Influence of ceramic material, thickness of restoration and cement layer on stress distribution of occlusal veneers

The influence of hygroscopic expansion of resin supporting dies on the fracture resistance of ceramic restorations during thermal cycling

OBJECTIVES

To evaluate the hygroscopic expansion characterization of resin composite dies during thermal cycling, and their influence on the fracture resistance of dental ceramic materials as well as the effect of pre-immersion on these measurements.

METHODS

Disc-shaped specimens (φ = 15.0 mm, h = 1.2 mm) and anatomical crown dies of four resin composites (epoxy, Z350, P60, G10) were fabricated. Disc-shaped samples were continuously soaked in distilled water and the volume expansion was measured at different time point by Archimedes method. Disc-shaped samples were pre-immersed for 0, 7, or 30 days, elastic modulus and hardness were measured using Nanoindentation test; thermal cycling (TC) test was performed (5 °C-55 °C, 104 cycles), and volume expansion during TC was measured. Four kinds of resin die with pre-immersion for 0, 7, or 30 days were cemented to 5Y-Z crown, or epoxy dies without pre-immersion were cemented to 5Y-Z, 3Y-Z and lithium disilicate glass (LDG) crowns, and load-to-failure testing was performed before and after TC. Finite element analysis (FEA) and fractography analysis were also conducted.

RESULTS

The hygroscopic expansion was in the order: epoxy > Z350 > P60 > G10. Except for G10, the other three resin composites exhibited different degrees of hygroscopic expansion during TC. Only the elastic modulus and hardness of epoxy decreased after water storage. However, only the fracture loads of 5Y-Z and LDG crowns supported by epoxy dies were significantly decreased after TC. FEA showed a stress concentration at the cervical region of the crown after volume expansion of the die, leading to the increase of the peak stress at the crown during loading.

SIGNIFICANCE

Only the hygroscopic expansion of epoxy dies caused by TC led to the decrease in the fracture resistance of the 5Y-Z and LDG crown, which may be related to the decrease in the elastic modulus of the epoxy die and the tensile stress caused by it.

Fracture resistance of chairside CAD-CAM lithium disilicate occlusal veneer with various designs after mechanical aging

PURPOSE

This study evaluated the fracture resistance of chairside computer-aided design and computer-aided manufacturing (CAD-CAM) lithium disilicate crown, onlay, and non-anatomical occlusal veneer (A-OV) with and without margin fabricated.

MATERIALS AND METHODS

Sixty-four CAD-CAM lithium disilicate restorations were designed as (1) complete coverage crown (CCC); (2) A-OV with margin; (3) non-A-OV with margin (NA-OV-M); and (4) non-A-OV without margin (NA-OV-NM), 16 of each. Restorations were crystallized and adhesively luted to resin dies using resin cement. Specimens were then subjected to 400,000 cycles of chewing in a mastication simulator. A universal testing machine was used to apply a compressive load at a crosshead speed of 1 mm/min to the long axis of the tooth with a stainless-steel sphere until fracture occurred. One-way ANOVA followed by post hoc tests were used to assess the impact of preparation design on the fracture load of CAD-CAM lithium disilicate restorations.

RESULTS

The highest fracture load was recorded for CAD-CAM lithium disilicate indirect restorations for non-A-OVs preparation with margin (2549 ± 428 N) and onlay (2549 ± 293 N) and the lowest fracture load was recorded for CCCs (2389 ± 428 N); however, there was no significant (p = 0.640) between groups.

CONCLUSIONS

CAD-CAM lithium disilicate restorations fabricated for anatomical and non-A-OV preparation display a fracture resistance similar to CCCs. Conservative partial coverage restorations may be considered an acceptable approach for posterior teeth.

Effect of abutment screw-access hole on the fatigue performance of implant-supported lithium-disilicate luted simplified restorations

The present study aimed to evaluate the impact of the existence of an abutment screw-access hole and the filling effects on the fatigue mechanical behavior of a luted lithium-disilicate glass-ceramic. Seventy-two discs (Ø = 10 mm, 1.0 mm in thickness) of lithium disilicate (IPS e.max CAD, Ivoclar AG) were obtained from prefabricated blocks. Thirty-six abutment specimens of an opaque zirconia (Yz - IPS e.max ZirCAD, Ivoclar AG) and titanium (Ti - Luminesse Ti-Cam discs, Talladium Inc.) were confectioned, and allocated according to 6 groups: Yz and Ti rigid (without screw access hole); Yz unfilled, Yz filled, Ti unfilled and Ti filled (with the screw access (Ø = 2.5 mm) in the center). For the unfilled groups, only a polytetrafluoroethylene tape was used. Resin composite (Tetric N-Ceram, Ivoclar AG) was applied to the screw access hole for the filled groups (Yz and Ti). A cyclic fatigue test was carried out (load of 200 N, 10,000 cycles each; 20 Hz of frequency, step size of 100 N until failure detection (radial/cone crack). The fatigue failure load (FFL) and number of cycles until failure (CFF) were recorded for statistical purposes. The stress distribution (MPa) was evaluated by finite element analysis. A statistically positive effect of the abutment material and the presence of the screw access hole was observed (p ≤ 0.05). The rigid groups (without screw access holes) depicted almost 100% of survival after the fatigue tests. Among the other groups, the Yz-filled group showed the best performance (p ≤ 0.05), followed by the Yz unfilled group. The Ti groups depicted lower values of FFL and CFF, with the Ti unfilled group showing the most unfavorable fatigue behavior (p ≤ 0.05). The lowest tensile stress concentration in the restorative material was observed with the use of rigid abutments, the filled groups depicted intermediate values, while unfilled groups showed the highest stress concentration (Yz rigid = 306.3 MPa; Ti rigid = 310.4 MPa < Yz filled = 490.7 MPa; Ti filled = 498.9 MPa < Yz unfilled = 707.6 MPa; Ti unfilled = 719.7 MPa). Therefore, the presence of a screw-access hole decreases the mechanical performance of a lithium disilicate ceramic regardless of the abutment material. In the presence of a screw-access hole, zirconia abutments depicted a higher fatigue failure load when compared with titanium. The filling of the abutment screw-access hole with resin composite increased the mechanical performance of the simulated restoration.

Influence of occlusal reduction design on the fracture resistance and biomechanical behavior of endocrowns restoring maxillary premolars

PURPOSE

To investigate the effect of different occlusal reduction design on stress distribution and fracture resistance of different endocrown systems.

MATERIAL AND METHODS

Sixty-four maxillary human premolars with endodontic treatment, prepared for endocrowns were divided into 2 groups (n = 32) according to the occlusal design: Butt joint preparation (B group) and Anatomical preparation (A group). Each group were subdivided into four groups according to ceramic systems: IPS E max CAD (EM group), monolithic zirconia (ZR group), Nacera Hyprid (NH group) and PEKKTON (PE group). After manufacturing of endocrowns and adhesive bonding the specimens were thermomechanically loaded and subsequently they were tested in a universal testing machine for evaluating the fracture resistance. The specimens failure mode was qualitatively assessed. The stress distribution in each group was assessed using three-dimensional finite element analysis (FEA). 1-way ANOVA and the Post Hoc Tukey HSD test were used to evaluate the data (a = .05).

RESULTS

The fracture resistance values between the groups showed statistically significant variations. The B PE and A PE groups had a higher ratio of fracture resistance values. Regarding failure mode, ceramic endocrowns recorded mainly irreparable failures. FEA showed that anatomical occlusal preparation have reduced the stress concentration under all endocrown systems.

CONCLUSION

Endocrowns could be used to restore endodontically treated maxillary premolars. PEKKTON endocrowns with anatomical preparations revealed most appropriate restoration. The tested new endocrown systems enhanced the biomechanical performance of the tooth.

CLINICAL SIGNIFICANCE

The innovative endocrown systems (PEKK, Nacera Hyprid) can be seen as a promising choice for restoration of severely-destructed endodontic treated premolars, with less stress transmit to the residual tooth structure. Although the traditional endocrown technology might increase the longevity of tooth bonding, it shouldn't be used for clenching cases since the risk of failure is too great overall.

Cyclic fatigue of a repaired 4 YSZ ceramic: Effect of the repair protocol on the adhesive and mechanical behavior

Objective

To evaluate the effect of different surface treatments on the morphology, shear bond, and flexural fatigue strength of a repaired translucent zirconia.

Methods

Monolithic disc-shaped specimens of translucent zirconia were prepared and ground to simulate repair areas. Four groups underwent different treatments: Air-MDP (air-abrasion with alumina particles and 10-MDP primer), Si-Sil (silica-coated alumina particles with MDP-containing silane), Si-MDP (silica coating with 10-MDP primer), and Uni adhe (universal adhesive). After roughness measurements and treatments, repairs were done using resin composite. Shear bond and flexural (n = 15) fatigue tests were performed. Surface topography, interfacial analysis, fractographic, and finite element analysis were conducted.

Results

The zirconia roughness was similar between the groups, however, the surface topography was modified according to the surface treatments. Si-Sil generated higher and more stable bond strength values (20.69 MPa) between translucent zirconia and resin composite when compared to Uni adhe (15.75 MPa) considering the fatigue bond strength scenario, while it was similar to Si-MDP (17.70 MPa) and Air-MDP (18.97 MPa). Regarding the mechanical behavior, Si-Sil (680.83 MPa) also showed higher and significantly different fatigue strength when compared to Uni adhe (584.55 MPa), while both were similar to Si-MDP (634.22 MPa) and Air-MDP (641.86 MPa).

Conclusion

The association of mechanical and chemical approaches is essential for long-term bond strength and optimized mechanical behavior, being air-abrasion protocols and the use of silane and/or MDP-based primers suitable for zirconia repair protocols. It was found that relying solely on a universal adhesive was not as effective as other options available.

Clinical significance

The surface treatment of repair protocols affects translucent zirconia's morphology. To enhance fatigue behavior in repaired monolithic zirconia, air abrasion is crucial. Exclusive use of a universal adhesive is less effective than other choices. A primer containing silane/MDP holds the potential for stable bond strength and optimized mechanical performance.

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.

Evaluation of Fracture Resistance of Occlusal Veneers Made of Different Types of Materials Depending on Their Thickness

Pathological tooth wear is an escalating social problem. Occlusal veneers can be an alternative to traditional prosthetic restorations such as crowns, inlays, and onlays.

BACKGROUND

The aim of this study is to assess the fracture resistance of occlusal veneers made of various materials depending on their thickness.

METHODS

In total, 120 occlusal veneers were examined. The restorations were made of four ceramics: leucite LC (IPS Empress Esthetic), hybrid HC (Vita Enamic), lithium disilicate LDC (IPS e.max Press), and zirconium oxide ZOC (Ceramill Zolid HT). A total of 30 veneers were made of each material, 10 for each of the three thicknesses: 1 mm, 1.5 mm, 2 mm. The restorations were cemented on identical abutments duplicated from the developed phantom tooth 35 (KaVo) with composite cement (All Bond Universal). The samples prepared in this way were subjected to a compressive strength test in a universal testing machine. Statistical analysis of the results was performed.

RESULTS

The average fracture resistance of occlusal veneers made of zirconium oxide ceramic was 1086-1640 N, of lithium disilicate ceramics 456-1044 N, of hybrid ceramics 449-576 N, and of leucite ceramics 257-499 N.

CONCLUSIONS

Occlusal veneers made of ceramics, zirconium oxide and lithium disilicate, had the highest resistance to fractures. Restorations made of leucite ceramics turned out to be the least resistant to forces. The greater the thickness of the ceramic occlusal veneers, the greater their fracture resistance.

Research Progress and Clinical Application of All-Ceramic Micro-Veneer

Anterior teeth problems affect the patient's daily eating, communication, social activities, self-confidence, and mental health. The trend in dentistry is to address anterior tooth problems with minimally invasive and aesthetic treatments. With the development of adhesive materials and ceramics, micro-veneers have been proposed as an alternative treatment for enhancing the aesthetic appearance and avoiding unnecessary tooth reduction. A micro-veneer is a veneer that can be cemented to the surface without or with minimal tooth preparation. These benefits include no need for anesthesia, postoperative insensitivity, good adhesion to enamel, reversibility of treatment, and higher patient acceptance. However, the micro-veneer repair is suitable only for specific cases and must be strictly controlled regarding indication. Treatment planning is a crucial step to achieving functional and aesthetic rehabilitation, and following the clinical protocol is helpful for the longevity and success of micro-veneer restorations. However, more precise and predictable tooth preparation methods, such as minimally invasive microscopic tooth preparation and digitally guided veneer preparation, are recommended rather than the traditional free-hand method. Therefore, this paper clarifies micro-veneers and compares them with other restorations to gain a deeper and more comprehensive understanding. The authors also review indications, materials, cementation, and effect evaluation of micro-veneers to provide clinicians with valuable information. In conclusion, micro-veneers are minimally invasive treatments that provide good restoration results when used appropriately and are worthy of promotion for the aesthetic restoration of anterior teeth.

Ceramic surface conditioning, resin cement viscosity, and aging relationships affect the load-bearing capacity under fatigue of bonded glass-ceramics

This study aimed to evaluate the influence of ceramic surface treatments, resin cement viscosities, and storage regimens on the fatigue performance of bonded glass-ceramics (lithium disilicate, LD; feldspathic, FEL). Ceramic discs (Ø = 10 mm; thickness = 1.5 mm) were allocated into eight groups per ceramic (n = 15), considering three factors: "ceramic surface treatment" in two levels - 5% hydrofluoric acid etching and silane-based coupling agent application (HF), or self-etching ceramic primer (E&P); "resin cement viscosity" in two levels - in high or low viscosity; and "storage regimen" in two levels - baseline, 24 h to 5 days; or aging, 180 days + 25,000 thermal cycles. Adhesive luting was performed onto glass fiber-reinforced epoxy resin discs (Ø = 10 mm; thickness = 2 mm) and the bonded assemblies were subjected to cyclic fatigue tests: initial load = 200 N; step-size = 25 N (FEL) and 50 N (LD); 10,000 cycles/step; 20 Hz. Scanning electron microscopy (SEM) inspections were performed. Regarding the LD ceramic, the fatigue behavior was reduced after aging for HF_HIGH and E&P_LOW conditions, while stable performance was observed for HF_LOW and E&P_HIGH. Regarding the FEL results, aging negatively affected HF_HIGH, E&P_HIGH, and E&P_LOW, being that only the HF_LOW condition presented a stable behavior. The failure initiated from defects on the etched surface of the ceramics, where the cross-sectional analysis commonly revealed unfilled areas. Long-term aging might induce a decrease in mechanical behavior. The 'ceramic microstructure/surface conditioning/resin cement viscosity relationships' modulate the fatigue performance of lithium disilicate and feldspathic glass-ceramics.

A Scoping Review on the Polymerization of Resin-Matrix Cements Used in Restorative Dentistry

In dentistry, clinicians mainly use dual-cured or light-cured resin-matrix cements to achieve a proper polymerization of the organic matrix leading to enhanced physical properties of the cement. However, several parameters can affect the polymerization of resin-matrix cements. The main aim of the present study was to perform a scoping review on the degree of conversion (DC) of the organic matrix, the polymerization, and the light transmittance of different resin-matrix cements used in dentistry. A search was performed on PubMed using a combination of the following key terms: degree of conversion, resin cements, light transmittance, polymerization, light curing, and thickness. Articles in the English language published up to November 2022 were selected. The selected studies' results demonstrated that restorative structures with a thickness higher than 1.5 mm decrease the light irradiance towards the resin-matrix cement. A decrease in light transmission provides a low energy absorption through the resin cement leading to a low DC percentage. On the other hand, the highest DC percentages, ranging between 55 and 75%, have been reported for dual-cured resin-matrix cements, although the polymerization mode and exposure time also influence the DC of monomers. Thus, the polymerization of resin-matrix cements can be optimized taking into account different parameters of light-curing, such as adequate light distance, irradiance, exposure time, equipment, and wavelength. Then, optimum physical properties are achieved that provide a long-term clinical performance of the cemented restorative materials.

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.

Overview of Several Typical Ceramic Materials for Restorative Dentistry

With the development of ceramic technology, prosthodontic ceramics are becoming a useful option for improving esthetic outcomes in dentistry. In this paper, various ceramic materials were reviewed and evaluated, and their advantages and disadvantages and indications in oral prosthodontics were analyzed objectively. The properties of resin-based ceramics, polycrystalline ceramics, and silicate ceramics were compared and analyzed. Resin-based ceramics may replace other ceramic materials in the CAD/CAM field.

Influence of restorative materials on occlusal and internal adaptation of CAD-CAM inlays

Aim: To evaluate the occlusal and internal marginal adaptation of inlay restorations made of different materials, using CAD-CAM. Methods: Preparations were made for MOD inlays of one-third intercuspal width and 4 mm depth in 30 third human molars. The teeth were restored using CAD-CAM materials (n=10) of nanoceramic resin (Lava Ultimate), polymer-infiltrated ceramic network (VITA ENAMIC), or lithium disilicate glass-ceramic (IPS e.max CAD). The specimens were cemented with dual resin cement and sectioned at the center of the restoration, after which the two halves were evaluated, and photographed The occlusal and internal discrepancy (μm) was determined at five points: cavosurface angle of the occlusal-facial wall (CA-O); center of the facial wall (FW); faciopulpal angle (FPA); center of the pulpal wall (PW); and center of the lingual wall (LW). The data were submitted to the Kruskal-Wallis and the Dunn tests (α=0.05). Results: No difference was observed among the materials regarding the occlusal discrepancy at the CA-O, FPA, or PW internal points. The e.max CAD measurement at FW showed larger internal discrepancy than that of Lava (p=0.02). The internal discrepancy at LW was greater for e.max CAD than VITA ENAMIC (p=0.02). Conclusion: Lithium disilicate glass-ceramic presented greater internal discrepancy in relation to the surrounding walls of the inlay preparations.

A comprehensive in vitro study on the performance of two different strategies to simplify adhesive bonding

Objective

The purpose of this study is to compare the bonding performance and mechanical properties of two different resin composite cements using simplified adhesive bonding strategies.

Materials and methods

Shear bond strength of two resin composite cements (an adhesive cement: Panavia V5 [PV5] and a self‐adhesive cement: RelyX Universal [RUV]) to human enamel, dentin, and a variety of restorative materials (microfilled composite, composite, polymer‐infiltrated ceramic, feldspar ceramic, lithium disilicate and zirconia) was measured. Thermocycle aging was performed with selected material combinations.

Results

For both cements, the highest shear bond strength to dentin was achieved when using a primer (PV5: 18.0 ± 4.2 MPa, RUV: 18.2 ± 3.3 MPa). Additional etching of dentin reduced bond strength for RUV (12.5 ± 4.9 MPa). On enamel, PV5 achieved the highest bond strength when the primer was used (18.0 ± 3.1 MPa), while for RUV etching of enamel and priming provided best results (21.2 ± 6.6 MPa). Shear bond strength of RUV to restorative materials was superior to PV5. Bonding to resin‐based materials was predominantly observed for RUV.

Conclusions

While use of RUV with the selective‐etch technique is slightly more labor intensive than PV5, RUV (with its universal primer) displayed a high‐bonding potential to all tested restorative materials, especially to resin.

Clinical significance

For a strong adhesion to the tooth substrate, PV5 (with its tooth primer) is to be preferred because etching with phosphoric acid is not required. However, when using a wide range of varying restorative materials, RUV with its universal primer seems to be an adequate option.

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.

Investigation of stress distribution within an endodontically treated tooth restored with different restorations

Background/purpose

Recently, metal-free restoration has become the standard in prosthetic treatment. However, it is still unclear which combination is most effective in preventing root fracture and secondary caries. The purpose of this study was to evaluate the influence of different post systems, crown materials, crown thickness and luting agents on the stress distribution around the crown margins, cervical dentin and the tip of the post.

Materials and methods

Ninety-six mandibular first premolar models were developed and analyzed using finite element analysis (FEA). Two designs of crowns, six kinds of crown materials, four types of post and core systems and two kinds of luting agents were included and evaluated for the stress distribution within the abutment teeth. The Von Mises stress magnitudes were compared among all models.

Results

The stress at the tip of the post decreased as the young's modulus of luting agent decreased; The stress concentrated more at the cervical area (dentin and crown), as the physical properties of the crown material increased.

Conclusion

Crowns fabricated using polyetheretherketone (PEEK) can reduce the stress concentration at the cervical area, so it may be possible to reduce the amount of tooth reduction during abutment tooth preparation. The stress distribution around the post tip is affected by the post and core systems and luting agent, regardless of crown materials and thickness. When inserting a post of the higher Young's modulus such as zirconia post, methyl methacrylate luting cement can reduce the stress concentration at the tip of the post.

A Systematic Review of Cementation Techniques to Minimize Cement Excess in Cement-Retained Implant Restorations

Background: The most used types of retention of implant-supported prostheses are screw-retained or cement-retained restorations. The advantages and disadvantages of both have been identified by various authors over the years. However, cement-retained implant crowns and fixed partial dentures are among the most used types of restorations in implant prostheses, due to their aesthetic and clinical advantages. When cemented prostheses are made on implants, the problem of cement residues is important and often associated with biological implant pathologies. The objective of this research was to establish to what extent the techniques to reduce excess cement really affect the volume of cement residues. Materials and Methods: This review was written following the PRISMA statement; a detailed search was carried out in three different electronic databases—PubMed, Scopus, and Cochrane Library. The inclusion criteria were prospective clinical studies, with at least 10 participants per group, and with at least 6 months of the follow-up period. Results: There have been many proposals for techniques supposed to reduce the amount of excess cement in the peri-implant sulcus and on the prosthetic components, but of these, which are exceptional in their in vitro capabilities, very few have been clinically validated, and this represents the real limitation and a great lack of knowledge regarding this topic. Three articles met the inclusion criteria, which were analyzed and compared, to obtain the information necessary for the purposes of the systematic review. Discussion: Extraoral cementation can reduce the excess cement, which, after a normal excess removal procedure, is, nevertheless, of such size that it does not affect the possibility of peri-implant pathologies developing. All these studies concluded that a small amount of cement residue is found in the gingival sulcus, and using eugenol-free oxide cements, the residues were only deposited on the metal surfaces, with a better peri-implant tissues health. Conclusion: Despite the limitations of this study, it was possible to carefully analyze these characteristics and obtain valuable suggestions for daily clinical practice. Resinous cements are considered, due to the free monomers present in them, toxic for the soft tissues. The provisional zinc-oxide cements, also eugenol-free, represent the ideal choice. The different grades of retentive forces provided by these cements do not seem to have clinical effects on the decementation of restorations.

Influence of Resin Cement Thickness and Elastic Modulus on the Stress Distribution of Zirconium Dioxide Inlay-Bridge: 3D Finite Element Analysis

The mechanical properties and the thickness of the resin cement agents used for bonding inlay bridges can modify the clinical performance of the restoration such as debonding or prosthetic materials fracture. Thus, the aim of this study was to evaluate the stress distribution and the maximum strain generated by resin cements with different elastic moduli and thicknesses used to cement resin-bonded fixed partial denture (RBFPD). A three-dimensional (3D) finite element analysis (FEA) was used, and a 3D model was created based on a Cone-Beam Computed Tomography system (CBCT). The model was analyzed by the Ansys software. The model fixation occurred at the root of the abutment teeth and an axial load of 300 N was applied on the occlusal surface of the pontic. The highest stress value was observed for the Variolink 0.4 group (1.76 × 10⁶ Pa), while the lowest was noted for the Panavia 0.2 group (1.07 × 10⁶ Pa). Furthermore, the highest total deformation value was found for the Variolink 0.2 group (3.36 × 10^-4 m), while the lowest was observed for the Panavia 0.4 group (2.33 × 10^-4 m). By means of this FEA, 0.2 mm layer Panavia F2.0 seemed to exhibit a more favorable stress distribution when used for cementation of posterior zirconium-dioxide-based RBFPD. However, both studied materials possessed clinically acceptable properties.

Rehabilitation of a deep bite patient with worn dentition using minimally invasive approach: A 3-year follow-up

Full mouth rehabilitation of severely worn teeth represents a challenging situation for dental clinicians. This case report describes the minimally invasive interdisciplinary approach for treatment of severely worn dentition with a loss of vertical dimension of occlusion. After 3 years of follow-up, no complication was observed.

Fracture Resistance and Failure Mode of Mandibular Molar Restored by Occlusal Veneer: Effect of Material Type and Dental Bonding Surface

This study assesses the effect of the material type (lithium disilicate, zirconia, and polymer-infiltrated ceramic) and dental bonding substrates (dentin, dentin with intra-coronal cavity, and dentin with composite filling) on the fracture resistance and failure mode of molars restored by occlusal veneers. Methods: Ninety occlusal veneers, fabricated from either lithium disilicate, zirconia, or polymer-infiltrated ceramic, were adhesively bonded to teeth prepared with either dentin, dentin with intra-coronal cavity, or dentin with composite filling. All specimens were thermally aged (5000 cycles), then load cycled (120,000 cycles). Each specimen was subjected to a compressive load through fracture, then was examined (×20) to identify the fracture type. Data were statistically analyzed. Results: Material type and dental substrate had no significant effect on the fracture resistance of adhesively retained occlusal veneer restorations. For each material, no significant differences were found between veneers bonded to dentin, dentin with intra-coronal cavity, and dentin with composite filling. Additionally, within each bonding substrate, there were no significant differences between lithium disilicate, zirconia, and polymer-infiltrated ceramic veneers. The adhesive failure was recorded mainly with zirconia occlusal veneer restorations. Conclusions: Considering the fracture results, lithium disilicate, zirconia, and polymer-infiltrated ceramic occlusal veneers perform well whatever the type of dental bonding surface. When the dental bonding surface varies, different occlusal veneer materials should be considered. Occlusal veneers bonded to dentin, dentin with composite filling, or dentin with an intra-coronal cavity exhibited a fracture resistance exceeding the average human masticatory forces in the molar area.

Effect of Cement Layer Thickness on the Immediate and Long-Term Bond Strength and Residual Stress between Lithium Disilicate Glass-Ceramic and Human Dentin

This study tested whether three different cement layer thicknesses (60, 120 and 180 μm) would provide the same bonding capacity between adhesively luted lithium disilicate and human dentin. Ceramic blocks were cut to 20 blocks with a low-speed diamond saw under cooling water and were then cemented to human flat dentin with an adhesive protocol. The assembly was sectioned into 1 mm² cross-section beams composed of ceramic/cement/dentin. Cement layer thickness was measured, and three groups were formed. Half of the samples were immediately tested to evaluate the short-term bond strength and the other half were submitted to an aging simulation. The microtensile test was performed in a universal testing machine, and the bond strength (MPa) was calculated. The fractured specimens were examined under stereomicroscopy. Applying the finite element method, the residual stress of polymerization shrinkage according to cement layer thickness was also calculated using first principal stress as analysis criteria. Kruskal–Wallis tests showed that the ‘‘cement layer thickness’’ factor significantly influenced the bond strength results for the aged samples (p = 0.028); however, no statistically significant difference was found between the immediately tested groups (p = 0.569). The higher the cement layer thickness, the higher the residual stress generated at the adhesive interface due to cement polymerization shrinkage. In conclusion, the cement layer thickness does not affect the immediate bond strength in lithium disilicate restorations; however, thinner cement layers are most stable in the short term, showing constant bond strength and lower residual stress.

Biomechanical Analysis of a Custom-Made Mouthguard Reinforced With Different Elastic Modulus Laminates During a Simulated Maxillofacial Trauma

Background/Aims

There is a lack of data regarding the influence of different laminates for mouthguard reinforcement in the mechanical response during an impact in the orofacial region. The aim of this study was to verify the influence of the laminate framework on the stresses and strains of the anterior teeth and displacement of ethylene-vinyl acetate (EVA) custom-made mouthguards during a simulated impact. The null hypotheses was that the different laminates reinforcement would present the similar effect in maxillary structures, regardless the elastic modulus.

Methods

A finite element model of human maxillary central incisors with an antagonist contact was used. A linear quasi-static analysis was used to simulate the force exerted during an impact. A total of 5 different layers were simulated inside the mouthguard at the labial portion according to the Elastic Modulus 1 MPa (Extremely flexible), 9 GPa (Low modulus reinforcement), 18 GPa (Without reinforcement), 50 GPa (Flexible alloy), 100 GPa (Titanium alloy) and 200 GPa (Hard material). The results were evaluated by means of Maximum Principal Stress (in the tooth and bone), Microstrain (periodontal ligament) and Displacement (mouthguard) criteria.

Results

The elastic modulus of the material inside the MG influenced the stress distribution on the enamel buccal face. However, it did not affect the bone tissue stress, periodontal ligament strain or root dentin tissue stress. Conclusion: The use of reinforcement inside the custom-made mouthguard can modify the stress generated in the enamel buccal surface without improvement to the root dentin, periodontal ligament or bone tissue.

Fatigue failure and success rate of lithium disilicate table-tops as a function of cement thickness

PURPOSE

Under thin, partial coverage restoration the proper cement thickness to be clinically employed still remains an issue. The aim of this study was to determine the failure and success rates of simplified lithium disilicate occlusal veneers as a function of cement thickness. The null hypothesis was that cement thickness has no effect on the fatigue resistance.

METHODS

Sound human molars were severed in a plane parallel to the occlusal surface to create a flat dentin surface surrounded by enamel edges. Forty-five occlusal veneers 1.0 mm thick (IPS e.max CAD LT) were luted to the teeth with Multilink Automix resin cement, creating 3 experimental groups (n=15) with cement thicknesses of 50, 100, and 200 µm. The restorations were fatigue-cycled using a ball mill machine containing zirconia and stainless steel spheres. Twelve 60 min cycles were performed. Survival statistics were applied to "failure" and "success" events, comparing the three groups using a log-rank Mantel-Cox test and a log-rank test for trends (alpha = 0.05).

RESULTS

The failure and success rates were not significantly influenced by cement thickness (P = 0.137 and P = 0.872, respectively); thus, the null hypothesis was accepted. However, when log-rank test for trends was applied to failure events, the tendency to have less failures with increasing thicknesses was found statistically significant (P = 0.047).

CONCLUSIONS

The cement thickness within the range adopted here did not have a significant effect on the failure or success rate of lithium disilicate occlusal veneers when exposed to randomized impact stresses generating fatigue phenomena.

Effect of Die Spacer Thickness on the Fracture Resistance of CAD/CAM Lithium Disilicate Veneers on Maxillary First Premolars

Objective

The purpose of this study was to compare the fracture resistance of ceramic veneers with digital die spacer settings at 20 µm, 40 µm, and 100 µm.

Materials and Methods

Eighteen sound maxillary first premolars were divided into three groups (n=6) according to their digital die spacer settings: group A=20 µm, group B=40 µm, group C=100 µm. Each tooth was prepared to a depth of 0.5 mm to receive lithium disilicate veneers (IPS e.max CAD, Ivoclar Vivadent). All groups were thermocycled (2500 cycles at 5-55°C) and subjected to fracture resistance test using a universal testing machine until failure. Failure modes were examined using a stereomicroscope.

Results

The values (N) for group A=1181.34±301.33, group B=1014.29±291.12, and group C=841.89±244.59. One-way ANOVA showed no statistical difference among the three groups (p=0.145). However, chi-square test showed that a significant difference was present in the modes of failure (p=0.009). Tukey's post hoc test indicated that the failure modes of group A were statistically different from those of group C, showing 83.3% adhesive failure for group A compared to 0% adhesive failures in group C. A p-value of ≤0.05 was considered as statistically significant.

Conclusion

Digital die spacer thickness did not influence the mean fracture resistance values of CAD/CAM lithium disilicate veneers. However, the way the failure occurred differed significantly at various die spacer thicknesses.

Effect of cement space on marginal discrepancy and retention of CAD/CAM crown

This research aimed to evaluate the effect of cement space on the marginal discrepancy and retention of computer-aided design/computer-aided manufacturing (CAD/CAM) crowns. A total of 30 premolar Frasaco teeth were machined to receive crowns with cement spaces of 70, 90, and 110 μm. The marginal discrepancy measurements were done before and after cementation. Pull-off test was conducted using universal testing machine (UTM). Data was analyzed using two-way mixed ANOVA with post-hoc Bonferroni test and Kruskal-Wallis test. The crowns with cement space of 70 μm showed a significantly higher absolute marginal discrepancy than those with 90 and 110 μm. No significant effect on the crown retention was found. Within the limitations of this study, modifying cement space to 90 μm and 110 μm may improve the marginal adaptation of CAD/CAM crown, whereas adjusting cement space from 70 to 110 μm did not significantly affect the crown retention.

Zirconia-reinforced lithium silicate (ZLS) mechanical and biological properties: A literature review

OBJECTIVES

This paper aimed to provide a literature review of the mechanical and biological properties of zirconia-reinforced lithium silicate glass-ceramics (ZLS) in Computer-aided design / Computer-aided manufacturing (CAD/CAM) systems.

DATA/SOURCES

An extensive search of the literature for papers related to ZLS was made on the databases of PubMed/Medline, Scopus, Embase, Google Scholar, Dynamed, and Open Grey. The papers were selected by 3 independent calibrated reviewers.

STUDY SELECTION

The search strategy produced 937 records. After the removal of duplicates and the exclusion of papers that did not meet the inclusion criteria, 71 papers were included.

CONCLUSIONS

After reviewing the included records, it was found that two types of ZLS (Vita Suprinity PC; Vita Zahnfabrik and Celtra Duo; Dentsply Sirona) are nowadays available on the market for CAD/CAM systems, similar in their chemical composition, microstructure, and biological-mechanical properties. ZLS is reported to be a biocompatible material, whose fracture resistance can withstand physiological chewing loads. The firing process influences the improvements of strength and fatigue failure load, with a volumetric shrinkage. To date, ZLS can be considered a viable alternative to other glass-ceramics for fixed single restorations.

CLINICAL SIGNIFICANCE

. As to biocompatibility and mechanical properties of ZLS, data are still scarce, often controversial and limited to short-term observational periods. These promising ceramics require further in vitro/in vivo studies to accurately define mechanical and biological properties, mainly in the long-term performance of restorations produced with such materials.

The influence of zirconia veneer thickness on the degree of conversion of resin-matrix cements: an integrative review

OBJECTIVE

The main aim of this study was to conduct an integrative review on the influence of the zirconia veneer thickness on the degree of conversion of resin-matrix cements.

MATERIALS AND METHOD

An electronic search was performed on PubMed using a combination of the following search items: zirconia, thickness, veneer, degree of conversion, resin cement, light curing, and polymerization. Articles published in the English language, up to July 2020, were included regarding the influence of ceramic veneer thickness on the degree of conversion of resin-matrix cements. Randomized controlled trials and prospective cohort studies were also evaluated.

RESULTS

Of the 21 selected studies, 9 investigated the light-curing effect, while five other articles evaluated the ceramic translucency. Three studies evaluated the degree of conversion of the resin-matrix cement while four articles assessed the veneer thickness. Results revealed a significant decrease of light transmission through the zirconia with a thickness ranging from 0.1 up to 1.5 mm. However, the ultra-thin thickness around 0.1 and 0.3 mm allowed a full polymerization of the dual-curing resin-matrix cement resulting in the integrity of the interface properties. The light-curing process of resin-matrix cements is also affected by the shade, chemical composition, and microstructure of zirconia and resin cement. Optimal conditions of light-curing are required to reach the threshold intensity of light and energy for polymerization of resin-matrix cements.

CONCLUSIONS

The increase in zirconia veneer thickness negatively affects the degree of conversion of resin-matrix cements. Also, shade and microstructure are key factor to improve the light curing of resin cements.

CLINICAL RELEVANCE

Clinicians should consider the zirconia thickness on resin-based cementation since a higher veneer thickness can negatively affect the light irradiation intensity towards the dual-curing resin-matrix cement. Thus, the degree of conversion of the resin-matrix cement can decrease leading to a low chemical stability (e.g., color instability) and poor mechanical properties.

Influence of Polymeric Restorative Materials on the Stress Distribution in Posterior Fixed Partial Dentures: 3D Finite Element Analysis

Background: This study evaluated the effect of interim restorative materials (acrylic resin (AR), resin composite (RC) or polyetheretherketone (PEEK) for dental computer-aided design/computer-aided manufacturing (CAD/CAM)) on the stress distribution of a posterior three-unit fixed partial denture. Methods: The abutment teeth (first molar and first premolar) were modeled using the BioCAD protocol containing 1.5 mm of axial reduction and converging axial walls. A static structural analysis was performed in the computer-aided engineering software, and the Maximum Principal Stress criterion was used to analyze the prosthesis and the cement layers of both abutment teeth. The materials were considered isotropic, linearly elastic, homogeneous and with bonded contacts. An axial load (600 N) was applied to the occlusal surface of the second premolar. Results: Regardless of the restorative material, the region of the prosthetic connectors showed the highest tensile stress magnitude. The highest stress peak was observed with the use of RC (129 MPa) compared to PEEK and AR. For the cement layers, RC showed the lowest values in the occlusal region (7 MPa) and the highest values for the cervical margin (14 MPa) compared to PEEK (21 and 12 MPa) and AR (21 and 13 MPa). Conclusions: Different interim restorative materials for posterior fixed partial dentures present different biomechanical behavior. The use of resin composite can attenuate the stress magnitude on the cement layer, and the use of acrylic resin can attenuate the stress magnitude on the connector region.

Influence of the foundation substrate on the fatigue behavior of bonded glass, zirconia polycrystals, and polymer infiltrated ceramic simplified CAD-CAM restorations

This study evaluated the influence of distinct substrates on the mechanical fatigue behavior of adhesively cemented simplified restorations made of glass, polycrystalline or polymer infiltrated-ceramics. CAD/CAM ceramic blocks (feldspathic - FEL; lithium disilicate - LD; yttria-stabilized zirconia - YZ; and polymer-infiltrated ceramic network - PICN) were shaped into discs (n = 15, Ø = 10 mm; thickness = 1.0 mm), mimicking a simplified monolithic restoration. After, they were adhesively cemented onto different foundation substrates (epoxy resin - ER; or Ni-Cr metal alloy - MA) of the same shape (Ø = 10 mm; thickness = 2.0 mm). The assemblies were subjected to fatigue testing using a step-stress approach (200N-2800 N; step-size of 200 N; 10,000 cycles per step; 20 Hz) upon the occurrence of a radial crack or fracture. The data was submitted to two-way ANOVA (α = 0.05) to analyze differences considering 'ceramic material' and 'type of substrate' as factors. In addition, a survival analysis (Kaplan Meier with Mantel-Cox log-rank post-hoc tests; α = 0.05) was conducted to obtain the survival probability during the steps in the fatigue test. Fractographic and finite element (FEA) analyzes were also conducted. The factors 'ceramic material', 'type of substrate' and the interaction between both were verified to be statistically significant (p < .001). All evaluated ceramics presented higher fatigue failure load (FFL), cycles for failure (CFF) and survival probabilities when cemented to the metallic alloy substrate. Among the restorative materials, YZ and LD restorations presented the best fatigue behavior when adhesively cemented onto the metallic alloy substrate, while FEL obtained the lowest FFL and CFF for both substrates. The LD, PICN and YZ restorations showed similar fatigue performance considering the epoxy resin substrate. A more rigid foundation substrate improves the fatigue performance of adhesively cemented glass, polycrystalline and polymer infiltrated-ceramic simplified restorations.

Influence of feldspar ceramic thickness on the properties of resin cements and restorative set

This in vitro study evaluated the influence of feldspathic ceramic thickness on the properties of light- and dual-cured resin cements. For each cement (RelyX Veneer, Allcem Veneer, RelyX Ultimate, and Allcem Dual), three ceramic specimens were prepared, with seven thicknesses for each (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 mm). The degree of conversion, Vickers microhardness, irradiance power, color variation (ΔE₀₀ ), and translucency parameters were assessed. Microhardness and irradiance power were analyzed using analysis of variance (ANOVA) with post hoc Tukey's test, while ΔE₀₀ , translucency parameters, and degree of conversion were analyzed using ANOVA of ranks with post hoc Duncan's Multiple Range Test. The relationship between each of the dependent variables (degree of conversion, ΔE₀₀ , and translucency parameter tests) and the specimen thickness was described using linear regression for each of the four resin cements. The significance level for all analyses was set at 5%. RelyX Ultimate yielded the lowest degree of conversion values among all resin cements. Allcem Veneer produced the lowest microhardness values, without statistical differences between thicknesses, of up to 1 mm. Allcem Dual produced the highest ΔE₀₀ and translucency parameter values. Feldspathic ceramic thickness influenced the mechanical properties of resin cements and optical aspects of the restorative set.

Long-term fracture load of all-ceramic crowns: Effects of veneering ceramic thickness, application techniques, and cooling protocol

Background

To evaluate, in vitro, the effects of the cooling protocol, application technique, and veneering ceramic thickness on the fracture resistance of ceramic crowns with Y-TZP frameworks.

Material and Methods

80 frameworks were made from zirconia by the CAD/CAM technique and divided into 8 groups (n = 10) according to the factors: “application technique” (stratified-L and pressed -P), “thickness” (1 mm and 2 mm), and “cooling protocol” (slow-S and fast-F) of the feldspathic veneering ceramic. After, all crowns were cemented over G10 preparations with resin cement (Panavia F, Kuraray), mechanically cycled (2x106 cycles, 200 N, 3Hz), and subjected to the axial compression resistance test (0.5 mm/min, 10 kN). The data (N) underwent descriptive statistical analysis by 3-way ANOVA and Tukey’s test (5%). Fracture analysis was performed to determine the possible origin of failure.

Results

The factors “cooling protocol” (P=0.0058) and “application” technique (P=0.0001) influenced the fracture resistance of the crowns. For pressed veneer technique, the P2S (4608.9±464.5). A presented significantly higher results than that P2F(3621.1±523.0)BCD (Tukey’s test). For the stratified technique, this difference was not observed (P>0.05). The thickness of the veneering ceramic was not significant regardless of the cooling protocol and technique (P>0.05). The predominant failure mode was chipping of the ceramic veneer originating in the subsurface.

Conclusions

The pressed technique, used with a slow-cooling protocol, leads to the best outcome for the veneering of all-ceramic crowns.

Key words:Zirconia, ceramics, cooling protocol, thickness, application technique.

Effect of aging on fracture resistance and torque loss of restorations supported by zirconia and polyetheretherketone abutments: An in vitro study

STATEMENT OF PROBLEM

New materials have been developed for digital processing, including implant abutments, but studies on their mechanical properties are lacking.

PURPOSE

The purpose of this in vitro study was to compare the effect of aging on the fracture resistance, failure mode, and torque loss of restorations made of zirconia and ceramic-reinforced polyetheretherketone (PEEK) abutments with titanium bases.

MATERIAL AND METHODS

Titanium-based PEEK and zirconia abutments were milled and veneered with composite resin or feldspathic porcelain in the form of maxillary first premolars (n=10). All the specimens were subjected to an aging process and were assessed after every 250 000 cycles under a stereomicroscope, and torque loss was recorded with an electronic torque meter. Fracture resistance was measured under static load (crosshead speed of 1 mm/min), and failure modes and final torque were determined. A t test was conducted for statistical analyses (α=.05).

RESULTS

The fracture resistance of the zirconia restorations was significantly greater than the PEEK-based restorations (P=.001). Torque losses were not significantly different after aging (P=.18); however, significant difference was recorded after the fracture test (P=.007). The effect of fracture load was significantly greater for the zirconia group (P<.001). Unlike with the zirconia group, the failure mode in the PEEK-based restorations was mainly adhesive and favorable (9 of 10 specimens).

CONCLUSIONS

Ceramic-reinforced PEEK abutments had acceptable resistance to fracture, a favorable failure mode, and successfully sustained the aging process.

The investigation of the stress distribution in abutment teeth for connected crowns

Background/purpose

With the advancement of an over aging society, the average number of remaining teeth has increased. However, these remaining teeth do not always have sufficient alveolar bone support, and sometimes fabricated connected crowns are applied. This study evaluated the influence of crown material, crown thickness, and alveolar bone resorption on the stress distribution within the abutment teeth of connected crowns.

Materials and methods

Using structural analysis software, a premolar crown model was fabricated. Three kinds of crown materials, two types of crown thickness, two types of post and core systems, and two levels of alveolar bone were assumed and evaluated for the stress distribution within the abutment teeth.

Results

The higher material properties crown was, the more stress was concentrated at the marginal area. The composite resin core showed larger stress values around the marginal area, and the metal core showed larger stress values at the tip of the post. Alveolar bone resorption progressed, the marginal area stress value increased.

Conclusion

The low elastic modulus crown material polyetheretherketone (PEEK) prevented stress concentrations at the marginal area of the crown and dentine, even with alveolar bone resorption. However, the amount of bone resorption has a great influence on the stress distribution around the tip of the post compared to the type of crown material.

Biaxial flexural strength, crystalline structure, and grain size of new commercially available zirconia-based ceramics for dental appliances produced using a new slip-casting method

OBJECTIVE

The aim of this study was to evaluate the biaxial flexural strength, the crystalline structure, and the grain size of zirconia-based ceramics produced using a new slip-casting method.

MATERIALS AND METHODS

Yttria-stabilized Tetragonal Zirconia Polycrystal (Y-TZP) and Alumina Toughened Zirconia (ATZ) ceramics were purchased from different manufactures. For the experimental group, ceramics produced using a patent pending slip-casting method (Slurry, Decema GmbH) was used. Slurry ceramics (n = 42) with a diameter of 14 ± 0.2 mm were produced by a proprietary colloidal shaping process, sintered, and subsequently polished with a lapping process using 15 μm diamond particles to a thickness of 1.2 ± 0.2 mm. For the control group, ceramics produced using the hot isostatic pressure method (HIP, Metoxit AG) were used. HIP ceramics discs (n = 42) with a diameter of 15.5 ± 0.02 mm were produced by classical HIP method and subsequently machined to a thickness of 1.99 ± 0.04 mm 32 discs of each ceramic were submitted to a biaxial flexural strength test using an universal testing machine at a crosshead speed of 0.5 mm/min. Statistical analyses using two-way ANOVA and Weibull distribution were performed. 2 discs of each ceramic were analyzed using X-ray diffraction for grain crystalline phase quantification. 2 discs of each ceramic were thermally etched and scanning electron microscopy images were obtained for grain size analysis (ISO 13383-1:2012). 6 discs of each ceramic were used for density measurement using the Archimedes' method.

RESULTS

For both ATZ and Y-TZP ceramics, the biaxial flexural strength and the characteristic strength of ceramics produced using the Slurry method were significantly higher than ones of the ceramic produced using HIP. The structure analysis confirmed the superiority of the Slurry ceramics which had only 1.2% tetragonal phase compared to 11-16% for the HIP ceramics. Grain size distributions covered a wide range 50-800 nm; the ZrO₂ grains of the Slurry ceramics were significantly smaller than the ones of the control ceramics, while the Al₂O₃ grain distributions were not affected by the manufacturing process. The manufacturing process had no influence on the density of both materials.

CONCLUSIONS

The Slurry method using a new proprietary slip-casting method to produce Y-TZP and ATZ dental ceramics presented higher biaxial flexural strength, less monoclinic phase and smaller ZrO₂ grains.

The impact of restorative material and ceramic thickness on CAD\CAM endocrowns

Background

Endocrown restorations as a conservative approach to restore endodontically treated teeth still need in vitro investigation under fatigue and made in different materials. This study evaluated the effect of restorative material and restoration thickness on the maximum fracture load of endocrowns subjected to cyclic loading.

Material and Methods

Sixty (60) third molar teeth received an endocrown preparation with three different heights of remaining dental tissue (1.5, 3.0 or 4.5 mm). A leucite-based ceramic (LEU) and a lithium disilicate (LD) based ceramic were selected to manufacture the CAD/CAM endocrown restorations, totaling 6 groups (n=10). The specimens were subjected to fatigue loading (200N, 2 x 106 cycles, water) and then to the single load to failure test (1 mm/min crosshead speed). Data were analyzed by using two-way ANOVA and Tukey tests (p< 0.05).

Results

All endocrowns survived the fatigue test. The thickness did not influence the restoration’s fracture load (p=0.548) instead the restorative material (p=0.003). LD showed higher mean values (1714.43 N)A than LEU (1313.47 N)B.

Conclusions

Endocrowns manufactured with CAD/CAM lithium disilicate blocks showed superior fracture load than the leucite-based blocks after mechanical fatigue. Nevertheless, both materials presented acceptable survival and fracture load as long as the material’s minimum thickness and the enamel adhesion are respected.

Key words:Endocrown, CAD/CAM, Endodontically treated teeth, Failure load, Minimal intervention dentistry.