Ergun, Nagas depends on the degree of hydrophobicity and po- ences in color measurements of these two studies rosity of the liner.3 Acrylic resin liners have high might be explained by the variations in the proper- water sorption and solubility. On the other hand, ties of the instruments used. This effect should silicon liners considered to be stable in aqueous be kept in mind while comparing the results of the environments for variable lengths of time and the researches about the color stability. hydrophobicity of them reduces water sorption.8,39 In the present study, acrylic and silicone These liners might be expected to possess more based liners exhibited significantly different color stain resistant than would the acrylic materials change values after aging. Silicone based liners tested. Kawano et al6 investigated the sorption and (UP, MP) seemed to be more resistant to staining solubility of 12 soft denture liners and reported than acrylic based liners (UH, TR, DL). Further- that MP had sorption values (0.23±0.01) less than more, comparing acrylic based liners, UH showed 0.8 mg/cm2 after 1 year and this met the 1 week marked change than the other acrylic based liners sorption requirements of ADA specification 12. In (TR, DL) after immersion in DW. This might lead us the current study, MP had low color change value to associate this result with the chemical compo- after immersion in DW (∆E* = 1.75) and after aging sition of UH, porosity formed by the air inclusions (∆E* = 4.78). This result might be attributed to the during mixing powder and liquid, surface rough- low sorption values of MP. Shotwell et al22 inves- ness and water absorption. tigated the color stability of long term soft liners Color stability of these materials may vary de- after accelerated aging and showed the appre- pending on their monomers’ chemical composi- ciable (marked change) NBS unit of MP (NBS unit, tion.34 Residual monomer is a well-known plasti- 3.3). Anil et al12 compared the color stability of cizer which remains in polymerized resin as free five denture reline materials and found the lower or unreacted monomer and it affects physical and NBS unit of MP (NBS unit, 1.94) than TR (NBS unit, mechanical properties of acrylics.40 According to 3.8) and UP (NBS unit, 56.05). However, UP was the previous research by Urban et al,40,41 com- the most color stable liner material in this study parisons between the residual monomer in hard (NBS unit, 3.18) and TR showed extremely marked acrylic relining resins (DL, TR, UH, Kooliner) and change (NBS unit, 11.96) after aging. The differ- heat-polymerized denture base resin (Lucitone Table 6. Mann-Whitney U test results of the liner materials. Immersion in DW Aging Material Mean ∆E* Mean rank Mean ∆E* Mean rank Mann-Whitney U Significance Table 7. Mann-Whitney U test results of acrylic and silicone based denture liner materials. Acrylic based liner materials Silicone based liner materials 16.30 13.01 8.68 4.78 3.46 12.33 6.58 149 European Journal of Dentistry 9.50 9.50 8.50 9.33 9.50 0.000 0.000 6.000 1.000 0.000 P=.004 P=.004 P=.055 P=.006 P=.004 Mann-Whitney U Significance 70.000 1.000 P=.108 P<.001 DL TR UH MP UP n=6, P< .01 Treatment Immersion in DW (Mean rank) Aging (Mean rank) n=6 July 2007 - Vol.1 0.50 1.19 7.13 1.75 0.41 3.50 3.50 4.50 3.67 3.50 17.61 21.44