Original Article |
Corresponding author: Maryam Jafari ( mery4000j@gmail.com ) © 2023 Mehrak Amjadi, Soolmaz Heidari, Maryam Jafari, Amin Jabbari.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Amjadi M, Heidari S, Jafari M, Jabbari A (2023) Comparative evaluation of fracture resistance and failure modes in endodontically treated molars restored with zirconia endocrown and onlays. Folia Medica 65(2): 260-268. https://doi.org/10.3897/folmed.65.e75621
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Introduction: Developments in dental materials, CAD/CAM technologies and adhesive dentistry have improved the application of conservative restorations such as endocrowns and onlays. Among ceramics, zirconia has properties such as high strength, transformation toughening, chemical and structural durability, and biocompatibility, which enable zirconia to be used in the posterior area.
Aim: This study is a comparative evaluation of fracture resistance and failure modes in endodontically treated molars restored with zirconia endocrown and onlays.
Materials and methods: This study was performed on 20 human mandibular first molars with similar dimensions. After root canal treatment, the samples were divided into two groups: endocrowns and onlays (n=10). Restorations were made using a CAD-CAM milling machine with zirconia CAD blocks and, after cementation, subjected to 10,000 thermocycling and 500,000 fatigue cycle procedures, respectively. Each specimen was placed on a Universal Testing Machine and subjected to axial compressive force applied at a crosshead speed of 0.5 mm/min. The mean loads of failure of each group were statistically compared using the Student t-test. Chi-square tests were used to compare frequencies of failure modes among groups.
Results: Fracture resistance showed a statistically significant difference between endocrown (5374.6810±670.03445 N) and onlay (3312.5000±804.01428 N) (p<0.001). No statistically significant difference was detected in the distribution of failure types among the groups (p>0.05).
Conclusions: The fracture resistance of endocrown is substantially higher than that of onlay, and failure type does not differ in both restorations. Zirconia is a reliable material to use in conservative restorations.
failure mode, fracture strength, zirconia onlay, zirconia endocrown
failure in endodontically treated posterior teeth (ETPT) occurs due to reduced fracture strength and stiffness. Increased cuspal deflection during loading and the enlargement of cavity preparation rather than dehydration and physical changes in dentin are the primary reasons.[
So far, no general agreement has been reached on the best restoration for reconstructing ETPT.[
Advances in adhesive dentistry and emphasis on minimally invasive principles for increasing the restoration’s longevity led to the development of conservative restorations.[
When the facial and lingual surfaces of an ETPT are intact, a conservative partial coverage restoration such as onlay can be designed instead of full coverage restoration.[
On the other hand, as proposed by Pissis in 1995,[
A wide range of materials including feldspathic ceramics, ceramics reinforced with lithium disilicate, zirconia and PEEK have been used in preparing endocrowns and onlays.[18,
Densely sintered Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) ceramics has mechanical properties such as flexural strength (700-1200 MPa) and fracture resistance (>2000 N) due to its transformation toughening effect. These properties are considerably higher than those of other dental ceramics.[
Although improved mechanical properties are critical for the durability of zirconia restorations, the cementation procedure plays an important role for its clinical success. Conventional methods of adhesive cementation are challenging because of the lack of silica and glass phase in zirconia.[
So far, various surface treatments such as airborne-particle abrasion, tribochemical silica coating, selective infiltrative etching and phosphate monomer-containing primers prior to cementation have been suggested.[
Although some studies have shown higher bond strength of lithium disilicate restorations[
The effect of the type of material, depth of the pulp chamber, and restoration preparations on the strength of restoration’s fracture has been investigated in several studies. Several in-vitro studies showed that the fracture resistance of endocrowns increased with the deeper extension into the pulp chamber.[
The aim of this study was to compare the fracture strength and failure mode of endodontically treated teeth restored with zirconia endocrown and onlay.
Twenty human mandibular first molar teeth were selected for this study. Freshly extracted teeth free of anomalies with similar mesiodistal and buccolingual dimensions were measured at the cemento-enamel junction (CEJ) and were included in the study. A maximum deviation of 10% in dimensions was allowed. Internal root resorption, calcified root canals, cracks or fractures were the exclusion criteria. The specimens were ultrasonically cleaned and stored in 0.5% chloramine T disinfectant solution at 4°C for one week.
By using a dental surveyor, each specimen was vertically embedded into autopolymerizing resin (Luxatemp, DGM, Hamburg, Germany) in a cylindrical chamber at 2 mm apical to the cemento-enamel junction to simulate bone level.[
Specimens were randomly divided into two groups (n=10) according to their restorative preparations: endocrowns and onlays.
A 2-mm occlusal reduction of all the specimens was performed with a tapered round-end diamond bur (018, 850 L, Teezkavan, Tehran, Iran). A butt-joint margin with no ferrule was designed.
The retentive form of the pulp chamber for endocrown restorations was achieved by eliminating all the undercuts with a tapered flat-end diamond bur (018, 847). The remaining undercuts were covered with glass ionomer material (GC Corporation, Tokyo, Japan). After a uniform taper of 7° in the pulp chamber was prepared, 4 mm height was standardized with a graduated periodontal probe.[
Before preparing the teeth for onlay restorations, according to the 4.0 mm height of the pulp chamber, 2.0 mm of the chamber was filled with composite resin material (GC Gradia Direct posterior A3, GC Corporation, Tokyo, Japan). The general principles for adhesive onlay restorations were applied.[
All the internal line angles of the preparations were rounded and a final finishing and polishing with polish bur and mullet (304514, 100, Composhine, Teezkavan, Tehran, Iran) was performed.
Restorations were fabricated with monolithic zirconia (Superfect Zir HT, AiditeR High technical ceramics Co Ltd, Qinhuangdao, China) blank.
A laboratory scanner (RainbowTM scanner prime, Dentium, Seoul South Korea) was used for making digital impressions of the teeth preparations. The data was kept as Standard Tessellation Language (STL) file and the restorations were designed. Cement film thickness of 80 µm was selected.[
The restorations were sandblasted (Aeroetcher sandblaster, Parkell INC, NY, USA) using 50 μm Al2O3 air abrasion at 2-bar pressure for 1 minute, and then cleaned in an ultrasonic water bath for 10 minutes.[
Enamel surfaces of the teeth were selectively etched with 37% phosphoric acid (Morva Etch, Morva Bon, Iran, Dentaj. IR) for 20 seconds followed by water rinse and air drying. Primer A and primer B (Ivoclar Vivadent AG, Schaan/Liechtenstein) were mixed with the equal ratio of 1:1 and gently applied on the teeth within 30 seconds.
After painting mono-component silane (Mono Bond N, Ivoclar Vivadent AG, Schaan/Liechtenstein) on the intaglio surface of the restorations within 60 seconds, the dual cure resin cement Multilink N (Ivoclar Vivadent AG, Schaan/Liechtenstein) was applied and seated on each tooth with finger pressure. The excess cement was removed after 4 seconds tack light curing by a scaler. Each surface was light cured (Woodpecker, DTE ®LUX E, Guilin China) definitively at 1200 mw/cm2 intensity for 40 seconds. The specimens were stored in a humid environment at 37°C for 72 hours to finalize the polymerization.
All specimens were thermocycled (SD mechatronic thermo cycler, Feldkirchen, Germany) for 10000 times between 5°C and 55°C with a dwell time of 30 seconds in each bath. The transfer time was 10 seconds. In addition, 500,000 cyclic loads (chewing simulator CS-4, SD Mechatronic, GmbH, Germany) were performed with a stainless steel ball (diameter of 4 mm). The magnitude of the force was 100 N, which was applied on the center of the occlusal surface at the frequency of 4 Hz and 0.6 mm cut off. The specimens were subjected to a universal testing machine (Zwick / Roell Z050, ULM, Germany) after no cracks or fractures was observed under the stereomicroscope. An axial compressive load was performed vertically on the centric fossa of the restorations by a metal sphere of 6 mm in diameter. The cross-head speed of 0.5 mm/min and 50 kN load cell was applied until the failure happened.
Fractured specimens were observed under a stereomicroscope (Nikon SMZ 1500, Tokyo, Japan) at magnifications 10× by one operator and the failure types were classified as follows:
Type 1: adhesive failure between restoration and teeth
Type 2: cohesive failure within the restoration
Type 3: mixed type failure above the CEJ
Type 4: mixed type failure below the CEJ
Fractures above the CEJ could be repairable, while fractures below the CEJ and extending to the root are considered irreparable.[
Fracture strength of the specimens was analyzed using the Student t-test. Evaluation of failure types was performed using chi-square test. The analysis was done using Statistical Package for Social Sciences (SPSS), version # 21(SPSS Inc.IL, USA). The level of significance was 0.05.
The study was approved by the research ethics committee of Qazvin University of Medical Science (IR.QUMS.REC.1398.157).
The mean fracture strength (load at fracture in Newton) for the two tested groups were as follows: endocrowns: 5374.6810±670.03445 N and onlays: 3312.5000 N (Table
According to the frequency of the failure modes, in the endocrown group, 10% type 1, 10% type 2 and 80% type 4 failure mode were detected. Furthermore, in the onlay group, 10% type 2 and 90% type 4 failure mode were observed (Table
Distribution of compressive fracture strength (N) by endocrown and onlay restorations
Mean | SD | Minimum | Maximum | p | |
Endocrown | 5374.6810 | 670.03445 | 4428.82 | 6870.30 | <0.001 |
Onlay | 3312.5000 | 804.01428 | 2149.87 | 4998.71 |
Stereomicroscopic image ×10 presenting type Ⅱ (cohesive failure within the restoration) failure mode of onlay specimen.
Stereomicroscopic image ×10 presenting type (cohesive failure within the restoration) failure mode of endocrown specimen.
Stereomicroscopic image ×10 presenting type (mixed type failure below the CEJ) failure mode of endocrown specimen.
There is no consensus regarding the most successful treatment procedure in restoration of endodontically treated posterior teeth (ETPT). It seems that the higher amount of preserved coronal tooth structure has a significant effect on the long-term survival of these teeth.[
Conservative restorations such as onlays and endocrowns allow a minimal tooth structure removal which can preserve and strengthen the remaining dental tissues.[
A monolithic zirconia restoration is stronger than the bi-layered one. Thus, they may be used for restoring teeth without removing an excessive amount of sound tooth structure.[
The mandibular molar teeth were used in this study as they are the best teeth for receiving endocrown restorations.[
In several studies, thermocycling was combined with cyclic loading to represent an artificial aging.[
After the aging procedure, all of the specimens were examined under a stereomicroscope and no fracture or cracks were detected. Wang et al.[
The mean fracture strength was 5374.6810±670.03445 N in endocrowns and 3312.5000±804.01428 N in onlay restorations, which both were in a higher range in comparison with other studies. The mean fracture strength range obtained from zirconia onlay and endocrown restorations in different studies was 1011.73-2568.76 N[
The type of zirconia material and the aging procedures, which are used in studies, are responsible for the different results. A presintered monolithic zirconia with 95% crystallization (Superfect Zir, AiditeR, High technical ceramics Co, Ltd, Qinhuangdao, China) with 10000 thermocycling procedure and 500,000 cyclic loads were used in this study. Elashmawy et al.[
Different preparation design and pulp chamber extension are the other critical factors.
The fracture strength of endocrown restorations was significantly higher than the strength of onlay restorations (p<0.001). This may be explained by the main difference between the groups, which was the depth of pulp chamber. The pulp chamber extension of endocrowns and onlays were 4 mm and 2 mm, respectively. In some previous studies,[
The result of the present study, which revealed higher fracture resistance in endocrown restorations, is in agreement with the results of some previous studies evaluating restorations made of different materials. Hamdy et al. reported higher fracture resistance of lithium disilicate endocrowns than the resistance of onlays and inlays.[
Similar observation of greater fracture resistance of hybrid ceramic endocrowns in comparison with onlays and inlays was also reported by Kassis et al.[
In this study, the monolithic zirconia restorations were luted with Multilink N, which is a dual cure self-adhesive resin cement. Besides the simplicity of cementation procedure of a self-adhesive cement, the self-cure mode ensures optimal polymerization and the phosphate monomers may guarantee a durable bonding both to enamel and dentin and to zirconia surface.[
The surface pretreatment protocol used for zirconia restorations in this study involved sandblasting, which has reported to be the most effective according to some previous studies.[
Stereomicroscopic analyses revealed the majority of type 4 failure mode under compressive loading in both groups, which was irreparable. None of the specimens showed type 3 failure mode. Both groups showed type 2 failure mode and type 1 was only detected in the endocrown group. In line with the current study, some previous studies[
In another study, Saridag et al.[
The results of this study are in contrast with the results reported by Erturk et al.[
Harsha et al.[
This study had a number of limitations. Thermocycling and static chewing simulator was used to simulate the conditions of oral environment, but it was limited to 10000 and 500000 cycles. Due to the high fracture strength of the zirconia restorations in the present study, more studies are needed to evaluate the effect of long-term fatigue test on this material. Also, involving artificial saliva with dynamic cyclic loading and non-axial loading directions are proposed for the further studies.
Compressive fracture strength of endodontically treated molar teeth reconstructed with zirconia endocrown restoration is higher than that of onlay restorations and the failure type mode was not seen to be different between the zirconia endocrown and onlay restoration.