Intra- and inter-brand color differences of denture teeth under different illuminations

Abstract Debonding, staining and wear are usually the reasons for denture teeth replacement by new ones from same or different brands. Objective This study investigates the possible differences in color of denture teeth of the same or different brands under different illuminations, since their metameric behavior in color under specific illumination may become unacceptable. Methodology For the purpose of this study, 10 denture teeth (#11), shade A3, of 4 different brands were selected (Creopal/KlemaDental Pro, Executive/DeguDent, Cosmo HXL/DeguDent, Ivostar/Ivoclar-Vivadent). Teeth stabilized in white silicone mold and the CIELAB color coordinates of their labial surface under 3 different illumination lights (D65, F2, A) were recorded, using a portable colorimeter (FRU/WR-18, Wave Inc). ΔE*ab values of all possible pairs of teeth of the same brand (n=45) or pair combinations of different brands (n=100) under each illumination light, in a dry and wet state were calculated. Data were analyzed statistically using 3-way ANOVA, Friedman’s and Wilcoxon’s tests at a significance level of α=0.05. Results The results showed that brand type affected significantly L*, a* and b* coordinates (p<0.0001), illumination a* and b* coordinates (p<0.0001), but none of them was affected by the hydration state of teeth (p>0.05). Intra-brand color differences ranged between 0.21-0.78ΔΕ* units with significant differences among brands (p<0.0001), among illumination lights (p<0.0001) and between hydration states (p=0.0001). Inter-brand differences ranged between 2.29-6.29ΔΕ* units with significant differences among pairs of brands (p<0.0001), illumination lights (p<0.0001) and hydration states (p<0.0001). Conclusions Differences were found between and within brands under D65 illumination which increased under F2 or A illumination affected by brand type and hydration status. Executive was the most stable brand than the others under different illuminations or wet states and for this reason its difference from other brands is the lowest. In clinical practice, there should be no blending of teeth of different brands but if we must, we should select those that are more stable under different illuminations

In Dentistry, studies on illuminant metamerism are limited. Metameric effects were investigated for direct restorative materials, 5,6 for ceramic materials, 7,8 between dentin and composite materials, 9 between natural teeth and shade tabs, 10 between porcelain and repair composites, 11 between shade guides and shade guide tabs, 12-14 on the opalescence of restorative materials, 15 on the translucency of porcelain and repairing resin, 16 and one on resin denture teeth in Chinese. 17 In most of the studies, the illuminant effect is measured by the degree of changes in color tristimulus values under the different illuminations.
Metamerism Index was modified in a few studies 9-11 , which considered a ΔE* ab greater than zero for the metameric pairs.
Replacing teeth on functioning dentures is not uncommon in clinical situations and knowing the degree of metameric effects of denture teeth of different or even of the same brand under natural and artificial light sources is useful for the behavior of the replacements under different illuminant conditions. Manufacturers use polymeric materials alone or in layers to achieve natural-looking denture teeth with long-lasting high mechanical and optical properties and good bonding to denture base materials. Classifying the materials used is difficult, since many new products can be classified into two or three different categories.
Simple PMMA, highly cross-linked PMMA, micro-filler reinforced polyacrylic (MRP), interpenetrating polymer network (IPN) and nano-hybrid composites (NHC) in a core or layered structure are the usual types of denture teeth. 18,19 Teeth with the same shade name may, therefore, behave as metamers due to differences in composition or texture. For similar reasons, teeth of the same brand but of different batch number may behave as metamers due to changes in the manufacturing processes. Differences among teeth of the same brand and batch are not expected but still possible due to internal manufacturing inconsistencies.
Finally, although denture teeth are usually selected in a dry state in which they may not show metameric effects, some brands may show metameric effects in wet state due to their higher ability to absorb water, which change the way the light interacts with the structure of the material.  To estimate color differences between dry and wet state, the teeth were stored for 48 hours in baths of tap water at 37 o C and measured again as previously described for the dry teeth. ΔΕ* ab calculations of wet intra-brand and inter-brand teeth were performed exactly as for the dry teeth.
Data were analyzed statistically and the mean with its standard deviation (SD) for each brand under all illumination modes and hydration states was calculated, as well as the shift of teeth color coordinates (L*, a*, b* and ΔΕ* ab ), when illumination was changed from D65 to F2 and A. Differences of teeth color coordinates under different illuminations were estimated using 3-way ANOVA, whereas differences in color among teeth of the same brand (intra-brand) or between different brands (inter-brand) were estimated using Friedman's two-way analysis of   Therefore, a three-way ANOVA at α=0.05 was performed and showed that L*, a* and b* coordinates were significantly affected by brand type (p<0.0001), a* and b* by Illumination light (p<0.0001), whereas no one was affected by the hydration state of teeth (p>0.05). The analysis also showed non-significant two-and three-factor interaction (p>0.05) for the L* coordinate, although a significant brand x Illumination interaction was found for a* and b* coordinates (p<0.001). Post-hoc multiple comparisons with Bonferroni correction indicated differences among brands or illuminations in L*, a* and b* coordinates, as also shown in Table 1