Influence of cooling rate on residual stresses and chipping of porcelain on zirconia restorations

Cohesive failure of veneering porcelain, or chipping, has been recognised as the main disadvantage of zirconia-based restorations. This is believed to be due to a propensity to develop large thermal gradients within the veneering porcelain layer when the restoration cools from the sintering temperature to room temperature. Recent literature draws attention to the vastly different thermo-physical properties of zirconia compared to other core materials, in particular to its very low thermal conductivity. When the veneering porcelain in a zirconia restoration is cooled at a fast rate from above to below the glass transition temperature during the final firing cycle, high residual tensile zones develop within the veneering porcelain. These tensile zones act as weakness zones within the veneering porcelain that increase the risk of fracture. Thus slow cooling protocols are currently recommended for zirconia-based restorations to reduce the development of high residual tensile zones, and therefore reduce the risk of chipping fractures. In addition to the cooling rate, the thickness of the veneering porcelain has also been identified as a contributing factor by further increasing thermal gradients from the outer cooling surface of the veneering porcelain to the inner porcelain close to the zirconia core. So far there are no studies that have integrated the relationship between veneering porcelain thickness, cooling rates, and differences in surface residual stresses on anatomical zirconia molar crowns. This research is composed of three parts: Part I (chapter 2) is a comprehensive review of the published clinical trials on zirconia-based restorations. Part II (chapter 3) investigates residual stresses on the cusp tips of fast cooled and slow cooled zirconia molar crowns with various cusp thicknesses. In part III (chapter 4), fast and slow cooled molar zirconia crowns were tested under static loading-to-failure in order to qualitatively compare differences in failure modes and fracture features using fractographic analysis. Furthermore, quantitative assessment of the maximum load-to-failure in both groups was also undertaken to establish any strengthening effects developed by the tempering process on the fast cooled zirconia crowns. Methods & Materials: Part I: A comprehensive review of in vivo trials involving zirconia-based restorations featuring in MEDLINE and PubMed between 1950 and June 2011 was completed. A manual hand search of relevant dental journals was also completed. Part II: Six identical Procera zirconia copings (Procera, Nobel Biocare), pressed with IPS e.max ZirPress (Ivoclar Vivadent) and layered with IPS e.max Ceram (Ivoclar Vivadent) based on a mandibular molar form, were divided into 3 groups to the following flattened cusp heights: 1mm, 2mm, and 3mm. Half the samples were slow cooled during the final glazing cycle and the other half fast cooled. To determine residual surface stresses, 4 cusps on each crown were indented using a Vickers microhardness indenter under a load of 10N. Crack measurements were made on calibrated photographic images taken immediately after each indentation using an optical light microscope and a mounted digital camera. Part III: Ten identical Procera zirconia copings, pressed with IPS e.max ZirPress based on a mandibular molar form, were divided into 2 groups. Half the samples were slow cooled during the final glazing cycle and the other half fast cooled. Following cementation of the zirconia crowns to epoxy resin abutments using Rely-X Unicem (3M ESPE), the disto-lingual cusp of each crown was subjected to a simulated occlusal adjustment prior to loading-to-failure using a universal testing machine. This was done using a 4mm diameter stainless steel ball at a cross-head speed of 0.1mm/min. Qualitative fracture analysis of each fractured sample was initially carried out by a visual interpretation. Two samples from each group were chosen for examination by scanning electron microscopy. Results: Part I: Nineteen clinical trials involving zirconia-based restorations were found. Fifteen were conducted on fixed partial dentures, and 4 on single crowns, of which 14 were based on soft-milled zirconia, and 5 on hard-milled zirconia. Chipping of veneering porcelain was a common occurrence, and framework fracture was only observed in soft-milled zirconia. Part II: Residual surface compressive stresses were recorded for all fast cooled crowns. In contrast, surface residual tensile stresses were recorded on the cusp tips of all slow cooled zirconia crowns. The highest residual compressive stresses were found on fast cooled 1mm thick porcelain cusps (-13.08 MPa) which was significantly higher than the 2mm and 3mm fast cooled crowns (P 0.05). The highest residual tensile stresses were recorded for the 2mm thick cusp crown in the slow cooled group (5.36 MPa), however there were no significant statistical differences in the various cusp heights for the slow cooled group (P = 0.05). Part III: The average maximum load-to-failure for the fast cooled group was slightly lower than that recorded for the slow cooled group (901.54 N and 1013.00 N respectively), however this difference was not statistically significant (P > 0.05). Fractographic analyses confirmed the presence of differential residual stress profiles in the veneering porcelain of fast cooled zirconia crowns by the presence of high concentrations of twist hackles, compression curls, and multi-plane fractures, which were not observed in the slow cooled zirconia crowns. Conclusions: Part I: Based on the limited number of short-term in vivo studies involving zirconia-based restorations, there is a high incidence of veneering porcelain chipping, affecting every brand of zirconia, in both single crowns and fixed partial dentures. These clinical trials report on restorations fabricated before the implementation of the modified manufacturers’ recommendation of slow cooling zirconia-based restorations. Further long-term prospective studies that address the cooling protocol around the glass transition temperatures of the veneering porcelains are necessary to establish the best manufacturing process for zirconia-based restorations. Part II: The results confirm that residual surface compressive stresses at the cusp tips were significantly higher in the fast cooled crowns than the slow cooled crowns. Increasing the thickness of the veneering porcelain in zirconia molar crowns did not cause an increase in the resultant surface residual stresses possibly due to variation in the anatomical contours of veneering porcelains. Moreover, a reverse trend was found when decreasing the thickness of veneering porcelain from 2mm to 1mm cusp tips in fast cooled crowns. Part III: Fast cooling zirconia crowns did not improve fracture resistance under static loading-to-failure. In addition, crack propagation and fracture patterns were highly influenced by the cooling protocols used