Effect of crown retention systems and loading direction on the stress magnitude of posterior implant-supported restorations: A 3D-FEA.

This study aimed to investigate the effect of four retention systems for implant-supported posterior crowns under compressive loading using three-dimensional finite element analysis. A morse-taper dental implant (4.1 × 10 mm) was designed with Computer Aided Design software based on non-uniform rational B-spline surfaces. According to International Organization for Standardization 14,801:2016, the implant was positioned at 3 mm above the crestal level. Then four models were designed with different crown retention systems: screw-retained (A), cement-retained (B), lateral-screw-retained (C), and modified lateral-screw-retained (D). The models were imported to the analysis software and mesh was generated based on the coincident nodes between the juxtaposed lines. For the boundary conditions, two loads (600 N) were applied (axial to the implant fixture and oblique at 30°) totaling 8 conditions according to retention design and loading. The von-Mises stress analysis showed that different retention systems modify the stress magnitude in the implant-supported posterior crown. There is a similar stress pattern in the implant threads. However, models C and D presented higher stress concentrations in the crown margin in comparison with A and B. The oblique loading highly increased the stress magnitude for all models. In the simulated conditions, part of the stress was concentrated at the lateral screw under axial loading for model C and oblique loading for model D. The results indicate a possible new failure origin for crown retained using lateral screws in comparison to conventional cement-retained or screw-retained systems.

Durability of staining and glazing on a hybrid ceramics after the three-body wear.

Hybrid ceramic is a promising material for monolithic restorations that could require an individualization through the extrinsic staining to improve aesthetics. Due to the possibilities to treat this ceramic prior to staining, this study evaluated the wear resistance of surface treatments prior to staining and glazing a hybrid ceramic. Thirty-two specimens (Vita Enamic) were divided into 8 groups according to the surface treatment prior to the staining (Polishing: Pol, Acid etching: Ac, Sandblasting with Al2O3: Sd or Self-etching silane: Ses) and glaze application (with: gl or without: gl). The specimens were submitted to the ACTA wear machine simulating the presence of food bolus and antagonist. The wear rate of the stain was determined after 7 intervals of 20,000 cycles, using a profilometer. The surface before and after staining, and after wear were inspected using Scanning Electron Microscopy (SEM). The rates were analyzed using three-way ANOVA and Tukey test. The wear was affected by surface treatment, glaze application and number of cycles (p < 0.001). 100% of the staining was removed after 20,000 cycles for Pol, 40,000 for Pol + gl, 60,000 for Ses + gl, 80,000 for Ac, 100,000 for Sd and Ses, 120,000 for Ac + gl and 140,000 for Sd + gl. SEM showed similar worn surfaces. Sandblasting followed by glaze application was the most durable treatment to maintain the external staining on the hybrid ceramic surface when subjected to three-body wear.

Survival Probability, Weibull Characteristics, Stress Distribution, and Fractographic Analysis of Polymer-Infiltrated Ceramic Network Restorations Cemented on a Chairside Titanium Base: An In Vitro and In Silico Study.

Different techniques are available to manufacture polymer-infiltrated ceramic restorations cemented on a chairside titanium base. To compare the influence of these techniques in the mechanical response, 75 implant-supported crowns were divided in three groups: CME (crown cemented on a mesostructure), a two-piece prosthetic solution consisting of a crown and hybrid abutment; MC (monolithic crown), a one-piece prosthetic solution consisting of a crown; and MP (monolithic crown with perforation), a one-piece prosthetic solution consisting of a crown with a screw access hole. All specimens were stepwise fatigued (50 N in each 20,000 cycles until 1200 N and 350,000 cycles). The failed crowns were inspected under scanning electron microscopy. The finite element method was applied to analyze mechanical behavior under 300 N axial load. Log-Rank (p = 0.17) and Wilcoxon (p = 0.11) tests revealed similar survival probability at 300 and 900 N. Higher stress concentration was observed in the crowns' emergence profiles. The MP and CME techniques showed similar survival and can be applied to manufacture an implant-supported crown. In all groups, the stress concentration associated with fractographic analysis suggests that the region of the emergence profile should always be evaluated due to the high prevalence of failures in this area.

Three-body wear effect on different CAD/CAM ceramics staining durability.

Regardless the materials properties, the vast majority of ceramic restorations could require an individualization through the extrinsic staining to improve aesthetics. This study aimed to compare the staining wear durability of different monolithic ceramics. Specimens of high translucent zirconia (YZHT), zirconia reinforced lithium silicate (ZLS), hybrid ceramic (HC) and feldspathic ceramic (FLD) were divided in five groups according to each material staining technique. The ZLS ceramic was tested with stained prior (ZLS1) and after crystallization (ZLS2). All specimens were extrinsically characterized, i.e. stained, and crystallized or sintered in specific ovens, according to the manufacturer's recommendation. The specimens were submitted to three-body wear tests in ACTA wear machine, simulating the presence of food bolus and antagonist (pH 7, 15 N, 1 Hz). The wear rate of the stain surface was determined after 5 intervals of 200,000 cycles, using a profilometer. The ceramic surface before and after staining, and after wear were inspected by Scanning Electron Microscopy (SEM). The wear rates were analyzed using two-way ANOVA and post-hoc Tukey test. The wear rates of the staining were affected by ceramic and the number of cycles (P < 0.001). 100% of staining was removed after 200,000 cycles for HC, and after 600,000 cycles for YZHT and ZLS1. Staining of ZLS2 and FLD remained on ceramic surface even after 1,000,000 cycles. Furthermore, FLD showed a significant higher staining durability than ZLS2. SEM revealed different surface morphologies for each group with and without staining and after the wear test. Ceramics with fired staining showed higher durability compared to the polymerized one. The feldspar ceramic presented superior staining durability, followed by zirconia reinforced lithium silicate and high translucent zirconia. The conventional two steps staining technique showed improved durability for zirconia reinforced lithium silicate.

Short communication: Influence of retainer configuration and loading direction on the stress distribution of lithium disilicate resin-bonded fixed dental prostheses: 3D finite element analysis.

The present study elucidates the mechanical performance of different designs of resin-bonded fixed dental prostheses made of lithium disilicate simulating masticatory loads of anterior or canine guidance. A three-dimensional model of maxilla was constructed containing central incisor and canine teeth, with edentulous space of the lateral incisor. Three designs of prosthesis were created: retained in central incisor (1-I), retained in canine (1-C) and fixed in both teeth (2-IC). The computational analysis was performed for load in canine and central incisor separately (100N, 45°). The tensile and shear stresses were calculated for the resin-bonded fixed dental prosthesis, bonding surface of each retainer and cement layer using 3D finite element analysis. The 20 highest stress values were analyzed using two-way ANOVA and post-hoc Tukey test, all with α = 5%. The computational analysis showed that 2-retainer resin-bonded fixed dental prosthesis presented the worst prognosis regardless of the mandibular movement. ANOVA showed that Mandibular movement*Retainer interaction influenced on the tensile and shear stresses values (p < 0.01). Higher stresses were observed in the connector region for all groups (13-82.2 MPa; 11-70.2 MPa). In order to reduce the stress concentration in the resin-bonded fixed dental prosthesis and the retainer made of lithium disilicate, the occlusion may serve as the selection criteria of the unitary abutment for better sustainability.

Short communication: Influence of restorative material and cement on the stress distribution of posterior resin-bonded fixed dental prostheses: 3D finite element analysis.

The goal of this study was to compare the mechanical response of resin-bonded fixed dental prosthesis (RBFDP) made in zirconia, metal, lithium disilicate and composite resin cemented using resin cements with different elastic modulus. For the finite element analysis, a three-dimensional model of partial right maxilla was used to create a model with edentulous space in the second premolar and the cavity's preparation on the first pre-molar and first molar to receive a RBFDP. The model was imported to the analysis software in which they were divided into mesh composed by nodes (371,101) and tetrahedral elements (213,673). Each material was considered isotropic, elastic and homogeneous. No-separation contacts were considered between restoration/resin cement and resin cement/tooth. For all other structures the contacts were considered ideal. The model fixation occurred at the base of the bone and an axial load of 300 N was applied on the pontic occlusal surface. To simulate polymerization shrinkage effects on the cement, the thermal expansion approach was used. The displacement and maximum principal stress (in MPa) were selected as failure criteria. The prosthesis made in composite resin showed higher displacement, while in zirconia showed higher stress concentration. Tensile stress between restoration/cement, cement and cement/cavity was directly proportional to the restorative material's elastic modulus. The more rigid cement increases the tensile zones in the cement layer but decreases the stress between prosthesis and cement. The molar cavity showed higher stress concentration between restoration/cement than the preparation in the pre-molar tooth. The use of composite resin for the manufacturing of RBFDP increases the displacement of the set during the loading. However, it reduces the amount of stress concentration at the adhesive interface in comparison with the other materials.

Silica coating followed by heat-treatment of MDP-primer for resin bond stability to yttria-stabilized zirconia polycrystals.

This study evaluated the influence of silica coating, primer type and its heat treatment on bond strength durability between resin cement and an yttrium-stabilized polycrystalline tetragonal zirconia (Y-TZP). Eighty (80) Y-TZP blocks were allocated into 16 groups considering four factors: silica coating (without and with); type of primer (RelyX Ceramic Primer, a silane-based primer; Single Bond Universal, a universal MDP-based primer); heat treatment of the primer (without and with); aging (without and with). After zirconia treatments, resin cement cylinders (RelyX ARC) (n = 20; N = 320) were built. Half of the samples were tested after 24 h, and another half were subjected to aging (thermocycling 5-55°C/5.000, and storage in water for 6 months). Shear bond strength test and failure analysis were performed. Bond strength data were submitted to four-way ANOVA and Tukey test (p = 0.05). All factors were statistically significant (p < 0.001) for bond strength (MPa): silica coating (7.3 ± 5.9) > no-treatment (3.6 ± 4.3); universal primer (6.7 ± 4.8) > silane (4.2 ± 5.8); heat-treatment (6.5 ± 6.3) > no-heating (4.4 ± 4.3); no-aging (8.2 ± 5) > aging (2.7 ± 4.4). Tukey test revealed that the association of silica coating + universal primer + heat-treatment promoted higher and stable resin bond strength. Silica coating, universal adhesive application and heat treatment improve/increase durability and bond strength of zirconia. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 104-111, 2019.

The Influence of Ceramic Re-pressing on Surface Properties, Bond Strength, and Color Stability of Leucite Ceramic.

PURPOSE: To evaluate the influence of recycling a pressed ceramic material on surface properties, color stability and bond strength to composite cement. MATERIALS AND METHODS: Forty-eight (48) ingots from a heat-pressed ceramic (PM9, Vita Zahnfabrik) were fabricated through the lost-wax technique and then polished with SiC sandpaper to standardize the dimensions (diameter: 12 mm; length: 4 mm). Leftover material from processing the pressed groups was retrieved and used to repeat heat pressing for the re-pressed groups. The ingots were randomly divided into 3 groups according to the number of injections - one, two, or three times. Ra, Rz, and RSm surface roughness parameters were evaluated through a contact rugosimeter. Topography was analyzed using SEM and the chemical constituents using EDS (energy dispersive spectroscopy). Color stability was evaluated according to CIE-Lab parameters to determine color variation (∆E). Composite ingots with the same dimensions were cemented over each ceramic, then this set was cut into sticks with a 1-mm2 cross-sectional area. Half of the sticks were submitted to microtensile bond strength (µTBS) testing after 24 h. The other half was tested after thermocycling (5000 cycles, 5°C-55°C) and storage in water at 37°C for 6 months. RESULTS: One-way ANOVA showed that the number of injections influenced both Rz and RSm roughness parameters (p = 0.00), but not Ra (p = 0.97). One injection exhibited higher (Rz) and grooves with less space between them (RSm) compared to the other groups. For color stability, the values of L*, a*, and b* were influenced by the number of injections (p = 0.00). ∆E = 5 was found for 2 injections and ∆E = 3 for 3 injections, using a control group as reference. One-way ANOVA showed that the number of injections significantly influenced wettability values: 1inj (63 ± 5.3)A > 2inj (49 ± 8.2)B > 3inj (45.8 ± 15.8)B. However, the evaluated factor did not influence the bond strength of non-aged groups. Furthermore, aged groups showed that 1inj (21 ± 37)a presented higher mean values than 2inj (10 ± 8)b or 3inj (12 ± 10.)b. CONCLUSION: Re-pressing the evaluated ceramic improves roughness and wettability, but compromises the color stability and decreases bond strengths after long-term aging.