Minimal tooth preparation for posterior monolithic ceramic crowns: Effect on the mechanical behavior, reliability and translucency

Impact of deglazing on biaxial flexural strength of high translucent zirconia: effect of hydrothermal aging, pH-cycling, and load-cycling

BACKGROUND

To compare the effect of deglazing and aging on biaxial flexural strength (FS) of high translucent (HT) zirconia.

METHODS

40 disc-shaped specimens (12 × 1.2 mm) of HT zirconia (3Y-PSZ, Upcera HT) were sintered after milling, glazed(G) and randomly assigned equally into 4 experimental groups: G) glazing; G-A) glazing, aging; DG-A) deglazing, aging; and NG-A) non-glazing, aging. A sequence of three methods was used for aging; hydrothermal, pH-cycling, and load cycling. FS(flexural strength) of all specimens was measured and one specimen from each group underwent X-ray diffraction (XRD) for assessment of phase transformation (t-m). Distribution of data was evaluated using the Kolmogorov -Smirnov test, and with respect to the confidence interval of 95%, data were analyzed using 1-way ANOVA with statistical software (IBM SPSS Statisticsv20.0) to compare the mean flexural strength values among the 4 groups. When significant differences were found, the mean values were compared using Tukey's honest significant difference (HSD) test (α = 0.05).

RESULTS

FS of glazed specimens (1092.78 ± 175.40 MPaa) did not decrease after aging (1141.11 ± 117.43 MPa, P = 0.872a). FS of DG-A (1338.22 ± 175.13 MPab) and NG-A (1320.44 ± 77.44 MPab) groups was significantly higher than that of G and GA groups (P < 0.005). XRD revealed monoclinic phase in DG-A (4.23%) and NG-A (7.63%) groups.

CONCLUSION

Loss of glaze layer and no glazing had no significant effect on FS of high translucent Y-PSZ after aging.

Fracture Resistance of Endodontically Treated Teeth Prepared With Biologically Oriented Preparation Technique Versus Horizontal Finish Lines: An In Vitro Study

INTRODUCTION

The biologically oriented preparation technique (BOPT) is a conservative tooth preparation method with no defined finish lines. Studies comparing the fracture resistance of endodontically treated teeth (ETT) prepared using BOPT or horizontal finish lines are lacking. The primary objective of this study was to compare the fracture resistance of ETT prepared using BOPT, chamfer finish line, or shoulder finish line and restored with monolithic zirconia (MZ) crowns. The secondary objective was to determine the modes of fracture of ETT with different finish lines.

METHODS

Forty-five maxillary premolar teeth were endodontically treated and randomly allotted to three tooth preparation methods: Group 1, BOPT; Group 2, chamfer finish line; and Group 3, shoulder finish line. MZ crowns were milled and cemented on their respective tooth preparations. The fracture resistance was tested using a universal testing machine. Maximum fracture load was recorded in Newtons (N). The fracture modes were classified using Burkey's codes and a newly proposed fracture grading.

RESULTS

The highest fracture resistance was seen in samples from Group 1, followed by Group 2, and the least in Group 3. Tukey's post hoc test showed a significant difference between Group 1 and Group 3 (p<0.05) and between Group 2 and Group 3 (p<0.05). No significant differences were found between Group 1 and Group 2 (p>0.05). Code V and Grade 3B fractures were highest in Group 3 samples.

CONCLUSIONS

Within the limitations of the present study, it can be concluded that ETT prepared with BOPT and chamfer finish lines had the greatest fracture resistance compared to shoulder finish lines. Teeth prepared with shoulder finish lines had more non-restorable fractures compared to BOPT or chamfer preparations.

Fatigue methods for evaluating translucent dental zirconia

OBJECTIVE

To investigate fatigue methods for the evaluation of translucent zirconia and to associate in vitro failures with clinically reported ones.

DATA

Studies published in English that used fatigue tests on dental translucent zirconia.

SOURCES

Two databases (MEDLINE/PubMed and Scopus) were electronically searched without any restriction on year of publication.

STUDY SELECTION

A total of 4555 studies were identified. After removal of duplicates (78) and irrelevant articles (4316) that did not meet the inclusion criteria, 161 articles were considered eligible based on their titles and abstracts. These articles were fully read, leading to the inclusion of 41 studies in the review.

RESULTS

The most widely used fatigue method for evaluation of translucent zirconia was step-stress (18 articles), followed by staircase (seven articles), and step-wise (two articles). Most studies had been conducted in a wet environment with the use of a stainless steel piston to apply load to cemented structures on a dentin-like substrate. Most fracture analyses indicated the fracture originated on the cementation or contact surface where the load was applied. Moreover, studies that utilized anatomical structures (dental crowns) reported fractures starting at the cervical margin of the crowns.

CONCLUSION

Most studies used the step-stress method. Only three studies reported failures similar to those found in clinical trials that used translucent zirconia restorations.

CLINICAL SIGNIFICANCE

The study findings can assist on correlating clinical failures to the ones observed in vitro.

Clinical efficacy, bone density, and follow-up in implant and orthodontic treatment for inclined adjacent teeth

BACKGROUND

Tooth defects can cause elongation of occlusal teeth, leading to insufficient repair space. The combination of dental implant restoration and orthodontic treatment of oblique adjacent teeth has a significant therapeutic effect.

AIM

To explore clinical efficacy, bone density, and follow-up of implant and orthodontic treatment for patients with inclined adjacent teeth.

METHODS

In total, 98 patients with oblique adjacent teeth were randomly assigned to implant restoration combined with orthodontic treatment (group A, n = 49) or to receive implant restoration alone (group B, n = 49). Changes in alveolar ridge bone density and apical bone density were observed before and after treatment in the two groups. Changes in chewing function and language function were compared between the two groups of patients. Follow-up lasted for 12 mo after repair to observe any adverse reactions in the oral cavity.

RESULTS

The clinical effective rates of group A and group B were 97.96% and 85.71%, respectively, with group A having a higher clinical effective rate than group B. After treatment, the bone density of the alveolar ridge and apical bone in both groups decreased compared to before treatment, while the chewing and language functions improved. The changes in various indicators in group A were more significant. After treatment, the satisfaction rate of group A (97.96%) was higher than that of group B (79.59%). The incidence of adverse reactions in group A (2.04%) was lower than that in group B (24.49%).

CONCLUSION

The amalgamation of implant restoration and orthodontic treatment for adjacent tilted teeth demonstrates notable clinical efficacy, diminishes alveolar bone resorption, and fosters patient functional rehabilitation while exhibiting negligible adverse reactions.

Effect of margin designs and loading conditions on the stress distribution of endocrowns: a finite element analysis

BACKGROUND

Margin designs and loading conditions can impact the mechanical characteristics and survival of endocrowns. Analyzing the stress distribution of endocrowns with various margin designs and loading conditions can provide evidence for their clinical application.

METHODS

Three finite element analysis models were established based on the margin designs: endocrown with a butt-joint type margin (E0), endocrown with a 90° shoulder (E90), and endocrown with a 135° shoulder (E135). The E0 group involved lowering the occlusal surface and preparing the pulp chamber. The E90 group created a 90° shoulder on the margin of model E0, measuring 1.5 mm high and 1 mm wide. The E135 group featured a 135° shoulder. The solids of the models were in fixed contact with each other, and the materials of tooth tissue and restoration were uniform, continuous, isotropic linear elasticity. Nine static loads were applied, with a total load of 225 N, and the maximum von Mises stresses and stress distribution were calculated for teeth and endocrowns with different margin designs.

RESULTS

Compared the stresses of different models under the same loading condition. In endocrowns, when the loading points were concentrated on the buccal side, the maximum von Mises stresses were E0 = E90 = E135, and when there was a lingual loading, they were E0 < E90 = E135. In enamel, the maximum von Mises stresses under all loading conditions were E0 > E90 > E135. In dentin, the maximum von Mises stresses of the three models were basically similar except for load2, load5 and load9. Compare the stresses of the same model under different loading conditions. In endocrowns, stresses were higher when lingual loading was present. In enamel and dentin, stresses were higher when loaded obliquely or unevenly. The stresses in the endocrowns were concentrated in the loading area. In enamel, stress concentration occurred at the cementoenamel junction. In particular, E90 and E135 also experienced stress concentration at the shoulder. In dentin, the stresses were mainly concentrated in the upper section of the tooth root.

CONCLUSION

Stress distribution is similar among the three margin designs of endocrowns, but the shoulder-type designs, especially the 135° shoulder, exhibit reduced stress concentration.

One-piece endodontic crown fixed partial denture: Is it possible?

STATEMENT OF PROBLEM

Whether the replacement of a missing tooth with a fixed partial denture supported by an endodontically treated abutment could be improved with 1-piece endodontic crowns is unclear.

PURPOSE

The purpose of the study was to evaluate the mechanical behavior of a fixed partial denture (FPD) according to the preparation of the abutment teeth (1-piece endodontic crown or complete crown) in terms of stress magnitude in the prosthesis, cement layer, and tooth.

MATERIAL AND METHODS

A posterior model with 2 abutment teeth (first molar and first premolar) was modeled with a computer-aided design (CAD) software program for conducting a 3-dimensional finite element analysis (FEA). To replace the missing second premolar, the model was replicated in different possible FPDs according to the abutment preparation design (complete crown [Conventional], 2 1-piece endodontic crowns [EC]) or a 1-piece endodontic crown on one of the abutment teeth (first molar [ECM] and first premolar [ECP]) for a total of 4 designs. All FPDs were in lithium disilicate. The solids were imported to an analysis software program (ANSYS 19.2) in the standard for the exchange of product data (STEP) format. The mechanical properties were considered isotropic and the materials to show linear elastic and homogeneous behavior. An axial load (300 N) was applied at the occlusal surface of the pontic. The results were evaluated by colorimetric stress maps of von Mises and maximum principal stress in the prosthesis, maximum principal stress and shear stresses on the cement layer, and maximum principal stress in the abutment teeth.

RESULTS

The von Mises stresses revealed that all FPD designs behaved similarly and that, considering the maximum principal stress criteria, the pontic was the most stressed region. For the cement layer, the combined designs presented an intermediate behavior, with the ECM more suitable to reducing the stress peak. The conventional preparation allowed less stress concentration in both teeth, and higher stress concentration in the premolar was observed with a 1-piece endodontic crown. The 1-piece endodontic crown decreased the risk of fracture failure. Considering the risk of debonding failure for the prosthesis, the 1-piece endodontic crown preparation was only able to decrease the failure risk when the EC design was used and when only the shear stress was considered.

CONCLUSIONS

Performing 1-piece endodontic crown preparations to retain a 3-unit lithium disilicate FPD is an alternative to conventional complete crown preparations.

The influence of pre-heating the restoration and luting agent on the flexural strength of indirect ceramic and composite restorations

Background

This study investigated the impact of luting procedure and restoration thicknesses on the flexural strength of CAD/CAM restorations. Traditional luting agents have been questioned in favor of pre-heated resin composites or flowable composites.

Materials and Methods

400 disc-shaped restorations (lithium disilicate [IPS e.max CAD] or resin composite [Tetric CAD, Ivoclar]) were cemented onto dentin analog discs using different procedures (n = 20): dual-curing resin cement (Panavia V5), light-curing resin cement (Panavia Veneer LC), pre-heated resin composite (Clearfil™ AP-X) with or without pre-heated restoration, and high-filled flowable composite (Clearfil Majesty™ Flow). The biaxial flexural strength was calculated.

Results

There were significant effects of material, thickness, and luting procedure on flexural strength (p < 0.001). Resin composite specimens exhibited lower flexural strength (90 MPa) compared to lithium disilicate specimens (571 MPa), with thicker restorations (338 MPa) being stronger than thinner ones (323 MPa). Light-curing cement showed the highest strength (408.8 MPa)A, followed by dual-curing cement (362 MPa)B, pre-heated cement with pre-heated composite (318 MPa)C, pre-heated composite (304 MPa)C, and flowable resin composite (259 MPa)D. The light-curing cement yielded similar results to the pre-heated resin composite associated or not with the pre-heated crown for the thicker lithium disilicate specimens, whereas for the thinner lithium disilicate specimens all luting procedures performed similarly. Thin resin composite discs showed higher flexural strength when luted with light-curing cement, whereas the luting procedure had less influence for the thicker restorations.

Conclusion

Luting procedures impact the flexural strength of CAD/CAM lithium disilicate and resin composite restorations. Pre-heated resin composite, with or without pre-heated restoration, can replace dual-curing cement. Nevertheless, light-curing cement is superior for resin composite and 1.5 mm lithium disilicate restorations.

Internal adaptation and mechanical properties of CAD/CAM glass fiber post-cores in molars: An in vitro study

OBJECTIVE

The purpose of this in vitro study was to evaluate the internal adaptation, fracture resistance, and fracture pattern of the residual roots and crowns of molars restored with computer-aided design/computer-aided manufacturing (CAD/CAM) glass fiber post-cores, and compare them with three other post-core restorations.

MATERIALS AND METHODS

We selected 32 extracted maxillary first molars and divided them into four groups according to the post-core system: traditional casting titanium (Ti) post-cores (TC group); Ti post-cores fabricated with selective laser melting (SLM group); CAD/CAM glass fiber post-cores of the split type (CCS group); and prefabricated glass fiber posts and composite resin cores (PF group). The internal adaptation was analyzed with microcomputed tomography. Teeth were restored with monolithic zirconia crowns and subjected to thermocycling and cyclic loading. A load was applied consistently along the long axis of the tooth until fracture to record the fracture resistance and pattern. For the statistical analysis, one- and two-way analyses of variance, Tukey's post hoc and chi-square tests were performed to compare the differences among the groups.

RESULTS

The CCS, TC, and SLM groups exhibited similar internal adaptations across all sections (P < 0.05). The FP group showed good fit with the root canals in the apical and middle sections but a poor fit with those in the cervical section. The fracture resistance was higher in the CCS, TC, and SLM groups compared to the PF group (P < 0.05). The proportions of restorable fractures in the CCS and PF groups were 62.5% and 50%, respectively. Unrestorable fractures were more frequent in the TC and SLM groups at frequencies of 100% and 87.5%, respectively.

CONCLUSION

The internal adaptation and fracture resistance of the CCS group were similar to those of the TC and SLM groups, and the fracture pattern was mostly restorable, thus meeting the clinical requirements for molar post-core restorations.

CLINICAL SIGNIFICANCE

CCS can be used to restore residual roots and crowns of molars and exhibit high efficacy in terms of adaptability and mechanical properties. More studies are required to evaluate the effectiveness of CCS.

Substrate Rigidity Effect on CAD/CAM Restorations at Different Thicknesses

OBJECTIVES

 This article evaluated the effect of substrates rigidities on the post-fatigue fracture resistance of adhesively cemented simplified restorations in lithium disilicate glass ceramic.

METHODS

 Precrystalized computer-aided design/computer-aided manufacturing ceramic blocks were processed into disc-shaped specimens (n = 10, Ø = 10 mm), mimicking a simplified restoration at two thicknesses (0.5 and 1.0 mm). Thereafter, the discs were cemented onto different base substrates (dentin analogue [control], dentin analogue with a central core build-up of resin composite [RC], or glass ionomer cement [GIC]). The specimens were subjected to mechanical cycling in a chewing simulator (100 N, 1 × 106 cycles, 4 Hz) and then subjected to thermocycling aging (10,000 cycles, 5/37/55°C, 30 seconds). After the fatigue protocol, the specimens were loaded until failure (N) in a universal testing machine. Finite element analysis calculated the first principal stress at the center of the adhesive interface.

RESULTS

 The results showed that "restoration thickness," "type of substrate," and their interaction were statistically significant (one-way analysis of variance; p < 0.001). Regardless the restoration thickness a higher fracture load was observed for specimens cemented to dentin analogue. Among the base materials, RC build-up presented the highest fracture load and lower stress magnitude for both restoration thicknesses in comparison with GIC build-up. The 0.5-mm restoration showed higher stress peak and lower fracture load when submitted to the compressive test.

CONCLUSION

 More flexible base material reduces the fracture load and increases the stress magnitude of adhesively cemented lithium disilicate restorations regardless the ceramic thickness. Therefore, more rigid substrates are suggested to be used to prevent restoration mechanical failures.

Thickness and Substrate Effect on the Mechanical Behaviour of Direct Occlusal Veneers

PURPOSE

This study aimed to evaluate the fracture resistance and stress magnitude of occlusal veneers made of conventional or flowable resin composites at different minimal thicknesses bonded on enamel or dentin.

MATERIAL AND METHODS

A total of 120 sound bovine incisors were flattened and used as substrates (enamel or dentin) for the restorations. The teeth were embedded into polymethyl methacrylate and allocated into 4 groups according to the resin composite (Clearfil AP-X PLT and Clearfil Majesty Flow, Kuraray Dental) and substrate. Further, the substrates were randomly subdivided in 12 groups (N = 120, n = 10) according to the occlusal veneer minimal thickness: 0.5, 1.0, or 2.0 mm. The teeth were directly restored with a standardised procedure. Then, the specimens were loaded until fracture in a universal testing machine (Instron 6022, Instron Corp.). A 3-way and a 1-way analysis of variance were used to determine significant differences for each factor. Three-dimensional finite element analysis was carried out following the in vitro boundary conditions to assess the stress magnitude in the restoration during compressive loading.

RESULTS

The fracture loads were recorded into initial load to failure (ILF) and fatal load to failure (FLF). Differences were found in material for ILF and FLF, leading to an overall equal good performance in fracture load and stress distribution for both materials, regardless of the substrate. Differences in thickness were apparent in both ILF and FLF.

CONCLUSIONS

Direct conventional and flow resin composite occlusal veneers present a promising mechanical behaviour when bonded on enamel or dentin. However, caution is advised when preparing 0.5-mm minimal thickness restorations.

Post-fatigue fracture load, stress concentration and mechanical properties of feldspathic, leucite- and lithium disilicate-reinforced glass ceramics

Objective

To evaluate the mechanical properties of different CAD/CAM ceramic systems and the post-fatigue fracture and stress distribution when used as cemented crowns.

Materials and methods

Sixty (60) CAD/CAM monolithic crowns were milled using three different ceramic materials (FD - Feldspathic [Vita Mark II]), LE - Leucite-based ceramic [IPS Empress CAD] and LD - Lithium Disilicate [IPS e.max CAD]) and adhesively cemented on resin composite dyes. Specimens were stored in distillated water (37 °C) for 7 days. After, half of the crowns were submitted to immediate fracture load test while the other half was submitted to fatigue cycling. The average cement layer of approximately 80 μm was assessed using scanning electron microscopy (SEM). The average thickness was used in the three-dimensional (3D) Finite Element Analysis (FEA). For each ceramic material, the density, Poisson ratio, shear modulus, Young modulus, fracture toughness, and true hardness were assessed (n = 3). The data was used to assess the Maximum Principal Stress throughout 3D-FEA according to each material during load to fail and post-fatigue. Data were submitted to two-way ANOVA and Tukey test (α = 0.05).

Results

LD showed the highest compression load, density, shear modulus, Young modulus, fracture toughness and true hardness values. While LE presented the lowest mechanical properties values. There is no difference in the Poisson ratio between the evaluated ceramics.

Conclusion

LD was susceptible to aging process but presented stronger physicomechanical properties, showing the highest post-fatigue fracture load and highest stress magnitude.

Reliability and lifetime of lithium disilicate, 3Y-TZP, and 5Y-TZP zirconia crowns with different occlusal thicknesses

OBJECTIVES

To assess the probability of survival of crowns made with a 3Y-TZP, a 5Y-TZP, and a lithium disilicate.

MATERIALS AND METHODS

CAD-CAM premolar crowns with occlusal thicknesses of 1.0 mm or 1.5 mm and cemented on a metal dye. Step-stress accelerated life testing (SSALT) was performed to access the use level probability Weibull curves and reliability were calculated for the completion of a mission of 100,000 cycles at 300 to 1200 N.

RESULTS

All ceramics showed a high probability of survival (87-99%) at 300 N, irrespective of thickness. 3Y-TZP shows no significant decrease in the probability of survival up to 1200 N (83-96%). Lithium disilicate presented lower reliability than zirconia under the 600 N mission. 5Y-TZP showed lower reliability than 3Y-TZP at 1200 N. There was no significant difference in the Weibull modulus (3.23-7.12). 3Y-TZP had the highest characteristic strength (2483-2972 N), followed by 5Y-TZP (1512-1547 N) and lithium disilicate (971-1154 N).

CONCLUSION

Zirconia ceramics have a high probability of survival (up to 900 N-load), while lithium disilicate survives up to 300 N load only, irrespective of the thickness (1.0 mm or 1.5 mm).

CLINICAL RELEVANCE

The probability of survival of posterior crowns made with zirconia ceramics resists extreme loads while glass ceramics resist normal chewing loads. In addition, crowns with thinner occlusal face showed sufficient mechanical behavior.

Effect of resin cement elastic modulus on the biaxial flexural strength and structural reliability of an ultra-thin lithium disilicate glass-ceramic material

OBJECTIVES

Photo- and dual-polymerized resin-based luting agent was evaluated for elastic moduli effects on ultra-thin lithium disilicate (LD) glass-ceramic strengthening, structural reliability, and stress distribution.

MATERIALS AND METHODS

One hundred-sixty LD discs (IPS e.max CAD, Ivoclar/Vivadent) were produced in ultra-thin thicknesses (half with 0.3 mm and the other half with 0.5 mm). The ultra-thin ceramic disks were coated with two different cement types (Variolink Veneer - V and Panavia F 2.0 - P). Two positive control groups were tested following hydrofluoric (HF) acid etching (LDt3, LDt5) and two negative control groups were tested for untreated ceramic (LD 3, LD 5). Biaxial flexural strength (BFS), characteristic strength (σ0) and Weibull modulus (m) were the response variables (n = 20) at the ceramic/resin cement interface (z = 0). Finite element analysis (FEA) was used to calculate maximum principal stress. Data were analyzed using two-way ANOVA, and Tukey's test. Scanning electron microscopy (SEM) was used to analyze the failed specimens using fractography and surface morphology.

RESULTS

The BFS of LD at either thickness was not affected by cement types, as also demonstrated by FEA. Structural reliability significantly improved in the positive control group (LDt5).

CONCLUSION

The cementation of ultra-thin LD with a resin-cement of varying elastic moduli did not influence BFS. LD surface modification by HF acid-etching increased the reliability.

CLINICAL RELEVANCE

Ultra-thin anterior veneer designs made from lithium disilicate have been widely proposed and the apparent success of LD ultra-thin veneers was not influenced by the cement choice in the current studies albeit the elastic moduli luting agents used were of similar values.

Stress distribution in resin-based CAD-CAM implant-supported crowns

OBJECTIVE

This study aimed to evaluate the influence of new resin-based CAD-CAM implant-supported materials on posterior crown restoration stress and strain concentrations.

METHODS

A previous 3D implant model was edited to receive a cement-retained posterior crown manufactured with different CAD/CAM materials (Estelite P Block, Estelite Block II or Estelite Layered Block). Each solid model was exported to the computer-aided engineering software and submitted to the finite element analysis of stress and strain. Material properties were assigned to each solid with isotropic and homogeneous behavior according to the manufacturer information. A vertical load of 600 N was applied in the occlusal region of the crown, via a simulated food bolus, and stress was calculated in Von Misses (σVM) for the implant, abutment and screw, Maximum (σMAX) Principal Stresses for the crown and microstrain for the bone.

RESULTS

All simulated materials showed acceptable stresses levels with a similar stress pattern among the models. At the crown intaglio region and cement layer, however, differences were observed: Estelite P Block showed a lower tensile and shear stresses magnitude when compared to other resin-based materials with lower elastic modulus.

SIGNIFICANCE

The stress effect of different resin-based CAD-CAM implant-supported crowns is predominant in the crown and cement layer, with Estelite P Block showing 7.4 % versus 9.3 % and 9.2 % for Estelite Block II and Estelite Layered Block of crown failure risk.

The Use of Polyetheretherketone (PEEK) as an Alternative Post and Core Material: Five-Year Follow-Up Report

This clinical report demonstrated the use of polyetheretherketone (PEEK) for manufacturing of custom-made post and core in weakened endodontically treated central incisors. The PEEK structure was manufactured using computer-aided design/computer-aided manufacturing (CAD/CAM). The optimal fit of this custom-made endodontic post allowed a thinner cement layer; and removed the need to manufacture a core build-up. While supplementary clinical trials and in vitro studies are needed to totally elucidate the advantages and limitations of PEEK as an option for post and core manufacturing, this case report showed that it can be promising for a predictable and simplified treatment with five years of success.

Sintering mode of a translucent Y-TZP: Effects on its biaxial flexure fatigue strength, surface morphology and translucency

OBJECTIVE

This investigation evaluated the effect of two sintering modes of a translucent zirconia (Y-TZP) on its surface roughness, topography, phase-transformation (t → m), translucency and biaxial flexure fatigue strength.

MATERIALS AND METHODS

To do so, 50 Y-TZP discs (Ø = 15 mm; thickness = 1.2 mm; IPS e.max ZirCAD LT) were prepared and divided into two groups: Standard mode (SM) and Fast mode (FM). Staircase fatigue testing was performed (piston-on-three balls set-up, ISO 6872:2015), as well as surface roughness, profilometry, scanning electron microscopy (SEM-FEG), energy dispersive X-ray spectroscopy (EDX), phase transformation (t → m) using X-ray diffraction analysis (XRD), translucency parameter analysis (TP and TP₀₀ ) and fractography.

RESULTS

The results showed no statistical significant differences for roughness parameters (p > 0.05, SM: Ra = 0.13 ± 0.02, Rz = 1.21 ± 0.26 and RSm = 24.91 ± 2.19; FM: Ra = 0.14 ± 0.03, Rz = 1.32 ± 0.25 and RSm = 24.68 ± 2.16) or flexural fatigue strength (SM: 512 (464-560) MPa; FM: 542 (472-611) MPa) between the groups. In addition, similarity in surface morphological features (SEM and profilometry), composition and phases (EDX and XRD) was observed between the firing protocols. Fractography showed that the failure origin occurred on the tensile side. Sintering mode did not affect the TP (F = 0.001, p = 0.97) and TP₀₀ (F = 0.12, p = 0.72).

CONCLUSIONS

Therefore, the fast-sintering mode is suggested as a viable alternative to the standard mode since it does not influence the evaluated surface morphology, microstructure, fatigue strength and translucency of a translucent monolithic zirconia.

CLINICAL SIGNIFICANCE

The fast sintering mode is a viable alternative for zirconia without compromising its topography, microstructure, mechanical performance or translucency.

Impact of different axial wall designs on the fracture strength and stress distribution of ceramic restorations in mandibular first molar

BACKGROUND

The purpose of this study was to investigate the fracture strength and stress distribution of four ceramic restorations.

METHODS

Forty human mandibular first molars were collected and randomized into four groups after establishing the distal defect: full crown group with 4 mm axial wall height (AWH) (FC4); short AWH crown group with 2 mm AWH (SC2); occlusal veneer group with 0 mm AWH (OV0); occlusal distal veneer group with only the distal surface prepared, and 4 mm AWH (OD4). The teeth were prepared according to the groups and the ceramic restorations were completed using celtra duo ceramic blocks. The ceramic thickness of the occlusal surface is about 1.5 mm and the edge is about 1 mm. The failure load values and fracture modes of each group were detected by mechanical test in vitro. According to the groups to establish three-dimensional finite element analysis (FEA) models, a 600 N loading force was applied vertically using a hemispherical indenter with a diameter of 6 mm. and compare the stress distribution under the condition of different restorations.

RESULTS

In vitro mechanical tests showed that the failure load values were SC2 (3232.80 ± 708.12 N) > OD4 (2886.90 ± 338.72 N) > VO0 (2133.20 ± 376.15 N) > FC4(1635.40 ± 413.05 N). The failure load values of the short AWH crown and occlusal distal veneer were significantly higher than that of occlusal veneer and full crown (P<0.05). The fracture modes of the full crown and occlusal veneer groups were mainly ceramic fractures and some were restorable tooth fractures. The short AWH crown and occlusal distal veneer groups presented with three fracture modes, the proportion of non-restorable tooth fracture was higher. The results of FEA show that under the spherical loading condition, the stress of ceramic was concentrated in the contact area of the loading head, the maximum von Mises stress values were FC4 (356.2 MPa) > VO0 (214.3 MPa) > OD4 (197.9 MPa) > SC2 (163.1 MPa). The stress of enamel was concentrated in the area where the remaining enamel was thinner, the maximum von Mises stress values was OD4 (246.2 MPa) ≈ FC4 (212.4 MPa) > VO0 (61.8 MPa) ≈ SC2 (45.81 MPa). The stress of dentin is concentrated in the root furcation and the upper third region of the root. However, stress concentration was observed at the tooth cervix in the full crown.

CONCLUSION

Under certain conditions, the occlusal distal veneer shows better performance than the full crown.

Annual review of selected scientific literature: A report of the Committee on Scientific Investigation of the American Academy of Restorative Dentistry

The Scientific Investigation Committee of the American Academy of Restorative Dentistry offers this review of the 2021 dental literature in restorative dentistry to inform busy dentists regarding noteworthy scientific and clinical progress over the past year. Each member of the committee brings discipline-specific expertise to coverage of this broad topical area. Specific subject areas addressed, in order of the appearance in this report, include COVID-19 and the dental profession (new); prosthodontics; periodontics, alveolar bone, and peri-implant tissues; implant dentistry; dental materials and therapeutics; occlusion and temporomandibular disorders; sleep-related breathing disorders; oral medicine and oral and maxillofacial surgery; and dental caries and cariology. The authors focused their efforts on reporting information likely to influence daily dental treatment decisions with an emphasis on future trends in dentistry. With the tremendous volume of dentistry and related literature being published daily, this review cannot possibly be comprehensive. Rather, its purpose is to update interested readers and provide important resource material for those interested in pursuing greater details on their own. It remains our intent to assist colleagues in negotiating the extensive volume of important information being published annually. It is our hope that readers find this work useful in successfully managing the patients and dental problems they encounter.

In Vitro Evaluation of the Shading Effect of Various Zirconia Surface Stains on Porcelain Crowns

Human teeth display various colors under natural light. Dental restorations, such as zirconia crowns, are generally used to rehabilitate the oral function of patients with tooth loss due to trauma or natural tooth falls. However, significant improvements in the color and translucency of zirconia are required to meet the clinical needs for dental restoration. In the past, a large amount of ceramic powder has been used to improve the appearance of zirconia. However, the interface between the ceramic powder and zirconia makes them prone to falling off. Therefore, the aesthetics of zirconia crowns remains a major challenge. Recently, substantial advances have been made in the field of dental materials, as special staining agents for zirconia have been introduced as alternatives to ceramic powders. Therefore, this study tested zirconia-specific staining agents that were used to produce zirconia samples with A1 and A3 colors. A dental colorimetric plate was used as the control group to assess the staining effects of the different brands of staining agents. Meanwhile, two hypotheses were proposed: that the staining effects of these special staining agents for zirconia met the criteria for clinical application and that there was no significant difference between the different staining agents for zirconia. The results showed that the coatings of different brands of staining agents were ultrathin, with a thickness of approximately 27–78 µm. In addition, the coloring effects of the zirconia staining agents were not significantly different from those of the colorimetric plates. After staining, the zirconia samples had decreased surface roughness and contact angle values, which improved surface smoothness and cleanliness. In summary, the results support the hypothesis of this study that zirconia stains can be used as an alternative to the current fabrication methods for clinical dental restorations. We sought to identify the clinical techniques that are easier to perform and to overcome the current problem of dental technicians requiring considerable dentin space for staining. It is expected that the results of this study will be useful in clinical dental restorations.

Biomechanical Assessment of Tilted Mandibular Second Molars with Full-Crown Adjacent to Implant-Supported Restoration: 3D Finite Element Analysis

Purpose

Materials and Methods

Results

Conclusion

Comprehensive CAD/CAM Prosthetic Rehabilitation Management in a Young Patient with Agenesis: A Case Report

With the advancement in ceramic restorations bonded to the tooth structure, the treatment has become a practicable and conservative option to restore teeth with shape alteration owing to the high demand for aesthetics, with minimum tooth preparation. This article describes the case of a 25-year-old man who was dissatisfied with his smile. After an assessment of the clinical situation, a decision was made to place a full-mouth prosthodontic rehabilitation (monolithic lithium disilicate glass-ceramic, IPS E-max CAD) with minimal tooth preparation, which figures as a conservative full-coverage approach. The restoration of an aesthetic smile resulted in the patient’s satisfaction. Bonding of all monolithic veenerlay and crowns onto minimally prepared teeth figures as conservative treatment in cases of hypodontia.

Effect of Different Ceramic Materials on Fatigue Resistance and Stress Distribution in Upper Canines with Palatal Veneers

OBJECTIVE

 The aim of this study was to evaluate, by means of a fatigue life test, different ceramic materials used in palatal veneers to restore the canine guidance.

MATERIALS AND METHODS

 Forty-five standardized anatomical preparations were made in extracted healthy human canines with 1.2 uniform thickness. Samples were scanned, restorations were designed and milled in polymer-infiltrated ceramic network (PICN, Vita Enamic), zirconia-reinforced lithium silicate (ZLS, Vita Suprinity), and high translucent yttrium oxide-stabilized tetragonal zirconia (YZHT, Vita YZHT). Dental preparations were etched, restorations were processed according to the manufacturers' recommendations, and adhesively cemented. Then, three samples of each group were tested with load-to-fracture to determine the fatigue parameters. In addition, the palatal veneers stresses were evaluated using numerical models through finite element analysis.

RESULTS

 The mean of the monotonic test for PICN, ZLS, and YZHT was 674.18 N, 560.5 N, and 918.98 N, respectively. The StepWise test was performed until specimen fracture or until suspension of the test after 1.2 × 10⁶ cycles. Regarding survival, using the Kaplan-Meier method, PICN presented results for the mean and median of 245.21 N and 225 N, respectively; ZLS had an average of 175.76 N and a median of 168 N, and YZHT with an average of 383.30 N and a median of 366 N. Regarding the Weibull method, PICN showed results of 5.43 β and 264 η for form and scale, respectively; ZLS had 36.14 β for form and 380.67 η for scale; and YZHT presented 4.95 β for form and 417.38 η for scale. The highest stress value was calculated for YZHT, ZLS, and PICN, respectively.

CONCLUSIONS

 It was possible to conclude that all tested materials have the possibility of being used for rehabilitation of upper canines' palatal surface.

Mechanical behavior and microstructural characterization of different zirconia polycrystals in different thicknesses

PURPOSE

To characterize the microstructure of three yttria partially stabilized zirconia ceramics and to compare their hardness, indentation fracture resistance (IFR), biaxial flexural strength (BFS), and fatigue flexural strength.

MATERIALS AND METHODS

Disc-shaped specimens were obtained from 3Y-TZP (Vita YZ HT), 4Y-PSZ (Vita YZ ST) and 5Y-PSZ (Vita YZ XT), following the ISO 6872/2015 guidelines for BFS testing (final dimensions of 12 mm in diameter, 0.7 and 1.2 ± 0.1 mm in thicknesses). Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed, and mechanical properties were assessed by Vickers hardness, IFR, quasi-static BFS and fatigue tests.

RESULTS

All ceramics showed similar chemical compositions, but mainly differed in the amount of yttria, which was higher as the amount of cubic phase in the diffractogram (5Y-PSZ > 4Y-PSZ > 3Y-TZP). The 4Y- and 5Y-PSZ specimens showed surface defects under SEM, while 3Y-TZP exhibited greater grain uniformity on the surface. 5Y-PSZ and 3Y-TZP presented the highest hardness values, while 3Y-TZP was higher than 4Y- and 5Y-PSZ with regard to the IFR. The 5Y-PSZ specimen (0.7 and 1.2 mm) showed the worst mechanical performance (fatigue BFS and cycles until failure), while 3Y-TZP and 4Y-PSZ presented statistically similar values, higher than 5Y-PSZ for both thicknesses (0.7 and 1.2 mm). Moreover, 3Y-TZP showed the highest (1.2 mm group) and the lowest (0.7 mm group) degradation percentage, and 5Y-PSZ had higher strength degradation than 4Y-PSZ group.

CONCLUSION

Despite the microstructural differences, 4Y-PSZ and 3Y-TZP had similar fatigue behavior regardless of thickness. 5Y-PSZ had the lowest mechanical performance.

Functional or Nonfunctional Cusps Preservation for Molars Restored with Indirect Composite or Glass-Ceramic Onlays: 3D FEA Study

Evidence regarding the effect of the onlay preparation design for different CAD/CAM restorative materials considering the preservation of cusps is lacking. Molars were 3D-modeled in four preparation designs for onlay restoration: traditional design with functional cusp coverage (TFC), non-retentive design with functional cusp coverage (NFC), traditional design with non-functional cusp coverage (TNFC) and non-retentive design with non-functional cusp coverage (NNFC). The restorations were simulated with two CAD/CAM restorative materials: LD—lithium disilicate (IPS e.max CAD) and RC—resin composite (GrandioBloc). A 100 N axial load was applied to the occlusal surface, simulating the centric contact point. Von Mises (VM) and maximum principal (Pmax) stress were evaluated for restorations, cement layer and dental substrate. The non-retentive preparation design reduced the stress concentration in the tooth structure in comparison to the conventional retentive design. For LD onlays, the stress distribution on the restoration intaglio surface showed that the preparation design, as well as the prepared cusp, influenced the stress magnitude. The non-retentive preparation design provided better load distribution in both restorative materials and more advantageous for molar structure. The resin composite restoration on thenon-functional cusp is recommended when the functional cusp is preserved in order to associate conservative dentistry and low-stress magnitude.

Comparison of Fracture Load of the Four Translucent Zirconia Crowns

Recently, translucent zirconia has become the most prevalent material used as a restorative material. This study aimed to compare the crown fracture load of the four most common different translucent zirconia brands available in the market at 1.5 mm thickness. Standardized tooth preparations for a full ceramic crown were designed digitally with software (AutoCAD) by placing a 1.0 mm chamfer margin and 1.5 mm occluso-cervical curvature for the crown sample manufacturing. Stylized crowns were chosen to control the thickness of the crown. The axial and occlusal thickness were standardized to 1.5 mm thickness except at the central pit, which was 1.3 mm thick. The STL file for the tooth dies was prepared using software (3Shape TRIOS^® Patient Monitoring, Copenhagen, Denmark). The tooth dies were printed with a resin material (NextDent Model 2.0, Vertex-Dental B.V., Soesterberg, The Netherlands) using a 3D printing software (3D Sprint^® Client Version 3.0.0.2494) from a 3D printer (NextDent™ 5100, Vertex-Dental B.V., Soesterberg, The Netherlands). The printing layer thickness was 50 µm. Then, a total of twenty-eight (N = 28) stylized crowns were milled out of AmannGirrbach (Amann Girrbach GmbH, Pforzheim, Germany) (n = 7), Cercon HT (Dentsply Sirona, Bensheim, Germany) (n = 7), Cercon XT (Dentsply Sirona, Bensheim, Germany) (n = 7), and Vita YZ XT (Zahnfabrik, Bäd Sackingen, Germany) (n = 7). Following sintering the crowns, sandblasting was performed and they were bonded to the tooth dies with the resin cement (RelyX U-200, 3M ESPE, Seefeld, Germany) and permitted to self-cure under finger pressure for 6 min. The crowns were loaded on the occlusal surface in a universal testing machine (MTS Centurion) with a stainless-steel ball indenter (7 mm radius) with a loading rate of 1 mm/min to contact the stylized crowns on each of the four cusps until failure. A rubber sheet (1.5 mm thickness) was positioned between the crown and indenter, which helped with the load distribution. Statistical analysis was done using SPSS version 20 (IBM Company, Chicago, USA). The fracture loads were analyzed using Dunnett’s T3 test, and the number of cracks was analyzed using the Mann–Whitney U test among the groups. The significant level was set at p value = 0.05. The mean fracture loads were 3086.54 ± 441.74 N, 4804.94 ± 70.12 N, 3317.76 ± 199.80 N, and 2921.87 ± 349.67 N for AmannGirrbac, Cercon HT, Cercon XT, and Vita YZ XT, respectively. The mean fracture loads for the surfaces with the greatest number of cracks (excluding the occlusal surfaces) were on the lingual surface for AmannGirrbach and Cercon HT, on the distal and mesial for Cercon XT, and on the buccal for Vita YZ XT. We found that the AmannGirrbach had the most overall cracks. Cercon XT had the greatest number of occlusal cracks and appeared to be the most shattered. Cercon HT had the least number of cracks. In conclusion, Cercon HT presented the best strength properties, the highest fracture load, and no visible cracks. AmannGirrbach presented the lowest strength properties.

Comparative Stress Evaluation between Bilayer, Monolithic and Cutback All-Ceramic Crown Designs: 3D Finite Element Study

Different all-ceramic crown designs are available to perform indirect restoration; however, the mechanical response of each model should still be elucidated. The study aims to evaluate the stress distribution in three different zirconia crown designs using finite element analysis. Different three-dimensional molar crowns were simulated: conventional bilayer zirconia covered with porcelain, a monolithic full-contour zirconia crown, and the cutback modified zirconia crown with porcelain veneered buccal face. The models were imported to the computer-aided engineering (CAE) software. Tetrahedral elements were used to form the mesh and the mechanical properties were assumed as isotropic, linear and homogeneous materials. The contacts were considered ideal. For the static structural mechanical analysis, 100 N occlusal load was applied and the bone tissue was fixed. Maximum principal stress showed that the stress pattern was different for the three crown designs, and the traditional bilayer model showed higher stress magnitude comparing to the other models. However, grayscale stress maps showed homogeneous stress distribution for all models. The all-ceramic crown designs affect the stress distribution, and the cutback porcelain-veneered zirconia crown can be a viable alternative to adequate function and esthetic when the monolithic zirconia crown cannot be indicated.

Toothbrushing Wear Resistance of Stained CAD/CAM Ceramics

Aim: To investigate the effect of toothbrushing on different stained Computerd Aided Design/Computer Aided Manufacturing CAD/CAM ceramics. Materials and Methods: Fifty specimens (high translucency zirconia, YZHT; zirconia-reinforced lithium silicate, ZLS; feldspathic, FDL; hybrid ceramic, HC; all from Vita Zahnfabrik) were divided into five groups according to their staining technique. ZLS allowed the composition of two groups: ZLS1 with crystallization and staining together; ZLS2, with the stain firing after the crystallization. YZHT received a stain firing after the sintering process, and FDL received stain firing directly. The HC was stained with acrylic staining, and was light-cured on its surface. The specimens were brushed in total for 150,000 cycles at 2.45 N with 180 strokes/min. Surface measurements to obtain Rz were performed after 50,000, 100,000 and 150,000 cycles, with five evaluation lines (5 mm) per specimen orthogonal to the brushing direction, covering brushed and unbrushed areas. The wear was analyzed using two-way ANOVA and Tukey tests (α = 5%). Scanning electron microscopy (SEM) was performed to access the surface profile. The wear was affected by material (p < 0.001) and time (p = 0.139). Superior wear (µm) was observed for HC (6.6 ± 4.4; 6.8 ± 4.0; 9.2 ± 3.5) compared to ZLS2 (1.0 ± 0.3; 1.2 ± 2.2; 1.3 ± 0.3), YZHT (1.0 ± 0.1; 1.2 ± 0.3; 1.2 ± 0.3), ZLS1 (0.9 ± 0.1; 1.1 ± 0.5; 1.2 ± 0.3) and FDL (0.9 ± 0.1; 0.9 ± 0.1; 1.0 ± 0.2) after 5, 10 and 15 years of simulation, respectively. SEM showed different wear patters for HC with the removal of the glaze layer. HC showed a higher staining wear rate compared to the glass-based and polycrystalline ceramics after 15 years. The extrinsic characterization of feldspathic ceramic showed its superior longevity compared to the evaluated high-translucency zirconia, zirconia-reinforced lithium silicate, and hybrid ceramic.