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 Table of Contents  
ORIGINAL ARTICLE
Year : 2019  |  Volume : 9  |  Issue : 1  |  Page : 9-17

Comparative evaluation of the effect of three medications on the color stability of two commercially available acrylic resin denture teeth after thermocycling – An in vitro study


Department of Prosthodontics, VSPM Dental College and Research Centre, Nagpur, Maharashtra, India

Date of Web Publication11-Oct-2019

Correspondence Address:
Dr. Neelam Pande
C/O Dr. Abhay S. Pande, 123, “Rujuta” Apartments, Flat No. 1, Pande-Lay-Out, Khamla, Nagpur - 440 025, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijmd.ijmd_1_19

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  Abstract 


Introduction: The purpose of this study is to estimate the color stability of two commercially available acrylic resin teeth and the effect of three types of medications after thermocycling, by spectrophotometric analysis.
Aim: The aim of this study is to evaluate the effect of three types of medications – iron syrup, cough syrup, and digestive syrup on the color stability of two commercially available acrylic resin denture teeth after thermocycling.
Materials and Methods: Total 160 samples were divided into two main groups. The samples were first thermocycled and randomly divided into subgroups according to the respective brands and were subjected to baseline spectrophotometric measurement and then immersed in respective medication solutions. After 4 weeks, color measurements were recorded for all samples. Color differences were determined by ΔE* and National Bureau of Standards units (NBS units).
Results: After ΔE evaluation, control groups had values A1 (0.509) and B1 (0.498). Color differences with ΔE values above 3.7 were observed in all the test subgroups. When the National Bureau of Standards (NBS) values were evaluated, iron syrup was observed to cause “much” change in both the subgroups A2 (7.835) and B2 (7.843), but more color change was observed in B2.
Conclusion: Acry Rock denture teeth were more color stable than Rolex in three medications. Most chromogenic staining medication was found to be iron syrup, which caused “much” color changes, followed by digestive syrup. Moreover, the least staining medication was cough syrup, which caused “appreciable” color changes according to the NBS critical remarks of color differences.

Keywords: Acrylic teeth; color stability; NSB values; spectrophotometer


How to cite this article:
Pande N, Kulkarni S, Jaiswal P, Chahande J. Comparative evaluation of the effect of three medications on the color stability of two commercially available acrylic resin denture teeth after thermocycling – An in vitro study. Indian J Multidiscip Dent 2019;9:9-17

How to cite this URL:
Pande N, Kulkarni S, Jaiswal P, Chahande J. Comparative evaluation of the effect of three medications on the color stability of two commercially available acrylic resin denture teeth after thermocycling – An in vitro study. Indian J Multidiscip Dent [serial online] 2019 [cited 2019 Dec 12];9:9-17. Available from: http://www.ijmdent.com/text.asp?2019/9/1/9/268983




  Introduction Top


In removable dentures, artificial teeth are a significant part of the overall esthetical outcome. In the maintenance of esthetical effects, the color stability of artificial tooth materials is one major factor. In particular resin, teeth used since the 1930s are known to have side-lined ceramic teeth from the market recently. This is because resin teeth gain an upper hand over ceramic teeth by virtue of these advantageous properties: better bond to the denture base, lighter weight, and lower fracture proneness.[1],[2] On the downside, these are inferior to porcelain teeth in maintaining adequate esthetic appearance with wear and discoloration and thus have a shorter durability period and service life. To overcome the wear problems of conventional plastic teeth, high-strength plastic teeth have since been developed and applied clinically.[3],[4]

Color stability is considered an important physical property of dental materials. The technique used for color determination can be visual or instrumental. Instrumental colorimetry eliminates subjective mistakes in color evaluation. Instrumental colorimetry eliminates subjective mistakes in color evaluation. It is more accurate as it assesses small differences in object color. Color changes in acrylic resins resulting from intrinsic factors are resin discoloration, including alterations to the matrix, and also seen due to the aging of material, usually results from exposure to physical and chemical conditions involving thermal changes and humidity. Extrinsic factors, such as absorption and adsorption, can promote discoloration, associated with the absorption of materials (e.g., tea, red wine, chlorhexidine, and iron salts) onto the surface cause extrinsic stain.[5],[6] Smoking, chewing tobacco, medications, and diet may affect the chemical stability of materials. Also other factors such as stain accumulation, water absorption, dehydration, leakage, rough surfaces, chemical and aging degradation, oxidation during the double reaction of carbon-producing peroxide composites, and permanent formation of pigments following product degradation, create extrinsic stains.[7],[8]

Within an aging population-specific disease patterns are emerging, with older people more likely to develop several chronic diseases. The dental practitioner will increasingly be faced with elderly patients requiring the administration of variety of drugs.[9] Numerous factors alter healing, host resistance, digestion and absorption, mastication, metabolic competency, renal and hepatic function, and excretory capability. Disorders of the gastrointestinal tract may give rise to nutritional deficiencies through loss of nutrients. Loss of iron is the most common cause of iron deficiency in the elderly.[10] Intake of liquid iron solutions in the treatment of iron-deficiency anemia causes heavy extrinsic staining of the teeth and acrylic restorations.[11] On this background, it may get necessary for elderly patients to consume syrups such as iron syrup, cough syrup, and digestive syrup for a long period of time.

Intraorally, prosthetic materials are subject to mechanical, chemical, and thermal influences through eating, drinking, and breathing.[12] Dentures are perpetually exposed to saliva which contains various ions, micro-organisms, and drugs, which penetrate the resin leading to discolorations. The current available literature does not show any studies on the effect of medications on the color stability on acrylic resin denture teeth. Color of acrylic teeth remains stable over a long span of life on exposure to oral environment and staining agents such as medications. However, acrylic teeth are susceptible to abrasion and pigments and tend to become gradually discolored by pigments contained in the diet with long-term use, causing an esthetic failure. The purpose of this study is to evaluate and compare the color stability of two commercially available acrylic resin denture teeth under the effect of three types of medications after thermocycling, by spectrophotometric analysis.

Aim

The aim of this study is to evaluate the effect of three types of medications – iron syrup, cough syrup, and digestive syrup on the color stability of two commercially available acrylic resin denture teeth after thermocycling.


  Materials Top


Materials

Reinforced acrylic teeth (Acry Rock, Ruthinium A1/S68 and Rolex, Ashoosons 22/7) [Figure 1] solutions used artificial saliva (MP Sai Enterprise, Mumbai-53), iron syrup (Dexorange, Cheryl Laboratories Pvt. Ltd. Plot No. A329/330, MIDC, Mahape, Navi Mumbai - 400 701, E15035), cough syrup (Asthalin, Cipla Ltd., Srinagar, Vijayawada, 520 007, B845028), and digestive syrup (Aristozyme, Aristo Pharmaceuticals Pvt. Ltd. 371, Kunbar Falia, Dabhel, Nani Daman - 396 210, D78A195) [Figure 2]. For tooth-positioning jig, autopolymerizing acrylic resin, polyvinyl siloxane impression material, separating medium, die stone, and pumice [Figure 3]. Equipment used thermocycling machine (LG, Model GC-051SA, Mahavir, India) [Figure 4], incubator (Lyzer, India), spectrophotometer (Perkin Elmer ultraviolet-visible) [Figure 5], and pressure pot (Puneet, India).
Figure 1: Maxillary central incisors (Acry Rock and Rolex)

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Figure 2: Medications used for immersion

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Figure 3: Materials used for tooth-positioning jig

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Figure 4: Thermocycling machine

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Figure 5: Spectrophotometer

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Method (Plates III, IV, and V)

  1. Thermocycling of acrylic teeth: Total 160 maxillary central incisors (Shade A1) were used for evaluating color stability for two different brands, 20 for each subgroup. All samples were initially thermocycled to simulate aging, in distilled water bath at 5°C and 55°C with 30-s dwell time, for 2000 cycles[5]
  2. Fabrication of tooth position acrylic jig: To facilitate repetitive measurements for each tooth in the same region. Customized brass dies with threaded hole, the jig was made and invested. It was packed with acrylic resin to form the resin block of dimensions 40 mm × 20 mm × 9 mm, according to the sample holder of spectrophotometer
  3. Color measurements after 24 h: After thermocycling, the samples were kept in artificial saliva for 24 h at 37°C in an incubator, rinsed with distilled water for 30 s, and divided randomly into subgroups with 20 samples for each solution and labeled. They were placed in the customized tooth-positioning jig and subjected to spectrophotometric analysis; these readings were taken as baseline
  4. Teeth immersion in respective medications: The samples were immersed in the 200 ml of respective medication solutions, fixed vertically on the floor of a container to avoid tooth-to-tooth contact[4] [Figure 6], and stored in labeled polyethylene containers. The solutions were carefully stirred twice a day to prevent sedimentation.[1] Artificial saliva was used as control. The samples were stored at 37°C in an incubator to simulate the temperature of the oral cavity and evaluated for color change after 4 weeks
  5. Color measurements after 4 weeks: Accelerated aging is utilized usually in a laboratory by controlled standard test methods. The samples were first rinsed in distilled water for 30 s and wiped clean dry. They were secured on tooth-positioning jig and subjected to spectrophotometric analysis. Average color change for two materials for each solution was then calculated using the CIE L*a*b* uniform color scale. Color difference data were completed using the National Bureau of Standards (NBS). Color differences (ΔE*) were determined using the following equation: ΔE* = ([ΔL]2+ [Δa]2+ [Δb]2)½. Values ΔE >3.7 was considered as clinically not acceptable. To relate the amount of color change (ΔE*) recorded by spectrophotometer to the clinical environment, the data were converted to NBS units through the equation:- NBS units= ΔE* ×0.92, where critical remarks of color difference expressed in terms of NBS units[13] as follows: 0.0–0.5 (trace) – extremely slight change, 0.5–1.5 (slight) – slight change, 1.5–3.0 (noticeable) – perceivable change, and 3.0–6.0 (appreciable) – marked change.
Figure 6: Teeth immersed in respective medications, stored in incubator

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  Results Top


The two brands of teeth were divided into 2 Groups:- A:- Acryrock and B:- Rolex, for purpose of the manuscript and statistics description.



For analysis of Group A, mean △E and NBS units [Table 1] were compared across four subgroups separately by performing one-way ANOVA test. Pairwise comparison between subgroups of each main group was done by Tukey's multiple comparison test. A similar test was applied for Group B [Table 2]. [Graph 1] shows the comparison of mean difference in △E and NBS units between four subgroups of A. Multiple comparison test showed that the difference between all subgroups was highly significant ( P < 0.001) with the highest seen between A1 versus A2 followed by A2 versus A3, A2 versus A4, A1 versus A4, and A1 versus A3. The least was between A3 versus A4 ( P = 1.000). Furthermore, multiple comparison test showed that the difference between all subgroups was highly significant ( P < 0.001) with the highest seen between B1 versus B2 followed by B2 versus B3, B2 versus B4, B1 versus B4, and B1 versus B3. The least was between B3 versus B4 ( P = 0.410) [Graph 2].
Table 1: Comparison of mean E and mean NBS units between four subgroups of group A

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Table 2: Comparisons of Mean E and Mean NBS units between four subgroups of Group B

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Mean △E and NBS units between each subgroup of A and B were compared by performing independent t-test and comparison of mean △E and mean NBS units between A1 and B1 by independent t-test [Graph 3] and [Graph 4]. It shows no significant difference between the two subgroups for △E ( P = 0.7890) and NBS units ( P = 0.8130). Similarly, in comparison between A2 and B2 by independent t-test, there was no significant difference between the two subgroups for △E ( P = 0.9821) and NBS units ( P = 0.9836). Between A3 and B3, there was no significant difference between the two subgroups for △E ( P = 0.3904) and NBS units ( P = 0.3892). Between A4 and B4, there was no significant difference between the two subgroups for △E ( P = 0.2117) and NBS units ( P = 0.2126). When ΔE values were evaluated, the control groups had the values A1 (0.509) and B1 (0.498). Color differences with ΔE values above 3.7 were observed in all the test subgroups. The highest was seen in B2 (8.527), followed by A2 (8.518), B4 (4.501), B3 (4.091), A4 (3.930), and A3 (3.739). When NBS values were evaluated, iron syrup was observed to cause “much” change in both the subgroups A2 (7.835) and B2 (7.843), but more color change was observed in B2. Immersion in cough syrup caused “appreciable” change in both subgroups A3 (3.440) and B3 (3.764), which showed more color change observed in B3. Immersion in digestive syrup also caused “appreciable” change in both subgroups A4 (3.617) and B4 (4.139), but more color change was observed in B4. The control groups A1 (0.468) and B1 (0.459) had only “trace” change. Thus, from statistical analysis, it is clear that Group A samples (Acry Rock) were more color stable than Group B (Rolex) after 4 weeks of immersion in all the three medications; however, the values were not statistically significant.




  Discussion Top


Acrylic teeth are most widely used owing to important characteristics, such as: Chemical bond to the denture base acrylic resin, which is resistant to cracks and organic solvents, has more natural appearance, less friability, reduced occlusal sounds, easy characterization, and greater impact resistance and flexural strength. On the other hand, these teeth have the disadvantages of low wear resistance, they easily absorb odors and do not have acceptable color stability, as they suffer the action of organic coloring agents, water, sunlight, and chemical agents.[6],[7]

The ability of denture teeth to remain color stable through wear and time is critical. The etiology of artificial tooth discoloration is multifactorial. Wear, lack of patient maintenance, the effect of compositional characteristics, exposure to stains, and time are factors that contribute to intrinsic and extrinsic staining. With an increase in esthetic demands and patient expectations of removable prostheses, the resistance of denture teeth to color changes plays a significant role in the selection of denture teeth.[14],[15],[16] Hersek et al.[17] suggested that most of the prosthetic materials are subject to sorption – a process of adsorption and absorption of liquids depending on environmental conditions. Sepúlveda-Navarro et al.[18] have explained that the discoloration of the denture base polymers in oral environment may be caused by the extrinsic or intrinsic factors. Intrinsic factors are related to the material's chemical stability and oxidation of polymer matrices. The intrinsic color can be altered as a result of accelerated aging conditions, mimicking sunlight, and water effects, such as ultraviolet irradiation, temperature, and humidity changes. Dietary factors and medications are commonly reported among the agents that cause extrinsic discoloration of the restorative materials.[19],[20],[21]

While a number of authors have reported on the color changes and staining of acrylic denture teeth materials under conditions of accelerated aging and exposure to oral fluids,[22],[23] the literature contains no references to the color stability of acrylic denture teeth materials in medications. Acry Rock consists of high molecular weight resin (polymethylmethacrylate [PMMA] cross-linked). It comes in double chromatic layers. Rolex is also available in double chromatic layers and is based on cross-linked PMMA resin. Yet, the degree of cross-linking within prosthetic teeth is somewhat greater than that within polymerized denture bases.[24]

Accelerated aging is a test that uses aggravated conditions to expedite the normal aging procedures of the tested materials, to help determine the long-standing outcome at anticipated levels of stress within a shorter time. It is utilized generally in a laboratory by controlled standard test methods. In this study, 2000 cycles of thermocycling were used simulating 2 years of clinical use.[25],[26] Changes in the molecular structure of polymers as a result of the aging process have been attributed to factors such as oxygen cross-linking, leaching of plasticizers, and water sorption. Thermocycling can promote volumetric contraction and expansion of materials, leading to degradation.[27]

Hersek et al.[17] and Crispin and Caputo[28] suggested that discoloration of the acrylic resin is possible if the contacting solution is colored or stained. Satoh et al.[2] employed 2% aqueous solution of no. 102 red food dye, 2% aqueous solution of coffee, and 2% aqueous solution of turmeric and found the turmeric solution tended to possess the strongest affinity for all the tested artificial tooth types. Koksal and Dikbas[4] used tea, instant coffee, and cola as staining solutions inferred that instant coffee was the most chromogenic agent among the solutions. Gregorius et al.[29] exposed high strength acrylic denture teeth to red wine and coffee and found that the denture teeth became darker, more chromatic, and redder due to the effects of red wine and coffee but with varied degrees of staining among them.

Specific disease patterns are emerging within an aging population, with older people more likely to develop several chronic diseases. Nutritional deficiencies may arise through loss of nutrients because of disorders of the gastrointestinal tract. Loss of iron is the most common cause of iron deficiency in the elderly.[10] Chronic obstructive pulmonary disease (COPD) is a term used to represent a number of conditions that result in progressive pulmonary airflow limitation. COPD is a leading cause of morbidity and is projected to rank fifth in 2020 as a worldwide burden of disease. These conditions include bronchitis, emphysema, and chronic asthma. Considering this, it may get necessary for elderly patients to consume medications for anemia, cough, and digestion for a long period of time. Problems that interfere with compliance include inability to take the medication. This can be the result of decreased manual ability with aging.[9] Hence, the patient may prefer consuming the syrup over pills.

A period of 4 weeks' immersion might be considered by some as too long an experimental period. However, in most in vitro studies, the final period is typically 4 weeks or more in order to achieve a cumulative staining effect and obtain distinct results.

Related to color changes, Okubo et al.[30] stated that color evaluation by visual comparison is an unreliable method as it can result in inconsistencies in color perception specifications among observers. Since instrumental measurements eliminate the subjective interpretation of visual color comparison, spectrophotometer was used instead of visual evaluation.[21],[31] This has been shown to be more accurate in measuring the color change than colorimeters as spectrophotometers contain monochromators and photodiodes that measure the reflectance curve of a product's color every 10 nm or less. Color changes were characterized using the Commission Internationale d'Eclairage L*a*b* color space (CIE L*a*b*). ΔE* represents the relative color changes that are observed for the materials after treatment or between time periods. ΔE value equal to 1 is considered visually detec[table 50]% of the time, whereas ΔE value >2 is perceptible 100% of the time. However, there is another threshold regarding the color stability of the materials. The upper limit of the acceptability in subjective visual evaluations has been confirmed by Um and Ruyter,[31] who suggested that a perceptible discoloration must be referred to as acceptable up to a value ΔE = 3.3, while Guler et al.[32] mentioned that a value of 3.7 should be considered as visually perceptible. Johnston and Kao[33] assessed appearance match by visual observation and clinical colorimetry. Judgments of appearance matching by means of the visual criteria established by the United States Public Health Service (USPHS) and by means of an extended visual rating scale were determined for composite resin veneer restorations and their comparison teeth. A colorimeter of 45°/0° geometry and the CIELAB color order system was used to calculate a color difference for every visual rating of the restorations and comparison teeth. Statistically significant relationships were found between each of the two visual rating systems and the color differences. The average CIELAB color difference of those ratings judged that a match by the USPHS criteria was found to be 3.7. Hence, in the present investigation, we have considered value ΔE >3.7 as clinically not acceptable. In dental literature, many authors[22],[33],[34] used ΔE* values to evaluate the “perceptibility” of color differences. However, it is noteworthy that the criteria of perceptibility adopted by each author were different. To counter such differences and disagreements in the criteria used, NBS rating system is a frequently used method to determine the degree of color difference since it offers absolute criteria by which ΔE* values can be converted to remarks with clinical significance. Therefore, the corresponding NBS units were also calculated to assess the color differences caused by immersion in the solutions.[4]

After evaluating alteration in the color in all the immersion solutions, color change in Rolex was more compared to Acry Rock, showing comparatively more color stability; however, the difference was not statistically significant. Artificial saliva, which was the control, by itself, caused color change in the specimens after 4 weeks. It may be speculated that staining was most likely caused by mucin.[35] However, according to the NBS units, it can be categorized as “trace” difference, which is clinically not significant. Color differences with ΔE values above 3.7 were observed in all the experimental subgroups of Acry Rock and Rolex, with the highest seen with Rolex samples in iron syrup (8.527). After evaluating NBS values, iron syrup was observed to cause “much” change in both subgroups of Acry Rock (7.835) and Rolex (7.843), more in Rolex samples. Immersion in cough syrup caused “appreciable” change in both subgroups of Acry Rock (3.440) and Rolex (3.764), more in Rolex. Immersion in digestive syrup also caused “appreciable” change in both subgroups Acry Rock (3.617) and Rolex (4.139), again more color change observed in Rolex. It is well known from literature that polymer materials undergo water sorption. The water may eventually cause irreversible damage to all forms of acrylic by the formation of microcracks as a result of repeated sorption/desorption cycles, which is followed by hydrolytic degradation of the polymer with scission of the ester linkages and gradual deterioration of the infrastructure of the polymer over time. This phenomenon may contribute to form acrylic zones with different optical properties, which can be visibly detected or esthetically unacceptable.[36] Acrylic resin teeth formed of polymethylmethacrylate present a high conversion rate and a low quantity of additional reagents, such as dibenzoyl peroxide, that remain after the reaction and may cause deterioration in color stability.[5],[23]

Certain teeth may have a higher degree of cross-linking than the others, causing less water-soluble dyes to be attracted to the surface of the teeth. Color change may alter the organic matrix of the material. Inhibition, hydrolysis, and breakdown of polymeric chains may result in division of main chains and separation of cross-linking.[5] The teeth evaluated in this study presented that similar chemical structures (PMMA) may be with different quantities of cross-linking agents, plasticizing, and pigments, which may explain the differences in color change for each brand of artificial teeth.[4]

Immersion in all the three medications caused a color change which was above 3.7 ΔE units. Out of these, the iron syrup caused the most color change, followed by digestive syrup, with the least in cough syrup. When NBS values after 4 weeks were evaluated, “much” changes were observed in both acrylic teeth subgroups immersed in iron syrup. Color changes in cough syrup and digestive syrup were “appreciable” according to the NBS rating system. Hence, iron syrup was found to be more chromogenic than other solutions. Between cough syrup and digestive syrup, there were no significant differences in these two solutions in their color change effects. In the present study, the iron syrup consisted of ferric ammonium citrate, cyanocobalamin, folic acid, alcohol (95%), and erythrosine. Nordbö et al.[11] observed that the intake of liquid iron solutions caused heavy extrinsic staining of the teeth and acrylic restorations. They mentioned iron (III) compounds, i.e., ferric compounds are generally more strongly colored than iron (II) or ferrous combinations. The alcohol has a plasticizer effect on the polymeric matrix, diminishing the adhesion of the filler content to the resin matrix. This may lead to the detachment of these particles, consequently, facilitating staining.[7]

The cough syrup consisted of salbutamol sulfate, flavored syrup base, and color – carmoisine. The contents of the digestive syrup were diastase, pepsin, flavored syrup base, and color – caramel. Medications may affect the chemical stability of materials. Oxidation in the structure of the material of unreacted pendant methacrylate, alteration of the matrix interface, matrix, and fillers and chemical bonding breakdown are some examples of chemical changes. Extrinsic factors, such as stain absorption and adsorption, may cause further discoloration.[7] According to Anil et al.,[8] the following factors may contribute to color changes: stain accumulation, dehydration, water absorption, rough surfaces, chemical/wear degradation, and oxidation during double carbon reactions, which produced peroxide compounds and the continuous formation of pigments due to the product's degradation. The color stability of an acrylic denture teeth material thus depends on the chemistry of that material, the surface finishing, and the environment it is subjected to.

Clinical implications

Analyzing the entire spectrum of this study, it becomes evident that the long-term consumption of medications may lead to color changes in the acrylic denture teeth which may not be in the clinically acceptable levels. In this study, Acry Rock was found to be more color stable than Rolex. Using silanized filler content might reduce the formation of microgaps and microfractures among the resin matrix-filler interfaces and thus decrease water uptake and staining probabilities.[29] When long-term consumption of medications is unavoidable, selection of the denture teeth material should be carefully done. Oral and denture hygiene instructions should be reinforced and the patient appropriately counseled.

Limitations

Being an in vitro study, oral environment was not simulated completely. Scope for further studies was as follows: (1) other tooth materials such as porcelain or composite can also be considered and (2) measurements can also be made at intervals or long-term evaluation.

Summary

The esthetic appearance of prosthesis is certainly an important feature required by patients and must satisfy their expectations. Acry Rock denture teeth were found to be more color stable than Rolex in the given test solutions.


  Conclusion Top


Within the limitations of this study, the following conclusions can be drawn:

  1. All tested acrylic teeth had color change values which were greater than clinically acceptable values (ΔE >3.7) in all the three medications
  2. Acry Rock acrylic resin denture teeth were more color stable than Rolex in the three medications; however, the difference was not statistically significant
  3. The most chromogenic staining medication was found to be iron syrup, which caused “much” color changes, and the least was cough syrup, which caused “appreciable” color changes, whereas the chromogenic potential of digestive syrup was almost similar to that of cough syrup, which also caused “appreciable” color changes according to the NBS critical remarks of color differences.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
 
 
    Tables

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