Effect of preparation design on marginal adaptation and fracture strength of ceramic occlusal veneers: A systematic review

Abstract Objectives This systematic review aims to investigate the effect of different preparation designs on the marginal fit and fracture strength of ceramic occlusal veneers. Materials and Methods Based on the PICO question and the search terms, an electronic search was performed in Google Scholar, PubMed (MEDLINE), Scopus, Cochrane Library, Web of Science, Science Direct, Wiley, Ovid, and SAGE for articles published up to July 2022. After including English in vitro studies that evaluated posterior ceramic occlusal overlays at the posterior with ceramic restorations by following the PRISMA statement, the extracted data was tabulated. The methodological quality of the included studies was evaluated. Risk of bias assessment was done independently by two authors using the modified MINORS scale. Results About 3138 search results were screened, of which 22 were selected due to their titles. Twenty‐one full‐text articles were assessed for eligibility. Seventeen in‐vitro studies were finalized for the extraction of quantitative data. All 17 articles had a low risk of bias and were retained. The influencing items for evaluating the research were different in most studies; therefore, qualitative synthesis of the results was feasible. They generally included preparation design, material thickness, depth of preparation in the tooth, internal divergence angle, and finish line. Meta‐analysis was not done due to heterogeneity of preparation types and evaluation methods. Results revealed that fracture resistance of occlusal veneers is higher than normal mastication force, and it is sufficient to prepare the occlusal surface, use a self‐etching primer for bonding, and an acceptable minimum ceramic thickness. The marginal discrepancy of occlusal veneers is clinically acceptable. However, this systematic review faces some limitations due to the lack of in vivo studies, different preparation designs in included studies, different follow‐ups, and lack of comprehensive explanations in articles. Conclusions The preparation design of occlusal veneers influences both marginal adaptation and fracture resistance. Various preparation designs are proven to have clinically acceptable fracture strength and marginal adaptation.

The fracture strength of the dental material is one of the essential criteria to raise the survival rate of conservative restoration (Falahchai, Babaee Hemmati, Neshandar Asli, & Rezaei, 2020).
In addition, with the advance of new and reliable adhesive bonding techniques (Van Dijken & Hasselrot, 2010) and dental materials, selecting conservative treatment modalities, including less invasive restorations rather than aggressive treatments, is preferable. Some less invasive alternative treatments such as conservative and esthetic partial coverage have been used currently, including inlays (no cuspal coverage), onlays (coverage of a minimum of one cusp), and overlays (all-cusp coverage; Felden et al., 1998). Also, novel occlusal veneers with a non-retentive design are used to restore the function and morphology of a defective occlusal surface (Johnson et al., 2014;Tsitrou & Van Noort, 2008).
To treat severe dental erosion, more conservative alternatives such as ultrafine restorations adhesively cemented have been preferred to traditional onlays or total crowns in posterior teeth (Johnson et al., 2014;Tsitrou & Van Noort, 2008), some studies have suggested occlusal veneers as they have high fracture resistance (Sasse et al., 2015). As many conventional preparation designs remove sound tooth structure, occlusal veneers with less invasive designs are increasingly sought-after.
A study by Albelasy et al. (2021) has shown that material type and restoration thickness affect fracture resistance of CAD/CAM overlays. Moreover, some literature has determined occlusal ceramic fractures as the most common reason for restoration failure (Felden et al., 1998;Krämer & Frankenberger, 2005), which is affected by preparation design, bonding techniques, and thickness of the allceramic restoration (Lima et al., 2013;Sasse et al., 2015).
Besides fracture resistance, marginal fitness is another main factor affecting long-term restoration success (Suarez et al., 2003). A marginal gap results in the dissolution of luting cement and may cause restoration failure in the long run (Gu & Kern, 2003). Lack of marginal adaptation increases cement dissolution, leading to microleakage, secondary caries, periodontitis, marginal discoloration, and pulpal inflammation in some cases (Beuer et al., 2009;Sener-Yamaner et al., 2017). Clinically, a marginal gap between 50 and 120 μm has been deemed acceptable (Heintze, 2007;Suarez et al., 2003).
Preparation design is an essential factor impacting fracture resistance in all-ceramic restorations, such as rounding off all sharp angles (Krämer & Frankenberger, 2005). Nowadays, preparation designs for all-ceramic restorations are modifications of conventional cast metal restorations (Stappert et al., 2008). Ultrathin one-step with no-prep polymer infiltrated ceramic network (PICN) overlays have indicated very high longevity and success (Ahmed et al., 2022).
According to available data, the effects of preparation designs for an overlay are limited (Falahchai, Babaee Hemmati, Neshandar Asli, & Rezaei, 2020). Therefore, in this systematic review, the effect of the preparation design has been evaluated.
A systematic review by Vagropoulou et al. (2018) has shown that ceramic fractures were one of the restorations' complications, followed by retention loss and porcelain chipping; however, the survival rate for crowns was 95.38%, and for inlays and onlays, 90.89% and 93.50%, respectively, for 5 years, which are very high. However, this systematic review is aimed to investigate the effect of preparation design on the marginal fit and fracture strength of occlusal veneers.

| MATERIALS AND METHODS
This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (McInnes et al., 2018). The focused PICO (population, intervention, control, outcome) question was whether specific preparation designs affect marginal adaptation and fracture strength of posterior occlusal ceramic veneers. The population, intervention, and outcome were defined as follows: the population was posterior occlusal veneers; the intervention was preparation design, and the outcome measures were marginal adaptation and fracture strength. all the included studies are in-vitro, the statement on ethical approval, the "protection of human subjects and animals in research," and informed consent are not indicated.
Data were extracted from the included studies and tabulated with the following information: authors, groups, specimen type, sample size, restoration material, occlusal reduction, finish line, evaluation method, and conclusion (Table 2).

| Quality assessment
Each included study was evaluated by three reviewers (Arghavan Navadeh, Samin Sirous, and Saeedeh Ebrahimgol) independently for the methodological quality, according to the following criteria: clearly stated aim, contemporary groups, clearly stated preparation method, baseline similarity of groups, randomization of specimens, clearly stated evaluation method (marginal gap/strength), blinding of the examiner, sample size calculation and power analysis, and adequate statistical analyses. The items were scored on a scale of 0-2; 0 if not reported; 1 when reported but inadequate; and 2 when reported and adequate. The overall score was considered from 18 for each study, and the articles were classified as high (0-6), medium (7-12), and low risk (13-18) of bias based on their score (Table 3).

| Identification of studies
A total of 3138 search results were screened. By evaluating the abstracts, 22 articles were selected. Of the 22 articles, one study was eliminated due to lack of data; another one was excluded since the full text could not be retrieved, and two case reports and one finite element were eliminated. Therefore, 17 articles were selected for extracting the quantitative data. The results of data extraction for each study are presented in Table 2.
As there was no specific scale to assess study risk or quality, in this systematic review, two authors (Samin Sirous/Faezeh Atri) independently evaluated the risk of bias by using the modified MINORS scale, which is a methodological index for non-randomized studies (Slim et al., 2003). Each item was scored between 0 and 2; 0 for not reported items; 1 when reported but inadequate; and 2 when both reported and adequate. 18 was the ideal score; if the study score was between 13 and 18, the authors determined it as low risk; 7-12 fell under medium risk, and scores lower than 6 were counted as high risk. As all the studies showed a low risk of bias, they were retained in the review (Table 3).
The included studies were in vitro. The specimens of studies varied from maxillary premolars (Guess et al., 2013), maxillary molars (Falahchai, Babaee Hemmati, Neshandar Asli, & Rezaei, 2020), or maxillary first molars , and so on. However, in the study by Abu-Izze et al. (2018), wherein specimens are simply described as posterior teeth, or in the study by Ioannidis et al. (2019), the tested teeth were mentioned as human molars (Table 2).
Falahchai, Babaee Hemmati, Neshandar Asli, and Rezaei (2020) and Falahchai, Babaee Hemmati, Neshandar Asli, and Neshandar Asli (2020)  The influencing items for evaluating the research were different in most of the studies. In general, they are as follows: preparation design, material thickness, depth of preparation in the tooth (cavity depth), internal divergence angle, and finish line.
Meta-analysis could not be calculated because of the heterogeneity of preparation types and evaluation methods.
Qualitative analysis of included studies came to the following results.
The marginal discrepancy and fracture resistance of occlusal veneers of all preparation designs are clinically acceptable. The strength of occlusal veneers is higher than normal mastication force (Johnson et al., 2014), and it is sufficient to prepare only the occlusal surface, use a self-etching primer for bonding, an acceptable minimum ceramic thickness, and as a conclusion, there is no need for extensive reduction.

T A B L E 3 Authors' judgments about risk of bias for each included study
Authors and year of publication The items are scored as follows: 0, not reported; 1, reported but not adequately; or 2, reported adequately.
primer when bonding to dentin and enamel. They proposed that the recent study's higher fracture strength in dentin could be due to the improvement of a newly developed so-called universal bonding system. On the other hand, the study by Walid Al-Zordk et al.
(2021) has shown that the fracture resistance within veneers bonded to dentin, dentin with intra-coronal cavity, and dentin with the composite filling was not significantly different. However, their fracture resistance is above human regular masticatory forces in molars. Also, by varying dental bonding surfaces, different overlays' materials have to be considered (Al-Zordk et al., 2021).

| Different thicknesses of cavity preparation
It is determined that overlays with various occlusal thicknesses could tolerate loads higher than human masticatory forces ranging from 585 to 880 N (Johnson et al., 2014). Although an increase in the occlusal thickness of overlay restorations can lead to higher fracture resistance, further reduction more than carious lesion or fracture to increase the occlusal thickness of the overlays is not recommended. In Albelasy's study, the thickness of both 1 and 1.5 mm occlusal veneers exhibited fracture resistance higher than functional and parafunctional masticatory forces (Albelasy et al., 2021). By evaluating the influence of restoration thickness on fracture resistance, Sasse et al. (2015) concluded that occlusal ceramic veneer thickness affects fracture resistance. They

| Different preparation design and finish line (marginal gap)
The reviewed studies also proved a relationship between finish line design and marginal adaptation. Falahchai, Babaee Hemmati, Neshandar Asli, and Rezaei (2020) showed that overlays prepared most conservatively with an anatomical occlusal reduction yielded the highest fracture resistance compared with those that included a rounded shoulder preparation or/and a central groove. In another study, Falahchai, Babaee Hemmati, Neshandar Asli, and Neshandar Asli (2020)  | 1401 extrapolated to humans. Also, this review had limitations due to using different teeth (premolars vs. molars), differently used materials, measuring techniques, and location of the teeth in included studies, and lack of comprehensive explanations in articles. Quantitative data analyses are prevented due to the high heterogeneity of the included articles. Some parameters have been considered based on a few studies or even one, and various other factors are not evaluated in studies; therefore, results should be drawn cautiously, and further studies and especially clinical trials with long periods of follow-up, are required to assess different modifications of overlay preparation design.

| CONCLUSION
Within the limitations of this systematic review, and according to the findings, the following conclusions were drawn: 1. The preparation design of overlays influences both marginal adaptation and fracture resistance.

Various preparation designs can be used with occlusal veneers
to safely restore posterior teeth as these restorations are proven to have clinically acceptable fracture strength and marginal adaptation.
3. Fracture resistance of occlusal veneers is higher than the normal range of human masticatory force; therefore, aggressive preparation to increase the fracture resistance is not recommended.

AUTHOR CONTRIBUTIONS
Faezeh Atri proposed the study concept. Arghavan Navadeh, Samin Sirous, and Saeedeh Ebrahimgol searched and gathered the relevant articles and analyzed them, and then Faezeh Atri approved the final ones. All the authors contributed to article evaluation and manuscript writing. The final manuscript is approved by all authors.

CONFLICT OF INTEREST
The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.