Effects of pulp capping materials on fracture resistance of Class II composite restorations.

OBJECTIVE: The aim of this study was to investigate the effect of cavity design and the type of pulp capping materials on the fracture resistance of Class II composite restorations.
MATERIALS AND METHODS: Sixty freshly extracted, sound molar teeth were selected for the study. A dovetail cavity on the mesio-occlusal and a slot cavity on disto-occlusal surfaces of each tooth were prepared, and the teeth were divided 4 groups which one of them as a control group. The pulp capping materials (TheraCal LC, Calcimol LC, Dycal) applied on pulpo-axial wall of each cavity, and the restoration was completed with composite resin. The teeth were subjected to a compressive load in a universal mechanical testing machine. The surfaces of the tooth and restoration were examined under a stereomicroscope. The data were analyzed using factorial analysis of variance and Tukey's test.
RESULTS: For pulp capping materials, the highest fracture load (931.15 ± 203.81 N) and the lowest fracture load (832.28 ± 245.75 N) were calculated for Control and Dycal group, respectively. However, there were no statistically significant differences among all groups (P > 0.05). The fracture load of the dovetail groups was significantly higher than those of the slot cavity groups (P < 0.05).
CONCLUSION: Dovetail cavity design shows better fracture resistance in Class II composite restorations, independent of used or not used pulp capping materials.

INTRODUCTION

There is a growing demand for new restorative materials in dental treatment procedures. With the development of new adhesives, resin composites are rapidly becoming the primary restorative material for direct restoration of posterior or anterior teeth due to their ability to bond to the dental structure.[12] Especially for Class II restorations, resin composites are the first choice in clinics.

Resin composites are technique-sensitive materials. Thus, it is difficult to use them in the proximal boxes of Class II cavity preparations and in irregular internal surfaces.[3] Due to this difficulty, based on the low elastic modulus of flowable material, flowable resin composites have been recommended to create an intermediate layer between the tooth, and packable composite resins.[456] The reduced filler content, enhanced flow capacity, and easy handling properties of flowable composites result in better sealing by forming a stress absorbing layer.[467] This stress absorbing layer reduces not only the polymerization shrinkage of the materials, but also the functional stress on the restored teeth.[18] However, some studies indicated that the liner or based materials, such as flowable composite or resin-modified glass ionomer, not reduce the polymerization shrinkage.[910] On the contrary, these liners lead to more stresses and a possible adhesive failure. Hence, the benefit of the intermediate layer become disputable.[11]

Pulp capping is a procedure which is performed for maintaining the vitality of the pulp when vital pulp is exposed or the remaining thin layer of dentin over a nearly exposed pulp during cavity preparation and removal of carious dentine.[1213] The pulp capping material applications can preserve the dentin-pulp complex against bacteria penetration due to microleakage and toxicity of restorative materials.[12] The success of the restorations, after pulp capping procedures, is closely related with protection of the dentin-pulp complex.[14] Pulp capping materials can also be used to show antibacterial activity, promote dentine bridge formation and protect the pulp tissue against thermal shocks.[121315] Calcium hydroxide is the most widely used pulp capping material in restorative dentistry due to its new dentin formation-inducing ability, protecting the pulp against thermoelectric stimuli, antibacterial effects, and alkaline pH.[16] Despite its popularity, the physical properties of conventional calcium hydroxide, such as its water solubility, bond strength to dental hard tissues, and compressive strength, are relatively poor.[17] Due to these disadvantages, light curing pulp capping materials were developed to treat deep cavities and in case of pulpal damage. In addition to aiding pulp healing, these resin-based materials serve as an intermediary layer in the cavity walls similar to that of flowable composite.[181920] The aforementioned light curing pulp capping materials generally contain calcium hydroxide, but nowadays a calcium silicate containing light curing pulp capping agent has been introduced.[1821] According to one study, TheraCal LC is a calcium-releasing material able to induce the formation of apatite and represents a promising material in direct pulp capping clinical procedures.[21] The ability to form apatite may play a positive role in new dentin formation. Another study indicated that it displays low solubility, low cytopathic effects, and sustained alkalinity.[21] Moreover, due to the low-temperature changes during polymerization, it might be preferable as an indirect pulp capping material in deep cavities.[22]

The fracture resistance of resin composites is a critical factor for the clinical success of restorations. The cavity design, anatomical contour of the tooth, pulp capping material used under the restoration, type of base and restorative material, type of bonding agent, configuration factor, composite placement technique, occlusal habits, and mastication forces affect the fracture resistance of composite restorations.[1352324]

Several researchers have investigated the fracture resistance of direct composite resin restorations in Class II preparations.[1232526] However, no study has investigated the effect of different pulp capping materials and cavity designs of Class II cavity preparation on the fracture resistance of composite restorations. Therefore, the purpose of this study was to investigate the effect of cavity design and the type of pulp capping materials on the fracture resistance of Class II composite restorations.

The null hypotheses were that (i) there would be no statistically significant difference among the various pulp capping materials used under the restoration, and (ii) there would be no statistically significant difference in their fracture resistance based on the cavity design.

MATERIALS AND METHODS

The present study was approved by the Research Ethics Committee of the Izmir Katip Celebi University, under report No. 2014/63. Sixty freshly extracted, sound, caries-free human molar teeth indicated for extraction because of periodontal problems were selected for the study. Calculus and soft tissue deposits were cleaned using a periodontal scaler and pumice slurry. The specimens were then immersed in thymol solution for 48 h for disinfection and stored in 4°C distilled water until used in the restorative and testing procedures. The buccolingual and mesiodistal dimensions of each tooth at the most prominent point of the tooth's surface were recorded using a digital caliper to determine the medium size range. The average buccolingual and mesiodistal mean widths were 10.83 mm and 9.68 mm, respectively. Each tooth was embedded in autopolymerizing acrylic resin (Vertex Dental, Zeist, The Netherlands) using a polyvinyl chloride cylinder (3 cm in height and 2 cm in diameter) up to 1 mm below the cement-enamel junction.

A dovetail cavity was made on the mesio-occlusal surface of each tooth and a slot cavity was made on the disto-occlusal surface of each tooth using a fissure type diamond bur, with a high-speed handpiece with oil-free water spray cooling. Standardized cavity preparations were prepared by a single operator. Facio-lingual dimension of the slot preparation was 3 mm, and mesiodistal dimension was 2.5 mm, the gingival floor was 1 mm above cementoenamel-junction. Facio-lingual dimension of the dovetail preparation was 3 mm (the narrowest part was 2 mm), the gingival floor was 1.5 mm wide, the axial wall was 2 × 2 mm and the gingival floor was 1 mm above cemento-enamel junction. The depths of the cavities were measured with a periodontal probe, and the widths of the cavities were measured with a caliper. The prepared specimens were divided into four groups of 15 teeth, with approximately equal mean dimensions in each group:

Group 1 (control group)

After preparing the cavity, a metal matrix held by a Tofflemire retainer (S.S. White Dental Manufacturing Company, Philadelphia, PA, USA) was placed around the tooth. A self-etch adhesive (Clearfil SE Bond, Kuraray Okayama, Japan) was applied on the cavities with the tips of a disposable microbrush according to the manufacturer's instructions. Light curing was performed using a light-emitting diode light curing unit (Valo, Ultradent Products Inc., South Jordan, UT, USA) for 20 s. Increments of the composite resin were first applied (Clearfil Majesty Posterior, Kuraray Tokyo, Japan) to the gingival wall and then to the pulpal wall. The final increment was applied flush with the contour of the tooth. All the increments were light cured. Immediately after filling, the samples were finished with diamond burs at low speed with air-water spray and polished with a disc system (OptiDisc System, Kerr Corporation, Orange, CA, USA).

Group 2 (Dycal Group)

The pulpoaxial wall of each Class II cavity was lined with a self-setting calcium hydroxide pulp capping material (Dycal® Dentsply Caulk, Milford, DE, USA) using a 1 mm diameter ballpoint instrument. The matrix band was placed and secured using a Tofflemire matrix retainer, and the remainder of the cavity was then restored with Clearfil SE bond and Clearfil Majesty Posterior as described for Group 1.

Group 3 (Calcimol LC Group)

The pulpoaxial wall of each Class II cavity was lined with a light curing calcium hydroxide pulp capping material (Calcimol LC, Voco GmbH, Cuxhaven, Germany) as described for Group 2. The restoration was then completed with Clearfil SE bond and Clearfil Majesty Posterior as described for Group 1.

Group 4 (TheraCal LC Group)

The pulpo-axial wall of each Class II cavity was lined with a light curing resin-modified calcium silicate pulp capping material (TheraCal LC, Bisco Inc., Schaumburg, IL, USA) as described for Group 2. The restoration was then completed with Clearfil SE bond and Clearfil Majesty Posterior as described for Group 1.

After the restoration procedures, the teeth were subjected to a compressive load in a universal mechanical testing machine (Shimadzu, Model AGS-X5kN, Shimadzu Corporation, Kyoto, Japan) connected to a personal computer with specially designed software. A 5 kN load cell was used, and the crosshead speed was kept constant at 1 mm/min. The load was applied at the marginal ridge of the restoration, making 13.5° angles with the longitudinal axis of the tooth. A special steel mold was prepared to hold the long axis of the teeth at a 13.5° angle to the vertical plane. A smooth cylindrical head (3 mm in diameter) was mounted in a specially constructed testing head. An increasing load force was applied until the restoration failed. The failure load of the restorations was recorded in Newton (N). The surfaces of the tooth and the restoration were examined microscopically and classified as adhesive, cohesive, or mixed fractures.

Statistical analysis

Prior to the data collection, a power analysis was performed with G*Power 3.0.10 (Franz Faul, Christian-Albrechts-Universität, Kiel, Germany) to estimate the sample size. The analysis indicated that a sample size of 15 teeth per group for eight groups achieved 90% power, with a medium effect size, and a significance level of α = 0.05l.

The results of the fracture resistance and failure modes were analyzed using the IBM SPSS Statistics version 20.0 statistical package (SPSS, Chicago, IL, USA). The dependent variable across the groups was examined in terms of existence normality assumption by performing a Shapiro–Wilk test and Levene's test for constant homogeneous variances. Accordingly, the data were analyzed statistically using a factorial analysis of variance and post-hoc Tukey's test at a significance level of P < 0.05.

RESULTS

The mean values for all the pulp capping materials and the cavity design types are presented together in Table 1. In the evaluation of cavity design factor, independently of others factors, the mean fracture resistance values of the dovetail and slot cavity groups are presented in Table 2. The fracture load of the dovetail group was significantly higher than that of the slot cavity group (P < 0.05).

Table 1

Mean and SD of the fracture load (Newton) of each group

Table 2

The fracture load (Newton) for cavity design factor and results of ANOVA, independent of other factors

For the pulp capping materials, the highest fracture load (931.15 ± 203.81 N) and the lowest fracture load (832.28 ± 245.75 N) were calculated for the control group and the Dycal group, respectively. However, there were no statistically significant differences among all groups (P > 0.05) [Table 3].

Table 3

The fracture load (Newton) for pulp capping materials factor and results of ANOVA, independent of other factors

The predominant fracture mode for all preparation groups was mix fracture (45.0%), for dovetail preparations was adhesive (45.0%) and mix fracture (43.3%), for slot cavity preparations was mixed fracture (46.7%) occurring in all four restorative procedures. The details of all the fracture modes are presented in Table 4.

Table 4

The distribution of fracture modes for all groups

DISCUSSION

Increasing demand for esthetic restorations has led dental researchers to focus on the development of materials that exhibit natural durability and strength. Among these, composite resins are clinically the most popular materials for the restoration of anterior and posterior teeth. However, for Class II cavities, the primary reason for the failure of composite resin restoration is fracture of the tooth or the restoration, wear off, loss of the bond between the tooth and the restoration, or loss of marginal adaptation.[3242728] Many studies have investigated the fracture resistance of Class II composite restorations because of their clinical relevance.[1232526] However, no study has investigated the effect of different pulp capping materials and the cavity design of Class II cavity preparation on the fracture resistance of composite restorations. Therefore, the main objective of this study was to evaluate the effect of cavity design and the type of pulp capping materials on the fracture resistance of Class II composite restorations.

For this study, human molar teeth extracted for periodontal problems were selected. While the differences between the anatomies of the teeth tested, such as the structure of the dental tissue, thickness of the enamel, geometry of the pulp chamber and external crown size, molar teeth which have comparable external crown size were selected, because the other variables could not be controlled. In the present study, the fracture resistance of composite restoration has been investigated. Because of the failure load of the restorative materials is associated with the bond strength of used bonding agent,[29] the same bonding system has was used in all the groups. Experimental comparative studies of the bond strength of various self-etching systems demonstrated superior adhesive performance of MDP-based bonding agents to enamel and superficial/deep dentine.[3031] Thus, an MDP based self-etching system was used in the current study to maintain ideal bond strength.

To assess the influence of the type of pulp capping materials and cavity design on the fracture resistance of Class II composite restorations, a compressive test was conducted. The load was applied at the marginal ridge of the restoration, making a 13.5° angle with the longitudinal axis of the tooth. This angle is the typical loading angle used in dental applications.[3]

According to the results of the present study, the first null hypothesis was accepted because the type of pulp capping material used under the restoration did not affect the fracture resistance of the Class II composite restorations. The use of pulp capping materials, which may act as flowable composite, did not significantly improve the fracture resistance of the composite restorations. As stated earlier, there is no study in the literature on the effect of pulp capping agents on the fracture resistance of composite restorations. However, the results of the present study are consistent with similar studies that used a flowable composite as a liner under restorations.[332] Ozgünaltay and Görücü compared the fracture resistance of Class II packable composite restorations with and without flowable liners and found no significant difference between the groups.[3] In that study, a micro hybrid flowable composite was used as a liner in the pulpoaxial wall of Class II slot restorations in a similar way with the present study. In another study, it has been reported that the fracture resistance of endodontically treated maxillary premolars has not been affected the use flowable composite as a base under composite restorations.[32] On the contrary, Akbarian et al. reported that the use of flowable liners increased the fracture resistance of teeth with MOD cavity preparations.[5] The discord between these studies may be related to the cavity design of the preparation, the site of the pulp capping material, the thickness of the material, and the design and application area of the cylindrical head.

Considering the findings of the present study, the second null hypothesis was rejected. We hypothesized that the cavity design of Class II cavity restorations does not affect the fracture resistance of composite restorations. The results of the study suggest that the dovetail cavity preparation may increase the fracture resistance of composite restorations. Contrarily, the resistance of the slot cavity preparations was not as strong as that of the dovetail preparations. This was apparently due to the dovetail preparations enhancing the bonding between the composite and dentin by increasing the available surface area. In the literature, some studies reported that the cavity design of the restorations influenced the fracture resistance of composite restorations[3334] and others reported that the type of cavity preparation does not affect the force required to cause fracture.[3536]

Today, the preservation of a sound tooth structure during cavity preparation is considered important. Thus, in the restoration of caries limited with approximal areas, slot cavity preparations are generally preferred. However, to increase the bond strength of the restorations, generating a secondary retention may play a vital role.[3738] Considering the data obtained from the present study, especially in cases where it is necessary to increase the durability of the restorations against occlusal forces, such as bruxism, part functional oral habits, grinding or clenching, dovetail preparations may be preferred for restoration of Class II approximal caries.

When evaluating the results of this study, it should be noted that the direct application of in vitro results to in vivo settings may not be possible. Basic laboratory methods are mainly employed to test the fracture resistance of teeth and restorations. The experimental method of occlusal loading during the fracture test is important, but forces applied in laboratory tests differ from intraoral forces. Such tests cannot simulate the repeated stress that occurs during oral functioning, in addition, axial and lateral forces. However, this methodology is largely employed in the literature, and it provides important information about the fragility of the various components. The real performance of restorations can be determined only in clinical trials. Thus, additional clinical studies are required to support the clinical application of slot or dovetail preparations and the use of pulp capping materials.

CONCLUSION

Dovetail cavity design shows better fracture resistance in Class II composite restorations, independent of used or not used pulp capping materials.