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Zirconia All-Ceramic Restorations:Do They Perform Well?Ahmad Jum’ahBDS(Hons), MSc/PhD (Clin) Student-First year

Restorative Dentistry Department, Leeds Dental Institute, University of Leeds, UK

dnaahj@leeds.ac.uk

especially, crowns and FPDs (Fixed Partial Dentures). This is largely attributed to the excellent mechanical properties the material exhibits and increasing popularity of CAD/CAM technology. The evidence from experimental work by Guazzato et al. 2004 indicates that the core of the all-ceramic crown dictates the fracture strength of the restoration. Thus, utilizing zirconia as substructure guarantees high success rate for such restorations. Clinical trials studying the performance of zirconia single crowns are few; most clinical trials examined the performance of multiunit restorations. In their 3-year clinical study, Beuer et al. 2010 reported that none of fifty single crowns failed at the end of the observation period. However, fractured lingual cusps of all-ceramic crowns used to restore mandibular molars is a frequent technical complication that has been reported by Raigrodski et al. 2006. Area-specific modification of the core design and thickness was suggested to increase support for the veneering porcelain. Silva et al. 2010 found increased reliability of modified Y-TZP (Lava) crowns while such an improvement wasn’t demonstrated by Lorenzonis’ et al. study in the same year.

In contrary to his findings regarding single crowns, Guazzato et al. 2004 stated that the strengthening action of the Y-TZP is outweighed by the weaker veneering porcelain in case of FPDs. This experimental finding has been substantiated by number of clinical studies that reported high incidence of chipping or complete delamination of veneering porcelain. Poor mechanical properties of the veneering porcelain and “a weak interface” between zirconia core and veneer are blamed for such complications. The former is a well-accepted justification for minor chipping especially when low fusing ceramics are used while some authors cast doubt that the weak interface even exists. Bond strength between zirconia core and veneering porcelain was found to be similar to the bond in metal ceramic samples used as controls (Al-Dohan et al. 2004). This in vitro finding was substantiated by Raigrodski et al.’s 2006 clinical study who reported absence of adhesive failures at the interface after 3 years follow-up period. However, the amount of evidence demonstrating the role of -“a weak interface”- in zirconia FPDs seems to be convincing (Aboushelib et al. 2007).

The extensive use of ceramics in industrial, medical and dental fields makes this period of time a good candidate to be named as the “Ceramic age” (Vagkopoulou et al. 2009). In dentistry, PFM (Porcelain Fused to Metal)restorations are among the most commonly prescribed and serviceable restorations with the longest and most traceable record of predictable performance. However, driven by the extraordinary increase of aesthetic demands, concerns about allergies and systemic effects of metal alloys, and adverse effects of destructive tooth preparation for PFM restorations, light has been shed on all-ceramic restorations and rigorous research and development in this field has been performed. The ambition of complete replacement of PFM restorations with all-ceramic counterparts was curbed by the brittleness and limited flexural strength of the latter especially in areas subjected to high occlusal forces. Development of high strength zirconia and alumina cores seems to be potentially effective to overcome this problem.

Partially stabilized zirconia based ceramics have distinct mechanical and optical properties and exhibit a very high biological compatibility with the oral environment: consequently, huge amount of research has been directed toward this material. Thus, clinicians should be aware of the current evidence and literature about zirconia all-ceramic restorations and their clinical serviceability in order to communicate with patients on solid scientific bases, clearly describe pros and cons of such restorations, and deal with the material in the proper way to guarantee better longevity.

As is the case with all new dental materials, clinicians should bear in mind that the evidence available about zirconia is largely based on in vitro studies that might be inapplicable to some clinical situations and the long term clinical trials are scarce. In general, clinical data about this material is of a short term and unfortunately, many contradictory findings are present. This article briefly envisages these findings, and attempts to establish a well-structured argument that will help the reader to get a broader image about the performance of these restorations in terms of their functional, aesthetic, and biological characteristics.

Recently, zirconia or “Ceramic steel” has been used extensively in fabrication of extra-coronal restorations

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FPDs (Komine et al. 2010). Lack of bonding also compromises the outcome of repairing delaminated veneering porcelain layer. in vitro studies investigated different surface treatments in attempts to overcome such problems, tribochemical silica coating (e.g. Rocatec® and CoJet®), plasma spraying, pre-treatment with phosphate containing primers, airborne particles and etching with CO2 laser were among those found to be effective. However, a recent report by Behr et al. 2011 found that silica coating and application of phosphate containing primers in addition to silanization was insufficient to attain a clinically acceptable bond strength which was predetermined as 10 MPa.

Owing to their excellent optical properties, zirconia all-ceramic restorations are aesthetically appealing.Absence of the black line at the gingival margins is an important advantage of using white zirconia frameworks over PFM restorations. Translucency and opacity of zirconia frameworks vary between different systems. For example, a 0.5mm thick core made of In-Ceram Zirconia or Cercon is completely opaque, it is thus recommended to be used in cases when masking discoloured cores is necessary. On the other hand, Lava frameworks are considered to be semi-translucent and they are advantageous as they enhance the accurate reproduction and depth of the natural tooth shade. The introduction of coloured zirconia frameworks hypothetically enhances the overall colour matching, disputes the concerns about “too white frameworks”, negates the need for liner application -which was found to weaken core-veneer interface- and reduces the thickness of veneering layer. Aboushelib et al. 2008 found that air-borne particle abrasion increased bond strength of the core-veneer interface when used with white zirconia framework in contrast to coloured ones. Furthermore, they stated that application of liner was found to enhance the bond in case of coloured frameworks yet, a high incidence of de-lamination was reported. Colored Cercon frameworks showed higher core-veneer bond strength than Lava despite the fact that their chemical composition was similar. They attributed this to the structural changes occurring as a result of different staining procedures. The significance of using coloured zirconia frameworks was assessed two years later by the same authors who concluded that this technique did not offer any direct advantage over the white zirconia frameworks layered with veneering porcelain. Finally, Jung et al. 2007 investigated soft tissue colour changes associated with veneered and unveneered zirconia. It was found that zirconia did not induce visible colour changes when thickness of mucosa was 2 and 3mm. Also, the colour change that occurred with 1.5mm thick mucosa was the least in case of zirconia when compared to titanium. This may indicate that gingival aesthetics are not affected when margins of zirconia restorations placed in a sub-gingival location. Results of in vitro and in vivo studies that proved high

Clinical studies reported 73.9-100% survival rate of zirconia all-ceramic FPDs over 2-5 years observation periods. The most common complication (15% in 3 studies) was veneer layer chipping or delamination (Sailer et al. 2007, Raigroski et al. 2006, and Vult von Steyern et al. 2005). Framework fracture was a far rarer complication and found to be as low as 0-2.2% in some studies (Sailer et al. 2006, 2007). The high complication rate in some studies may be worrying especially when compared to survival rate of PFM FPDs which is 94.4% for 5 years as estimated in the systematic review by Sailer et al. 2007. However, Denry and Kelly 2008 remarked that replacement of any restoration due to veneer layer crazing or chipping was not needed. The oscillation of complication and survival rates between different studies necessitates careful interpretation and paying attention to what is deemed to be failure. From a clinical point of view, the restoration is successful if it does not need further intervention or remaking, and it maintains health, function and aesthetics (Ahmad 2006).

The amount of research tackling mechanical properties of zirconia dental ceramics far exceeds that investigating wear properties of this material. This is largely attributed to the fact that wear properties of veneered zirconia restorations are primarily dictated by those of the veneering porcelain and the clinical applications of unveneered zirconia are not so popular and limited to cases of lack of interocclusal clearance, compromised abutments and resin-bonded and inlay-retained zirconia FPDs. In 2010, two papers published on the wear of unveneered zirconia, one investigated the antagonistic tooth wear (wear of the tooth structure opposing to restoration) and found that unveneered zirconia is associated with a lower antagonistic tooth wear when compared to polished feldspathic porcelain (Jung et al.2010). The other study was by Albashaireh et al. who found that wear resistance of the zirconia ceramics is the highest when compared to others. These studies indicate that zirconia ceramics are compatible to opposing dentition and at the same time structurally stable. These results may be promising and encourage the use of unveneered zirconia especially in the era of improved shading techniques of zirconia frameworks; however the phenomenon of low temperature degradation –alternatively called aging- that causes slow material deterioration when exposed to wet environment is still troublesome and concerning.

Bonding to zirconia poses a big challenge in some clinical situations as surface treatments used with glass or silica containing ceramics, e.g., hydrofluoric acid etching and silanization, have been found to be ineffective (Blatz et al. 2007). Use of adhesive cementation was found to have no effect on the performance of zirconia restorations, however it is still necessary when dealing with cases of compromised retention as in the case of short abutments or when using resin bonded zirconia

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biological properties outweigh the risk of restoration failure -mainly delamination- which was found to be very high in some studies? Thirdly, is there any guarantee that zirconia restorations will not massively fail in a manner analogous to zirconia heads used in total hip arthroplasty due to low temperature degradation? Answering these questions should be done through conducting long term clinical trials exploiting the results of in vitro research in hot topics like improving core-veneer interface utilizing different surface treatments and testing aging-free zirconia ceramics namely, zirconia toughened alumina and ceria-doped zirconia.

Acknowledgment I would like to acknowledge with gratitude Dr. Brian Nattress for the clinical photographs, Professor David Wood for editing the paper and continuous support and Dr. Hassan Maghireh for his kindness and motivation.

biocompatibility of zirconia coupled with reduced bacterial and plaque colonization when compared to titanium encouraged the use of zirconia in implant and restorative dentistry as a material with periointegrative properties. Readers are referred to the excellent review by Hisbergues et al. 2008 for more information about the biocompatibility of zirconia and its applications in implant dentistry.

Finally, there remain debatable questions to be asked when all-ceramic zirconia restorations are concerned. Firstly, if the tooth preparation guidelines are basically similar to PFM restorations and zirconia has no advantage over other types of all-ceramic restorations in cases of limited inter-occlusal space and para-functional habits, do we really get a benefit from substituting PFM restorations with zirconia in terms of conservation of tooth structure and dealing with difficult cases? Secondly, does the gain from excellent aesthetic, mechanical and

A clinical case of three-unit all-ceramic zirconia FPD over implants with an excellent aesthetic outcome. Zirconia abutments were used.