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Changes in Resin-infiltrated Dentin Stiffness after Water Storage

¹ Faculty of Dentistry, Srinakharinwirot University, Thailand;
² Guanghua School of Stomatology & Institute of Stomatological Research, Sun Yatsen University, Guangzhou, China;
³ Department of Oral Biology and
⁵ Department of Oral Rehabilitation, School of Dentistry, Medical College of Georgia, Augusta, GA 30912-1129, USA;
⁴ Pediatric Dentistry and Orthodontics, University of Hong Kong, Hong Kong SAR, China;
⁶ Department of Operative Dentistry, Faculty of Dentistry, Mahidol University, Thailand; and
⁷ Department of Mechanical Engineering, University of Maryland Baltimore County, Baltimore, MD, USA

View larger version (37K): [in this window] [in a new window]   Figure 1. Three-point flexure. A schematic showing the miniature three-point flexure device consisting of a supporting base with a 5-mm span and a loading piston milled from aluminum blocks. The diameter for both supports and the loading piston was 1 mm. The supporting base was fixed with cyanoacrylate glue to a glass Petri dish, which in turn was attached to a metal fixture. This design enabled testing to be performed in either air or a liquid medium. Both polymer beams and macro-hybrid layers were prepared to 0.30 ± 0.01 mm thick, to establish a span-to-depth ratio of approximately 16:1, to minimize shear and local deformation effects during three-point flexure (Mujika, 2007). (B) A schematic depicting three-point flexure of the polymer beams and macro-hybrid layers to 2% strain in water during the repeated flexure part of the experiment. These schematics are not drawn to scale.

View larger version (21K): [in this window] [in a new window]   Figure 2. Degree of conversion and stress-strain curves. Superimposition of FTIR results on the degree of conversion, and (B) conversion rate of the 3 experimental neat resin blends with increasing hydrophilic characteristics. Results represent the mean of 5 readings for each resin blend. (C) Representative stress-strain curves of the 3 polymerized neat resin beams that were stressed to failure under dry condition. (D) A representative stress-strain curve of a macro-hybrid layer fabricated with Resin A that was stressed to 3% strain under dry conditions before being unloaded. Plastic deformation resulted in the exhibition of a residual strain after unloading. Similar results were observed for the other two resin blends (not shown). Thus, subsequent experiments on repeated flexure of the macro-hybrid layers after water storage were conducted at 2% strain.

View larger version (11K): [in this window] [in a new window]   Figure 3. Regressions and correlations. (A) Pearson’s correlation between the mean percent decrease in flexural modulus of the polymer beams (N = 12; horizontal standard deviations) and that of the macro-hybrid layers (N = 12; vertical standard deviations) after 1–4 wks of water storage. Dotted lines: 95% confidence intervals. Reductions in the flexural modulus of the macro-hybrid layers after water storage tend to reflect the neat resins from which they were prepared, with the reductions being more extensive in the resin-infiltrated dentin than in the polymerized resins. (B) The percent decreases in flexural modulus of the macro-hybrid layers after 4 wks of water storage were consistent with a one-phase exponential association model: Y = Ymax*[1–exp(-KX)], where Ymax represents the maximum anticipated percent reduction in flexural modulus for a particular macro-hybrid layer, and K is a rate constant. The half-lives [ln(2)/K] shown along the X axis represent the times required for the flexural modulus of these macro-hybrid layers to drop to half of their maximum anticipated percent changes in flexural modulus. (C) Linear regression analyses of the half-lives of macro-hybrid layers vs. their Hoy’s solubility parameters for polar forces (p) and total intermolecular attraction forces (t). As adhesive resin blends are rendered increasingly hydrophilic, the percent reductions in the stiffness of the corresponding resin-dentin interfaces become more extensive after water storage. The rate of reduction in the flexural modulus also significantly increases in a manner that is proportional to the overall hydrophilicity of the respective resin-dentin interfaces and, specifically, to the polarity of these interfaces.

Journal of Dental Research, Vol. 87, No. 7, 655-660 (2008)
DOI: 10.1177/154405910808700704


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