Polymerization Shrinkage Of Composite Resin And Its Shrinkage Stress In Class Ⅰ Restoration

Composite resin has been widely used in dental restoration due to its good mechanicalproperties and excellent aesthetic performance. But the polymerization shrinkage is oneof the major drawbacks of composite resin because it will produce shrinkage stress in theinterface between resin and tooth structure and subsequent gaps around the restoration. Inorder to reduce the shrinkage stress, some strategies have been proposed, such assoft-start light curing method. The efficacy of soft-start light curing in reducing shrinkagestress has been studied in a filling with a high configuration value, which is different fromthe situation in clinic. In this paper, the polymerization shrinkage of a commercialcomposite resin was tested in filling with different C value and the shrinkage stressaround Class I cavity was tested using photoelastic method.1.The polymerization shrinkage of composite resin filling with lower C valueAim: To test the polymerization shrinkage of a resin composite with lower C valuelight-cured with different curing modes. Method: Bonded-discs method was used to test the polymerization shrinkages of a resin composite light-cured with ten curing modes:high continuous40s (800mW/cm240s), high continuous20s (800mW/cm220s), highramping (200mW/cm2to800mW/cm2in5s+800mW/cm215s), high pulse (800mW/cm220s), moderate continuous (400mW/cm240s), moderate ramping (200mW/cm2to400mW/cm2in5s+400mW/cm215s), moderate pulse (400mW/cm220s), two-step1(100mW/cm220s+800mW/cm220s), two-step2(200mW/cm210s+800mW/cm220s),two-step3(200mW/cm220s+800mW/cm220s). Results: The three two-step modespresented the highest polymerization shrinkage (P<0.01) and two-step1presented ahigher shrinkage compared to two-step2and two-step3; high continuous mode presentedhigher polymerization shrinkage than high ramping and high pulse; no significantdifference existed between three moderate modes; no significant difference existedbetween high continuous40s and high continuous20s. Conclusion: The built-in rampingand pulse light-curing modes are unable to increase the pre-gel shrinkage of resincomposite Charisma and a longer lower light intensity is able to increase the pre-gelshrinkage.2. Polymerization shrinkage stress around Class I composite resin restorationA cavity,4mm in diameter and2mm in depth, was prepared in a polycarbonate plate3cm in side-length and3mm in thickness and100cavities were prepared. A compositeresin was filled and the interference fringes in the PC plate around the filling wereobserved under a photoelastic analysis instrument. The stress around the filling wascalculated according to the diameter of the interference fringes. Results: Soft-startlight-curing modes, including the three two-step modes, did not decrease the stressaround composite resin restoration significantly. Conclusion: For the filling restorationwith a high C value, soft-start light-curing mode is unable to decrease polymerizationstress.3. The surface flow compensation of Class I composite resin restorationA Class I cavity,2mm in depth and4mm in diameter, was prepared in a PC plate. Acomposite resin was filled into the cavity and light-curing with different light-curingmodes. Then the surface of the restoration was scaned with a three-dimensional topography scanner (PS50type, Nanovea Co., USA) and the volume of depressed partwas determined and the volume percent of the depressed part to the whole volume of therestoration was calculated. Results: The three two-step modes produced the leastdepressed volume (P<0.05) and medium continuous mode the highest depressed volume.Other soft-start modes presented no effect on the depression of the restorationsurface(P>0.05). Conclusion: For the filling restoration with a high C value, such asClass I cavity, soft-start light-curing mode is unable to produce more surface flowcompensation for polymerization shrinkage.