|Year : 2023 | Volume
| Issue : 5 | Page : 469-475
Fracture strength of direct and indirect composite veneers after aging: An in vitro study
Aishwarya Menon, Sowmya M Kumar, Chethan Hegde
Department of Prosthodontics, Nitte (Deemed to be University), AB Shetty Memorial Institute of Dental Sciences (ABSMIDS), Mangalore, Karnataka, India
|Date of Submission||20-Jul-2023|
|Date of Decision||25-Sep-2023|
|Date of Acceptance||25-Sep-2023|
|Date of Web Publication||30-Oct-2023|
Dr. Sowmya M Kumar
Department of Prosthodontics, Nitte (Deemed to be University), AB Shetty Memorial Institute of Dental Sciences (ABSMIDS), Mangalore, Karnataka
Source of Support: None, Conflict of Interest: None
Aim: The number of patients seeking veneers for minor changes in the color, shape, and size of their anterior teeth has increased. On the basis of this requirement, with the advances in physical and optical properties, composite resins have become viable materials to be used for veneers. There is a paucity of studies that highlight the use of composite resins for anterior esthetic rehabilitations. Hence, this study aimed to compare the fracture strengths of direct and indirect composite veneers. Materials and Methods: This study was conducted on a total of 56 maxillary central incisor samples, divided into two groups: direct composite (n = 28) and indirect composite (n = 28) veneers. Tooth preparations were done, and composite veneers were made via the two techniques. Thermocycling was carried out to age the restorations for half the samples from both groups. The fracture test was performed in a universal testing machine where the load was applied from the incisal direction at the veneer–tooth interface. Results: Direct composite veneers showed a mean fracture strength of 485 ± 147 N before and 438 ± 199 N after thermocycling. Indirect composite veneers showed a mean fracture strength of 409 ± 179 N before and 254 ± 135 N after thermocycling. The most common mode of failure for direct composite veneers was veneer chipping and fracture, whereas debonding was most frequently noted for indirect composite veneers. Statistical analysis used: Unpaired t test was used to study the fracture strengths among the two groups before and after thermocycling. The comparison of frequencies of a mode of failure was investigated using the chi-square test. Conclusions: The direct composite resin veneers showed superior fracture strength, both before and after thermocycling.
Keywords: Direct composite, fracture strength, indirect composite, thermocycling
|How to cite this article:|
Menon A, Kumar SM, Hegde C. Fracture strength of direct and indirect composite veneers after aging: An in vitro study. J Int Oral Health 2023;15:469-75
|How to cite this URL:|
Menon A, Kumar SM, Hegde C. Fracture strength of direct and indirect composite veneers after aging: An in vitro study. J Int Oral Health [serial online] 2023 [cited 2023 Dec 2];15:469-75. Available from: https://www.jioh.org/text.asp?2023/15/5/469/388787
| Key messages:|| |
Composite resins can serve as durable materials for use in the treatment of dental veneers. They have optimal functional and esthetic properties, along with being cost-effective for the patient and time-effective for the clinician.
| Introduction|| |
Abnormalities in the color, form, structure, and position of the anterior teeth can cause patients to have significant aesthetic issues. The development of techniques aimed at restoring or enhancing the natural look of human dentition is a direct outcome of the patient’s desire for cosmetic dentistry treatments. Dental veneers have played a pivotal role in providing the best aesthetic outcomes while being conservative. The use of a minimally invasive laminate veneer is indicated for teeth that need morphologic adjustments due to their unfavorable shape, lack of volume, or size, minor tooth alignment, discoloration that is resistant to bleaching techniques, diastema closure, restoration of localized enamel malformations, minor chipping and fractures, and misshapen teeth. Veneers should not be used on teeth that are badly malaligned, have extensive previous restorations, have significant parafunctional activity, have deep vertical overlap anteriorly without any horizontal overlap, or have soft-tissue pathology. The decision of which material to use depends on the patient’s socioeconomic conditions and the time available for treatment, among other factors. Throughout the years, researchers and manufacturers have worked to develop novel materials with improved aesthetic properties. The materials presently used for veneers are usually ceramics and composite resins. Although ceramics have good wear resistance and color stability, a smooth surface, and superior aesthetics, they are inherently brittle and abrasive for the opposing natural tooth structure. Modern composite resins have good aesthetic and physical properties, are simple to use, do not require complicated equipment, and provide a relatively inexpensive treatment alternative for people of all ages.
Composite veneers can be implemented by two main techniques: the direct technique and the indirect technique. Direct composite (DC) veneer procedures can be completed in one appointment, do not require lengthy laboratory procedures, and are much less expensive when compared to ceramics. Indirect composite (IC) veneers have better abrasion resistance than DC veneers and are easier to prepare outside the oral cavity, as well as to lute and repair, and thus both materials are a viable alternative to ceramics.
The oral cavity is subject to fluctuating temperatures during the consumption of hot and cold food and drinks. This could lead to thermally induced changes between the tooth and restoration, such as chip fractures of the composite. According to the literature, the most persistent causes of veneer failure are fracture and debonding. Both DC and IC resins are now routinely used for the fabrication of veneers, but there is limited evidence comparing their fracture strengths after aging. Hence, the study aimed to evaluate the fracture strength of DC and IC veneers before and after thermocycling and to determine the manner of sample fracture.
| Materials and Methods|| |
Sample size estimation was done using nMaster software version 2 and 56 maxillary central incisors were selected and randomly assigned to two groups: Group A were DC veneers (n = 28) and Group B were IC veneers (n = 28) [Table 1]. Ethical clearance was obtained from the Institutional Ethical Clearance Board, with approval number ABSM/EC/92/2021. All the teeth were stored in water at room temperature until use. Teeth were embedded in polymethylmethacrylate resin (RR Cold Cure, DPI, India) up to 1 mm below the cementoenamel junction. Before preparation, an impression was made of each tooth using a high-precision condensation silicone (Zeta Plus, Zhermack, Italy) to obtain an index to ensure the consistency of tooth reduction.
The first step in preparing teeth for veneers was to use a depth cutting bur (DM-305, Mani, Japan), to establish 0.5 mm depth orientation grooves from the cervical to the incisal edge, without reducing the incisal length [[Figure 1]A]. A tapered round-end diamond bur (TR-12, Mani) was used to remove a uniform thickness of facial enamel by joining the depth orientation grooves. The tooth preparation was finished and smoothened using a tapered round-end diamond bur of extra fine grit (TR-13EF, Mani). This was done to avoid any areas of stress concentration within the tooth preparation.
|Figure 1: (A) Depth orientation grooves of 0.5 mm. (B) Die stone cast. (C) Indirect composite veneer on the cast|
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Composite veneer build-up
The teeth of Group A were restored immediately after preparation. The enamel was etched for 15 s using 37% phosphoric acid (Scotchbond Etchant, 3M ESPE, Minnesota), rinsed for 15 s, and then gently air dried. Following this, primer and adhesive (Prime&Bond NT, Dentsply Sirona) were applied. According to the manufacturer’s instructions, the surface was kept completely wet for 20 s after application, and then air dried for 5 s and light cured for 20 s. Following this, the DC veneer was built up on the tooth using nanohybrid composite resin (Beautifil II, Shofu, Japan) and light cured for 20 s. The thickness of the veneer was confirmed with the use of the index made before tooth preparation. The veneers were then finished and polished with aluminum oxide-impregnated discs (2382F and 2382SF, Sof-Lex, 3M ESPE).
For Group B, first, an impression of the tooth preparation was made using polyvinylsiloxane impression material. As per ISO 4823, light body/type 3 (Express XT, 3M ESPE) was used to achieve accurate detail reproduction of the tooth preparation margins, and putty/Type 0 (Soft Putty, 3M ESPE) was used to complete the impression of the tooth. It was then cast using die stone (Kalrock, Kalabhai Karson Pvt Ltd, India). Once the cast was attained [[Figure 1]B], a single layer of die spacer (Ceramage Spacer, Shofu) was applied to the tooth preparation. It dries in 1-2 minutes and forms a silicone-like layer to provide space for the luting agent. Following this, a layer of separating medium (Ceramage Sep, Shofu) was applied, avoiding the margin region. This helps with isolation and facilitates the removal of the veneer from the cast. The veneer was then built up on the cast [[Figure 1]C] using IC resin (Ceramage, Shofu), and its thickness was verified. It was cured in the light cure unit (bre. Lux Power Unit 2, Bredent, UK) for 3 min. Etching and bonding of the tooth substrate were performed in the manner described for the DC veneers. After the completion of the cure, the IC veneer was removed from the cast and luted onto the tooth using a light-cured composite luting agent (Calibra Veneer Esthetic Resin Cement, Dentsply Sirona). The luting agent was tack cured for 2 s to facilitate the removal of excess resin, then cured for 20 s.
Thermocycling was used on samples from Groups A2 and B2 to age the restorations in a clinical setting (TC-4, SD Mechatronik GmbH, Germany). The restorations underwent 1,000 cycles between 5°C and 55°C [Figure 2].
Fracture strength test
To determine the fracture strength and mechanism, each sample underwent a fracture test. A Universal Testing Machine (Zwick/Roell Z020, Germany) was used to conduct the test. At the veneer–tooth interface at the incisal edge, the load was applied using a stainless-steel spherical load parallel to the long axis of the tooth at a crosshead speed of 1.0 mm/min [Figure 3]. The load (N) at failure was measured when there was either a noticeable drop in the load curve or visible and/or auditory indications of fracture. The load was applied until fracture occurred.
The data were analyzed using a statistical package for the social sciences software version 26 (SPSS, Chicago, Illinois). Normality of numerical data was checked using Shapiro-Wilk test and it was found that the data followed a normal curve; hence parametric tests have been used for comparisons. An unpaired t test was used for intergroup comparison to study the mean differences in fracture strengths among the two groups before and after thermocycling. The comparison of frequencies of a mode of failure was investigated using the chi-square test. The significance threshold was set at P < 0.05.
| Results|| |
The values of fracture strength of DC and IC veneers are presented in [Table 2].
There was a statistically non-significant difference seen for the values between the groups before thermocycling (P > 0.05) [Table 3].
|Table 3: Intergroup comparison of mean fracture strength of composite veneers|
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There was a statistically highly significant difference seen for the values between the groups (P < 0.01) after thermocycling with higher values in Group A2 [Graph 1].
In Groups A1 and B1, there was a statistically highly significant difference seen for the frequencies of a mode of fracture between the groups (P < 0.01) with a higher frequency for chipping of veneer in Group A1 and debonding in Group B1 [Table 4].
In Groups A2 and B2, there was a statistically highly significant difference seen for the frequencies of a mode of fracture between the groups (P < 0.01) with a higher frequency for veneer fracture in Group A2 and debonding in Group B2 [Table 5].
| Discussion|| |
The overall decline in caries rates, superior periodontal disease control, and improved oral hygiene measures have all contributed to prolonging the lifespan of dentition. With the advent of cosmetic dentistry, patients are now more aware, and eager to undergo procedures in dentistry exclusively for aesthetic purposes. The most commonly sought-after treatment is that of dental veneers. The survival rate of veneers depends on a variety of factors such as the thickness of the remaining tooth structure, the material used, the preparation design, tooth function and occlusion, the degree of tooth damage, the vitality of the tooth to be treated (vital or non-vital), and the level of skill of the clinician.
Various tooth preparation designs were introduced to improve aesthetic outcomes. The main designs commonly followed are (1) window preparation, (2) feather preparation, (3) bevel preparation, and (4) incisal overlap preparation. The decision on which tooth preparation design should be followed would differ from case to case and is primarily based on the clinician’s preference and judgment. In this study, a feather tooth preparation design of 0.5 mm depth was done, from the cervical to the incisal edge, with a chamfer margin. This design has the advantage of being conservative, preserving the tooth structure while enhancing its aesthetics. Most of the literature recommends maintaining the interproximal contact to preserve more tooth structure and provide a positive seat for cementation. The finish lines, when placed facially to the contact area, also effectively conceal the restorative margins. However, in certain situations such as diastema, caries, or malaligned teeth, better aesthetic outcomes may be obtained by removing the interproximal contact. Tooth preparation for veneers is also generally limited to the enamel which provides superior bond strengths due to the high inorganic mineral content of enamel.
The choice of which material to use must be based on evidence. Porcelain is one of the most widely used materials and provides predictable aesthetic and functional outcomes. Morimoto et al. identified 899 studies in a systemic review, of which 13 were examined. The overall survival rate after 9 years was 89%; glass-ceramic veneers had a greater survival rate (94%) than feldspathic porcelain veneers (87%). The failure modes included debonding (2%), fracture (4%), caries (1%), and discoloration (2%). A major drawback of ceramic veneers is that they cannot be repaired. This renders a small chip or fracture of the veneer a tedious task to correct.
The field of composite resins has three distinct phases: the polymerizable resin, the filler, and the filler-resin interface. The filler content of resin restorations is responsible for the majority of their mechanical characteristics. As a result of their ability to combine nanoparticles with submicron particles to improve mechanical, chemical, and optical properties, nanohybrid resin composites are frequently used in this context. When added to resins, silica-based fillers give the resins a good elastic modulus, a greater strength, and an acceptable refractive index. Hence, these DC resins serve as feasible alternatives to porcelain for veneers. They have several other advantages, such as ease of use, being simple to repair, requiring a lesser number of appointments, and the participation of dental laboratories and technicians becoming unwarranted, which further reduces the cost of treatment. It also involves little to no reduction of tooth structure as a minimum thickness of the material is not required to be met. The DC resin used in this study was a DC resin with nanohybrid fillers, which provides good shade stability, better polishability, and an outstanding surface shine. Unfortunately, DC resins come with their limitations. Bis-GMA-based resin composites show significant polymerization-induced linear shrinkage of approximately 1%–6% which can cause debonding at the restoration/tooth interface and the formation of marginal gaps, leading to discoloration and even secondary caries. It may also cause postoperative sensitivity in vital teeth. They require periodic repolishing to increase their longevity, especially shade and surface luster. To overcome these limitations, IC resins were introduced in the market. These materials undergo a post-cure with extra-oral light and heat which increases the degree of conversion and reduces the incidence of polymerization shrinkage. The only shrinkage that may influence interfacial adaptation occurs in the cement layer. Other clinical advantages include accurate marginal integrity, better wear resistance, optimum proximal contacts, superior anatomic morphology, and excellent aesthetics. In comparison to porcelain, IC resins have shown a stronger ability to withstand compressive loading forces and lower impact forces by 57%. Gresnigt et al. found that ceramics have a greater elastic modulus (65–90 GPa) than composite resins (1.6–12.4 GPa), which means that the former is stiffer and less resilient than the latter. As more durable materials, composite resins can dissipate stresses more effectively which is shown in this study. Da Viega et al. evaluated the durability and clinical outcomes of DC and IC restorations in posterior teeth in a systematic review and meta-analysis. There was no indication that DC and IC restorations had different clinical lifespans. Crins et al. assessed the survival and failure behavior of minimally invasive DC and IC resin restorations in a randomized trial in people with moderate to severe tooth wear. After a 3-year follow-up period, they concluded that IC restorations in anterior teeth and DC restorations in both anterior and posterior teeth performed satisfactorily. It was also reported that individuals with severe tooth wear should not use IC for molar restorations.
Thermocycling is an in-vitro process that aims to age the restorations in a way that simulates intra-oral use. Various clinical studies show that the most frequent reasons for the failure of composite veneers are their fracture. Demarco et al. systematically reviewed the long-term success and causes of failure of anterior composite restorations. The aesthetics of a tooth or restoration are, either directly or indirectly, related to failures in anterior restorations. In the studies that were taken into consideration, restoration failure was consistently attributed to either tooth or restoration fracture, with annual rates of failure ranging from 0% to 4.1%. With composite resins being used more frequently for veneers, it opens the question of whether DC or IC, when used as veneers, perform better in terms of fracture resistance. Test for fracture strength in this study was performed with a universal testing machine. The load at failure along with the mode of failure were analyzed, before and after thermocycling. DC and IC veneers showed fracture strength values of 438 N and 254 N, respectively, after thermocycling. This value was statistically significant. This could be ascribed to the ability of DC to adhesively bond to the underlying tooth structure and reinforce the tooth, hence showing higher values. This was confirmed by Tomer et al. The filler content of DC resin was nanohybrid, whereas that of IC resin was a micro-hybrid. Hence, the higher filler content could also be attributed to the greater fracture strength of DC resin. DC veneers showed a mean fracture strength of 485 N before and 438 N after thermal cycling. Thermocycling did not have a statistically significant effect on DC veneers. This can be due to the direct adhesion of the composite to the underlying tooth structure, without any intervening layer in between. On the contrary, thermocycling led to a statistically significant difference in the values for IC veneers’ fracture strength. Before thermocycling, IC veneers had a fracture strength of 409 N and 254 N after. The presence of a resin luting agent between the veneer and tooth structure might influence the fracture strength values. However, Nandini et al. concluded that given that luting cement and composites have a comparable composition, the marginal adaptability of composites is superior to that of ceramics. The overall high values for fracture strength could also be accredited to the minimal tooth reduction carried out. Excessive tooth preparation leads to an increased dependence on dentin adhesive systems to effectively retain the restoration.
The nature of the veneers’ fracture was also investigated. In Group A1, veneer chipping occurred more often before thermocycling, whereas veneer fracture occurred more frequently in Group A2. This shows that as restorations age, their capacity to withstand loads decreases, making more severe forms of failure, like fracture more likely. Additionally, it has been reported that failure rates in non-vital teeth are almost double those seen in vital teeth. Fracture of the tooth occurred only in two samples in Group A2 and in all the other cases of veneer chipping, fracture, and debonding, the advantage of DC remains that it is reparable intraorally. In Groups B1 and B2, debonding was most frequently noted. IC veneers mostly underwent debonding of the veneers, without chipping or fracture, at lower loads of force. If all other conditions are favorable, veneers that debond without being damaged clinically could be luted back. It is noteworthy that the loads at which the veneers failed were above the usual loads borne by the anterior teeth. Therefore, it can be concluded that both DC and IC are suitable materials for the fabrication of anterior veneers. It can be inferred based on the various benefits and drawbacks of these two materials that clinicians may prefer to use DC over IC for veneers due to their adequate fracture strength and the fact that the treatment can be completed within one appointment.
This study has some limitations. First, it is difficult to properly duplicate the intraoral environment via thermocycling because of its complex and distinctive features. Second, the fracture load applied was in a static dimension, which does not accurately recreate the dynamic forces occurring intra-orally. Third, the study was carried out on maxillary central incisor teeth. Depending on the tooth being evaluated, the fracture strength values would differ correspondingly.
| Conclusion|| |
DC veneers showed a mean fracture strength of 485 ± 147 N before, and 438 ± 199 N after thermocycling, whereas IC veneers showed a mean fracture strength of 409 ± 179 N before, and 254 ± 135 N after thermocycling. DC veneers showed higher values of fracture strength than IC veneers, both before and after thermocycling. The values of fracture strength after thermocycling for Groups A and B were statistically significant.
The authors thank Nitte (Deemed to be University), AB Shetty Memorial Institute of Dental Sciences (ABSMIDS) for their support.
Financial support and sponsorship
Conflict of interest
There are no conflicts of interest.
Dr. Aishwarya Menon: concept, design, experimental study, data acquisition, literature search, data analysis, manuscript writing. Dr. Sowmya M.K.: design, manuscript preparation, manuscript review. Dr. Chethan Hegde: concept, statistical analysis, manuscript editing, and manuscript review.
Ethical policy and Institutional Review Board statement
This experimental study was approved (No. ABSM/EC/92/2021) by the Nitte (Deemed to be University), AB Shetty Memorial Institute of Dental Sciences (ABSMIDS), Mangalore, India.
Patient Declaration of consent
Data availability statement
The data presented in this study are available on request from the corresponding author.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]