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. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 689-699, 2017.

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.

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.

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.

Bladder tissue biomechanical behavior: Experimental tests and constitutive formulation.

A procedure for the constitutive analysis of bladder tissues mechanical behavior is provided, by using a coupled experimental and computational approach. The first step pertains to the design and development of mechanical tests on specimens from porcine bladders. The bladders have been harvested, and the specimens have been subjected to uniaxial cyclic tests at different strain rates along preferential directions, considering the distribution of tissue fibrous components. Experimental results showed the anisotropic, non-linear and time-dependent stress-strain behavior, due to tissue conformation with fibers distributed along preferential directions and their interaction phenomena with ground substance. In detail, experimental data showed a greater tissue stiffness along transversal direction. Viscous behavior was assessed by strain rate dependence of stress-strain curves and hysteretic phenomena. The second step pertains the development of a specific fiber-reinforced visco-hyperelastic constitutive model, in the light of bladder tissues structural conformation and experimental results. Constitutive parameters have been identified by minimizing the discrepancy between model and experimental data. The agreement between experimental and model results represent a term for evaluating the reliability of the constitutive models by means of the proposed operational procedure.

Constitutive modeling of time-dependent response of human plantar aponeurosis.

The attention is focused on the viscoelastic behavior of human plantar aponeurosis tissue. At this purpose, stress relaxation tests were developed on samples taken from the plantar aponeurosis of frozen adult donors with age ranging from 67 to 78 years, imposing three levels of strain in the physiological range (4%, 6%, and 8%) and observing stress decay for 240 s. A viscohyperelastic fiber-reinforced constitutive model with transverse isotropy was assumed to describe the time-dependent behavior of the aponeurotic tissue. This model is consistent with the structural conformation of the tissue where collagen fibers are mainly aligned with the proximal-distal direction. Constitutive model fitting to experimental data was made by implementing a stochastic-deterministic procedure. The stress relaxation was found close to 40%, independently of the level of strain applied. The agreement between experimental data and numerical results confirms the suitability of the constitutive model to describe the viscoelastic behaviour of the plantar aponeurosis.

Investigation of the mechanical behaviour of the foot skin.

BACKGROUND/PURPOSE: The aim of this work was to provide computational tools for the characterization of the actual mechanical behaviour of foot skin, accounting for results from experimental testing and histological investigation. Such results show the typical features of skin mechanics, such as anisotropic configuration, almost incompressible behaviour, material and geometrical non linearity. The anisotropic behaviour is mainly determined by the distribution of collagen fibres along specific directions, usually identified as cleavage lines. METHODS: To evaluate the biomechanical response of foot skin, a refined numerical model of the foot is developed. The overall mechanical behaviour of the skin is interpreted by a fibre-reinforced hyperelastic constitutive model and the orientation of the cleavage lines is implemented by a specific procedure. Numerical analyses that interpret typical loading conditions of the foot are performed. The influence of fibres orientation and distribution on skin mechanics is outlined also by a comparison with results using an isotropic scheme. RESULTS: A specific constitutive formulation is provided to characterize the mechanical behaviour of foot skin. The formulation is applied within a numerical model of the foot to investigate the skin functionality during typical foot movements. Numerical analyses developed accounting for the actual anisotropic configuration of the skin show lower maximum principal stress fields than results from isotropic analyses. CONCLUSION: The developed computational models provide reliable tools for the investigation of foot tissues functionality. Furthermore, the comparison between numerical results from anisotropic and isotropic models shows the optimal configuration of foot skin.

Characterization of the anisotropic mechanical behaviour of colonic tissues: experimental activity and constitutive formulation.

The aim was to investigate the biomechanical behaviour of colonic tissues by a coupled experimental and numerical approach. The wall of the colon is composed of different tissue layers. Within each layer, different fibre families are distributed according to specific spatial orientations, which lead to a strongly anisotropic configuration. Accounting for the complex histology of the tissues, mechanical tests must be planned and designed to evaluate the behaviour of the colonic wall in different directions. Uni-axial tensile tests were performed on tissue specimens from 15 fresh pig colons, accounting for six different loading directions (five specimens for each loading direction). The next step of the investigation was to define an appropriate constitutive framework and develop a procedure for identification of the constitutive parameters. A specific hyperelastic formulation was developed that accounted for the multilayered conformation of the colonic wall and the fibre-reinforced configuration of the tissues. The parameters were identified by inverse analyses of the mechanical tests. The comparison of model results with experimental data, together with the evaluation of satisfaction of material thermomechanics principles, confirmed the reliability of the analysis developed. This work forms the basis for more comprehensive activities that aim to provide computational tools for the interpretation of surgical procedures that involve the gastrointestinal tract, considering the specific biomedical devices adopted.

Analysis of the biomechanical behaviour of gastrointestinal regions adopting an experimental and computational approach.

An integrated experimental and computational procedure is provided for the evaluation of the biomechanical behaviour that characterizes the pressure-volume response of gastrointestinal regions. The experimental activity pertains to inflation tests performed on specific gastrointestinal conduct segments. Different inflation processes are performed according to progressively increasing volumes. Each inflation test is performed by a rapid liquid in-flaw, up to a prescribed volume, which is held constant for about 300s to allow the development of relaxation processes. The different tests are interspersed by 600s of rest to allow the recovery of the specimen mechanical condition. A physio-mechanical model is developed to interpret both the elastic behaviour of the sample, as the pressure-volume trend during the rapid liquid in-flaw, and the time-dependent response, as the pressure drop during the relaxation processes. The minimization of discrepancy between experimental data and model results entails the identification of the parameters that characterize the viscoelastic model adopted for the definition of the behaviour of the gastrointestinal regions. The reliability of the procedure is assessed by the characterization of the response of samples from rat small intestine.

Biomechanical behaviour of ankle ligaments: constitutive formulation and numerical modelling.

This study was aimed at the definition of a constitutive formulation of ankle ligaments and of a procedure for the constitutive parameters evaluation, for the biomechanical analysis by means of numerical models. To interpret the typical features of ligaments mechanical response, as anisotropic configuration, geometric non-linearity, non-linear elasticity and time-dependent behaviour, a specific fibre-reinforced visco-hyperelastic model is provided. The identification of constitutive parameters is performed by a stochastic-deterministic procedure that minimises the discrepancy between experimental and computational results. A preliminary evaluation of parameters is performed by analytical models in order to define reference values. Afterwards, solid models are developed to consider the complex histo-morphometric configuration of samples as a basis for the definition of numerical models. The results obtained are adopted for upgrading parameter values by comparison with specific mechanical tests. Assuming the new parameters set, the final numerical results are compared with the overall set of experimental data, to assess the reliability and efficacy of the analysis developed for the interpretation of the mechanical response of ankle ligaments.

Analysis of heel pad tissues mechanics at the heel strike in bare and shod conditions.

A combined experimental and numerical approach is used to investigate the interaction phenomena occurring between foot and footwear during the heel strike phase of the gait. Two force platforms are utilised to evaluate the ground reaction forces of a subject in bare and shod walking. The reaction forces obtained from the experimental tests are assumed as loading conditions for the numerical analyses using three dimensional models of the heel region and of the running shoe. The heel pad region, as fat and skin tissues, is described by visco-hyperelastic and fibre-reinforced hyperelastic formulations respectively and bone region by a linear orthotropic formulation. Different elastomeric foams are considered with regard to the outsole, the midsole and the insole layers. The mechanical properties are described by a hyperfoam formulation. The evaluation of the mechanical behaviour of the heel pad tissues at the heel strike in bare and shod conditions is performed considering different combinations of materials for midsole and insole layers. Results allow for the definition of the influence of different material characteristics on the mechanical response of the heel pad region, in particular showing the compressive stress differentiation in the bare and shod conditions.

Investigation on the load-displacement curves of a human healthy heel pad: In vivo compression data compared to numerical results.

The aims of the present work were to build a 3D subject-specific heel pad model based on the anatomy revealed by MR imaging of a subject's heel pad, and to compare the load-displacement responses obtained from this model with those obtained from a compression device used on the subject's heel pad. A 30 year-old European healthy female (mass=54kg, height=165cm) was enrolled in this study. Her left foot underwent both MRI and compression tests. A numerical model of the heel region was developed based on a 3D CAD solid model obtained by MR images. The calcaneal fat pad tissue was described with a visco-hyperelastic model, while a fiber-reinforced hyperelastic model was formulated for the skin. Numerical analyses were performed to interpret the mechanical response of heel tissues. Different loading conditions were assumed according to experimental tests. The heel tissues showed a non-linear visco-elastic behavior and the load-displacement curves followed a characteristic hysteresis form. The energy dissipation ratios measured by experimental tests (0.25±0.02 at low strain rate and 0.26±0.03 at high strain rate) were comparable with those evaluated by finite element analyses (0.23±0.01 at low strain rate and 0.25±0.01 at high strain rate). The validity and efficacy of the investigation performed was confirmed by the interpretation of the mechanical response of the heel tissues under different strain rates. The mean absolute percentage error between experimental data and model results was 0.39% at low strain rate and 0.28% at high strain rate.

A numerical model for investigating the mechanics of calcaneal fat pad region.

The present paper pertains to the definition of a numerical model of the calcaneal fat pad region, considering a structure composed of adipose and connective tissues organized in fibrous septae and adipose chambers. The mechanical response is strongly influenced by the structural conformation, as the dimension of adipose chambers, the thickness of connective septae walls and the mechanical properties of the different soft tissues. In order to define the constitutive formulation of adipose tissues, experimental data from pig specimens are considered, according to the functional similarity, while the mechanical response of connective tissue septae is assumed with regard to the mechanical behaviour that characterize ligaments. Different numerical models are provided accounting for the variation of chambers dimensions, septae wall thickness and tissues characteristics. The spiral angles of collagen fibres within the septae influence the capability of the structure to withstand the bulging of chambers. The analysis considers different orientation of the fibres. The response of calcaneal fat pad region is evaluated in comparison with experimental data from unconfined compression tests. The present work provides a preliminary approach to enhance the correlation between the structural conformation and tissues mechanical properties towards the biomechanical response of overall heel pad region.

Constitutive formulation and numerical analysis of the heel pad region.

The aim of this work is to provide a numerical approach for the investigation of the mechanical behaviour of the heel pad region. A visco-hyperelastic model is formulated with regard to fat pad tissue, while a fibre-reinforced hyperelastic model is considered for the heel skin tissue. Bone components are defined by means of an orthotropic linear elastic model. Particular attention is paid to the evaluation of constitutive parameters within different models adopted in consideration of experimental tests data. Preliminarily, indentation tests on a skinless cadaveric foot are considered with regard to fat pad tissue. Indentation tests on an intact heel pad of a cadaveric foot are subsequently adopted for the final identification of constitutive parameters of fat pad and skin tissues. A numerical model of the rear foot is defined and different loading conditions are assumed according to experimental data. A comparison between experimental and numerical data leads to the evaluation of the real capability of the procedure to interpret the actual response of the rear foot.

Investigation of the mechanical properties of the plantar aponeurosis.

INTRODUCTION: The aim of this work was to obtain a preliminary investigation of the mechanical properties of the human plantar aponeurosis based on regional observation, in order to rationally plan a subsequent larger experimental campaign and develop suited constitutive models to characterize the mechanical response of this tissue. MATERIALS AND METHODS: Different in vitro mechanical tests were developed on eleven samples taken from the plantar aponeurosis of human cadaver (man, age 78 years). The samples were tested along the distal-proximal direction. Range of elasticity of the tissue, development of damage phenomena and stress relaxation at different levels of strain were evaluated. RESULTS: The strength of the tissue was found in the order of that proposed in previous works, with peak stress of about 12.5 MPa. The compliance of the plantar aponeurosis was in line with in vivo evaluation. A softening behaviour appeared for tensile strain larger than 12%. In relaxation tests, the stress was reduced of 35-40% in 120 s. The percentage stress relaxation was found independent on the level of the applied strain. DISCUSSION: The evaluation of the mechanical characteristics is fundamental for a subsequent development of numerical models of the plantar aponeurosis. Such approach is helpful to understand its response to overuse, but also to understand the clinical results of different manual and physical therapies that use warm, pressure or stretch to modify this tissue.

Biomechanical behaviour of heel pad tissue: experimental testing, constitutive formulation, and numerical modelling.

This paper deals with the constitutive formulation of heel pad tissue and presents a procedure for identifying constitutive parameters using experimental data, with the aim of developing a computational approach for investigating the actual biomechanical response. The preliminary definition of constitutive parameters was developed using a visco-hyperelastic formulation, considering experimental data from in vitro compression tests on specimens of fat pad tissue and data from in vivo tests to identify the actual trend of tissue stiffness. The discrepancy between model results and experimental data was evaluated on the basis of a specific cost function, adopting a stochastic/deterministic procedure. The parameter evaluation was upgraded by considering experimental tests performed on the fat pad tissues of a cadaveric foot using in situ indentation tests at 0.01 and 350 mm/s strain rates. The constitutive formulation was implemented in a numerical model. The comparison of data from in situ tests and numerical results led to an optimal domain of parameters based on an admissible discrepancy criterion. Numerical results evaluated for different sets of parameters inside the domain are reported and compared with experimental data for a reliability evaluation of the proposed procedure.

Investigation of foot plantar pressure: experimental and numerical analysis.

The analysis of interaction phenomena occurring between the plantar region of the foot and insole was investigated using a combined experimental-numerical approach. Experimental data on the plantar pressure for treadmill walking of a subject were obtained using the Pedar(®) system. The plantar pressure resultant was monitored during walking and adopted to define the loading conditions for a subsequent static numerical analysis. Geometrical configuration of the foot model is provided on the basis of biomedical images. Because the mechanical behaviour of adipose tissues and plantar fascia is the determinant factor in affecting the paths of the plantar pressure, specific attention was paid to define an appropriate constitutive model for these tissues. The numerical model included sole and insole, providing for friction contact conditions between foot-insole and insole-sole pairs as well. Two different numerical analyses were performed with regards to different loading conditions during the gait cycle. The plantar pressure peaks predicted by the numerical model for the two loading conditions are 0.16 and 0.12 MPa, and 0.09 and 0.12 MPa in the posterior and anterior regions of the foot, respectively. These values are in agreement with experimental evidence, showing the suitability of the model proposed.

Constitutive formulation and analysis of heel pad tissues mechanics.

This paper presents a visco-hyperelastic constitutive model developed to describe the biomechanical response of heel pad tissues. The model takes into account the typical features of the mechanical response such as large displacement, strain phenomena, and non-linear elasticity together with time-dependent effects. The constitutive model was formulated, starting from the analysis of the complex structural and micro-structural configuration of the tissues, to evaluate the relationship between tissue histology and mechanical properties. To define the constitutive model, experimental data from mechanical tests were analyzed. To obtain information about the mechanical response of the tissue so that the constitutive parameters could be established, data from both in vitro and in vivo tests were investigated. Specifically, the first evaluation of the constitutive parameters was performed by a coupled deterministic and stochastic optimization method, accounting for data from in vitro tests. The comparison of constitutive model results and experimental data confirmed the model's capability to describe the compression behaviour of the heel pad tissues, regarding both constant strain rate and stress relaxation tests. Based on the data from additional experimental tests, some of the constitutive parameters were modified in order to interpret the in vivo mechanical response of the heel pad tissues. This approach made it possible to interpret the actual mechanical function of the tissues.

Interaction phenomena between oral implants and bone tissue in single and multiple implant frames under occlusal loads and misfit conditions: A numerical approach.

An investigation is carried out on the effects induced in bone tissue surrounding oral implants placed in the premolar region of a mandible by using a numerical approach. In particular, a single implant and a multiple implant frame under loading are considered. The effects of accuracy in the coupling of the connecting bar and implants are evaluated. The mechanical response of the bone-oral implant system, depending on the different mechanical properties assumed for the peri-implant bone tissue during the evolutionary trend of osseointegration, is studied. A further task regard to the comparison of the mechanical state induced in the bone depending on the loading conditions considered. Effects of physiological occlusal loads are compared with ones given by framework defects arising from the specific manufacturing process, such as misfit between the implants and the connecting bar. The investigation offers the basis for an integrated clinical and biomechanical evaluation of the effects induced on peri-implant bone, depending on bone properties, implant system configuration, and the actions induced. Analyses performed show that stress states induced by the investigated type of misfit are comparable to those arising from the application of physiological loading conditions.

Experimental-numerical analysis of minipig's multi-rooted teeth.

The paper pertains to the analysis of the biomechanical behaviour of the periodontal ligament (PDL) by using a combined experimental and numerical approach. Experimental analysis provides information about a two-rooted pig premolar tooth in its socket with regard to morphological configuration and deformational response. The numerical analysis developed for the present investigation adopts a specific anisotropic hyperelastic formulation, accounting for tissue structural arrangement. The parameters to be adopted for the PDL constitutive model are evaluated with reference to data deducted from experimental in vitro tests on different specimens taken from literature. According to morphometric data relieved, solid models are provided as basis for the development of numerical models that adopt the constitutive formulation proposed. A reciprocal validation of experimental and numerical data allows for the evaluation of reliability of results obtained. The work is intended as preliminary investigation to study the correlation between mechanical status of PDL and induction to cellular activity in orthodontic treatments.