1

Introduction

Veneered zirconia systems are becoming widely used in dentistry due to their high strength provided by transformation toughening and the great optical characteristics (esthetics) of the aluminosilicate glass-based veneers. Veneered zirconia crowns represented 10% of the over 1 million indirect restorations fabricated in 2011 by the largest dental laboratory in the U.S. . Zirconia frameworks offer higher translucency than do metal copings, allowing a more natural appearance .

Data from clinical studies reported survival rates within a range of 81–100% for 3 years of observation time , 74% for 5 years , 79–85% and 95% in 4 years . Among the main problems for veneered-zirconia reported in literature (i.e. biological complications, fracture of the core, loss of retention), of special concern was the potential susceptibility to premature fracture of the porcelain . The values for porcelain chipping (15–62%), cracking (25–50%), delaminations (<10.7%) and large fractures (3–33%) are excessively high when compared to the porcelain fractures observed for metal–ceramic restorations (for the same mode of failures: 20–34%; 3–18%; 3–14% and 3–11%, respectively) for a period of evaluation varying from 2 to 5 years .

Several possible causes of porcelain early fracture have been proposed in previous literature, including: (1) weakness of the porcelain material (lower elastic modulus and fracture toughness) ; (2) porcelain thickness ; (3) thermal expansion mismatch or incompatibility ; (4) poor wetting and weak bond strength of porcelain to zirconia ; (5) insufficient support by the framework due to inadequate, non-anatomical design ; (6) improper firing ; and (7) inappropriate cooling rate .

A systematic review of clinical survival of veneered metal and zirconia fixed partial dentures (FPD) reported that the following did not influence the frequency of porcelain chipping: (1) type of tooth (position in the mouth); (2) number of FPD units; (3) luting agent type; (4) sandblasting; or (5) thickness and support of the veneer. However, the veneering material used and its processing was suggested as potential explanations for the wide variation of chipping prevalence observed among different research centers (0–30%, depending on the research group).

Long-term cooling is a common practice for glasses and glass-matrix materials to prevent or anneal residual stresses. An annealing point is known as the temperature at which the viscosity is 10 ^13.4 poise and at which 90% of any stress in the porcelain would anneal in 15 min . In practice, the annealing temperature is reported as being approximately 50 °C above the porcelain glass transition temperature ( T _g ) . Since structural rearrangements require time and thermal energy, the location of the glass transition is strongly affected by the cooling rate, which can have pronounced effect on the magnitude of residual stresses of ceramics . Thus, it is necessary to take into account the effect of cooling rate on the glass transition temperature to obtain an accurate annealing point.

Basic characteristics (such as density, elastic modulus, strength) of the veneering porcelains for metal and zirconia are quite similar. The only real difference is that the veneering porcelains for zirconia have a lower coefficient of thermal expansion/contraction ( α ≈ 10.5 × 10 ⁻⁶ °C ⁻¹ ) than those for metals ( α ≈ 12–14 × 10 ⁻⁶ °C ⁻¹ ), CTE may vary depending on the manufacturer. Therefore, other possible differences between the framework materials become the focus of examinations regarding their influence on porcelain stress states during processing as do differences between casting alloys and zirconia.

Comparing the mechanical behavior of the materials, zirconia has an elastic modulus of 200 GPa, which is quite close to the value for base-metal alloys (204 GPa) and much higher than noble alloys (high gold content, 93 GPa) . In addition to the higher rigidity, zirconia has a brittle nature (allow minimum deformation under stress), characteristic of ceramic materials, while metals have active slip systems allowing for some elastic and/or plastic deformation.

As to thermal compatibility, manufacturers are careful to develop veneering ceramics having a coefficient of thermal expansion (CTE) “matching” the framework . Many studies described the risk of porcelain cracking when it is applied to a lower thermal expansion framework (negative mismatch, − Δ _α = α _c − α _v ) . An α _v (porcelain CTE) slightly lower than the α _c (framework CTE) has been generally used by manufacturers for decades and was found experimentally to be favorable to the performance of metal–ceramic systems during firing . Within a limit of <10% (0.1 × 10 ⁻⁶ K ⁻¹ ) of dimensional difference, the residual compression of the veneer, leaving the metallic core in tension, increases the durability of restorations . However, matching the CTE may be more critical for all-ceramic systems because of the brittleness of the framework materials (ceramics). Ceramics do not tolerate tensile forces as well as metals, thus a lowest dimensional mismatch is recommended to avoid tensile stresses in the structure .

Remarkable differences in important thermal properties do exist between zirconia and other framework materials. The thermal conductivity ( κ – W/m K) and specific heat ( c – J/kg K), are very different between zirconia ( κ = 2; c = 450) and noble metal alloys ( κ = 200; c = 130) . Even alumina has significantly higher thermal conductivity and lower specific heat than zirconia ( κ = 30; c = 775) . Since the veneering process involves multiple firings at temperature ranges far above the T _g of the porcelain (500–600 °C, depending on the composition), inhomogeneous temperatures distribution (thermal gradients) can be generated during heating or cooling throughout the dental restoration structure . Thermal gradients are frequently related to the development of transient and residual stresses in the porcelain that are potentially related to its premature fracture.

The present study used thermocouple readings to evaluate the temperature distribution of veneered restorations during different heating and cooling rates, both externally and internally. Based on the low values of zirconia thermal conductivity, this study tested the hypothesis that higher thermal gradients (temperature differences between outside and inside) are generated through the porcelain–zirconia crown than in the metal–ceramic crown, especially for the thicker restorations under the fast heating and cooling rates typically used for processing of both metal–ceramic and alumina–ceramic prostheses. It also examined whether a manufacturer-recommended slow-cooling protocol altered thermal gradients.