A finite element analysis of diaphragmatic hernia repair on an animal model.

The diaphragm is a mammalian skeletal muscle that plays a fundamental role in the process of respiration. Alteration of its mechanical properties due to a diaphragmatic hernia contributes towards compromising its respiratory functions, leading to the need for surgical intervention to restore the physiological conditions by means of implants. This study aims to assess via numerical modeling biomechanical differences between a diaphragm in healthy conditions and a herniated diaphragm surgically repaired with a polymeric implant, in a mouse model. Finite Element models of healthy and repaired diaphragms are developed from diagnostic images and anatomical samples. The mechanical response of the diaphragmatic tendon is described by assuming an isotropic hyperelastic model. A similar constitutive model is used to define the mechanical behavior of the polymeric implant, while the muscular tissue is modeled by means of a three-element Hill's model, specifically adapted to mouse muscle fibers. The Finite Element Analysis is addressed to simulate diaphragmatic contraction in the eupnea condition, allowing the evaluation of diaphragm deformation in healthy and herniated-repaired conditions. The polymeric implant reduces diaphragm excursion compared to healthy conditions. This explains the possible alteration in the mechanical functionality of the repaired diaphragm. Looking to the surgical treatment of diaphragmatic hernia in human neonatal subjects, this study suggests the implementation of alternative approaches based on the use of biological implants.

Effects of working gas pressure on zirconium dioxide thin film prepared by pulsed plasma deposition: roughness, wettability, friction and wear characteristics.

In joint arthroplasty one of the main issues related to the failure of prosthetic implants is due to the wear of the ultra-high molecular weight polyethylene (UHMWPE) component. Surface treatments and coatings have been recognized as enhancing methods, able to improve the tribological properties of the implants. Therefore, the main objective of this work was to investigate the possibility to fabricate yttria-stabilized zirconia (YSZ) coatings on a metal (AISI 316-L) substrate by means of Pulsed Electron Deposition, in order to improve the tribological behavior of the polymer-metal coupling, by reducing the initial wear of the UHMWPE component. In order to optimize the coating characteristics, the effects of working gas pressure on both its morphological and tribological properties were analyzed. Morphological characterization of the films was evaluated by Atomic Force Microscopy (AFM). Coating wettability was also estimated by contact angle (CA) measurement. Tribological performance (coupling friction and wear of UHMWPE) was evaluated by using a ball-on-disc tribometer during highly-stressing tests in dry and lubricated (i.e. NaCl and serum) conditions; friction and wear were specifically evaluated at the initial sliding distances - to highlight the main effect of coating morphology - and after 100m - where the influence of the intrinsic materials properties prevails. AFM analysis highlighted that the working pressure heavily affected the morphological characteristics of the realized films. The wettability of the coating at the highest and lowest deposition pressures (CA ~ 60°, closed to substrate value) decreased for intermediate pressures, reaching a maximum CA of ~ 90°. Regarding tribological tests, a strong correlation was found in the initial steps between friction coefficient and wettability, which decreased as the distance increased. Concerning UHMWPE wear associated to coated counterpart, at 100m a reduction rate of about 7% in dry, 12% in NaCl and 5% in presence of serum was obtained compared to the uncoated counterpart. Differently from what highlighted for friction, no correlation was found between wear rate and morphological parameters. These findings, in agreement with literature, underlined the effect of the deposition pressure on the morphological properties, but suggested that physical characteristics are influenced too. Further research on the deposition process will be required in order to improve the tribological performance of the coating at long distances, addressing - above all - orthopedic applications.

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.

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 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.

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.

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.

A numerical approach to resonance frequency analysis for the investigation of oral implant osseointegration.

Experimental devices based on vibration testing are employed as non-destructive procedures for evaluating implants osseointegration. Their behaviour was evaluated considering the outcome of numerical analysis. The purpose was to use the finite element method for assessing the ability of frequency analysis in detecting the degree of oral implant osseointegration. A three-dimensional model of a mandible was obtained from tomographic survey. A single implant was considered in canine region. Two configurations were analysed, with and without a mass linked to the implant as a cantilever, reproducing experimental devices. Simulation consisted of analysing the response to impulse forces for different osseointegration levels, thus evaluating the biomechanical efficiency of the implant-bone compound. A good correlation between frequency response and osseointegration level was obtained. This was carried out by providing an impulse excitation of the implant that resulted in a vibration pattern. Within the limit of finite element analysis, the outcomes showed that numerical investigation provides understanding the behaviour of testing devices based on frequency measurements, confirming the potential of vibrations technique as non-invasive analysis for osseointegration process.

Anisotropic elasto-damage constitutive model for the biomechanical analysis of tendons.

The aim of this work is to investigate the instantaneous mechanical response of tendons by the use of an anisotropic elasto-damage constitutive model. This study addresses the analysis of the mechanical behaviour of healthy tendons during physiological loading and to degeneration phenomena. These are correlated with aging or traumatic events such as chronic or acute overloading during sporting activities. Histo-morphometric considerations suggest the adoption of a transversally isotropic constitutive model that describes the anisotropy of the material. The non-linearity of its overall mechanical response is taken into account by using a hyperelastic approach and also evaluates softening behaviour related to damage phenomena. The values of the parameters adopted within the analytical model are estimated both for human tendons previously subjected to cyclical loading and for specimens not subjected to cyclical loading. The results obtained by adopting this analytical model are compared with the experimental data in order to evaluate the capability of the model to describe the mechanical response of the tissue.

A transversally isotropic elasto-damage constitutive model for the periodontal ligament.

A numerical formulation of an elasto-damage constitutive model was developed and implemented in a finite element software to investigate the biomechanical response of the periodontal ligament (PDL). The mathematical framework accounts for the description of large strains, anisotropy and inelastic phenomena. The anisotropic mechanical response is caused by the spatial orientation of the sub-structures of the tissue, such as collagen fibres. Inelastic behaviour, induced by high level strains, is modelled by means of damage models. In vitro experimental testing on PDL samples from pigs was performed to obtain tensile stress-strain curves. A finite element analysis is presented in order to define a general numerical approach. A comparison of numerical and experimental data is provided in order to show the reliability and effectiveness of the formulation assumed.

A comparative analysis based on different strength criteria for evaluation of risk factor for dental implants.

A numerical analysis is developed to study the interaction phenomena between endousseus titanium dental implants and surrounding jawbone tissue. The interest is focused on the most appropriate evaluation of the stress state arising in the tissue because of the implant under physiological loading. The problem is considered with regard to linear elastic response of the one and to short time effect. Different configurations of bone-implant system are described, using axial-symmetrical and three-dimensional models, by means of finite and geometric element method. The investigation attains to the stress states induced in bone that lead to a limit condition near the effective failure surface. The parameter commonly adopted in literature, such as the Von Mises stress, represents an excessive simplification of problem formulation, leading to an incorrect evaluation of the real failure risk for the implant, due to the assumption of the isotropic and deviatoric nature of the adopted stress measure. More suitable criterion can be assumed, such as the Tsai-Wu criterion, to take into account the anisotropy that characterises the response of bone, as well as the influence of a hydrostatic stress state. The analysis developed offers a comparison of results by using different criteria, leading to an evaluation of reliability of the procedure to be followed and addressing also to an evaluation of a risk factor for the implant investigated.