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Article

Fracture Resistance Evaluation and Failure Modes Rating Agreement for Two Endocrown Designs: An In Vitro Study

by
Saeed J. Alzahrani
1,*,
Maher S. Hajjaj
1,
Hanin E. Yeslam
1 and
Thamer Y. Marghalani
2
1
Department of Restorative Dentistry, Faculty of Dentistry, King Abdulaziz University, Jeddah P.O. Box 80200, Saudi Arabia
2
Department of Oral and Maxillofacial Prosthodontics, Faculty of Dentistry, King Abdulaziz University, Jeddah P.O. Box 80200, Saudi Arabia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(5), 3001; https://doi.org/10.3390/app13053001
Submission received: 23 January 2023 / Revised: 11 February 2023 / Accepted: 21 February 2023 / Published: 26 February 2023
(This article belongs to the Special Issue Current Advances in Dentistry)
Browse Figures
Versions Notes

Abstract

:
This in vitro study aimed to evaluate the fracture resistance and mode of failure of endocrowns with proximal extension design compared to the conventional design, and to assess the agreement of evaluators on the failure mode. Twenty mandibular third molars free of caries with approximately similar root lengths and crown dimensions were sectioned parallel to the occlusal plane 2 mm above the cementoenamel junction (CEJ). Then, pulp chambers and canals were accessed, cleaned, and smoothed for the path of insertion. To achieve a symmetrical pulp chamber with an average depth of 3 mm, chambers were filled with flowable resin composite. Then, teeth were randomly divided into two groups (n = 10). The control group has all the walls at the same level. The test group had a box extension on the proximal wall 2 mm apical to the buccal and lingual wall level. Endocrowns (n = 20) of two different designs (conventional and proximal extensions) were fabricated using lithium disilicate (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein). After cementation, specimens were loaded with a round-end vertical loading tip at a rate of 0.5 mm per minute until fracture (INSTRON, Norwood, MA, USA). Then, the fractured specimens were collected for evaluation and assessment. Statistical analyses were performed using the Mann-Whitney U-test (α = 0.05) for fracture test analysis and Cohen’s Kappa for inter-rater reliability. The Mann-Whitney U-test showed a non-significant difference between the two groups (p = 0.142). However, the mode of failure of the endocrown with proximal extension showed more catastrophic failures than the conventional design. Adding proximal boxes to the endocrown design did not negatively impact the fracture resistance of the restoration. Visualization of the fracture line and tracing their apical end by direct vision or other diagnostic tools is an essential part of the evaluation of failures of endocrowns. Endodontically treated molar teeth with proximal caries extension might be restored with an endocrown with proximal extension. Clinicians should take extra care when visualizing the fracture lines and tracing their apical end as it might be very misleading.

1. Introduction

Cuspal coverage of posterior endodontically treated teeth is recommended to minimize tooth fracture and restore function [1,2,3]. However, the choice of appropriate restorative treatment modality in these situations is not simply reached. The reduced dental structural integrity in such cases is brought about by extensive caries, mechanical trauma, and/or existing large defective restorations [4,5,6,7]. Caries lesions and extensive defective restorations extending gingivally and proximally in posterior teeth are of particular concern with highly variable prognoses, as they lead to inadvertent deviation from classical tooth preparation designs for definitive restorations [7,8]. This led to full coverage crowns reportedly being the restorative treatment of choice for such cases [9,10]. In a systematic review, single crowns versus direct restoration on endodontically treated teeth showed 81% and 63% survival rates, respectively [11].
Post and core is a well-established technique to restore endodontically treated teeth with significant loss of tooth structure [12]. Metal and ceramic posts both have an elastic modulus that is much higher than that of dentin, which could ultimately lead to root fracture when the tooth is overloaded. Fiber posts have been recommended for a more conservative approach, with their tooth-similar modulus of elasticity and the utilization of adhesive luting agents that could better distribute occlusal stresses along the tooth structure [13,14,15]. However, prefabricated posts require the removal of tooth structure during post space preparation, risk of root perforation, additional costs, and custom post and core technique require several visits [16]. Improvement in adhesive dentistry led to more conservative tooth preparation for endodontic treatment such as endocrowns [17]. Endocrown is a monoblock crown restoration retained by the intra-coronal pulp chamber space and bonding to the remaining tooth structure [18,19]. Endocrown was introduced in 1995 as a more conservative and convenient treatment option requiring no intra-radicular post space preparation [15]. Since then, endocrowns have continued to grow in popularity among dental practitioners [20,21,22]. There is limited data in the literature concerning endocrowns that established a fully detailed standardized cavity depth within the pulp chamber [18,23]. However, the Shoulder finish line combined with a short axial wall has been reportedly advantageous to the fracture resistance of endocrowns [19]. Additionally, the addition of a ferrule in the coronal preparation of the tooth to increase fracture resistance has been argued, with studies both advocating and/or advising against further tooth structure loss associated with such preparation design [9,10,20,22,24,25,26]. Generally, the endocrown preparation comprises a cervical butt-joint margin, cuspal reduction, and an intra-pulp-chamber conservative cavity [23,24,25].
Govare and Contreposi [26] found in their systematic review that endocrown for molars is a reliable alternative to conventional post-and-core crowns. A recent meta-analysis compared endocrown with conventional post-and-core, and crowns showed no significant difference in overall survival or success estimates [27]. Many studies evaluated the influence of different modifications of the endocrown design on fracture resistance, including adding ferrule design [20,24,25], different pulp chamber depths [10,23,28], various restorative materials, and types of teeth [6,10,15,21,29,30,31,32,33].
Perception and determination of the extent and seriousness of failures of restorations and fixed prostheses into catastrophic or restorable failures often vary between individuals depending on the background and experience of the clinician [34,35,36]. Unfortunately, only a few studies have reported on the variation in detecting the extent and restorability of failures and cracks associated with conventional crowns or prostheses and, more specifically, endocrowns [37,38,39,40].
When gingivally extending proximal caries lesions or existing restorations are present, proper isolation for both core build-up, digital and/or traditional impression, and adhesive cementation of posts and/or crowns builds up to be a major challenge [4,41]. Elevation of the proximal box using composite resin was introduced in 1998, in an effort to establish a supragingival margin preparation that allows proper impression and cementation of full-coverage crowns [27]. Recently, this approach was tested for the elevation of the proximal box in indirect restorations for posterior teeth and in premolar endocrowns [1,28,29]. However, modification of conventional endocrown preparation designs to endocrown with proximal boxes extension has not been investigated yet in molar teeth. Thus, the purpose of this in vitro study was to compare the conventional endocrown preparation design with endocrown preparation with mesial and distal extension boxes on the fracture resistance of molar teeth. One of the additional aims of this study is to check the agreement of the evaluators on the ability to detect and evaluate failures in vitro, and decide on the restorability or catastrophic extent of endocrown failures based on experience without previous consensus and then after consensus. The null hypothesis is that there were no differences in the fracture resistance of the two designs and the failure mode between the endocrown designs and the conventional and proximal extensions.

2. Materials and Methods

The study was carried out after obtaining the ethical approval of its design from the research ethics committee of the Faculty of Dentistry at King Abdulaziz University (IRB protocol #362-12-21).
Twenty extracted mandibular third molars free of caries with approximately similar root lengths and crown dimensions were collected from local clinics; teeth were extracted for routine clinical indications. Extracted teeth with cracks in the crown or roots of teeth identified after extraction were excluded from the study. First, teeth were cleaned using an ultrasonic scaler. Then, teeth were sectioned parallel to the occlusal plane 2 mm above the cementoenamel junction (CEJ) using a low-speed diamond saw (Allied TechCut 4 Low-Speed Diamond Saw, Rancho Dominguez, CA, USA) under water coolant. Next, pulp chambers and canals were accessed and cleaned from the remaining soft tissue. All specimens were ultrasonically cleaned (PowerSonic 405, Hwashin, Seoul, Republic of Korea). Finally, the pulp chamber walls were smoothed for the path of insertion. To achieve a symmetrical pulp chamber with an average depth of 3 mm, chambers were filled with flowable resin composite (Tetric N-Flow Bulk Fill, Ivoclar Vivadent, Amherst, NY, USA) and light cured (E-Morlit.Apoza, NewTaipei, Taiwan).
Then, teeth were randomly divided into two groups (n = 10). The control group has all the tooth structure walls at the same level. Test group: box extension on the proximal walls was 2 mm apical to the level of the buccal and lingual walls. The remaining tooth structure was measured using a digital caliper. For the conventional endocrown design, the ranges of thicknesses were as follows: buccal wall width ranged 2–3.6 mm, lingual wall width ranged 1.7–3.3 mm, mesial wall thickness ranged 2.3–3.3 mm, distal wall thickness ranged 2–3.2 mm. For the endocrown with mesial and distal extension boxes, the ranges of thicknesses were as follows: buccal wall width ranged 1.8–3.1 mm, lingual wall width ranged 1.7–2.8 mm, mesial box depth ranged 2–2.5 mm, distal box depth ranged 2–2.5 mm, mesial box width range 2.5–4 mm, distal box width range 2–5 mm. The remaining wall thicknesses for both conventional endocrown and boxed designs were homogeneous among the specimens for each wall thickness, which were analyzed using the normality test Shapiro-Wilks.
The root portion of the teeth was covered with a thin layer (0.35 mm) of light polyvinyl siloxane material (Virtual extra light body, Ivoclar Vivadent, Schaan, Liechtenstein) to simulate the periodontal ligament [42]. Then, the teeth were embedded in auto-polymerizing acrylic denture base resin (Probase Cold, Ivoclar Vivadent, Schaan, Liechtenstein) at 2 mm apical to the cementoenamel junction (CEJ) to simulate alveolar bone level [43]. Mean chamber depth and wall thickness were measured. Specimens were scanned using an intraoral scanner (Cerec AC Bluecam; Sirona Dental Systems GmbH, Bensheim, Germany), and acquired data were transferred to software for restoration design (Cerec SW 4.0, Sirona Dental Systems GmbH, Bensheim, Germany). Endocrowns were designed with approximately 4 mm height established for all specimens. The crown material was lithium disilicate (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and was milled using (Cerec MC XL; Sirona Dental Systems GmbH, Bensheim, Germany). The post-milling firing was accomplished following the manufacturer’s protocol in ceramic furnaces (Programat S, Ivoclar Vivadent, Schaan, Liechtenstein). Abutment preparations for conventional endocrown design and endocrown design with mesial and distal extension boxes are shown in Figure 1 and Figure 2, respectively.
Silicon indicating paste (Fit Checker Advanced Blue cartridge pack, GC America, St Alsip, IL, USA) was applied to check the seating, and adjustment was made if needed. The intaglio surface of endocrowns was etched with 9% hydrofluoric acid for 20 s. Then, each surface was rinsed with water and dried. Next, a silane coupling agent was applied (Bisco two-bottle silane primer, BISCO Inc., Schaumburg, IL, USA). The teeth were etched with 37% phosphoric acid gel. Then, each surface was rinsed with water and dried, followed by the application of the adhesive resin (ALL-Bond Adhesive, BISCO Inc., Schaumburg, IL, USA). All endocrowns were cemented with dual-cure resin cement RelyX Unicem Capsules (3M ESPE, Seefeld, Germany) under a firm finger pressure [25]. Tack cure for 2 s was accomplished with E-Morlit (Apoza, NewTaipei, Taiwan), the excess cement was removed, and additional light curing was performed for 20 s on each surface for complete curing. Restorations were stored in an incubator at 37 °C and 100% humidity for twenty-four h.
Afterward, specimens were placed into fixtures mounted on the universal testing machine (INSTRON, Norwood, MA, USA), as shown in Figure 3. The buccal cusps were loaded with a round-end vertical loading tip at a rate of 0.5 mm per minute until fracture. After the fracture resistance test was applied, specimens were collected for evaluation and assessment.
Two evaluators, TYM (1) and SJA (2), were assigned to examine and evaluate the fractures and failure modes independently based on their clinical experiences and understanding, terms, and classifications made for this project, without previous agreement on the details. The evaluation of mode of failure was done basically on two stages: independent evaluation and after a consensus evaluation. First catastrophic failure evaluation was done with evaluator 1 without the removal of the tooth from the simulated socket and PDL, resin, and light bodied silicone. In addition, evaluator 1 was using magnification loupes, therefore simulating the real clinical situation. Evaluator 2 removed the tooth from the resin socket and simulated PDL, and used the magnifying loupes to examine the specimen. The assessment criteria were the most apical fracture location, the presence of fracture line/crack across various materials (tooth/endocrown), classification of the restorability into catastrophic (non-restorable) failure or repairable fracture, the number of and separation of pieces after fracture, and the fracture direction. After the wash period, three weeks, a consensus was reached for the disagreed upon specimens by thoroughly discussing the differences and mutual agreement on the differences to a specific term and decision. To reach consensus, both evaluators had to have a second round where they both used direct examination of the whole tooth out of the simulated socket with magnifying loupes and agree on the rating by consensus. Each examiner observation was checked for agreement with the consensus agreement reference. Descriptive statistical data analyses were performed, and assumptions of random specimens, independence, normality, and homogeneity of the variance were met. The Mann-Whitney U-test was performed utilizing Statistical Package for Social Sciences SPSS 28 (SPSS v28; IBM Corp, Armonk, NY, USA) at (a = 0.05) and Cohen’s Kappa (SPSS v28; IBM Corp, Armonk, NY, USA) for inter-rater reliability between the two evaluators.

3. Results

Descriptive statistics of endocrown designs’ fracture resistance (load to fracture) of conventional and proximal boxes in control and test groups are demonstrated in Figure 4.
Results of the Mann-Whitney U-test of fracture resistance test in Newtons (N) are detailed in Table 1. The test showed an insignificant difference between the two endocrown preparation design groups (p = 0.142).
Cohen’s Kappa agreement between the two evaluators on the assessment criteria after the fracture resistance test criteria is presented in Table 2.
a.
The estimation of the weighted kappa uses linear weights (0–0.2 = non-agreement, 0.21–0.39 = minimal agreement, 0.4–0.59 = weak agreement, 0.6–0.79 = moderate agreement, 0.8–0.9 = strong agreement, >0.9 almost perfect agreement).
b.
Value does not depend on either null or alternative hypotheses.
c.
Estimates the asymptotic standard error assuming the null hypothesis that weighted kappa is zero.
In comparing the test group (endocrown with proximal boxes) with the control group (no proximal boxes), we can observe the following: Endocrown with proximal extension boxes had the apical location of the fracture line, mainly occurring below the simulated bone level. Almost one-third of the specimens were at the finish line. None were detected at or below, or above CEJ. That may reflect on the fracture classification as catastrophic or non-restorable, as shown in Table 3, as 80% of fractures were classified as catastrophic. Agreement represented by Kappa of catastrophic failure mode was 0.255 between the two evaluators. However, when comparing each previous rater’s rating to the reached consensus rating, evaluator one kappa was 0.315 in agreement with the consensus rating and evaluator two was 0.787. Fracture lines were primarily involved in both materials (tooth and ceramic) at 80%. None was in tooth structure only. The separation of pieces into more than two pieces non-attached (separated) occurred in half of the specimens. The remaining other half were distributed among the other classifications. The fracture direction was either mesiodistal or multidirectional, with an abundance of multidirectional at 70%.
Endocrown with conventional design had the apical location of the fracture at or below the simulated bone level in more than half of the specimens. A third of the specimens were at the finish line, and only one specimen showed fracture below CEJ and above the simulated bone level, which may reflect on the classification into catastrophic or non-restorable teeth being almost equally distributed. Fracture lines were mostly involved in both materials (tooth and ceramic) at 77.8%, 22.2% of the fracture lines was in the endocrown material itself, and none in the tooth structure only. The separation of pieces into more than two pieces non-attached (separated) was 55% of the specimens, while 33.3% was fractured into two pieces, and only one specimen was fractured into two pieces attached. The fracture direction was predominantly multidirectional at 44.4%. Except for the distolingual ones, the remaining directions were distributed almost equally among the other five directions. Table 3 shows the mean fracture resistance values of specimens for each mode of failure. Figure 5 shows the failure mode of two specimens.

4. Discussion

Endocrowns are considered good conservative options for restoring endodontically treated posterior teeth, especially molars, with inadequate remaining tooth structure [39]. Altier et al. evaluated both ceramic and composite endocrown fracture resistance, and they were comparable to intact natural teeth used in the study [44]. Several studies reported superior fracture resistance of endocrowns compared to post-and-core supported restorations [45,46,47]. Lithium disilicate endocrowns showed higher fracture resistance values than the average masticatory force in several previous studies [6,48]. However, endocrowns are contraindicated in cases with less than 3 mm deep pulp chambers, less than 2 mm wide cervical margins, and cases where adhesion is compromised [49]. Unfortunately, there is limited data regarding the mechanical properties of endocrowns with different designs and materials. The purpose of this in vitro study was to compare the conventional endocrown preparation design with endocrowns prepared with mesial and distal extension boxes on the fracture resistance of molar teeth. One of the control specimens was not stable during testing and therefore was eliminated from the group. The null hypothesis was accepted.
There were no differences in the fracture resistance of the two endocrown designs: the conventional and proximal extension. Furthermore, there was no statistically significant difference between the two endocrown designs in the fracture resistance test (p = 0.142). This result is similar to a previous study by de Kuijper et al. [30], investigating endocrown preparation modification effects on restoration fracture resistance and failure. They explored various pulp chamber extension designs and found no significant differences in fracture resistance between the different designs. In the current study, a 3 mm standardized preparation depth into the pulp chamber was ensured for all samples. Deeper pulp chamber preparation designs were found to enhance restorative retention due to the increased bonding surface and high fracture resistance values, but transmitted the loads to the root dentin [28,50].
However, the failure mode in the current study, fractographic inspection, showed different results between the conventional and proximal boxes endocrown designs (Table 3). This difference is seen with other changes in endocrown design. Lin et al. found that increasing the thickness of lithium disilicate endocrowns enhances the restoration’s fracture resistance, but increases the chance of tooth fracture [51]. Previous studies in the literature mostly investigated the effect of the ferrule preparation design on the fracture resistance of endocrowns. Einhorn et al. [25] found that a 1 mm ferrule increased the fracture resistance and retention of endocrowns. Minimal proximal adjustments, where the amount of remaining coronal tooth structure is contraindicative of a conventional endocrown preparation, were proposed in a case report by Biacchi et al. [52]. Another study investigated the effect of the type of preparation finish line on endocrown fracture resistance and found that shoulder finish lines enhanced fracture resistance of the restorations [53].
Comparing our results with other studies in the literature was complicated due to variations in the teeth selected (anterior versus posterior), the depth of the pulp chamber, study design (finite element analysis, in vitro study), presence of ferrule, design of the study (in vitro versus finite element analysis), restorative materials (glass-based, resin-based, or zirconia-based), presence of periodontal ligaments replica, type of fracture load (static loading versus cyclic loading), and loading direction (axial versus oblique). The periodontal ligament (PDL) allows some degree of tooth displacement under the occlusal load [54]. Without simulating such an effect, load test results would be misleadingly high [55]. In the current study, polyvinyl siloxane material was used to simulate the PDL, which was demonstrated to be a good option by Rathi et al. [42]. In the current study, acrylic resin with an elastic modulus of around 2000 MPa [56] was used to embed the teeth, resembling the alveolar bone [43,57]. Various resins have been used in the literature, with elastic modulus values lower than the elastic modulus of alveolar bone (about 10,000–14,000 MPa [58,59]). However, their elastic moduli are still considered much closer to the alveolar bone than those of metal alloys (around 100,000 MPa [60]). The use of acrylic and polyurethane resins to simulate alveolar bone during restorative mechanical testing was deemed adequate by previous studies, and PDL simulation significantly influenced fracture test results more than embedding resin [61,62,63].
In the current study, extracted human molars were used to evaluate the fracture resistance of endocrowns with two preparation designs, since the pulp chamber of molars is larger than premolars, providing more surface for bonding to the restoration [44]. To reduce variability between specimens, lithium disilicate endocrowns were milled from prefabricated CAD/CAM blocks (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) using the same software and milling machine for Cerec CAD/CAM system, and all endocrowns had the same occlusal anatomy.
Lithium disilicate ceramic material is considered one of the best esthetic materials for fabricating indirect single-tooth restorations [44,47]. Using lithium disilicates for endocrowns offers the ability to bond to the remaining tooth structure, besides their excellent mechanical properties [26,27]. For conventional lithium disilicate endocrown design restorations in molar teeth with static loading, the fracture resistance in the current study showed higher mean values than Güngör et al. [47] (915.91 ± 182.06 N), Einhorn et al. [25] (638.5 ± 238.5 N), and Rayyan et al. [64] (584.48 ± 5.80 N). However, Rayyan et al. evaluated endocrowns on anterior teeth rather than molars. El-damanhoury et al. [33] found higher mean values (1368.77 ± 237.34 N) but with more catastrophic failures (70%( than the present study (44%). Sahebi et al. [65] found higher mean values (1618.3 N ± 585 N) using zirconium lithium silicate blocks than those seen with lithium disilicate used in the current study. [65]. Altier et al. [44] demonstrated the more favorable failure mode of composite endocrowns compared to lithium disilicates, even though the latter exhibited higher mean fracture resistance values (3320.35 ± 961.21 N). This could be attributed to the composite material’s bending under load and its elastic modulus resembling dentin. Magne et al. concluded that CAD/CAM composite material exhibited higher fatigue resistance than lithium disilicate [66]. On the other hand, Gresnigt et al. [67] found lithium disilicate endocrowns to be more durable under lateral loading than composite. This could be explained by the results of stress concentration findings from a finite element analysis (FEA) study by Dartora et al. comparing endocrowns fabricated from different materials [68].
The statistical test used, in the current study, was the non-parametric Mann-Witney U-test, because it is like the independent sample t-test but for small sample size studies [69]. That test yielded a non-significant comparison, a p-value > 0.05. Another run using the t-test yielded a non-significant comparison between the independent variables, a p-value > 0.05. An independent sample T-test would have been used if the availability and number of tested specimens was large enough.
The agreement between the evaluators to assess the specimens for the presence of fracture line involvement in endocrown/tooth structure after the fracture resistance test was minimal. However, even after the discussion, each evaluator could identify cracks the other evaluator did not notice at the first evaluation. Evaluating the fracture directly by visualizing the coronal and radicular portion of the tooth is essential and crucial. Although, it is sometimes difficult to apply that clinically without using other sophisticated diagnostic tools such as cone beam computed tomography radiographs (CBCT), transilluminators, microscopes, direct vision (exploratory flap/surgery), and staining dyes to trace. The agreement between the two evaluators regarding the number and the separation of pieces after fracture was moderate agreement. During the evaluation, two of the samples were separated after removing the root socket for the second evaluator to examine them visually, which might lead to the slight difference between the two evaluators’ agreement with the reference observation. The agreement between the two evaluators regarding the fracture direction was weak agreement. This is probably due to visualizing the fracture using magnifying loupes and agreement on terminologies and classifications. The divergence between the evaluators, regarding the catastrophic or non-catastrophic rating, was mainly because the way each evaluator examined the failure mode initially and independently before reaching the consensus in the second stage. Therefore, the agreement was better before and after the consensus for evaluator two than the first evaluator, because evaluator two had another chance to examine the specimen in the same way he did before. Evaluator one had a first chance without full examination of the specimen, and a second chance with the direct examination of the specimen in question. Thus, the differences and divergences in the agreement originates.
The study’s results showed that the extension of the proximal box in the endocrown design did not influence fracture resistance. Furthermore, the fracture load was higher than the average maximum biting forces [70,71]. Previous studies utilized different methods for ensuring the seating of indirect restoration during cementation: finger pressure, control static loading using the universal testing machine, or not mentioned [15,21,23,32]. In the current study, seating using finger pressure was used. This may lead to different cement film thicknesses compared to other studies and may lead to possible differences from studies using other methods. Since finger pressure was found to differ from person to person [72], all endocrowns in the current study were seated by one investigator to reduce the chance of pressure variability. This procedure was adopted in several previous studies in the literature [73,74,75].
Although the total mean fracture resistance showed no differences between the two tested endocrown designs, the failure mode variations seemed intriguing to investigate further. Therefore, a detailed view of the fracture resistance sorted by the failure modes was initiated. It showed how different failure modes relate to the fracture resistance in Table 3. This initial view of the variability of patterns of failure modes and extent may further be investigated or observed by future similar studies of endocrowns and various design characteristics, either in vitro or in vivo. Additionally, the load was applied in a single axial direction in the current study. Application of load from two directions could produce different fracture modes and/or strength values. Previous studies showed that axial loading produced different fracture resistance values compared to lateral loading of lithium disilicate endocrowns [40,50], which could be attributed to the material’s flexural properties, including a high elastic modulus and rigidity [44,50].
One of the limitations of this study is that the detailed effects of marginal and internal discrepancy of the endocrowns extension box design were not thoroughly studied or evaluated in relation to the fracture resistance or failure modes of endocrowns. First, because of the extension of the restoration into the pulp chamber, marginal and internal fit are usually influenced by the cavity depth [76,77]. Second, most of the measurements were taken using a digital caliper (buccal wall width, lingual wall width, mesial box depth, distal box depth, mesial box width, and distal box width) and using standardized restoration through digital planning (occlusal thickness, cusp angle, and height). However, the surface area of the pulp chamber was not measured, which might influence the fracture resistance and the failure mode [23]. Although the current study is an in vitro lab study, and in one part it might not be duplicated clinically, the examination of the roots visually gives an indication of how difficult, complex, and complicated the detection of failures and cracks are in clinical situations. The differences between the evaluators of tooth fracture can be very extreme due to many factors which may affect the decision of classifying the tooth into restorable or non-restorable based on the catastrophic nature of the fracture line extension. Among these factors is the visualization of the fracture line and tracing their ends apically by direct vision or other diagnostic tools. Extra care and enough time should be taken into consideration during the examination of the occurrences of possible fractures on teeth, especially endocrowns, which were examined in this study, to ensure thorough detection and decision on teeth restorability.
It should not be taken lightly when the treatment option of endocrowns encounters failure of the restoration. Therefore, the authors suggest further studies to evaluate additional proximal boxes to endocrown design to further apply what is observed and known from in vitro studies before the general, imprudent, or injudicious use or application of various endocrown designs in any clinical situation.

5. Conclusions

Based on the finding of this study, the following can be concluded:
  • There was no significant difference in fracture resistance between the two endocrown designs. However, there were differences in the failure mode.
  • Differences between the evaluators of tooth fracture can be very extreme due to many factors which may affect the decision of classifying the tooth into restorable or non-restorable based on the catastrophic nature of the fracture line extension.
  • Visualization of the fracture line and tracing their apical end by direct vision or other diagnostic tools is an essential part of the evaluation of failures of endocrowns. Extra care and enough time should be taken into consideration during the examination of possible fractures of teeth, especially endocrowns, which were examined in this study, to ensure thorough detection and decision on teeth restorability.

Author Contributions

Conceptualization, S.J.A., M.S.H., H.E.Y. and T.Y.M.; methodology, S.J.A., M.S.H., H.E.Y. and T.Y.M.; software, S.J.A., M.S.H., H.E.Y. and T.Y.M.; validation, S.J.A., M.S.H., H.E.Y. and T.Y.M.; formal analysis, S.J.A., M.S.H., H.E.Y. and T.Y.M.; investigation, S.J.A., M.S.H., H.E.Y. and T.Y.M.; resources, S.J.A., M.S.H., H.E.Y. and T.Y.M.; data curation, T.Y.M.; writing—original draft preparation, S.J.A., M.S.H. and H.E.Y.; writing—review and editing, T.Y.M.; visualization, S.J.A.; supervision, S.J.A.; project administration, S.J.A.; funding acquisition, S.J.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, Saudi Arabia, under grant No. (G: 249-165-1443).

Institutional Review Board Statement

Ethical approval of the study design was obtained from the research ethics committee of the Faculty of Dentistry at King Abdulaziz University (IRB protocol #362-12-21).

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Applsci 13 03001 g001 550
Figure 1. Abutment preparation for Conventional Endocrown design.
Figure 1. Abutment preparation for Conventional Endocrown design.
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Figure 2. Abutment preparation for Endocrown design with mesial and distal extension boxes.
Figure 2. Abutment preparation for Endocrown design with mesial and distal extension boxes.
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Figure 3. Specimens were placed into fixtures mounted of the universal testing machine.
Figure 3. Specimens were placed into fixtures mounted of the universal testing machine.
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Figure 4. Analysis of endocrown designs’ fracture resistance of conventional and proximal boxes.
Figure 4. Analysis of endocrown designs’ fracture resistance of conventional and proximal boxes.
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Figure 5. (A,B) Two specimens with different failure modes of conventional endocrown design and endocrown with proximal extension.
Figure 5. (A,B) Two specimens with different failure modes of conventional endocrown design and endocrown with proximal extension.
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Table
Table 1. Mann-Whitney U-test results of fracture resistance test in Newtons (N).
Table 1. Mann-Whitney U-test results of fracture resistance test in Newtons (N).
Endocrown DesignNMean (N)Standard Deviationp *
Conventional91196.971318.3640.142
proximal boxes101019.766266.348
* Significant at p < 0.05.
Table
Table 2. Agreement between the two examiners and each one to the consensus for each assessment point using Kappa statistics.
Table 2. Agreement between the two examiners and each one to the consensus for each assessment point using Kappa statistics.
AssessmentWeighted KappaAsymptoticZcSig.95% Asymptotic Confidence Interval
Std. ErrorLower BoundUpper Bound
Location/End Level Apically
Between the two evaluators0.3120.1362.2010.0280.0460.578
Between evaluator 1 and consensus0.5710.1454.076<0.0010.2870.855
Between evaluator 2 and consensus0.5960.2043.408<0.0010.1960.995
Fracture line involvement Endocrown/tooth structure
Between the two evaluators0.3210.1951.5930.111−0.0610.704
Between evaluator 1 and consensus0.5650.2222.4890.0130.1310.999
Between evaluator 2 and consensus0.6460.1523.313<0.0010.3480.944
Catastrophic/non-restorability
Between the two evaluators0.2550.1761.4280.153−0.0900.600
Between evaluator 1 and consensus0.3150.1441.9900.0470.0330.598
Between evaluator 2 and consensus0.7870.1393.509<0.0010.5131.060
Number of pieces and separation
Between the two evaluators0.7340.1224.714<0.0010.4940.974
Between evaluator 1 and consensus0.7820.1185.077<0.0010.5511.013
Between evaluator 2 and consensus0.9440.0566.121<0.0010.8341.053
Fracture direction
Between the two evaluators0.4910.1462.8360.0050.2040.777
Between evaluator 1 and consensus0.8850.0724.512<0.0010.7441.026
Between evaluator 2 and consensus0.4830.1542.8040.0050.1810.786
Table
Table 3. Comparison between the conventional and the proximal boxes extension groups of each examined (observed) item and the number of specimens after consensus.
Table 3. Comparison between the conventional and the proximal boxes extension groups of each examined (observed) item and the number of specimens after consensus.
AssessmentCriteriaProximal BoxesConventional
Mean (N)CountMean (N)Count
Fracture Location/End Level Apically
  • Above Cementoenamel Junction
----
b.
Below Cementoenamel Junction and above the Simulated bone level (not at the finish line)
--1240.131
c.
At Cementoenamel Junction
----
d.
Below Simulated Bone Level
1104.0571161.885
e.
At the finish line
823.1031241.073
Fracture line involvement Endocrown/tooth structure
a.
Tooth Structure only
 ---
b.
Endocrown Ceramic only
746.8521435.622
c.
In Both Materials
1087.9981128.797
Catastrophic/Non-reparability
a.
Yes
1053.8881169.364
b.
No
883.3321219.065
Number of pieces and separation
a.
Two pieces separated
791.0711071.443
b.
>two pieces separated
1238.8651215.485
c.
Two pieces attached
739.6721481.011
d.
>two pieces attached
866.482--
Fracture direction
a.
Mesiodistal
756.803991.982
b.
Buccolingual
--1481.011
c.
Buccomesial
--1420.841
d.
Distobuccal
--1450.401
e.
Distolingual
-- -
f.
Multidirectional
1132.4771109.144
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Alzahrani, S.J.; Hajjaj, M.S.; Yeslam, H.E.; Marghalani, T.Y. Fracture Resistance Evaluation and Failure Modes Rating Agreement for Two Endocrown Designs: An In Vitro Study. Appl. Sci. 2023, 13, 3001. https://doi.org/10.3390/app13053001

AMA Style

Alzahrani SJ, Hajjaj MS, Yeslam HE, Marghalani TY. Fracture Resistance Evaluation and Failure Modes Rating Agreement for Two Endocrown Designs: An In Vitro Study. Applied Sciences. 2023; 13(5):3001. https://doi.org/10.3390/app13053001

Chicago/Turabian Style

Alzahrani, Saeed J., Maher S. Hajjaj, Hanin E. Yeslam, and Thamer Y. Marghalani. 2023. "Fracture Resistance Evaluation and Failure Modes Rating Agreement for Two Endocrown Designs: An In Vitro Study" Applied Sciences 13, no. 5: 3001. https://doi.org/10.3390/app13053001

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