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Synthetic surgical meshes used in abdominal wall surgery: Part I—materials and structural conformation

Surgical implants are commonly used in abdominal wall surgery for hernia repair. Many different prostheses are currently offered to surgeons, comprising permanent synthetic polymer meshes and biologic scaffolds. There is a wide range of synthetic meshes currently available on the market with differing chemical compositions, fiber conformations, and mesh textures. These chemical and structural characteristics determine a specific biochemical and mechanical behavior and play a crucial role in guaranteeing a successful post-operative outcome. Although an increasing number of studies report on the structural and mechanical properties of synthetic surgical meshes, nowadays there are no consistent guidelines for the evaluation of mechanical biocompatibility or common criteria for the selection of prostheses. The aim of this work is to review synthetic meshes by considering the extensive bibliography documentation of their use in abdominal wall surgery, taking into account their material and structural properties, in Part I, and their mechanical behavior, in Part II. The main materials available for the manufacture of polymeric meshes are described, including references to their chemical composition, fiber conformation, and textile structural properties. These characteristics are decisive for the evaluation of mesh-tissue interaction process, including foreign body response, mesh encapsulation, infection, and adhesion formation.

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.

Experimental investigation of the biomechanics of urethral tissues and structures.

What is the central question of this study? Prostheses for treatment of urinary incontinence elicit complications associated with an inadequate mechanical action. This investigation aimed to define a procedure addressed to urethral mechanical characterization. Experimental tests are the basis for constitutive formulation, with a view to numerical modelling for investigation of the interaction between the tissues and a prosthesis. What is the main finding and its importance? Horse urethra, selected for its histomorphometric similarity to human urethra, was characterized by integrated histological analysis and mechanical tests on the biological tissue and structure, leading to constitutive formulation. A non‐linear, anisotropic and time‐dependent response was found, representing a valid basis for development of a numerical model to interpret the functional behaviour of the urethra.Urinary dysfunction can lead to incontinence, with relevant impact on the quality of life. This severe dysfunction can be surgically overcome by using an artificial urinary sphincter. However, several complications may result from an inappropriate prosthesis functionality, in many cases due to an unsuitable mechanical action of the device on urethral tissues. Computational models allow the investigation of mechanical interaction between biological tissues and biomedical devices, representing a potential support for surgical practice and prosthesis design. The development of such computational tools requires experimental data on biological tissues and structures mechanics, which are rarely reported in the literature. The present activities aim at providing a procedure for the mechanical characterization of urethral tissues and structures. The experimental protocol includes the morphometric and histologic analysis of urethral tissues, the mechanical characterization of tissues response by tensile and stress relaxation tests and the evaluation of urethral structural behavior by inflation tests. Results from preliminary experimental activities are processed adopting specific model formulations, also providing the definition of parameters that identify elastic and viscous tissues behavior. Different experimental protocols, leading to a comprehensive set of experimental data, allow for a reciprocal assessment of reliability of the investigation approach. This article is protected by copyright. All rights reserved

A numerical investigation of the healthy abdominal wall structures

The present work aims to assess, via numerical modeling, the global passive mechanical behavior of the healthy abdominal wall under the action of pressures that characterize different daily tasks and physiological functions. The evaluation of a normal range of intra-abdominal pressure (IAP) during activities of daily living is fundamental because pressure alterations can cause several adverse effects. At this purpose, a finite element model is developed from literature histomorphometric data and from diagnostic images of Computed Tomography (CT), detailing the different anatomical regions. Numerical simulations cover an IAP up to the physiological limit of 171 (0.0223MPa) mmHg reached while jumping. Numerical results are in agreement with evidences on physiological abdomens when evaluating the local deformations along the craniocaudal direction, the transversal load forces in different regions and the increase of the abdominal area at a IAP of 12mmHg. The developed model can be upgraded for the investigation of the abdominal hernia repair and the assessment of prostheses mechanical compatibility, correlating stiffness and tensile strength of the abdominal tissues with those of surgical meshes.

Mechanics of the urethral duct: tissue constitutive formulation and structural modeling for the investigation of lumen occlusion

Urinary incontinence, often related to sphincter damage, is found in male patients, leading to a miserable quality of life and to huge costs for the healthcare system. The most effective surgical solution currently considered for men is the artificial urinary sphincter that exerts a pressure field on the urethra, occluding the duct. The evaluation of this device is currently based on clinical and surgical competences. The artificial sphincter design and mechanical action can be investigated by a biomechanical model of the urethra under occlusion, evaluating the interaction between tissues and prosthesis. A specific computational approach to urethral mechanics is here proposed, recalling the results of previous biomechanical experimental investigation. In this preliminary analysis, the horse urethra is considered, in the light of a significant correlation with human and in consideration of the relevant difficulty to get to human samples, which anyway represents the future advance. Histological data processing allow for the definition of a virtual and a subsequent finite element model of a urethral section. A specific hyperelastic formulation is developed to characterize the nonlinear mechanical behavior. The inverse analysis of tensile tests on urethra samples leads to the definition of preliminary constitutive parameters. The parameters are further refined by the computational analysis of inflation tests carried out on the entire urethral structure. The results obtained represent, in the light of the correlation reported, a valid preliminary support for the information to be assumed for prosthesis design, integrating surgical and biomechanical competences.

Biomechanical properties of synthetic surgical meshes for pelvic prolapse repair.

Synthetic meshes are widely used for surgical repair of different kind of prolapses. In the light of the experience of abdominal wall repair, similar prostheses are currently used in the pelvic region, to restore physiological anatomy after organ prolapse into the vaginal wall, that represent a recurrent dysfunction. For this purpose, synthetic meshes are surgically positioned in contact with the anterior and/or posterior vaginal wall, to inferiorly support prolapsed organs. Nonetheless, while mesh implantation restores physiological anatomy, it is often associated with different complications in the vaginal region. These potentially dangerous effects induce the surgical community to reconsider the safety and efficacy of mesh transvaginal placement. For this purpose, the evaluation of state-of-the-art research may provide the basis for a comprehensive analysis of mesh compatibility and functionality. The aim of this work is to review synthetic surgical meshes for pelvic organs prolapse repair, taking into account the mechanics of mesh material and structure, and to relate them with pelvic and vaginal tissue biomechanics. Synthetic meshes are currently available in different chemical composition, fiber and textile conformations. Material and structural properties are key factors in determining mesh biochemical and mechanical compatibility in vivo. The most significant results on vaginal tissue and surgical meshes mechanical characterization are here reported and discussed. Moreover, computational models of the pelvic region, which could support the surgeon in the evaluation of mesh performances in physiological conditions, are recalled.

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 PermacolTM 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 400s. 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.

Urethral lumen occlusion by artificial sphincteric devices: a computational biomechanics approach

The action induced by artificial sphincteric devices to provide urinary continence is related to the problem of evaluating the interaction between the occlusive cuff and the urethral duct. The intensity and distribution of the force induced within the region of application determine a different occlusion process and potential degradation of the urethral tissue, mostly at the borders of the cuff. This problem is generally considered in the light of clinical and surgical operational experience, while a valid cooperation is established with biomechanical competences by means of experimental and numerical investigation. A three-dimensional model of the urethra is proposed aiming at a representation of the phases of the urethral occlusion through artificial sphincters. Different conformations of the cuff are considered, mimicking different loading conditions in terms of force intensity and distribution and consequent deformation caused in soft tissues. The action induced in the healthy urethra is investigated, as basis for an evaluation of the efficacy and reliability of the sphincteric devices. The problem is characterized by coupled nonlinear geometric and material problem and entails a complex constitutive formulation. A heavy computational procedure is developed by means of analyses that operate within an explicit finite element formulation. Results reported outline the overall response of the urethral duct during lumen occlusion, leading to an accurate description of the phenomenon in the different phases.

Evaluation of the mechanical behaviour of Telemark ski boots: Part II – structural analysis

This work reports the evaluation of the mechanical behaviour of Telemark ski boots, by means of an integrated approach, considering polymeric material characterisation, reported in a previous study (Part I), and numerical structural analysis, reported in this study (Part II). Telemark boots entail a complex procedure for analysis of the mechanical response, with regard to both material assumption and overall structural behaviour, and represent a reference problem within sky footwear in consideration of this peculiar complexity. A visco-elastic constitutive model is formulated to describe the material mechanical response defined in accordance with experimental test performed. Solid models define the morphology of the ski boots and represent the basis for numerical modelling. Specific boundary conditions are assumed to mimic the binding effect. The numerical analysis leads to an interpretation of the global and local responses of a specific Telemark ski boot, considering material properties and structural conformation. The results provide valid information on the ski boot mechanical response, taking into account also the effects of temperature on material response and strain rate with regard to loading conditions. The evaluation of variations in shape and material of the different components can be performed representing a fundamental support for the design.

Evaluation of the mechanical behaviour of Telemark ski boots: Part I – materials characterization in use conditions

This study reports the first part of the analysis for the evaluation of the mechanical behaviour of ski boots by means of an integrated approach that considers polymeric materials characterization, in Part I, and numerical structural analysis, in Part II. In the present Part I, different techniques are adopted to characterize the mechanical behaviour of the polymeric materials used for ski boots, to define the elastic, visco-elastic, temperature and weathering-dependent characteristics. Experimental data provide fundamental information on mechanical response, in particular taking into account the effect of the environmental conditions, due to temperature variation, ultraviolet radiation and water absorption, which are correlated to the definition of reliability and durability of the materials. In more detail, experimental results from tensile tests and dynamic mechanical analysis are reported, evaluating mechanical response and chemical conformation of the polymers. Materials properties are correlated with the specific use conditions and boot structure, to be able to evaluate and preserve overall performances and general safety requirements of ski boots. This activity represents a reference procedure for the evaluation of the material mechanical behaviour that must be considered within the structural analysis.