A Comparative Analysis of Marginal Adaptation Values between Lithium Disilicate Glass Ceramics and Zirconia-Reinforced Lithium Silicate Endocrowns: A Systematic Review of In Vitro Studies

Background

Endocrowns are monolithic conservative prostheses proposed as an alternative to the typical prosthodontic approach for rigorously compromised root canal treatment (RCT) teeth [1]. An endocrown consists of the crown and core in a single unit and is created with circumferential butt margin preparation and a retentive cavity inside the pulp chamber, without extending into it. While the pulp chamber is used for macromechanical retention, adhesive applications provide micromechanical retention [2,3]. Ceramic endocrown durability and success rates have been reported to be the same or better than that of a post-and-core and crown, with the advantages of conservative preparation, adhesive retention mechanism, fewer clinical and laboratory sessions, and possibility of use in teeth without ferrule [4]. Similarly, endocrowns have been suggested to restore RCT teeth with structurally variated bases or calcified or curved root canals that can prevent the insertion of a conventional post and core [5]. The main indications for endocrowns are lack of coronal dimension, inadequate interocclusal space, and considerable loss of tooth shape that results in insufficient ferrule extent [6].

Marginal adaptation is mostly evaluated by assessing the marginal gap, which is described as the distance between the internal surface of the restoration and the finish line of the preparation [7]. McLean and von Fraunhaufer concluded that 120 μm was the highest acceptable marginal opening [8]. Inadequate marginal adaptation can cause plaque accumulation, microleakage, caries, and endodontic inflammation, finally resulting in restoration failure [9]. Issues affecting marginal integrity are the extent of persisting enamel and dentin, form of scanning, cementation method, mode and bulk of materials, and preparation scheme [10,11]. Marginal bonding areas can be evaluated by different methods. Because it is a nondestructive and repeatable technique, stereomicroscope [12–18] followed by direct digital microscopic [19–21] examination of the marginal area is the most widely used method to measure the endocrown marginal adaption in different bonding areas, while micro-computed topography [22–25], scanning electron microscopy (SEM) [26,27], and other [28] systems can be used for endocrown marginal gap measurements.

Various ceramic systems are used for endocrown fabrications, such as feldspathic ceramics [26,27], lithium disilicate glass ceramics (LDGC) [13,15–19,22,23,25], and different types of zirconia [13,14,21], which have high mechanical strength and can be acid-etched. Zirconia and LDGC are considered the best choices owing to their mechanical and aesthetic features, including good anti-abrasion, anti-aging, and anti-corrosion properties, translucency, and biocompatibility with natural teeth. Other materials have been used for endocrown construction in laboratory studies [12,20].

The adhesive endocrown restoration option has been widely used recently as a successful alternative for the coronal seal restoration of RCT teeth. Conservatism, aesthetics, and minimum clinical and laboratory methods give this system its uniqueness compared with conventional post-and-core and crown procedures [12–28]. Most endocrowns are fabricated by computer-aided-design/computer-aided-manufacturing (CAD/CAM) or heat-press techniques [19,25,26] using various ceramic systems, such as LDGC and zirconia-reinforced lithium silicates, which are the most common all-ceramic materials used for endocrown constructions [29,30].

Endocrown ceramic material and adhesive system are used to obtain the monoblock concept, while some studies mentioned that there are various factors manipulating the prosthesis fitting, containment of preparation configuration, restriction location, and cement layer thickness and form [11,31]. The CAD/CAM system is vital, specifically for the marginal area. Defective prostheses milling makes the prostheses unable to seat correctly and can cause an installation with occlusal inconsistency. Moreover, additional milled material around the marginal contacts, microleakage, will result [25,26,32].

The general clinical steps of endocrown treatment and construction are shown in Figure 1, with the following images: (Figure 1A) a preoperative view of right mandibular first molar, (Figure 1B) view after RCT and intra-pulpal extension, (Figure 1C) view with cemented endocrown, and (Figure 1D) postoperative bite-wing radiograph showing the adaptation between the endocrown and tooth structure with a butt-joint preparation. In this systematic review, we aimed to identify and analyze in vitro studies on the marginal adaptation values of lithium disilicate glass ceramics and zirconia-reinforced lithium silicates in endocrown restorations.

Material and Methods

REVIEW QUESTION:

The present review was conducted following the guidelines of the Preferred Reporting Items for Systematic Review (PRISMA; www.prisma-statement.org) statement [33,34]. The Population, Intervention, Comparison, Outcome (PICO) format was used to formulate the focused question according to Methley et al [35]: “Whether CAD/CAM or heat-press LDGC all-ceramic endocrowns (Intervention) used to restore RCT teeth (Population) exhibit superior marginal adaptation or minimum leakage (Outcome) compared with CAD/CAM or heat-press zirconia all-ceramic endocrowns (Comparison).”

SELECTION CRITERIA:

The inclusion and eligibility criteria were as follows: (i) in vitro investigations assessing the fitting of all-ceramic endocrowns for RCT teeth, (ii) investigations assessing the marginal adaptation and marginal and axial discrepancy of RCT teeth, typodont, or natural teeth expressed as mean±the standard deviation and measured in micrometers (μm), and (iii) studies published in English. Studies involving RCT teeth treated for bovine teeth or using finite element analysis were excluded. Reviews, case reports, pilot studies, case series, editorials, clinical trials, and studies published in languages other than English were also excluded from the present systematic review.

LITERATURE SEARCH:

The search strategy was performed using combinations of Medical Subject Headings (MeSH) terms and free keywords together with Boolean operators (ie, AND, OR, and NOT) with respect to the PICO question. An electronic literature search was performed on January 15, 2023, by 3 operators, which included a senior librarian specializing in health sciences database searches utilizing the following databases: Clarivate Analytics’ Web of Science, Elsevier’s Scopus, and PubMed (MEDLINE), without a restriction to studies after 2015 publication year. A manual search was then conducted by checking the bibliographies of all initially selected studies to identify articles that may have been missed during the electronic search.

STUDY SELECTION:

The article selection process went through 3 stages: (i) selection based on title relevance, (ii) selection based on abstract relevance, and (iii) full-text analysis. All articles retrieved by manual and electronic searches were gathered and assessed for inclusion according to the eligibility criteria.

DATA EXTRACTION AND ANALYSIS:

A standardized spreadsheet (Microsoft Office Excel software) was used to extract data of interest from the included articles. These data included (i) researchers and publication years, (ii) abutment type, arch, and tooth type, (iii) endocrown material and fabrication system used, (iv) endocrown preparation details, (v) method of preparation, impression type, use and thickness of die spacers, (vi) type of testing system and magnification size, (vii) mean values of marginal adaptation, marginal and axial discrepancy, or gap in different areas between the endocrown and materials used before and after cementation, expressed as mean±standard deviation in micrometers.

QUALITY ASSESSMENT:

The methodological quality of the incorporated studies was evaluated using the parameters of previously published relevant systematic reviews involving in vitro investigations [36–38]. The risk of bias was judged according to the following variables: (i) sample randomization, (ii) single-operator protocol application, (iii) existence of a control group, (iv) blinding of the testing machine operator, (v) standardization of the specimen preparation, (vi) assessment of the failure mode, (vii) use of all materials in accordance with the manufacturer’s recommendations, and (viii) sample size calculation. A “yes” classification was assigned to an assessed variable if it was present in an article. If the information was not reported in the article, a “no” classification was assigned to the variable. The risk of bias of each individual article was categorized based on the total number of “yes” classifications received as follows: 1–3 (low quality/high risk of bias); 4–6 (medium quality/moderate risk of bias); and 7–8 (high quality/low risk of bias).

Results

LITERATURE SEARCH AND STUDY SELECTION:

The primary search resulted in 428 items. After removing 355 irrelevant and duplicate items and titles, we read the abstracts of 73 studies to exclude ineligible studies. A total of 56 studies were excluded, and 17 studies were selected for full-text retrieval. Finally, 17 studies were included in the present review [12–28]. Those studies measured the marginal adaptation or gap of endocrowns made by LDGC or zirconia ceramic materials measured in micrometers. The flowchart of the literature search process is illustrated in Figure 2.

CHARACTERISTICS OF THE STUDIES:

All included studies were in vitro studies [12–28]. The highest number of publications, 7 studies (41%), was from the year 2023 [12–15,19,20,22], followed by years between 2020 and 2022, with 6 studies (35%) [20,21,25–28]. Fourteen (76%) studies reported using natural teeth [12–15,17–23,25–27], 12 studies (71%) were for mandibular arches [13–17,19–20,22–25,27], and 11 studies (65%) were for molars [13–16,19,22–25,27,28]. LDGC and zirconia were the most common all-ceramic materials used for endocrown constructions, with 35% and 30%, respectively, and either alone or in comparisons with other all-ceramic materials. The manual abutment preparation for endocrowns was used in approximately 75% of studies [13–16,18,21–28], scanning was used in 90% of studies of endocrown preparation, while the stereomicroscope system was used in 40% of the total studies to record and assess the marginal adaptation in micrometers [12–18]. The information regarding the involved in vitro studies is presented, along with other characteristics, in Table 1. Figure 3 shows the detailed data of the included papers.

MARGINAL ADAPTION OUTCOMES:

Figure 4 shows the values of marginal adaptation, or gap, of the endocrown materials and systems used in this review. Regarding the pulpal extension of the endocrown design and type of created finish line, the LDGC endocrowns had a slightly lower recorded marginal adaptation (55.66 μm) than did zirconia (62.34 μm) endocrowns. The pressable all-ceramic endocrowns had higher recorded marginal adaptation values than did the CAD/CAM systems, with 73.17 μm and 52.30 μm, respectively.

RESULTS OF BIAS ASSESSMENT:

Five studies did not have a single or blinded operator, and most of them did not have sample randomization or a control group. All studies recorded a standard sample size and marginal integrity and followed the manufacturer’s instructions. Out of the studies, only 1 study had a medium risk of bias [28], while the remaining scored a risk of bias of only 3, which is interpreted as a high level of bias [13,17,19,27]. Table 2 shows the quality bias assessment of the in vitro studies included in this systematic review.

Discussion

From a biomimetic perspective, preservation and conservation of the tooth structure are essential for maintaining the balance between biological, mechanical, adhesive, functional, and aesthetic factors of the dental element to be restored [39]. Endocrowns constructed by CAD/CAM or heat-pressed techniques are currently used for crowning of posterior maxillary and mandibular teeth after RCT. These endocrowns have a significant rate of survivability, and their success rate is described as equal or better than that of the post-and-core and crown, with the advantages of conservative preparation, adhesive retention devises, fewer clinical and laboratory sittings, and opportunity of practicing in teeth without ferrule [4,18,28]. In the present study, we conducted a systematic review to assess the behavior of marginal adaptation or marginal gap of machinable or pressable endocrowns constructed from LDGC and zirconia-reinforced lithium silicate ceramic materials.

Overall, and based on the outcomes of the in vitro studies included in this systematic review, the recorded values of the marginal fitting, gap, and marginal adaptation of both LDGC and zirconia endocrowns were within the clinically acceptable value of <120 μm [40,41]. Some in vitro studies documented equal or slightly higher marginal gap values, as documented by Jalali et al, with 160 μm for zirconia CAD/DAM [14], and by Shin et al, with 138 μm for LDGC CAD/CAM [23]. An equal value of marginal adaptation or slightly lower values were obvious in laboratory studies conducted by Sağlam et al for both LDGC and zirconia CAD/CAM [27], LDGC endocrowns constructed with CAD/CAM or heat-press systems [25], and LDGC manufactured by CAD/CAM [22]. Most of the studies recorded values ranging from 30 to 70 μm, as shown in Table 1. On the other hand, heat-pressed or CAD/CAM LDGC endocrowns had lower recorded marginal adaptation values than did zirconia endocrowns constructed by the same systems, as reported by Shafi and Rayyan and Ioannidis et al [42,43]. Those values are in agreement with the outcome of this review. Also, Riccitiello et al recorded similar marginal gap values of 65.0±23.0 μm, 69.0±41.0 μm, and 85.0±26.0 μm for CAD/CAM zirconia, CAD/CAM LDGC, and heat-pressed LDGC all-ceramic materials, respectively [44]. Recently, the mechanical behavior of LDGC endocrowns has been found to be parallel to that of post-and-core crowns [45].

CAD-CAM endocrown restorations that can affect CAD/CAM marginal fitting using intraoral optical scanners have been introduced. Recently, a systematic review by Turkyilmaz et al in 2023 showed that LDGC and zirconia all-ceramic CAD/CAM crowns offer parallel marginal gap values, but LDGC material presents an excellent internal fit, compared with that of zirconia [41]. Also, using SEM, the marginal gap documented by the zirconia material was 56.21±15.07 μm, as stated by Chouksey et al [46], while a lower value (21.45±12.58 μm) was recorded by Ferrini et al in 2023 for the same material, and marginally similar values were recorded for LDGC (62.28±51.8 μm) [40].

Marginal adaptation, defined as the gap between the finish line and the prostheses margin, is measured as one of the main criteria affecting the long-term prognosis of ceramic CAD/CAM prostheses [47]. Presence of a significant marginal gap between the tooth and the endocrown luting material will be visible to the oral location, resulting in luting dissolution and microleakage, leading to irritation of the gingival tissues, secondary caries, and finally endocrown failure [15–17,21–23,26–28]. The assessment of the marginal adaptation of prostheses is regularly accomplished using a simple technique with a digital microscope [19–21], stereomicroscope [12–15,17,18], or more advanced systems, such as SEM [26,27] and microcomputed tomography [22–25], as shown in Table 1. All systems used for endocrown marginal adaptation showed a high degree of accuracy and usually used the micrometer for assessments.

Endocrowns can be fabricated by CAD/CAM or heat-press techniques using various ceramic systems, such as lithium silicates and zirconia-reinforced lithium silicates [14,48,49]. A variety of the products are used in dental practice and produce prostheses with better accuracy of marginal variation [13,19,21]. The precision of CAD/CAM-fabricated endocrown fitting differs depending on the inspecting procedure, milling type step, post-milling dimensional differences, and configuration of the program used. Equally, the outline and width of the milling devices limit the machining of the inner outline, which can influence the final outcome in relation to the endocrown fitting [50]. To guarantee well-fitting prostheses, endocrowns can need reliable material dimensions, as they are somewhat brittle, and this can cause breakdown of the final products. The fit accuracy of prostheses assembled by heat-pressed porcelain resources has been shown to affect the durability and survival rate of the prostheses [21,32]. This is obvious in values recorded for machinable CAD/CAM materials as compared with pressable materials, as shown in Figure 4.

A second sintering step is required for most ceramic restorations involving CAD/CAM and heat-press techniques. Nevertheless, several reports have determined that the crystallization firing practice causes a substantial increase in the marginal gap size, possibly as shrinkage of the ceramic during the crystallization process [4,19,49,51]. However, other studies mentioned that there are various components affecting the fit of prostheses, incorporating preparation layout, parameter positioning, and cement nature and gap. CAD/CAM machinability is an essential factor, specifically for the part related to the marginal zone. Defective restoration milling indicates the lack of ability of the prostheses to seat correctly and can result in some occlusal inconsistency. Microleakage will be the outcome if too much material is milled around the marginal area of any restoration [21,32].

Attar et al, Soliman et al, and Oh et al determined that the scanning approach influences restoration precision [13,21,52]. According to the conclusions of Abduljawad et al and Son and Lee, the margin point, equigingival or subgingival, is able to impact restoration accurateness [25,53]. Likewise, the software model, record managing, and image triangulation technique affect the resolution and surface features of the last digital impression generated [54]. Other studies have shown that the 5-axis CAM element offered outstanding accuracy compared with the 4-axis element [12,14,55]. Moreover, the marginal adaptations of endocrowns are increased with additional preparation features and increased cavity height and divergence; therefore, declaring the preparation as simple as cavity design lead is proposed [56]. Many endocrown designs were used in this systematic review in relation to the height of ferrule, presence of butt-joint preparation, shape and length of the intra-pulpal extension, and material used with CAD/CAM or heat-press. Regardless, the marginal gap or adaptation recorded in all studies involved in this systematic review were similar and did not exceed the clinically acceptable recommended distance in micrometers.

Marginal discrepancy of endocrowns is intensified with adding preparation features, higher cavity depth and increasing the divergence. Fracture resistance of endocrowns is increased with more occlusal reduction and cavity depth

This systematic review, which used in vitro laboratory studies had some significant limitations. First, the methodologies used had high heterogeneity among the included studies. Second, most of the included studies were graded as having “high” quality bias. Third, variations in the dimensions (maxillary or mandibular) and types of endocrown teeth (molars or premolars) among the included studies may have resulted in differences in the marginal adaptation values stated. All of those limitations can affect the bonding strength and marginal durability of LDGC and zirconia.

Conclusions

From this systematic review, the following conclusions were drawn: all-ceramic LDGC and/or zirconia endocrowns fabricated for posterior teeth in both arches using CAD/CAM or heat-press had marginal adaptation values within an acceptable range. New in vitro investigations in which consistent material and measurement attempts are implemented are needed to strengthen the evidence currently presented in the literature regarding the durability of all-ceramic endocrowns.