In 20 Wistar rats, a defect was produced in the abdominal wall, with the integrity of the parietal peritoneum. For correction, the meshes were fixed with surgical glue (2-octyl cyanoacrylate) (subgroup C1), or polypropylene suture (subgroup C2). The two subgroups of 10 animals were euthanized on the 90th postoperative day, and the fragments of the abdominal wall were submitted to macroscopic, histological, and tensiometric analysis.

RESULTS

Macroscopic analysis did not show any abnormalities. Tensiometry on the 90th postoperative day in subgroup C1 showed mean rupture tension of 28.47N and in subgroup C2 32.06N (p=0.773). The inflammatory process score revealed that both groups are in the subacute phase (p=0.380).

CONCLUSION

The fixation of a polypropylene macroporous mesh to repair an abdominal wall defect can be performed with surgical glue (2-octyl cyanoacrylate) or polypropylene suture, both methods being equally effective.

Combined numerical and experimental approach to determine numerical model of abdominal scaffold

A proper junction of the prosthesis and the abdominal wall is important in successful hernia repair. The number of tacks should be balanced to assure appropriate mesh fixation and not to induce post-operative pain. Numerical simulations help to find this balance. The study is aimed at creating a proper numerical model of a knitted surgical mesh subjected to boundary conditions and load occurring in the abdominal cavity. Continuous, anisotropic constitutive relation is considered to reflect the mesh behaviour. Different sets of material law parameters are determined on the basis of different bi-axial tests setups. Force- and displacement-controlled tests with different ratios are considered. Consequently, some numerical model variants are obtained featuring various reaction distributions in the scaffold fixation points. The proper variant is selected based on comparison of the position of maximal reaction force in the numerical model and in the reference physical model of operated hernia. Force-driven tests have shown anisotropic mesh behaviour, while equibiaxial displacement-driven test has demonstrated reduced anisotropic response. Within seven scenarios of constitutive parameters identification (based on single or combined experimental data), the equibiaxial force-controlled test appeared to produce the most relevant model to follow the prosthesis behaviour under pressure. The position of maximal reaction force in such model is similar to obtained in the physical hernia model. The equibiaxial force-driven test provides most suitable data for Gasser-Ogden-Holzapfel constitutive model identification of a considered surgical mesh to be used to model the mesh under pressure.

An approach to evaluating and benchmarking the mechanical behavior of a surgical mesh prototype designed for the repair of abdominal wall defects

Ventral hernia repair is a common surgical procedure in abdominal surgery in which surgical mesh has become an essential tool to improve outcomes. To avoid recurrences the mesh needs to mimic the mechanical behavior of the abdominal wall. In this scenario the mechanical properties at the interface between the mesh and its surrounding tissue is critical for the performance of the device and, therefore, the success after surgery. We aimed to characterize and compare the mechanical behavior of the patented prototype mesh Spider and four commercial meshes at the mesh-tissue interface. The prototype mesh was designed based on the hypothesis that the best performance for a large-sized defect in a ventral hernia is obtained when the mesh presents an isotropic behavior. In contrast, commercial meshes presented significant anisotropic behavior. Mechanical properties of the meshes were characterized through uniaxial tensile tests. Longitudinal and transverse axes were defined for each mesh, and samples were cut in each axis orientation. Samples underwent uniaxial tensile testing, from which the elastic modulus in each axis was determined. The degree of anisotropy was calculated as the ratio between the elastic modulus in each axis. An in silico model of the ventral hernia defect was designed to simulate the mesh-tissue interface behavior via finite element method. Meshes were modeled by an hyperelastic orthotropic constitutive model, which allowed isotropic symmetry as particular case for the prototype mesh. Abdominal wall was modeled using a Neo-Hookean model. Once the simulations were launched, mesh-tissue interface behavior was evaluated through the difference between Von Mises stress values on either size of the interface, both on the external and the internal face of the mesh and abdominal wall. Mechanical response was anisotropic for all commercial meshes and isotropic for the Spider prototype. Among commercial, Ultrapro^® was highly anisotropic. Tests revealed Gore-Tex^® to be the stiffest, followed by Repol Angimesh^®, Spider and Ultrapro^®; Duramesh™  was found to be the most compliant. Concerning mesh-tissue behavior, simulation results revealed the Spider prototype and Duramesh™  to be the best; Spider due to its uniformity and lower stress difference thanks to its nearly isotropic behavior, and Duramesh™  due to its compliant behavior. Our results suggest that the compromise between stiffness and anisotropy must be considered in order to improve the mechanical performance of the meshes, bearing in mind that for large-sized ventral defects, nearly isotropic mesh ensures better performance.

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.

BETWEEN PROLENE®, ULTRAPRO® AND BARD SOFT® MESHES WHICH PRESENTS THE BEST PERFORMANCE IN THE REPAIR OF THE ABDOMINAL WALL?

BACKGROUND

In the definition of the mesh to be used to correct hernias, porosity, amount of absorbable material and polypropylene should be considered in the different stages of healing process.

AIM

To evaluate the inflammatory reaction in the use of macro and microporous meshes of high and low weight in the repair of defects in the abdominal wall of rats.

METHODS

Ninety Wistar rats (Rattus norvegicus albinus) were used. The animals were submitted to similar surgical procedures, with lesion of the ventral abdominal wall, maintaining the integrity of the parietal peritoneum and correction using the studied meshes (Prolene®, Ultrapro® and Bard Soft®). Euthanasia was performed at 30, 60 and 120 days after surgery. The abdominal wall segments were submitted to histological analysis using H&E, Masson's trichrome, immunohistochemistry, picrosirius red and tensiometric evaluation.

RESULTS

On the 120th day, the tensiometric analysis was superior with Ultrapro® macroporous mesh. The inflammatory process score showed a significant prevalence of subacute process at the beginning and at the end of the study. Microporous meshes showed block encapsulation and in macroporous predominance of filamentous encapsulation.

CONCLUSION

The Ultrapro® mesh showed better performance than the others in healing process of the abdominal wall.

Tailoring electrospun mesh for a compliant remodeling in the repair of full-thickness abdominal wall defect - The role of decellularized human amniotic membrane and silk fibroin

Tailored electrospun meshes have been increasingly explored for abdominal wall defect repair in preclinical and clinical studies. However, the fabrication of a bioengineered mesh adapts to the intraperitoneal repair for a compliant remodeling remains a great challenge. In this study, we fabricated a functional mesh by combining polycaprolactone (PCL) with silk fibroin (SF) and decellularized human amniotic membrane (HAM) proportionally via electrospinning. SF was integrated with PCL (40:60 w/w) to regulate the structural flexibility. Micronized HAM was incorporated to PCL/SF (10:90 w/w) to provide a biocompatible milieu with functions being conferred to facilitate intraperitoneal repair. After the blend electrospinning, the PCL/SF/HAM mesh was characterized in vitro and implanted into the rat model with a full-thickness defect for a comprehensive evaluation in comparison to the PCL and PCL/SF meshes. The results demonstrated that electrospinning fabricated PCL stabilized the mechanical elongation toward approximating the native counterparts after integrating with SF. After integrating with HAM, which is coupled with diverse biomolecular compositions, the developed PCL/SF/HAM mesh provided a better microenvironment for cell proliferation and vasculogenic network over other meshes without HAM addition and possessed the functions capable of inhibiting transforming growth factor β1 (TGF-β1) expression and collagen secretion under inflammatory conditions. Moreover, the functional mesh developed less-intensive adhesion along with histologically weaker inflammatory response and foreign body reaction than the PCL and PCL/SF meshes after 90 days in vivo. During the remodeling process, the bioactive structure induced more pronounced neovascularization and remarkable incorporation of collagen and elastin fibers and contractile filaments for a mechanically sufficient and physiologically stiffness-matched healing. This tailor-made mesh expands the intraperitoneal applicability of conventional electrospun meshes for a compliant remodeling in the repair of abdominal wall defects.

Polymer Hernia Repair Materials: Adapting to Patient Needs and Surgical Techniques

Biomaterials and their applications are perhaps among the most dynamic areas of research within the field of biomedicine. Any advance in this topic translates to an improved quality of life for recipient patients. One application of a biomaterial is the repair of an abdominal wall defect whether congenital or acquired. In the great majority of cases requiring surgery, the defect takes the form of a hernia. Over the past few years, biomaterials designed with this purpose in mind have been gradually evolving in parallel with new developments in the different surgical techniques. In consequence, the classic polymer prosthetic materials have been the starting point for structural modifications or new prototypes that have always strived to accommodate patients’ needs. This evolving process has pursued both improvements in the wound repair process depending on the implant interface in the host and in the material’s mechanical properties at the repair site. This last factor is important considering that this site—the abdominal wall—is a dynamic structure subjected to considerable mechanical demands. This review aims to provide a narrative overview of the different biomaterials that have been gradually introduced over the years, along with their modifications as new surgical techniques have unfolded.

Regulatory science for hernia mesh: Current status and future perspectives

Regulatory science for medical devices aims to develop new tools, standards and approaches to assess the safety, effectiveness, quality and performance of medical devices. In the field of biomaterials, hernia mesh is a class of implants that have been successfully translated to clinical applications. With a focus on hernia mesh and its regulatory science system, this paper collected and reviewed information on hernia mesh products and biomaterials in both Chinese and American markets. The current development of regulatory science for hernia mesh, including its regulations, standards, guidance documents and classification, and the scientific evaluation of its safety and effectiveness was first reported. Then the research prospect of regulatory science for hernia mesh was discussed. New methods for the preclinical animal study and new tools for the evaluation of the safety and effectiveness of hernia mesh, such as computational modeling, big data platform and evidence-based research, were assessed. By taking the regulatory science of hernia mesh as a case study, this review provided a research basis for developing a regulatory science system of implantable medical devices, furthering the systematic evaluation of the safety and effectiveness of medical devices for better regulatory decision-making. This was the first article reviewing the regulatory science of hernia mesh and biomaterial-based implants. It also proposed and explained the concepts of evidence-based regulatory science and technical review for the first time.

Improved tissue integration of a new elastic intraperitoneal stoma mesh prosthesis

Parastomal herniation is a frequent complication in colorectal surgery, occurring with a prevalence of 30-80%. The aim of the study was to create a new intraperitoneal colostoma mesh prosthesis (IPST) with enhanced elastic properties made with thermoplastic polyurethane (TPU) monofilaments. We performed open terminal sigmoid colostomies reinforced with either a 10 cm by 10 cm polyvinylidene fluoride (PVDF) or a new TPU/PVDF composite mesh in a total of 10 minipigs. Colostoma was placed paramedian in the left lower abdomen and IPST meshes were fixed intraperitoneal. After 8 weeks, the animals were euthanized after laparoscopic exploration and specimen were explanted for histological investigations. Implantation of a new IPST-mesh with enhanced elastic properties was feasible in a minipig model within an observation period of 8 weeks. Immunohistochemically, Collagen I/III ratio as a marker of tissue integration was significantly higher in TPU-group versus PVDF group (9.4 ± 0.5 vs. 8.1 ± 0.5, p = 0.002) with a significantly lower inflammatory reaction measured by a smaller inner granuloma at mesh-colon interface (17.6 ± 3.3 μm vs. 23 ± 5 μm, p < 0.001). A new TPU/PVDF composite mesh with enhanced elastic properties as IPST was created. Stoma surgery and especially the evaluation of the new stoma mesh prosthesis are feasible with reproducible results in an animal model. Tissue integration expressed by Collagen I/III ratio seems to be improved in comparison to standard-elastic PVDF-IPST meshes.

Implementation of a new constitutive model for abdominal muscles

BACKGROUND AND OBJECTIVE

Abdominal hernia repair is one of the most often performed surgical procedures worldwide. Numerical simulations of the abdominal wall mechanics can be a valuable tool to devise actions aimed at preventing hernia formation. A first step towards this goal is the development of consistent constitutive models for the tissues that form the human abdominal wall. In this study we propose, for each of the tissues involved, a new formulation of the so-called transversely isotropic hyperelastic model (TIHM).

METHODS

We propose a new TIHM for the human abdominal wall tissues and we present a systemic view of the methodology that we have implemented in the present study. First we consider the mathematical background of the TIHM. The novelty of our formulation is that both the isotropic and the fiber contributions to the strain energy function are characterized exclusively by polynomial convex functions of certain invariant quantities. Then, we provide a detailed description on how the constitutive model is implemented into an open source finite element (FE) software. In our approach we use the specific interface provided by the MFront software to incorporate our TIHM formulation into the Code Aster FE solver. For each of the tissues considered, the values of the TIHM constants are adjusted by means of a numerical simulation of previous experimental data from tensile tests.

RESULTS

We studied the following abdominal wall tissues: linea alba, rectus sheath, external oblique muscle, internal oblique muscle, transversus abdominis muscle and rectus abdominis muscle. Our formulation closely reproduces tensile test data for each tissue in the corresponding FE numerical simulation.

CONCLUSIONS

The new TIHM formulation is suitable for a future numerical investigation of the abdominal wall, which will in turn help us to assess the best zone to practice a colostomy. The methodology implemented in the present study can be easily extended in the future to develop and implement a TIHM for active muscles and/or a different type of constitutive model which might be suitable to characterize other tissues of biomedical interest.

Pre-clinical assay of the tissue integration and mechanical adhesion of several types of cyanoacrylate adhesives in the fixation of lightweight polypropylene meshes for abdominal hernia repair

INTRODUCTION

Lightweight (LW) polypropylene (PP) meshes better adapt to host tissue, causing less fibrosis and inflammatory responses than high-density meshes. Mesh fixation using tissue adhesives (TA) that replace conventional sutures may improve the process of hernia repair and tissue trauma. This preclinical study compares the behavior of different cyanoacrylate-based adhesives in the fixation of LW-PP meshes for hernia repair.

METHODS

Partial abdominal wall defects were repaired using LW-PP Optilene meshes in New Zealand rabbits. The following groups were established according to the mesh fixation method: Suture (control), Glubran 2 (n-butyl), Ifabond (n-hexyl), SafetySeal (n-butyl) and Evobond (n-octyl). At 14, 90 and 180 days after surgery, the recovered implants were examined to assess the host tissue integration, the macrophage response and the biomechanical strength.

RESULTS

All the groups showed optimal host tissue incorporation regardless of the fixation procedure. Significantly increased levels of collagen 1 and collagen 3 gene expression (p<0.001) were observed at 14 days compared to the medium- and long-term durations, where the Suture and Glubran groups showed the highest expression of collagen 1. All the adhesives increased the macrophage reaction (p<0.001) compared to sutures at all implant times. Maximal macrophage response was observed in the short-term Glubran group (p<0.01) compared to the rest of the groups. Although SafetySeal and Evobond did not reach the biomechanical resistance of sutures at 14 days, all the adhesives did reach this level in the medium- to long-term periods, providing significantly higher resistance (p<0.05).

CONCLUSIONS

All the cyanoacrylates, despite inducing a significantly increased macrophage response versus sutures, showed optimal host tissue integration and long-term mechanical behavior; thus, they might be good choices for LW-PP mesh hernia repairs.

Surface modification of polypropylene surgical meshes for improving adhesion with poloxamine hydrogel adhesive

Tissue adhesive has notable clinical benefits in hernia repair fixation. A novel poloxamine tissue adhesive was previously shown to successfully bond collagen tissue with adequate adhesive strength. In application related to attachment of polypropylene (PP) mesh, the adhesive strength between the mesh and poloxamine hydrogel adhesive is limited by the hydrophobicity of PP monofilaments and lack of covalent bond formation. The purpose of this study was to compare two different surface modifications [bovine serum albumin (BSA) adsorption and poly-glycidyl methacrylate/human serum albumin (PGMA/HSA) grafting] of PP mesh for improving the adhesive strength between poloxamine hydrogel adhesive and PP mesh. The PGMA/HSA surface modification significantly improved the adhesive strength for meshes attached with poloxamine hydrogel tissue adhesive compared with unmodified meshes and meshes modified by BSA adsorption. An area of 1 cm² adhesive provided for a maximum adhesive strength of 65-70 kPa for meshes modified by PGMA/HSA, 4-13 kPa for meshes modified by BSA, and 22-45 kPa for unmodified meshes. Optical microscopy and infrared spectroscopy (FTIR) confirmed the improved adhesive strength was achieved through mechanical interlock of the hydrogel tissue adhesive into the PP mesh pores and chemical bonding of the albumin after successful PGMA/HSA grafting onto the PP monofilaments. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1047-1055, 2019.

TENSIOMETRIC ANALYSIS OF MESHES USED IN ABDOMINAL VENTRAL WALL DEFECTS IN RATS

BACKGROUND

Among the various strategies to avoid exaggerated foreign body reaction in the treatment of hernias is the limitation of the amount of polypropylene or the use of absorbable material.

AIM

To evaluate the healing of defects in the abdominal wall of rats, comparing microporous polypropylene, macroporous polypropylene and polypropylene/polyglecaprone at the 30º, 60º and 120º postoperative day.

METHODS

Wistar rats were submitted to defect production in the ventral abdominal wall, with integrity of the parietal peritoneum. Prolene®, Ultrapro® and Bard Soft® meshes were used in the correction of the defect. Nine subgroups of 10 animals were submitted to euthanasia at 30th, 60th and 120th postoperative day. Fragments of the abdominal wall of the animals were submitted to tensiometric analysis.

RESULTS

The tensiometry at the 30th postoperative day showed greater resistance of the tissues with Bard Soft® (macroporous mesh) in relation to the tissues with Prolene® (microporous mesh). On the 60th postoperative day Bard Soft® maintained the superior resistance to the tissues comparing to Prolene Mesh®. On the 120th postoperative day the tissues repaired with Ultrapro® (macroporous mesh) proved to be more resistant than the ones by Prolene® (microporous mesh) and Bard Soft® (macroporous mesh).

CONCLUSION

The tissues repaired with macroporous meshes showed greater resistance than with microporous meshes at all stages, and at 120 days postoperative Ultrapro® performed better than the others.

Lightweight, Highly Permeable, Biocompatible, and Antiadhesive Composite Meshes for Intraperitoneal Repairs

Ventral hernia is a public health issue and millions of meshes are used to repair abdominal wall defects every year. Polypropylene-based composite meshes represent an important class of materials for intraperitoneal repair, but the meshes generally give rise to infection, seroma, migration, and adhesion, leading to severe consequence or even reoperation. Here, a facile and versatile one-way fabrication of lightweight, highly permeable, and biocompatible composite meshes with superior antiadhesion properties is proposed by modifying polypropylene meshes with well-defined polydopamine nanocoating. The resulting composite meshes are found to significantly enhance the biocompatibility and antiadhesion effect in rat model. The scalable production and excellent biomedical properties of composite meshes make them a promising candidate for future-generation ventral hernia repair materials.

Mechanical behavior of surgical meshes for abdominal wall repair: In vivo versus biaxial characterization

Despite the widespread use of synthetic meshes in the surgical treatment of the hernia pathology, the election criteria of a suitable mesh for specific patient continues to be uncertain. Thus, in this work, we propose a methodology to determine in advance potential disadvantages on the use of certain meshes based on the patient-specific abdominal geometry and the mechanical features of the certain meshes. To that purpose, we have first characterized the mechanical behavior of four synthetic meshes through biaxial tests. Secondly, two of these meshes were implanted in several New Zealand rabbits with a total defect previously created on the center of the abdominal wall. After the surgical procedure, specimen were subjected to in vivo pneumoperitoneum tests to determine the immediate post-surgical response of those meshes after implanted in a healthy specimen. Experimental performance was recorded by a stereo rig with the aim of obtaining quantitative information about the pressure-displacement relation of the abdominal wall. Finally, following the procedure presented in prior works (Simón-Allué et al., 2015, 2017), a finite element model was reconstructed from the experimental measurements and tests were computationally reproduced for the healthy and herniated cases. Simulations were compared and validated with the in vivo behavior and results were given along the abdominal wall in terms of displacements, stresses and strain. Mechanical characterization of the meshes revealed Surgipro^TM as the most rigid implant and Neomesh SuperSoft® as the softer, while other two meshes (Neomesh Soft®, Neopore®) remained in between. These two meshes were employed in the experimental study and resulted in similar effect in the abdominal wall cavity and both were close to the healthy case. Simulations confirmed this result while showed potential objections in the case of the other two meshes, due to high values in stresses or elongation that may led to discomfort in real tissue. The use of this methodology on human surgery may provide the surgeons with reliable and useful information to avoid certain meshes on specific-patient treatment.

Characterization of the anisotropic mechanical behavior of human abdominal wall connective tissues

Abdominal wall sheathing tissues are commonly involved in hernia formation. However, there is very limited work studying mechanics of all tissues from the same donor which prevents a complete understanding of the abdominal wall behavior and the differences in these tissues. The aim of this study was to investigate the differences between the mechanical properties of the linea alba and the anterior and posterior rectus sheaths from a macroscopic point of view. Eight full-thickness human anterior abdominal walls of both genders were collected and longitudinal and transverse samples were harvested from the three sheathing connective tissues. The total of 398 uniaxial tensile tests was conducted and the mechanical characteristics of the behavior (tangent rigidities for small and large deformations) were determined. Statistical comparisons highlighted heterogeneity and non-linearity in behavior of the three tissues under both small and large deformations. High anisotropy was observed under small and large deformations with higher stress in the transverse direction. Variabilities in the mechanical properties of the linea alba according to the gender and location were also identified. Finally, data dispersion correlated with microstructure revealed that macroscopic characterization is not sufficient to fully describe behavior. Microstructure consideration is needed. These results provide a better understanding of the mechanical behavior of the abdominal wall sheathing tissues as well as the directions for microstructure-based constitutive model.

Modifying hernia mesh design to improve device mechanical performance and promote tension-free repair

PURPOSE

Approximately 348,000 ventral hernia repairs are performed annually in the United States and the incisional hernia recurrence rate is approximately 20% as a result of suture and mesh device failure. Device failure is related to changes at the suture/tissue interface that leads to acute or chronic suture pull-through and surgical failure. To better manage mechanical tension, we propose a modified mesh design with extensions and demonstrate its mechanical superiority.

METHODS

Comparative uniaxial static tensile testing was conducted on polypropylene suture and a modified mesh. Subsequently, a standard of care (SOC) mesh and modified mesh were evaluated using a tensometer in an acute hernia bench-top model.

RESULTS

Modified mesh breaking strength, extension knot breaking strength, extension disruption, and extension anchoring were superior to suture (p < .05). Modified mesh ultimate tensile strength of anchoring was superior to SOC mesh (p < .05). Various stitch patterns and modifications in device design significantly improved device tension-free performance far beyond clinically relevant benchmarks (p < .05).

CONCLUSIONS

Testing demonstrates that the modified mesh outperforms SOC mesh and suture in all tested failure modes. SOC hernia mesh tears through tissue at stress levels below maximum physiologic stress, whereas, the modified hernia mesh is up to 200% stronger than SOC mesh at resisting suture tearing through tissue and maintains anchoring at stresses far beyond clinically relevant benchmarks. Modifying hernia mesh design significantly improves device mechanical performance and enhances tension-free repair.

Mechanical properties of a nanoporous membrane used in implantable medical devices. Correlation between experimental characterization and 2D numerical simulation

Nanoporous membranes are used for the elaboration of implantable medical devices. In order to guaranty their integrity after implantation in a patient body, it is necessary to characterize the microstructure and the mechanical behavior of such membranes. They present randomly distributed pores around 1µm in diameter at the surface. X-ray nanotomography permits to get the geometry of the pores through the thickness with a reduction of the diameter in the core. A multiscale study is done to characterize the membranes: macroscopic tensile tests permit to get the behavior law of the non porous material and in situ tensile tests are carried on in a Scanning Electron Microscope in order to observe the evolution of pores and cracks during loading. A 2D Finite Element Model is also developed in parallel. The confrontation between experiments and numerical simulations permit to validate the accuracy of the model. The latter is then used to simulate several types of loadings considering various pore distributions and sizes.

BIOMECHANICAL PROPERTIES OF MESHES FOLLOWING IMPLANTATION IN THE RAT ABDOMINAL WALL MODEL

The objective of our study was to (1) evaluate mesh strength and collagen incorporation after 4 and 12 weeks of implantation in a rat abdominal wall model and (2) determine the relationship between collagen deposition and mechanical strength of a chitosan-coated polypropylene mesh. We implanted 0.5% chitosan-coated polypropylene mesh (PPM), collagen-coated PPM (PelvitexTM; C.R. Bard), and PPM (Avaulta Solo[Formula: see text]; C.R. Bard) using a rat abdominal defect model. Mechanical properties were determined from uniaxial tensile testing and collagen deposition of each mesh was evaluated 4 and 12 weeks post-implantation. We found that after implantation, the neo tissue of Ch-PPM is stiffer than the commercially available meshes. We also observed no significant difference in the ratio of collagen types I/III between mesh samples at 4 weeks or 12 weeks. We found no relationship between the ratio of collagen types I/III and the mechanical strength of mesh samples after implantation. The increased stiffness with chitosan coating could be due to increased muscle tissue ingrowth.

MECHANICAL CHARACTERIZATION OF ANIMAL DERIVED GRAFTS FOR SURGICAL IMPLANTATION

Bioprostheses obtained from animal models are often adopted in abdominal surgery for repair and reconstruction. The functionality of these prosthetic implants is related also to their mechanical characteristics that are analyzed here. This work illustrates a constitutive model to describe the short-term mechanical response of Permacol[Formula: see text] bioprostheses. Experimental tests were developed on tissue samples to highlight mechanical non-linear characteristics and viscoelastic phenomena. Uni-axial tensile tests were developed to evaluate the strength and strain stiffening. Incremental uni-axial stress relaxation tests were carried out at nominal strain ranging from 10% to 20% and to monitor the stress relaxation process up to 400[Formula: see text]s. The constitutive model effectively describes the mechanical behavior found in experimental testing. The mechanical response appears to be independent on the loading direction, showing that the tissue can be considered as isotropic. The viscoelastic response of the tissue shows a strong decay of the stress in the first seconds of the relaxation process. The investigation performed is aimed at a general characterization of the biomechanical response and addresses the development of numerical models to evaluate the biomechanical performance of the graft with surrounding host tissues.

Synthetic surgical meshes used in abdominal wall surgery: Part II-Biomechanical aspects

This work reports the second part of a review on synthetic surgical meshes used for abdominal hernia repair. While material and structural characteristics, together with mesh-tissue interaction, were considered in a previous article (Part I), biomechanical behavior is described here in more detail. The role of the prosthesis is to strengthen the impaired abdominal wall, mimicking autologous tissue without reducing its compliance. Consequently, mesh mechanical properties play a crucial role in a successful surgical repair. The main available techniques for mechanical testing, such as uniaxial and biaxial tensile testing, ball burst, suture retention strength, and tear resistance testing, are described in depth. Among these methods, the biaxial tensile test is the one that can more faithfully reproduce the physiological loading condition. An outline of the most significant results documented in the literature is reported, showing the variety of data on mesh mechanical properties. Synthetic surgical meshes generally follow a non-linear stress-strain behavior, with mechanical characteristics dependant on test direction due to mesh anisotropy. Ex-vivo tests revealed an increased stiffness in mesh explants due to the gradual ingrowth of the host tissue after implant. In general, the absence of standardization in test methods and terminology makes it difficult to compare results from different studies. Numerical models of the abdominal wall interacting with surgical meshes were also discussed representing a potential tool for the selection of suitable prostheses. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 892-903, 2017.

Comparison of two different concepts of mesh and fixation technique in laparoscopic ventral hernia repair: a randomized controlled trial

BACKGROUND

Patients' need to improve outcomes and to reduce the number of complications triggers the development of new materials and surgery concepts. Currently, there are many implants and fixation systems dedicated for intraperitoneal onlay mesh procedure. The aim of this study was to compare two different mesh/fixation system concepts (PH: Physiomesh/Securestrap and VS: Ventralight ST/SorbaFix) for laparoscopic ventral hernia repair with respect to pain.

METHODS

A single-center, prospective, randomized study was designed to include 50 patients per group with a planned interim analysis for safety after 25 patients. The endpoints were pain occurrences and intensity, which was measured with the visual analogue scale 7 days, 30 days, 3 months and 6 months after surgery. The safety parameters included the number of recurrences and postoperative complications.

RESULTS

During the interim analysis, the study was stopped due to safety reasons. We observed five (20 %) recurrences in the PH group in first 6 months and none in the VS group. We observed a significantly higher pain rate in the PH group after 3 months (p < 0.0001) and no difference after 7 days (p = 0. 7019). The pain intensity decreased significantly over time (p < 0.0001) and was significantly higher in the PH group (p < 0.0001).

CONCLUSIONS

Although this clinical trial was terminated prior to the preplanned recruitment goal, the obtained results from the enrolled patients indicate that the PH system associated with significantly greater hernia recurrences and postoperative pain compared with the VS system. This confirms the superiority of the elastic mesh concept, which may be a safer and more efficacious option for laparoscopic ventral hernia repairs.

Mechanical properties of mosquito nets in the context of hernia repair

The paper deals with issue of applying mosquito nets as implants in hernia repair, which have already been used in resource-poor developing countries. Uniaxial tensile tests have been conducted on polyester mosquito meshes in two orthogonal directions. Non-linear elastic constitutive laws parameters have been identified to be applied in dense net material models. Mechanical performance of tested mosquito nets has been compared with properties of commercial implants used in treatment of hernia and with properties of human tissue. This study contributes to mechanical knowledge of hernia repair issue by investigation of cheaper alternative to commercial implants.

Biomechanical behavior of pericardial human tissue: a constitutive formulation

This work aims to present a constitutive model suitable to interpret the biomechanical response of human pericardial tissues. The model is consistent with the need of describing large strains, anisotropy, almost incompressibility, and time-dependent effects. Attention is given to human pericardial tissue because of the increased interest in its application as a substitute in reconstructive surgery. Specific, even limited, experimental investigation has been performed on human samples taken from surgical grafts in order to verify the capability of the constitutive model in supplying a correct description of tissue mechanical response. Experimental data include uni-axial tensile tests and stress relaxation tests up to 300 s, developed along different directions of the tissue. The grafts tested show different mechanical characteristics for what concern the level of anisotropy of the tissue. The constitutive model proposed shows to adapt to the different configurations of the human pericardium grafts, as emerged by experimental data considered, and it is capable to describe the variability of the mechanical characteristics.

ECM hydrogel coating mitigates the chronic inflammatory response to polypropylene mesh

Polypropylene has been used as a surgical mesh material for several decades. This non-degradable synthetic polymer provides mechanical strength, a predictable host response, and its use has resulted in reduced recurrence rates for ventral hernia and pelvic organ prolapse. However, polypropylene and similar synthetic materials are associated with a chronic local tissue inflammatory response and dense fibrous tissue deposition. These outcomes have prompted variations in mesh design to minimize the surface area interface and increase integration with host tissue. In contrast, biologic scaffold materials composed of extracellular matrix (ECM) are rapidly degraded in-vivo and are associated with constructive tissue remodeling and minimal fibrosis. The objective of the present study was to assess the effects of an ECM hydrogel coating on the long-term host tissue response to polypropylene mesh in a rodent model of abdominal muscle injury. At 14 days post implantation, the ECM coated polypropylene mesh devices showed a decreased inflammatory response as characterized by the number and distribution of M1 macrophages (CD86+/CD68+) around mesh fibers when compared to the uncoated mesh devices. At 180 days the ECM coated polypropylene showed decreased density of collagen and amount of mature type I collagen deposited between mesh fibers when compared to the uncoated mesh devices. This study confirms and extends previous findings that an ECM coating mitigates the chronic inflammatory response and associated scar tissue deposition characteristic of polypropylene.

Varying degrees of nonlinear mechanical behavior arising from geometric differences of urogynecological meshes

Synthetic polypropylene meshes were designed to restore pelvic organ support for women suffering from pelvic organ prolapse; however, the FDA released two notifications regarding potential complications associated with mesh implantation. Our aim was to characterize the structural properties of Restorelle and UltraPro subjected to uniaxial tension along perpendicular directions, and then model the tensile behavior of these meshes utilizing a co-rotational finite element model, with an imbedded linear or fiber-recruitment local stress-strain relationship. Both meshes exhibited a highly nonlinear stress-strain behavior; Restorelle had no significant differences between the two perpendicular directions, while UltraPro had a 93% difference in the low (initial) stiffness (p=0.009) between loading directions. Our model predicted that early alignment of the mesh segments in the loading direction and subsequent stretching could explain the observed nonlinear tensile behavior. However, a nonlinear stress-strain response in the stretching regime, that may be inherent to the mesh segment, was required to better capture experimental results. Utilizing a nonlinear fiber recruitment model with two parameters A and B, we observed improved agreement between the simulations and the experimental results. An inverse analysis found A=120 MPa and B=1.75 for Restorelle (RMSE=0.36). This approach yielded A=30 MPa and B=3.5 for UltraPro along one direction (RMSE=0.652), while the perpendicular orientation resulted in A=130 MPa and B=4.75 (RMSE=4.36). From the uniaxial protocol, Restorelle was found to have little variance in structural properties along these two perpendicular directions; however, UltraPro was found to behave anisotropically.

Characterizing the ex vivo mechanical properties of synthetic polypropylene surgical mesh

The use of synthetic polypropylene mesh for hernia surgical repair and the correction of female pelvic organ prolapse have been controversial due to increasing post-operative complications, including mesh erosion, chronic pain, infection and support failure. These morbidities may be related to a mismatch of mechanical properties between soft tissues and the mesh. The aim of this study was to gain a better understanding of the biomechanical behavior of Prolene polypropylene mesh (Ethicon, Sommerville, NJ, USA), which is widely used for a variety of surgical repair procedures. The stiffness and permanent deformation of Prolene mesh were compared in different directions by performing uniaxial tensile failure tests, cyclic and creep tests at simulated physiological loads in the coursewise (0°), walewise (90°) and the diagonal (45°) directions. Failure tests suggest that the mechanical properties of the mesh is anisotropic; with response at 0° being the most compliant while 90° was the stiffest. Irreversible deformation and viscoelastic behavior were observed in both cyclic and creep tests. The anisotropic property may be relevant to the placement of mesh in surgery to maximize long term mesh performance. The considerable permanent deformation may be associated with an increased risk of post-operative support failure.