The aim of this study was to develop a new method to measure the polymerization shrinkage of light cured composites and to evaluate the overall utility and significance of the technique.
An optical instrument to measure the linear polymerization shrinkage of composites without directly contacting the specimen was developed using a particle tracking method with computer vision. The measurement system consisted of a CCD color video camera, a lens, an image storage device, and image processing and analysis software. The shrinkage kinetics of a commercial silorane-based composite (P90) and two conventional methacrylate-based composites (Z250 and a flowable Z350) were investigated and compared with the data measured using the “bonded disc method”.
The linear shrinkage of the composites was 0.33–1.41%. The shrinkage value was lowest for the silorane-based (P90) composite and highest for the flowable Z350 composite. The estimated volume shrinkages of the materials were comparable to the axial shrinkages measured with the bonded disc method.
The new instrument was able to measure the true linear shrinkage of composites without sensitivity to the specimen geometry and the viscosity of the material. Therefore, this instrument can be used to characterize the shrinkage kinetics for a wide range of commercial and experimental visible-light-cure materials in relation to the composition and chemistry.
Restorative light-cured resin composites undergo a volumetric shrinkage of 2–5% during curing. Polymerization shrinkage from direct composite restorations produces intense stress on cavity walls, which can cause debonding, microgaps, enamel microcracks, and cuspal deplection. As a result, restorative failures such as postoperative hypersensitivity, pulpitis, and secondary caries may occur .
Polymerization shrinkage occurs because, as the polymerization reaction proceeds, the distances between monomers weakly bound by van der Waals forces decrease as monomer molecules become bound by covalent bonds within the polymer. Much research has been performed to develop new monomers or to improve the filler system in an effort to decrease this polymerization shrinkage . As a result, measurement of the polymerization shrinkage strain of composites is very important.
Many devices have been used to measure the polymerization shrinkage of composites, with each device having its own advantages and disadvantages . The standard device that has been used by investigators to measure volumetric polymerization shrinkage is the mercury dilatometer. In this instrument, the volume change of a liquid in a reservoir is amplified into a change in length through a capillary tube and is read like a thermometer. When small amounts of composite samples are investigated with this method, small temperature changes can affect the liquid volume and may result in significant errors. Specimen preparation is very tedious, and specimens with low viscosity such as flowable composites cannot be measured. In addition, environmental mercury contamination poses a concern. Lee et al. developed a new method to measure the volumetric polymerization shrinkage of composites in real time by measuring the buoyancy change of the specimen in distilled water.
When restoring a cavity, the immediate cause of shrinkage stress is linear shrinkage acting in normal directions on the cavity walls . When shrinkage occurs identically in all directions, the relationship between volumetric shrinkage and linear shrinkage is given by <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='γp=13−(1−αp)3=3αp−3αp2+αp3′>γp=13−(1−αp)3=3αp−3α2p+α3pγp=13−(1−αp)3=3αp−3αp2+αp3
γ p = 1 3 − ( 1 − α p ) 3 = 3 α p − 3 α p 2 + α p 3
( γ p , volume shrinkage; α p , linear shrinkage strain), where α p is relatively small. When the second and third power terms on the right side of the equation are ignored, γ p ≈ 3 α p . Therefore, volumetric shrinkage is approximately equal to three times the linear shrinkage.
Linear shrinkage is sometimes measured with linear displacement transducers such as an LVDT (linear variable differential transformer) or a laser sensor and can be converted into volumetric shrinkage. However, this method is applicable only in situations of isotropic contraction. If anisotropic contraction occurs, the measured shrinkage deviates from the real value depending on the geometry of the specimen. For most methods of measuring linear contraction, the sensor that measures the change in length must contact the composite specimen, either directly or indirectly. This affects the specimen geometry and shrinkage direction and results in the reporting of many different shrinkage values for the same composite brand .
In the “bonded disc method”, which is currently the most widely used method for measuring axial linear shrinkage, shrinkage in the radial direction of the thin disc-shaped specimen is restricted by friction due to limitations in the geometric configuration in which the composite is sandwiched between two plates. Therefore, shrinkage in the axial direction is measured to be 1/3 to 1 times the volumetric shrinkage depending on the aspect ratio (the ratio of diameter to height). This effect becomes more complicated when the viscosity of the composite is also being considered and can lead to inaccurate measurements .
When using contact displacement sensors such as a strain gage , there is a minimum amount of mechanical resistance that must be overcome in order to displace the sensor. Therefore, contraction prior to the gel point is compensated by flow, and only post-gel shrinkage is recorded after gel formation in which the elastic modulus of the composite increases beyond a certain point.
Most recently, 3D micro-CT imaging techniques or an optical method using digital image correlation have been used for the non-contact shrinkage measurement of dental composites . However, shrinkage strain during light curing could not be measured.
The aim of the study was to develop a new method to measure the linear polymerization shrinkage kinetics of composites during light curing without direct contact with the specimen using a particle tracking method with computer vision and to evaluate the overall utility and significance of the technique. Using this new instrument, the polymerization shrinkage values of three commercially available light-cured composites were measured and compared.
Materials and methods
System configuration and working principles of the instrument
The computer vision system for measuring polymerization shrinkage consisted of a 768 × 494 pixel CCD color video camera (CS-5260 BD, Tokyo Electronic Industry Co., Tokyo, Japan), a lens without magnification (MML1-110D, Moritex Corp., Tokyo, Japan), a frame grabber for image storage (IMAQ PCI-1411, National Instrument, Austin, TX, USA), and a custom made software for image processing and analysis (IMAQ Vision and Labview 7.0, National Instrument, Austin, TX, USA) ( Fig. 1 a ).
A marker particle placed on the specimen ( Fig. 1 b) is detected by the CCD camera and recorded as RGB color data onto the image storage device, as in the left picture of Fig. 2 . After analyzing the color histogram, binarization is performed according to preset RGB threshold values, as in the right picture of Fig. 2 . For noise reduction, particles below a certain size are removed, and the X , Y coordinates of the marker are obtained using the “area center method” ( Fig. 3 ). In this method, the center coordinates of the marker particle are determined by dividing the total sum of the x , y coordinates of each pixel by the total number of pixels.