Experimental evaluation of the bond strength between a CoCrMo dental alloy and porcelain through a composite metal–ceramic graded transition interlayer

Influence of inner crown thickness on the bonding strength of porcelain fused to Co-Cr alloy endocrown

PURPOSE

The purpose of this study was to investigate the influence of inner crown thickness on the bonding strength of porcelain fused to Co-Cr alloy endocrown by shear strength test.

METHODS

According to the molar average anatomical data to obtain a simplified model for endocrowns, porcelain fused to Co-Cr alloy endocrowns were fabricated with four inner crown thicknesses: 0.3 mm, 0.5 mm, 0.8 mm, and 1.0 mm group, eight specimens were fabricated for each thickness group. Shear bond strength test was used to observe the metal-porcelain bonding strength, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analysis were performed after the test to evaluate the interface of metal-porcelain.

RESULTS

The shear bond strength means (in MPa) were: 10.50 ± 2.55 (0.3 mm), 13.21 ± 2.27 (0.5 mm), 14.47 ± 1.46 (0.8 mm), and 15.78 ± 2.81 (1.0 mm), one-way ANOVA showed that there was a statistically significant difference among the thicknesses (P < 0.05). The SEM observation showed tight bonding, and the EDS analysis showed element diffusion phenomenon at the metal-porcelain interface.

CONCLUSION

The bonding strength of porcelain fused to Co-Cr alloy endocrown varied according to the different thickness of inner crown and the thinner inner crown was recommended for tooth defects with limited prosthodontic space.

Biomechanical behavior of functionally graded S53P4 bioglass-zirconia dental implants: Experimental and finite element analyses

OBJECTIVES

The aim of this work was to evaluate the biomechanical behavior of one-piece zirconia implants with a functionally graded bioglass (BG) layer as compared to monolithic zirconia and BG-coated implants, using the finite element method (FEM).

METHODS

Zirconia disks were infiltrated with bioglass S53P4 and then morphologically inspected by scanning electron microscopy (SEM) followed by mechanical analyses on micro-indentation tests for further biomechanical validation using the finite element method (FEM). On modeling, zirconia dental implants anchored into mandibular bone were simulated on occlusal loading as recorded under mastication. Three types of implants were simulated: i) free of BG coating, ii) with 100 μm or 150 μm thick conventional BG coatings; and iii) with graded BG coatings involving 3 different chemical composition distributions. The stress state at both implant and bone were evaluated using the FEM. The mechanically-induced bone remodelling was analyzed through the bone strain results.

RESULTS

Infiltration of BG into a zirconia structure resulted in a ∼100 μm thick layer with an exponential-like gradation of chemical composition and properties. Regarding the FEM calculations, the BG coating induced up to 30% decrease on stress in the implant body when compared to the monolithic zirconia implant. The gradient of chemical composition also improved the stresses' distribution. The stresses distribution towards the BG-coatings were significantly high and could lead to failure. Stresses on the bone were recorded down to its strength threshold, with insignificant influence of the coating layer. The bone strain values on all models indicates further bone remodelling although BG-coated and BG-graded zirconia implants showed the highest strain magnitude that may enhance the mechanical stimulation for bone maintenance.

SIGNIFICANCE

Graded BG-zirconia dental implants showed enhanced overall biomechanical behaviour as compared to the BG-coated or monolithic zirconia dental implants. Also, such biomechanical improvements noticed for the BG-graded system should be considered in combination with the well-known osseointegration benefits of bioactive glasses.

Functionally graded biomimetic biomaterials in dentistry: an evidence-based update

Design and development of novel therapeutic strategies to regenerate lost tissue structure and function is a serious clinical hurdle for researchers. Traditionally, much of the research is dedicated in optimising properties of scaffolds. Current synthetic biomaterials remain rudimentary in comparison to their natural counterparts. The ability to incorporate biologically inspired elements into the design of synthetic materials has advanced with time. Recent reports suggest that functionally graded material mimicking the natural tissue morphology can have a more exaggerated response on the targeted tissue. The aim of this review is to deliver an overview of the functionally graded concept with respect to applications in clinical dentistry. A comprehensive understanding of spatiotemporal arrangement in fields of restorative, prosthodontics, periodontics, orthodontics and oral surgery is presented. Different processing techniques have been adapted to achieve such gradients ranging from additive manufacturing (three dimensional printing/rapid prototyping) to conventional techniques of freeze gelation, freeze drying, electrospinning and particulate leaching. The scope of employing additive manufacturing technique as a reliable and predictable tool for the design and accurate reproduction of biomimetic templates is vast by any measure. Further research in the materials used and refinement of the synthesis techniques will continue to expand the frontiers of functionally graded membrane based biomaterials application in the clinical domain.

Discrepancy in alloy composition of imported and non-imported porcelain-fused-to-metal (PFM) crowns produced by Norwegian dental laboratories

Purpose: Even though the use of full ceramic crowns have become a well-established practice in dental clinics compare to the last decade, the use of imported casted porcelain-fused-to-metal (PFMs) crowns is still prevalent. The use of imported PFMs is often economically driven; however, when dentists order PFMs, they do not have capabilities to examine its true alloy content. Therefore, we raise the questions whether cheaper imported PFMs have more discrepancies in alloy content compared to domestically produced PFMs? Materials and Methods: This study included 62 porcelain-fused-to-metal crowns: 41 produced in Norway and 21 imported. Their alloy-composition was determined non-destructively by EDX and SEM. Results and Conclusions: Imported PFMs demonstrated larger deviations compared with non-imported PFMs. Significant deviation was found in key metallic elements in the different alloys (W, In, Pd, Ag). The detected deviations in key element such as Wolfram and Indium could influence the PFMs service time. These finding may be of international concern.

Bond strength enhancement of zirconia-porcelain interfaces via Nd:YAG laser surface structuring

OBJECTIVES

The aim of this study was to evaluate the effect of laser surface structuring on the bond strength of feldspar-based porcelain to zirconia, as compared to conventional sandblasting treatment.

MATERIALS AND METHODS

Thirty cylindrical zirconia substrates, previously sintered, were divided in three groups according to the type of surface conditioning: 1) sandblasting with 50 µm Al₂O₃; 2) laser structuring (Ø25 µm holes); and 3) laser structuring (Ø50 µm holes). Porcelain was injected onto the zirconia substrates. X-ray diffractometry (XRD) was used to evaluate the influence of the laser treatment on zirconia crystallographic phases. Shear bond strength test was performed. Micrographs using SEM were used to evaluate the zirconia surface after each surface treatment and to evaluate the fracture surface after the shear test.

RESULTS

The laser-structured groups presented the highest shear bond strength (65 ± 16 MPa and 65 ± 11 MPa, for the 25 µm and 50 µm holes, respectively). The sandblasting samples presented shear bond strength of 37 ± 16 MPa. XRD analysis showed that there was no phase transformation on the thermally affected surface due to laser action. Microcracks were created at some holes due to the high temperature gradient generated by laser.

SIGNIFICANCE

Laser structuring significantly increased (up to 75%) the shear bond strength of zirconia to veneering porcelain as compared to conventional sandblasting treatment. Therefore, laser structuring arises as a surface conditioning method for producing stronger and long lasting zirconia-porcelain interfaces.

THERMAL RESIDUAL STRESSES IN BILAYERED, TRILAYERED AND GRADED DENTAL CERAMICS

Layered ceramic systems are usually hit by residual thermal stresses created during cooling from high processing temperature. The purpose of this study was to determine the thermal residual stresses at different ceramic multi-layered systems and evaluate their influence on the bending stress distribution. Finite elements method was used to evaluate the residual stresses in zirconia-porcelain and alumina-porcelain multi-layered discs and to simulate the 'piston-on-ring' test. Temperature-dependent material properties were used. Three different multi-layered designs were simulated: a conventional bilayered design; a trilayered design, with an intermediate composite layer with constant composition; and a graded design, with an intermediate layer with gradation of properties. Parameters such as the interlayer thickness and composition profiles were varied in the study. Alumina-porcelain discs present smaller residual stress than the zirconia-porcelain discs, regardless of the type of design. The homogeneous interlayer can yield a reduction of ~40% in thermal stress relative to bilayered systems. Thinner interlayers favoured the formation of lower thermal stresses. The graded discs showed the lowest thermal stresses for a gradation profile given by power law function with p=2. The bending stresses were significantly affected by the thermal stresses in the discs. The risk of failure for all-ceramic dental restorative systems can be significantly reduced by using trilayered systems (homogenous or graded interlayer) with the proper design.

Influence of interlayer design on residual thermal stresses in trilayered and graded all-ceramic restorations

Residual thermal stresses are formed in dental restorations during cooling from high temperature processing. The aim of this study was to evaluate the influence of constructive design variables (composition and interlayer thickness) on residual stresses in alumina- and zirconia-graded restorations. Restorations' real-like cooling conditions were simulated using finite elements method and temperature-dependent material properties were used. Three different designs were evaluated: a bilayered restoration (sharp transition between materials); a trilayered restoration with a homogenous interlayer between core and veneer; and a trilayered restoration with a graded interlayer. The interlayer thickness and composition were varied. Zirconia restorations presented overall higher thermal stress values than alumina ones. Thermal stresses were significantly reduced by the presence of a homogeneous interlayer. The composition of the interlayer showed great influence on the thermal stresses, with the best results for homogeneous interlayers being observed for porcelain contents in the composite ranging between 30%-50% (vol.%), for both alumina and zirconia restorations. The interlayer's thickness showed a minor contribution in the thermal stress reduction. The graded interlayer showed an optimized reduction in restorations' thermal stresses. The use of graded interlayer, favoring enhanced thermal stress distributions and lower magnitude is expected to reduce the risk of catastrophic failure.

The bending stress distribution in bilayered and graded zirconia-based dental ceramics

The purpose of this study was to evaluate the biaxial flexural stresses in classic bilayered and in graded zirconia-feldspathic porcelain composites. A finite element method and an analytical model were used to simulate the piston-on-ring test and to predict the biaxial stress distributions across the thickness of the bilayer and graded zirconia-feldspathic porcelain discs. An axisymmetric model and a flexure formula of Hsueh et al. were used in the FEM and analytical analysis, respectively. Four porcelain thicknesses were tested in the bilayered discs. In graded discs, continuous and stepwise transitions from the bottom zirconia layer to the top porcelain layer were studied. The resulting stresses across the thickness, measured along the central axis of the disc, for the bilayered and graded discs were compared. In bilayered discs, the maximum tensile stress decreased while the stress mismatch (at the interface) increased with the porcelain layer thickness. The optimized balance between both variables is achieved for a porcelain thickness ratio in the range of 0.30-0.35. In graded discs, the highest tensile stresses were registered for porcelain rich interlayers (p=0.25) whereas the zirconia rich ones (p=8) yield the lowest tensile stresses. In addition, the maximum stresses in a graded structure can be tailored by altering compositional gradients. A decrease in maximum stresses with increasing values of p (a scaling exponent in the power law function) was observed. Our findings showed a good agreement between the analytical and simulated models, particularly in the tensile region of the disc. Graded zirconia-feldspathic porcelain composites exhibited a more favourable stress distribution relative to conventional bilayered systems. This fact can significantly impact the clinical performance of zirconia-feldspathic porcelain prostheses, namely reducing the fracture incidence of zirconia and the chipping and delamination of porcelain.

Dental prostheses mimic the natural enamel behavior under functional loading: A review article

Alumina- and zirconia-based ceramic dental restorations are designed to repair functionality as well as esthetics of the failed teeth. However, these materials exhibited several performance deficiencies such as fracture, poor esthetic properties of ceramic cores (particularly zirconia cores), and difficulty in accomplishing a strong ceramic–resin-based cement bond. Therefore, improving the mechanical properties of these ceramic materials is of great interest in a wide range of disciplines. Consequently, spatial gradients in surface composition and structure can improve the mechanical integrity of ceramic dental restorations. Thus, this article reviews the current status of the functionally graded dental prostheses inspired by the dentino-enamel junction (DEJ) structures and the linear gradation in Young's modulus of the DEJ, as a new material design approach, to improve the performance compared to traditional dental prostheses. This is a remarkable example of nature's ability to engineer functionally graded dental prostheses. The current article opens a new avenue for recent researches aimed at the further development of new ceramic dental restorations for improving their clinical durability.

Evaluation of the shear bond strength of dental porcelain and the low magnetic susceptibility Zr-14Nb alloy

The aim of this study was to investigate the bond strength of dental porcelain and the preheated Zr-14Nb alloy, and compare this strength with that of titanium. White oxide layers, which were predominantly composed of monoclinic zirconia, were formed on the preheated sample groups, and exhibited a greater roughness than the control samples. At the metal-ceramic interface, a greater Nb diffusion range was observed than in the control samples. The bond strengths of the samples subjected to 20min preheating treatment were the lowest (33.6 ± 3.2 MPa), which may be ascribed to the formation of a brittle thick oxide layer under excessive heat treatment. The samples subjected to this heat treatment for 5 min exhibited the highest mean bond strength (43.7 ± 5.9 MPa), which was significantly higher than that of titanium (35.3 ± 3.5 MPa). Thus, the Zr-14Nb alloy is a promising candidate for fixed dental prosthesis, as long as the appropriate treatment conditions are adopted.

Fracture and shear bond strength analyses of different dental veneering ceramics to zirconia

The purpose of this work was to evaluate the interaction of different layering porcelains with zirconia via shear bond strength test and microscopy. Four different groups of dental veneering porcelains (VM9, Zirkonzanh, Ceramco, IPS) were fused onto forty zirconia-based cylindrical substrates (8mm in diameter and 12 mm in height) (n=10), according to the manufacturer's recommendations. Additionally, layered dental porcelain (D-sign, Ivoclar) was fired on ten Ni-Cr cylindrical substrates Shear bond strength tests of the veneering porcelain to zirconia or Ni-Cr were carried out at a crosshead speed of 0.5mm/min. After the shear bond tests, the interfaces were analyzed by scanning electron microscopy (SEM). The fracture type exhibited by the different systems was also assessed. The results were statistically analyzed by ANOVA at a significant level of p<.05. The shear bond strength values of the porcelain-to-NiCr interfaces (25.3±7.1 MPa) were significantly higher than those recorded for the following porcelain-to-zirconia systems: Zirkonzanh (18.8±1 MPa), Ceramco (18.2±4.7 MPa), and IPS (16±4.5 MPa). However, no significant differences were found in the shear bond strength values between the porcelain-to-NiCr and porcelain (VM9)-to-zirconia (23.2±5.1 MPa) groups (p>.05). All-ceramic interfaces revealed mixed failure type, cohesive in the porcelain and adhesive at the interface. This study demonstrated that all-ceramic systems do not attain yet the same bond strength standards equivalent to metal-ceramic systems. Therefore, despite the esthetic appeal of all-ceramic restorations, the adhesion between the porcelain and zirconia framework is still an issue considering the long term success of the restoration.