Franco Maceri

The influence of implant diameter and length on stress distribution of osseointegrated implants related to crestal bone geometry: A three- dimensional finite element analysis

STATEMENT OF PROBLEM Load transfer mechanisms and possible failure of osseointegrated implants are affected by implant shape, geometrical and mechanical properties of the site of placement, as well as crestal bone resorption. Suitable estimation of such effects allows for correct design of implant features. PURPOSE The purpose of this study was to analyze the influence of implant diameter and length on stress distribution and to analyze overload risk of clinically evidenced crestal bone loss at the implant neck in mandibular and maxillary molar periimplant regions. MATERIAL AND METHODS Stress-based performances of 5 commercially available implants (2 ITI, 2 Nobel Biocare, and 1 Ankylos implant; diameters of 3.3 mm to 4.5 mm, bone-implant interface lengths of 7.5 mm to 12 mm) were analyzed by linearly elastic 3-dimensional finite element simulations, under a static load (lateral component: 100 N; vertical intrusive component: 250 N). Numerical models of maxillary and mandibular molar bone segments were generated from computed tomography images, and local stress measures were introduced to allow for the assessment of bone overload risk. Different crestal bone geometries were also modelled. Type II bone quality was approximated, and complete osseous integration was assumed. RESULTS Maximum stress areas were numerically located at the implant neck, and possible overloading could occur in compression in compact bone (due to lateral components of the occlusal load) and in tension at the interface between cortical and trabecular bone (due to vertical intrusive loading components). Stress values and concentration areas decreased for cortical bone when implant diameter increased, whereas more effective stress distributions for cancellous bone were experienced with increasing implant length. For implants with comparable diameter and length, compressive stress values at cortical bone were reduced when low crestal bone loss was considered. Finally, dissimilar stress-based performances were exhibited for mandibular and maxillary placements, resulting in higher compressive stress in maxillary situations. CONCLUSIONS Implant designs, crestal bone geometry, and site of placement affect load transmission mechanisms. Due to the low crestal bone resorption documented by clinical evidence, the Ankylos implant based on the platform switching concept and subcrestal positioning demonstrated better stress-based performance and lower risk of bone overload than the other implant systems evaluated.

A unified multiscale mechanical model for soft collagenous tissues with regular fiber arrangement.

In this paper the mechanical response of soft collagenous tissues with regular fiber arrangement (RSCTs) is described by means of a nanoscale model and a two-step micro-macro homogenization technique. The non-linear collagen constitutive behavior is modeled at the nanoscale by a novel approach accounting for entropic mechanisms as well as stretching effects occurring in collagen molecules. Crimped fibers are reduced to equivalent straight ones at the microscale and the constitutive response of RSCTs at the macroscale is formulated by homogenizing a fiber reinforced material. This approach has been applied to different RSCTs (tendon, periodontal ligament and aortic media), resulting effective and accurate as proved by the excellent agreement with available experimental data. The model is based on few parameters, directly related to histological and morphological evidences and whose sensitivity has been widely investigated. Applications to simulation of some physiopathological mechanisms are also proposed, providing confirmation of clinical evidences and quantitative indications helpful for clinical practice.

Stress-based performance evaluation of osseointegrated dental implants by finite-element simulation

Abstract In this paper biomechanical interaction between osseointegrated dental implants and bone is numerically investigated through 3D linearly elastic finite-element analyses, when static functional loads occur. Influence of some mechanical and geometrical parameters on bone stress distribution is highlighted and risk indicators relevant to critical overloading of bone are introduced. Insertions both in mandibular and maxillary molar segments are analyzed, taking into account different crestal bone loss configurations. Stress-based performances of five commercially-available dental implants are evaluated, demonstrating as the optimal choice of an endosseous implant is strongly affected by a number of shape parameters as well as by anatomy and mechanical properties of the site of placement. Moreover, effectiveness of some double-implant devices is addressed. The first one is relevant to a partially edentulous arch restoration, whereas other applications regard single-tooth restorations based on non-conventional endosteal mini-implants. Starting from computer tomography images and real devices, numerical models have been generated through a parametric algorithm based on a fully 3D approach. Furthermore, effectiveness and accuracy of finite-element simulations have been validated by means of a detailed convergence analysis.

A consistent theory of thin piezoelectric plates

In this paper a theory of thin piezoelectric plates is obtained throigh a rational derivation from the three-dimensional linear theory of piezoelectricity. The coupling between the ekictric and mechanical fields is taken into account, leading to a consistent definition of the bending and stretching stiffnesses. In particular, it is shown that a piezoelectric plate has a different stretching stiffness when it is used as an actuator or as a sensor. The procedure used to derive the field equations governing the piezoelectric plate problem is based on the initial functions method, in conjunction with a rescaling of the applied loads. The field equations are then rewritten in a variational form, according to a generalized statement of the virtual work principle, in order to deduce the compatible boundary conditions. The theory established here is used to find closed-form expressions of the solutions of some technical problems, involving piezoelectric plates used as sensors or actuators.

Age-Dependent Arterial Mechanics via a Multiscale Elastic Approach

A multiscale mechanical model for arterial walls is proposed to describe their age-dependent elastic behavior. The model accounts for nanoscale mechanisms related to molecular and cross-link stretching, as well as for micro- and macroscale effects, by employing homogenization techniques. Such a model uses only a few measurable histological parameters, and allows the reproduction of well-established experimental evidence, highlighting that stiffness of collagen fibrils is related to both cross-link density and their mechanical properties. In the case of aortic walls, the model allows to account for histological alterations occurring with age, fully reproducing available experimental results. Proposed evidence also gives a clear mechanical interpretation of the influence of cross-link density and stiffness on arterial tissue elastic modulus and arterial compliance.

An insight on multiscale tendon modeling in muscle–tendon integrated behavior

This paper aims to highlight the need for a refined tendon model to reproduce the main mechanical features of the integrated muscle–tendon unit (MTU). Elastic nonlinearities of the tendon, both at the nano and microscale, are modeled by a multiscale approach, accounting for the hierarchical arrangement (from molecules up to the fibers) of the collagen structures within the tissue. This model accounts also for the variation of tendon stiffness due to physical activity. Since the proposed tendon model is based on tissue-structured histology, the training-driven adaptation laws are directly formulated starting from histological evidences. Such a tendon description is integrated into a viscoelastic Hill-type model of the whole MTU. A fixed-end contraction test is numerically simulated, and results based on both linear and nonlinear tendon elastic model are compared. Sound and effective time-histories of muscle contractile force and fiber length are obtained only accounting for tendon elastic nonlinearities, which allow to quantitatively recover some experimental data. Finally, proposed numerical results give clear indications toward a rational explanation of the influence of tendon remodeling induced by physical activity on muscular contractile force.

EIT-Inspired Microfluidic Cytometer for Single-Cell Dielectric Spectroscopy

A new microfluidic cytometer for single-cell dielectric spectroscopy is proposed in this paper and analyzed in silico by means of a finite-element model. The device, inspired by electrical impedance tomography, includes two circumferential arrays of electrodes instead of just two pairs of coplanar or parallel-facing electrodes, thus allowing a great versatility in stimulation and measurement patterns. In particular, using stimulation patterns with different spatial orientation provides information on cell morphology, besides quantitative cell-volume estimation. Moreover, the performance limitation at low frequency due to electrode polarization is overcome, owing to a peculiar recording scheme: Current is injected between an electrode pair, and the resulting voltages are measured at remaining electrodes using high-input impedance differential amplifiers. These features significantly enhance the cytometer discrimination capabilities.

A layer-wise Reissner–Mindlin-type model for the vibration analysis and suppression of piezoactuated plates

A layer-wise model of three-layer piezoelectric sandwich plates is presented. Each layer is modeled according to a first-order shear deformation theory. Both a variational formulation and a locking-free finite-element formulation of the sandwich-plate problem are developed. The latter is based on a new bilinear quadrangular four-node finite element with 13 degrees of freedom per node and is validated by comparing the numerical and the analytical solution of a special problem. The proposed model is applied to the analysis of the vibration suppression problem of a thick cantilever steel plate equipped with a piezoelectric actuator, and turns out to be especially useful when a thick piezoelectric actuator is used.

D-PANA: a convergent block-relaxation solution method for the discretized dual formulation of the Signorini-Coulomb contact problem †

Signorinis law of unilateral contact and Coulombs friction law constitute a simple and useful framework for the analysis of unilateral frictional contact problems of a linearly elastic body with a rigid support. For quasi-static, monotone-loadings, the discrete dual formulation of this problem leads to a quasi-variational inequality, whose unknowns, after condensation, are the normal and tangential contact forces at nodes of the initial contact area. A new block-relaxation solution technique is proposed here. At the typical iteration step, shown to be a contraction for small friction coefficients, two quadratic programming problems are solved one after the other: the former is a friction problem with given normal forces, the latter is a unilateral contact problem with prescribed tangential forces. The contraction principle is used to establish the well-posedness of the discrete formulation, to prove the convergence of the algorithm, and to obtain an estimate of the convergence rate.  2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS D-PANA : une methode convergente de relaxation par blocs pour la resolution du probleme de Signorini-Coulomb discret en formulation duale

Modelling and Simulation of Long-Span Bridges under Aerodynamic Loads

This paper deals with the dynamic stability of long-span bridges under non-stationary aerodynamic loads. Some generalizations of classical models are presented in order to check the critical conditions both in the case of flutter and divergence for long-span bridges with particular reference to the cable-stayed scheme. Furthermore, a numerical model based on a finite volume formulation of the flow problem around the girder cross-section is developed, able to simulate the steady and non-steady wind load conditions on the bridge. Good agreement with wind tunnel test results is found for the Normandy Bridge design cross-section models.