Shrinkage of Dental Composite in Simulated Cavity Measured with Digital Image Correlation

Summary

In order to understand the spatial development of polymerization shrinkage stress in dental resin-composite restorations, Digital Image Correlation was used to provide full-field displacement/strain measurement of restored model glass cavities by correlating images of the restoration taken before and after polymerization.

Abstract

Polymerization shrinkage of dental resin composites can lead to restoration debonding or cracked tooth tissues in composite-restored teeth. In order to understand where and how shrinkage strain and stress develop in such restored teeth, Digital Image Correlation (DIC) was used to provide a comprehensive view of the displacement and strain distributions within model restorations that had undergone polymerization shrinkage.

Specimens with model cavities were made of cylindrical glass rods with both diameter and length being 10 mm. The dimensions of the mesial-occlusal-distal (MOD) cavity prepared in each specimen measured 3 mm and 2 mm in width and depth, respectively. After filling the cavity with resin composite, the surface under observation was sprayed with first a thin layer of white paint and then fine black charcoal powder to create high-contrast speckles. Pictures of that surface were then taken before curing and 5 min after. Finally, the two pictures were correlated using DIC software to calculate the displacement and strain distributions.

The resin composite shrunk vertically towards the bottom of the cavity, with the top center portion of the restoration having the largest downward displacement. At the same time, it shrunk horizontally towards its vertical midline. Shrinkage of the composite stretched the material in the vicinity of the “tooth-restoration” interface, resulting in cuspal deflections and high tensile strains around the restoration. Material close to the cavity walls or floor had direct strains mostly in the directions perpendicular to the interfaces. Summation of the two direct strain components showed a relatively uniform distribution around the restoration and its magnitude equaled approximately to the volumetric shrinkage strain of the material.

Introduction

Resin composites are widely used in restorative dentistry because of their superior aesthetics and handling properties. However, despite being bonded to the tooth tissues, the polymerization shrinkage of resin composites remains a clinical concern as the shrinkage stress developed may cause debonding at the tooth-restoration interface^1-2. Consequently, bacteria can invade and reside in the failed areas and result in secondary caries. On the other hand, if the restoration is well bonded to the tooth, the shrinkage stress may cause cracking in the tooth tissues. Either of these failures will jeopardize the service life of the dental restoration, which will be subjected to a large number of cycles of thermal and mechanical loading.

Measurement of polymerization shrinkage strain and stress has thus become indispensable in the development and evaluation of dental resin composites^3-4. Various measuring techniques or methods have been developed^5-11 with the main purpose of providing a simple setup for measuring the shrinkage behavior of resin composite materials reliably. While the measurements they provide may be sufficient for comparing the shrinkage behaviors of different materials, they do not help in the understanding of how and where shrinkage stress develops in actual restored teeth. Specifically, a question of great interest is how the cavity walls constrain the shrinkage of composites and leads to the creation of shrinkage stress in dental restorations¹². Note that, to create shrinkage stress, part of the shrinkage strain of the resin composite has to be converted into tensile elastic strain. It would therefore be useful if this component of the strain in the restoration can be measured. Recently, the optical full-field strain-measuring technique, Digital Image Correlation (DIC), has been applied to the measurement of free shrinkage of resin composites as well as material flow in dental restorations^13-15. The basic idea of DIC is to track and correlate visible patterns on the sample surface from sequential images taken during its deformation whereby the displacement and strain fields over that surface can be determined. Full-field measurement is one of the main advantages of the DIC method, which is especially useful in observing non-uniform deformation and strain patterns¹³. In this study, DIC was used to uncover the strain patterns in dental resin composite restorations, with the aim of understanding the development of shrinkage stress and identifying potential sites for debonding. This information is not directly available in the works cited above^14-15, which only measured the displacement of the restoration due to polymerization shrinkage. The measurement was conducted using models that simulated teeth with mesial-occlusal-distal (MOD) tooth cavities as an attempt to replicate the stress or strain in real dental restorations. Although the use of real teeth is more anatomically representative, the disadvantage of that is the significant inherent differences among teeth in anatomy, mechanical properties, degree of hydration as well as invisible internal defects¹⁴ that result in large variations in the results. To overcome such a drawback, some studies have tried to standardize tooth samples by grouping them in terms of the buccal size¹⁶ or replaced the teeth altogether with models of a surrogate material¹⁷. For example, aluminum models which have a similar Young’s modulus to enamel (69 and 83 GPa, respectively) have been employed in shrinkage stress measurement, with the level of shrinkage stress being indicated by the cusp deflection¹⁷. In this study, silica glass models (cavities) were used instead because the material also has a similar Young’s modulus (63 GPa) to human enamel and, as it is transparent, any debonding or cracking in the specimens can be readily observed.

Protocol

Note: Three dental resin composites were studied using the glass cavities: Z100, Z250 and LS, as listed in Materials List. Among them, LS is known to be a low-shrinkage resin composite with a volumetric shrinkage of around 1.0%, much lower than those of Z250 and Z100 (~2% and ~2.5%, respectively)^18-19. The equipment and other materials used in this study are also given in Materials List.

1. Model Cavity Preparation

1. Cut a long cylindrical glass rod, 10 mm in diameter, into 10-mm long short rods using a low-speed diamond saw.
2. Cut a Mesial-Occlusal-Distal (MOD) cavity (Figure 1) measuring 3 mm (width) x 2 mm (depth) in each specimen using an adapted low-speed diamond saw.
3. Polish down each cylindrical specimen to create a flat surface perpendicular to the length of the cavity, with dimensions as shown in Figure 1. The flat surface allows precise focusing and image calibration on the restoration. Henceforth, it will be called the observation surface.
4. Prepare three specimens for each of the three materials tested: Z100, Z250 and LS; see Materials table.

2. Cavity Filling with Resin Composite

1. Apply a thin layer of Ceramic Primer with a brush to silanize all the glass cavity surfaces. This allows bonding between the glass surfaces and the resin composites.
2. After about 1 min, apply a thin layer of adhesive. Use LS Adhesive system for composite LS and Adper Single Bond Plus for composite Z100 and Z250.
3. Cure the adhesive with a curing light and duration (10-20 sec) based on the manufacturer’s instructions (Materials table).
4. Cover all the glass surfaces surrounding the restoration with black tape except the observation surface, as shown in Figure 2. The purpose is to avoid the curing light reaching the resin composite through the surrounding transparent glass, which does not happen in real teeth.
5. Bulk-fill the cavity with resin composite and scrape off any excess to flatten all the surfaces.

3. Surface Painting

1. Spray a thin layer of white paint onto the observation surface, which now includes part of the resin composite.
2. Sprinkle immediately some black fine charcoal powder onto the paint to create high-contrast speckles. The irregular shapes of the speckles will help the DIC software to identify them and track their movements.

4. Sample Mounting, Curing, and Photographing

1. Referring to Figure 2, place a specimen (E) into the holder (C) and tighten it with a screw (D). Then, place the whole unit at the end of a large horizontal beam.
2. Secure a CCD camera and a yellow illumination LED light onto the same beam such that they face the observation surface.
3. Using a stand with adjustable clamps, position the curing light such that its tip is about 1 mm above the sample.
4. Take a picture of the specimen to provide the reference image prior to curing.
5. Cure the resin composite for 20 sec.
6. Take another picture at 5 min after curing.
7. Place a calibration block at the same position as the observation surface and take a picture. The calibration block contains an array of circular dots with size and spacing precisely known.

5. Image Analysis with DIC Software

1. Import the two pictures taken for each sample, one before and one after curing, into the DIC software.
2. Calibrate the dimensions of the images and correct for image distortion using the image of the calibration block. .
3. Define the area of interest within the observation surface for analysis.
4. Define the size of the square subset windows as 64 x 64 pixels for the first iteration and 32 x 32 pixels for the second iteration²⁰. Define the overlap as 50%.
5. Correlate the image taken after curing with the reference image taken before curing to calculate the displacement and strain distributions.

Results

Three specimens were tested for each material. After each test, the specimen was examined by eyes or, if necessary, using a microscope. No apparent debonding at the “tooth-restoration” interface or cracking was found.

The resolution of the pictures was 1,600 x 1,180 pixels with a pixel size of 5.8 mm. With a subset window size of 32 pixels, the spatial resolution of the displacement distributions was around 186 mm.

Figure 3 shows a typic...

Discussion

The use of glass cavities with the same shape and dimensions for shrinkage strain measurement was to minimize the variation in results due to differences in size, anatomy and material properties of natural human teeth. In addition, the fused silica glass used in this study has a similar Young’s modulus to enamel, making it a suitable simulant material for natural teeth as far as mechanical behavior is concerned^21-22. Although in real tooth restorations, the resin composite is mostly bonded to dentin rath...

Materials

Name	Company	Catalog Number	Comments
Dental composite Z100	3M ESPE	N362979	volume shrinkage ~ 2.5%, Young's modulus ~ 14 GPa
Dental composite Z250	3M ESPE	N326080	volume shrinkage ~ 2.0%, Young's modulus ~ 11 GPa
Dental composite LS	3M ESPE	N240313	volume shrinkage ~ 1%, Young's modulus ~ 10 GPa
Ceramic Primer	3M ESPE	N167818	Rely X
LS System Adhesive	3M ESPE	N391675	Adhesive for compoiste LS
Adper Single Bond Plus	3M ESPE	501757	Adhesive for compoiste Z100 and Z250
Glass rod	Corning Inc.	Pyrex 7740 borosilicate
Curing light	3M ESPE	Elipar S10
White paint	Krylon Product Group		Indoor/Outdoor, Flat white
Charcoal powder	Sigma Aldrich, Co.	BCBH6518V	Fluka activated charcoal
CCD camera	Point Grey Research, Inc.	Point Grey Gras-20S4C-C