The aim of the work was to assess the influence of coping and veneer thickness on the fracture resistance of porcelain–metal and porcelain–zirconia crowns in a clinically representative model.
A total of 30 zirconia and 30 precious metal copings were fabricated. There were 10 copings in each group of 0.5 mm, 1 mm and 1.5 mm thickness. Each group of 10 was further divided into two groups with a total thickness of three and four millimeters inclusive of veneering ceramic. The specimens were cemented to a titanium abutment with zinc oxide cement and tested using a microindenter. Crack length, hardness and spalling (chipping) were recorded using an optical microscope from which fracture toughness was calculated.
Kruskal–Wallis tests and simple linear regression analysis were used to analyze the results, revealing a significant difference between zirconia (ZrCC) and metal (MCC) specimens for crack length. 0.5 mm coping thicknesses and MCC specimens showed the highest fracture toughness values. Simple linear regression analysis showed a limited effect of the overall thickness or veneer thickness on crack length and hardness; however coping thickness showed a positive correlation with both. Spalling was higher in zirconia specimens.
Residual stresses were higher for ZrCC specimens and showed a positive correlation with crack length. The ratio of veneer to coping thickness was negatively correlated with residual stress.
Residual stresses due to thermal mismatch between the coping and the veneering ceramic should be kept to a minimum. The ZrCC specimens were found to have lower apparent fracture toughness than MCC specimens. Thermal mismatch caused a larger drop in apparent fracture toughness than the inherent differences in the materials used.
CAD/CAM zirconia restorations are arguably the greatest advance in dental materials in recent times. Zirconia ceramic was first introduced to the medical profession in 1969 in the field of orthopedics as a proposed material for hip replacements . Since then it has been described as “ceramic steel” and is widely used in both dentistry and medicine . Due to its impressive flexural and compressive strengths of 900–1200 MPa and 2000 MPa, respectively it has developed into an efficient core material for all-ceramic restorations.
Review papers comment consistently that the most common cause of technical failure in zirconia restorations is veneer chipping or cracking . Failure rates of 5.7% at 5 years have been calculated, compared with porcelain fused with metal restorations which showed 4.4% failure at 5 years .
Many possible explanations for these fracture characteristics have been postulated and examined. These explanations can be broadly categorized into: geometric, including coping design and veneer/coping ratios : thermal, including differences in sintering time, coefficients of thermal expansion and cooling rates ; and structural, including internal flaws of the ceramic, wettability of the coping and bond between veneer and coping .
Fracture toughness is a quantitative measure of the ability of a material to resist propagation of a pre-existing crack. Brittle materials, the limiting stress that can be applied to them is therefore dictated by flaw size rather than strength. The flaws that will propagate in such materials at reasonable engineering stresses are typically in the range of a few micrometers and so the toughness is pivotal in the chipping of a veneered zirconia all-ceramic crown or fixed partial denture. The literature to date on toughness of veneering ceramics generally assesses it in isolation from other geometric factors. That is to say that previous studies have been performed on bilayer discs and bars , and, to date, there have been no assessments of fracture toughness in a clinically representable model. Only a few of the more recent articles have used a control for their assessments by also testing the gold standard of the porcelain fused to metal restoration .
There is a consensus that a coping should have a higher coefficient of thermal expansion than its veneering porcelain, thus leading to a positive thermal mismatch . Such a mismatch would result in compressive stresses in the veneering porcelain which should inhibit the growth of cracks. This is verifiable when compared to a negative thermal mismatch , but the value of a significant compressive stress is disputed by other authors who have shown that thermal mismatch should be kept to a minimum, irrespective of its sense, to limit the general level of residual stresses within the porcelain . Thermal mismatch has been shown not to affect bond strength of porcelain to zirconia copings which correlates well with evidence that thermal mismatch does not affect the phase of zirconia at the core/veneer junction , although this has been challenged by other authors . No quantitative assessment of the effect of thermal mismatches on the fracture resistance of veneering ceramics has been made.
Given that the aim of the current investigation is to assess the influence of coping and veneer thickness on the fracture resistance of porcelain–metal and porcelain–zirconia crowns in a clinically representative model, the measurement of fracture toughness in situ is of major importance.
Materials and methods
A veneered zirconia and a veneered precious metal crown were assessed in this study. The coping and veneer thicknesses of the crowns were varied. A clinically representative model was used to replicate an implant-supported, cement-retained crown within experimental limitations.
Fabrication of mounting jig
A 5 cm × 5 cm × 5 cm cube was constructed in self-cure acrylic resin (Meadway Methylmethacrylate; Mr Dental LOT: 20226) using a lost wax technique.
A 5 mm diameter drill bit was used to create space for an implant analog in the middle of one of the cubes surfaces. The implant analog was 4.5 mm in diameter coronally (Astra-tech) and secured within the cube using a self-curing acrylic resin.
A CAD/CAM titanium abutment was selected for the study. A 5 mm high abutment (margin to tallest point) with a 6° taper and a diameter of 6 mm was fabricated (Atlantis; Astra Tech). This was designed to represent an abutment which might be fabricated clinically for an implant in the first molar region. The long axis of the abutment was designed to be parallel with that of the analog. Once received, the abutment was torqued into place at 35 N cm.
Design of CAD/CAM copings
Following dusting with zirconium dioxide powder, the experimental jig was scanned using the Lava ST optical scanner (3M ESPE).
A coping was designed using the Lava ST software to mimic the shape of the abutment. The design parameters for the coping are presented in Table 1 . This design was sent to a local milling center (ZMC, DTS International, Glasgow) where 30 zirconia 3Y-TZP (LAVA, 3M ESPE) and 30 resin copings were milled and printed, respectively. The quality of the internal surface of the zirconia copings was inspected and they were then tried on the abutment individually to assess marginal fit.
|Coping thickness (mm)||Margin reinforcement (mm)||Cement gap||Cement gap expansion|
|Thickness (mm)||Begin above margin (mm)||Thickness (mm)||Begin above margin (mm)|
Upon receipt and inspection, the resin copings were sprued, invested and cast in precious metal alloy (Endurance 52). The fit surface of the casting was inspected and any internal casting flaws were removed.
Application of veneering ceramic
A mold was constructed to allow consistent application of veneering ceramic to the coping.
Two wax-ups were made directly onto the abutment. The first wax-up was 3 mm taller and 2 mm wider than the abutment itself. While the second was 1 mm taller than the first. Both had smooth, flat surfaces on the axial and coronal aspects. Location grooves were cut in the coronal surface of the acrylic block and a vinylpolysiloxane putty mold was made of this wax-up.
Prior to veneering with dentin porcelain, an opaquer (Paste Opaque 3203 A3; Vita VMK Metallkeramik) was applied to the metal using a ball ended applicator and fired according to the manufacturer’s instructions. One layer of the opaquer was applied to each coping. A liner material (LAVA modifier; 3M ESPE) was applied to the zirconia in the same manner, again being fired according to the instructions provided by the manufacturer.
Once cooled, the copings were tacked to the abutment with a minimal amount of adhesive wax and the corresponding veneering ceramic (Vita VMK 95 and Lava Ceram for the metal and zirconia, respectively) was vibrated into the putty mold. Shade D2 was used for both LAVA and metal copings. The specimens were blotted with tissue to draw off excess water to reduce porosity within the veneering ceramic. Upon removal from the mold, the copings were teased from the abutment and fired according to the manufacturers’ instructions.
The zirconia copings were veneered with Lava ceram veneering ceramic, constituting zirconia ceramic crowns (ZrCC). The metal copings were veneered with Vita VMK veneering ceramic, constituting metal ceramic crowns (MCC).
The coronal surface of each specimen was ground and polished using consecutively finer diamond paste, with a final size of 3 μm. Prior to testing, each specimen’s occlusal thickness was measured at nine approximately equidistant points on the internal surface of the crown using digital calipers (Electronic digital calipers; Fino Gmbh, Mangelsfield, Germany). A mean measurement of the final occlusal thickness of each specimen was calculated in millimeters.
A zinc oxide eugenol cement was selected to lute the specimens to the abutment (TempBond; Kerr). Equal amounts of base and catalyst were mixed and the axial walls of the fit surface of the specimens were lightly coated. After seating the crown manually onto the abutment, it was immediately transferred to a cementation jig which applied a 5 kg static axial load to the center of the occlusal surface. This load was sustained for 3 min. Any cement at the margin of the crown was removed with a sharp curette. The specimens were tested immediately following removal of cement. After testing, the specimen was removed from the abutment by hand and the abutment was steam cleaned in preparation for a new specimen.
Using a microhardness indenter (Micro Vickers Hardness Tester DHV-1000) the specimens were loaded at 10 N for 20 s. During each indentation, the machine diplayed a hardness value determined from the geometry of the indenter and the depth of penetration. The specimens were then examined at various magnifications using an optical microscope (Nikon, with N50 monochrome camera). Using software associated with the microscope (a4i Docu; Olympus) images of the indentations were recorded to scale and any cracks measured in micrometers. Any subsurface failures were noted. If the indentation was indistinguishable from the fracture, the crack was recorded as immeasurable.
Calculation of fracture toughness
A generic toughness equation used for radial-median cracks was used to calculate K IC (Eq. (1) ) :
where P is the indentation load, c is the crack length from the crack tip to the middle of the indentation and k is an empirical constant obtained by data fitting.
The value of k was calculated for the ZrCC using the K IC provided by the manufacturer of the LAVA Ceram, so the measure of fracture toughness is not independent, all measures being relative to the published value for Lava Ceram. However, no value for K IC could be obtained from the manufacturer of VITA VMK 95, or from the literature. Therefore the constant k was calculated using Eq. (2) , by using the fracture toughness of LAVA Ceram and the elastic modulus of the two materials:
k VITA = E VITA E LAVA 0.25 k LAVA or k VITA = E VITA E LAVA 0.667 k LAVA