Performance of Zirconia for Dental Healthcare

Recent advances in dental zirconia: 15 years of material and processing evolution

OBJECTIVES

The objective was to discuss the research on zirconia published in the past 15 years to help the dental materials community understand the key properties of the types of zirconia and their clinical applications.

METHODS

A literature search was performed in May/2023 using Web of Science Core Collection with the term "dental zirconia". The search returned 5102 articles, which were categorized into 31 groups according to the research topic.

RESULTS

The current approach to improving the translucency of zirconia is to decrease the alumina content while increasing the yttria content. The resulting materials (4Y-, 5Y-, and above 5 mol% PSZs) may contain more than 50% of cubic phase, with a decrease in mechanical properties. The market trend for zirconia is the production of CAD/CAM disks containing more fracture resistant 3Y-TZP at the bottom layers and more translucent 5Y-PSZ at the top. Although flaws located between layers in multilayered blocks might represent a problem, newer generations of zirconia layered blocks appear to have solved this problem with novel powder compaction technology. Significant advancements in zirconia processing technologies have been made, but there is still plenty of room for improvement, especially in the fields of high-speed sintering and additive manufacturing.

SIGNIFICANCE

The wide range of zirconia materials currently available in the market may cause confusion in materials selection. It is therefore imperative for dental clinicians and laboratory technicians to get the needed knowledge on zirconia material science, to follow manufacturers' instructions, and to optimize the design of the prosthetic restoration with a good understanding where to reinforce the structure with a tough and strong zirconia.

Bonding effect of a Zr/Si coating prepared on zirconia using a sol-gel method

STATEMENT OF PROBLEM

Zirconia has been widely used as a dental prosthetic material. However, bonding to zirconia is challenging, and whether a Zr/Si coating would improve bonding is unclear.

PURPOSE

The purpose of this in vitro study was to prepare a Zr/Si coating on zirconia ceramics using a sol-gel method and to determine whether the bonding to resin is improved.

MATERIAL AND METHODS

Presintered zirconia specimens were prepared and divided into 5 groups: 4 experimental groups with ratios of the binary sol-gel precursor (zirconium oxychloride/tetraethoxysilane) set as 2:1 (Z2), 1:1 (Z1), 0.5:1 (Z0.5), and 0.25:1 (Z0.25) and Group C as the control group. In addition to surface roughness measurements, scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and X-ray diffraction (XRD) were carried out to characterize the surface. Each group was divided into 2 subgroups according to whether a silane coupling agent was applied. Half of the bond specimens were stored in deionized water for 24 hours; the remaining half were aged using 5000 thermocycles. The shear bond strength (SBS) of resin bonded to specimens was tested for the initial and durable bond strength, and the bonding interface was also observed by SEM after debonding. Data were subjected to 1-way ANOVA and the post hoc Tukey honestly significant difference test (α=.05).

RESULTS

The Zr/Si coating formed on zirconia ceramics. Z0.5 had the greatest mean ±standard deviation roughness (2.13 ±0.15 μm) and had the highest silicon content (21.7 ±0.21%). t-ZrO₂, m-ZrO₂, c-SiO_2, and ZrSiO₄ were detected by XRD in Z1. The SBS values were decreased by aging but were significantly increased by Zr/Si coating, especially for Z0.5, with the application of silane (initial: 22.92 ±2.79 MPa; aged: 9.91 ±0.92 MPa).

CONCLUSIONS

The Zr/Si coating significantly improved the initial and aged bond strength, and the optimal Zr/Si ratio of the sol-gel appeared to be 0.5:1.

Assessment of Intra-Oral Repair Systems for Veneered Zirconia and Zirconia Only

The aim of this study was to compare bond strength resin composites to porcelain laminate veneers in the indirect repair method to composite resins used in the direct repair method for cases of porcelain veneer fracture of zirconia-based fixed dental prostheses. In the study, the groups were formed with different percentages of areas to be repaired to mimic porcelain fractures in the mouth. The experimental group of veneered zirconia were as follows: Group A = 100% Zr surface; Group B = 70% Zr, 30% porcelain surface; Group C = 50% Zr, 50% porcelain surface; Group D = 30% Zr, 70% porcelain surface; Group E = 100% porcelain surface. The repairs of the specimens were made using composite resin systems in half of the groups and using porcelain laminate veneers in the other half. Specimens were embedded in acrylic blocks before surface treatments and repairs were applied. After surface conditioning, laminate veneers were applied to the first half of the groups, and composite repair systems were applied to the second half of the groups. After all specimens were aged by thermal cycling, their bond strength values were measured using a Universal Testing Machine, and the obtained data were recorded. The specimens were examined with a stereomicroscope and classified according to failure types (adhesive/cohesive/mixed). Bond strength values were evaluated based on independent-samples t-test statistics. According to the comparisons among the groups, the bond strength of the indirect repairs made with the laminate material was higher than the bond strength of the repairs made with the composite. There was a statistically significant difference in favor of the indirect repair groups among all groups except for Group C. The highest bond strength was found in Group A in the indirect repair method, while the lowest bond was found in the direct repair method in Group E. Adhesive failure was mostly seen in the groups that were repaired with the composite.

An In Vitro Comparison of Zirconia and Hybrid Ceramic Crowns With Heavy Chamfer and Shoulder Finish Lines

Purpose This in vitro study aimed to compare the marginal fit and internal adaptation of computer-aided designed and computer-aided manufactured (CAD-CAM) zirconia and hybrid ceramic crowns on heavy chamfer and shoulder finish line designs using silicon replica method. Materials and methods Forty die samples were divided into four groups of 10 dies each. Out of 40 diecasts scanned, zirconia crowns were milled on 20 casts (10 prepared with shoulder and 10 prepared with heavy chamfer finish line design), while hybrid ceramic crowns were milled on the rest of the 20 casts. After milling crowns, the silicone replica technique measured the marginal fit and internal adaptation. Results The heavy chamfer finish line design provided a better marginal fit than the shoulder finish line design for zirconia and hybrid ceramic crowns. Hybrid ceramic crowns had a better marginal fit and internal adaptation than zirconia crowns, both at heavy chamfer and shoulder finish line design. The gap at the margin was less than the axial and occlusal walls, and the maximum gap was observed in the occlusal area. In addition, the marginal gap was less than the internal gap, which showed a positive correlation with each other. Conclusion The study concluded that the difference in CAD-CAM materials and finish line designs influences marginal fit and crown restoration's internal adaptation. A heavy chamfer finish line design provides a better marginal fit for zirconia and hybrid ceramic crowns than a shoulder finish line design. Hybrid ceramic crowns have a better marginal fit and internal adaptation than zirconia crowns in heavy chamfer and shoulder finish lines.

Influence of CAD/CAM milling, sintering and surface treatments on the fatigue behavior of lithium disilicate glass ceramic

This paper reports on the process-fatigue relation of lithium disilicate glass ceramic (LDGC) using low-cycle, high-load Hertzian indentations with a rigid indenter to simulate teeth grinding/clenching of LDGC restorations with different surface asperities obtained in CAD/CAM milling, sintering, polishing and glazing. The maximum contact stresses were evaluated as functions of the number of load cycles and surface treatments using the Hertzian model. Indentation-induced surface damage was viewed using scanning electron microscopy (SEM) to understand the relationships among microstructures, surface asperities, crack morphology and propagation. Different processes and surface treatments significantly affected the maximum contact stresses of indented LDGC surfaces (ANOVA, p < 0.05), which were all significantly reduced with the number of cycles (ANOVA, p < 0.05). Quasi-plastic deformation was dominant in single-cycle indentation of all processed and treated surfaces. In higher cycle indentations, inner cone cracks were formed on all surfaces; median and transverse cracks were formed on the roughest surfaces processed by CAD/CAM milling and sintering. Ring cracks, fretting, pulverization, micro-bridges, surface smearing and wedging, and edge chippings were also propagated on all surfaces. The process-fatigue relation provides an understanding of the mechanical functions of surface asperities produced in different processes and treatments. It indicates that the mechanically assisted growth of surface asperities with different roughness strongly affected the indentation-induced surface damage. Finally, the smoothest surfaces produced by CAD/CAM milling, polishing and sintering sustained the highest contact stresses and the least fatigue damage at higher cycles, ensuring their superior fatigue performance compared to other processed LDGC surfaces.

A simple 3-point flexural method for measuring fracture toughness of the dental porcelain to zirconia bond and other brittle bimaterial interfaces

PURPOSE

Porcelain fused to zirconia prostheses are widely used, but porcelain chipping, fracture, spalling and delamination are common clinical problems. Conventional bond strength testing is inherently unsuited for studying interfacial failure by cracking in brittle materials. Instead, fracture toughness is a more meaningful parameter because it can assess the robustness of the interface when subjected to loading, but fracture mechanics approaches have only rarely been used. Our purpose was to develop a novel, simple, 3-point flexural methodology and mathematical analysis to measure the fracture toughness of the porcelain to zirconia interface.

METHODS

Equations were derived to estimate the fracture toughness of the bond by computing the interfacial energy release rate for a novel simple 3-point flexural test model. The test was validated using two different configurations of layered zirconia/porcelain beams (n = 10), approximating the dimensions of a fixed dental prosthesis, fabricated from a tetragonal polycrystalline zirconium dioxide partially stabilized with yttria and a feldspathic dental porcelain.

RESULTS

Cracking along the bimaterial interface was produced and measured as a discrete event. Fracture toughness means (standard deviations) computed from the measured energy release rate, for the porcelain to zirconia interface in two different specimen configurations were 7.9 (1.3) and 5.3 (1.6) J/m².

CONCLUSIONS

Equations were derived to measure interfacial fracture toughness of brittle materials using a novel simple 3-point flexural test method. The test was then validated; estimates for the fracture toughness for the porcelain to zirconia bond, overlapped with previously published data derived from more complex 4-point notched tests.

Comparative study of new bone formation capability of zirconia bone graft material in rabbit calvarial

PURPOSE

The purpose of this study was to compare the new bone formation capability of zirconia with those of other synthetic bone grafts.

MATERIALS AND METHODS

Twelve rabbits were used and four 6-mm diameter transcortical defects were formed on each calvaria. Each defect was filled with Osteon II (Os), Tigran PTG (Ti), and zirconia (Zi) bone grafts. For the control group, the defects were left unfilled. The rabbits were sacrificed at 2, 4, and 8 weeks. Specimens were analyzed through micro computed tomography (CT) and histomorphometric analysis.

RESULTS

The Ti and Zi groups showed significant differences in the amount of newly formed bone between 2 and 4 weeks and between 2 and 8 weeks (P<.05). The measurements of total bone using micro CT showed significant differences between the Os and Ti groups and between the Os and Zi groups at 2 and 8 weeks (P<.05). Comparing by week in each group, the Ti group showed a significant difference between 4 and 8 weeks. Histomorphometric analysis also showed significant differences in new bone formation between the control group and the experimental groups at 2, 4, and 8 weeks (P<.05). In the comparison of newly formed bone, significant differences were observed between 2 and 4 weeks and between 2 and 8 weeks (P<.05) in all groups.

CONCLUSION

Zirconia bone graft material showed satisfactory results in new bone formation and zirconia could be used as a new synthetic bone graft material.

Surface quality of yttria-stabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes

This paper studied the surface quality (damage, morphology, and phase transformation) of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) in CAD/CAM milling, and subsequent polishing, sintering and sandblasting processes applied in dental restorations. X-ray diffraction and scanning electron microscopy (SEM) were used to scan all processed surfaces to determine phase transformations and analyse surface damage morphology, respectively. The average surface roughness (R_a) and maximum roughness (R_z) for all processed surfaces were measured using desk-top SEM-assisted morphology analytical software. X-ray diffraction patterns prove the sintering-induced monoclinic-tetragonal phase transformation while the sandblasting-induced phase transformation was not detected. The CAD/CAM milling of pre-sintered Y-TZP produced very rough surfaces with extensive fractures and cracks. Simply polishing or sintering of milled pre-sintered surfaces did not significantly improve their surface roughness (ANOVA, p>0.05). Neither sintering-polishing of the milled surfaces could effectively improve the surface roughness (ANOVA, p>0.05). The best surface morphology was produced in the milling-polishing-sintering process, achieving R_a=0.21±0.03µm and R_z=1.73±0.04µm, which meets the threshold for bacterial retention. Sandblasting of intaglios with smaller abrasives was recommended as larger abrasive produced visible surface defects. This study provides technical insights into process selection for Y-TZP to achieve the improved restorative quality.

A Novel Technique for the Connection of Ceramic and Titanium Implant Components Using Glass Solder Bonding

Both titanium and ceramic materials provide specific advantages in dental implant technology. However, some problems, like hypersensitivity reactions, corrosion and mechanical failure, have been reported. Therefore, the combining of both materials to take advantage of their pros, while eliminating their respective cons, would be desirable. Hence, we introduced a new technique to bond titanium and ceramic materials by means of a silica-based glass ceramic solder. Cylindrical compound samples (Ø10 mm × 56 mm) made of alumina toughened zirconia (ATZ), as well as titanium grade 5, were bonded by glass solder on their end faces. As a control, a two-component adhesive glue was utilized. The samples were investigated without further treatment, after 30 and 90 days of storage in distilled water at room temperature, and after aging. All samples were subjected to quasi-static four-point-bending tests. We found that the glass solder bonding provided significantly higher bending strength than adhesive glue bonding. In contrast to the glued samples, the bending strength of the soldered samples remained unaltered by the storage and aging treatments. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses confirmed the presence of a stable solder-ceramic interface. Therefore, the glass solder technique represents a promising method for optimizing dental and orthopedic implant bondings.

A Critical Review of Dental Implant Materials with an Emphasis on Titanium versus Zirconia

The goal of the current publication is to provide a comprehensive literature review on the topic of dental implant materials. The following paper focuses on conventional titanium implants and more recently introduced and increasingly popular zirconia implants. Major subtopics include the material science and the clinical considerations involving both implant materials and the influence of their physical properties on the treatment outcome. Titanium remains the gold standard for the fabrication of oral implants, even though sensitivity does occur, though its clinical relevance is not yet clear. Zirconia implants may prove to be promising in the future; however, further in vitro and well-designed in vivo clinical studies are needed before such a recommendation can be made. Special considerations and technical experience are needed when dealing with zirconia implants to minimize the incidence of mechanical failure.

New Coating Technique of Ceramic Implants with Different Glass Solder Matrices for Improved Osseointegration-Mechanical Investigations

Ceramics are a very popular material in dental implant technology due to their tribological properties, their biocompatibility and their esthetic appearance. However, their natural surface structure lacks the ability of proper osseointegration, which constitutes a crucial process for the stability and, thus, the functionality of a bone implant. We investigated the application of a glass solder matrix in three configurations—consisting mainly of SiO₂, Al₂O₃, K₂O and Na₂O to TZP-A ceramic specimens. The corresponding adhesive strength and surface roughness of the coatings on ceramic specimens have been analyzed. Thereby, high adhesive strength (70.3 ± 7.9 MPa) was found for the three different coatings. The obtained roughness (R_z) amounted to 18.24 ± 2.48 µm in average, with significant differences between the glass solder configurations. Furthermore, one configuration was also tested after additional etching which did not lead to significant increase of surface roughness (19.37 ± 1.04 µm) or adhesive strength (57.2 ± 5.8 MPa). In conclusion, coating with glass solder matrix seems to be a promising surface modification technique that may enable direct insertion of ceramic implants in dental and orthopaedic surgery.

Resin zirconia bonding promotion with some novel coupling agents

OBJECTIVES

To evaluate and compare three novel coupling agents: 2-hydroxyethyl methacrylate, itaconic acid and oleic acid to two silane coupling agents, one commercial silane product and 3-acryloxypropyltrimethoxysilane on the bond durability of resin composite to zirconia.

METHODS

Zirconia samples were silica-coated by air abrasion and each of the five coupling agents was then applied to give five test groups. Resin composite stubs were bonded onto the conditioned zirconia surfaces. The samples were stored: dry storage, 30 days in water and thermocycled to give a total of fifteen test groups. The shear bond strengths were determined using a universal testing machine and data analyzed by two-way ANOVA and Tukey HSD (p<0.05) with shear bond strength as dependent variable and storage condition and primers as independent variables. The bond formation of the five coupling agents to zirconia was examined by X-ray photoelectron spectroscopy (XPS).

RESULTS

Two-way ANOVA analysis showed that there was a significant difference for different primers (p<0.05) and different storage conditions (p<0.05) on the shear bond strength values measured. XPS analysis showed a shift in binding energy for O(1s) after priming with the five coupling agents which revealed different bond formations related to the functional groups of the coupling agents.

SIGNIFICANCE

The shear bond strength values measured for all coupling agents after water storage and thermocycling exceed the minimum shear bond strength value of 5MPa set by ISO. The silane coupling agent, 3-acryloxypropyltrimethoxysilane, showed the highest bond strength of the three storage conditions.

Correlation between Structural Properties and In Vivo Biocompatibility of Alumina/Zirconia Bioceramics

The aim of our study is the characterization and comparison of structural properties of two novel alumina/zirconia ceramics prepared by Spark Plasma Sintering and biocompatibility evaluation by using an animal model (Wistar rats). SEM, XRD and FTIR spectroscopic results are reported for structural characteristics. In vivo tests demonstrated the biocompatibility and osseointegration of the composites by complementary SEM and histological analysis of the defects in rat femur respectively the connective tissue.

Performance of dental ceramics: challenges for improvements

The clinical success of modern dental ceramics depends on an array of factors, ranging from initial physical properties of the material itself, to the fabrication and clinical procedures that inevitably damage these brittle materials, and the oral environment. Understanding the influence of these factors on clinical performance has engaged the dental, ceramics, and engineering communities alike. The objective of this review is to first summarize clinical, experimental, and analytic results reported in the recent literature. Additionally, it seeks to address how this new information adds insight into predictive test procedures and reveals challenges for future improvements.

Modified Y-TZP core design improves all-ceramic crown reliability

This study tested the hypothesis that all-ceramic core-veneer system crown reliability is improved by modification of the core design. We modeled a tooth preparation by reducing the height of proximal walls by 1.5 mm and the occlusal surface by 2.0 mm. The CAD-based tooth preparation was replicated and positioned in a dental articulator for core and veneer fabrication. Standard (0.5 mm uniform thickness) and modified (2.5 mm height lingual and proximal cervical areas) core designs were produced, followed by the application of veneer porcelain for a total thickness of 1.5 mm. The crowns were cemented to 30-day-aged composite dies and were either single-load-to-failure or step-stress-accelerated fatigue-tested. Use of level probability plots showed significantly higher reliability for the modified core design group. The fatigue fracture modes were veneer chipping not exposing the core for the standard group, and exposing the veneer core interface for the modified group.