In Vitro Evaluation of Lithium Disilicate Endocrowns and Post and Core Crowns—A Systematic Review

The aim of this systematic review was to summarize the results of the studies that have compared the physical and mechanical properties of lithium disilicate (LDS) endocrowns constructed for posterior teeth to those retained by post-and-core retention systems. The review was conducted following the PRISMA guidelines. The electronic search process was conducted on PubMed-Medline, Scopus, Embase and ISI Web of Knowledge (WoS) from the earliest available date till 31 January 2023. Additionally, the studies were assessed for their overall quality and risk of bias using the Quality Assessment Tool For In Vitro Studies (the QUIN). The initial search resulted in 291 articles, out of which, only 10 studies met the eligibility criteria. In all studies LDS endocrowns were compared with various kinds of endodontic posts and crowns made from other materials. There were no definite pattern or trends observed in the fracture strengths of tested specimens. There was no predilection observed in failure patters among the experimental specimens. No predilection was observed in the fracture strengths of LDS endocrowns when compared to post-and-core crowns. Furthermore, no differences in failure patterns could be observed when both types of restorations were compared. The authors propose standardized testing of endocrowns against post-and-core crowns in future studies. In conclusion, long-term clinical trials are advocated to compare the survival, failure and complication rates of LDS endocrowns and post-and-core restorations.


Introduction
There are several options to restore extensively damaged or decayed teeth [1]. Conventionally, crowns made from ceramic (usually porcelain) or porcelain-fused-to-metal (PRF) crowns are constructed and cemented or bonded on to prepared crowns. For endodontically treated crowns, a post-and-core restoration is fabricated to improve the retention of the restoration [2]. Several types of endodontic posts have been used. Pre-fabricated post-and-core, customized cast, all-ceramic and fiber-reinforced posts are some types of endodontic retention systems that have been used [3]. The main advantage of a post-retained restoration is the increased retention [4]. However, preparing the tooth for a post weakens the tooth structure and increases the probability of vertical root fractures [5]. These fractures are usually not reparable, and the only usual choice left for their management is usually extraction of the tooth [6].
More recently, endocrowns have been advocated as alternatives for post-and-core crowns [7]. Endocrowns take advantage of adhesives and monolithic ceramic, composite or other materials. An endocrown is a one-piece restoration which requires minimal preparation of the root canal [8]. As opposed to conventional posts (which requires extensive removal of gutta percha and dentine along with extension into the root canal), the endocrown extends just 2-3 mm in to the pulp chamber but not in to the root canal [9]. Nevertheless, endocorowns have some limitations as well. Their use in the anterior or premolar region has not been seen as much as in the molar region [7]. Furthermore, tooth preparations for these crowns are usually technique-sensitive and produced by specialists. Higher costs may limit their use as well. Endocrowns are usually constructed via computer-aid-design and computer-aided-manufacture (CAD-CAM) systems, giving them an additional advantage of being able to be produced 'chairside' [10]. Reduction for endocrowns usually require a 2 mm occlusal reduction and and 3 mm axial depth in to the pulp chamber [7]. The gingival margin usually has a butt-joint design. Although several materials such as resin composites and zirconia have been used to construct endocrowns, lithium disilicate (LDS) is a more recent material introduced for their manufacture [10]. In addition to possessing superior flexural and fracture resistance along with excellent esthetic properties, they are able to be milled in to endocrowns [11]. Some studies suggest that LDS has fracture resistance and favours favourable (reparable) fractures compared to post-and-core [12] while others have found no difference. Although prior systematic reviews have been published on endocrowns, none of them focused specifically on LDS endocrowns. Therefore, the aim of this systematic review is to summarize the results of the studies that have compared the physical and mechanical properties of LDS endocrowns constructed for posterior teeth to those retained by post-and-core retention systems.

Focused Question
Using the Participants, Intervention, Control and Outcomes principal described in the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines [13], a focused question was constructed. The question was 'In patients who need prosthodontic replacement of premolars and molars (participants), are the mechanical and physical properties (outcomes) of LDS endocrowns (intervention) better or worse than (or comparable) to post-and-core-crowns)?'.

Search Strategy
An exhaustive literature search was conducted for mechanical and physical properties of LDS endocrowns. Online electronic databases such as PubMed-MEDLINE, Embase, Scopus and ISI Web of Knowledge (WoS) were explored from the earliest available date till 31 January 2023 without restriction on language. Additional sources like Google Scholar, unpublished studies, conference proceedings and cross-references were explored. Contact with authors was done for any unpublished studies.
Medical subject headings (MeSH) terms, keywords and other free terms combined with Boolean operators (OR, AND) were used for searching articles. Identical keywords were used for all search platforms following the syntax rules of each database. Databases were searched using the following Medical Subject Headings (MeSH) terms and keywords, [((molar) OR (molars) OR (premolars) OR (posterior teeth)) AND (lithium disilicate) AND (endocrown) AND ((endodontic post) OR (post) OR (post-and-core) OR (post and core)) AND ((strength) OR (fracture) OR (fracture strength) OR (fracture resistance) OR (fatigue) OR (failure) OR (mechanical strength))]. The search strategy was tailored for each database [ Table 1]. The search results were downloaded to a bibliographic database to facilitate duplicate removal. The entire search process was carried out by two investigators (AM and GM) independently. Any disagreements were solved by discussion. If a conflict persisted, the judgement of a third reviewer (ZQ) was considered decisive. Initially, the titles of the articles in the primary search were read for eligibility. Any irrelevant articles or duplicates were excluded. After exclusion of these articles, the abstracts of the remaining items were read for eligibility to further exclude ineligible articles. Full texts of potentially eligible articles were downloaded and read comprehensively. Furthermore, reference lists of these articles were also read to further find further articles eligible for inclusion. The level of agreement between the two reviewers, calculated by Cohen's kappa (k), was 0.92 for titles and abstracts and 0.90 for full texts.

Inclusion Criteria
It was decided to include the following studies: (1) Prospective clinical studies, (2) Laboratory studies and (3) Studies that focused on the comparison of mechanical and physical properties of LDS endocrowns with those of post-and-core crowns.

Exclusion Criteria
Letters to the editors, commentaries, finite-element analysis studies and case reports were excluded. Studies not in English were excluded via application of language filter. Those papers that fulfilled all selection criteria were processed for data extraction.

Data Extraction
Using pre-calibrated data extraction forms utilising Microsoft Excel software (version 2303, Microsoft, Redmond, WA, USA), general information corresponding to the following criteria were extracted from the included studies: author/year of publication, type(s) of teeth restored/population details, experimental groups, materials used and tooth preparation/post details. The following outcomes data were extracted: Fracture resistance measurements, marginal adaptation outcomes, failure patterns and overall qualitative outcomes. The data extraction was carried out by two investigators (AM and GM) independently. The data extraction was overseen by a subject matter expert (SME) and any disagreements were solved by discussion with the of third reviewer. (ZQ)

Quality Assessment
The risk of bias in the included studies was assessed using the Quality Assessment Tool For In Vitro Studies (the QUIN) developed by Sheth et al. [14]. Briefly, the following criteria were critically assessed in each study: the aims and objectives, sampling technique, comparison group details, detailed explanation of methodology, operator details, randomization details, outcome measurement, outcome assessor details, blinding and statistical analysis. Overall quality assessments were given to each study corresponding to the range of scores received: high (1-4), medium (5-8) and low (9)(10)(11)(12).
The systematic review was registered with the International Prospective Register of Systematic Reviews on 10 November 2022, which was in accordance with the guidelines. (Registration Number CRD42023390072).

Literature Search
The literature search process is illustrated in Figure 1. The primary search resulted in 291 items. After removal of 71 duplicate items, titles and abstracts of 220 studies were read to exclude further ineligible studies. At this point, 200 studies were further excluded; therefore, 20 studies were selected for full-text retrieval. Of these, one study was excluded because it was a case report [15] and nine studies were excluded because they did not have an experimental group with endodontic posts [10,[16][17][18][19][20][21][22][23]. Therefore, 10 studies were eventually selected for inclusion in this review [11,12,[24][25][26][27][28][29][30][31] No studies were found within the reference lists of the selected articles.

Fracture Strength Outcomes
There were no definite patterns or trends observed in the fracture strengths of tested specimens. In the study by Forberger & Gohring, no significant difference was observed between the fracture strengths of endocrowns and post-and-core crowns [24]. In another study, the fracture resistance of endocrowns and post-and-core crowns were similar but still lower than that of unprepared teeth [25]. Lise et al. observed that all specimens survived after 120,000 chewing cycle simulations and there was no difference between endocrowns and post-and-core crowns in terms of fracture strength [12]. In another study, no significant difference was observed between the fracture strengths of endocrowns and post-andcrowns, but all of them performed better than overlay crowns [26]. In the study by Ghoul et al., LDS endocrowns had similar fracture strength to nanoceramic-reinforced endocrowns, zirconia reinforced endocrowns and LDS endocrowns, but all had a higher fracture strength than post-and-core crowns [27]. In contrast, in one study, LDS with a resin composite core (without post) had the highest fracture strength when compared to LDS with a post and LDS endocrowns [28]. In one study, LDS endocrowns had higher oblique fracture strength when compared to post-and-core crowns (with and without ferrule) [29]. When compared to bulk-fill endocrowns, endocrowns were found to have a lower fracture strength [30]. When compared to post-retained zirconia crowns and resin composite endocrowns, LDS endocrowns were found to have a lower fracture strength [31]. Finally, in one study, postretained crowns, zirconia or LDS endocrowns with 1.5 circumferential ferrule and zirconia crowns with flat occlusal table had comparable higher fracture strengths, but a buccal ferrule resulted in compromised fracture strength [11].

Failure Patterns
There was no predilection observed in failure patterns among the experimental specimens. In five studies, there were equal numbers of root fractures among endocrowns and post-retained crowns [11,12,24,25,29]. When compared to endocrowns and post-and-core crowns, intact teeth were found to have more reparable fractures [25,30]. In two other studies, a higher number of irreparable fractures were observed in endocrowns than in postand-core crowns [27,28]. In another study, both LDS endocrowns and LDS post-and-crowns had lower numbers of irreparable fractures than did Zr crowns of either design [31].

Marginal Adaptation
Marginal adaptation was measured in just two studies. In one study, post-and-core crowns had higher marginal adaption than endocrowns [24]. In the other study, no difference was observed between the two types of prostheses [26].

Discussion
The results from studies included failed to draw a conclusive comparison between the mechanical properties of lithium disilicate endocrowns and conventional post-and-core restorations [11,12,[24][25][26][27][28][29][30][31]. Nevertheless, the results suggest that LDS endocrowns have similar fracture strength and failure modes to those of post-and-core crowns in majority of the studies [11,12,[24][25][26][27][28][29][30][31]. The authors decided to include only molars and premolars in the inclusion criteria of studies because of the similar occlusal forces experienced by them in the posterior region of the dental arch. It is difficult to translate the results of in vitro studies to clinical practice because of several factors that can not be replicated in the laboratory. These include parafunctional habits, oral hygiene, dental practitioner skills and saliva [32][33][34]. Nevertheless, even after undergoing 120,000 chewing cycles, no appreciable differences have been found between the fracture strengths of LDS endocrowns and post-and-core crowns [12].
Marginal adaptation, defined as the distance between the finish line and the restoration margin, is considered one of the major criteria affecting the long term prognosis of ceramic restorations [35]. If a significant marginal gap is present between the tooth and the restoration, luting material will be exposed to the oral environment resulting in its dissolution and consequent microleakage, leading to inflammation of the periodontal tissues, secondary caries and eventually prosthesis failure [35][36][37][38][39]. The assessment of the marginal adaptation of restorations is usually performed using either invasive techniques such as cross sectioning and impression replica or noninvasive techniques such as direct-viewing [39].
Although to date, no comparative clinical trials have been carried out to compare the survival rate of LDS endocrowns and conventional post-and-core crowns, a retrospective study found the 10-year survival rate of predominantly LDS endocrowns to be as high as 99% [18]. A prior systematic review attempted to gauge the performance of endocrowns [40]. However, the authors of that review did not evaluate the material, performance, survival and mechanical properties of LDS endocrowns relative to post-and-core restorations. Before advocating a preference of endocrowns over post-and-core crowns, it is pertinent to synthesize evidence regarding their comparative long-term clinical survival rates, fracture strength and mode of failure. For reparability, it would be preferable for crowns to fail without damaging the tooth structure, particularly avoiding vertical root fractures [41]. In the studies reviewed in the current review, a majority of the studies did not find a significant difference in failure patterns among endocrowns and post-and-core crowns [11,12,24,25,29,31]. To date, very few studies have been carried out to ascertain the impact of pulp chamber extension on the survival or strength of endocrowns [42]. However, a general consensus is that the pulp chamber extension should not exceed the pulpal floor [34]. To date, no studies have attempted to investigate the impact of variability of tooth or pulp shape on the survival of endocrowns. In addition, teeth with deep subgingival margins or root caries may not be suitable for endocrown placement. Nevertheless, this parameter has not been explored significantly in LDS crowns in comparison to other materials. Furthermore, variables such as flexural [43] and compressive strengths [44] have only been partially explored. Finally, no clinical or laboratory studies have attempted to investigate the impact of actual or simulated bruxism and other parafunctional habits. Therefore, future studies should investigate the impact these variables on the performance of LDS endocrowns.
The current review has some limitations. Firstly, due to the heterogeneity of the methodology, results and outcome measurements, a meta-analysis could not be carried out. Therefore, it is difficult to infer any meaningful conclusions regarding the relative properties of the included groups in the studies. Secondly, due to the language restrictions of the investigators, studies only in English were reviewed, further limiting the scope of this review. There were several potential sources of bias detected within the studies. The biggest concerns would be the lack of blinding in majority of the studies which could have influenced the outcomes of the studies. Therefore, the authors propose standardized testing of endocrowns against post-and-core crowns in future studies. Furthermore, long-term clinical trials are advocated the comparison of the survival, failure and complication rates between LDS endocrowns and post-and-core restorations.

Conclusions
Within the limitations of this review, no predilection was observed in the fracture strengths of LDS endocrowns when compared to post-and-core crowns. Furthermore, no differences in failure patterns could be observed when both types of restorations were compared. Long-term clinical trials are required before LDS endocrowns can be deemed superior to post-and-core restorations.