Fit of tooth‐supported zirconia single crowns—A systematic review of the literature

Abstract Purpose The purpose of this study is to systematically map all the factors that influence the fit and adaptation of zirconia crowns and/or copings. Materials and methods The investigational strategy involved carrying out an electronic search between December 1, 2009 and September 1, 2019 through the Embase and Medline databases using Boolean operators to locate appropriate articles. Results A total of 637 articles were discovered after the removal of duplicates, and 46 of these were selected for evaluation. Further, a quality assessment was performed using GRADE evaluation criteria. Conclusions Shoulder finish line preparations had slightly better marginal fit compared to chamfer finish lines. Crowns obtained from digital impressions had comparable to superior marginal adaptation compared to conventional impressions. Increasing cement space showed to improve zirconia crown adaptation. Cementation and veneering zirconia frameworks found to increase the marginal and internal gaps. Limited information is available on the effect of the alteration of sintering time/Temperature and/or sintering techniques on the adaptation of zirconia crowns. Most of the selected studies had a moderate quality assessment evaluation. Future studies could investigate the chair‐side, ultra‐fast sintering effect on the marginal gap of zirconia crowns.

. Misfit of the crown prosthesis margin generates a potential space (marginal gap) between the restoration and the prepared tooth. Bacterial contamination can easily accumulate in the marginal gap and jeopardize the longevity of the treatment (Sailer, Makarov, Thoma, Zwahlen, & Pjetursson, 2015). Furthermore, adjustments of the prosthesis by grinding to achieve a proper fit can lead to stress concentrations; this may reduce the resistance to fracturing of the crown and, consequently, lead to clinical failure (Pak, Han, Lee, Kim, & Yang, 2010).
The importance of the marginal fit lies in the fact that the major causes of zirconia restorations failure are secondary caries and loss of retention (Sailer et al., 2015), which are factors closely related to the dissolution of the luting cement and deficiencies in marginal adaptation. Meanwhile, a minimum and uniform internal gap is also a desirable and important aspect of indirect restorations; this is because large and inhomogeneous internal gaps may negatively affect the retention or resistance of the restoration (Pedroche et al., 2016). It has been proposed that secondary caries (recurrent caries) are linked to microleakage at the tooth-restoration interface and depend on the size of the marginal gap; however, this is not supported by either clinical studies or laboratory models (Nassar & Gonzalez-Cabezas, 2011).
In contrast, larger marginal gaps have been discovered to have a significant effect on the development of secondary caries in vitro and can be modified by fluoride applications (Nassar & Gonzalez-Cabezas,-2011). Although a positive correlation has been found between the marginal gap size and the development of secondary caries (Totiam, Gonzalez-Cabezas, Fontana, & Zero, 2007), no conclusive evidence has been found to relate the marginal gap size and/or placement (supra/subgingival) to secondary caries. Moreover, studies were not even able to find an association between marginal gap and microleakage (Cristian, Jeanette, Francisco, & Guillermo, 2016;Karl, Graef, Schubinski, & Taylor, 2012).
Routinely in clinical dentistry, a mirror and probe are used to evaluate the restoration margins. However, standardized criteria, such as those from the Modified US Public Health Service (USPHS) (Ryge, 1980) and California Dental Association (CDA) (California Dental Association, 1977), are used for the clinical evaluations of marginal dental prostheses in clinical studies to ensure quality. Owing to the fact that the USPHS and CDA criteria do not consider the presence of secondary caries, Zoellner, Bragger, Fellmann, and Gaengler (2000) proposed a clinical diagnostic index for secondary caries at the crown gingival margin that can assist in the management of secondary caries according to location and severity.
Among dental ceramics, zirconia has seen a marked increase in use in dentistry because of its white color and functional outcomes (Tabatabaian, 2018). In contemporary dentistry, the use of computeraided design/computer-aided manufacturing (CAD/CAM) is the only technique for fabricating zirconia restorations. The most popular route for fabricating zirconia prostheses is to use partially sintered zirconia blanks using the soft-milling technique, and the blank must be sintered to achieve the final density and maximum strength of the material. This sintering procedure is accompanied by a relatively high sintering shrinkage of approximately 20%-30% (Suttor, Bunke, Hoescheler, Hauptmann, & Hertlein, 2001). This sintering shrinkage is managed by instructing the software to mill an enlarged crown by an appropriate factor to compensate for this shrinkage after sintering (Besimo, Spielmann, & Rohner, 2001). Consequently, a certain degree of misfit is expected when trying-in a crown on the original preparation.
It has been well established that misfit of prostheses may occur as a result of several different clinical and laboratory variables; these include deviation from the recommended guidelines of tooth preparation, inaccuracy of the impression taken, and/or firing cycles. Assessment of the post-sintering dimensional change in zirconia in dentistry has been accomplished by means of measuring the marginal and internal fit of the prosthesis. Inadequate fitting of crowns is usually managed by accepting a larger cement line and/or by making postsintering bur adjustments to the crown to compensate for discrepancies. Post-sintering adjustments can trigger tetragonal to monoclinic (tàm) phase transformation, which can lead to potentially disastrous consequences for the prosthesis. The purpose of this study is to systematically review zirconia crowns and/or coping studies related to marginal and internal fit; further, this study is aimed to map all the factors that influenced the fit of zirconia crowns and/or copings.

| Focused question and search strategy
The focused question was determined according to the well-

| Selection criteria
The titles and abstracts of all articles were reviewed by two independent reviewers (W.A. and A.G.). Disagreement between the two reviewers was resolved by discussion. Upon identification of an abstract for possible inclusion, the full text of the article was reviewed and cross-matched against the predefined inclusion criteria. Figure 1 illustrates the process of identifying the included articles in the review as a flow diagram. Table 2 describes the reasons for excluding studies.

| Data extraction
The following data were extracted from each article: type of fabrica-

| Quality assessment
The interobserver calibration was evaluated by Cohen's Kappa and the chosen cut-off point was 80%. GRADE criteria were used to provide a framework for quality assessment of the selected studies (Balshem et al., 2011). The quality levels were High (H), Moderate (M), Low (L), and Very Low (VL). Quality reflects our confidence that the estimate of the effect is correct. GRADE separates the process of quality assessment of evidence from the process of formulating recommendations (Balshem et al., 2011). Decisions about the guideline developments relayed on more than just the quality of evidence.

| RESULTS
The electronic search collectively revealed 1,047 articles, including 731 from Medline and 316 from Embase of which, after the removal of duplicates, 382 studies were processed for review based on an analysis of titles and abstracts. Searching manually and checking the references of the selected articles did not provide any further articles; therefore, only the articles from the electronic search were considered. The articles that did not meet the inclusion criteria were articles of in vivo studies, fixed dental prosthesis of two or more units, implant-supported prostheses, testing other types of all-ceramic restorations that were not zirconia, non-English studies, non-peerreviewed studies, and testing bond strength or scanning accuracy.
Consequently, 46 articles were selected for quality assessment of marginal and internal fit of zirconia crowns. Most of them had moderate quality assessment, with a Kappa score of 0.8.
Assessing measuring points varied between studies even within each measuring technique, as shown in the summary table of included articles ( Table 3). The same was true for the cement space selection.
Owing to the high heterogeneity of the methodologies between the selected studies, it was difficult to draw a solid conclusion regarding the best methodology to evaluate the fitting accuracy of zirconia crowns. However, some studies confirmed the similarity between direct external and internal viewing techniques in measuring marginal fit of zirconia crowns and indicated that using the external viewing is adequate and accurate for measuring the marginal fit without the need of destroying the specimens (Ortega et al., 2017).

| DISCUSSION
The purpose of this systematic review is to map all the factors influencing the fit of zirconia crowns and/or copings and to update the latest review published in 2011 by Abduo et al. (2010). A clinically acceptable marginal gap for dental prostheses has been debated in the literature (Christensen, 1966;Fransson, Oilo, & Gjeitanger, 1985;McLean & Fraunhofer, 1971;Nawafleh, Mack, Evans, Mackay, & Hatamleh, 2013 (Ahmed et al., 2019) found a significant interaction between crown thickness, finish line width and sintering protocol on the marginal fit of zirconia crown; meanwhile, another study (Khaledi et al., 2019) did not find a significant difference among three different sintering times on the adaptation of zirconia copings.
Sixty percent of the reviewed studies scored "moderate" according to GRADE evaluation criteria. Evaluators were moderately confident in the effect estimate owing to the high variability of methodologies, absence of many important details, and/or quality of the peer-reviewed journal. Seventy four percent of the studies were found to be missing the sintering protocol details, including the time/ Temperature (t/T) ratio, as well as the technique used for sintering.
Thirty percent of the studies investigated the effects of using different manufacturing systems (Euan et al., 2012;Ha & Cho, 2016;Lins et al., 2015), or zirconia types (Ortega et al., 2017;Schriwer et al., 2017) for fabricating zirconia prostheses or compared digital and conventional impression techniques using either a direct technique or replicas (Dahl et al., 2017;Ortega et al., 2017;Schriwer et al., 2017). Other studies investigated the effects of veneering within different manufacturing systems. Ha and Cho (2016) evaluated the fit accuracy of two zirconia systems (Ceramill and Zirkonzhan) and studied the effect of pressed veneering over zirconia copies, comparing it to monolithic zirconia crowns. This was the only study to use the weight technique to determine the overall fit accuracy by weighing the silicone impression of the cement space. The marginal gap was smaller with Ceramill, and the internal gap was smaller with Zirkonzhan. The marginal and internal gaps were higher after veneering compared with before veneering.
In this study, the authors used the manufacturer instructions to design and fabricate the prostheses, which may account for the differences in the weight techniques between the two systems. Multiple studies demonstrated superior (Kocaagaoglu et al., 2017;Pedroche et al., 2016) comparable (Dauti et al., 2016) or inferior Dahl et al., 2017) marginal fit of digital impressions compared with conventional impressions. Consideration should be given to the study design, methodological parameters, and measurement tools when comparing the results, keeping in mind that increasing the processing steps in fabricating a restoration allows for the accumulation of errors. Therefore, direct digitalization would be anticipated to yield better marginal and internal fit of the restoration. Deformation of conventional impression materials would be expected while removing the impression from the prepared tooth and during casting procedures, in addition to expansion and shrinkage of the materials used.
Few studies have been reported investigating the effect of cement or cementation on the marginal gap. One study measured the absolute marginal gap of crowns cemented by four types of cement and was not able to find a linear correlation between microleakage and absolute marginal discrepancy (Cristian et al., 2016). A second study found that increasing the cement space increases the observed fit, and that group 25-50, which has a cement space of 25 μm at the margin and 50 μm starting 1 mm above the margin and elsewhere,  Ji et al. (2015) found that the fabrication system and the finish line configuration significantly influenced the absolute marginal discrepancy (p < .05). In contrast to most studies, the study found that the chamfer finish line yielded a better marginal adaptation compared with the shoulder finish line. Vojdani et al. (2015) found that the marginal fit of shoulder copings was significantly better than that of chamfer copings; however, there was no significant difference between the two margins after firing the porcelain. Euan et al. (2012) and Euan et al. (2014)  Five studies evaluated the influence of veneering on the fit of zirconia restorations (Cho, Nagy, Goodman, Solomon, & Koike, 2012;Ha & Cho, 2016;Pak et al., 2010;Regish et al., 2013;Torabi et al., 2015). One study found a significant difference between the absolute marginal gaps of chamfer and shoulder finish line groups before (49.87, 35.20 μm) and after porcelain firing (68.24, 63.06 μm), and found that the absolute marginal fit of shoulder copings was significantly better than that of chamfer copings but with no significant difference between the two margins after firing the porcelain . The second demonstrated a significant effect of the veneering technique on the vertical marginal gap was demonstrated an increase in the vertical MG after porcelain veneering that was highest in the layering technique (63.06 μm) compared with the presson (50.64 μm) and CAD-on (51.50 μm) techniques (Torabi et al., 2015). The third study compared marginal gaps between nickel chromium and zirconia copings before and after veneering and found that nickel chromium had a better marginal fit, but both deteriorated after porcelain veneering (Regish et al., 2013). The fourth study did not find an effect on marginal accuracy before or after veneering with the same system (Pak et al., 2010). However, the fifth study investigated the effect of multiple firing cycles on the marginal fit and found no significant difference in terms of ceramic type, finish line design, margin location, or their interactions (Cho et al., 2012). According to the previous studies, there was no consensus on how many measurements should be taken per specimen to obtain an accurate and clinically relevant conclusion on marginal and internal gaps. In addition, our findings highlighted the importance of obtaining individual measurements without combining the results, which complicates interpretation of the results in terms of their clinical relevancy. Groten, Axmann, Probster, and Weber (2000) suggested that 50 measurements along the crown margin can provide clinically relevant information if the measurements are taken at equal distances or are randomly selected. In contrast, Gassino, Monfrin, Scanu, Spina, and Preti (2004)

Clinical Significance
Precise marginal fit is an essential component for the clinical success of dental restorations. Misfit of the prosthesis margin generates a potential space between the restored and prepared tooth; this gap can accumulate bacterial plaque and consequently jeopardize the longevity of the treatment. Therefore, understanding the factors influencing the marginal and internal adaptability of zirconia prostheses will assist in the improved preparation, designing, and processing zirconia prostheses for achieving clinical success.