An evalution of compressive strength of newer nanocomposite: An in vitro study.

AIM: The purpose of the study is to assess and compare compressive strength of newer nanocomposites (FiltekZ350, Ceram X Mono, Ceram X Duo) with microhybrid (Tetric Ceram) and to compare difference in compressive strength of newer nanocomposites.
MATERIALS AND METHODS: Forty eight specimens of composite were fabricated using customized biparpite brass mold measuring 5mm x 5mm and were grouped with twelve specimens in each Group I : Tetric Ceram, Group II: Filtek Z 350, Group III : Ceram X Mono, Group IV : Ceram X Duo. Composite resins are placed in cylindrical recesses and covered with mylar strip and are cured using QHL light curing unit. Compressive strength is evaluated using Instron machine. Results are statistically analyzed using One way Anova and Student t test. Analysis demonstrated that nanocomposites have better compressive strength than micro hybrid (P<0.001).
RESULTS: Within the limitations of the study, it can be concluded that nanocomposites have better compressive strength than microhybrid composite and nanocomposite showed optimal compressive strength of 312 - 417 Mpa.

INTRODUCTION

The ultimate goal of dental restorative material is to replace the biological, functional and esthetic properties of healthy tooth structure. Dental amalgam and gold alloys, which have a long record of clinical success, have been used as dental restorative materials for more than 100 years, especially in posterior teeth, because their mechanical properties match those of natural teeth; however, these metallic materials are not esthetic. Since their introduction into the dental market 40 years ago, dental resin composites have proven to be successful. It is expected that the usage of resin composites in posterior teeth will continue to grow. Although considerable improvements have been made in the properties of dental resin composite over the years, no fundamental change in monomer systems has occurred since Bowen introduced dimethacryltaes the form of bis – GMA in 1962. Major developments come from improvements in filler systems. Resin composites have gone through generations of traditional macrofilled composites, microfilled, hybrid, microhybrid and nanocomposites.[1]

No composite materials are able to meet both the functional needs of posterior class I and class II restoration and the superior esthetics required for anterior restorations. Nanocomposites thus have been introduced to serve these functional needs through the application of nanotechnology.[2]

Nanotechnology is the production of functional materials and structures in the range of 0.1 -100 nanometers – nanoscale – by various physical and chemical methods. The usage of nanomaterials stems from the idea that they may be used to manipulate the structure of materials to provide dramatic improvements in the electrical, chemical, mechanical and optical properties.[210]

Nanocomposites have improved mechanical properties i.e. better compressive strength, diametrical tensile strength, fracture resistance, wear resistance, low polymerization shrinkage, high translucency, high polish retention and better esthetics.[314]

The aim of this study is to measure and compare the compressive strength of newer nanocomposites.

MATERIALS AND METHODS

Materials used in the study are given in Table 1.

Table 1

Name and product details of the materials used

Operative procedure

Preparation and grouping of the specimens for compressive strength

Four groups are made of four different composite materials having 12 specimens in each group, thus fabricating 48 specimens using customized bipartite brass mold measuring (5mm × 5mm).

Preparation of the specimens

The composite resins used for this study were grouped as follows:

Group I: Tetric Ceram (micro hybrid composite resin)

Group II: Filtek Z350 (nanocomposite)

Group III: Ceram X mono (nanocomposite)

Group IV: Ceram X Duo (nanocomposite)

The composite resins are placed in cylindrical recesses. The composites are covered with a mylar strip. A glass slide (1mm thick) is then placed over composites and pressure is applied to accommodate the material into the mold and to extrude excess material. After removing the glass slide, the composites were then irradiated from the top and bottom surfaces through the mylar strip as per the manufactures instructions using the QHL light curing unit. The specimens are taken out of the brass mould and light cured in the middle of the specimen at opposing sides.

In total, 48 specimens are fabricated according to the grouping done. Study is performed in controlled temperature by keeping it in a distilled water bath for 24h at 37°C.

Testing procedure

All specimens are transferred to the universal Instron testing machine individually and subjected to compressive strength analysis at crosshead speed of 1.0mm/min. [model no. 4206]

RESULTS

Data obtained in the present study is subjected to statistical analysis using one way ANOVA and inter group comparison is done using Student t test.

The compressive strength of experimental groups is compared with one way ANOVA test, p value < 0.001 is obtained which indicates highly statistically significant difference between tested material [Table 2.]

Table 2

Mean and standard deviation values for compressive strength using one way Anova

Further inter group comparison is carried out using Student t test. Results demonstrated that group I differed significantly only with group II (P value = 0.001); however, there was no statistically significant difference between group I and group III and group I and group IV with p values of 0.078 and 0.185, respectively [Table 3].

Table 3

Comparison of compressive strength value of microhybrid (Group I) with Nano composite (Group II , III, IV) using Student t test

Group II showed significant differences with all the other 3 groups having a p value of 0.001 in all cases. No statistical significant difference was found between group III and group IV (p value is 0.490) [Table 4].

Table 4

Comparison of compressive strength values within nanocomposites between Group II, III, IV

DISCUSSION

During the last decades, the increasing demand for esthetic dentistry has led to the development of resin composite materials for direct restorations with improved physical and mechanical properties, esthetics and durability.[48]

The latest development in the field has been the introduction of nano filled materials by combining nanometric particles and nano clusters in a conventional resin matrix.

Restorations in posterior areas are constantly subjected to functional loading.[2]

Nano filled materials are believed to offer excellent wear resistance, strength and ultimate esthetics due to their excellent polishability, polish retention and lustrous appearance. Nano filled resin composites show mechanical properties at least as good as those of universal hybrids and could thus be used for the same clinical indications along with anterior restorations due to their high esthetic properties. Mechanical properties of a material describe its response to loading. Although most clinical situations involve complicated three-dimensional loading situations, it is common to simply describe the external load in terms of a simple dimension as compression. Compressive strength is particularly important because of chewing forces. It is one of the measures of strength of material in different force conditions, increased value represents increased strength of the material.[5]

Hence in this study the compressive strength of nanocomposites is evaluated and compared with micro hybrid composite. Compressive strength is measured using Instron Universal testing machine.

Tetric Ceram is a microhybrid, light curing, radiopaque fine particle hybrid composite for the restrorative therapy. Filtek Z 350 is a nano filled composite with a combination of nanomer sized particles to the nano cluster formulations which reduces the interstitial spacing of the filler particles. This provides increased filler loading, better physical properties when compared to composites containing only nanoclusters. Average filler particle size is 5- 20nm. Ceram X is a nano hybrid containing organically modified ceramic nano particles comprising polysiloxane back bone. These nanoceramic particles can be best described as inorganic – organic hybrid particles where the inorganic siloxane part provides strength and the organic methacrylic part makes the particles compatible and polymerizable with the resin matrix. The good resistance to micro crack propagation might be related to strengthening effect of the nano – ceramic particles. Average filler particle size is 10nm.

In this in vitro study, Filtek Z 350 has the highest compressive strength and Tetric Ceram has the least compressive strength among the composites evaluated. Ceram X Mono and Duo had comparable compressive strength with that of Tetric Ceram.

This study is in accordance with the studies done by Lu et al, Mitra et al, Beun et al.

However, it is in contrast with the result discussed by Ruddell et al, who stated that pre polymerized nanoparticles have the potential to improve wear properties; however, there was reduction in mechanical properties as compared with conventional composites. The reason stated for this drop in the mechanical properties was micro cracking present in some nanoparticles which was introduced during impregnation procedures resulting in inbuilt flaws.

Typical microfill fillers are made using pyrogenic processes, which produce materials with an average primary particle size of about 40 nm, but in which the primary particles typically aggregate in fibrous, low-density, chain-like secondary structures. The fibrous structures of microfill fillers limits paste filler loadings and results in poor handling and lower mechanical properties than that are demonstrated by hybrids and microhybrids such as Tetric Ceram[12] . Commercial microfills generally contain prepolymerized resin particles previously filled with fumed silica (commonly known as “organic filler”) to improve the handling characteristics. Because of the small primary particle size, microfills display high gloss retention, but poor bonding between the organic filler particles and the resin matrix lowers the mechanical properties. Thus, indications for microfills usually are limited to low stress-bearing anterior restorations.[2]

The use of spheroidal nanoclusters fillers with their broad particle distribution produced high filler loading, desirable handling characteristics and physical properties comparable with those of commercial hybrid composites.[2]

The performance of a dental composite depends on filler type, resin composition, filler matrix bonding and cure conditions (Wendt 1987, Pallav et al, 1989, Ferracane et al, 1998, Watts and Hindi 1999, Lim et al, 2002).[6]

The differences obtained between the various study groups could be explained by the nanofiller content (wt%).

Micro hybrid composite (Tetric Ceram) has 50 wt% of inorganic phase compared to 80 wt% for the nano filled. Nano fillers have higher contact surface with the organic phase when compared to mini filled composites, consequently improving the material strength.[6]

Mechanical behavior depends upon the concentration and particle size of the inorganic filler.[81113] Owing to the reduced dimension of the particles and to a wide size distribution, an increased filler load can be achieved in nano composites (Filtek Z 350, Ceram X), with out increasing their viscosity and increasing the mechanical properties such as tensile strength, compressive strength and other mechanical properties.[97]

The filler particle size of Filter Z350 is 5-20nm and Ceram X is 10 nm, which is lower in comparison with the filler particle size of Tetric Ceram which is 0.7μm. A spherical shape is known to have many advantages such as to allow an increased filler load in composites and also enhance their fracture strength since mechanical stresses tend to concentrate on the angles and protuberances of the filler particles. Also the spherical shaped filler particle is seen in Filtek Z 350 and most nano composites have the advantage of increased filler load, which is not seen in Tetric Ceram which has irregular shaped filler particles.[7]

The results obtained in this study could be attributed to the differences in the shape, size and concentration of the fillers in the experimental groups.

It is hypothesized that a composite with high mechanical properties will better resist to occlusal loads than a composite with low mechanical properties. Filtek Z 350 has better compressive strength and Ceram X mono and duo has comparable compressive strength to Tetric Ceram. From the above results, it can thus be expected that the nanofilled composites are able to resist these stresses better than micro hybrid composite.[7]

Conclusions

Within the limitations of this in vitro study; it can be concluded that

Nanocomposites have shown better compressive strength than micro hybrid composite. Filtek Z 350 has shown the highest compressive strength and Tetric Ceram has shown the least compressive strength among the tested materials.

Among the nanocomposites, Ceram X Duo had the least compressive strength as compared to Ceram X Mono and Filtek Z350.

Nanotechnology has helped to develop a dental filling material that will be used in all areas of the mouth with initial polish retention, as well as excellent mechanical properties suitable for high stress bearing restorations.

Further in-vitro studies should be carried out to improve the knowledge of the mechanical behavior of nanofilled composites and in-vivo studies to determine their clinical performance.