Comparison of color changes,fracture strengths,and failure modes of conventional endocrowns and endocrowns with different design modifications

Abstract

BACKGROUND:

Endocrowns (ECs) are alternatives for rebuilding severely damaged teeth and show superior efficacy in molars over premolars.

OBJECTIVE:

The objective of this in vitro study is to evaluate the effects of different preparation designs with short pulp chambers on the mean color change ( $\Delta E$ ), fracture resistance, and failure types of mandibular molar ECs.

METHOD:

A total of 40 extracted mandibular molars were treated endodontically and divided into four groups. Samples in groups 1, 2, 3, and 4 had occlusal preparation depths of 5 mm, 3 mm, 3 mm with ferrule, and 3 mm with boxes, respectively. The samples were immersed in coffee and their $\Delta E$ values were measured by using the Commission Internationale de l’Eclairage color system. They were also subjected to a fracture test. Next, all specimens were examined visually under a stereomicroscope to evaluate their failure modes and identify their fracture origins. Data were entered and analyzed by using Statistical Package for Social Sciences.

RESULTS:

Among all groups, group 4 (3 mm $+$ boxes) presented the highest $\Delta E$ (4.15) after immersion in coffee. Moreover, ANOVA revealed that the $\Delta E$ of group 4 (occlusal preparation depth of 3 mm with boxes) was significantly different ( $p<$ 0.05) from that of group 2 (3 mm $+$ ferrule, 3.07). The EC with a 3 mm chamfer and ferrule showed the highest maximum load of 2847.68 $\pm$ 693.27 N, whereas that with a 5 mm chamfer finish line had a marginally reduced load at fracture of 2831.52 $\pm$ 881.83 N. The EC with a 3 mm chamber and boxes had a slightly increased maximum load of 2700.75 $\pm$ 436.40 N, whereas that with the 3 mm chamber had the lowest maximum load at fracture of 2385.97 $\pm$ 465.61 N. One-way ANOVA showed that different EC preparation designs had no effect on maximum fracture load (F [3,16] $=$ 0.550, $p=$ 0.6).

CONCLUSION:

The recorded $\Delta E$ values of ECs in all groups were equal or marginally higher than the acceptable values. The EC with a 3 mm chamfer and ferrule displayed the highest mean maximum load. The EC with a 5 mm chamfer finish line had a marginally lower maximum load at fracture than other ECs. Failures, such as ceramic fracture, split fracture, and ceramic and tooth splitting above the cemento–enamel junction (CEJ) or vertically were predominant in samples with occlusal preparation depths of 3 mm with ferrule and 5 mm.

Keywords

Endocrown color measurement fracture strength failure type endocrown design

1. Introduction

Teeth requiring endodontic treatment necessitate intricate restorations. Given that the use of posts, cores, and crowns have numerous contraindications, dentists must consider alternative restorations. Endocrowns (ECs) are alternatives for rebuilding severely damaged teeth and show superior efficacy in molars over premolars. They have gained popularity in the treatment of molars with unsuitable roots for post placement due to excessive tooth structure loss and restricted interocclusal space [1, 2].

ECs are partial crowns that provide comprehensive occlusal coverage and are maintained by the intaglio cover of the pulp chamber and cavity borders of root canal treated (RCT) teeth. Increasing the cavity depth within the pulp chamber can enhance stability and retention by expanding the bonding surface area [2, 3]. ECs are indicated when inadequate interocclusal space or clinical crown length and extensive tooth structure loss may prevent the use of an acceptable ferrule [4]. They are not recommended for teeth with pulp chambers less than 3 mm in depth [5]. CAD/CAM ECs have been recently promoted as good alternatives for post-and-core crowns [6]. The trends of the fracture forces or failure patterns of ECs and post-and-core crowns have been observed [7]. ECs fabricated through a systematic approach that includes good design, technique, and materials are superior to full-coverage crowns for severely compromised molars [8].

ECs offer the benefits of conserving tooth structure, minimizing the need for additive macroretentive features, and saving patient and operator time because they involve few clinical steps and do not need the same laboratory procedures required to fabricate conventional crowns. Their short-term survival is comparable to that of full-coverage crowns supported by posts and cores [9, 10].

Three factors primarily determine the clinical efficacy and quality of restorations: aesthetic value, fracture resistance, and marginal adaptation. Poor marginal adaptation can result in plaque accumulation, microleakage, caries, and endodontic inflammation, ultimately leading to the failure of restorations [11]. The preparation design of ECs could provide restorations with sufficient stability, structural durability, and retention. These adhesively dependent restorations require minimal preparation. Not all of the principles of EC preparation design have been established. Previously described preparation designs include a cuspal reduction of 2–3 mm, 90^∘ butt margins, smooth internal transitions, 6^∘ pulp chamber tapers, flat pulpal floors with sealed radicular spaces, and supragingival enamel margins when possible [12].

Lithium disilicate (LDS) glass-ceramics with enhanced mechanical strength, strong bonding strength to the tooth structure, and superior aesthetic appearance are one of the main materials used for EC constructions in the posterior area [13, 14]. The use of EC restorations has become increasingly prevalent in the conservative treatment of posterior molar teeth with the development of adhesive and bonding techniques. The fracture resistances and failure modes of endodontically treated molars restored without posts have been determined to be comparable to those of molars restored with posts [15, 16]. In a recent 2023 study, Koosha et al. applied force to the occlusal surface perpendicular to the long axis of specimens to simulate the forces in the molar region and observed that specimens with three axial walls had considerably higher fracture resistance than those with two walls and that overall, fracture forces fell within the range of 1000–1200 N [17]. A group of studies used molar teeth and examined the fracture forces of ECs fabricated with LDS glass–ceramics. The forces they recorded were below or approximately 1000 N [18]. However, Alzahrani et al. and El Ghoula et al. [16, 19] recorded values higher than 1000 N. Meanwhile, Haralur et al. recorded values of as high as over 2000 N [13], and Rayyan et al. reported values of as low as 500 N [20]. All the above-mentioned values were affected by the extension inside the pulp chambers and the height of the ECs as well as modifications to mesial and distal tooth preparation. Most of the studies conducted worldwide used the same method to test the fracture forces of EC restorations.

The Commission Internationale de l’Eclairage (CIE) LAB color system is a universal color investigation system that is used to interpret color values clinically. In this system, mean color change ( $\Delta E$ ) and color spaces are represented by three coordinates: the lightness, brightness, and black/white color character L* and the chromatic color characters a* and b* [21, 22]. Color stability is an indicator of the efficacy of aesthetic zoneestorations. Coffee is a beverage that discolors prostheses that are made primarily of ceramics [21]. A number of recent studies have established the color stability of LDS glass-ceramic materials with occlusal preparation depths of 3 mm or more. Alghazali et al. reported that Arabic coffee altered the color of glazed or polished ceramic materials of various varieties and that coffee immersion substantially altered the hues of zirconia ceramics [22]. In another paper published in 2023, Alrabeah et al. reported that among all tested materials, home bleach had the least significant effect on the color stability of LDS ceramics [23]. A year earlier, Donmez et al. compared the load-to-failure resistance and optical properties of nanolithium disilicate (NLD) with those of LDS and zirconia-reinforced lithium silicate (ZLS) before and after thermocycling in coffee and discovered that among the tested materials, ZLS had the highest load-to-failure resistance, whereas the difference between LDS and NLD was nonsignificant [24]. Compared with those of glass ceramics, the color change values of resin-based nanoceramics showed greater reductions as material thickness increased from 2 mm to 4 mm [25].

The required pulp chamber depth for retention/resistance in ECs should be maximized without undermining the border of the axial walls of the preparations. An increase in pulpal chamber depth had a detrimental effect on the marginal adaptation and internal fit of CAD/CAM ECs [26, 27]. Haralur et al. concluded that increasing occlusal thickness drastically increased the fracture strength of ECs composed of polymer-infiltrated ceramics and LD [14]. Although a radicular extension of 2 mm considerably improved the fracture strength of zirconia, it may result in the development of unfavorable failure types [14]. Koosha et al. concluded that the number of staying axial walls had a substantial effect on fracture resistance. Although most of the fractures in their study were catastrophic, these fractures occurred because they used forces that were greater than those usually imposed [17].

Most of the fracture types of CAD/CAM restorations are usually examined under stereomicroscopy and different magnifications to evaluate their failure modes [13, 14]. Although no conclusive studies regarding the extension of pulp chambers exist, adequate depth is necessary for the optimal retention and resistance of ECs. Thus, the objective of the present in vitro study is to evaluate the effects of different preparation designs with short pulp chambers on the $\Delta E$ , fracture resistance, and failure types of mandibular molar ECs. The null hypothesis of this work is that $\Delta E$ , fracture resistance, and failure types will not differ among EC restorations with different preparation designs.

2. Materials and methods

2.1 Study design

This in vitro study was conducted on 40 extracted human mandibular molars with approximately similar dimensions collected from the Department of Oral and Maxillofacial Surgery, College of Dentistry, King Khalid University. Authorization from the institutional ethical review board was obtained for the study protocol (SRC/GTH/2018-19/125).

2.2 Inclusion criteria

The inclusion criteria for the extracted teeth were almost the same dimensions of crowns and absence of caries, restorations, cracks, or previous RCT. The teeth had almost the same occluso–cervical height, molar root length, canal morphology, and CEJ dimensions and the mean buccolingual dimensions of 10.47 $\pm$ 0.27 mm, mesiodistal dimensions of 11.13 $\pm$ 0.40 mm, and root length of 14 mm as confirmed clinically and by intraoral radiography. Noncarious teeth extracted for prosthodontic or periodontic therapeutic purposes were included and stored in 10% formalin [28].

2.3 Endodontic treatment

All tooth samples were subjected to RCT following conventional protocol. A total of 40 extracted mandibular molars of approximately equal size were sectioned with a slow-speed diamond saw at the facial–lingual height of the contour perpendicular to the tooth long axis approximately 2 mm above the CEJ. The pulp chamber was accessed by using a high-speed handpiece and a diamond bur with copious water spray. Root canal shaping was finalized with a ProTaper Next X2 file (ProTaper, Dentsply Maillefer, USA). The root canals were disinfected with 5 ml of 1% NaOCI, irrigated with distilled water, and dried with paper points. Access to the pulp chamber was accomplished by using a high-speed handpiece and diamond bur with copious water spray. Pulpal remnants were removed with barbed broaches and gross instrumentation with hand files and a rotary system then through single cone obturation. The coronal access was filled with a temporary filling and stocked at 37^∘C and 100% humidity for 1 week [13, 14].

2.4 Teeth preparation

The RCT samples were vertically encased in no-shrink epoxy resin with the assistance of a vertical holding machine. The CEJ was maintained at 2 mm above the resin block surface. The occlusal preparation depth was completed in accordance with grouping and standardized with a graduated periodontal probe. The prepared teeth were randomly subdivided into four groups comprising 10 samples in accordance with different preparation depths (Fig. 1).

Figure 1.

Depth of Pulp Chamber with Modifications where the upper row represents the EC preparation type, and the lower row shows the restoration type.

Group 1: samples had an occlusal preparation depth of 5 mm (overall reduction in the height of the occlusal surface in the axial direction),

Group 2: samples had an occlusal preparation depth of 3 mm,

Group 3: samples had an occlusal preparation depth of 3 mm with a ferrule, and

Group 4: samples had an occlusal preparation depth of 3 mm with boxes.

2.5 Endocrown construction

ECs were fabricated with the injection technique in accordance with the manufacturer’s instructions. Pressed ECs were fabricated with pressable LDGC. Full anatomical contour wax designs were constructed directly over the cut extracted tooth samples by a single dental technician. A digital caliper (finoPraCeci caliper; FINO GmbH, Bad Bocklet, Germany) was used to confirm the dimensions from the central groove to the pulpal floor in the wax pattern (Geoclassic Opak, Renfert, Germany). The occlusal anatomy of all specimens was standardized with the assistance of an individual silicone template. Wax patterns were cast (Pressvest speed, IvoclarVivadent, Liechtenstein, Germany), burnt out in a furnace, and heat-pressed with an IPS Empress Programat EP 5000 furnace (IvoclarVivadent AG Schaan, Liechtenstein, Germany) at 920 ${{}^{\circ}}$ C. The pressed ECs were finished, polished, and adjusted through brief sandblasting with 100 Al₂O₃ at 1 bar (15 psi) pressure and steam cleaned. The EC samples were soaked in an ultrasonic cleaner with distilled water for 10 min and covered with a layer of neutral shade glaze through firing at 765 ${{}^{\circ}}$ C. The average thickness of the ECs was 4.0 $\pm$ 0.5 mm (area of color measurements) without extension inside the pulp chamber or mesial and distal boxes.

2.6 Color change measurements

All samples were immersed in Nescafe coffee from Saudi Arabia for 15 days, and the coffee was changed twice a day. One operator measured the color of each sample against a gray backdrop. The term “lightness” is denoted by the L* value. A high L* value is indicative of great lightness. Positive and negative a* values indicate red and green colors, respectively. Yellow and blue colors are represented by positive and negative b *values, respectively [29, 30, 31].

The CIE LAB color system, which provides the mathematical values for 3D color measurements, was used to record the parameters for each EC design modification [30, 31, 32]. The color readings of the molars were measured two times by a single operator: at the baseline (L1*, a1*, and b1*) and after 15 days (L2*, a2*, and b2*). The means of the three readings were taken. The spectrophotometer (VITA Easyshade 3 Advance, Vita Zahnfabrik, Bad Säckingen, Germany) used in this work was calibrated before the color measurement of each sample. Color differences between the mandibular molar at baseline and each molar after 15 days were determined by using the following equation:

$\displaystyle\Delta E*=(L1*-L2*)^{2}+(a1*-a2*)^{2}+(b1*-b2*)^{2}\times 1/2$

where the average color change values denoted by $\Delta E$ *, L1*, a1*, and b1* are the color coordinates of the EC samples at the baseline, and L2*, a2*, and b2* are the color coordinates of the EC samples after 15 days of coffee staining [21, 22], as shown in Fig. 2.

Figure 2.

Endocrowns after 15 days coffee staining.

2.7 Cementation of the endocrown bonding surface

Pressed restorations were adjusted, and intaglio surfaces (bonding surfaces) were cleaned, dried, etched with 5% hydrofluoric acid for 20 s (IPS ceramic etching gel), rinsed, and dried. After being etched, the ECs were washed and cleaned in distilled water in an ultrasonic bath for 10 min. The etched ceramic surfaces were then silanated for 60 s (Monobond Plus) and air-dried. Tooth surfaces were cleansed with a pumice/water slurry and rinsed/dried. The restorations were luted by using a self-adhesive resin cement (RelyXUnicem, 3M/ESPE). Excess cement was removed, and margins were then finished. Samples were wiped dry with tissue paper and left in place for complete dryness [13, 14].

2.8 Fracture test measurements

All the specimens were subjected to 10 000 aging cycles by a thermocycling machine (Thermocycler, SD Mechatronik, Feldkirchen-Westerham, Germany) in a cold and hot water bath at 5^∘C–55^∘C with the dwell time of 30 s before fracture and failure tests [21, 31]. After 24 h, the specimens were placed in a vise fixture on a universal testing machine (Instron, Norwood, MA, USA) for the application of static load at a crosshead speed of 1 mm/min along the long axis of the tooth. The occlusal surfaces were loaded with a hardened, stainless steel piston with a 3 mm diameter and 0.5 m radius of curvature at a rate of 0.5 mm/min with fracture. The maximum load at fracture was recorded in N [13, 14].

2.9 Failure type measurement

After the fracture test, all specimens were examined under a stereomicroscope (Olympus/DeTrey, Germany) at magnifications of 8 $\times$ and 25 $\times$ to evaluate failure mode and determine fracture origin. Failure modes were classified into three types as mentioned earlier [33]:

Type I: only the ceramic fractured or the ceramic and tooth fractured above the CEJ;

Type II: the ceramic and tooth fractured below the CEJ (nonrepairable); and

Type III: split fracture, the ceramic and tooth split vertically (nonrepairable).

2.10 Statistical analysis

Data were entered and analyzed using Statistical Package for Social Sciences for Windows, version 28.0 (IBM Corp., Armonk, NY, USA). Confidence intervals were set at 95%, and $p\leqslant$ 0.05 was considered statistically significant. Data normality was explored by using the Shapiro–Wilk test. Categorical variables were presented in the form of a frequency table. Continuous variables were presented in the form of mean $\pm$ std. deviation. One-way ANOVA was used to compare the $\Delta E$ and maximum load of different EC types.

Table 1
Pairwise comparison of mean color change ( $\Delta E$ )

Mean color change ( $\Delta E$ )	G1 (5 mm)	G2 (3 mm)	G3 (3 mm $+$ ferrule)	G4 (3 mm $+$ boxes)
G1 (5 mm)	–	0.13	0.98	0.11
G2 (3 mm)	0.13	–	0.06	0.03*
G3 (3 mm $+$ ferrule)	0.98	0.06	–	0.23
G4 (3 mm $+$ boxes)	0.11	0.03*	0.23	–

^*Indicates statistical significance; Post hoc analysis.

Figure 3.

Mean color change ( $\Delta E$ ) values after immersion of coffee.

3. Results

Figure 3 shows the values of the $\Delta E$ of all groups. $\Delta E$ value was the highest in group 4 (3 mm $+$ boxes) after immersion in coffee (4.15). One-way ANOVA revealed a statistically significant difference ( $p<$ 0.05) between the $\Delta E$ values of group 2 (3 mm, 3.07 $\pm$ 0.6) and group 4 (3 mm $+$ ferrule, 4.15 $\pm$ 0.7) (Table 1). The EC with the 3 mm chamfer design and ferrule (group3) showed the highest maximum load of 2847.68 $\pm$ 693.27 N, whereas EC with a 5 mm depth in pulp chamber preparation had a marginally reduced (2831.52 $\pm$ 881.83 N) load at fracture. Group 4 (3 mm chamber $+$ boxes) showed a slight improvement in maximum load (2700.75 $\pm$ 436.40 N), whereas the EC with a 3 mm depth in chamber had the lowest maximum load at fracture (2385.97 $\pm$ 465.61 N) (Fig. 4). One-way ANOVA (Table 2) showed that different EC preparation designs had no effect on the maximum fracture load (F [3,16] $=$ 0.550, $p=$ 0.6). Type III (split fracture, wherein the ceramic and tooth split vertically) and type I (only the ceramic fractured, or the ceramic and tooth fractured above the CEJ failure predominated (70%) in group 3 (3 mm A $+$ ferrule) and group 1 (5 mm), respectively. A total of 50% of the samples in group 4 (3 mm $+$ boxes) and group 2 (3 mm) had type I (only the ceramic fractured, or the ceramic and tooth fractured above the CEJ) and type III (split fracture, ceramic and tooth split vertically) fractures (Fig. 5).

Table 2
One way ANOVA of maximum load at fracture for different groups

	Sum of squares	Df*	Mean squares	$F$	$p$ -value
Between groups	687135.41	3	229045.13	0.550	0.65
Within groups	6662052.64	16	416378.29
Total	7349188.06	19

*Df: Degrees of freedom, $p\geqslant$ 0.05.

Figure 4.

Descriptive statistics of the Maximum load at fracture (N) in different groups.

Figure 5.

Graphical images of different failure modes; where, numbers in each bar represent the number of failed samples per group for each type of failure (blue, type I failure; orange, type II failure; green type III failure).

4. Discussion

Endodontically treated teeth display several physiological changes in dentin composition and microstructure. These changes increase their susceptibility to a number of risk factors, including decreased retention and stability, increased tooth fragility, compromised substrate adhesion, and ultimately prosthesis failure [2]. ECs are used primarily to achieve an all-ceramic bonded repair that is minimally invasive relative to root canals. In consideration of this aim, the preparation of ECs is different from that of traditional full-coverage crowns. ECs are monolithic ceramic restorations that are bonded and possess supragingival butt junctions to maximize the maintenance of enamel for improved adhesion. Given that ECs invade only pulp chambers, the shape and cavity of pulp chambers account for the stability and retention of ECs [4]. The present in vitro study evaluated the effects of different preparation designs with short pulp chambers on the $\Delta E$ , fracture resistance, and failure type of mandibular molar LDS glass–ceramic ECs. Overall, this in vitro study found significant differences in $\Delta E$ with different fracture patterns but not in the values of fracture forces in relation to the different EC designs ( $p=$ 0.65) of the tested samples and within different groups. Those outcomes partially reject the null hypothesis, i.e., no differences would be observed in $\Delta E$ , fracture resistance, and failure types among EC restorations with different preparation designs.

This study assessed the optical and mechanical properties of CAD/CAM ECs because all the related published works tested only the mechanical behaviors of ECs. In this study, samples in group 4, that is, samples with an occlusal preparation depth of 3 mm and boxes, had the highest $\Delta E$ of 4.15 after immersion in coffee. Groups 2 and 4 had statistically significantly different $\Delta E$ values of 3.07 $\pm$ 0.6 and 4.15 $\pm$ 0.7, respectively. Thus, the null hypothesis that the $\Delta E$ of EC restorations with different preparation designs would not differ was partially rejected. In this study, the ECs were manufactured by using pressable LDGC and analyzed by applying a spectrophotometer. The spectrophotometer was utilized for color measurement due to its superior precision, numerical expression of color, and lack of subjective bias [34, 35]. In this work, color was expressed in accordance with the CIE LAB system because this system incorporates all perceivable colors and was developed to serve as a device-independent reference. In this system, a* and b* values denote the chromatic range of green-magenta and blue-yellow, respectively. L* represents the lightness of a color. The independent measurement of these three coordinates permits the measurement of infinitely many possible colors in a three-dimensional real number space [30, 34, 35]. Alencar-Silva et al. reported that the $\Delta E$ of glazed and polished CAD-CAM LDGC due to beverages is below the perceptibility threshold of 1.30 [36]. The findings of the present work are similar to those of the study of Haralur et al., wherein coffee caused the least discoloration of pressed glazed LD e-max restorations and resulted in a $\Delta E$ value of 1.78 [37]. Aldosari et al. found a similar value of 1.96 in their study on Vita Suprinity ceramic materials [21]. This finding was further verified in studies by Alrabeah et al. and Donmez et al., who found concordant mean values likely because the uneven surfaces of unglazed pressed ceramics allowed for water infiltration and subsequent silica network disintegration, decreasing crystallinity and increasing coloring pigment absorption [23, 24].

The thicknesses of the ceramics used in the above-mentioned studies were not lower than the thickness of the ECs in the present study. The findings of this work are in agreement with those reported by Kang et al. because the thicknesses of the samples in the present study were equal to or more than those of the samples in the work of Kang et al. The $\Delta E$ recorded by Aldosari et al. and Alghazali et al. [21, 22] after immersion in coffee solution were lower than those obtained in the present study likely due to the different thicknesses of the LDS ceramics used. All the recorded $\Delta E$ values of ECs were equal or marginally or slightly higher than the acceptable clinical value of 4.2 [30]. The color of a ceramic material is determined by its chemical structure; matrix or matrix/load interphase variation; and physical–chemical reactions, which are intrinsic factors in the deep portions of CAD/CAM ceramic materials [38]. The results of the present study were also consistent with those of a coffee study by Haraluer et al., who showed that zirconia had high $\Delta E$ due to aging. They also found that LDGC was highly aesthetic in terms of color stability [37]. Extrinsic factors, such as dietary habits and coloring agents, inorganic particulate characteristics, surface topography, and glazing or sintering type (superficial changes encouraging surface degradation and promoting diffusion), can also influence color. The discoloration of CAD/CAM ceramic materials aggravates with the prolongation of storage in various types of liquid beverages; this behavior alters color stability [39].

In vitro investigations revealed that the fracture resistance of ECs was comparable to that of conventional crowns [40]. A statistically significant difference was seen between groups 2 and 4. The EC with a 3 mm chamfer and ferrule presented the highest maximum load, whereas that with a 5 mm chamfer finish line had a marginally reduced load at fracture. The maximum load of the EC with the 3 mm chamber and boxes had slightly increased, whereas the EC with the 3 mm chamber presented the least maximum load at fracture. Different EC preparation designs had no effect on maximum fracture load. Similar findings have been reported by Alzahrani et al., who compared the fracture resistance and failure mode of LDGC ECs with a proximal extension design with those of ECs with the conventional design and found that adding proximal boxes to the EC design did not negatively affect the fracture resistance of the restorations [16]. de Kuijper et al. investigated the influence of the extension of pulp chambers in restorations and the type of outline (enamel or dentin) on the load-to-failure resistance of LDS ECs after extensive cyclic loading in a chewing simulator and found that pulp extension or outline had no significant main effect on fracture load [18]. The findings of another study by El Ghoul et al. agree with the results of the present work. El Ghoul et al. determined that under axial and lateral loading, conventional preparations had the maximum fracture loads of 2914 and 1516 N, respectively, whereas the modified preparation had the maximum fracture loads of 3329 and 1871 N, respectively. Thus, in their study, the modified EC design showed higher fracture resistance than the conventional EC design [19]. Haralur et al. also demonstrated that LDS ECs with 4.5 mm occlusal thickness and 2 mm radicular extension had high mean fracture strengths of 3770 and 3877 N, respectively [13]. In a 2012 study, Biacchi and Basting compared the fracture strengths of indirect conventional crowns retained by glass fiber posts with those of ECs. They concluded that ECs exhibited greater resistance to compressive forces than conventional crowns [41]. Koosha et al. evaluated the fracture resistance of four different preparation schemes of EC restorations in mandibular molars. In the lingual group, the lingual wall was removed, and in the mesiodistal group, mesial and distal walls at up to 1 mm above the CEJ were removed. They concluded that the lingual groups showed significantly higher fracture resistance than the mesiodistal groups. However, the type of finish line did not have a significant effect on fracture resistance in the experimented groups [17].

Failure mode in any type of restoration is an indicator of the values of the fracture forces that the restoration can withstand before it fractures and is an indication of the durability of the restoration in the oral cavity before fracture. Failure modes were classified into three types in a study conducted by Demachkia et al. [33], who recorded similar types and percentages of failures in most of the samples. The present study found that type III, i.e., split fracture wherein the ceramic and tooth split vertically, and type I, i.e., wherein only the ceramic fractured or the ceramic and tooth fractured above the CEJ, were the predominant (approximately 70%) failure modes in groups 3 and 1, respectively, whereas types I and III failure modes were predominant (50%) in groups 4 and 2. In a systematic review that included three clinical and five in vitro investigations, Sedrez-Porto et al. compared ECs with conventional restorations, such as posts, inlays/onlays, and direct composites. The three clinical trials included in a systematic review showed that the efficacy rate of ECs ranged between 94% and 100% [39]. Bindl et al. also reported that for molar teeth, EC restorations could be clinically acceptable with an 87.1% survival rate, whereas classic crowns had a 97.0% survival rate after 55 $\pm$ 15 months of follow-up [43]. Donmez et al. also discovered that ZLS had the highest load-to-failure resistance ( $p\leqslant$ 0.002), whereas the difference between LDS and NLD was nonsignificant ( $p=$ 0.776) [24]. Similar findings were recorded by Haralur et al. for cases with extensions in the mesial and distal walls for molar ECs constructed from similar materials [13].

For ECs, the goal is to obtain a uniform, stable, and broad surface that is resistant to the compressive forces that are most prevalent on molars. The prepared surface is matched to the occlusal plane to guarantee stress resistance alongside the main axis of the tooth [12]. The cavity of the pulpal chamber ensures retention and stability. Its trapezoidal shape enhances the restoration’s stability in mandibular molars. Additional preparation is not needed. The pulpal floor’s saddle-shaped design increases its stability. This anatomy and the adhesive properties of the bonding material preclude the continued use of posts with root canals. Given that root canals do not require a particular shape, they are not weakened by drilling and are not subject to the stresses associated with post placement. The compressive stresses distributed over the cervical butt joint and pulp chamber walls are reduced [44, 45].

The presence of ferrules in full-coverage crowns supported by posts and cores increases fracture resistance and the number of fatigue cycles to failure. Einhorn et al. explored the consequence of ferrule incorporation on the failure resistance of molar ECs. Their findings demonstrated that adding ferrules to preparations increased the bondable dentin surface [12]. However, milling limitations prevented the reproduction of the interior surfaces of ECs. Consequently, the adaptation of the EC inner surface to the preparation appeared to simplify the preparation design due to ferrule addition. They concluded that although ferrule-containing EC preparations revealed substantially greater failure loads than regular EC restorations, the groups did not show differences in the calculated failure stress based on the existing surface area for adhesive bonding [14, 15]. In addition, EC preparations with a 1 mm ferrule were associated with reduced instances of failures [12]. These findings are similar to the results of Magne et al., who reported EC failure loads of 2606 N, and the findings of El-Damanhoury et al., who reported a mean fracture load of 1368 N for their LDS material [46, 47]. ECs are more likely to resist occlusal loading than conventional crowns according to Motta et al., who showed that the fracture resistance of the system increased with the occlusal thickness of the restoration. In contrast to conventional crowns, which have an occlusal portion that ranges from 1.5 mm to 2 mm in thickness, ECs have an occlusal portion that is 3 mm to 7 mm thick [48]. Nevertheless, the load-to failure resistance values reported in this study could be reached given that occlusal forces alternate from 600 N to 800 N in the posterior region [49] or even exceed these values in patients with bruxism [50].

The limitations of the present study are as follows: Given that it was an in vitro study, it could not recreate an oral environment. Only one material was used in this work. This material was isotropic and homogeneous in composition and optical and mechanical behaviors. The use of other systems or materials might have resulted in different outcomes. Additional comparisons between different materials can be performed. Further different in vivo and in vitro studies using other CAD/CAM materials with different preparation modes are required to test the clinical performance of EC restorations.

5. Conclusion

The following can be concluded within the limitations of this in vitro study:

Samples with an occlusal preparation depth of 3 mm and boxes showed the highest $\Delta E$ after immersion in coffee. The $\Delta E$ values of ECs were equal or marginally higher than the acceptable values.

The EC preparation with a 3 mm chamfer and ferrule displayed the highest maximum load. The EC with a 5 mm chamfer finish line had a marginally lower maximum load at fracture than other ECs. The preparation with a 3 mm chamber and boxes presented a slight improvement in maximum load, whereas the EC with a 3 mm chamber had the lowest maximum load at fracture.

Failures, such as ceramic fracture, split fracture, ceramic and tooth splitting above the CEJ or vertically, were predominant in samples with an occlusal preparation depth of 3 mm and ferrule and an occlusal preparation depth of 5 mm. Thus, in a clinical set up, ECs can be a reliable treatment option as mandibular molar crown.

Author contributions

NMA, AHS, and TSG: conceptualization, methodology, funding acquisition, resources, and supervision; JAA and MMA: sample preparation and collection and data curation; NMA and TSG; writing–original draft, investigation, formal analysis; BBW, JL, and HY: software, investigation, and formal analysis; ENA, ASA and SMA; tooth collection, laboratory work and supervision.

Funding

This research did not receive any funds.

Ethical compliance

Authorization for the study protocol was obtained from the institutional ethical review board (SRC/ GTH/2018-19/125).

Footnotes

Acknowledgments

None to report.

Conflict of interest

The authors declare no conflicts of interest.

References

Biacchi

Mello

Basting

. The endocrown: an alternative approach for restoring extensively damaged molars. J Esthet Restor Dent. 2013; 25: 383-90.

Elagra

. Endocrown preparation: review. Int J Appl Dent Sci 2019; 5: 253-6.

Lander

and Dietschi

. Endocrowns: a clinical report. Quintessence Int. 2008; 39: 99-106.

Bindl

and Mörmann

. Clinical evaluation of adhesively placed Cerec endo-crowns after 2 years – preliminary results. J Adhes Dent. 1999; 1(3): 255-65.

Alomran

. Endocrowns: a review article. Sch J Dent Sci. 2018; 5: 306-9.

Papalexopoulos

Samartzi

Sarafianou

. A Thorough Analysis of the Endocrown Restoration: A Literature Review. J Contemp Dent Pract. 2021; 22(4): 422-426.

Qamar

Alghamdi

AMS

Haydarah

NKB

Balateef

Alamoudi

Abumismar

Mathur

Minervini

. In Vitro Evaluation of Lithium Disilicate Endocrowns and Post and Core Crowns-A Systematic Review. J Funct Biomater. 2023; 14(5): 276.

Mezied

Alhazmi

Alhamad

Alshammari

Almukairin

Aljabr

Barakat

Koppolu

. Endocrowns Versus Post-core Retained Crowns as a Restoration of Root Canal Treated Molars – A Review Article. J Pharm Bioallied Sci. 2022; 14(Suppl 1): S39-S42.

Bernhart

Bra¨uning

Altenburger

and Wrbas

. Cerec3D endocrowns – Two-year clinical examination of CAD/CAM crowns for restoring endodontically treated molars. Int J Comp Dent. 2010; 13(2): 141-154.

10.

Göhring

Peters

. Restoration endodontically treated teeth without posts. Am J Dent. 2003; 16: 313-8.

11.

Contrepois

Soenen

Bartala

Laviole

. Marginal adaptation of ceramic crowns: A systematic review. J Prosthetic Dentistry. 2013; 110(6): 447-54.

12.

Einhorn

DuVall

Wajdowicz

Brewster

Roberts

. Preparation Ferrule Design Effect on Endocrown Failure Resistance. J Prosthodont. 2019; 28(1): e237-e242.

13.

Haralur

Alamrey

Alshehri

Alzahrani

Alfarsi

. Effect of different preparation designs and all ceramic materials on fracture strength of molar endocrowns. J Appl Biomater Funct Mater. 2020A; 18: 2280800020947329.

14.

Haralur

Alamri

Alshehri

Alzahrani

Alfarsi

. Influence of Occlusal Thickness and Radicular Extension on the Fracture Resistance of Premolar Endocrowns from Different All-Ceramic Materials. App Sci. 2020B; 10(8): 2696.

15.

Sevimli

Cengiz

Oruc

. Endocrowns: review. J Istanb Univ Fac Dent. 2015; 49(2): 57-63.

16.

Alzahrani

Hajjaj

Yeslam

Marghalani

. Fracture Resistance Evaluation and Failure Modes Rating Agreement for Two Endocrown Designs: An In Vitro Study. Appl Sci. 2023; 13: 3001.

17.

Koosha

Mostafavi

Jebelizadeh

Ghasemi

Hayerimaybodi

. Fracture Resistance and Failure Mode of Endocrown Restorations with Different Remaining Walls and Finish Lines. Open Dent J. 2023; 17: e187421062212260.

18.

de Kuijper

MCFM

Cune

Tromp

Gresnigt

MMM

. Cyclic loading and load to failure of lithium disilicate endocrowns: Influence of the restoration extension in the pulp chamber and the enamel outline. J Mecha Beha Biomedical Materials. 2020; 105: 103670.

19.

Ghoula

Ozcan

Tribstc

JPM

Salameh

. Fracture resistance, failure mode and stress concentration in a modified endocrown design. Acta Biomater Odontologica Scandinavica. 2020; 7(1): 110-119.

20.

Rayyan

Alauti

Abanmy

AlReshaid

, et al. Endocrowns versus post-core retained crowns for restoration of compromised mandibular molars: an in vitro study. Int J Comp Dent. 2019; 22(1): 39-44.

21.

Aldosari

Alshadidi

Porwal

, Ahmari

, Moaleem

Suhluli

, et al. Surface roughness and color measurements of glazed or polished hybrid, feldspathic, and Zirconia CAD/CAM restorative materials after hot and cold coffee immersion. BMC Oral Health. 2021; 21(1): 1-13.

22.

Alghazali

Hakami

AlAjlan

Alotaibi

Alabdulwahab

AlQuraishi

, et al. Infuence of the arabic-cofee on the overall color of glazed or polished porcelain veneers-study. Open Dent J. 2019; 13(1): 364-70.

23.

Alrabeah

Shabib

Almomen

Alhedeithi

Alotaibi

Habib

. Effect of Home Bleaching on the Optical Properties and Surface Roughness of Novel Aesthetic Dental Ceramics. Coatings. 2023; 13(2): 330.

24.

Donmez

Olcay

Demirel

. Load-to-Failure Resistance and Optical Characteristics of Nano-Lithium Disilicate Ceramic after Different Aging Processes. Materials. 2022; 15: 4011.

25.

Kang

Ryu

Kim

Park

. Effect of thickness on the translucency of resin-based composites and glass-ceramics. Dent Mater J. 2023; 42(1): 30-41.

26.

Shin

Park

Kim

Park

Roh

. Evaluation of the marginal and internal discrepancies of CAD-CAM endocrowns with different cavity depths: an in vitro study. J Prosthet Dent. 2017; 117: 109-15.

27.

Gaintantzopoulou

El-Damanhoury

. Effect of preparation depth on the marginal and internal adaptation of computer-aided design/computerassisted manufacture endocrowns. Oper Dent. 2016; 41: 607-16.

28.

Lee

Lin

Chiang

Chiu

. Crown morphology of the mandibular first molars with disto lingual roots. J Dent Sci. 2016; 11(2): 189-195.

29.

CIE ICoI. Colorimetry: official recommendations of the international commission on illumination. Paris: Bureau Central de la CIE; 1971.

30.

Alghazali

Burnside

Moallem

Smith

Preston

Jarad

. Assessment of perceptibility and acceptability of color difference of denture teeth. J Dent. 2012; 40(Suppl.1): e10-e17.

31.

Moaleem

Adawi

Alsharif

Alhazmi

Alshahrani

Hadi

RMA

, et al. Impact of Smokeless Tobacco on the Color Stability of Zirconia, Zirconia-Reinforced Lithium Silicate and Feldspathic CAD/CAM Restorative Materials: An In-Vitro Study. Coatings. 2022; 12: 207.

32.

Odaira

Itoh

Ishibashi

. Clinical evaluation of a dental color analysis system: The Crystaleye Spectrophotometer . J Prosthodontic Research. 2011; 55(4): 199-205.

33.

Demachkia

Velho

Valandro

Dimashkieh

Samran

Tribst

JPM

, de Melo

. Endocrown restorations in premolars: influence of remaining axial walls of tooth structure and restorative materials on fatigue resistance. Clin Oral Investig. 2023; 27(6): 2957-2968.

34.

AlGhazali

Burnside

Smith

Preston

Jarad

. Performance assessment of Vita Easy Shade spectrophotometer on colour measurement of aesthetic dental materials. Eur J Prosthodont Restor Dent. 2011; 19(4): 168-74.

35.

Alghazali

Preston

Moaleem

Jarad

Aldosari

Smith

. The Effects of Different Spectrophotometric Modes on Colour Measurement of Resin Composite and Porcelain Materials. Eur J Prosthodont Restor Dent. 2018; 26(4): 163-173.

36.

Alencar-Silva

Barreto

Negreiros

Silva

Pinto-Fiamengui

LMS

Regis

. Effect of beverage solutions and toothbrushing on the surface roughness, microhardness, and color stainability of a vitreous CAD-CAM lithium disilicate ceramic. J Prosthet Dent. 2019; 121: 711e1-711.e6..

37.

Haralur

Raqe

Alqahtani

, Mujayri

. Effect of Hydrothermal Aging and Beverages on Color Stability of Lithium Disilicate and Zirconia Based Ceramics. Medicina (Kaunas). 2019; 55(11): 749.

38.

Palla

Kontonasaki

Kantiranis

Papadopoulou

Zorba

Paraskevopoulos

Koidis

. Color stability of lithium disilicate ceramics after aging and immersion in common beverages. J. Prosthet. Dent. 2018; 119: 632-642.

39.

Nasim

Neelakantan

Sujeer

Subbarao

. Color stability of microfilled, microhybrid and nanocomposite resins–an in vitro study. J Dent. 2010; 38(Suppl.2): e137-e142.

40.

Mörmann

Bindl

Lüthy

and Rathke

. Effect of preparation and luting system on all-ceramic computergenerated crowns. Int J Prosthodont. 1998; 11: 333-9.

41.

Biacchi

Basting

. Comparison of fracture strength of endocrowns and glass fiber postretained conventional crowns. Oper Dent. 2012; 37: 130-6.

42.

Sedrez-Porto

Rosa

da Silva

Münchow

Pereira-Cenci

. Endocrown restorations: A systematic review and meta-analysis. J Dent. 2016; 52: 8-14.

43.

Bindl

Richter

Mörmann

. Survival of ceramic computer-aided design/manufacturing crowns bonded to preparations with reduced macroretention geometry. Int J Prosthodont. 2005; 18: 219-224.

44.

Zogheib

, Gde

Cardoso

Valera

Araújo

. Resistance to compression of weakened roots subjected to different root reconstruction protocols. J Appl Oral Sci. 2011; 19(6): 648-54.

45.

Fernandes

Dessai

. Factors affecting the fracture resistance of post-core reconstructed teeth: a review. Int J Prosthodont. 2001; 14(4): 355-63.

46.

Magne

Carvalho

Bruzi

Anderson

Maia

Giannini

. Influence of no ferrufe and no-Post buildup design on the fatigue resistance of endodontically treated molars restored with resin nanoceramic CAD/CAM crowns. Oper Dent. 2014; 39: 595-602.

47.

El-Damanhoury

Haj-Ali

Platt

. Microleakage of Endocrowns Utilizing Three CAD-CAM Blocks. Oper Dent. 2015; 40: 201-210.

48.

Motta

Pereira

Duda

, of substructure design and occlusal reduction on the stress distribution in metal ceramic complete crowns: 3D finite element analysis. J Prosthodont. 2014; 23(5): 381-389.

49.

Zimmermann

Ender

Attin

Mehl

. Fracture load of three-unit full-contour fixed dental prostheses fabricated with subtractive and additive CAD/CAM technology. Clin Oral Investig. 2020; 24: 1035-1042.

50.

Pizolato

Gavião

Berretin-Felix

Sampaio

, Junior

. Maximal bite force in young adults with temporomandibular disorders and bruxism. Brazilian Oral Research. 2007; 21: 278-83.

Comparison of color changes,fracture strengths,and failure modes of conventional endocrowns and endocrowns with different design modifications

Abstract

BACKGROUND:

OBJECTIVE:

METHOD:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Materials and methods

2.1 Study design

2.2 Inclusion criteria

2.3 Endodontic treatment

2.4 Teeth preparation

2.6 Color change measurements

2.8 Fracture test measurements

2.9 Failure type measurement

2.10 Statistical analysis

Table 1 Pairwise comparison of mean color change ( Δ ⁢ E )

Table 2 One way ANOVA of maximum load at fracture for different groups

5. Conclusion

Author contributions

Funding

Ethical compliance

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Pairwise comparison of mean color change ( $\Delta E$ )

Table 2
One way ANOVA of maximum load at fracture for different groups