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Effects of Thermal Cycling on Surface Hardness, Diametral Tensile Strength, and Porosity of an Organically Modified Ceramic (ORMOCER)-Based Visible Light Cure Dental Restorative Resin

P. P. LIZYMOL

Scientist F and in Charge, Division of Dental Products, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Poojappura, Thiruvananthapuram, 695012, Kerala, India, Tel.: +91-471-2520221, Fax: +91-471-2341814,

E-mails: This email address is being protected from spam bots, you need Javascript enabled to view it ; This email address is being protected from spam bots, you need Javascript enabled to view it (P. P. Lizymol)

ABSTRACT

Dental restorative material placed in the tooth cavity is exposed to cyclic changes of temperature. The present study aimed to find the effect of thermal cycling on surface hardness, diametral tensile strength and porosity of a visible light cure composite.

INTRODUCTION

Dental restorative polymer composite materials based on polymerizable bisphenol-A glycidyl methacrylate (Bis-GMA) monomers [1,2] and quartz/radiopaque glass fillers has been the most popular materials used in dentistiy, since Bowen [1,2] introduced (Bis-GMA) in the 1960s. Though they have good aesthetic and physical properties [3], attempts including few structural variations in the organic matrix of dental composites are going on to improve the clinical performance of restorative materials [4-14]. Among these modifications, urethane dimethacrylates (UDMAs), [4] urethane tetra- methacrylates, [5] organically modified ceramics (ORMOCERS) [6,-13], and bioactive materials [14] are included.

ORMOCERS are very promising materials. Although ORMOCERS are veiy promising, few investigations [6] have confirmed the potential of ORMOCERS as biomaterials or low-contraction materials applied to teeth restoration. The two composites (Definite® (Degussa AG, Hanau, Germany) or Admira® (Voco GmbH, Cuxhaven, Germany) available in the market based on ORMOCER technology. Admira composite contained 78% inorganic particles (barium and aluminum silicate) with an average size of 0.7 p and the organic fraction composed of 65.5%, conventional organic dimethacrylates such as BisGMA, and UDMA along with 34.5% of triethylene glycol dinrethacrylate (TEGDMA). The concept of ORMOCER [7] is to combine properties of organic polymers with glass-like materials to generate new/synergistic properties. The processing steps are based on sol-gel type reaction. Our previous studies reported [8-12] the development of a noncytotoxic and biocompatible organically modified ceramic composite with lower polymerization shrinkage compared to a composite containing BisGMA.

VOCO, the Dentalists [13] claimed that they have developed a new radiopaque light-curing composite, Admira Fusion which based on a nanohybrid silicate and ORMOCER technology with 1.25% polymerization shrinkage (volumetric)) is suitable for posterior and anterior restoration. Though the popularity of tooth-colored polymeric restorations is increasingly in dentistry, about half of the restorations have to be removed or replaced due to the failed restorations. The poor clinical performance of the restorative composite is due to various reasons. Polymerization shrinkage in the oral cavity, low monomer conversion, presence of residual monomer, cytotoxic effects of leachants, intensity of light, exposure time, shelf life of material food habits, bacterial infection and the handling characteristics have significant effect on the durability of restoration in the oral cavity. Current research [15] in dental materials reached from the traditional bioinert materials for restorative purposes to replace the decayed tooth to bioactive materials has a therapeutic function. As per the reported studies of Maktabi et al. [16], when the dental composite is placed in the oral cavity incrementally, the performance of the light-curing procedure is low, which leads to the formation of biofihn and secondaiy caries. Studies by Shimokawa CA [17] showed that microhardness of bulk-fill resin-based composites (RBCs) has a positive correlation on the performance of the light-curing unit by using four different light-curing units (LCUs. Microhardness is directly related to the monomer conversion.

Our previous studies [18] showed that the selection of photo-initiator has a significant effect on monomer conversion and clinical performance. Compared to CQ/amine, photo-polymerization of BisGMA is found to be more efficient with TPO. Both exposure and storage times were important variables in CQ/amine, but not in TPO. Free radicals generated by CQ/ Amine showed more radiative and nonradiative energy loss compared to TPO photolysis. The better monomer conversion of TPO based system reduces the adverse toxicological effects due to chemical and mechanical degradation.

The clinical durability of composite restorative materials is significantly affected by cyclic temperature changes. The composite restorative material has to expose to cyclic temperature changes in the oral environment during normal eating and drinking probably from low temperature as 5°C to about 55°C. During this process, the materials may undergo degradation and decrease in properties.

As per the Reported studies [19], the observed changes in the marginal gap of crowns made from light-cured resin during thermal cycling are comparatively less than autopolymerized resin. They suggested that the improved characteristics of the light-polymerized material may improve the longevity of provisional crowns during clinical applications.

The effect of thermal cycling on degradation of the commercially available visible light cure dental restorative material (Admira®) was evaluated in terms of Vickers hardness number (VHN) and DTS. We have carried out an accelerated thermal cycling test on cured dental composites. Though we have carried out the aging studies of various dental restorative materials [12], changes on porosity, and physico-mechanical properties of visible light cure dental restorative composites based on ORMOCER technology has not reported.

MicroCT (pCT) evaluation was carried out to find out whether thermal cycling has any effect on internal structure of the cured composite. Micro-CT is a non-destructive 3D imaging technique [20] that can be used to inspect the internal structures of small objects with high spatial resolution and unprecedented speed. In the present study, thermally cycled samples were examined using pCT technique to evaluate the increase in porosity, which can also be the reason for loss of properties. The change in porosity was correlated with the decrease in VHN and DTS.

EXPERIMENTAL

15.2.1 MATERIALS

Commercially available dental resin Admira (VOCO, Cuxhaven, Germany) was used for the studies.

15.2.2 PREPARATION OF SAMPLES

Samples with 6 mm (dia) x and 3 mm (thick) prepared as per the reported procedure [8] was cured of 40 seconds and stored at 37°C for 24 hours and used for thermal cycling.

15.2.3 THERMAL CYCLING

The prepared samples were allowed to expose cyclic temperature changes in distilled water from 5°C to 55°C in a dental thermal cycler (Willytec, Germany). The dwell time used was 15 sec at 5°C and 55°C. A 15 sec time was given as the drain time at 22°C. Samples were subjected to 500, 1000, 1500, and 2000 cycles. The thermo cycled samples were used for microCT, microhardness, and DTS evaluation. For microCT evaluation, the same sample before and after thermal cycling was evaluated at each cycles up to 2000 cycles of thermal cycling.

15.2.4 MICROARCHITECTURAL ANALYSIS

A micro Computed Tomography (MicroCT) system (pCT 40, ScancoMedical, Bassersdorf, Switzerland) was used to non-destuctively image and quantifies the 3D microstructural morphology of each sample. The samples were scanned using with 45 [KeV] energy, 177 [pA] intensity, and 12 pm slice thickness. 2D images reconstructed using the Isotropic slice data generated by the system were used for the qualitative analysis and to get the 3D images. Porosity (%) and total pore volume were calculated using the equations:

where, Tv is the total volume and Bv is the bone volume (volume excluding the pores).

15.2.5 EVALUATION OF SURFACE HARDNESS (VHN)

Vickers microhardness tester (Model HMV2, Shimadzu, Japan was used to evaluate the suiface hardness [Vickers hardness (VHN)] of each side of samples. A load of 100 gm was applied for 15 sec. and the measurement was done as per the reported procedure [8] Vickers hardness was calculated from the equation:

where Hv = hardness number; F = Test load (N); d = mean diagonal length of the indentation (mm).

The mean and standard deviation for six measurements was recorded.

15.2.6 EVALUATION OF DTS

The diametral tensile strength (DTS) was determined as described before [8] using a Universal Testing Machine (Instron, Model 1011, UK). DTS was calculated using the following equation:

where, P is the load at break in Newtons, D is the diameter and L is the thickness of the specimen in mm. Statistical analysis was carried out using ANOVA (analysis of variance) single factor to determine significant changes (P < 0.05).

RESULTS AND DISCUSSION

Dental restorative material is expected to remain lifelong in the oral environment, which is a thermal cycling system with temperature variation from 5 to 55°C, which changes with the food habits. Variation in temperature can make irreversible changes in properties of the restorative material which adversely affects the clinical performance.

The variation of surface hardness (VHN) of Admira with thermal cycling given in Figure 15.1 shows that hardness decreases with thermal cycling up to 1000 cycles (P = 2.35E-14) and increases after 1000 cycles.

Compared to samples without thermal cycling (0 cycles) significant decrease in VHN is observed up to 2000 cycles. P-value obtained in statistical analysis is given in Table 15.1.

Effect of thermal cycling on surface hardness (VHN)

FIGURE 15.1 Effect of thermal cycling on surface hardness (VHN).

TABLE 15.1 Effect of Thermal Cycling on VHN: Analysis of Variance (P Value)

VHN 0 CYCLES

VHN 500 CYCLES

2.22E-11

VHN 0 CYCLES

VHN 1000 CYCLES

2.35E-14

VHN 0 CYCLES

VHN 1500 CYCLES

1.36E-07

VHN 0 CYCLES

VHN 2000 CYCLES

4.32E-08

VHN 500 CYCLES

VHN 1000 CYCLES

5.68E-11

VHN 1000 CYCLES

VHN 1500 CYCLES

0.000688

VHN 1500 CYCLES

VHN 2000 CYCLES

0.00084

The variation in surface hardness with thermal cycling indicated the initial surface softening in the aqueous medium after prolonged exposure (> 500 cycles). Surface hardness is related to the amount of water taken up, the greater the uptake the weaker the resultant swollen polymer. But later long term exposure after 1000 cycles indicated that the surface is supersaturated with the adsorbed moisture and further exposure has no deteriorating effect. The increase in VHN after 1000 cycles may be due to thermally induced cross-linking at the surface. Hardness and monomer conversion are directly related [6, 10]. The greater the monomer conversion, the more will be the hardness. Hardness values have been used [10] as an indirect measure of degree of conversion and degradation in aqueous medium. Figure 15.2 shows that DTS of Admira has no significant change up to 500 cycles. After 500 cycles, DTS decreases with increase in cycles.

Micro-CT nondestructively and reproducibly provided 3D representations of structural characteristics. Microarchitecture parameters including volume fraction, strut density, strut thickness, and degree of anisotropy were calculated using 3D stereology. Figure 15.3 shows the threshold inversion which represents the porosity after deducting the Admira from the scarmed image using the attached software (Micro-CT evaluation programme version 6.0).

Figure 15.4 shows porosity distribution of Admira from 0-2000 cycles. Up to 500 cycles, no change in porosity (%) and pore volume (Figure 15.4) was observed. Pore size is found to be 24 pm up to 500 cycles. After 500 cycles both pore size and pore volume are found to increase. Figure 15.4 shows that pore size is 90 pm after 500 cycles. Total porosity (%) and pore volume of Admira (Table 15.2) are found to increase with increase in thermal cycling. Increase in total porosity (%) and pore volume may be due to the dissolution of filler particles and uncured resin. This increase in pore size may be the reason for decrease in DTS.

Effect of thermal cycling on DTS

FIGURE 15.2 Effect of thermal cycling on DTS.

TABLE 15.2 Porosity and Total Pore Volume of Thermocycled Admira

Sample Code

Porosity (%)

Pore Volume (mm5)

Admira-0

2.24

1.96

Admira-500

2.25

1.98

Admira-1000

3.40

3.00

Admira-15 00

3.41

2.99

Admira-2000

3.57

3.14

Effect of thermal cycling on porosity

FIGURE 15.3 Effect of thermal cycling on porosity.

 
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