Ergun, Nagas mersed in water, these materials undergo 2 pro- This requirement might be difficult to achieve cesses; leaching of plasticizers and other soluble because most of the liners show noticeable color materials into water and water imbibition by the changes when exposed to ultraviolet light.28 Hence, polymer. Eventually, the physical and mechanical the purpose of this in vitro study was to compare properties of the materials change with time in the the effect of accelerated aging on the color stabil- patient’s mouth.9 ity of silicone and acrylic denture liners by using a An ideal processed denture liner should be re- colorimeter. sistant to imbibing oral fluids or releasing com- pounds into the saliva. High water sorption and MAtErIALs And MEtHods solubility of lining material decrease mechanical Two silicone and three acrylic based lining ma- properties such as hardness, transverse strength terials used in this study are listed in Table 1. Sixty and fatigue limit. Discoloration, dimensional disc-shaped samples, with uniform size of 10 mm change and separation from a denture base can diameter and 2 mm in thickness, were prepared also appear because of the high values of sorption for each lining material according to the manu- and solubility parameters.3,11 Thus, sorption, solu- facturers’ instructions. All of the samples were bility and discoloration properties are important to stored in distilled water (DW) at 37±1°C for 24 evaluate the longevity of a liner.6 hours before color measurement. Denture relining materials often show changes Six samples from each tested liner material in mechanical properties with aging.12 Their lon- served as control groups and these samples were gevity was evaluated by immersion in water, accel- immersed in DW at 37±1°C in a dark room for 900 erated weather testing and thermocycling.13-16 The hours. The remaining 6 samples from each group use of accelerated aging process has increased were subjected to accelerated aging in Wheather- in dental researches recently.17-20 This process Ometer instrument (QUV accelerated Weathering simulates the effects of long-term exposure to Tester, The Q-Panel Company 26200 First St., SN: environmental conditions through an accelerated 92-7475-44, Cleveland, Ohio, USA.) and exposed weathering process that involves ultraviolet light to continuous ultraviolet and visible-light, at a exposure, temperature and humidity changes.21 temperature of 110°F and intermittent DW spray Besides, previous studies have shown the effect of was used for a period of 18 minutes within each accelerated aging on the physical and mechanical 2-hours period. The manufacturer of the weath- properties of the liner materials.12,22 ering instrument estimates that 300 hours of ag- Color stability is an important property of a ing intraorally is equivalent to 1 year of clinical denture liner to retain its color in specified en- service.17,29 The samples were kept for 900 hours vironment, especially in long term use. Besides, either in weathering machine or immersed in DW color assessment and its reproduction is one of (equal to 3 years of service). the most challenging aspects of dentistry.23 Visu- Before and after either aging or immersion in al color matching is still the primary method for DW, color measurements of the samples were evaluating the color and may cause dissatisfying evaluated with the colorimeter (Spectrometer, results.20,24 Photoelectric tristimulus colorimeters Gretag Machbeth, SN: KH 1435, Regensdolf, Swit- have the potential to remove some of the variables zerland). Before each measurement session, the found when the visual method is used exclusively colorimeter was calibrated with its white refer- and provide accurate and repeatable measure- ence tile according the manufacturer’s instruc- ments.25,26 tions. The same examiner made three measure- Color changes in the oral environment need to ments for each sample. be investigated further in order to predict which The color change values of all samples were materials will provide the best clinical service in calculated by measuring the mean and standard long term use. According to ANSI/ADA Specifica- deviation of ∆E* values with the use of CIELAB col- tion No. 12, the denture liner should pass the color or system.20,30 The critical remark of color change stability test, which requires that the specimen (∆E*) were quantified by The National Bureau of show no more than a slight color change after an Standards (NBS) rates the way that a color change exposure of 24 hours.27 is evaluated by the human eye (Table 2). As such, July 2007 - Vol.1 145 European Journal of Dentistry