Effects of different designs of orthodontic clear aligners on the maxillary central incisors in the tooth extraction cases: a biomechanical study.

BACKGROUND: Controlling the 3D movement of central incisors during tooth extraction cases with clear aligners is important but challenging in invisible orthodontic treatment. This study aimed to explore the biomechanical effects of central incisors in tooth extraction cases with clear aligners under different power ridge design schemes and propose appropriate advice for orthodontic clinic. METHODS: A series of Finite Element models was constructed to simulate anterior teeth retraction or no retraction with different power ridge designs. These models all consisted of maxillary dentition with extracted first premolars, alveolar bone, periodontal ligaments and clear aligner. And the biomechanical effects were analysed and compared in each model. RESULTS: For the model of anterior teeth retraction without power ridge and for the model of anterior teeth no retraction with a single power ridge, the central incisors exhibited crown lingual inclination and relative extrusion. For the model of anterior teeth no retraction with double power ridges, the central incisors tended to have crown labial inclination and relative intrusion. For the model of anterior tooth retraction with double power ridges, the central incisors exhibited a similar trend to the first kind of model, but as the depth of the power ridge increased, there was a gradual decrease in crown retraction value and an increase in crown extrusion value. The simulated results showed that von-Mises stress concentration was observed in the cervical and apical regions of the periodontal ligaments of the central incisors. The clear aligner connection areas of adjacent teeth and power ridge areas also exhibited von-Mises stress concentration and the addition of power ridge caused the clear aligner to spread out on the labial and lingual sides. CONCLUSIONS: The central incisors are prone to losing torque and extruding in tooth extraction cases. Double power ridges have a certain root torque effect when there are no auxiliary designs, but they still cannot rescue tooth inclination during tooth retraction period. For tooth translation, it may be a better clinical procedure to change the one-step aligner design to two-step process: tilting retraction and root control.

Nonlinear visco-elastic finite element analysis of porcelain veneers: a submodelling approach to strain and stress distributions in adhesive and resin cement.

PURPOSE: To assess under load the biomechanical behavior of the cementing system of feldspathic vs alumina porcelain veneers. MATERIALS AND METHODS: A 3D model of a maxillary central incisor, the periodontal ligament (PDL) and the alveolar bone was generated. Incisors restored with alumina and feldspathic porcelain veneers were compared to a natural sound tooth. Enamel, cementum, cancellous and cortical bone were considered isotropic elastic materials; conversely, dentin was designated as orthotropic. The nonlinear visco-elatic behavior of the PDL was considered. The adhesive layers were modelled using spring elements. A 50-N load at a 60-degree angle to the tooth's longitudinal axis was applied and validated. Stress concentration in the interfacial volumes of the main models was identified and submodelled in a new environment. RESULTS: Regarding tooth structure, strain concentrations were observed in the root dentin below the CEJ. As to the cement layer, tensile stresses concentrated in the palatal margin of the adhesive complex. CONCLUSION: Despite the effects on tooth deformation, the rigidity of the veneer did not affect the stress distributions in the cement layer or in the adhesive layers. In both cases, the palatal and cervical margins seemed to be the most stressed areas.

Non-linear viscoelastic finite element analysis of the effect of the length of glass fiber posts on the biomechanical behaviour of directly restored incisors and surrounding alveolar bone.

The study aimed at estimating the effect of insertion length of posts with composite restorations on stress and strain distributions in central incisors and surrounding bone. The typical, average geometries were generated in a FEA environment. Dentin was considered as an elastic orthotropic material, and periodontal ligament was coupled with nonlinear viscoelastic mechanical properties. The model was then validated with experimental data on displacement of incisors from published literature. Three post lengths were investigated in this study: root insertion of 5, 7, and 9 mm. For control, a sound incisor model was generated. Then, a tearing load of 50 N was applied to both sound tooth and simulation models. Post restorations did not seem to affect the strain distribution in bone when compared to the control. All simulated post restorations affected incisor biomechanics and reduced the root's deforming capability, while the composite crowns underwent a higher degree of deformation than the sound crown. No differences could be noticed in incisor stress and strain. As for the influence of post length, it was not shown to affect the biomechanics of restored teeth.

Three-dimensional finite element analysis of strain and stress distributions in endodontically treated maxillary central incisors restored with different post, core and crown materials.

OBJECTIVES: The present comparative analysis aimed at evaluating which combination of restorative materials resulted in the most homogeneous stress and strain distributions. METHODS: A three-dimensional finite element analysis was performed. All the nodes on the external surface of the root were constrained in all directions. Eighteen experimental models with different material properties and configurations were simulated. An arbitrary load of 10N was applied at 60 degrees angle with tooth longitudinal axis on the palatal surface of the crown. Von Mises (equivalent stresses) energetic criterion was chosen. RESULTS: In all the models the values of both strain and stress recorded at the middle third of the buccal aspect of the root surface were at their maxima. On the contrary, the minimum values were noticed at level of both the apical portion of the post and the root apex. The maximum stresses were evidenced at level of the cemento-enamel junction (CEJ) on both the buccal and palatal aspects of root cement and dentin. Stress progressively decreased from the outer to the inner part of the root and from the CEJ towards the incisal margin of the crown as well. SIGNIFICANCE: The results of the present study would allow clinicians to make an informed choice from among available materials to restore endodontically treated teeth.

In vitro biological response to a light-cured composite when used for cementation of composite inlays.

OBJECTIVE: To define the cytotoxicity of a photo-cured composite when used as a bonding system under a composite inlay. METHODS: Composite specimens were photo-cured with or without a 2 mm composite inlay interposed between them and the light source. Samples were extracted in complete cell culture medium and the obtained eluates applied to primary cultures of human pulp and gingival fibroblasts. After 72 h of incubation, cell viability was evaluated by MTT assay. Survival rates were calculated with respect to negative controls. RESULTS: Both shielded and unshielded composite samples were cytotoxic to pulp and gingival cells. The inlay shielded composite samples reached a significantly higher level of cytotoxicity compared to the unshielded ones. SIGNIFICANCE: The results suggested that the cytotoxicity of a light-cured composite resin used as a bonding system for indirect composite restorations may be significantly increased as a result of an inlay light-shielding effect.

Combined microcomputed tomography, biomechanical and histomorphometric analysis of the peri-implant bone: a pilot study in minipig model.

OBJECTIVES: To present a practical approach that combines biomechanical tests, microcomputed tomography (µCT) and histomorphometry, providing quantitative results on bone structure and mechanical properties in a minipig model, in order to investigate the specific response to an innovative dental biomaterial. METHODS: Titanium implants with innovative three-dimensional scaffolds were inserted in the tibias of 4 minipigs. Primary stability and osseointegration were investigated by means of insertion torque (IT) values, resonance frequency analysis (RFA), bone-to-implant contact (BIC), bone mineral density (BMD) and stereological measures of trabecular bone. RESULTS: A significant positive correlation was found between IT and RFA (r=0.980, p=0.0001). BMD at the implant sites was 18% less than the reference values (p=0.0156). Peri-implant Tb.Th was 50% higher, while Tb.N was 50% lower than the reference zone (p<0.003) and they were negatively correlated (r=-0.897, p=0.006). SIGNIFICANCE: µCT increases evaluation throughput and offers the possibility for qualitative three-dimensional recording of the bone-implant system as well as for non-destructive evaluation of bone architecture and mineral density, in combination with conventional analysis methods. The proposed multimodal approach allows to improve accuracy and reproducibility for peri-implant bone measurements and could support future investigations.

Light shielding effect of overlaying resin composite on the photopolymerization cure kinetics of a resin composite and a dentin adhesive.

OBJECTIVES: The purpose of this study was to simultaneously determine the impact of exposure times and incremental resin composite overlaying thickness on the cure kinetics of a light activated composite and a dentin adhesive at selected depths of a simulated restoration. METHODS: Levels and kinetics of polymerization of a light activated resin composite (Z250, 3M-ESPE) and dentin adhesive (Excite, Ivoclar) cured with a halogen light unit (Demetron, Kerr, USA) operating at low/medium intensity (500 mW/cm2) for different exposure durations (20 and 60 s) were measured at selected depths (0.3, 0.6 and 1mm) using a modified calorimetric analyzer (DSC 25, METLLER-TOLEDO). RESULTS: Final polymerization levels of materials up to 1mm through the composite are not significantly different while the light shielding effect of incremental resin composite overlaying progressively reduces their reaction rates. SIGNIFICANCE: Prolonged irradiation time at low/medium energies is effective for proper conversion of a resin composite and dentin adhesive; 60 s irradiation time provides the maximum obtainable conversion through the dental resin composite for thicknesses up to 1mm.

3D FEA of cemented steel, glass and carbon posts in a maxillary incisor.

OBJECTIVES: A comparative study on the stress distribution in the dentine and cement layer of an endodontically treated maxillary incisor has been carried out by using Finite Element Analysis (FEA). The role of post and cement rigidity on reliability of endodontic restorations is discussed. METHODS: A 3D FEM model (13,272 elements and 15,152 nodes) of a central maxillary incisor is presented. A chewing static force of 10 N was applied at 125 degree angle with the tooth longitudinal axis at the palatal surface of the crown. Steel, carbon and glass fiber posts have been considered. The differences in occlusal load transfer ability when steel, carbon and glass posts, fixed to root canal using luting cements of different elastic moduli (7.0 and 18.7 GPa) are discussed. RESULTS AND SIGNIFICANCE: The more stiff systems (steel and carbon posts) have been evaluated to work against the natural function of the tooth. Maximum Von Mises equivalent stress values ranging from 7.5 (steel) to 5.4 and 3.6 MPa (respectively, for carbon posts fixed with high and low cement moduli) and to 2.2 MPa (either for glass posts fixed with high and low cement moduli) have been observed under a static masticatory load of 10 N. A very stiff post works against the natural function of the tooth creating zones of tension and shear both in the dentine and at the interfaces of the luting cement and the post. Stresses in static loading do not reach material (dentine and cement) failure limits, however, they significantly differ leading to different abilities of the restored systems to sustain fatigue loading. The influence of the cement layer elasticity in redistributing the stresses has been observed to be less relevant as the post flexibility is increased.

Influence of tooth preparation design on the stress distribution in maxillary central incisors restored by means of alumina porcelain veneers: a 3D-finite element analysis.

AIM: The present study aimed at providing 3D-FEA engineering tools for the understanding of the influence of tooth preparation design on the stress distribution and localization of critical sites in maxillary central incisors restored by means of alumina porcelain veneers under functional loading. METHODS: A 3D-FEM model of a maxillary central incisor is presented. An arbitrary chewing static force of 10 N was applied with an angulation of 60 and 125 degrees to the tooth longitudinal axis at the palatal surface of the crown. The model was considered to be restored by means of alumina porcelain veneers with different tooth preparation designs. The differences in occlusal load transfer ability of the two restorative systems are discussed. RESULTS: The maximum Von Mises equivalent stress values were observed in the window restorative system for both 125 and 60 degrees load angulations. When the chamfer with palatal overlap preparation was simulated, the stress distributed uniformly in the cement layer, whereas in the window preparation the stress mainly occurred in the incisal area of the cement layer. SIGNIFICANCE: When restoring a tooth by means of porcelain veneers, the chamfer with palatal overlap preparation better restores the natural stress distribution under load than the window technique.

Inlay shading effect on the photopolymerization kinetic of a dental composite material used as bonding system in an indirect restoration technique.

OBJECTIVES: To define the inlay shading effect on the polymerization levels and kinetics of a light activated bonding system for an indirect restoration technique. MATERIALS AND METHODS: For the bonding system, an adhesive: Excite (Ivoclar-vivadent) and a composite: Z250 (3M-ESPE, St Paul Minnesota, USA) were investigated. A Demetron (Kerr USA) light curing unit was used. The composite inlay blocks of 2 mm thick were used for the experiment (Artglass A2 Heraeus, Kulzer, Dormagen, Germany). The bonding composite was photocured using a 2 mm composite inlay block as a shielding system while the adhesive was shielded by a 2.3 mm thick wafer, composed of the inlay material and the previously cured bonding composite. The kinetics and levels of polymerization were measured by a differential scanning calorimeter technique (DSC 25, Mettler, Orange, CA, Toledoh, küsnacht, switzerland). RESULTS: The inlay shielded dental composite reaches a significantly lower level of polymerization compared to the unshielded composite. Inlay shielded composite, has a slower polymerization kinetic compared to unshielded composite. The resin adhesive shielded by the inlay-composite wafer reaches polymerization values not significantly different from those of the unshielded adhesive. SIGNIFICANCES: The degree of cure of the light-cured composite resins for use as a base for indirect composite restorations, may be severely reduced as a result of inlay shielding.

Development of hybrid materials based on hydroxyethylmethacrylate as supports for improving cell adhesion and proliferation.

A novel hydrogel based on 2-hydroxyethylmethacrylate and fumed silica nanoparticles is presented. The filler was mixed at increasing amount (3-40% w/w) to the organic monomer, before accomplish thermal polymerization. The hybrid composite materials obtained were characterized as far as concern the physical-chemical stability and sorption behaviour in water and water solutions. The novel hybrid hydrogels were compared to poly(hydroxyethylmethacrylate) (pHEMA) on cytocompatibility and ability to elicit cell adhesion and proliferation. These in vitro assays showed that the first ones were supporting cell growth better then pHEMA, moreover experiments on murine fibroblasts showed improved adhesion and proliferation with the increase of the nanomeric filler content. For a more physiological response, the in vitro tests should match biomaterials with cell populations typical of the implant site. Therefore, in view of future applications of these composites as scaffolds for bone engineering, in a successive step of our research we selected primary cultures of human osteoblasts (OB) as the most appropriate models to study the in vitro performance of these materials. The preliminary results obtained confirmed the remarkable improvement of OB adhesion properties of the new hybrids with respect to pure pHEMA.

The importance of cortical bone orthotropicity, maximum stiffness direction and thickness on the reliability of mandible numerical models.

AIM: To identify mechanical and geometrical variables affecting the biofidelity of numerical models of human mandible. Computed results sensibility to cortical bone orthotropy and thicknesses is investigated. METHODS: Two mandible numerical models of different bone complexities are setup. In the low-complexity model, cortical bone is coupled with isotropic materials properties; constant thickness for cortical bone is adopted along the mandible structure. In the higher complexity model, the cortical bone is considered as an orthotropic material according to an independent mechanical characterization performed on fresh human dentate mandibles. Cortical thickness distribution, the values of the principal elastic moduli and principal directions of orthotropy are considered as piecewise heterogeneous. Forces for masseter (10 N), medial pterigoid (6 N), anterior (4 N) and posterior (4 N) temporalis muscles are applied to the models. Computed strains fields are compared with those experimentally measured in an independent test performed on a real human mandible in the same loading conditions. RESULTS: Under closure muscles forces both models shows similar strain distribution. On the contrary, strain fields values are significantly different between the presented models. CONCLUSIONS: The mandible structure is sensible to compact bone orthotropy and thickness at the facial side of condylar neck, retro molar area and at the lingual side of middle portion of the corpus in molars area, anterior margin of the ramus. In these areas, it is advisable to use orthotropic properties for cortical bone to accurately describe the strain state.

Non-linear elastic three-dimensional finite element analysis on the effect of endocrown material rigidity on alveolar bone remodeling process.

OBJECTIVES: In healthy conditions, modeling and remodeling collaborate to obtain a correct shape and function of bones. Loads on bones cause bone strains which generate signals that some cells can detect and respond to. Threshold ranges of such signals are genetically determined and are involved in the control of modeling and remodeling. The present study aimed at assessing the deformations transferred to surrounding bone by endodontically treated maxillary central incisors restored with endocrowns made up of high or low elastic modulus materials. METHODS: The solid model consisted of a maxillary central incisor, the periodontal ligament (PDL) and the surrounding cortical and cancellous bone. Both composite and alumina endocrowns were simulated under load and compared to a sound tooth. Dynamic non-linear analyses were performed to validate discretization processes. Non-linear analyses were performed on teeth and surrounding bone to estimate strain variations according to restorative techniques. RESULTS: Strain values in cortical bone, spongy bone, alveolar cortex and tooth were evaluated. PDL allowed models to homogeneously transfer loads to bone. Strains developing in highly rigid restorations were estimated to activate bone modeling and remodeling. SIGNIFICANCE: The higher deformability of composites could enable restorative systems to transfer limited strains to compact and spongy bone of tooth socket. Although composites could not prevent the physiological resorption of the alveolar bone, they could successfully reduce strain arising in tooth socket when compared to alumina. The PDL prevented bone to undergo high deformations, resulting in natural flexural movements of teeth.

Nonlinear visco-elastic finite element analysis of different porcelain veneers configuration.

This study is aimed at evaluating the biomechanical behavior of feldspathic versus alumina porcelain veneers. A 3D numerical model of a maxillary central incisor, with the periodontal ligament (PDL) and the alveolar bone was generated. Such model was made up of four main volumes: dentin, enamel, cement layer and veneer. Incisors restored with alumina and feldspathic porcelain veneers were compared with a natural sound tooth (control). Enamel, cementum, cancellous and cortical bone were considered as isotropic elastic materials; on the contrary, the tubular structure of dentin was designed as elastic orthotropic. The nonlinear visco-elatic behavior of the PDL was considered. The veneer volumes were coupled with alumina and feldspathic porcelain mechanical properties. The adhesive layers were modeled in the FE environment using spring elements. A 50N load applied at 60 degrees angle with tooth longitudinal axis was applied and validated. Compressive stresses were concentrated on the external surface of the buccal side of the veneer close to the incisal margin; such phenomenon was more evident in the presence of alumina. Tensile stresses were negligible when compared to compressive ones. Alumina and feldspathic ceramic were characterized by a different biomechanical behavior in terms of elastic deformations and stress distributions. The ultimate strength of both materials was not overcome in the performed analysis.

Biological response of human bone marrow stromal cells to sandblasted titanium nitride-coated implant surfaces.

Titanium nitride (TiN) coating has been proposed as an adjunctive surface treatment aimed to increase the physico-mechanical and aesthetic properties of dental implants. In this study we investigated the biological response of primary human bone marrow stromal cells (BMSC) to TiN-coated sandblasted (TiN-SB) compared to uncoated sandblasted (SB) surfaces. SB and TiN-SB disks were qualitatively and quantitatively analyzed by atomic force microscopy. BMSC were obtained from healthy donors and their adhesion and proliferation on the titanium disks were evaluated by scanning electron microscopy and viability assay. The osteoblastic differentiation, in terms of alkaline phosphatase activity, osteocalcin synthesis, and extracellular mineralization, was assessed by specific immunoenzymatic or spectrophotometric assays. No difference (P > 0.05) between TiN-SB and SB disks was found in terms of any of the investigated parameters. TiN-coating showed to maintain the topographical characteristics of sandblasted titanium surfaces and their biological affinity toward bone precursors.

Mandibular flexure and stress build-up in mandibular full-arch fixed prostheses supported by osseointegrated implants.

The biomechanical effect of mandibular functional flexure on stress build-up in implant-supported fixed restorations is discussed. The relative deformations and stress distributions in six different designs of implant-supported prosthetic systems (six or four implants, with or without distal cantilevers, cross-arch or midline-divided bar into two free-standing bridges) were analysed by a three-dimensional finite element (FE) model of a human edentulous mandible. A significant amount of stress in the more distal implants and the superstructure at the symphysis arises as a consequence of mandible functional flexure. The analysis of the stress distributions generated by the different restorative patterns suggests that a division of the superstructure at the level of the symphysis significantly restores the natural functional flexure of the mandible.