The overall goal of this procedure is to use digital image correlation to reveal the shrinkage strain distribution in a model tooth cavity restored with resin composite when it is cured. This is accomplished by first creating a model tooth with a filled cavity and a surface that will allow the needed observations. The second step is to create a speckled pattern on the specimen surface under observation, so that its deformation can be captured by imaging.
Next, capture images of the speckled surface before and after curing of the resin composite, along with images for calibration. The final step is to determine the deformation of the tooth model specimen by comparing the images taken before and after curing. Using the principles of image correlation, ultimately, results can be obtained that show the areas of strain concentrations were de bonding of the resin composite from the tooth surface could occur.
The main advantage of this technique over the existing methods like strain gauge, is that DIC can provide the visual images of the strain distribution over the entire observation surface. This method can help answer key questions in the development of resin encompasses for dental restorations, such as the level of shrinkage, stress induced by the polymerization process. Demonstrating the procedure will be Ian Huxford, a junior scientist from my laboratory As the first step create tooth models.
Sica glass tooth models begin with a long cylindrical glass rod. 10 millimeters in diameter use a low speed diamond saw to cut the rod into 10 millimeter long cylinders. Once the cylinders are the correct length as shown here, adapt the saw to cut a three millimeter wide, two millimeter deep groove across one end of each to form a mesial occlusal distal cavity.
Next on the cylinder ends with a cavity polish, a flat surface perpendicular to the cavity length. This observation surface will allow focusing and image calibration of the restoration. The next step is to fill the cavities.
After cleaning a cavity, prepare a brush and ceramic primer. Use the brush to apply a thin layer of ceramic primer to silent eyes. All the glass cavity surfaces and allow bonding with the resin composites.
After about one minute, apply a thin layer of adhesive appropriate to the resin composite. Use a curing light and follow the manufacturer's instructions to cure the adhesive. About 10 to 20 seconds proceed by covering all of the glass surfaces except the observation surface with black tape.
This will prevent the curing light, reaching the resin composites from unrealistic paths. The cavity is now ready to be filled bulk. Fill each cavity with resin composite.
Scrape off any excess so that all surfaces are flat. For the next steps, obtain white paint that can be sprayed on and find charcoal powder. Begin with the paint.
Spray a thin layer of white paint onto the observation surface, which now includes some resin composite. Immediately sprinkle some of the charcoal powder onto the paint to create high contrast speckles That can be tracked in software. The charcoal should be thinly and evenly spaced on the observation surface.
The experiment will be conducted using a prefabricated platform with a specimen holder and a large horizontal beam. A CCD camera and light are mounted on the beam so as to face the position of a sample holder that can be attached to one end. To begin mounting the specimen, prepare the holder.
Place the model cavity specimen in the holder. Then tighten the screw to secure it. Next, mount the holder on the end of the horizontal beam.
The specimen observation surface should face the camera lens, turn on the light and position it to reduce glare from the resin surface. Now use a stand with adjustable clamps to position a curing light over the tooth model. The curing lamp tip should be about one millimeter above the sample at after everything is in place.
Use the CCD camera to take a reference image of the specimen prior to curing. Then turn on the lamp to cure the resin composite for 20 seconds. After turning off the lamp, wait five minutes and take another image of the specimen for calibration.
Obtain a calibration block. The block has features of known size and spacing. Remove the specimen and place this block at the same position as the observation surface.
Take a picture of the block. Analysis of the specimen will be done in digital image correlation software. Load the before and after curing images and register them using the calibration data.
Next, define an area of interest on the image of the observation surface. Define the size of the square subset windows for each of two iterations. Use a larger size 64 by 64 pixels.
For the first iteration, choose a smaller size 32 by 32 pixels for the second iteration, define the overlap between neighboring subset windows as 50%correlate the image taken after curing with a reference image taken before curing to calculate the displacement and strain distributions. This is a typical plot of the displacement vectors of a cured restoration made with Z two 50. The plot is representative of all resins used.
The arrows indicate the direction of a displacement, and the colors indicate magnitude as shown on the right. The resin composite has withdrawn to the bottom of the cavity with the largest downward displacement in the top center portion. At the same time, the resin composite has contracted horizontally towards the vertical midline, which has zero displacement.
Horizontal tensile strain concentrations denoted by red are along the two vertical tooth restoration surfaces. Higher horizontal contraction strain denoted by bluer colors is seen adjacent to the vertical sidewalls and at the top of the restoration. This depiction of vertical contraction strain shows that it increases gradually with the depth of the cavity while vertical tensile strain is seen on the bottom surface.
When the two strain components are summed referred to as the mean in plain total direct strain, a relatively uniform distribution of contraction strain is seen within the restoration area. Measurement of the mean in plain total direct strain of the three resin composites along the same horizontal line across the samples produced these curves. The composite LS produced the lowest in plain total contraction strain, followed by Z two 50 and then Z 100.
The three materials showed similar tensile strain concentrations of about 1%at the interfaces. While attempting this procedure, it is very important to remember to create the observation surface with the nice speckle pattern and calibrate its dimensions. After watching this video, you should have a good understanding of how to use digital image correlation to review the shrinkage strain distribution in the tooth restored with resin composite when it is cured.
