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Table of Contents   
ORIGINAL ARTICLE  
Year : 2022  |  Volume : 25  |  Issue : 3  |  Page : 258-263
Fracture resistance of lab composite versus all-ceramic restorations in class II inlay cavity preparations: An in vitro study


1 Department of Conservative Dentistry and Endodontics, Genesis Institute of Dental Sciences and Research, Ferozepur, Punjab, India
2 Department of Pediatric and Preventive Dentistry, Dashmesh Institute of Research and Dental Sciences, Faridkot, Punjab, India
3 Department of Conservative Dentistry and Endodontics, Hazaribag College of Dental Sciences and Hospital, Hazaribagh, Jharkhand, India

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Date of Submission18-May-2021
Date of Decision22-Jan-2022
Date of Acceptance23-Jan-2022
Date of Web Publication13-Jun-2022
 

   Abstract 

Aim: The aim of this study was to evaluate and compare the fracture resistance of inlay preparations restored with indirect lab composite, conventional and translucent monolithic zirconia-based ceramics.
Materials and Methods: Fifty freshly extracted human maxillary premolars were selected for the study. Standardized inlay cavities were prepared and restored with indirect lab composite, conventional monolithic zirconia-based ceramic and translucent monolithic zirconia-based ceramic. After restoration each sample was subjected to axial compressive load with Universal testing machine. The force required to induce fracture was recorded in Newton (N).
Statistical Analysis Used: The data were analyzed using the one-way ANOVA test and Post hoc Bonferroni multiple comparison test.
Results: Results revealed that fracture resistance of prepared inlay cavities restored with conventional monolithic zirconia-based ceramics was found to be best followed by other groups. Group I > Group IV > Group V > Group III > Group II.
Conclusion: The fracture resistance of conventional monolithic zirconia-based ceramic inlays and translucent monolithic zirconia-based ceramic inlays were comparable with intact teeth but, indirect lab composite inlays showed lower fracture resistance than all.

Keywords: Conventional monolithic zirconia-based ceramic; indirect lab composite; inlay technique; translucent monolithic zirconia-based ceramic

How to cite this article:
Bhanot S, Mahajan P, Bajaj N, Monga P, Sood A, Yadav R. Fracture resistance of lab composite versus all-ceramic restorations in class II inlay cavity preparations: An in vitro study. J Conserv Dent 2022;25:258-63

How to cite this URL:
Bhanot S, Mahajan P, Bajaj N, Monga P, Sood A, Yadav R. Fracture resistance of lab composite versus all-ceramic restorations in class II inlay cavity preparations: An in vitro study. J Conserv Dent [serial online] 2022 [cited 2022 Jul 4];25:258-63. Available from: https://www.jcd.org.in/text.asp?2022/25/3/258/347341

   Introduction Top


Inlay is defined as an intra-coronal cast restoration that is designed to restore occlusal and proximal surfaces of posterior teeth without involving the cusps. Indirect technique is used to prepare inlay which helps in overcoming problems associated with direct filling techniques such as inadequate proximal or occlusal morphology, insufficient wear resistance, or inferior mechanical properties of directly placed filling materials.[1] This study made use of nanocomposite-based material named GC Solare (GC Solare, Hongo, Bunkyo-ku, Tokyo). It is a universal sculptable composite with advanced light scattering technology that contains single dispersion nanofillers homogeneously dispersed to provide high flexural strength and wear resistance.

Other materials used in this study were VITA In-Ceram YZ (VITA In-Ceram® YZ, North America) and Cercon xt – extra translucent zirconia (Cercon® xt, Dentsply DeTrey, Switzerland). Both these materials are composed of zirconium dioxide, yttrium oxide (Y2O3), hafnium oxide, aluminum oxide (Al2O3) and silicon dioxide. Only difference in composition is that VITA In-Ceram YZ has Y2O3 9% while the latter has 5% Y2O3.

Literature revealed lack of information regarding the effect of light-cured indirect lab composite and ceramic-based indirect inlay restorations on the fracture resistance of the teeth. Hence, the present study aimed to evaluate and compare the fracture resistance of light-cured indirect lab composite, conventional monolithic zirconia-based ceramic, and translucent monolithic zirconia-based ceramic materials when used as indirect inlay restorations.


   Materials and Methods Top


Fifty freshly extracted human maxillary premolars were selected for the study. The roots of the teeth were embedded in polyvinyl chloride ring (1.4 cm × 2 cm) using an autopolymerizing acrylic resin up to 2 mm below the cementoenamel junction.

The teeth were randomly divided into five groups of 10 samples each. Out of these, Group I served as positive control group which consisted of intact teeth. Inlay cavities were prepared in the teeth of remaining four groups.

Standardized inlay cavity was prepared for each sample. To ensure cutting efficiency, No. 271 bur was replaced after every four preparations.

Cavity preparations of all the groups were standardized in dimensions as per [Table 1]. This was achieved by measuring pulpal floor depth and occlusal isthmus width with calibrated Williams probe.
Table 1: Standardized of cavity preparations

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Impressions of the teeth with prepared inlay cavity were taken using vinyl-polysiloxane impression material. The impressions were poured using Type IV gypsum material to make a die model. All restorations were fabricated according to manufacturer's instructions for respective materials.

Group I (n = 10): Positive control group.

Samples of this group neither received any cavity preparation nor any restoration.

Group II (n = 10): Negative control group.

Inlay cavity was prepared for each sample of this group but left unrestored.

Group III (n = 10): Indirect lab composite inlay restoration.

Each prepared inlay cavity was restored with indirect lab composite.

For preparation of indirect composite inlay, composite material was condensed in increments and each increment was light cured for 40 s on die model. Occlusal anatomy was formed and then the inlay was heat cured in an oven at 100°C for 15 min. Each inlay was carefully finished with carbide burs and fine diamond points at low speed and light pressure.

Group IV (n = 10): Conventional monolithic zirconia-based ceramic inlay.

Each prepared cavity was restored with conventional monolithic zirconia-based ceramic.

Each die model was sprayed with antireflection coating which was necessary for achieving an optical image. Model was scanned with fully automated optical strip-light scanner connected to computer-aided design (CAD)/computer-aided manufacturing (CAM) machine [Figure 1]. All the margins of the cavity were marked accurately on the digital image and then the design of the framework was made by a special computer software program for inlays. After designing, the frameworks were milled from VITA In-Ceram YZ blocks. Then the milled frameworks were finished with a carbide bur. After the drying stage, all the frameworks were placed on a firing tray and sintered in a sintering furnace at 1530°C for approximately 7.5 h including the cooling phase at 200°C. Finally, all restorations were fired in a vacuum furnace at 700°C and glazed at 500°C until the glaze firing cycle was completely carried out.

Group V (n = 10): Translucent monolithic zirconia-based ceramic inlay.
Figure 1: Computer-aided design computer-aided manufacturing procedure

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Each prepared cavity was restored with translucent monolithic zirconia-based ceramic [Figure 2].
Figure 2: Inlay fabricated

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After preparing similar inlay designs, as explained in Group IV, the frameworks were milled from Cercon xt blocks by milling burs inside CAD/CAM milling machine. Further after drying, all the frameworks were placed on a firing tray and were sintered at 1500°C for 4.5 h including the cooling phase at 200°C. Finally, all restorations were fired in a vacuum furnace at 920°C and glazed at 450°C until sandblasting was done with Al2O3.

After the try-in procedures, inlay restorations were luted with dual-cure luting cement.

After luting of inlay restorations, the specimens were stored for 1 week and thermocycled. Then each specimen was subjected to axial compressive loading in a Universal testing machine having a metal sphere of 8-mm diameter applied vertically with crosshead speed of 1 mm/min. The force required to induce fracture was recorded in Newton (N). Fractured samples were observed under stereomicroscope and were classified as follows: Cohesive fracture of the tooth (CS), adhesive fracture at the interface of restoration and tooth (AD), cohesive failure of the restorative material (CM) and complete fracture of the sample (CO).

Data obtained were tabulated and put to statistical analysis.

The two-way ANOVA parametric test was carried out to test group variability. The level of significance was set at P < 0.05. The post hoc Bonferroni multiple comparison test was applied to check significance of difference between each group.


   Results Top


The results showed that the sound intact teeth (Group I) showed maximum fracture resistance compared to other groups. When results were compared, Group III (indirect lab composite inlays) showed less fracture resistance than Group I (intact teeth), Group IV (conventional monolithic zirconia-based ceramic inlays), and Group V (translucent monolithic zirconia-based ceramic inlays). Results were found to be statistically significant. When Group IV and Group V were compared to Group I and to each other, fracture resistance was higher for Group I and fracture resistance was slightly higher for Group IV than Group V [Graph 1]. The results were statistically nonsignificant.




   Discussion Top


Tooth loss due to caries, fracture and/or cavity preparation has been associated with decrease in fracture resistance of teeth.[2] In addition, if there is loss of marginal ridges, the decrease in fracture resistance is much more.[3] It has been found that loss of marginal ridges results in around 46% loss in tooth rigidity.[4] This loss increases the risk of fracture of the concerned tooth.

Direct restorations have few disadvantages such as lack of proper contact, poor wear resistance in contact areas, compromised marginal integrity which may result in postoperative sensitivity, and development of recurrent caries along the margins.[5]

Among the various esthetic dental restorative materials available in the market, we chose indirect lab composite and ceramic-based materials.[6] While selecting the type of restorative material, not only the patient's esthetic expectations, but also the mechanical properties of the materials were taken into consideration.

Indirect lab composite resin restoration could be a viable option to overcome the problem of polymerization shrinkage seen in direct composite resin restorations.[7] The secondary polymerization of the composite inlay, at a high temperature, improved the final set of material. Further, it allows the initial polymerization shrinkage and releases postcure stress before insertion of the inlay. It has also been seen that prevalence of postoperative sensitivity is less in indirect composite inlays compared to direct composite restorations.[8] Therefore, in this study indirect lab composite was chosen for restoration of prepared inlay cavity.

To evaluate fracture resistance, mesio-occlusal-distal (MOD) cavities of similar dimensions were prepared following a standardized procedure in each specimen. MOD cavity simulates the loss of the marginal ridges in clinical condition.

In our study, we measured the fracture resistance as it is probably one of the most important characteristics, capable of producing the durability of these restorations. Group I was taken as positive control group in which no cavity preparation was done. The mean fracture strength value of this group samples was 1207.76 ± 28. This value was well within the range of 882 N and 1568 N that was observed in previous studies.[9] Group I samples showed maximum values of fracture resistance as compared to other groups due to preservation of healthy tooth structure.

Group II served as negative control group in which standardized inlay cavity was prepared but left unrestored. Hence, the fracture resistance values of samples of this group were lowest (613.39 ± 91.51) among all other groups. It is due to loss of tooth structure, and no restoration was placed to replace the lost tooth structure.[10]

In Group III of this study, we used indirect lab composite (GC Solare). This composite material had nanofillers which could significantly improve its mechanical properties. It contains 30%–40% of silane-coated, 300-nm size strontium nanoceramic fillers.[11]

In this study, two types of zirconia, namely VITA In-Ceram YZ (Group IV – conventional monolithic zirconia-based ceramic) and Cercon xt (Group V – translucent monolithic zirconia-based ceramic) restorative materials, were used. They are both esthetic materials and have polycrystalline structure.[6] Polycrystalline ceramics are densely sintered zirconium oxide-based materials and are characterized by the absence of glass in their composition. The monolithic forms were used because they may have high flexural strength and fracture toughness. This is due to the presence of partially stabilized zirconia-based core material and it has property of transformation toughening in which yttria partially stabilizes zirconia polycrystalline structure and gets transformed from a tetragonal crystalline structure to a more voluminous monoclinic structure. This stops the subcritical crack propagation and also prevents stress corrosion, which are the main problems associated with other types of the ceramic materials.[12]

The development of a crack could also be facilitated by any preexisting faults in the restoration, which may occur during the manufacturing process and cyclic loading test. Taking this into account, CAD/CAM technology was used in this study. This fabricates an indirect restoration in a single session thus, avoiding crack formation. Similarly previous studies had also reported higher flexural strength or fracture load strength of zirconia which is about 2.5 times greater than pressable glass ceramic-based materials. Hence, zirconia-based materials could be more suitable for stress bearing restorative areas.[13]

When Group III (indirect lab composite) was compared with other groups, fracture resistance values were inferior to Group I, Group IV, and Group V. Difference in values was statistically significant. This could be due to lower modulus of elasticity of the laboratory-processed resin used which presented less restoration stiffness and greater distribution of stresses to adjacent tooth structures.[14] Many studies have demonstrated that samples restored with indirect composite resin showed more severe fracture as compared to samples restored with ceramic material.[15] This behavior was also found in the present study, in which premolars restored with indirect composites presented catastrophic fractures.

This result was in accordance with previous studies who had also concluded that zirconia-based ceramic inlays could recover tooth structure rigidity better than composite inlays.[14],[16]

When Group III was compared with Group II (negative control group), fracture resistance values were more for Group III, and the results were statistically significant. This meant that cavities restored with the indirect inlays improved the fracture resistance of tooth, but, as already had been explained, resistance values were less than intact teeth and teeth restored with ceramic-based inlays.

When Group IV (conventional monolithic zirconia-based ceramic) was compared with other groups, fracture resistance values were comparable (slightly lower) with Group I as results were statistically nonsignificant.

When Group V (translucent monolithic zirconia-based ceramic) was compared with other groups, fracture resistance values were comparable (slightly lower) with Group I as results were statistically nonsignificant.

Results of our study were in accordance with a previous study which had also reported that the fracture resistance of teeth restored with zirconia ceramic inlays was similar to that of intact teeth.[17]

When Group IV was compared with Group V, values were higher for Group IV, but, here also, results were statistically nonsignificant. This may be due to lower content of Y2O3 in conventional form as compared to traditional one.

The higher amount of yttria content in Cercon xt materials leads to the formation of more cubic phase in the crystal structure which increases translucency, decreases the flexural strength and fracture toughness of the materials.[18] Sandblasting could also be the reason for reduced biaxial flexural strength. This could be a possible explanation for the reduced flexural strength values of translucent monolithic zirconia-based ceramic.[19],[20]

In addition to the fracture resistance, it was important to analyze failure modes in samples to predict clinical performance and prognosis of the restored teeth. The most frequent failure mode observed in samples restored with indirect lab composite was complete fracture of the sample. The adhesively cemented lab composite restorations first absorbed the stresses and instantly transferred it to the tooth structure. Therefore, tooth and restoration both fractured. These similar forms of fractures were reported in other studies also.[21]

The adhesive failure also occurred in samples restored with indirect lab composite. It was mainly due to extra-oral polymerization which reduced double bond availability for chemical adhesion with the luting cement.[22] Hence, two types of fracture mode were seen in samples restored indirect lab composite: Complete failure and adhesive failure.

For zirconia-based ceramic inlay restorations most frequent failure mode observed was complete fracture of the sample. This finding agreed with Banditmahakun et al. who reported a similar type of fracture mode in their study.[23]

Based on the results of this study, we may use ceramic inlays for restoration of teeth with loss of both marginal ridges. Few limitations of the study are stated below:

  1. This study was conducted in in vitro conditions. Therefore, fracture resistance values may vary in clinical conditions
  2. In this in vitro study, only vertical axial forces were applied, but clinically lateral forces and fatigue loading, also play an important role
  3. Fracture resistance values of indirect restorations should have been compared to fracture resistance of direct restorative materials.



   Conclusion Top


Within limitations of the study, we can say that indirect zirconia-based ceramic materials provide adequate fracture resistance, but further studies are required to investigate fracture resistance of these materials against various types of forces before using them clinically for restoration of teeth.

Acknowledgments

The authors deny any conflict of interest regarding the publication of this paper.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
   References Top

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Correspondence Address:
Dr. Smridhi Bhanot
Department of Conservative Dentistry and Endodontics, Genesis Institute of Dental Sciences and Research, Ferozepur, Punjab
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcd.jcd_261_21

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