1

Introduction

Despite the success in developing high strength ceramic cores for bilayered all-ceramic restorations in the posterior of the mouth , chipping of the veneering porcelain in zirconia-based restorations has been reported to be higher than that for metal-ceramics and other all-ceramic restorations . Molin and Karlsson found the incidence of chipping fractures to be 35% in zirconia-based fixed partial dentures (FPDs) over 5 years , while Larsson et al. reported an incidence of 54% in 1 year . Reuter and Brose reported a chipping rate of 2.5% for metal-ceramic FPDs after 5 years , whereas no veneering porcelain chipping was observed for glass infiltrated ceramic-based frameworks after 5 years in two other studies . The chipping of veneering porcelain has been identified as a major setback for zirconia-based restorations, instigating a plethora of studies investigating the causes and its prevention.

The literature has indicated a number of reasons why zirconia-based all-ceramic restorations have a higher incidence of chipping fractures, being due to cohesive failures within the veneering porcelain rather than adhesive failure between the zirconia core and veneer. These include mismatch of the coefficient of thermal expansion between the zirconia core and veneering porcelain , mechanically defective microstructural regions in the porcelain, areas of porosities , surface defects or improper support by the framework , overloading and fatigue , and low fracture toughness of the veneering ceramic . Nevertheless the most accepted explanation so far is the development of high residual tensile stresses within the veneering porcelain caused by fast cooling zirconia restorations . Indeed, since the introduction of zirconia restorations in dentistry, manufacturers have introduced slow cooling firing programs in order to reduce the risk of chipping fractures.

Zirconia is a very poor thermal conductor compared to metal alloys and even other all-ceramic core materials ( Table 1 ). It is important to note that the rate at which the inner veneering porcelain in a bilayered restoration cools below its glass transition temperature ( T _g ) during the end of a firing cycle will depend on the neighboring core material and its thermal conductivity properties. For instance, when a metal-ceramic restoration is fast cooled by air-bench cooling, the veneering porcelain cools rapidly both from the outside and inside of the restoration because of the high thermal conductivity of the metal core. On the other hand, when a zirconia-based restoration is fast cooled, the center of the veneering porcelain close to the zirconia core remains at temperatures above T _g for longer. A large thermal gradient forms between the outer surface of the veneering porcelain and the inner regions, influencing the type and magnitude of residual stresses in the veneering porcelain . It was known as early as 1979 that thermal conductivity of the core material was a contributing factor in the development of thermal stresses in metal-ceramic restorations . However, this factor was not previously investigated since all metal alloys are relatively good thermal conductors and it is unlikely that differences amongst them had any clinical significance.

Just as commercial tempering is used to strengthen glass for windscreens and glass doors/windows , tempering of metal-ceramic restorations by removing them from the furnace at high temperatures and allowing them to bench-cool in air at ambient temperatures, has been established as common practice by dental laboratories to strengthen the veneering porcelain . This process in effect toughens the veneering porcelain by the development of compressive stresses on the outer surface of the veneer. As a result, applied tensile loads that may fracture non-strengthened glass initially have to exceed the surface compressive stresses before surface cracks begin to be placed in tension, and therefore the strength of the tempered material will be approximately increased by the extent of the reinforcing surface compressive stresses. It is important to note however, that although the overall “effective” fracture toughness of tempered glass increases, once a crack begins to grow through the thin compressed superficial layer, the glass can spontaneously shatter, as internal tensile stresses rapidly accelerate and cracks bifurcate .

In terms of the classic literature pertaining to metal-ceramics, transient and residual stresses in dental porcelain cooled at various rates investigated using porcelain disks , and metal-porcelain disks , report analogous results. Regardless of the exact values reported in each study, surface residual compressive stresses are observed with faster cooling rates, while slow cooled samples exhibit residual tensile stresses. Consequently, the aforementioned studies concluded that fast cooling metal-ceramic restorations is preferable in order to strengthen the veneering porcelain, thereby its clinical life, and indeed has been the established procedure for decades.

Taskonak et al. determined residual stresses in zirconia-based bilayered disks under both fast and slow cooling rates using fracture mechanics (biaxial flexural strength test) . They also found that fast cooling generated surface residual compressive stresses with an upper compressive limit of −21 MPa, and slow cooling generated surface residual tensile stresses with an upper tensile limit of +19 MPa. The authors concluded that residual stresses can be altered using different heat treatments, and that these changes are a direct result of the viscoelastic behavior of the glass veneer during various cooling rates. The authors nevertheless did not make any technical recommendations regarding cooling rates for zirconia-based restorations. These results may suggest that bilayered zirconia restorations behave similarly to metal-ceramics during fast cooling, however stress profiles in bilayered zirconia and metal disk samples have been found to be different , and to also exhibit opposite trends when the veneering porcelain thickness was varied . In the mean time, the practice of fast cooling has been recognized as being the offending factor when considering the cause of zirconia-based restorations chipping . In vitro studies using mathematical modeling and finite element analysis (FEA) , optical polarimetery , fracture load resistance testing , shear bond strength in veneer/zirconia disks , and Vickers indentation of sectioned FPD samples confirm the relationship between residual tensile stresses in the veneering porcelain and fast cooling zirconia restorations. Using zirconia spheres and 5 zirconia compatible veneering porcelains, Guazzato et al. found that the incidence of cracking of veneering porcelains increased when using a faster cooling rate, demonstrating that the superficial compressive strength generated with fast cooling may be less of an advantage than the hazardous tensile stresses developed within the veneers in spherical geometries . This was also identified with the finite element numerical modeling of planar and spherical bilayered objects by DeHoff and Anusavice . Therefore it has been proposed that fast cooling zirconia restorations have a direct influence on the development of high compressive residual surface stresses and compensating central residual tensile stresses in the veneering porcelain, thus placing the system at a high risk of chipping by the development of subsurface cracks. In addition to cooling rates, mismatch of the coefficient of thermal expansion (CTE) and thickness of the veneering porcelain are also critical factors. These factors determine whether the residual stresses in the veneering porcelain are in compression or tension, influencing the overall fracture toughness of the restorations .

It has been well established for metal-ceramic systems, that the veneering porcelain layer should not exceed 1.5–2 mm of thickness . Temperature gradients throughout a cooling veneering porcelain thickness will result in areas that are above and below its T _g , and the thicker the porcelain is, the greater are the thermal gradients and residual stresses . A number of studies have examined the relationship between veneering porcelain thickness in various geometries and its influence on residual stress development. Many have preferred to use simple models such as monolayer and bilayered plates to look at this interaction, because of their simplicity in explaining complex thermo-mechanical principles. In reality however, crowns and FPDs are more sphero-cylindrical hollow forms with varying porcelain and framework thickness throughout the anatomical makeup, demonstrating far more complex structures than simple plates. What is more is that residual stresses are additive in nature, and are geometry-dependent, with cylindrical and spherical geometries generating higher stress values than disk geometries . As a result, residual stresses may vary at different locations in a restoration due to variations in the thermal properties of the porcelain resulting from different cooling rates , and irregular porcelain/core thickness ratios , with the potential for an increase in tensile stress being directly influenced by an abrupt change in geometry . When considering a monolayered porcelain plate, the temperature difference between the surface and center of a porcelain plate is proportional to the square of the plate half-thickness. Because of this, thicker plates need longer to complete the liquid-to-glass transformation sequentially from the surface to the center while the layers of porcelain reach T _g during cooling . As a consequence, it has been found that since the temperature difference is greater in a thicker plate, then the surface residual compressive stresses and central residual tensile stresses are also greater .

In a bilayer plate model using four different ceramic core materials, Swain demonstrated that compression in the outer surface of the veneer increased as the thickness of the veneering porcelain increased . This model assumed a positive CTE mismatch of 1 ppm/K and T _g of 500 °C for all four groups, and showed that by keeping these variables constant, increasing the thickness of the veneering porcelain increases the residual compressive stress on the surface of the veneering porcelain in a bilayer model. Nevertheless, the author acknowledged that this is a very simplistic model, as it does not consider a number of important influencing factors such as the complex sphero-cylindrical form of dental restorations. In view of this, it is of great importance to investigate the relationship between residual thermal stresses and veneering porcelain thickness resulting from various cooling rates in clinically representative crown forms, in order to relate the results to the basic flat-plate models commonly used.

Currently there are no simple techniques to directly measure residual stresses within veneering ceramics. This is because in spherical like objects (such as crowns), the resultant residual internal stresses are locked within the porcelain, and are considered in terms of three principle stress components: hoop, radial and axial . As a result, studies have mainly relied on indirect means to determine residual stress formation in veneering porcelains. Methods used include mathematical modeling , FEA , fracture mechanics , optical imaging systems measuring polarized light transmitted through thin porcelain sections , and more recently using a hole-drilling method adapted from the engineering industry . Residual surface stresses have also been determined using micro-hardness indentation techniques developed by Marshal and Lawn . This technique is one of the most commonly used methods for evaluating the fracture toughness of ceramics because of its simplicity and convenience, causing minimal surface damage, without the need for special specimens or preparations. As a result, this technique has been used in a large number of studies investigating tempering induced residual stresses in dental monolayer and bilayer porcelains disks , flat non-anatomical crowns , and in sectioned zirconia FPD forms . However, it has been limited to the flat model because of the need for a perpendicular flat surface during indentation by the indenter, which does not suit cuspal inclines and curved surfaces. In addition, the geometry of dental crowns and FPDs are not flat, and thus precludes utilizing indentation for determining surface residual stress type and magnitude directly on clinically representative dental restorations. It is also recognized that this method of measuring the absolute value of the fracture toughness in glass and ceramics has been challenged . It is argued that because of the discrepancies in fracture toughness values amongst some studies, the Vickers indentation fracture toughness technique is not reliable for all ceramics and other brittle materials, although these authors in many instances found good agreement between indentation and traditional methods for determining fracture toughness. So while this indentation technique may not be satisfactory for absolute ranking of different materials, they can give information about crack formation and comparative surface residual stress trends in a group of samples within a study, as opposed to comparing the results among different studies in the literature. The aim of this study was to investigate the influence of cooling rate on surface residual stresses in duplicate bilayered zirconia molar crowns with flattened cusp tips of different heights. Cusp heights of 1 mm, 2 mm, and 3 mm were investigated using fast and slow cooling protocols during the final glazing cycle. Vickers indentation was used to calculate the type and magnitude of residual stresses for each cusp height. The null hypothesis for this study was that residual stresses would be the same for all cusp heights, and for both cooling protocols.

2

Materials and methods

An epoxy resin (Masterflow 622 Heavy Duty Epoxy Resin Grout, Degussa, Hanau, Germany) jig with multiple indexing features was prepared, duplicating the anatomy of a mandibular right first molar tooth from a dental model kit (Nissin Dental Study Model Mould 305, Nissin Dental Products, Kyoto, Japan). Zirconia crown preparation was carried out on the resin duplicate model using course and fine-grit diamond burs (Komet Diamonds, Brasseler GmbH & Co., Lemgo, Germany), comprising 2 mm occlusal reduction, 1.5 mm axial reduction, 1.2 mm shoulder, 4 mm height and 8 mm width mimicking an average molar zirconia full crown preparation. Using a Piccolo Procera scanner (Nobel Biocare AB, Gothenburg, Sweden) the resin abutment was scanned and 6 duplicate 0.7 mm thick Procera zirconia crown copings (Nobel Biocare) were milled, allowing for die spacing of 20 μm. The copings were veneered with a wash layer of IPS e.max ZirLiner (Ivoclar Vivadent, Schaan, Liechtenstein) mixed with IPS e.max ZirLiner Liquid (Ivoclar Vivadent) and then sintered in a Programat P500 furnace (Ivoclar Vivdent) following the manufacturers instructions. The first wash firings were done on “as delivered” zirconia copings with no pre-treatment to the zirconia surfaces. Using a duplicate of the unprepared master epoxy resin model, the thickness of the veneering porcelain was modified using blue modeling wax (S-U-Gnatho-Wax Blue, Schuler Dental, Ulm, Germany) to the final anatomy of flat cusped crowns with 1 mm, 2 mm, and 3 mm thick veneering porcelain. Individual split putty keys were then made of each crown form using polyvinylsiloxane putty material (Express STD, 3M ESPE, Seefeld, Germany) relined with light bodied polyvinylsiloxane impression material (Exahiflex Injection type, GC America, Alsip, USA), to accurately capture the crown anatomy and indexing features on the jig. Molten blue modeling wax was poured into the silicone putty keys to produce pairs of geometrically identical waxed zirconia copings with 1 mm, 2 mm, and 3 mm cusp heights ready for spruing and heat-pressing the veneering porcelain ( Fig. 1 ).

Split putty key used to wax-up 2 mm cusps group and occlusal view of completed waxed coping ready for pressing.

The zirconia copings were hot-press veneered with IPS e.max ZirPress (Ivoclar Vivadent) pressable feldspathic porcelain using a Programat EP500 pressing machine (Ivoclar Vivadent) following the manufacturers recommendations. After cooling the investment to room temperature, divesting was done in a sandblasting unit (EasyBlast, Bego Dental, Bermen, Germany) using 50 μm glass beads at 2-bar pressure (Renfert, Hilzingen, Germany). The reaction layer formed during the pressing was removed by immersing the crowns into a hydrofluoric acid solution (IPS e.max Press Invex Liquid, Ivoclar Vivadent) in an ultrasonic cleaner for 5 min. Subsequently the crowns were cleaned under running water for 2 min and dried under compressed air pressure. Sprues were cut using a high-speed fine-grit diamond bur (Komet Diamonds) under copious water-cooling. The flat cusps on each pressed crown were cut back by 0.5 mm using a high-speed fine-grit pear-shaped diamond bur (Komet Diamonds) under water cooling to allow for a thin layer of IPS e.max Ceram veneer (Ivoclar Vivadent). This was done to overcome a severe porosity problem on the surface of the pressed porcelain noted on a number of crowns. A slurry of IPS e.max Ceram porcelain powder mixed with modeling liquid was condensed on the cut-back cusp tips by a combination of blotting with absorbent paper and using an ultrasonic device (Ceramosonic II Condenser, Shofu Inc., Kyoto, Japan) to ensure minimal porosity development within the veneer surface. The flat cusp tips were then polished to a mirror finish using 1200, 2000 and 4000 grit silicon carbide rotary polishing disks (Struers Inc., Copenhagen, Denmark) in a TegraSystem polishing machine (Struers Inc.), while the remainder of the surfaces were polished using diamond impregnated rubber burs (OptraFine, Ivoclar Vivadent).

Each pair of crowns were then divided into a fast and slow cooled group by being subjected to a final glaze cycle according to the firing protocols shown in Table 2 .

The Austromat D4 (Dekema Dental, Freilassing, Germany) porcelain furnace was used for the slow cooling protocol because of the accuracy of control of these furnaces in bringing the crowns down through the glass transformation temperature range. The firing table mechanism allows the table to exit the furnace in a vertical downwards direction so the crowns are subjected to high radiant heat from the furnace muffle resulting in even heating/cooling of the crown. However, this mechanism is limited when wanting to carry out a fast cooling cycle because it takes too long for the table to exit the furnace muffle and the radiant heat from the muffle influences the rate of cooling through the T _g temperature. In contrast, the “clam” lid design of the Programat P500 furnaces opens immediately at the end of the program so the crowns can be removed from the firing table, minimizing exposure to radiant heat, and allowing fast cooling. This design is less suitable for slow cooling because the opening of the lid muffle at an angle to the crowns on the firing table results in one side being subjected to radiant heat and the other side cooling faster with less control. Crowns in the slow group were cooled at the rate of 20 °C/min after the completion of the glazing cycle from 725 °C to 400 °C. This slow cooling rate insured that the veneering porcelains in this system (IPS e.max ZirPress and IPS e.max Ceram) reached their glass transition temperatures at a slow rate throughout the entire thickness of the veneer. According to the manufacturer’s data sheets, the T _g values for IPS e.max ZirPress and IPS e.max Ceram are 530 ± 10 °C and 490 ± 10 °C respectively. If we were to take the lower value for each porcelain, then cooling from the 725 °C glazing temperature to T _g of IPS e.max ZirPress takes just over 10 min, with a further 2 min for the temperature to reach the glass transition temperature of IPS e.max Ceram. The temperature was then held at 400 °C for further 5 min before crowns were removed from the furnace to insure that the entire veneering layer reaches a temperature below T _g before the crowns are allowed to cool in ambient air.

After the completion of glazing cycles, composite cores (Z100, 3M ESPE) were made for each crown ensuring that the base of the composite core was parallel to the flattened cusps. This was accomplished by using two glass slides (75 mm × 25 mm × 1.2 mm) that were held apart using two rectangular bars (30 mm × 12 mm × 0.8 mm) made of type III dental stone. Composite bases were built over the composite cores with the crowns inverted on their flat cusps touching the bottom slide, while the final composite cure was done with the top slide flattening the base parallel to the plane of the cusps.