Abstract
Objectives
Stickiness is a vital rheological parameter for the clinical handling behavior of unset resin composite restoratives. The aim of this study was to investigate the stickiness of three different resin composites at 23 °C and 37 °C tested on steel, dentin and dentin covered with different bonding agents.
Methods
The stickiness instrument, used in this study consists of a vertical cylindrical stainless steel rod, with a flat circular end, and a platform with a cylindrical mold (diameter: 6.1 mm, depth: 2.2 mm). The test-material surface temperature and the speed of the rod can be modified. It moves slowly into the prepared mold which is filled with unset composite materials. The degree of stickiness is deducted from the height of the “elevation” the material forms when the plunger is withdrawn from the mold until the steelhead detaches itself from the composite. In this study, stickiness was tested directly to the steel plunger and to dentin slices (uncovered or covered with two different bonding agents) fixed to the plunger rod with a clamp.
Results
The coefficients of variation (CVs) were generally less than 0.10, indicating that the stickiness instrument offers an adequately reproducible way of testing stickiness. The tested composite materials varied significantly in stickiness. For all investigated materials a decrease of peak heights with increasing speed was found (for all three materials: p < 0.0001). Generally the stickiness increased when increasing the temperature from 23 °C to 37 °C. Comparing the areas under the curve ( Fig. 2 ), stickiness of resin composites was higher on dentin than on steel and least on bonded dentin. The order of stickiness of composites was not affected by testing the stickiness on the different materials.
Significance
This method allows the characterization of composite resin materials stickiness to steel, as equivalent to dental steel instruments, and to bonded dentin as equivalent to the tooth cavity after preparation. An ideal material should have a sufficient difference between stickiness on steel and dentin so that it remains in the cavity and is not pulled back by the steel instrument.
1
Introduction
The use of resin composite materials for dental restorations has increased during the last decade primarily as an esthetic alternative to dental amalgam but also as a low cost alternative to gold and ceramic restorations. According to Ruddell et al. , the advantages of dental composites are relatively low costs, easy handling characteristics and high esthetics. The handling behavior of resin composite materials plays an important part in the clinical use. Resin composites have a lot of different handling characters that are often determined by the method of curing and by the size of their filler particles. Some handling characteristics are for instance pack-ability, flow, viscosity, thixotropy and shape stability. While plenty of refinements have been made in resin composite materials two handling characteristics of composites have not existed until recently: non-stickiness and fluid inject-ability .
The quality of being non-sticky is a fundamental part for the practitioner during the preparation. The resin composite material should not be sticking on the plunger while filling the cavity with the resin composite material; on the other hand the material should be sticky enough to stay in the prepared cavity. “The current flowable materials, while easily syringed into place, are difficult to manipulate because of their stickiness. Any attempt to move or smooth these materials is complicated by their sticking to instrument surface.” Manufacturers try to minimize the stickiness by reducing the viscosity of the material . Lee et al. list three important conclusions: Firstly the connection between the viscosity and the stickiness; the more viscous a material is, the less sticky is it; secondly the exponential increase of viscosity with the increase of the percentage of the filler volume; thirdly the exponential decrease of the composite’s viscosity corresponding to a higher temperature. The last finding of Lee et al. defines one requirement for the handling of composite materials: the material should not tack while transferred from the packing containers to the prepared cavity into the mouth by not sticking to the instrument. Inserted in the cavity the material should have more sticky characters to tack onto the surrounding walls. The temperature change between the room and the mouth helps to achieve this requirement.
Another problem in clinical dentistry with dental composites are porosities and voids in restorations. Opdam et al. mention that the risk of voids and porosities increases when the material sticks to filling instruments because air will entrap. They have found porosities in 373 out of 480 restorations. Furthermore Tyas et al. point out that a marginal opening appears when the material sticks to the condenser.
Until now not much attention was given to the handling characteristic stickiness. Al Sharaa and Watts focussed on stickiness in their study. They gained values for stickiness from the elevated profile of the resin composite material moved up from a stainless steel instrument (2 cm/s) and the projected areas of elevation which discriminated the stickiness between 12 commercial composite materials.
In this present study, using an advanced instrument for testing stickiness, the speed of elevating the composite materials can be adjusted. Furthermore stickiness on steel, on dentin and on bonded dentin, using different bonding agents has been observed.
The major objective of this study was not only to investigate the stickiness of different resin composite materials but also the even more important question if stickiness changes under different conditions concerning: (1) speed of the instrument elevating the composite material; (2) temperature of the resin composites during placement; (3) surface material of the instrument’s contacting head. In line with these objectives the following null hypotheses were formulated: (1) the speed of the instrument elevating the composite materials does not influence stickiness; (2) composite materials tested at 23 °C and 37 °C show the same stickiness; (3) different materials (steel, dentin, dentin coated with bonding materials) covering the contacting head do not influence stickiness.
2
Materials and methods
2.1
Stickiness instrument and test procedure
In this study three different composite materials were selected out of 13 commercial resin composites, which have been tested on stickiness in a previous study by Watts . Estelite, Filtek Supreme XT and Premise were chosen according to their widely spread stickiness characters, ranging from extremely low to extremely high stickiness ( Table 1 ).
| Material | Batch and shade | Classification | % fillers by weight | % filler by volume | Mean particle size | Monomer matrix | Manufacturer |
|---|---|---|---|---|---|---|---|
| Estelite | ESA39428 (A3) | Submicron filled composite | 82 | 71 | 0.2 μm (silica–zirconia and composite filler, particle size range from 0.1 to 0.3 μm) | bis-GMA and Triethylene glycol dimethacrylate | Tokuyama Dental Corporation, Japan |
| Premise | 443493 (A3,5) | Tri-modal composite restorative | 84 | 71.2 | 0.4 μm (prepolymerized filler 30–50 μm, barium glass 0.4 μm, silica filler 0.02 μm) | Ethoxylated bis-phenol-A-dimethacrylate, TEGDMA | Kerr Italia S.r.l., Italia |
| Filtek Supreme XT | 6HM (A3B) | Universal restorative | 72.5 | 57.7 | 0.6–1.4 μm average cluster particle size, 5–20 nm primer particle size | bis-GMA, UDMA, TEGDMA, and bis-EMA | 3M-Espe, Dental Products, USA |
An apparatus was designed specifically to measure the stickiness of resin composites. This apparatus is based on a rectangular support aluminium base (23.3 cm × 5 cm). On the front of the instrument, a magnifying viewing window is fixed. A circular temperature control stage, with a temperature range from 10 °C to 50 °C, is fixed on the rear of the base. The upright aluminium bar behind supports the linear drive assembly in turn. There are two adjustable arms, with two light emitting diode (LED) lights attached, projecting from the rear upright. The unit fixed on the rear part of the upright bar allows a flat-ended stainless steel rod to move a vertical-axial direction via a linear motor. The movement of the rod is supervised by the control unit. The rod tip’s surface fits into a prepared cylindrical cavity, of 6 mm in diameter and 3 mm in depth, constructed in a polytetrafluoroethylene (PTFE) disk placed in the temperature control stage.
The resin composites were carefully packed into the PTFE mold such that minimal porosity and no external debris were incorporated. The probe moves axially down within a selected speed range 0.1–5.0 mm/s until its surface contacts the material. Dunking completely in the material the direction of the rod is changed which elevates the test material until the contact between the rod and the material is lost. Immediately after detaching the material from the instrument, a LED light cure unit (Stopford Rheology Ltd., Manchester) was used to cure the resin composites at 600 mW/cm 2 for 40 s. This fixed the elevated composite material into position, allowing subsequent measurement of ‘peak height’ as an index of stickiness. Six different speeds for the upward movement of the rod, for each material, were chosen for the measurements.
The distance x between the initial position of the rod and the mean peak height of the elevation was measured by letting down the rod from its upper position, until the peak height (mm) of the material’s elevation was reached and measured. For comparing the data the ‘areas under the curves of peak height vs. probe speed’ (mm 2 /s) were calculated and analyzed.
For measurements investigating stickiness of resin composites to surfaces other than stainless steel, a precision fixing clamp was attached to the rod, enabling disk surfaces of bovine dentin to be attached and lowered onto the resin composite paste surface, as detailed below.
2.2
Stickiness of resin composite pastes to various rod surfaces
Estelite, Filtek Supreme XT and Premise were tested at 23 °C and 37 °C under various conditions ( Table 2 ). The first test procedure was done on steel. The similarity of the steel rod to the working part of a dental instrument results in data that are close to clinical use. The stickiness of composite materials was further tested on dentin slices. The dentin slices were produced by the Bernhard Gottlieb University Clinic of Dentistry (Vienna, Austria). Bovine teeth were cut into small slices (width: 1 mm). The slices were ground until they had a circular shape with a diameter of 6 mm. Then the dentin slices were fixed to the steel rod with the help of an additional part (fixing clamp), which was produced by the Technical University of Vienna. The fixing clamp (diameter: 12 mm) has an impression with a diameter of 6 mm and a depth of 2 mm. On the other side of the fixing clamp is an elevation (diameter: 6 mm) which corresponds to the area of the flat-ended stainless steel rod of the stickiness instrument. On this elevation the small bovine dentin slices were attached with glass-ionomer luting cement (Ketac Cem Applicap, 3M Espe). Another test procedure was carried out on dentin slices, which were covered with two different bonding systems (a total etch system and a self etch system, Table 3 ). The test procedure for testing the stickiness on dentin slices was identical as described above.
| Steel | Dentin | Bonding Optibond Solo Plus | Bonding Optinbond All in one | |
|---|---|---|---|---|
| Estlite | 23 °C and 37 °C | 23 °C and 37 °C | 23 °C and 37 °C | |
| Filtek Supreme XT | 23 °C and 37 °C | 23 °C | 23 °C and 37 °C | 23 °C and 37 °C |
| Premise | 23 °C and 37 °C | 23 °C and 37 °C | 23 °C and 37 °C |
| Material | Contribution/filler by weight | Procedure | Manufacturer |
|---|---|---|---|
| OptiBond Solo Plus | 15% filled with 0.4 μm barium glass | Etch 15 s, dry lightly, apply adhesive for 15 s, air blow 3 s, light cure 20 s | Kerr, Italia S.p.A. |
| OptiBond All in One | 7% | Scrub 20 s, apply second application, scrub 20 s, air blow 5 s, light cure 10 s | Kerr, Italia S.p.A. |
2.3
Statistical methods
To investigate the repeatability of the peak height for speed = 1.905 mm/s, specimens were measured three times. Coefficients of variation (CVs) were calculated to measure the repeatability. To investigate the influence of speed and temperature on the peak height ANOVAs were calculated for the different materials (Estelite, Filtek Supreme XT, Premise). The stickiness of the different classes and materials was compared in terms of the area under the peak height-curve (as a function of speed). To investigate the influence of materials, classes and temperature on these ‘areas under the curve’, again an ANOVA was performed.
2
Materials and methods
2.1
Stickiness instrument and test procedure
In this study three different composite materials were selected out of 13 commercial resin composites, which have been tested on stickiness in a previous study by Watts . Estelite, Filtek Supreme XT and Premise were chosen according to their widely spread stickiness characters, ranging from extremely low to extremely high stickiness ( Table 1 ).
| Material | Batch and shade | Classification | % fillers by weight | % filler by volume | Mean particle size | Monomer matrix | Manufacturer |
|---|---|---|---|---|---|---|---|
| Estelite | ESA39428 (A3) | Submicron filled composite | 82 | 71 | 0.2 μm (silica–zirconia and composite filler, particle size range from 0.1 to 0.3 μm) | bis-GMA and Triethylene glycol dimethacrylate | Tokuyama Dental Corporation, Japan |
| Premise | 443493 (A3,5) | Tri-modal composite restorative | 84 | 71.2 | 0.4 μm (prepolymerized filler 30–50 μm, barium glass 0.4 μm, silica filler 0.02 μm) | Ethoxylated bis-phenol-A-dimethacrylate, TEGDMA | Kerr Italia S.r.l., Italia |
| Filtek Supreme XT | 6HM (A3B) | Universal restorative | 72.5 | 57.7 | 0.6–1.4 μm average cluster particle size, 5–20 nm primer particle size | bis-GMA, UDMA, TEGDMA, and bis-EMA | 3M-Espe, Dental Products, USA |
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