In vitro Comparative Study of Compressive Strength of Different Types of Composite Resins in Different Periods of Time

1. Introduction

     Mechanical properties of core build up restorative materials have important role in efficacy and longevity of the tooth and restoration. A badly broken down tooth in anterior or posterior region of oral cavity which has happened because of caries or root canal therapy, needs to be restored with a suitable restorative material which can resist complicated forces of mastication [1, 2]. Since the majority of mastication forces in posterior region are particularly compressive, the restored endodontically treated tooth or the complex and extensive restoration should bear these kinds of forces [2, 4]. It is said that compressive strength is the most important mechanical property of core build up materials. A restorative material with lower compressive strength than tooth, tends to fail, fracture and it ends with periodontal problems or extraction of the broken tooth [3, 4].

     Compressive strength is a useful property to compare materials which are brittle and generally weak in tension such as amalgams, cements or composite resins. Amalgam has been the core material of choice in posterior region for a long time but in recent years core buildup glass ionomers and posterior composite resins have been introduced as a core build up material. During the past recent years many new composite resins have been introduced to market such as condensable composite resins, core build up types and fiber reinforced composite resins in order to restore the coronal portion of a vital or endodontically treated tooth with an adhesive restoration [2, 4].

     Composite resins are improving everyday because of their chemical ingredients, bonding ability, conservative preparation, preservation of tooth structure and esthetics. In anterior region composite resins are the materials of choice but in posterior region the composite resins should have mechanical properties like tooth structure and they should have a compressive strength equal or more than tooth structure to resist the mastication forces [2].

    Many researches have been undertaken to evaluate the compressive strength of the different restorative materials. The studies in this area showed different results. In some studies amalgam showed the highest compressive strength [1], but in some of them composite resins had the higher strength [5], however, some have shown no significant difference between amalgam and composite resin. Also the compressive strength has changed with time of evaluation [6-14].

The aim of this study was to compare the compressive strength of different types of composite resins which have introduced as posterior composite resins in different times.

Table1. Mean , standard deviation and CV of  compressive strength of composite resins after 1 hour(Mpa)

2. Materials and methods

     Compressive strength of five commonly used composite resins in Iran was measured at 1 h, 24 h, 7 days and 1 month. Six samples were prepared for each time group of related composite resins, so they were 24 specimens for each composite resin and 120 specimens totally. The groups were as follows:

Group 1: Nulite -F (NSI, Australia), light cure fiber reinforced composite resin [15]. Group 2: P60 (3MESPE, USA), light cure posterior composite resin [16].

Group3: Z250 (3MESPE, USA), light cure hybrid composite resin [17].

Group 4: Biscore (Bisco,USA) , two components core build up self cure hybrid composite resin [18].

Group 5: Tetric Ceram HB (Vivadent , Liechtenstein), light cure hybrid composite resin [19].

2.1. Specimens preparation

     According to ISO9917 standard specification [12], specimens prepared with 8.0 mm length and 4.0 mm diameter. A two part stainless steel cylindrical mold was used to prepare the specimens. The mold was placed on a glass slab and a mylar matrix was placed under the mold to obtain flat surface. Composite resins were applied in the mold in 2 mm layers to fill the mold. For the last layer a mylar matrix was placed over the layer.

     For light cure composite resins light curing was done with Coltolux 2.5 (Coltene, Switzerland) light curing unit for 40 seconds per layer with the intensity of 400 mW/cm². In order to have maximum curing, each specimen was post-cured 10 minutes after preparation for 60 seconds at all directions.

    After the end of sample preparation in each group of composite resin they divided into 4 groups of 6 samples and stored at 37±1 °C in an incubator for different times prior to compressive strength testing.

Table 2. Mean , standard deviation and CV of compressive strength of composite resins after 24 hours (Mpa)

2.2. Measurement of compressive strength

   Compressive strength of specimens was measured after 1 h, 24 h, 1 week and 4 weeks. Test was carried out on a universal testing machine (Zwick/Roell,Z020,Germany) at a crosshead speed of 0.5 cm/min.Ultimate Compressive strength (UCS) was calculated from the formula UCS=4f/πd², where F is maximum applied load(N) and D the cylindrical Specimen diameter (mm)[3, 4, 12]. Data were analyzed by ANOVA one-way and Scheffe test.

3. Results

Table 3. Mean , standard deviation and CV of compressive strength of composite resins after 1 week (Mpa)

4. Discussion

       Compressive strength is one of the most important mechanical properties of a core buildup material which restores the structure of a tooth in posterior region [1-4, 20]. A core buildup material should have the same mechanical properties as tooth structure. A material with higher or lower amount of a property will adversely affect on longevity of the tooth structure and the restoration and a premature failure of each will happen.

      The results of this study revealed that the compressive strength of all the composite resins increased with time from 1 h to 24 h. This is the same as the result of the studies of other researchers (1, 6, 8, 9, 11) but even after 24 h none of the composite resins reached to the amount of the compressive strength of the tooth structure which is 384 Mpa for enamel and 297 Mpa for dentin. After 7 days only p60 and Z250 reached to dentin's amount.

     It seems that the compressive strength is related to the type of composite resin. P60 and Z250 had the highest compressive strength at all the times. Biscore placed in second position and Tetric ceram HB and Nulite-F had the lowest compressive strength. In recent years composite resins have introduced as core build up materials because they have some advantages to dental amalgam. Composite resin needs more conservative preparation and adheres to tooth structure due to the application of the adhesive agent prior to it and this will help to preserve the tooth structure [3]. But they have some negative points which need more careful techniques in restoration such as polymerization shrinkage, water sorption, high sensitivity to contamination during the restoration and incomplete polymerization especially in light cure composite resins which light does not reach to the deep parts of the cavity [2-4]. All of these factors will affect on the mechanical properties of the composite restorations and lead to premature failure than amalgam restorations. So it is necessary to select the right composite resin to apply as a core build up material in complex and extensive cavities.

     In this study we evaluated the compressive strength of five different types of composite resins which each of them has been introduced for some applications in posterior region but they have some structural and chemical differences. One of the composite resins ingredients which affects on the mechanical properties is filler size, type and content [2-4]. P60 is a packable composite resin which can use as a direct and indirect material, core buildup, cusp buildup and splinting the teeth [16]. It has 61% V filler [16]. P60 had the highest compressive strength among the groups at all the times examined.