Viviana Avila-Gnau General Dentist
Chipping of veneering porcelain is an ongoing problem with all-ceramic restorations, while zirconia framework fractures are reported only in systems using soft-milling.
While porcelain fractures are problematic, zirconia is suitable for single crowns, fixed partial dentures (FPD), and implant abutments but is not recommended for inlay-retained FPDs and cantilever FPDs.
Are zirconia-based restorations clinically successful in regard to framework fracture and veneering porcelain chipping, and which milling method yields the best results?
The authors sought clinical studies on zirconia restorations published between 1950 and June 2009. They searched one database and hand-searched the literature for clinical trials on Y-TZP (3 mol% yttria-containing tetragonal zirconia polycrystalline). The authors determined inclusion and exclusion criteria. The inclusion criteria included trials on Y-TZP used on single crowns, FPDs, or implant abutments, written in English.
The authors included 17 studies in the review, of which only three were randomized controlled trials. Thirteen of the studies investigated all-ceramic FPDs (ranging from 3 to 10 units). Two studies investigated single crowns, and one study examined implant-supported zirconia FPDs. Among the 17 studies, the authors identified eight different brands of Y-TZP. They found encouraging results with zirconia implant abutments in the only two studies conducted on single-tooth implant abutments using both hot isostatic pressed (HIPed) and non-HIPed zirconia. The authors found short-term success rates for zirconia single crowns in a study with a small sample size of 15 and in another study based solely on private practice patient records rather than clinical evaluation. Clinical trials have reported a relatively small number of framework fractures. Fracture of Y-TZP substructures mostly occurred in FPDs and was only reported in five studies with two zirconia brands. The incidence of framework fracture was directly related to the design of the FPD. Inlay-retained FPDs showed the highest failure rate of 10 percent after only 12 months. Debonding of the inlay pontic has been determined to be the cause of the framework fractures. This review indicated that the most common complication observed in zirconia-based restorations was fracture of the veneering porcelain. All eight of the investigated zirconia brands exhibited chipping fractures. Loss of retention was seen in 7 of 16 studies involving the cementation of zirconia where different brands of cement were used.
While chipping of the veneering porcelain is an ongoing problem with all-ceramic restorations, zirconia framework fractures only have been reported in systems utilizing soft-milling. Zirconia is suitable for posterior single crowns, FPDs and implant abutments but cannot be recommended for inlay-retained FPDs and cantilever FPDs.
Source of Funding:
No support statement is provided in the paper.
Importance and Context:
Three types of zirconia-containing ceramics are used in dentistry; 3 mol% yttria-containing tetragonal zirconia polycrystalline (Y-TZP), is utilized most. The zirconia can be soft-milled or hard-milled. Supporters of soft-milling using non-HIPed zirconia claim that hard-milling of HIPed zirconia may introduce framework microcracks. In contrast, hard-milling supporters claim a superior marginal fit because no shrinkage is involved in the manufacturing process. It is important for dentists to know which method is least harmful to the final product. More manufacturers choose soft-milling to fabricate dental zirconia products, while only a few have used HIPed zirconia.
Strengths and Weaknesses of the Systematic Review:
This systematic review had several shortcomings. The authors included only English-language articles, culled from only one database. In addition, they made no 1) mention of gray literature; 2) listing or explanation of excluded studies; 3) description of dual independent data extraction; or 3) quality assessment of included studies, which weakens the systematic review evidence.
Strengths and Weaknesses of the Evidence:
Seventeen Y-TZP restoration studies were found; only three were randomized controlled trials. The authors stated that 'in-vivo' trials met the inclusion criteria for the review, a vague term which does not clearly define the included study design(s). The longest follow-up was five years (in only two papers). Thirteen studies investigated all-ceramic posterior FPDs, two examined single crowns, and two investigated implant-supported zirconia FPDs. Among the 17 studies, eight different brands of Y-TZP were identified. The review included 11 studies for soft-milling but only 6 for hard-milling, with sample sizes from 13 to 204. Readers should view the data with caution due to small sample sizes and varying number of studies of each type. The studies included reported patient –oriented outcomes. Since all 17 studies excluded bruxism, this indicates a clinical limitation of this all-ceramic system .
Implications for Dental Practice:
There is an ongoing problem with chipping of the veneering porcelain with zirconia all-ceramic restorations, and although more dental zirconia manufacturers supply non-HIPed Y-TZP, HIPed Y-TZP has shown more favorable results due to absence of any framework failures in the short term. Zirconia is suitable for posterior single crowns, FPDs, and implant abutments but cannot be recommended for inlay-retained FPDs and cantilever FPDs.