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

[Influence of two methods of smear layer removal on the surface properties of dentin]

OBJECTIVE

To explore the effects of two methods of smear layer removal on the surface properties of dentin.

METHODS

Sixty extracted sound third molars were collected in this study, and were prepared as uniform dentin specimens with smear layer. All specimens were randomly divided into three groups: Control group, ultrasonic treatment (UT) group and etched treatment (ET) group. Scanning electron microscope (SEM) were used to observe the surface micromorphology of all three groups. Then, the surface elements, mineral phases and functional groups were analyzed by energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and flourier transformed infrared spectrometer (FTIR) respectively. The mechanical properties, hydrophilicity and biocompatibility were also further evaluated.

RESULTS

It was revealed that dentin tubules of UT and ET groups were exposed, but lots of dentin debris piled up on the surface of the control one which covered up dentin tubules on the surface. The EDX results should that the weaker peak value of calcium and phosphorus in ET group than control and UT groups. Characteristic peaks of hydroxyapatite could be seen by XRD in all of the three groups, but lower distinctive peaks of amide Ⅰ, Ⅱ and Ⅲ bands of collagen of the dentin surface in control group than in ET and UT groups. The microhardness results showed that ET group was lower than control and UT groups, the difference was significant (P < 0.05). Better hydrophilicity of ET group was investigated (P < 0.05) than control group and UT group. Cells could be observed to adhere normally to dentin surface of each group which meant that all of the three groups had good biocompatibility.

CONCLUSION

Both UT and ET could effectively remove the smear layer on the surface of dentin and had no adverse effect of the dentin micromorphology and biocompatibility. The ultrasonic removal of the smear layer did not influence the mineral structure, hydrophilicity and mechanical properties of dentin surface. Although ET can effectively improve the hydrophilicity of dentin but decreased mechanical properties and the content of calcium and phosphorus.

Interfacial Effects Between Dental Impression and Die Materials and Their Role in the Internal Fit of Indirect Resin-Based Composite Restorations

Background/Objectives: This study tested a method for evaluating the internal fit of indirect resin-based composite (RBC) restorations, as well as the influence of different combinations of impression and die materials on the reproducibility of the topography of teeth prepared for indirect RBC restoration. Methods: Bovine incisors received flattened and cavitated areas at the cervical and middle thirds of the buccal surface, respectively. The samples were randomly assigned to two groups according to the material used for impression taking (n = 5): irreversible hydrocolloid and polyvinyl siloxane (PVS). Die replicas were obtained with Type IV gypsum or elastomeric material. RBC restorations were fabricated through an indirect technique (test) and a direct-indirect technique as the control. The internal fit of restorations was assessed by measuring the cementation line thickness with a digital caliper (simulated cementation protocol with ultra-light PVS) and validated using scanning electron microscopy (SEM). Surface topography (Sa, Sq, and Sz) was analyzed via optical profilometry, and wettability was assessed through the water contact angle method. The data were analyzed using t-test, ANOVA, and Pearson correlation tests (α = 5%). Results: The simulated cementation resulted in internal gap values positively correlated to the values from SEM (R2 = 0.958; p = 0.0102). The internal gap of restorations was not significantly correlated with the discrepancies between the topography of the die and tooth substrate (p ≥ 0.067). The combination of irreversible hydrocolloid and gypsum resulted in restorations with the lowest cementation line thickness, although in terms of roughness, this combination was the only one that resulted in significant differences from the control (p ≤ 0.028). The internal mean gap values of restorations were significantly correlated to the cumulative wettability difference of materials used during impression taking, fabrication of die replica, and restoration build-up (R2 = 0.981; p = 0.003). Conclusions: The reproducibility of topographical characteristics of the tooth in the die replica did not affect the internal adaptation of indirect RBC restorations, whereas surface wettability of materials presented a more relevant effect on the overall gap formation. The simulated cementation technique tested in the study shows potential as a simpler, cost-effective, and non-destructive method for evaluating the adaptation of indirect RBC restorations.

Cement layer thickness and load-bearing capacity of tooth restored with lithium-disilicate glass ceramic and hybrid ceramic occlusal veneers

OBJECTIVES

To evaluate the influence of cement layer and veneer thickness on load-bearing capacity of tooth restored with hybrid ceramic (HC) and lithium-disilicate glass ceramic (LDGC) occlusal veneer restorations.

METHODS

Cement layer thickness was set at either 50 µm or 200 µm and tooth restored either with 0.5 mm or 1.8 mm thick HC Cerasmart270 (GC) or LDGC IPS e.max CAD (Ivoclar Vivadent) occlusal veneers. For this study, 64 extracted human molar teeth were selected and divided into 8 groups. Prepared teeth were scanned, and occlusal veneers were manufactured using CAD/CAM technology (Cerec, Dentsply-Sirona). Finished veneers were luted to preparations using self-adhesive resin cement (G-CEM ONE) according to manufacturers' instructions. Teeth were loaded quasi-statically and ultimate fracture loads were recorded. Fracture types were analyzed and classified visually. Statistical analysis was performed using two-way ANOVA.

RESULTS

With HC occlusal veneers, thickness of both veneer and cement layer had no significant influence on fracture load. The lowest mean ultimate fracture load value was found in 0.5 mm thick LDGC veneers group with 200 µm cement layer, which was significantly lower loading value compared with that of 1.8 mm thick LDGC veneers or any of HC veneers (p ≤ 0.0280). LDGC veneers with 0.5 mm thickness showed fractures within the veneer, whereas in other groups fractures of the tooth substance was also detected.

CONCLUSIONS

Within the limitations of this study, it can be concluded that thin HC occlusal veneers provided higher load-bearing capacity than LDGC counterparts of the same thickness. HC veneers were also less sensitive to the effect of cement layer thickness.

Microleakage of luting cements in CAD/CAM pediatric zirconia crowns: an in vitro study

The durability of pediatric zirconia crowns for primary teeth is influenced by the choice of luting cement, with the effectiveness of the cement being directly correlated to its ability to reduce microleakage. This in vitro study aimed to assess and compare the microleakage of custom-made zirconia crowns (CZCs) and prefabricated zirconia crowns (PZCs) on primary maxillary incisors when luted with self-adhesive resin cement, resin-modified glass ionomer cement (RMGIC), and bioactive cement. Sixty primary maxillary incisors were prepared and allocated into two groups, each corresponding to the two types of crowns. These groups were further divided into three subgroups each to test the different luting cements. Following the cementation process and thermocycling, the specimens were immersed in a 2% methylene blue solution for microleakage evaluation. The analysis involved sectioning the teeth and examining them under a stereomicroscope. Statistical analysis, using two-way ANOVA and post hoc Dunnett T3 tests (p < 0.05), revealed significant differences in microleakage among the cements. The study found that PZCs luted with RMGIC showed the highest level of microleakage, whereas those luted with bioactive cement exhibited the lowest, positioning bioactive cement as the preferable choice for minimizing microleakage. This finding illustrates the critical importance of selecting appropriate luting cements to optimize the clinical outcomes of zirconia crown restorations in pediatric dentistry, focusing on reducing microleakage to ensure the restoration's durability and success.

Clinical Survival Rate and Laboratory Failure of Dental Veneers: A Narrative Literature Review

There is a vast amount of published literature concerning dental veneers; however, the effects of tooth preparation, aging, veneer type, and resin cement type on the failure of dental veneers in laboratory versus clinical scenarios are not clear. The purpose of the present narrative review was to determine the principal factors associated with failures of dental veneers in laboratory tests and to understand how these factors translate into clinical successes/failures. Articles were identified and screened by the lead author in January 2024 using the keywords ''dental veneer", "complication", "survival rate", "failure", and "success rate" using PubMed/Medline, Scopus, Google Scholar, and Science Direct. The inclusion criteria included articles published between January 1999 and January 2024 on the topics of preparation of a tooth, aging processes of the resin cement and veneer, translucency, thickness, fabrication technique of the veneer; shade, and thickness of the resin cement. The exclusion criteria included articles that discussed marginal and internal fit, microhardness, water sorption, solubility, polishability, occlusal veneers, retention, surface treatments, and wear. The results of the present review indicated that dental veneers generally have a high survival rate (>90% for more than 10 years). The amount of preserved enamel layer plays a paramount role in the survival and success rates of veneers, and glass-ceramic veneers with minimal/no preparation showed the highest survival rates. Fracture was the primary failure mechanism associated with decreased survival rate, followed by debonding and color change. Fractures increased in the presence of parafunctional activities. Fewer endodontic complications were associated with veneer restorations. No difference was observed between the maxillary and mandibular teeth. Clinical significance: Fractures can be reduced by evaluation of occlusion immediately after cementation and through the use of high-strength veneer materials, resin cements with low moduli, and thin layers of highly polished veneers. Debonding failures can be reduced with minimal/no preparation, and immediate dentin sealing should be considered when dentin is exposed. Debonding can also be reduced by preventing contamination from blood, saliva, handpiece oil, or fluoride-containing polishing paste; through proper surface treatment (20 s of hydrofluoric acid etching for glass ceramic followed by silane for 60 s); and through use of light-cured polymerization for thin veneers. Long-term color stability may be maintained using resin cements with UDMA-based resin, glass ceramic materials, and light-cure polymerization with thin veneers.

Effect of Resin Cement at Different Thicknesses on the Fatigue Shear Bond Strength to Leucite Ceramic

OBJECTIVES

 This in vitro study was performed to evaluate fatigue survival by shear test in the union of leucite-reinforced feldspathic ceramic using different cement thicknesses.

MATERIALS AND METHODS

 Leucite-reinforced glass ceramics blocks were sectioned in 2-mm thick slices where resin cylinders were cemented. The samples were distributed in two experimental groups (n = 20) according to the cement thickness (60 and 300 μm). The specimens of each group were submitted to the stepwise fatigue test in the mechanical cycling machine under shear stress state, with a frequency of 2 Hz, a step-size of 0.16 bar, starting with a load of 31 N (1.0 bar) and a lifetime of 20,000 cycles at each load step.

RESULTS

 The samples were analyzed in a stereomicroscope and scanning electron microscopy to determine the failure type. There is no significant difference between the mean values of shear bond strength according to both groups. Log-rank (p = 0.925) and Wilcoxon (p = 0.520) tests revealed a similar survival probability in both cement layer thicknesses according to the confidence interval (95%). The fracture analysis showed that the mixed failure was the most common failure type in the 300-μm thickness group (80%), while adhesive failure was predominant in the 60-μm thickness group (67%). The different cement thicknesses did not influence the leucite ceramic bonding in fatigue shear testing; however, the thicker cement layer increased the predominance of the ceramic material failure.

CONCLUSION

 The resin cement thicknesses bonded to leucite ceramic did not influence the long-term interfacial shear bond strength, although thicker cement layer increased the ceramic material cohesive failure. Regardless the cement layer thickness, the shear bond strength lifetime decreases under fatigue.

The Clinical Potential of 3D-Printed Crowns Reinforced with Zirconia and Glass Silica Microfillers

The development of 3D-printed crown resin materials with improved mechanical and physical properties is an area of growing interest in dentistry. This study aimed to develop a 3D-printed crown resin material modified with zirconia glass (ZG) and glass silica (GS) microfillers to enhance overall mechanical and physical properties. A total of 125 specimens were created and divided into five groups: control unmodified resin, 5% either ZG or GS reinforced 3D-printed resin, and 10% either ZG or GS reinforced 3D-printed resin. The fracture resistance, surface roughness, and translucency parameter were measured, and fractured crowns were studied under a scanning electron microscope. The results showed that 3D-printed parts that were strengthened with ZG and GS microfillers demonstrated comparable mechanical performance to unmodified crown resin but resulted in greater surface roughness, and only the group that contained 5% ZG showed an increase in translucency. However, it should be noted that increased surface roughness may impact the aesthetics of the crowns, and further optimisation of microfillers concentrations may be necessary. These findings suggest that the newly developed dental-based resins that incorporate microfillers could be suitable for clinical applications, but further studies are necessary to optimise the nanoparticle concentrations and investigate their long-term clinical outcomes.

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.

Dental Luting Cements: An Updated Comprehensive Review

The cementation of indirect restoration is one of the most important steps in prosthetic and restorative dentistry. Cementation aims to bond the prosthetic restoration to the prepared enamel or enamel and dentine. Successful cementation protocols prevent biofilm formation at the margin between tooth and restoration and minimize mechanical and biological complications. With the advancements in dental cements, they have been modified to be versatile in terms of handling, curing, and bond strengths. This review presents updates on dental cements, focusing on the composition, properties, advantages, limitations, and indications of the various cements available. Currently, dental restorations are made from various biomaterials, and depending on each clinical case, an appropriate luting material will be selected. There is no luting material that can be universally used. Therefore, it is important to distinguish the physical, mechanical, and biological properties of luting materials in order to identify the best options for each case. Nowadays, the most commonly used dental cements are glass-ionomer and resin cement. The type, shade, thickness of resin cement and the shade of the ceramic, all together, have a tangible influence on the final restoration color. Surface treatments of the restoration increase the microtensile bond strength. Hence, the proper surface treatment protocol of both the substrate and restoration surfaces is needed before cementation. Additionally, the manufacturer's instructions for the thin cement-layer thickness are important for the long-term success of the restoration.

Comparative Stress Analysis of Polyetherketoneketone (PEKK) Telescopic Crowns Supported by Different Primary Crown Materials

The present study aimed to investigate the stress distribution of secondary telescopic crowns made of polyetherketoneketone (PEKK) combined with different primary crown (PC) materials (Zirconia, CoCr, Titanium, and PEKK) using finite element analysis. The geometric model was composed of bone tissue, periodontal ligament, root dentin, cement layer, primary crown, and secondary telescopic crown (SC). A total of four models were evaluated in which the secondary crowns were simulated in PEKK. The models were designed in CAD software and exported to the computer aided engineering software for the statistic structural analysis simulation. The materials were considered isotropic, with linear behavior and elastic properties. The model was fixed in the bone base and the load was applied at the occlusal surface of the crowns with 600 N. The results were required in von-Mises stress for the primary crown, secondary crown, cement layer, and Equivalent Strain to the periodontal ligament and bone tissue. Results show that the material influenced the stress distribution. The higher the PC elastic modulus, the higher the stress magnitude on the SC and cement layer. In the present study, the use of milled high-density polymer for primary crown presented a promising biomechanical behavior as an alternative material for double-crown design.

Shear bond strength of resin bonded zirconia and lithium disilicate - effect of surface treatment of ceramics and dentin

Objectives:The purpose of the study was to investigate the effect of ceramic surface pretreatment, effect of resin cement and dentin surface roughness on shear bond strength.

Methodology: Zirconia rods (n = 140) were randomly assigned to air born particle abrasion with aluminum oxide (Al₂O₃) or hot etching with potassium hydrogen difluoride (KHF₂). Lithium disilicate rods (LDS; n = 50) etched with hydrofluoric acid served as reference material. In Part 1 of the study, ceramic rods were cemented to bovine dentin using 5 dual-polymerizing resin cements (Variolink Esthetic, Multilink Automix (Ivoclar Vivadent), Duo-Link (BISCO Dental), Panavia F2.0 (Kuraray Dental), RelyX Unicem (3 M)). Shear bond strength was tested and fracture morphology determined. In Part 2 of the study, test groups with the highest frequency of adhesive fractures between cement and dentin were selected for further bond strength testing with different surface roughness of dentin; ground with P1200 or P80 silicon carbide paper. Dentin samples were fractured vertically to the cemented surface and the adherence between cement and dentin was studied.

Results: The results of Part 1 showed that hot etching of zirconia significantly improved bond strength to Duo-Link cement. In Part 2, RelyX Unicem showed significantly higher bond strength to P1200 compared to P80 ground dentin. For Variolink Esthetic, bond strengths to P1200 and P80 ground dentin were similar. Adhesive fracture between cement and dentin dominated.

Conclusions: A smooth dentin surface (P1200) improved bond strength to RelyX Unicem. Surface roughness was not important for Variolink Esthetic.