Fracture resistance of injectable resin composite versus packable resin composite in class II cavities: an in vitro study

Introduction

Restorative materials are constantly evolving due to strong demand for improving the esthetic quality, mechanical properties, and bonding longevity of materials. The purpose of a restorative material is not restricted to replacing the carious tooth structure, but also to provide mechanical durability and marginal seal to the remaining tooth structure. Introduction of adhesive protocols and resin composite materials have made resin compo­site restorations the optimal filling for posterior teeth [1]. The strength of a tooth is greatly affected by the amount of dentin lost due to caries or fracture. Stiffness values of a tooth are largely affected by the loss of marginal ridges. The loss of a single marginal ridge due to decay or trauma reduces tooth stiffness values by 46%, while the loss of both marginal ridges causes a 63% reduction in tooth stiffness values. Therefore, posterior cavi­ties involving loss of marginal ridges pose a great risk for tooth fracture [2]. In addition, losing the marginal ridge complicates the restoration procedure in terms of compensating tooth strength and chair time [3]. According to previous clinical studies evalua­ting the clinical performance of direct resin composite fillings, caries and fracture were the main causes for resin composite replacement. A dentist must cautiously restore posterior cavities by choosing a suitable type of resin composite with superior mechanical, marginal, and handling pro­perties [4].
Conventional resin composites are not the ideal substitute for amalgam because of their inherent drawbacks. Resin composites are more technique-sensitive. A successful class II resin composite restoration requires good manipulation skills, tight proximal contacts, and intimate marginal adaptation. These parameters need more chair time and clinical dexterity [5]. Previous gene­rations of packable resin composites were thought to have similar properties to amalgam in terms of adaptation to cavity margins. However, packable resin composites attained a stiffer consistency as compared to conventional resin composites. Therefore, they may fail to perfectly seal gingival margins at the depth of class II cavities [6].
In order to overcome this problem, flowable resin composites with reduced viscosity are used as liners underneath packable resin composite to enhance adaptation [7]. However, long-term prognosis of this technique is questionable due to inferior mechanical performance and bonding longevity of such restorations [8]. According to flowability, resin composites can be classified as packable, universal, and flowable [9].

Objectives

The introduction of universal resin composites may bridge the gap between packable resin composites and flowable resin composites. Manufacturers claim that universal resin composites can be used from liners to small and large class I and II cavities. However, the lite­rature is lacking evidence regarding the performance of universal injectable resin composites in posterior cavi­ties. Therefore, conducting a study comparing fracture resistance of teeth with class II cavities restored with universal injectable resin composite versus packable resin composite might be of value.
The null hypothesis of this study is that there is no significant difference between the fracture resistance of teeth restored with injectable resin composite and teeth restored with packable resin composite in small and large class II cavities.

Material and methods

The materials used in this study are presented in Table 1.

Study design

Sample size calculation was performed using G*Power version 3.1.9.2. Power analysis employed fracture strength as the primary outcome. Effect size of 0.5568224 was calculated based on the results from a study by Laegreid et al. in 2011 [10], and assuming standard deviation within each group of 371 using  level of 5% and  level of 80%, i.e., power of 80%. The minimum estimated sample size was 45 samples (9 samples per group).
A total of 50 sound extracted maxillary premolars were selected for the study. Teeth were extracted for orthodontic or periodontal reasons and were collected from the Misr International University (MIU) teeth bank. Teeth with caries, cracks, fractures, or restorations were excluded. Selected teeth were of close bucco-lingual (BL) dimensions ranging between eight and nine millimeters. Teeth were cleaned of visible blood and gross debris, and then placed into 5.25% sodium hypochlorite (NaOCl) solution for five minutes immediately after extraction. They were then stored in 0.9% saline solution at room temperature until further use [1, 11, 12].
The study tested two variables, such as restorative material and cavity design. The selected teeth were randomly divided into five groups of ten teeth (n = 10). A restorative material was assigned to each group, and cavity design was prepared. Group 1 (m0c0), as a nega­tive control, included sound unprepared teeth, group 2 (m1c1) injectable resin composite in small class II box-only cavities, group 3 (m1c2) injectable resin compo­site in large class II occluso-mesial (OM) cavities, group 4 (m2c1) packable resin composite in small class II box-only cavities, and group 5 (m2c2) packable resin composite in large class II OM cavities.

Cavity preparation

In this study, a pear-shaped high-speed diamond stone (830L.314.016, Komet Dental, Germany) was employed to prepare the teeth. The diamond stone had dimensions of 1.6 mm width and 5 mm height. Prior to cavity preparation, the teeth were outlined with a pencil to accurately identify their location and extent. The initiation of cavities was guided by the central groove. Small cavity groups (c1) had box preparations with dimensions of approximately 2 mm BL (Figure 1), 2 mm axial depth, and 3 mm height (2 × 2 × 3 mm). Large cavity groups consisted of OM cavities with the occlusal part approximately one-third of the BL width of the tooth. The proximal box had dimensions of approximately 4 mm BL (Figure 2), 2 mm axial depth, and 3 mm height (4 × 2 × 3 mm). Cavity floors were perpendicular to long axes of the teeth, and internal angles were rounded matching the shape of the bur.
The teeth were then embedded in cylindrical acrylic resin blocks of standard dimensions. Blocks had a length of approximately 27 mm and a width of nearly 16 mm. To create the blocks, polyvinyl chloride (PVC) cylinders of standard dimensions were cut and filled with self-cure acrylic resin. The teeth were positioned in soft resin, leaving a gap of approximately 2 mm between the resin and cemento-enamel junction (CEJ). Once the resin hardened, PVC molds were removed from the resin blocks.

Restorative procedures

A Tofflemire No. 8 matrix band (Universal, USA) was used to build the mesial proximal walls of cavities (Figure 3). Total-etch procedure was performed for bonding by etching enamel and dentin for 15 seconds, and a universal adhesive was applied. Centripetal build-up technique was used to build the mesial wall first with injectable or packable resin composite, followed by incremental application. Each resin composite increment was cured for 20 seconds using a multi-wave LED light cure (Eighteeth LED Curing Pen, Eighteeth Co., China), with a light intensity of 1000 mW/cm2. Finishing and polishing discs (Sof-Lex discs, 3M Oral Care, USA) were utilized for the finishing and polishing procedure of occlusal and proximal surfaces.

Fracture resistance assessment

A universal testing machine (model 3343, Instron Cor­poration, Canton, MA, USA) was used to load the teeth. The loading tip was a stainless-steel ball with 4 mm dia­meter. Loading was perpendicular to the marginal ridge of the restoration contacting the marginal ridge, buccal, and palatal cusp inclines (Figure 4). Loading rate was 1 mm per min until initial or complete fracture. The maximum load of the unit was 1 kN.

Mode of failure

Each sample was examined at 30× magnification using a stereomicroscope (Nikon MA100, Japan) with an image analysis software (Omnimet, Buchler, USA), to detect whether the fracture was restorable or non-restorable (Figure 5). The fracture was classified as restorable when it was coronal, or 1 mm, or less apical to the CEJ. The fracture was classified as non-restorable when it passed more than 1 mm apical to the CEJ [13-15].

Statistical analysis

Numerical data were investigated for normality by verifying the distribution of data, and normality tests, such as Kolmogorov-Smirnov and Shapiro-Wilk tests were applied. Fracture resistance data showed non-normal (non-parametric) distribution. Data were presented as median and range values. Kruskal-Wallis test was employed to compare between different groups, while Fisher’s exact test was utilized to compare between failure modes of different groups. Failure mode data were presented as frequencies and percentages. Significance level was set at p ≤ 0.05. Statistical analysis was performed with IBM SPSS Statistics for Windows, version 23.0 (IBM Corp., Armonk, NY, USA).

Results

Fracture resistance

Table 2 demonstrates the comparison between the fracture resistance values of different restorative materials in small and large class II cavities. The statistical analysis showed no significant difference between the groups: p-value = 0.924, effect size = 0.007, and p-value = 0.200, effect size = 0.106, respectively. Additionally, Figure 6 displays the median and range values of the fracture resistance for all five groups.
Table 3 shows the comparison between the fracture resistance values of restored cavity designs using either injectable or packable resin composites. The statistical analysis indicated that there was no significant difference between the groups: p-value = 0.247, effect size = 0.082, and p-value = 0.133, effect size = 0.081, respectively.

Mode of failure

Within each group, most of the failures were restorable. There was no statistically significant difference in the failure modes between the different groups (p-value = 1, effect size = 0.115). The distribution of failure modes is presented in Table 4.

Discussion

Class II restorations may be one of the most challenging posterior cavity procedures. A clinician’s role is to meticulously remove decay and design the cavity, considering biomechanics to withstand occlusal stresses. Simulation of these clinical conditions involves various factors, such as tooth anatomy, cavity size, bonding protocol, and restorative materials’ properties [16, 17]. Despite many studies investigating flowable resin composites [18-23], none have explored the fracture resistance of universal injectable resin composite. Therefore, the current study primarily aimed to assess the impact of restoring class II cavities with universal injectable resin composite on the fracture resistance.
Regarding bur material used during cavity preparation, there is a controversy between diamond and carbide burs. Previous studies showed no significant difference between them in terms of dentin surface roughness, hydraulic conductance, and restoration inter-facial gap [24-26]. Despite diamond burs creating a thicker smear layer, it is fully removed by phosphoric acid etchant during total-etch procedure, not affecting dentin surface topography or bonding strength [24, 27]. Moreover, although periodontal ligament fibers cushion occlusal stresses, their simulation in vitro has shown no impact on tooth fracture resistance tests [28]. Consequently, this study molded teeth two millimeters below the CEJ without simulating these ligaments.
According to a study by Taha et al. from 2011 [29], in vitro testing of fracture strength by static loading has been proved to be an effective method for examining and comparing the effect of cavity design and restora­tive materials. However, laboratory-simulated occlusal stresses may not completely resemble intra-oral occlusal stresses regarding magnitude, frequency, and direction. Nonetheless, it is the most frequently used testing me­thod in fracture resistance studies in the literature [30]. Accordingly, during the present study, the fracture resistance was tested using static axial loading of teeth with a universal testing machine.
Moreover, the fracture resistance test acting on class II restorations requires suitable selection of loading points, loading rate, and direction for proper clinical interpretation of the results [16, 17]. Most studies in recent literature have utilized round or spherical stainless-steel points to load the teeth (premolars or molars) [1, 12, 16, 17, 30, 31-35]. Laegreid et al. [10] and Göktürk et al. [31] investigated the fracture resistance of maxillary premolars using a round stainless-steel ball of four millimeters dia­meter. They claimed that it contacted the occlusal surface as well as the buccal and lingual cusps. Al-Nahedh and Alawami [16, 17] loaded the marginal ridge of premolars to assess the fracture resistance of occluso-distal class II restorations. Therefore, in the present study, maxil­lary premolars were loaded at the marginal ridge contacting the restoration and cuspal inclines simultaneously.
Considering the findings of this study (Table 2, Figure 6), there were no statistically significant differences between the fracture resistance of teeth restored with injectable resin composite and packable resin compo­site. Therefore, the null hypothesis was accepted. The results of this study are consistent with similar research by Fráter et al. in 2021 [1], who reported no statistically significant differences in the fracture resistance when capping fiber reinforced resin composite with universal injectable resin composite or packable resin composite. They attributed these findings to the high filler loading (69% by weight) of universal injectable resin composites. The present data also support these assumptions.
The results of the present study were also consistent with those of Rocha Gomes Torres et al. [36], who conducted a randomized clinical trial and found no statistical significance between two-year performance of highly-filled flowable resin composites and conventional paste-type resin composites. Additionally, Lawson et al. [37] proved in their clinical study that there was no difference between two-year clinical performance of flowable resin composites and highly-filled conventional resin composites in class I cavities. The current study results are in line with these findings.
However, another in vitro study disagree with our study by reporting superior mechanical properties of highly-filled flowable resin composites with small filler particles as compared with conventional nano-hybrid resin composites. The authors stated that the conventional resin composite used in their study contained pre-polymerized filler particles, which lack chemical bonding with the resin matrix, leading to an increased susceptibility to stress fractures [18].
Furthermore, here, no statistically significant diffe­rences in the fracture resistance between the control (intact) group and resin composite groups (Table 3) were observed. This is in line with the results of Forster et al. [11], who concluded that a three millimeters cavity depth can be restored to the same values of fracture resistance as intact teeth, regardless of cavity wall thickness.
Concerning cavity design, large class II cavities exhibited higher median fracture resistance values in both the materials used (Table 3, Figure 6). Although the differences were not statistically significant, these higher values may partially support previous studies [30, 38, 39]. Kucukyilmaz et al. [30] proved better fracture resistance of dovetail cavities as compared with slot cavity designs. This was reasoned by a greater surface area of dovetail cavities for resin composite bonding. Therefore, higher median values found in this study may be due to the enhanced bonding of resin composite restorations in large cavities, which are equivalent to the dovetail design. Moreover, Matuda et al. [38] demonstrated in their finite element analysis that OM class II cavities exhibited lower stress values as compared with vertical slot designs, which resemble small (box-only) cavities in the current study. In addition, Babaei et al. [39] in a finite element analysis of their study showed that double-curved restorations significantly reduced the peak stresses as compared with straight and single-curved designs. This is in agreement with the present findings, where the box-only preparations may resemble straight or single-curved designs.
However, cavity design findings of this study do not support the results of other studies [40, 41]. For instance, Larson et al. [40] observed that increasing cavity isthmus width in class II cavities greatly dimini­shed fracture resistance values. This is in contradiction to the findings of the present study. Additionally, Valian et al. [41] proved that the greater the mesio-distal extension of the cavities, the greater the stresses on the enamel. This also contradicts the findings of the current study, where there was no statistically significant difference in the fracture resistance between the large OM cavities and small (box-only) cavities.
Furthermore, here, there was no significant diffe­rence in the failure modes between all tested groups (Table 4). Most of the fractures in the five teeth groups were restorable. This may be supported by evidence stating that bonded restorations aid in reinforcing the residual tooth structure by supporting the dentin [41].
When analyzing the findings of this study, it is needless to mention that complete dependence of in vitro studies for clinical interpretation is impossible. This is due to the differences between static occlusal loading of laboratory and intra-oral occlusal loading with cyclic fatigue. The static occlusal loading may neglect the frequency and lateral direction of stresses that occur intra-orally [17, 28, 30, 42]. These tests may also disregard the fact that most teeth and restorations fail clinically due to reproduction of defects [42]. Therefore, there is still a substantial need for further clinical studies to better evaluate the clinical performance of injectable resin composite, and to justify their use in restoring class II cavities.

Conclusions

Under the limitations of this study, the teeth restored with injectable resin composite in class II cavities showed comparable fracture resistance with the teeth restored with packable resin composite. Additionally, teeth with small and large class II cavities performed similarly in terms of fracture resistance, when restored with injectable resin composite.
Based on the outcomes of this study, injectable resin composite may be a promising time-convenient alternative to conventional packable resin composite that may be used successfully in restoring stress-bearing class II cavities in premolars, while maintaining sufficient flow.

Disclosures

1. Institutional review board statement: The study was approved by Misr International University Institutional Review Board, with approval number: MIU-IRB-2122-140.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: The authors declare no potential conflicts of interest concerning the research, author­ship, and/or publication of this article.

References