An in vitro investigation on the reinforcing potential of contemporary composites in weakened bicuspids.

Context: This study was carried out to assess the currently available composite systems in India for resistance to fracture in teeth with large cavities. Aim: To evaluate the fracture toughness of weakened maxillary premolars restored by contemporary composites. Settings and Design: In-vitro study was done in the department of conservative dentistry and endodontics. Materials and
Methods: Freshly extracted 44 human maxillary bicuspids were randomly divided into four groups where Group I included intact teeth. The teeth in the other three groups were subjected to a standardized mesio-occluso-distal cavity preparation. The cavities were filled in Group II with bioactive, nanohybrid composite with Giomer chemistry (Beautifil II LS, Shofu Inc., Kyoto, Japan), in Group III with highly filled, nanohybrid composite (Prevest Fusion Universal, Prevest DenPro Limited, Jammu, India) and in Group IV with fiber-reinforced composite with optimized fiber-aspect ratio technology (everX Posterior, GC EUROPE N. V., Leuven, Belgium). All the specimens were then subjected to thermocycling followed by incubation procedures. Fracture resistance was measured in Newton (N) using the universal testing machine. Statistical Analysis Used: Data obtained were tabulated and subjected to one-way analysis of variance (ANOVA) test followed by Tukey's test.
Results: Highest mean fracture resistance was observed with Group I (2294.06), followed by Group II (1708.72), Group IV (1195.82), and Group III (825.38). One-way ANOVA test showed a statistically significant difference (P ≤ 0.05) between all the four groups. Post hoc Tukey test was used for intergroup comparison and showed significant difference (P ≤ 0.05) in mean fracture resistance between groups.
Conclusion: The results suggest that the highest compressive fracture resistance was shown by Group II (Beautifil II LS composite resin). Copyright:

INTRODUCTION

Today, various restorative materials are available in the market for the restoration of extensively carious teeth in high-stress-bearing areas. As such, heavily restored posterior teeth are more susceptible to bulk fracture, to choose the best restorative material based on terms of longevity and fracture resistance is a challenge.[1]

In literature, a number of recommendations on the use of adhesive restorations to reinforce dental tissues have been published. Studies have shown that adhesively bonded composite resin reinforces the tooth as it has cusp stabilizing effect that decreases the risk of tooth fractures by reducing crack propagation.[23]

Evolution in filler and polymer technology has led to the development of materials like fiber-reinforced composites with improved fracture toughness.[45] They also have minimum polymerization shrinkage and resultant shrinkage stress.

Although studies comparing the fracture toughness of different composite systems in large cavities are present in literature, there is no study comparing fracture resistance of newer composite systems, which are nanohybrid, bioactive composite with giomer chemistry (Beautifil II LS),[6] highly filled, nanohybrid composite (Prevest Fusion Universal)[7] and a fiber-reinforced composite with optimized fiber-aspect ratio technology (GC everX Posterior).[89] The aim of the current study was to evaluate and compare the fracture resistance of maxillary premolar teeth weakened by large mesio-occluso-distal (MOD) cavities that have been restored with various contemporary composites. The null hypothesis was that there was no difference in compressive fracture resistance of intact teeth and teeth with MOD cavities restored with the different recent composite systems.

MATERIALS AND METHODS

The study protocol was approved by the institutional ethical committee. A total of 44 human maxillary premolars, freshly extracted for orthodontic purposes were taken. Only noncarious teeth with mature apices and intact enamel and dentine were included, whereas the teeth with developmental defects/cracks or teeth with anatomical variation/resorption were excluded from the study. Software was used to calculate the sample size (G Power version 3.0.10, Dusseldorf, Germany).

All the specimens were cleaned using ultrasonic scaler and were stored in normal saline. A thin layer of light body elastomeric impression material (Dentsply Aquasil Ultra LV Light Body, North Carolina, U. S.) was made around the root surface of all the clean specimens to simulate the periodontal ligament. The specimens were then mounted in 2 cm × 2 cm blocks of cold-cure acrylic resin (DPI RR cold cure, India) up to 1.5 mm apical to the cementoenamel junction [Figure 1a].

Figure 1

Specimen mounting and cavity outline (a); mesio-occlusal-distal cavity preparation (b); Measurement of cavity dimensions occlusal view (c), mesial view (d); Application of etchant (e); Application of bonding agent (f); Placement of Tofflemire matrix band retainer and application of composite material (g); Light curing (h)

All the 44 specimens were randomly allotted to one of the four groups.

Group I (control group) contained intact teeth which were kept unprepared.

MOD cavities were prepared in the remaining teeth. Cavities with standardized dimensions of 2 mm ± 0.3 mm pulpal width, 1.5 mm ± 0.3 mm gingival width, and 2 mm ± 0.3 mm bucco-lingual width were prepared using an air-rotor handpiece (NSK, Japan) with straight diamond bur (Mani, Inc., Japan) [Figure 1b]. Dimensions were verified using periodontal probe [Figure 1c and d]. Facial and lingual walls were prepared parallel to each other with a 90°cavosurface angle.

Prepared cavities were acid-etched for 15 s (3M Scotchbond Universal etchant, Germany) [Figure 1e] and rinsed thoroughly for 15 s and then dried with cotton pellets. Universal bonding agent (3M ESPE Scotchbond Universal Adhesive, Germany) was applied to the prepared tooth surface and rubbed for 20 s with a disposable applicator tip [Figure 1f] and air-dried for 5 s (to evaporate the solvent) followed by curing cycle of 10 s with curing light (LED light cure Dentsply Sirona). Matrix band retainer (GDC Tofflemire universal matrix retainer MRTU, India) was applied and the teeth were then divided into three groups (n = 11) according to the restorative material used.

The application and curing technique for all the test materials were followed according to the manufacturer's instructions [Figure 1g and h].

Group II-MOD cavity restored with bioactive giomer, nanohybrid composite (Beautifil II LS, Shofu Inc., Kyoto, Japan): Composite material was placed within the prepared cavity in 2 mm increments and cured.

Group III-MOD cavity restored with highly filled, nanohybrid composite (Prevest Fusion Universal, Prevest DenPro Limited, Jammu, India): It was placed within the cavity in <2 mm increments and cured.

Group IV-MOD cavity restored with fibre-reinforced, microhybrid composite (Ever X posterior, GC EUROPE N. V., Leuven, Belgium): In this group, first, the missing walls were built with composite. Following which the material was applied to the core and cured.

All the specimens were subjected to thermocycling with 500 cycles at 5°C ± 2°C to 55°C ± 2°C with 30 s dwell time and 5 s transfer time. Thermocycling was followed by incubation at 37°C and 100% humidity for 24 h.

A universal testing machine (Instron 3345, USA) was used with a plunger of 5-mm diameter round stainless steel ball at a strain rate of 2 mm/min.[4] The specimen was mounted on the machine such that the ball should contact the inclined planes of the facial and palatal cusps beyond the margins of the restoration.[4] The axial compressive force at which the cusp or cusps fractured was recorded in Newton (N).

Statistical analysis

Data obtained were collected, tabulated, and evaluated by statistical analysis using IBM SPSS Statistics professional software (Armonk, New York, United States). For multiple group comparisons, one-way analysis of variance was used, which was followed by Post hoc Tukey test for group-wise comparisons.

RESULTS

Maximum mean fracture resistance was observed with control group I (intact teeth: 2294.06) followed by Group II (Beautifil II LS: 1708.72), Group IV (GC everX Posterior: 1195.82), and Group III (Prevest Fusion Universal: 825.38) with a statistically significant difference (P < 0.05) [Table 1]. Intergroup comparisons [Table 2] showed that the mean fracture resistance between all groups was significantly different. The graphical representation of Tables 1 and 2 is depicted in Figure 2.

Table 1

Comparison of compressive strength among study groups

Groups	Sample size	Mean (n)	SD	95% CI for mean		P
				Lower bound	Upper bound
I (intact teeth)	11	2294±0.06	236±0.80	2134±0.98	2453±0.15	0.001*
II (beautifil II LS)	11	1708±0.72	236±0.17	1550±0.06	1867±0.39	0.001*
III (prevest fusion universal)	11	825±0.38	102±0.82	756±0.30	894±0.45	0.001*
IV (GC everX Posterior)	11	1195±0.82	354±0.86	957±0.42	1434±0.22	0.001*

*Significant difference at P≤0.05. SD: Standard deviation, CI: Confidence interval

Table 2

Pairwise comparison of compressive strength

Comparison groups	Mean difference in CS	P
Group I versus Group II	585±0.33	0.001*
Group I versus Group III	1468±0.68	0.001*
Group I versus Group IV	1098±0.24	0.001*
Group II versus Group III	883±0.34	0.001*
Group II versus Group IV	512±0.90	0.001*
Group III versus Group IV	−370±0.44	0.001*

*Significant difference at P≤0.05. CS: Compressive Strength

Figure 2

Graphs showing comparison of compressive strength among study groups (a) and pairwise comparison of compressive strength (b)

DISCUSSION

In this study, three recent composite systems were chosen to fill the cavities as they are aesthetic as well as designed to withstand high masticatory stresses.

Maxillary premolars were selected for this study because their anatomy and morphology render them more susceptible to fracture under occlusal load during mastication.[1] According to Lopes et al., premolar teeth with large MOD cavities have reduced cusp stiffness (approximately one-third) than that of sound teeth.[10] A standardized cavity design was made to provide consistency for the restorative materials to be tested and imitated the ideal intraoral cavity design. To maintain uniformity, all the prepared specimens were etched and bonded using 3M Scotchbond Universal etchant and 3M ESPE Scotchbond Universal Adhesive, respectively. Furthermore, all the specimens were subjected to thermocycling process to mimic 20–25 days usage in the oral cavity.

To simulate the forces of centric occlusion, the axial compressive force was applied on each specimen using the universal testing machine. Steel cylinders, steel spheres, round steel ball, wedge-shaped devices are the various metallic load devices in the universal testing machine which have been used in previous studies.[11] Burke et al. concluded that universal testing machine with a round steel ball of a specific diameter is the best method to determine fracture resistance in premolar teeth.[12]

In results, Group I (intact teeth) showed maximum mean fracture resistance followed by Group II (Beautifil II LS), Group IV (GC everX Posterior), and Group III (Prevest Fusion Universal); hence, the null hypothesis was rejected. Pairwise comparison showed that the compressive fracture resistance of intact teeth was significantly higher than the three experimental groups. Larson et al. suggested that restorative procedures weaken the tooth due to the loss of tooth structure.[13] Santos and Bezerra stated that as the cavity dimensions increase, the remaining tooth structure becomes more susceptible to cuspal deformation during mastication.[14] In the MOD cavity, loss of marginal ridge integrity causes approximately 54% reduction in fracture strength of the tooth.[15] The results of the present study are in accordance with the above-mentioned studies and showed that teeth with MOD cavities cannot be restored to their physiological fracture resistance with currently available composite restorations. However, Gelb et al. suggested that etching, bonding, and composite restoration mechanically splint the cusps together, and hence, such teeth showed fracture strength as high or higher than that of sound, unprepared teeth.[16]

Group II (Beautifil II LS) showed significantly higher fracture resistance than Group III and Group IV (Prevest Fusion Universal and GC everX Posterior). Beautifil II LS is a bioactive giomer. It is based on prereacted glass-ionomer (PRG) technology to form a stable phase of glass ionomer in the restoration. It is of the S-PRG (reaction of only the glass surface) category, which chemically consists of fluoroboroaluminosilicate glass (81.5% w/w) reacted with polyalkenoic acid in water before its inclusion into the resin base of bisphenol A glycidyl methacrylate (bis-GMA) and Triethylene glycol dimethacrylate (TEGDMA).[17] This chemistry reduces their polymerization shrinkage, increases strength and wear resistance. Furthermore, it imparts improved abrasion resistance and antagonist-friendly surface hardness.[6]

Differences in strengths may be related to the differences in their filler content, filler size, and distribution. Studies have shown that the filler volume percentage affects the flexural strength of composite resin.[18] The filler loading of Beautifil II LS is 87.0% by weight which is greater than that of Prevest fusion universal (78% by weight) and GC everX Posterior (53.6% by weight).

GC everX Posterior showed significantly higher fracture resistance than that of Prevest fusion universal despite its lower filler load. This may be because GC everX Posterior consists of a combination of E-glass fibers and barium glass filler with an optimal fiber aspect ratio. This filler technology enables to obtain sufficient stress transfer from the matrix to the fibers, thereby hindering crack propagation and thus efficiently reinforcing the restoration,[89] whereas Prevest Fusion universal is a nanohybrid, highly filled composite resin containing silica nanoparticles. They have increased polishability and decreased shrinkage.[7] The improved physical and mechanical properties of the material were explained to be due to the increased filler load as well as due to appropriate fibers length and diameter.[19] Peterson suggested that fibers with length more than certain minimum length incorporated into a material greatly enhance its mechanical properties.[20]

Being an in-vitro study, variations in anatomy and morphology of the teeth like enamel thickness, dentin thickness and angulations of cuspal inclines were not taken into consideration. The universal testing machine could not simulate the actual masticatory load. Furthermore, the handling of materials by the operator during restoration has the potential to affect its mechanical properties.

CONCLUSION

It can be concluded that the extent to which contemporary composites are able to reinforce a weakened tooth is variable. Despite using modern composites manufactured by advanced technological processes, the results of this study suggest that their reinforcing ability was not the same. Within the limitations of this investigation, it was observed that the nano-hybrid composite with Giomer chemistry provided the highest reinforcement of weakened maxillary bicuspids. However, for better reliability, clinical trials are recommended.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.