Clinical trials in zirconia: a systematic review

Journal of

Oral Rehabilitation

Journal of Oral Rehabilitation 2010 37; 641–652

Review Article

Clinical trials in zirconia: a systematic review B. AL-AMLEH, K. LYONS & M. SWAIN

Department of Oral Rehabilitation, Faculty of Dentistry, University of

Otago, Dunedin, New Zealand

SUMMARY Zirconia is unique in its polymorphic crystalline makeup, reported to be sensitive to manufacturing and handling processes, and there is debate about which processing method is least harmful to the final product. Currently, zirconia restorations are manufactured by either soft or hard-milling processes, with the manufacturer of each claiming advantages over the other. Chipping of the veneering porcelain is reported as a common problem and has been labelled as its main clinical setback. The objective of this systematic review is to report on the clinical success of zirconia-based restorations fabricated by both milling processes, in regard to framework fractures and veneering porcelain chipping. A comprehensive review of the literature was completed for in vivo trials on zirconia restorations in MEDLINE and PubMed between 1950 and 2009. A manual hand search of relevant dental journals was also completed. Seventeen clinical trials involving

Introduction In dentistry, gold and metal-alloys have passed the test of time and are recognized as predictable and wellestablished clinical materials for the restoration of various fixed prostheses (1). Indeed, metal-ceramic systems require relatively little special knowledge for their routine use which has led to their worldwide acceptance and use since their inception. Nevertheless, the increasing aesthetic demand in dentistry has driven the development of a number of ceramics for their aesthetic capability, biocompatibility, colour stability, wear resistance and low thermal conductivity (2). As far back as 1885, porcelain jacket crowns were first used for single crowns for the anterior teeth because of their aesthetic and natural appearance (3). However, ª 2010 Blackwell Publishing Ltd

zirconia-based restorations were found, 13 were conducted on fixed partial dentures, two on single crowns and two on zirconia implant abutments, of which 11 were based on soft-milled zirconia and six on hard-milled zirconia. Chipping of the veneering porcelain was a common occurrence, and framework fracture was only observed in soft-milled zirconia. Based on the limited number of short-term in vivo studies, zirconia appears to be suitable for the fabrication of single crowns, and fixed partial dentures and implant abutments providing strict protocols during the manufacturing and delivery process are adhered to. Further long-term prospective studies are necessary to establish the best manufacturing process for zirconia-based restorations. KEYWORDS: zirconia, fracture, porcelain chipping, crowns, fixed partial dentures, implant abutments Accepted for publication 13 March 2010

ceramics cannot withstand deformation strain of more than 0Æ1%–0Æ3% without fracturing and are susceptible to fatigue fracture. It is this brittleness, because of the ionic-covalent atomic bonding, which has limited their use in dentistry for decades (4). The most recent introduction to the dental ceramics family is zirconia, which in its pure form is a polymorphic material that occurs in three temperature-dependant forms that are: monoclinic (room temperature to 1170 C), tetragonal (1170 C–2370 C) and cubic (2370 C – up to melting point) (5). However, when stabilizing oxides such as magnesia, ceria, yttria and calcium are added to zirconia, the tetragonal phase is retained in a metastable condition at room temperature, enabling a phenomenon called transformation toughening to occur. The partially stabilized crystalline doi: 10.1111/j.1365-2842.2010.02094.x

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B . A L - A M L E H et al. tetragonal zirconia, in response to mechanical stimuli, such as tensile stress at crack tips, transforms to the more stable monoclinic phase with a local increase in volume of approximately 4%. This increase in volume closes the crack tips, effectively blunting crack propagation. It is this transformation-toughening process which gives zirconia its strength and toughness, exceeding all currently available sintered ceramics. Compared to alumina, zirconia has twice the flexural strength, partly because of its grain size and the transformation-toughening mechanism. (6). To date, there are three types of zirconia-containing ceramics which are used in dentistry. Glass-infiltrated zirconia-toughened alumina ceramics, magnesiumdoped partially stabilized zirconia and 3 mol% yttria containing tetragonal zirconia polycrystalline (Y-TZP), with the latter being the most utilised form in dentistry because of its higher flexural strength reported to range from 900 to 1200 MPa (7). Y-TZP has been used in root canal posts (8), frameworks for all-ceramic posterior crowns and fixed partial dentures (FPDs) (9–12), implant abutments (13, 14) and dental implants (15). Advances in CAD ⁄ CAM technology has made it possible to more readily use zirconia in dentistry. This technology enables complex shapes to be milled out of pre-made zirconia blanks (or blocks), where the prepared abutment is first scanned, then using computer software, the desired framework is designed prior to milling (16). There are two types of zirconia milling processes available: (i) soft-milling and (ii) hard-milling. Softmilling involves machining enlarged frameworks out of pre-sintered blanks of zirconia, also called the ‘‘green’’ state. These are then sintered to their full strength, which is accompanied by shrinkage of the milled framework by approximately 25% to the desired dimensions. Hard-milling involves machining the framework directly to the desired dimension out of densely sintered (higher strength and more homogenous) zirconia blanks, typically these have been hot isostatic pressed (HIPed). However, because of the extreme hardness of sintered zirconia, a robust milling system is required that needs an extended milling period compared to the soft-milling process as well as placing heavy demands on the rigidity of the cutting instruments. The relative ease and speed of soft-milling may be why more manufacturers chose this method to fabricate their dental zirconia products, while only a smaller number have used HIPed zirconia. Among the

common representative systems utilizing soft-milling are Lava,* Procera zirconia,† IPS e.max ZirCAD‡ and Cercon.§ Systems that utilize hard-milling of HIPed zirconia include DC-Zirkon¶ and Denzir.** Supporters of soft-milling claim that hard-milling may introduce microcracks in the framework during the milling process. In contrast, hard-milling supporters claim a superior marginal fit because no shrinkage is involved in their manufacturing process. Nevertheless, in vitro studies support the use of both HIPed and nonHIPed zirconia for all-ceramic FPDs, crowns and implant abutments for the posterior of the mouth because of their high flexural strength and fracture toughness (17). The most utilized zirconia in dentistry, Y-TZP, has been found to withstand cyclic fatigue testing, where posterior all-ceramic FPDs spanning up to 5-units, had a lifetime comparable to that achieved with metalceramic restorations (2), and it has been predicted, based on the results of this study, to have a lifetime longer than 20 years (18). However, early clinical findings show that there are two main drawbacks for zirconia restorations compared to metal-ceramics. The first is a high incidence of veneering porcelain fracture, manifesting clinically as chipping fractures, and the other is an inherent accelerated ageing problem that has been identified to occur in zirconia in the presence of water. This ageing phenomenon is known as low-temperature degradation (LTD), which causes a decrease in physical properties by spontaneous phase transformation of the zirconia crystals from the tetragonal phase to the weaker monoclinic phase putting zirconia frameworks at risk of spontaneous catastrophic failure (19). The objective of this systematic review is to report on the clinical success of HIPed and non-HIPed Y-TZPbased restorations (single crowns, FPDs and implant abutments), focusing on the incidence of framework fracture and chipping of the veneering porcelain in both groups. In addition, recent in vitro studies conducted in an attempt to solve some of the reported problems in zirconia-based restorations are also discussed. *3M †

ESPE, Seefeld, Germany. Nobel Biocare AB, Carolinsk, Sweden. ‡ Vivadent-Ivoclar, Ellwangen, Germany. § Dentsply-Degudent, Hanau, Germany. ¶ DCS Dental AG, Allschwil, Germany. **Decim AB, Skelleftea˚, Sweden. ª 2010 Blackwell Publishing Ltd

CLINICAL TRIALS IN ZIRCONIA

Materials and methods A search was performed in MEDLINE and PubMed for in vivo trials on zirconia restorations published between 1950 and June 2009. The main keywords used for the search and the number of articles produced were: 1 ‘‘zirconia AND clinical’’- 329 articles 2 ‘‘zirconia AND fixed partial dentures’’- 130 articles 3 ‘‘zirconia AND FPD’’- 23 articles 4 ‘‘zirconia AND implant abutments’’- 61 articles 5 ‘‘zirconia AND single crowns’’- 73 articles In addition, a manual hand search was conducted through the literature to identify any possible clinical trials on Y-TZP which may have not been listed on MEDLINE and PubMed. The articles found were read to identify ones which satisfied the following inclusion and exclusion criteria: Inclusion Criteria: 1 Human in vivo only 2 Conducted on Y-TZP 3 Fixed prosthetics (single crowns, FPDs or implant abutments) 4 Study has a set inclusion and exclusion criteria 5 Study has a materials and methods 6 In the English language Exclusion Criteria: 1 Case reports 2 In vitro trials 3 Animal studies 4 Trials