d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 271–280

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Influence of veneering porcelain thickness and cooling rate on residual stresses in zirconia molar crowns Basil Al-Amleh ∗ , J. Neil Waddell, Karl Lyons, Michael V. Swain Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. The aim of this study was to investigate the influence of increasing veneer-

Received 28 October 2012

ing porcelain thickness in clinically representative zirconia molar crowns on the residual

Received in revised form

stresses under fast and slow cooling protocols.

30 April 2013

Methods. Six veneered zirconia copings (Procera, Nobel Biocare AB, Gothenburg, Sweden)

Accepted 27 November 2013

based on a mandibular molar form, were divided into 3 groups with flattened cusp heights that were 1 mm, 2 mm, or 3 mm. Half the samples were fast cooled during final glazing; the other half were slow cooled. Vickers indentation technique was used to determine

Keywords:

surface residual stresses. Normality distribution within each sample was done using

Zirconia

Kolmogorov–Smirnov & Shapiro–Wilk tests, and one-way ANOVA tests used to test for signif-

Porcelain

icance between various cusp heights within each group. Independent t-tests used to evaluate

Crowns

significance between each cusp height group with regards to cooling.

Residual stresses

Results. Compressive stresses were recorded with fast cooling, while tensile stresses with

Chipping

slow cooling. The highest residual compressive stresses were recorded on the fast cooled

Thickness of veneer

1 mm cusps which was significantly higher than the 2 and 3 mm fast cooled crowns (P < 0.05).

Cooling protocol

There was a significant linear trend for residual stress to decrease as veneering porcelain

Vickers indentation

thickness increased in the fast cooled group (P < 0.05). No significant differences were found between the various cusp heights during slow cooling (P ≥ 0.05). Significance. Cooling rate and geometric influences in a crown anatomy have substantially different effects on residual stress profiles with increasing veneering porcelain thickness compared to the basic flat plate model. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Despite the success in developing high strength ceramic cores for bilayered all-ceramic restorations in the posterior of the mouth [1,2], chipping of the veneering porcelain in zirconiabased restorations has been reported to be higher than that for

metal-ceramics and other all-ceramic restorations [3]. Molin and Karlsson found the incidence of chipping fractures to be 35% in zirconia-based fixed partial dentures (FPDs) over 5 years [4], while Larsson et al. reported an incidence of 54% in 1 year [5]. Reuter and Brose reported a chipping rate of 2.5% for metal-ceramic FPDs after 5 years [6], whereas no veneering

∗ Corresponding author at: Department of Oral Rehabilitation, Faculty of Dentistry, PO Box 647, Dunedin 9054, New Zealand. Tel.: +64 3 479 5284; fax: +64 3 479 5079. E-mail addresses: [email protected], [email protected] (B. Al-Amleh). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.11.011

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Table 1 – Thermal conductivity of common dental materials from highest to lowest (modified from Swain [15]). Material Gold alloys Base metals Alumina In-Ceram alumina Zirconia Feldspathic porcelains

Thermal conductivity (W m−1 K−1 ) 200 40 30 14 2 2

porcelain chipping was observed for glass infiltrated ceramicbased frameworks after 5 years in two other studies [7,8]. 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 [9], mechanically defective microstructural regions in the porcelain, areas of porosities [10], surface defects or improper support by the framework [11,12], overloading and fatigue [13], and low fracture toughness of the veneering ceramic [14]. 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 [15]. 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 (Tg ) 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 Tg 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 [16–18]. 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 [19]. 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 [20], 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 [21,22]. 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 [23,24]. In terms of the classic literature pertaining to metalceramics, transient and residual stresses in dental porcelain cooled at various rates investigated using porcelain disks [22,25–28], and metal-porcelain disks [21,29], 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 zirconiabased bilayered disks under both fast and slow cooling rates using fracture mechanics (biaxial flexural strength test) [30]. 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 [31], and to also exhibit opposite trends when the veneering porcelain thickness was varied [32]. 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 [15,33]. In vitro studies using mathematical modeling and finite element analysis (FEA) [17,18], optical polarimetery [16], fracture load resistance testing [34], shear bond strength in veneer/zirconia disks [35], and Vickers indentation of sectioned FPD samples [36] 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

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 271–280

in spherical geometries [37]. This was also identified with the finite element numerical modeling of planar and spherical bilayered objects by DeHoff and Anusavice [25]. 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 [29,38–40]. It has been well established for metal-ceramic systems, that the veneering porcelain layer should not exceed 1.5–2 mm of thickness [41]. Temperature gradients throughout a cooling veneering porcelain thickness will result in areas that are above and below its Tg , and the thicker the porcelain is, the greater are the thermal gradients and residual stresses [42,43]. 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 [39,44,45]. 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 [46], and irregular porcelain/core thickness ratios [32], with the potential for an increase in tensile stress being directly influenced by an abrupt change in geometry [40,47]. 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 Tg during cooling [42]. 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 [24]. 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 [15]. This model assumed a positive CTE mismatch of 1 ppm/K and Tg 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

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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 [48]. As a result, studies have mainly relied on indirect means to determine residual stress formation in veneering porcelains. Methods used include mathematical modeling [15,18,42], FEA [39,40,48], fracture mechanics [30], optical imaging systems measuring polarized light transmitted through thin porcelain sections [16,47], and more recently using a hole-drilling method adapted from the engineering industry [31,32,49,50]. Residual surface stresses have also been determined using micro-hardness indentation techniques developed by Marshal and Lawn [51]. 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 [22,24,27–29], flat nonanatomical crowns [52], and in sectioned zirconia FPD forms [36]. 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 [23]. 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.

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Fig. 1 – Split putty key used to wax-up 2 mm cusps group and occlusal view of completed waxed coping ready for pressing.

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 ␮m. 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). 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 ␮m 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 Tg 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

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Table 2 – Final glazing firing protocols for fast and slow cooled crowns. Fast cooling Furnace used Preheating temperature (◦ C) Drying time (min) Heating rate (◦ C/min) Firing temperature (◦ C) Holding time (min) Cooling protocol

Slow cooling

Programat P500 403

Austromat D4 403

1 60 725 5 Each crown immediately removed from the furnace to bench cool at the end of the firing temperature holding time

1 60 725 5 Cooling rate at 20 ◦ C/min from firing temperature until reached 400 ◦ C, then held for extra 5 min before crowns allowed to bench cool

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 Tg 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 Tg 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 Tg 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.

2.1.

Indentation fracture toughness testing

A total of 5 indentation cracks were made on each flattened cusp tip, resulting in a total of 20 indentations per sample (4 cusps per tooth, 5 indentations per cusp) (Fig. 2). To aid in crack visualization and measurements, the samples were sputter coated with gold-palladium prior to indentation. The indentations were made with a Vickers hardness indenter (Shimadzu Corp., Kyoto, Japan) using a standard 136◦ pyramidal diamond

Fig. 2 – Example of a completed crown demonstrating flattened cusps ready for indentation; MB, mesio-buccal cusp; ML, mesio-lingual cusp; DB, disto-buccal cusp; DL, disto-lingual cusp.

indenter at a load of 10 N for 15 s, oriented perpendicular to the flattened cusps surfaces. Digital photographic images were taken immediately after each indentation using a digital camera (PowerShot A640, Canon, Tokyo, Japan) that was fixed onto an optical light microscope (Alphaphot-2 YS2, Nikon, Tokyo, Japan). Using Adobe Photoshop CS3 software (Adobe Systems Inc., San Jose, USA) each indentation was measured at a later date with minimal error of crack lengths due to continuing crack propagation in the presence of residual indentation stress and environmental moisture during direct crack measurements. Indents showing material spalling were excluded from the analysis and an alternative indentation was made. To avoid influence between each indent, a distance of at least twice the crack length between each indentation was performed. Furthermore, the typical size of cracks measured were less than 110 ␮m, well below the thickness of the veneering porcelain layers tested, thus only residual stresses in the outer layer of the porcelain were evaluated.

2.2.

Residual stress calculation

In order to calculate the residual surface stresses on the various cusp tips using the indentation technique developed by Marshall and Lawn (1977), the fracture toughness or the critical stress intensity value (KIC ) for IPS e.max Ceram was first determined via Vickers indentations on two slow cooled IPS e.max Ceram discs (10 mm × 2.5 mm) and using Eq. (1):

KIC = 

 E 0.5 P H

c3/2

(1)

where E is the elastic modulus of the material, P is the load applied, H is the hardness which is determined by dividing P with the measured indent length squared, c is the length of radial crack measure from center of indent, and  is a constant 0.016. The KIC value for IPS e.max Ceram was calculated to be 0.61 MPa m1/2 . Following this, residual stress calculations were made for each indent on the flattened cusps by measuring the

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Table 3 – Average results for each cusp per slow cooled sample with standard deviation in brackets. Sample

Cusps

Crack length (␮m)

Slow cooled 1 mm

DB DL MB ML

97.75 (4.58) 96.45 (3.40) 99.18 (6.04) 95.62 (4.40)

39.07 (0.35) 38.84 (0.32) 39.46 (0.90) 38.66 (0.16)

4.44 (1.85) 4.14 (1.26) 4.64 (2.34) 3.88 (1.90)

Slow cooled 2 mm

DB DL MB ML

107.26 (7.01) 96.38 (7.26) 104.58 (4.78) 99.04 (4.67)

39.66 (1.27) 39.23 (0.56) 39.45 (0.71) 38.74 (0.69)

7.09 (2.41) 3.66 (2.43) 6.57 (1.28) 5.21 (1.27)

Slow cooled 3 mm

DB DL MB ML

95.10 (4.68) 95.01 (5.37) 101.64 (4.99) 91.33 (2.97)

38.70 (0.62) 37.84 (0.55) 38.51 (0.45) 37.69 (0.63)

3.62 (1.93) 4.17 (2.07) 6.18 (1.74) 2.79 (1.27)

average crack lengths and indent sizes that were then inserted into Eq. (2):

KIC = 

 E 0.5 P H

c3/2

√ ± Y c

(2)

where Y is the factor for a half-penny surface crack (Y = 1.985), and  is the applied or residual stress present. The second term in Eq. (2) may be positive or negative depending upon whether the stresses on the surface are tensile or compressive respectively. The above approach was used to determine the residual stresses that developed in the veneering porcelains as a consequence of fast and slow cooling protocols for each group.

2.3.

Statistical analysis

All statistical analyses were made using Statistical Package for Social Sciences for Windows (SPSS 17.0, SPSS Inc., Chicago, USA). To test for residual stresses normality distribution within each sample, the Kolmogorov–Smirnov & Shapiro–Wilk tests were used. One-way ANOVA tests were used to test for significance between the various cusp heights in the fast cooled group and the slow cooled group, between the four cusps on each sample. Independent t-tests were used to evaluate significance between each cusp height group (1 mm, 2 mm and 3 mm) with regards to the cooling protocols.

3.

Results

The average crack size, indent length, and residual stresses calculated using Eq. (2) with standard deviations are shown for each cusp on each sample in Tables 3 and 4 (slow cooled and fast cooled respectively). Table 5 shows the average results for all indentations in each sample for both cooling protocols. In general, surface residual compressive stresses were recorded for all fast cooled crowns, and in contrast, residual surface tensile stresses were recorded on the cusp tips of all slow cooled zirconia crowns (Fig. 3). Statistical normality testing for the residual stresses were done using the Kolmogorov–Smirnov & Shapiro–Wilk tests. Both the fast and slow cooled groups had normally distributed results, thus one-way ANOVA and independent t-tests were used to determine statistical significance. There was no statistical significant difference in

Indent size (␮m)

Residual stress (MPa)

residual stress recorded between each indented cusp on each sample for both the slow and fast cooled groups (P < 0.05) (Tables 3 and 4). The compressive residual stresses in the 1 mm fast cooled crown were statistically significantly higher than the 2 mm and 3 mm fast cooled samples (P < 0.05). Furthermore, there was a significant linear trend for residual stress to decrease with increasing thickness of the veneering porcelain in the fast cooled group (P < 0.05). However, there was no statistically significant difference between the 2 mm fast cooled and 3 mm fast cooled samples (P ≥ 0.05). Similarly, the one-way ANOVA test for the means for the various cusp thicknesses in the slow cooled group did not show any statistical significant differences (P = 0.05), moreover there was no significant linear trend with increasing veneering thickness (P ≥ 0.05). There was a statistically significant difference (P < 0.05) between the means of the fast and slow cooled crowns for each cusp height tested (1 mm, 2 mm and 3 mm).

4.

Discussion

Tempering of porcelain by fast cooling has been used to incorporate residual compressive stresses on the surface of porcelains, effectively toughening it. However compensating tensile stresses within the body of the porcelain must balance the compressive stresses formed on the porcelain surface, thus resulting in a net residual force of zero [43]. It follows then that for tempered bilayered dental crowns, the higher the residual surface compressive stress, the higher the compensating residual tensile stresses within the crown cusps and/or supporting framework must be. It is also well established that for flat plate geometries, the thicker the porcelain, the greater the thermal gradients throughout the cooling porcelain which results in higher residual stress formation than in thinner porcelain samples [24,42] and bilayered zirconia crown forms [18]. A statistically significant difference in surface residual stresses was observed between the fast and slow cooled bilayered zirconia crowns for each cusp height tested (1 mm, 2 mm and 3 mm) (P < 0.05). Each of the fast cooled crowns exhibited residual compressive stresses on the surface of the flattened cusp tips, while the residual stresses on the slow cooled crowns were in tension. These findings corroborated the results of others on tempered bilayered porcelain flat samples using the indentation technique

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Table 4 – Average results for each cusp per fast cooled sample with standard deviation in brackets. Sample

Cusps

Fast cooled 1 mm

DB DL MB ML

72.32 (6.00) 73.32 (1.55) 71.78 (3.99) 71.27 (4.08)

38.43 (1.59) 38.56 (0.40) 40.15 (0.51) 39.84 (0.95)

−12.08 (5.18) −10.72 (1.78) −14.64 (4.80) −14.85 (4.74)

Fast cooled 2 mm

DB DL MB ML

75.98 (3.94) 72.24 (2.50) 75.57 (2.92) 77.07 (4.67)

39.44 (1.01) 38.73 (0.55) 38.42 (0.30) 39.06 (0.45)

−9.26 (2.81) −12.12 (2.41) −8.45 (2.69) −8.01 (4.04)

Fast cooling 3 mm

DB DL MB ML

75.46 (1.33) 76.97 (1.25) 71.94 (1.76) 72.93 (4.32)

38.93 (0.74) 37.97 (0.40) 37.50 (0.53) 37.77 (1.20)

−9.02 (1.32) −6.58 (1.10) −10.86 (2.22) −10.47 (4.70)

Crack length (␮m)

Indent size (␮m)

Residual stress (MPa)

Table 5 – Average results for each sample with standard deviation in brackets. Samples

Crack length (␮m)

Indent size (␮m)

Residual stress (MPa)

Slow cooling

1 mm 2 mm 3 mm

97.25 (4.53) 101.81 (6.87) 95.77 (5.68)

39.01 (0.56) 39.27 (0.86) 38.19 (0.68)

4.27 (1.75) 5.63 (2.24) 4.19 (2.07)

Fast cooling

1 mm 2 mm 3 mm

72.17 (3.94) 75.22 (3.79) 74.33 (3.08)

39.25 (1.19) 38.91 (0.70) 38.04 (0.90)

−13.07 (4.37) −9.46 (3.25) −9.23 (3.04)

[24,27–29], as well as the fracture mechanics approach [30]. However, in contrast to others, increasing the thickness of the veneering porcelain did not result in an increase in residual stresses in the crown samples in this study. To the contrary, the fast cooled group demonstrated a significant trend for residual compressive stress to decrease with increasing thickness of veneering porcelain from 1 mm to 2 mm (P < 0.05). However, increasing the veneering cusp thickness beyond 2 mm showed no statistically significant differences in residual surface stresses in the fast cooled samples (P ≥ 0.05). Also the residual surface stresses on the slow cooled crowns did not show any statistically significant differences with changing the veneering porcelain thickness (P = 0.05). Therefore the null hypothesis, that the residual stresses would be the same for all cusp heights, and for both cooling protocols, is

accepted for the slow cooled group, but rejected in the fast cooled. In terms of increasing the veneering porcelain thickness, these results are somewhat in disagreement with what is reported by others. This may be explained by the fact that residual stress profiles of porcelain bodies are geometrydependent and are additive in nature [32,53]. Studies on spherical and cylindrical geometries have been found to generate higher residual stress values than disk geometries [40,44,45]. Equally, residual stresses may vary at different locations in a restoration, due to irregular porcelain/core thickness ratios, and variations in the thermal properties of the porcelain resulting from different cooling rates [46]. Furthermore, abrupt changes in shape and thickness in the ceramic contours of a dental crown or FPD are reported to be areas

Fig. 3 – Calculated residual stresses on fast and slow cooled 1 mm, 2 mm, and 3 mm crowns.

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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 271–280

of potential tensile stress raisers in the system that increase the risk of fracture of the veneering porcelain [39,40,47]. It is important to realize that the results in this study are limited to local residual surface stresses and do not report on maximum principle stresses existing within each system. Using mathematical modeling and FEA, authors are able to determine maximum principle stresses in various geometries, however these studies do so using many assumptions and therefore arbitrarily selected values are used [39,40,42]. In one FEA study on various geometries, that included molar crowns and 3unit FPDs, De Hoff et al. found that maximum global tensile stresses were higher in FPD forms than in disks (44.0 MPa and 5.4 MPa, respectively) [39]. Similarly, tensile stresses were higher at the core/veneer interface compared to those at the surface. This may explain the relatively low tensile stresses recorded at the surfaces of the slow cooled crowns in this study. Decreasing the cusps heights in the test samples from 3 mm to 1 mm effectively changed the various anatomical contours that influence residual stresses. This change may have eliminated the potential stress concentration zones in the 1 mm fast cooled crown, allowing the porcelain to contract evenly throughout the veneer during cooling. One limitation of this study was that indentations were done on the cusp tips only, which does not allow for measurement of residual surface stresses at other locations, such as the fossa region, where sharp “v” type notches exist between the cusps. It is possible that despite having lower residual compressive stresses on the surface of the cusp tips in the fast cooled samples, stress concentrations existed for the thicker veneered crowns in the fissure regions, influencing global hoop stress profiles on the cusps. Furthermore, despite a number of studies demonstrating that thicker porcelain develops higher residual stresses, in this study, increasing the veneering layer from 2 mm to 3 mm in the fast cooled crown samples did not have a significant effect on residual global surface stresses. The various complex anatomical features may have limited residual stress differences by directing stress concentrations away from the cusp tips, highlighting differences in results when using clinically relevant geometries rather than flat planar models for in vitro investigations. Belli et al. reported stress concentrations in the fissure regions of fast cooled bilayered zirconia molar crowns using photoelastic assessment when the zirconia/veneer CTE mismatch was low (+0.3 ppm/ ◦ C) [47]. However different stress profiles were reported when a high mismatch CTE porcelain was used (+1.4 ppm/ ◦ C). According to the manufacturers data, the CTE mismatch in our system was +0.55 ppm/◦ C, therefore based on the findings by Belli et al., a similar stress profile with stress concentration within the fissure regions is probable and may explain our findings. In addition, slow cooling by the same group resulted in stress concentration zones that were different to that seen in fast cooled samples, located at the veneering porcelain over curved surfaces of the zirconia core at both the cuspal and fissures regions. This also explains our finding that varying the veneering porcelain in the slow cooled samples, from 1 mm to 3 mm, played less of a role in varying the surface stress profile since slow cooling allows more even relaxation of stresses within the veneering layer rather than concentrating stresses in the fissure regions. A very slow cooling protocol was implemented in this study to

insure that the entire veneering layer reached a temperature below Tg before the crowns were allowed to cool in ambient air (Table 2). In addition to using a cooling rate of 20 ◦ C/min, the furnace temperature was then held at 400 ◦ C for a further 5 min before crowns were removed. This thorough slow cooling protocol may account for the minimal difference in residual surface stresses recorded for the slow cooled crowns despite increasing the veneering porcelain thickness by 1 mm increments. In contrast to the fast cooled group, the 1 mm, 2 mm, and 3 mm slow cooled crowns demonstrated residual tensile stresses on the flattened cusp surfaces (4.27, 5.63, and 4.19 MPa respectively). This is a concern in terms of the likely mechanical behavior of these restorations since tensile stresses accelerate the propagation of cracks in ceramics. Studies investigating residual surface stresses created by various cooling rates on disks also report tension on the surface of slow cooled samples. Using fracture mechanics to calculate residual stresses in zirconia bilayered disk samples, Taskonak et al. reported high residual tensile stresses for samples slow cooled from Tg , as well as several temperatures above and below Tg [30]. In comparison, all fast cooled samples exhibited residual compressive stresses. Tensile stresses have also been found using the Vickers indentation technique to measure residual surface stresses in slow cooled opaque porcelain-body porcelain disks [27,28], and metal-porcelain disks [29]. The residual tensile stresses reported by those studies ranged from 8.4 MPa to 47.5 MPa, which were higher than that observed in our results. This can be explained by the geometric influences found in the crown anatomy compared to disks. Flat plane disk geometries are reported to be influenced by two temperature gradients in the system, one being vertical and the other horizontal, a phenomena more recently highlighted by Mainjot et al. [32]. While it may be argued that indentation results on brittle materials are not an absolute reliable means for determination of fracture toughness [23], there are virtually no other simple methods for spatially quantifying residual stresses in brittle materials. It would therefore be inappropriate to make any direct comparisons between the absolute stress magnitudes found in this study to that of previously mentioned studies. Nevertheless, results verify the consistent development of tensile stresses on the surface of slow cooled samples, and compressive stress on fast cooled samples, regardless of their geometry. In vitro trials investigating cooling rates using zirconia spherical and crown geometries have reported the advantage of slow cooling with significantly improved failure loads and crack resistance [34,37]. In contrast others have found slow cooling to negatively influence the zirconia/veneer interface adhesion [35,54]. Further studies are needed to establish the critical role of cooling rate on residual stress profiles and in zirconia/veneer interface adhesion. Part of the sample preparation process, was to produce highly polished crowns with flattened tips on all four cusps. Polishing was done using rotary carbide disks of various grits, however polishing porcelain has been shown to create a layer of compression on the surface [55,56], which may directly influence the indentation results. To overcome this problem, all samples were auto-glazed during the final glazing cycle for 5 min rather than the recommended 1 min. Extended glazing

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 271–280

was done for two reasons. First, it served to anneal the system and relax any surface compressive stresses created during the polishing process. Annealing however has been reported to be a complex procedure, and effective annealing temperature and duration is material specific [57,58]. It is therefore difficult to confirm that residual compressive stresses from the polishing process were completely removed by the 5 min annealing time, however to establish this was beyond the scope of this research. Secondly, it allowed sufficient time for thorough heating of the entire sample at the glazing temperature before the cooling protocols were initiated. The recommended 1 min glazing time may have not been long enough for the porcelain in the thickest regions to reach its visco-elastic state, thus influencing the final residual stresses in the thicker crowns. The three main influencing factors that determine residual stresses in bilayered dental restorations are reported to be differences in the coefficient of thermal expansion (CTE) between the core substructure and the veneering porcelain, cooling rate of the restoration to room temperature, and thickness of the veneering porcelain [15,24,27,29,42,46]. It follows then that the various cusp heights in the slow cooled group, should have observed different residual stress magnitudes. It would appear however, that the latter factor (thickness of the veneering porcelain) is only critical during fast cooling rates but not during slow cooling. This is because thermal gradients that influence the residual stresses can only be created during differential cooling in thicker porcelains. As a consequence, no significant differences in surface residual stresses in 1 mm, 2 mm and 3 mm cusped crowns would be expected when they are slow cooled, which is confirmed by the findings in this study.

5.

Conclusions

In this study, the Vickers indentation technique was used to determine surface residual stress type and magnitudes on clinically relevant bilayered zirconia crowns with various cusps heights subjected to two cooling protocols. Results can be concluded as: 1. Fast cooled zirconia crowns exhibited residual surface compressive stresses on the cusp tips, while tensile stresses were recorded on the cusps of slow cooled crowns. 2. In the fast cooled group, there was a trend for residual surface compressive stresses to decrease with increasing veneering porcelain thickness from 1 mm to 2 mm, which does not support results from flat plane disk models. An increase in thickness beyond 2 mm did not have any further influence. 3. Increasing the veneering porcelain thickness in slow cooled crowns did not influence residual surface stresses at the cusp tips of zirconia crowns.

Acknowledgments The authors wish to thank Mr. Ludwig Jansen van Vuuren and the late Dr Lihong He for their assistance, as well as Ms. Sawsan Al-Shamaa and Dr Momen Atieh for the statistical

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analyses. This work has been supported by Nobel Biocare with research grant 2010-949 and by a Fuller Scholarship from the Sir John Walsh Research Institute, University of Otago.

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