Abstract

Aim: This study aims to provide valuable insights into the effect of waterpipe exposure on the color change of CAD/CAM plastic dental materials. Method: Three types of crowns were used in this study for color change after they were exposed to waterpipe whole-body exposure machine for 6-months. The materials are PEEK, Zircon, and Acrylic denture teeth. PEEK and Zircon were prepared using Computer Aided Design/Computer Aided Manufacturing (CAD/CAM) while Acrylic teeth were taken from a kit. Results: The findings of this study are important for clinicians. Color changes were measured using a colorimeter. The lightness values generally decreased after exposure across all materials, PEEK, Zircon, and Acrylic denture teeth. The differences in lightness values (L*) among the materials were statistically significant at all exposure durations (p < 0.001). Redness values (a*) and yellowness values (b*) have been investigated with significant differences in redness between PEEK, Zircon, and Acrylic denture teeth (p < 0.001). While yellowness values (b) fluctuate, with no significant 4difference in yellowness among materials. The AE values were reported for each material, and the results show a specific extent of color change after exposure to waterpipe smoke over different periods. The results showed that despite having the highest baseline AE values, Acrylic exhibited relatively minor color changes after exposure. Chroma values for all materials tend to fluctuate with exposure time with no clear increasing or decreasing trend. Conclusion: The exposure to waterpipe smoke showed that PEEK, Zircon, and Acrylic denture teeth are susceptible to staining to different extents depending on the duration of exposure.

1. Introduction

Color is among the most critical factors for the aesthetic appearance of prosthetic restorations. Discoloration of teeth and prosthetic restorations is often associated with pigments in food, beverages, and other substances that frequently contact dental hard tissues in the oral cavity ¹. The use of tobacco products is also a known risk factor for tooth discoloration, and smokers are more likely to exhibit tooth discoloration than non-smokers. It has been approved that tobacco products are one of the factors that affect the color of the teeth ^{2, 3}. Smoking habits have been reported in almost 1.3 billion people worldwide, and they are associated with the discoloration of teeth and prostheses ⁴. Dental materials can be discolored by exposure to staining agents such as tea, coffee, or cigarette smoke¹, which is considered a threat to the esthetic properties of dental materials. The global trend of cigarette smoking is stable or declining, but the usage ot tobacco in other forms, especially waterpipe smoking, is dominant ^{5, 6, 7}. Waterpipe smoking was the third most common source of tobacco usage, following cigarettes and cigars in USA ⁸. It is a global public health threat. The prevalence of daily waterpipe smoking is estimated to be 100 million globally, with an increasing trend seen in childhood ^{8, 9}. It has been adept for centuries, and its usage is increasing worldwide, although it is more widespread in the Eastern Mediterranean region ¹⁰. Waterpipe has become a common practice in the Arab world, China, Turkey, Pakistan, India, and Bangladesh ^{11, 12}, particularly among young people due to its social acceptance, entertainment purposes, relaxation, and free time ¹³ or social and relieving academic stress ¹⁴. Several studies attempted to investigate the effects of cigarette smoke on the color stability of dental materials ^{1, 4, 15}. Previous research has documented the tendency of dental materials, such as acrylic resins, to pick up stains over time. Through adsorption, liquid molecules adhere to resin materials depending on the environmental conditions, and consequently, possible discolorations may occur ^{16, 17}. Minimizing color change is a criterion when selecting dental materials for the best esthetic effect. This criterion may provide crucial information on the serviceability of materials ^{18, 19}. All ceramic restorations have become more commonly disseminated due to their high esthetic potential and their excellent biocompatibility ^{20, 21}. Today, various framework structures for prosthctic restorations are fabricated in computer-aided design (CAD)/computer-aided manufacturing (CAM) procedures, which means that a main part of the working sequence is carried out by industrial machines ^{21, 22}. Zircon and PEEK crowns are examples of restorations produced by CAD/CAM. There is increasing interest in the capacity of dental restorative materials to resist discoloration, as color stability is an essential factor in treatment success for dental restoration in the esthetic zone ^{19, 23}. Multiple studies have reported the negative effect of food, daily beverages, and cigarettes on the color of dental prostheses. However, there is rare research examining the effects of waterpipe smoking on PEEK and Zircon material for color stability compared with conventional types of material, Acrylic, which is a gap this study intends to investigate.

2. Materials and Methods

Preparation of samples

This in vitro experimental study assessed the color change of PEEK and Zircon teeth. A die stone die of 1st molar was scanned by a dental laboratory scanner (D1000; 3Shape, Copenhagen, Denmark). A full-contour crown was designed using dental CAD Software (Dental Designer; 3Shape, Copenhagen, Denmark). Parameters for restorations are as follows: cement gap 20 μm, extra cement gap 20 μm, distance to margin line 1.2 mm, smooth distance 0.3 mm, drill radius 0.5 mm, drill compensation offset 1.0 mm, margin line offset 50 μm, offset angle 70°, and extension offset 50μm. PEEK and Zircon discs were used to mill the crown's 3D data. PEEK disc was used to fabricate 28 crowns by a 4-axis milling machine. The produced PEEK crowns have advantages such as a pleasant esthetic, biocompatibility, and no need for grounding and polishing. The 28 crowns of Zircon were also produced. This material is used for esthetic crowns and bridges. However, 28 acrylic denture teeth were used for this study, and the teeth were taken from a denture teeth kit made from acrylic resin. Samples of each material were divided into four subgroups: a control group and one, two, and three-hour waterpipe exposure for six months. Table 1 describes the shades of materials used in the study. The shade guide available in dentistry that is used for acrylic resin denture teeth is Vita Classical (Figure 1).

Waterpipe preparation

For this study, a waterpipe smoke exposure system was designed based on the previously established system developed by Khabour and his colleagues in 2012. The apparatus consists of a waterpipe smoking machine and an exposure chamber. The smoking machine uses a diaphragm pump to draw smoke from a waterpipe and discharge it into the chamber. The pump is automatically controlled to provide 171 puffs each 2.6 seconds, with an inter-puff interval of 17 seconds. The diaphragm pump flow rate was manually adjusted to maintain the 530 mL puff volume during each exposure session. This smoking exposure system was chosen because it is approximately similar to the human puff topography during waterpipe smoking ²⁴. The waterpipe group was exposed for one, two, and three hours for six months using a waterpipe exposure apparatus. Before waterpipe exposure, control subgroups for each material were measured for color change using a colorimeter (Eoptis, Italy) calibrated according to the manufacturer’s instructions. The experimental subgroups were also measured after six months of color change. Various methods are generally used to measure the color of teeth, ranging from visual comparisons using shade guides to instrumental measurements such as spectrophotometers, colorimeters, spectroradiometers, and digital image analysis techniques ²⁵.

Colorimeters

Colorimeters measure tristimulus values (C1E X YZ) by filtering the reflected light from an object into red, green, and blue areas of the visible spectrum and typically convert these to Commission Internationale de l'Eclairage, or the International Commission on Illumination (CIELAB) values. Thus, ClELAB measurements evaluate the color of each sample at a given time. They also enable the calculation of the amount of color change (AE) between two-time intervals by using a formula where AL*, Aa*, and Ab* are the differences in L*, a*, and b* values, which are measured before and after exposure to waterpipe smoke. AE is the shortest distance in the CIELAB color system between the colors which is compared to the AE= (AL*2+ Aa*2 +\h*2)1/2 (Table 2). The key optical elements in the colorimeter include a light source and a detector consisting of three filters intended to match the CIE color-matching functions or linear combination closely. Colorimeters are generally reliable, have good repeatability, and accurately measure color difference. For example, a colorimeter's repeatability for measuring shade tabs in vitro is 99.0%, with an accuracy of 92.6% ²⁶. The measurement of the color in vivo with a colorimeter also showed excellent repeatability ²⁷. The strengths of a colorimeter include its ease of use and sensitivity in detecting and measuring small color differences between samples of similar color ²⁸. Therefore, it is ideal for many aspects of color research in dentistry. Indeed, colorimeters have found widespread use in denial research for measuring the color of teeth, restorative materials, and soft tissues ^{29, 30, 31, 32, 33}. Some disadvantages of colorimeters have been described, including the fact that they are designed to measure flat surfaces, while teeth are often not flat and can have surface anomalies; teeth are translucent, which can lead to light loss at the edge of the tooth sample being measured, giving incorrect color values: and inter-instrument agreement is relatively poor ^{25, 34}. The CIE L*a*b* (CIELAB) color space is a uniform color space derived from the tristimulus values of X, Y, and Z, with L*, a*, and b*12 coordinates. It is one of the color spaces within the international standard color specification system by the CIE and is useful for colorimetric assessments of natural teeth and dental restorative materials. The L* axis describes lightness, which ranges from black (0) to white (100), while the a* axis represents red (+a*) to green (-a*), and the b* axis represents yellow (+b*) to blue (b*). The change in tooth color following tooth whitening treatments has been described in both in vitro ³⁵ and in vivo studies ^{36, 37}. For longitudinal tooth whitening clinical studies, the use of custom-made mouthguards with apertures aligned to the anterior teeth are frequently used to ensure accurate realignment of the colorimeter measuring head onto the tooth surface before and after treatment.

Measuring the samples

The color was measured by a colorimeter (options) and recorded in the L*, a*, b* color system. This color system consists of a luminance or Lightness component (L*) and two chromatic components: the a* component for green (-a) to red (+a) and the b* component from blue (-b) to yellow (+b) colors. The colorimeter was calibrated throughout the study using a standard white ceramic reference (CIE L* = 97.91, a* = -0.68, and b* = +2.45). Seven samples were used for each subgroup of each material. 208 Color was measured and repeated three times for each sample, and the average was calculated. In addition, the total color difference (AE) and chroma were calculated using the following equations: AE = [(Aa) 2 + (Ab) 2 + (AL) 2]'/2, chroma = [(a) 2 + (b) 2\/i.

Statistical analysis

The collected data was analyzed using SPSS statistical software version 23.0 (SPSSINC, Chicago, IL, USA). The primary outcome measure was color values (lightness, redness, and yellowness), the overall color difference (AE), and chroma values of three experimental materials, PEEK, Zircon, and Acrylic denture teeth, before and after exposure to the waterpipe smoke with different exposure times. Differences in color values, AE values, and chroma values among the three subgroups were compared in PEEK, Zircon, and Acrylic denture teeth at various exposure times using analysis of variance and post hoc Fisher’s least significant difference (LSD) test.

3. Results

1. Effect of waterpipe exposure on color measurement for PEEK. Zircon, and Acrylic denture teeth:

The effect of waterpipe exposure on the color change of PEEK, Zircon, and Acrylic denture teeth is evaluated using three color parameters (lightness (L*), redness (a*), and yellowness (b*). The findings in Table 3 present color values (lightness, redness, and yellowness) for three materials (PEEK, Zircon, and Acrylic denture teeth) at baseline (no exposure) and after 6 months of exposure for different durations (1 hour, 2 hours, and 3 hours). Across all materials, the lightness values generally decreased after exposure. At baseline. Acrylic generally shows darker appearances (with the lowest L* value: -1.331±0.178), compared to PEEK and Zircon (-0.511 ±0.681, -0.770±0.649, respectively) at baseline. Meanwhile, Zircon exhibits the highest lightness compared to PEEK and Acrylic across all exposure times. The results of multiple comparisons between different material groups using post hoc Least Significant Difference (LSD) analysis indicate that the differences in lightness values (L*) among the materials were statistically significant at all exposure durations, indicating distinct inherent lightness. characteristics of each material (p < 0.001). The Post hoc analysis shows significant differences in effect in lightness between PEEK, Zircon, and Acrylic denture teeth (p < 0.001), suggesting that Acrylic tends to appear darker than the other materials and has consistent results across all the measured time. However, Zircon and PEEK have less consistency in lightness change (Supplementary, Table 1). Exposure duration has a cumulative daring effect on the material's lightness. The findings of post hoc analysis revealed a significant decrease in lightness values, mainly after short-term and intermediate-term exposure (1 hr and 2 hr, respectively), with no significant difference observed in lightness with more prolonged exposure durations (3 hr of exposure) compared with baseline (no exposure) (p < 0.05) (Supplementary, Table 2). These findings might suggest that the most significant darkening happens within the first two hours of exposure. Redness values (a*) differ with exposure and. Acrylic consistently exhibited slightly decreased redness compared to baseline at exposure durations. Zircon generally decreases over time, while PEEK shows the highest redness values compared to the baseline across all exposure durations. After 3 hours of exposure, Acrylic has the highest redness compared to Zircon and PEEK. The post hoc analysis for redness and time of exposure presented an overall increase in the redness among all materials after exposure to the waterpipe, with high, more pronounced redness found after a longer duration of exposure (3 hr), (Supplementary, Table 3). The post hoc analysis revealed significant differences in redness between PEEK, Zircon, and Acrylic denture teeth (p< 0.001), indicating that Acrylic tends to have the highest redness value compared to the other materials. While there is no significant difference in redness between Zircon and PEEK (p > 0.05). (Supplementary, Table 4) Yellowness values (b*) fluctuate, with some materials increasing and others displaying fluctuations. Zircon exhibited an increase in b* after 1 hour of exposure (1.720±0.779), while resin showed an increase after 2 hours of exposure (at t=2 hr: 1.242±1.044) and then returned near to the baseline by 3 hours (at t=3 hr: 1,776±0.205, at baseline: 1.786±0.305). However, there is a trend towards increased yellowness with longer exposure; the post hoc analysis shows no significant effect between exposure duration and yellowness values across all materials (P<0.05). Additionally, there is no significant difference in yellowness values across all materials (p>0.05), (Supplementary, Table 5). Yellowness values remained relatively stable across different exposure durations, indicating minimal influence of exposure duration on yellowness. The results show that exposure to waterpipes and smoking can change color in polymer materials. PEEK and Zircon show more pronounced alterations in redness and relatively stable color properties compared to the other materials. Acrylic presented more stable color values with less deviation from the baseline.

2. Effects of exposure on color change (AE values)

The results in Table 4 present the AE values of PEEK, Zircon, and Acrylic denture teeth at baseline (no exposure). After 6 months of exposure, higher AE values indicate greater color differences, suggesting more significant changes in appearance due to exposure. Generally, all materials showed an increase in AE values with prolonged exposure time, suggesting a noticeable color change. At baseline (no exposure), Acrylic and PEEK had higher AE values (2.08±0.!82, 2.11±0.301, respectively) compared to Zircon (1.91 ±0.363). Zircon exhibits the lowest AE among all exposure times compared to Acrylic and PEEK (1.91±0.363, 2.22±0.268, 2.03±0.223, 2.02±0.281, respectively).From the results, Acrylic appears to be the most susceptible to color changes, followed by PEEK, while Zircon exhibits the most stable color properties among the materials examined. Post hoc analysis confirms significant differences in AE values between Acrylic and both Zircon (p < 0.001) and PEEK (p < 0.05), with no significant difference between Zircon and PEEK (p>0.05). This indicates that each material undergoes a distinct level of color change over the exposure period. The exposure time significantly increased in AE values, indicating more noticeable color change with longer exposure time. However, when comparing the AE values among the three materials, it is noticeable that the difference/deviation between AE values at baseline and after different times of exposure is significantly higher for Zircon and PEEK, while the change of AE values for Acrylic from baseline to after exposure is relatively low.

Effects of exposure on Chroma value

The materials (PEEK, Zircon, and Acrylic denture teeth) exhibit different responses to exposure, indicating varying susceptibility to environmental factors. Longer exposure durations generally lead to changes in chroma values, although the specific effects vary depending on the material, as presented in Table 5. At baseline (no exposure), PEEK exhibits the highest mean chroma value (1.93±0.360), followed by Zircon (1.63±0.361) and Acrylic (1.59±0.160). After exposure, Zircon exhibited a consistent increase in chroma values with longer exposure durations, showing the highest chroma values after 3 hr exposure (1,91±0.323). PEEK displayed varying chroma values with exposure time, showing a decrease after 1 hr exposure, followed by an increase after 3 hr exposure, but remaining relatively stable after 2 hr exposure. Acrylic showed an increase in chroma values after 1 hr and 3 hr exposures compared to baseline, with a slight decrease after 2 hr exposure. The post hoc analysis shows a significant difference in chroma value between Acrylic and Zircon (p=003) and a slightly significant difference with PEEK (p=0.037), while no significant difference is shown in chroma value between Zircon and PEEK (p=0397) (Supplementary Table 9 & Table 10).

The results show that chroma values for all materials tend to fluctuate with exposure time with no clear increasing or decreasing trend. PEEK exhibits fewer fluctuations compared to Acrylic and Zircon. This fluctuation suggests that exposure time had no significant impact on the color change of the materials. Further studies are needed to explore the factors that could impact color saturation in this material.

4. Discussion

The aesthetics of teeth and their color is an essential topic for many individuals. Dentists want to select the correct tooth shade and esthetic restorative material to maximize the recreation of natural tooth structure; dental technician wants to replicate the form and quality of the tooth appearance, and patients who desire to enhance their smiles ³⁸. This study investigated the color stability and waterpipe smoking on different restorative CAD/CAM dental materials was investigated by examining the change in Lightness (L*), redness (a*), and yellowness (b*). The CIELAB color system, recommended by the American Dental Association (ADA) represents the international standard for color measurement due to its suitability for determining minor color differences ³⁹. The results indicate that exposure to waterpipe smoke for different exposure times increased the staining of all tested restorative materials. PEEK, Zircon, and Acrylic denture teeth are susceptible to staining, but different behaviors were observed about the three axes of the CIE L*a*b* system. The color measurements of different CAD/CAM dental materials showed that after waterpipe smoking, the lightness (L*) values for all groups increased, indicating darker surfaces after smoking exposure. The results of this study revealed that all three materials exhibited some level change in lightness after exposure to waterpipe smoking. There is a significant difference in the behavior of each material in response to exposure. Specifically, the results demonstrated that Acrylic generally exhibited the greatest decrease in lightness, mainly after 2 hours of exposure. This suggests that Acrylic darkens more significantly than Zircon and PEEK under the same conditions. Zircon showed the highest lightness values across all exposure times, suggesting good resistance characteristics toward darkness. After 3 hours of exposure. Acrylic had the highest redness value for redness and yellowness compared to Zircon and PEEK. However, Acrylic has minor deviations in redness, suggesting that it might be more stable and less influenced by exposure to waterpipe smoking than PEEK and Zircon. PEEK and Zircon exhibited the highest deviation values in redness, meaning PEEK and Zircon can significantly produce color changes after exposure. Yellowness showed variability across all materials, with significant changes over time, especially in Zircon, which increased in yellowness rapidly after the first hour of exposure. Our findings align with previous studies indicating that exposure to smoke products can trigger surface changes in ceramic restorations ⁴⁰ and dental composites ⁴¹, leading to a reduction in the lightness and darker color in the specimens, with increased L* values and shift into redness and darker color clinically. According to Patil and his colleagues (2013), discoloration of these materials may be related to tar, nicotine, and metals, such as arsenic, along with the smoke produced by the burning of the charcoal, which produced dark components of smoke and a range of chemicals that are deposited on the materials' surface, being responsible for the change in surface's color ⁴².

Moreover, Mahross and his colleagues (2015) demonstrated that waterpipes or hookah tobacco contain flavorings and additives that can damage surfaces and increase stainability ⁴³. In addition, it is also noteworthy that temperature changes alter the staining of dental materials. This moist smoke, combined with the heat, can exacerbate the penetration of staining compounds into porous dental materials, deepening the stains and potentially causing more permanent discoloration ⁴⁴. Furthermore, previous studies have shown the effect of pH on discoloration; for example, Patil et al. (2013) revealed that smoking mixed with saliva can produce an acidic pH solution, which might damage the surface integrity of the materials, thus producing favorable conditions for discoloration ⁴². The same observations were also demonstrated by Sulieman (2005) ³⁶ and Zhao et al. (2019) ⁴⁵, who suggested that low pH may affect the surface integrity of dental hard tissues and enable easier penetration of pigments into the tissue matrices, leading to internalized discoloration that cannot be removed by mechanical means. Given the different initial L*a*b* values, evaluating the color change in the specimens as AE values rather than the final post-waterpipe smoke exposure L*a*b* values is important. The AE values, indicating the overall color difference before and after exposure, further supported these observations. In dentistry, a discoloration that is more than visually perceptible (AE > 1.0) will be referred to as clinically acceptable up to the subjective visual evaluation ^{45, 46, 47}. In the current study, the AE values of all materials are less than 3.3. Acrylic appears to be the most susceptible to color changes, followed by Zircon and PEEK. Despite Acrylic showing the highest AE values, it exhibited relatively minor changes when comparing its baseline AE value (2.08±0.182) to those after exposure (AE values at t=l hr: 2.38±0.216,t=2hr: 2.38±0227,t=3 hr: 2.23±0.308). This indicates that while Acrylic is susceptible to color changes, its variation is less pronounced than in Zircon, Zircon showed a significant shift from its baseline AE value (1.91±0.363) to the values post-exposure (AE at t=l hr: 2.22±0.268, t=2 hr: 2.03±0.223, t=3 hr: 2.02±0.281). As previous studies demonstrated, these differences in color change are related to each material's structural properties, chemical composition, and fabrication process. For example, the polymers present in the composition of Zircon, known to uptake water, therefore, may be more prone to absorbing the pigments of staining solutions, leading to a greater color change (AE values) and more yellowish surface (b*) after 1 hour of 396 exposure (Arocha et al.,2014). Additionally, Patil et al. (2013) and Mathias et al. (2010) demonstrated that polymeric teeth are insoluble in oral fluids, but they are soluble to 398 some extent in aromatic hydrocarbons such as tar, which contains aromatic hydrocarbons ^{42, 48}. So, it was deduced from this 400 study that such surface-dissolving substances might be causative factors of discoloration. The distinct responses of PEEK, Zircon and Acrylic to waterpipe exposure highlight the importance of material selection based on the specific environmental conditions they will encounter. However, there are limitations to this study. The overall accumulation of waterpipe smoking residues on the materials’ surfaces is one of the limitations. The nature of the waterpipe and smoking tends to accumulate heterogeneously, leading to inconsistencies within each specimen’s post-exposure resulting surface. Additionally, the materials have different physical and mechanical properties, and the effect of smoking on color stability and stainability may be different. Another limitation of the present study is the environment conducted in this study. This research is tin in vitro study: the environment in which smoking and soaking occurred was carefully constructed to replicate the oral environment best; however, the oral environment is impossible to replicate exactly in an in vitro study. Therefore, careful speculation and hypotheses of these materials' performance in an oral environment should be considered.

5. Conclusion

In conclusion, this study provides critical insights into different restorative materials' color stability after exposure to waterpipe smoke. Therefore, the present in vitro study can guide clinicians in choosing appropriate restorative materials for smoker patients for greater longevity and better esthetic outcomes. Further studies have been needed to examine multi-factorial factors and other staining exposures, such as coffee and certain mouth rinses, which would provide a more comprehensive insight into the color stability of these materials as well. The correlation between color stability, stainability, and surface roughness should also be considered.

References