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Fracture Strength of Two Oxide Ceramic Crown Systems After Cyclic Pre-Loading and Thermocycling

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Fracture Strength of Two Oxide Ceramic Crown Systems After Cyclic Pre-Loading and Thermocycling Journal of Oral Rehabilitation 2006 33; 682–689 Fracture strength of two oxide ceramic crown systems after cyclic pre-loading and thermocycling P. VULT VON STEYERN*, S. EBBESSON†,J.HOLMGREN†,P.HAAG*& K. NILNER* *Department of Prosthetic Dentistry, Faculty of Odontology, Malmo¨ University, Malmo¨, Sweden and †Commercial dental laboratory, Malmo¨ , Sweden SUMMARY The aim of the present study was to zirconia 910 (P >0Æ05). Total fractures were more investigate the fracture resistance of zirconia frequent in the alumina group (P <0Æ01). Within the crowns and to compare the results with crowns limitations of this in vitro study, it can be concluded made of a material with known clinical perform- that there is no difference in fracture strength ance (alumina) in away that reflects clinical aspects. between crowns made with zirconia cores com- Sixty crowns were made, 30 identical crowns of pared with those made of alumina if they are alumina and 30 of zirconia. Each group of 30 was subjected to load without any cyclic pre-load or randomly divided into three groups of 10 crowns thermocycling. There is, however, a significant that were to undergo different treatments: (i) difference (P ¼ 0Æ01) in the fracture mode, suggest- water storage only, (ii) pre-loading (10 000 cycles, ing that the zirconia core is stronger than the 30–300 N, 1 Hz), (iii) thermocycling (5–55°, 5000 alumina core. Crowns made with zirconia cores cycles) + pre-loading (10 000 cycles, 30–300 N, have significantly higher fracture strengths after 1 Hz). Subsequently, all 60 crowns were subjected pre-loading. to load until fracture occurred. There were two KEYWORDS: dental ceramics, dental porcelain, all- types of fracture: total fracture and partial fracture. ceramic crowns, aluminium oxide, zirconium-di- Fracture strengths (N) were: group 1, alumina 905/ oxide, thermocycling zirconia 975 (P ¼ 0Æ38); group 2, alumina 904/zirco- nia 1108 (P <0Æ007) and group 3, alumina 917/ Accepted for publication 25 October 2005 Consequently, when ceramics are used in dental Introduction reconstructions, it is important to address these incon- A need for non-metallic restorative materials with veniences when deciding which ceramic systems or optimal aesthetics and characteristics such as biocom- designs to employ. patibility, colour stability, high wear resistance and low Several new materials and techniques have been thermal conductivity are often forwarded as reasons for introduced in the last 20 years to meet the requirements the use of ceramics in dentistry (1, 2). Ceramics are, for use in the oral cavity. Ceramics have now, by and however, brittle materials because of atomic bonds that large, been developed to a point where strength and do not allow the atomic planes to slide apart when toughness fulfil the demands for use in, e.g. all-ceramic subjected to load. Thus, ceramics cannot withstand fixed partial dentures (FPDs), although indications for deformation of >0Æ1% without fracturing (3). Further- the use of these types of reconstructions are consider- more, ceramics in general have pre-existing flaws that ably broader today than before (5–7). High-strength vary in both type and size. Such imperfections act as ceramics for the cores of crowns and FPDs have been starting points for crack formation whenever ceramic introduced and tested, both in vitro and in vivo(1, 8–10). constructions are loaded above a certain level (4). Such high strength ceramics based on alumina or ª 2006 Blackwell Publishing Ltd doi: 10.1111/j.1365-2842.2005.01604.x FRACTURE RESISTANCE OF TWO OXIDE CERAMIC CROWNS 683 zirconia, meets the requirements of strength and based on the same technology as the alumina system toughness, which when compared with other ceramics described above, but the core is made of yttrium- makes them more suitable for use as ceramic cores and stabilized zirconia instead of alumina. The copings copings when extensive loads are expected (1, 11, 12). made of zirconia are veneered with a specially devel- oped porcelain. In the search for new ceramics with improved Alumina strength and long-term clinical performance, it is Aluminium oxide (alumina) has been used for the essential to address the ability of the material to resist purpose of increasing strength of dental porcelains for slow crack growth. Although the properties of zirconia >4 decades (3). Then 15 years ago a new all-ceramic are promising, the tooth-restoration unit forms a system was introduced, employing a technique where laminate system consisting of many layers; (i) veneer high purity alumina crown copings or FPD cores are material, (ii) the interface between core and veneer (the fabricated using computer-aided design/computer- compatibility between those two materials), (iii) core aided manufacturing (CAD/CAM) techniques (13, 14). material, (iv) the cement and its interfaces and (v) the CAD/CAM makes it possible to use industrially manu- abutment tooth. When loading such complex multilay- factured ceramic materials with highly defined quality; ered system in the clinical situation it is subjected to to store all production steps electronically; and to attain fatiguing stresses. As the response to such stresses good reproducibility, accuracy and precision. All these differs between different laminates, it is not possible to measures are considered important, especially when extrapolate strength data from one material system to working with ceramics (15). Subsequent to CAM, the another. It is therefore important to investigate the alumina substructures are densely sintered and ven- fracture resistance of zirconia and to compare the eered with dental porcelain to create the appearance of results with a material with known clinical perform- a natural tooth(2). Clinical studies have indicated that ance (alumina) in a way that reflects clinical aspects as such alumina-based crowns may be used for crowns in nearly as possible regarding test specimens, environ- all locations of the oral cavity (2, 16). Yet, the best mental influences and test mode. The aim of the mechanical properties of all the dental ceramics are present study therefore was to evaluate the strength of attained with yttrium-stabilized zirconiumdioxide (12, one zirconia and of one alumina crown system in a 17). standardized way by mimicking the clinical situation with cyclic mechanical pre-loading and thermocycling under the null-hypothesis that there is not any differ- Zirconia ence between the fracture resistance between crowns Zirconiumdioxide (zirconia) is well known as an with a core of alumina or of zirconia. orthopaedic implant material and has been used in hip surgery for many years (18). By adding a small Materials and methods amount of Y2O3 to ZrO2, it is possible to stabilize the ceramic in a tetragonal phase that normally is unstable Sixty crowns designed as stylized norm crowns were to at room temperature. Several studies have indicated be made for the study, 30 identical crowns of alumina* that flexural strength values of 1200 MPa and fracture and 30 of zirconia (ProceraÒZirconia*). Each group of toughness values of 9 MPa m1/2, which are possible 30 was randomly divided into three groups of 10 with zirconia, are substantially higher than for other crowns that were to undergo different treatments ceramics and that this material therefore could be used according to a test protocol. for highly loaded, all-ceramic restorations. Hence, suggestions have been made that zirconia could also Producing crown copings be a viable alternative to metal in reconstructive dentistry, especially for crowns in the molar region A master die resembling a molar crown preparation was and FPDs (12, 15, 19). made of die stone† for the production of 60 crown A new, strong, all-ceramic alternative that is based on yttrium-stabilized zirconiumdioxide has been devel- *ProceraÒAlumina; Nobel Biocare, Gothenburg, Sweden. oped for heavily loaded restorations. The material is †Vell-mix; Kerr, Romulus, MI, USA. ª 2006 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 33; 682–689 684 P. VULT VON STEYERN et al. copings. The master die was scanned once with a mechanical CAD/CAM scanner (Procera Scanner 40*) and the scanned data were sent via the World Wide Web to the ProceraÒ manufacturer who subsequently produced and delivered the crown copings. Building up the porcelain veneer A replica of the master die was cast in a metal alloy‡ to hold and support the crown copings in a reproducible position during porcelain build-up. The shape and dimensions of the veneer build-up were determined by using a specially made knife to shape the porcelain in a standardized way, according to a modification of the technique described by Yoshinari and De´rand (20; Fig. 1). AllCeramTM porcelain* was used as veneer material for the alumina crowns and Cercon-Ceram STM§ was used for the zirconia crowns, both according to the manufacturer’s recommendations (Fig. 2). In each case, one liner firing and two dentine firings were carried out. Before the dentine was fired a second time, the porcelain was blasted with 50 lmAl2O3 under two bars of pressure. In the last step, the crowns were autoglazed. All firing cycles were made according to the manufacturer’s recommendations in a calibrated porcelain furnace¶. The furnace was calibrated as recommended by the manufacturer, which briefly means that a wedge-shaped porcelain build-up is fired. If the wedge turns clear during firing, this indicates that it is fired in an accurately calibrated furnace. Cementation The norm crowns were to be cemented on separate dies that were specially made by moulding an inlay pattern resin** in 60 A-silicone impressions made of the master Fig. 1. Building up the porcelain veneer. (a) The master die. (b) die††. All crowns were cemented to the Duralay dies Metal replica (crown-holder) and the knife. (c) An alumina-core ‡‡ using zinc phosphate cement under a standardized positioned on the metal-replica.
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