The Effect of Isolation and Dehydration on Shade Matching

The use of rubber dam in dentistry has expanded well beyond its original intended purpose for mainly endodontic and prosthodontic procedures. The techniques involving placement of these rubber dams have also advanced. These techniques include the use of accessory clamps, floss ligatures, and cross-arch stabilization, all allowing the provider to achieve better visualization and isolation for dental procedures (Figure 1 and Figure 2). Due to this increased ease of placement, combined with the unparalleled isolation they provide, rubber dams may be used for nearly any imaginable restorative procedure, including crown and veneer preparation and cementation.

When using rubber dams in restorative procedures, it is imperative for clinicians to critically evaluate potential side effects. Rubber dams act to seal the teeth from the remaining part of the oral cavity and, specifically, protect against salivary contamination. Saliva plays a key role in hydration of teeth in the oral cavity. Therefore, reduction of saliva by the rubber dam ultimately results in temporary dehydration of the teeth.

Dehydration is also critical during the try-in of porcelain veneers. Between the time when topical anesthetic is applied, provisional restorations are removed, and the veneers have been tried in, enough time can elapse for dehydration to set in. The restorations, therefore, may appear brighter than their true color. After a few days when the teeth have fully rehydrated, the restorations may appear darker resulting in an unesthetic appearance and a dissatisfied patient.

To understand the effects of hydration on shade matching, it is important to understand the layers of natural teeth that contribute to shade. The dentin layer is primarily responsible for the chroma of a natural tooth.¹ The enamel layer varies the appearance of the chromatic dentin layer through scattering of light. If light is more scattered, enamel appears more translucent. If light scattering is reduced, enamel appears opaquer, thus masking the chromatic layer of dentin.¹

Refractive indices give an indication of light scattering. When teeth are hydrated, water is present in the inter-prism spaces between hydroxyapatite crystals in the enamel layer.¹ When teeth are isolated with rubber dams, the water in these spaces is replaced by air.² The refractive index of water is greater than the refractive index of air, with values of 1.33 and 1.00, respectively.³ Therefore, when isolated in a rubber dam, the enamel layer of a tooth reflects light at a lower index. This reduction of light scattering increases the opacity of the enamel layer thus masking the chromatic dentin layer and giving the tooth a brighter appearance.¹

The purpose of this study was to determine the extent that shade is altered from baseline due to dehydration from rubber dam use during restorative procedures, and furthermore to determine the length of time required for teeth to rehydrate back to their baseline shade.

To be included in the study, the patient needed to: (1) be willing to participate in the study, (2) be at least 18 years of age, (3) be available for a 24-hour period to complete photographs at specific intervals, (4) have no periodontal disease, and (5) have no decay or restorations present on his or her maxillary central incisors.

Patients were excluded from the study if they: (1) were underage (<18 years old), (2) were unable to be available for a series of photographs over a 24-hour period, (3) had active periodontal disease, or (4) had decay or restorations present on his or her maxillary central incisors.

After a patient was selected, an informed consent was reviewed. A short clinical examination was completed to ensure participants fulfilled inclusion criteria for the study. Next, a timeline was established to determine the start of the photograph series based on a 24-hour interval.

The experimental procedure was as follows: A baseline photograph was taken at minute 0 (Figure 3). Latex-free rubber dam (Insti-Dam^®, Zirc, zirc.com) was then immediately placed on one of the participant's maxillary central incisors and held in place with widgets. The rubber dam was left in place for 30 minutes, and photographs were taken at the following intervals: 1 minute, 2 minutes, 3 minutes, and 5 minutes (Figure 4 through Figure 7). After 30 minutes, the rubber dam was removed, and a photograph was immediately taken (Figure 8). Additional photographs were taken after rubber dam removal at the following intervals from baseline: 1 hour, 3 hours, 6 hours, 12 hours, and 24 hours (Figure 9 through Figure 13). All photographs were taken with a Canon 80D with APS-C sensor, 100 mm macro lens, Canon ring flash (Canon, canon.com), polar_eyes^® cross-polarization filter (Bio-Emulation^™, emulation.me), and white_balance^® reference card (eLAB, elabprime.com).

For the second part of the study, one subject was retracted with surgical retractors (Hu-Friedy, hufriedygroup.com), and photographs were taken at baseline 0, then at 3 minutes and 10 minutes (Figure 14 through Figure 16), in the same fashion as the photographs described above. A second set of photographs was taken with the same subject more than 48 hours later at baseline 0 and at 10 minutes (Figure 17 and Figure 18) with the same retractors, however during the 10-minute interval the teeth were wrapped in wet gauze (Figure 19).

As stated earlier, a Canon 80D with APS-C sensor camera with a 100 mm macro lens was used for photography of each participant. As per the guidelines established by Hein et al, an aperture of F22, shutter speed of 1/125s, and ISO of 100 were selected. A Canon ring flash was set to maximum output (1/1) in manual mode.⁴ A polar_eyes cross-polarization filter was placed over the ring flash. The aspect ratio was set to the width of the white_balance reference card, or approximately 1/2.5.

Five volunteers were selected for the study. All photographs were taken with a white_balance reference card at camera settings indicated previously. The baseline photograph (Figure 3) was taken immediately before placement of rubber dam. Individuals had a latex-free rubber dam (Insti-Dam) placed on one maxillary central incisor tooth for 30 minutes. All teeth selected were unrestored. Photographs were taken at 1-, 2-, 3-, and 5-minute intervals with the rubber dam in place (Figure 4 through Figure 7). Rubber dam was removed at the 30-minute mark, and a photograph was taken immediately upon removal (Figure 8). Photographs were then documented at the 1-, 3-, 6-, 12-, and 24-hour marks (Figure 9 through Figure 13) for comparison to baseline photograph.

Images for each participant were uploaded and calibrated to the CIE L*a*b* value of the white-balance gray reference card set used, with L* = 79, a* = 0, b* = 0.⁵ The gray card set is designed for use in dental photography to develop a robust workflow for shade estimation, allowing the lab technician to calibrate the shade photograph to allow for a highly exact shade match in the final restoration. Following calibration, the color difference, or Delta E, was measured between the baseline photograph and each interval photograph. In this study, the following parameters were set to evaluate the thresholds for change in shade from baseline: Delta E <1.0 is clinically imperceptible, Delta E 1.0-2.0 is clinically acceptable, and Delta E >3.0 indicates an appreciable difference.^6,7

Table 1 through Table 5 present the results of the L*a*b* value of the tooth for each given time interval over the 24-hour period for each participant. In total, five participants were evaluated over a 24-hour period. From the L*a*b* values, Delta E comparisons were generated using the Delta E Calculator software developed by Pereira.⁸ Each photograph was compared to the baseline photo for each participant, as seen in Table 6 through Table 10.

Patients were instructed to drink only water during the 24-hour period in which the experiment was being conducted and were told specifically to avoid dark-staining liquids, including but not limited to coffee, black tea, red wine, and soda. No other restrictions were placed on the participants. They were instructed to continue with normal oral hygiene maintenance.

Interpretation of the Delta E values showed variation among the participants in the return of shade levels to a clinically imperceptible level compared to baseline. Delta E value of 2.0 was set as the threshold for "clinically imperceptible" or "imperceivable to the human eye."^7,9

No participant returned to a clinically imperceptible level prior to 3 hours post rubber dam removal. Sample 4 returned to a clinically imperceptible level between 3-6 hours after rubber dam removal (Table 9), and Sample 2 returned to a clinically imperceptible level between 12-24 hours after rubber dam removal (Table 7). Sample 5 returned to a clinically imperceptible level at approximately 24 hours (Table 10). The final two participants, Sample 1 (Table 6) and Sample 3 (Table 8), did not return to a clinically imperceptible level within the 24-hour period documented.

For the second part of the dehydration analysis, a subject was placed in metal retractors for 10 minutes. Photographs were taken at baseline, 3 minutes, and 10 minutes. Table 11 shows the L*a*b* values of the subject at those intervals. Table 12, which shows the Delta E compared to baseline, indicated significant perceptible difference in shade at 2.58 and 3.22 at 3 minutes and 10 minutes, respectively.

The third part of this analysis evaluated the teeth at baseline 0 and at 10 minutes where the teeth were wrapped in wet gauze to "avoid" dehydration. Table 13 shows the L*a*b* values at baseline and 10 minutes. Delta E analysis, as seen in Table 14, is 0.48. Therefore wrapping teeth in gauze for 10 minutes resulted in clinically imperceptible shade change from baseline.

The goal of this study was to measure the effect of dehydration on the shade of teeth over a 24-hour period, and to evaluate the effect of keeping the teeth "hydrated" prior to try-in of anterior esthetic restorations. Shade selection is an extremely subjective process. Protocols using the white_balance reference card developed by Hein combined with cross-polarization techniques were used to assign numeric values, thus allowing for an objective analysis for shade comparison.^5,10 This analysis was conducted based on the principles of CIE LAB color space developed by the Commission on Illumination in 1976.⁹

CIE LAB is a 3-dimensional analysis of color. The white_balance reference card represents known L*a*b* values. Because of this, photographs of teeth could be calibrated and L*a*b* values generated for the teeth being studied in this experiment. L* stands for lightness, a* represents the red/green value, and b* denotes the blue/yellow value. By assigning L*a*b* values to shades, differences in color can be quantified and, therefore, more easily compared.^9,10

Delta E represents the comparison between these shades. It is a single number that signifies the "distance" between two colors based on their L*a*b* values.^5,10 Delta E is measured on a scale from 0 to 100. Lower Delta E values indicate closer shade matching, and higher Delta E values indicate distortion between shades. Delta E values less than or equal to 1.0 represent imperceptible differences in shade to the human eye. Values between 1.0 and 2.0 represent perceptible differences only through close observation. Values greater than 2.0 represent perceptible differences at a glance.⁷

In this study, thresholds of Delta E were redefined. Delta E values of 2.0 or lower were defined as "clinically imperceptible," and Delta E values higher than 2.0 were defined as "clinically perceptible." According to the data collected during this study, all five participants had a shade change determined "clinically perceptible" based on the parameters set by the experimenters for a period no shorter than 3 hours after removal of the rubber dam. These results suggest that dehydration plays a significant and lasting effect on the shade of natural teeth.

Based on these findings, shade selection for all restorative procedures should be completed before rubber dam placement. If shade selection is not taken before the start of the procedure, the provider should schedule the patient to return in 48 hours to select shade. This amount of time will allow for rehydration of the teeth so that better shade match of material may be obtained. Also, clinicians should avoid letting teeth dehydrate during try-in of anterior esthetic restorations. It appears that wrapping the teeth in wet gauze during administration of topical anesthetic, injection administration, and following removal of the provisional restorations prior to veneer try-in, will allow for a more accurate shade evaluation during esthetic try-in.

Adamo E. Notarantonio, DDS
Clinical Instructor, Honors Program in Aesthetic Dentistry, New York University College of Dentistry, New York, New York; Fellow, American Academy of Cosmetic Dentistry; Private Practice, Huntington, New York

Devin McClintock, DDS
Associate Dentist, Williamsburg, Virginia

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