Dehydration may influence the true color perception of a clinician due to the temporary dental color change, and it may result in an irreversible complication such as color mismatch during the restorative treatment [18]. This study clinically investigated the quantitative color changes of maxillary central incisors and evaluated the related dental color perception due to dehydration at short-term periods and the reverse effect of rehydration up to 24 h, using different dental photography techniques and a spectrophotometer. According to the results of the present study, the clinical perception of tooth color was affected by both the level of tooth dehydration and rehydration. Therefore, the first and second null hypotheses of the study were rejected. Some of the dental photography techniques presented similar results to the spectrophotometer outcomes, while some presented significant differences in clinical color detection. Therefore, the third hypothesis of the study was partially accepted.

Visual color analysis may cause inconsistency in color evaluation due to the subjectiveness, and the most precise, practical, and adaptable tool for dental color measurement was considered the dental spectrophotometers [25, 30]. The clinical contact-type spectrophotometers were found repeatable and reliable in measuring dental color parameters quantitatively [9, 25, 31]. A clinical hybrid-type (with cross-polarization feature) dental spectrophotometer was used as the control in the present study. The older versions with the same working principle were previously mentioned as the clinical standard for the non-hybrid and non-polarized feature color assessment devices due to the ability to avoid ambient light and, thereby, provide high accuracy and reliability [32]. A previous version of the spectrophotometer with the same features (SpectroShade) was reported to have 96.9% reliability and 80.2% accuracy [33]. Even though the accuracy was inferior to the non-polarized spectrophotometer (VITA EasyShade), the observed reliability was superior to it. The lower rate for accuracy was probably due to the difference in the analysis area of the two devices. EasyShade device’s small tip measures an area of only 5 mm in diameter more precisely, while hybrid-type spectrophotometers can measure the entire tooth surface more realistically and provide average tooth shade, shade by region (thirds), or detailed color map [33]. Because of the variety of colors on a particular tooth surface, similar to the previous clinical studies that focused on the middle third of the maxillary central incisors, the tip of the RP device was located on the cervical- and mid-third surface including the cervical gingival area in this study [11, 14, 34, 35]. The standardization in the positioning of all measurement equipment and camera and shooting parameters was done to avoid bias [36].

The level of color change due to dehydration or rehydration was considered time-dependent [13, 37]. Alamé et al. [16] mentioned that the more the dehydration time elapses, the more the color difference compared to the baseline increases. Du et al. [34] measured the level of dehydration in vitro at 2 h and 4 h using a colorimeter and observed a significant color change for both periods. Ahmed et al. [38] investigated the effect of dehydration in vitro with a spectrophotometer at baseline, 1 h, and 2 h and observed significant dental color changes at both periods. However, decreasing the time intervals for more precise results was recommended [17, 38]. In a clinical study, Russel et al. [11] mentioned significant color changes in maxillary central incisors after 15 min of dehydration. Burki et al. [14] observed significant color changes clinically with a spectrophotometer in maxillary central incisors after 10 min of dehydration. Ibrahim and Abou Steit [17] also used a spectrophotometer clinically to evaluate dehydration at 10 and 30 min for central incisors. They observed a significant increase in ∆E* at 10 min, supporting the results of Burki et al. [14], but no difference between 10 and 30 min. Consistent with these results, in the present study, RP ∆E00 values gradually and significantly increased till the 15th min (Table 4). Hatırlı et al. [13] conducted another clinical study for dehydration and rehydration of maxillary incisors using a hybrid-type spectrophotometer and evaluated dehydration for a total of 30 min. They observed significant color changes for each of the 10-min intervals supporting Burki et al.’s [14] and Ibrahim and Abou Steit’s [17] results, and the ∆E00 at the 10th min was above the AT (≥ 1.8). Sharmila et al. [39] conducted a clinical study with a similar methodology to Hatırlı et al. [13] comparing also younger and older patients. They presented perceptible and clinically unacceptable color changes after 10 min of dehydration like the results of Hatırlı et al. [13]. Whereas the color changes of older patients were clinically acceptable up to 30 min of dehydration, that the color brightening was less pronounced for the older patients. The RP color change levels after 15 min of dehydration in the present study were consistent with these previous findings and were above the AT, supporting the results of Hatırlı et al. [13]. However, due to the inclusion criteria of this study (participants at the age of 20–40), no statistical data were obtained for the differences between young and old patients.

All the previous study results mentioned above reported the long-term effect of dehydration on dental color. However, the dehydration effect on dental color starts at the earliest minutes of the dental appointment, which means the color should be selected before any restorative procedure [1, 2, 4, 14]. Moreover, there is limited clinical evidence in the literature regarding shorter term intervals to evaluate dehydration and longer term intervals for rehydration [12, 15, 17, 38]. Accordingly, this study investigated the short-term intervals to evaluate the dehydration clinically and the influence on the tooth color. Ruiz-López et al. [15] conducted a clinical study with short-term dehydration intervals of 2, 4, 6, and 8 min and evaluated the color changes of maxillary incisors due to dehydration using a spectroradiometer. They presented ∆E00* values higher than PT (≥ 0.8) for 50% of the teeth after 2 min of dehydration and 95.8% after 10 min. Their results were beyond the AT after 6 min of dehydration, consistent with the control method RP results of this study. Buldur et al. [18] evaluated the effect of dehydration and rehydration with short-term intervals (1, 2, 3, 5, 7, 10, and 15 min for each) on maxillary immature permanent central incisors using a spectrophotometer. They observed 50% surpassing the AT at the 5th min of dehydration for the teeth with completed root development. Suliman et al. [12] investigated short-term dehydration intervals of 1, 2, 3, 5, 7, 10, and 15 min clinically for maxillary incisors using a spectrophotometer and observed significant color changes within the 1st min of dehydration, which were 87% beyond the PT (0.8) and 72% beyond the AT (1.8). Inconsistent with these results, in the RP results of the present study, ∆E00* was beyond the PT for only 23.4% of the teeth after 3 min of dehydration but 100% at 5 min (Table 4). Therefore, a fast change in color perception was observed due to the dehydration between the 3rd and the 5th min. At the 5th min, only 16.7% of the teeth were beyond the AT, and thus, most of the teeth’s color was clinically acceptable, inconsistent with the results of Buldur et al. [18] and Suliman et al. [12]. Surpassing the AT for 100% was observed at the 15th min but not before, according to the RP outcomes. The inconsistency in the observed intervals with the previous studies for perceptibility and acceptability might be due to the differences in the standardization of the clinical setups or the type and specification of the spectrophotometer used. Especially, the hybrid spectrophotometer (RP) device used in this study was particularly produced for the clinical color assessment by combining spectrophotometric analysis with cross-polarization photography. More standardized clinical assessments could be achieved using the features of auto-perceiving of the tooth crown and related cervical gingiva and guided angulation during the shooting. On the other hand, color matching is a crucial step for restorative success, and dental color selection mistakes are irreversible during restorative procedures. Therefore, it might be wiser to consider the perceptibility clinically more important than the acceptability, which is affected in the longer term. Accordingly, the RP results of the present study revealed that it might be better to select the dental color within the very first 3 min of the dental appointment to avoid the color change due to the dehydration effect.

Following dehydration, the clinical time needed for complete rehydration is also very critical for the clinical workflow and success, and this study also clinically investigated the level of color reversal after the dehydration. Buldur et al. [18] and Suliman et al. [12] considered more than 15 min of rehydration was required to regain the original color after 15 min of dehydration. However, in these studies, the rehydration period was limited only to 15 min like most of the previous clinical studies. Inconsistent with them, Russel et al. [11] mentioned that the original color was regained after 30 min of rehydration and Burki et al. [14] supported their result. The RP results of the present study are opposed to these previous findings because 100% of the teeth were below the AT but still beyond the PT at the 30th min of rehydration. Even after the 60th min of rehydration, although ∆E00 significantly decreased, supporting the results of Ibrahim and Abou Steit [17], the situation regarding PT and AT was completely the same as mentioned above. However, after at least 24 h of rehydration, ∆E00 values according to the RP were below the PT, and 70% of the teeth regained their original color (Table 5). It can be interpreted that, even after 24 h of rehydration, there is still a probability that the original tooth color cannot be restored. This result is also consistent with the results of Hatırlı et al. [13], reporting that the color returned to the original level at least after 24 h of rehydration. 90% of the teeth were below PT at the 24th h in their results, while it was 70% in the present study, which can be considered more critical for clinical practice. Moreover, the recent results of Sharmila et al. [39] were completely consistent with the results of this study, in which they reported the ∆E* was below the PT only after 48 h of rehydration.

Digital dental photography has been used to assess the clinical assessment of dental color besides the spectrophotometers [7, 40,41,42]. Moazam et al. [41] clinically compared a spectrophotometer (VITA EasyShade) and DP for evaluating the color of maxillary incisors and considered digital photography a highly reliable clinical assessment method. Although this is an important outcome, they did not use the CP filters for the DP technique. Yilmaz et al. [28] compared a spectroradiometer, DP (with ring flash), and CP photography in vitro and used WB calibration for all the photographs. They observed similar outcomes for the spectroradiometer and CP photography, especially for the high-value shades. This result might be due to the ability of the CP filter to avoid unwanted light reflections on the surface, providing an advantage for the teeth with high value and gloss clinically. Korkut et al. [43] compared a spectrophotometer (VITA EasyShade V) and CP photography for the color assessment of composite samples and observed a very high positive correlation between the two techniques. He et al. [32] clinically compared a clinical hybrid-type spectrophotometer (ShadepilotTM, DeguDent, Germany), DP (with twin-flash), and CP photography in tooth color assessment and mentioned that the CP photography presented a high agreement with the spectrophotometer outcomes as control. The spectrophotometer they used was an older version of the one used in the present study. In this study, supporting He et al. [32] and Yilmaz et al. [28], all the photography techniques, including CP filters, presented very high agreements with the spectrophotometric control method RP (Table 6). Therefore, the DP/CP technique might be a clinical replacement for the spectrophotometric color analysis. The results revealed that the use of the CP filters led to more standardized images and, thereby, color assessments for monitoring both dehydration and rehydration clinically. Evaluating the DP and MDP techniques, only the assessments through the cross-polarized images were like the assessments with the RP results (Tables 4 and 5). Hein et al. [9] introduced the eLAB system, focusing on standardizing digital camera images using a specific gray card to calibrate the WB, and considered it a promising method for quantitatively measuring dental color in dental practice. Supporting their results, Swarowsky et al. [42] and Yung et al. [43] considered WB calibration to improve the determination of color differences through the photographs. However, in the present study, WB calibration individually did not improve the determination of color changes but improved when used together with the CP filters. The results obtained solely from the CP filters without the WB calibration also presented a high agreement with the RP outcomes. Thus, the DP/WB/CP technique may provide another clinical option for the spectrophotometric color analysis. He et al. [32] mentioned that CP photography can remove specular highlights and thereby, aid in the observation of subtle color changes that may not be evident in regular flash photography. The quantitative assessment through the collected images might be influenced by the glossy tooth surfaces even when calibrating the white balance [43]. This might be the reason for the lower effectiveness of WB calibration, compared to the use of CP filters in this study. The previous clinical results of Sampaio et al. [7] and Korkut et al. [43] completely agreed with these findings by considering the WB calibration not necessary when using CP filter photography. They also reported less accurate photography results with the smartphone photography and ring flash photography compared to the CP photography, consistent with our results (Table 6). Whereas better results for both the MDP and DP techniques were obtained in this study when they were used with the CP filters. This inconsistency was probably due to the absence of external illumination like the MDP device and the CP filters for the smartphone photography technique in Sampaio et al.’s study [7]. Supporting the results of Yung et al. [43], the results of this study can be interpreted that smartphone photography (MDP/CP or MDP/WB/CP technique) can be used effectively for clinical color assessment and as an alternative to the spectrophotometers when it is used together with an extraoral illumination like an MDP device and the CP filters. Jorquera et al. [23] supported this by considering MDP (with the CP filter) and CP photography methods comparable and reliable for dental shade selection in a clinical study. Consistent with their statement, WB cards, and CP filters can be thereby beneficial tools to standardize dental colors in terms of communication with dental technicians [23]. On the contrary, two recent clinical studies considered CP photography alone was not beneficial for color assessment, inconsistent with our results [31, 43]. However, Yung et al. [43] used no spectrophotometric control method in their study and captured all the images under fluorescence lightning, but not the daylight at 5500 Kº. They also used the old CIELab76 formula to calculate the color changes which was not as definitive as the CEIDE2000. Moreover, they captured all the images under fluorescence lightning, but not the daylight at 5500 Kº. Saygılı et al. [31] used a non-polarized spectrophotometer device (EasyShade) as a control which can analyze only a limited surface area of 5 mm and be affected by the ambient light that could have influenced the obtained color data. CP photography might be affected by the experience level of the observer, depending on their reports. In addition, both studies investigated only the young participants and did not mention the dehydration level during the photo shooting procedures, which might also have affected their results and caused the inconsistency. These might be the possible reasons for the inconsistency of these studies with our results.

In the present study, there were no major differences in effectiveness between the investigated dental photography techniques that involved the CP filters, and those were considered clinically promising, whereas the ones not containing the CP filters were not considered clinically convenient under the limitations of this study. However, there might be some limitations. It might be more accurate to assess the dental color changes by also using other clinical spectrophotometric analysis devices and to check the agreement level with the current techniques. Moreover, other brands of cross-polarization filters, white balance calibration cards, and dental photography equipment should be investigated, which may influence the outcomes. In addition, other computer-based software programs can be used for the quantitative analyses, and the influence on the outcomes should be evaluated.