Colour is a phenomenon of visual perception that responds to the light reflected or transmitted from an object (1). The colour of natural tooth is the result of the combination of light reflected from the enamel surface and light scattered and reflected from both enamel and dentine (2). Three factors can influence the perception of tooth colour – the light source, the object being viewed and the observer viewing the object (2).
There are two ways of assessing colour, one subjective, using different shade guides, and other objective, using different digital shade-matching devices. The use of shade guides to measure shades in dentistry is a subjective process and many variables may affect the results: the surrounding illumination, the angle of view of the tooth and the tab, clothing, make-up and the chromatic perception of the dentist such as previous eye exposure and metamerism (3-5). Additional uncontrolled factors such as fatigue, ageing and emotions also influence the observer's interpretation of colour stimulus (5-8). In addition, shade guides do not repesent the entire gamut of natural teeth colour (9-11).
Dental shade matching instruments have been brought to market to reduce or overcome imperfections and inconsistencies of traditional shade matching (5, 12). They encompass spectrophotometers, colorimeters and imaging systems.
Spectrophotometers are amongst the most accurate, useful and flexible devices for colour matching in dentistry (7, 13). They measure the amount of light energy reflected from an object at 1-25 nm intervals along the visible spectrum (14, 15). Compared with observations by the human eye, or conventional techniques, it was found that spectrophotometers offered a 33% increase in accuracy and a more objective match in 93.3% of cases (5). Some authors suggest that instrumental and visual colour matching methods should be used together, as they complement each other (16, 17).
The aim of this study was to evaluate the intra-device repeatability and accuracy of a dental shade-matching device (VITA Easyshade® Advance 4.0) using both in vitro and in vivo models. The null hypothesis for the intradevice repeatability is that the type of the in vitro and in vivo model tested presents no difference in the colour measuring and matching of the dental shade-matching device as determined by the colour parameters.
Materials and methods
In vivo model for assessment of repeatability
Before measuring, the teeth of each of the ten patients participating in the study were cleaned and polished to remove any accumulated extrinsic stain (Proxyt RDA 83; Ivoclar Vivadent, Liechtenstein). After the treatment they were instructed to place their heads against the headrest of the treatment chair and to keep their mouths slightly open during measurement. They were instructed to keep the tongue in a relaxed position away from the maxillary teeth during measurement to prevent false measurements. The central region of the labial surface of maxillary right central incisors of each patient was measured twice with an interval of 1 hour (Figure 1). Patients were provided with water after each sequence to prevent dehydration of the teeth. Following tooth colors were measured: B1, A1, A2, A3, C1 and C3. An intraoral spectrophotometer VITA Easyshade® Advance 4.0 (VITA Zahnfabrik, Bad Sackingen, Germany) was used to capture CIELAB colour coordinates. The shade-matching device was operated according to the manufacturer's instructions.
In vitro model for assessment of repeatability
The in vitro evaluation was based on measurements of the shade tabs in a simulated oral envionment. Three VITA Classical shade guides (VITA Zahnfabrik, Bad Sackingen, Germany) were cleaned with soap and distilled water. Simulated clinical conditions were created for colour measurement. It was conducted in the same room under the same conditions as in the in vivo model.
Six Vitapan Classical tabs (same colours measured in the in vivo model) were selected for the study. These tabs created a variety of levels of lightness, chroma and hue. Each of these six shade tabs was removed from three different shade guides. The shade tab to be measured was then placed in the middle of custom made acrylic gingival matrix (GC RelineTM GC Europe N.V., Leuven, Belgium) with shade tabs of the same nominal shade from additional shade guides placed on both sides. The measurements were made inside a black box (25 X 14 X 10 cm) to simulate the oral cavity. Two measurements were made of the central region of each centrally positioned shade tab with an interval of 1 hour (Figure 2). An intraoral spectrophotometer VITA Easyshade® Advance 4.0 (VITA Zahnfabrik, Bad Sackingen, Germany) was used to capture CIELAB colour coordinates. The shade-matching device was operated according to the manufacturer's instructions in tooth colour mode.
In vitro model for assessment of accuracy
For the accuracy study, each shade tab from 3 shade guides (VITA Classical; VITA Zahnfabrik, Bad Sackingen, Germany) was measured once by the shade-matching device under the same in vitro conditions mentioned before (Figure 3).
A measurement was considered to be accurate if the shade-matching device selected a shade identical to the shade tab measured. For the accuracy evaluation, a total of 48 colour measurements were made, each measured once. The accuracy of the device was calculated as a percentage of the total number of measurements made for each shade tab (n=48) that were an exact match.
All measurements were made by a single trained operator under standardized test conditions. Before any measurement, the device was calibrated on its own white ceramic block. According to the CIE standard, the daylight illumination conditions (Just Normlicht, Weilheim an der Teck, Germany) were set at 6500 K and 1000 Lux. Natural daylight was excluded using a room with no windows.
Colour quantification was based on CIE Lab values. Colour differences (dE) were calculated using the formula:
dE = .
Data were imported into statistical program SPSS 19.0 (SPSS, Chicago, IL, USA). To estimate the in vivo and in vitro repeatability of the device in measuring and matching tooth colour, paired t-test and t-test for independant samples as well as the intraclass correlation coefficient (ICC) were calculated. All tests were performed at an alpha of .05.
The mean L*a*b* values for both measurements of colours measured in the in vivo and in vitro models are given graphically in Figure 4. Paired t-tests performed on these values showed significant differences among the repeated measurements only for the in vitro measurements of L* component (p=0.019; Table 1). The difference between two in vitro measurements for b* component almost reached the level of significancy, too (p=0.058; Table 1).
|Within measurements (paired t-test)/Unutar mjerenja (t test za zavisne uzorke)|
In vivo t vrijednost/p
In vivo t vrijednost/p
The mean colour differences (dE) for the in vivo and in vitro models were 3.51 and 1.25, respectively.
ICCs of in vitro and in vivo models based on L*a*b* measurements of teeth are shown in Table 2. Intradevice ICCs (device repeatability) were very high for both models (from 0.858 to 0.994), but higher for all the in vitro components (Table 2).
|Device repeatability/Ponovljivost uređaja|
There were only three measurements (6.25%) which were not an exact match in accuracy evaluation (48 shade tabs, each measured once). Therefore, the accuracy tested for VITA Easyshade® Advance 4.0 (VITA Zahnfabrik, Bad Sackingen, Germany) was 93.75%.
The colour of the restorations is a significant factor affecting dental appearance of maxillary anterior teeth in patients and therefore the clinician has to be very careful in choosing the right one using reliable tools (18).
Repeatability measurements enable an evaluation of the consistency of the shade-matching device in making repeated measurements of the same shade tab. In Kim-Pusateri's study the reliability of four different shade-matching devices compared in controlled setting in the in vitro model ranged between 87.4% and 99.0%, with ShadeScan having significantly lower reliability and VITA Easyshade with 96.4% (1). Another in vitro study, conducted by Lagouvardos et al. reported significantly higher measuring repeatability of VITA Easyshade in comparison to another shade-measuring device only for the L* parameter (ICC = 0.928, p<0.05), alhough ICCs for both a* and b* parameters were slightly, but not significantly higer (22).
Lehman et al. reported that for the in vivo model intradevice repeatability for VITA Easyshade ICC for L* was 0.845, a* = 0.916, b* = 0.914 and E = 2.49 (13).
In this study, the results do not support the null hypothesis that the type of the in vivo and in vitro models tested presents no difference in the colour measuring and matching of the dental shade-matching device as determined by the colour parameters. The results revealed no statistically significant difference within both in vivo and in vitro measurements with the exception of L* component in the in vitro model, (p<0.001; Figure 4, Table 1). Generally, L* parameter for the in vivo model was higher than for the in vitro model proving that the natural teeth were lighter than the matching shade tabs (Figure 4). At the same time a* and b* components were lower for the in vivo model (a* component was even negative) meaning that the natural teeth were more greenish and less saturated in comparisson to the matching shade tabs (Figure 4).
The color difference of the in vivo and in vitro measurements had an average value of 3.51 and 1.25 dE units, respectively (t=5.8; p<0.001). These values remain below the 50% acceptability level of 5.5 dE units, but exceed the 50% of perceptibility threshold for a clinical mismatch in the in vivo model (varies from 2.6 to 3.3 dE units) (5, 23-27). Since the examiner in this study was trained and well experienced in the color measurements with the tested device and we achived the standardized conditions and illumination, this finding for the in vivo model may be attributed to the tooth variance in surface morphology and the different tooth layers’ thickness and transparency. Although ICCs for the in vivo model in this study coincided in all CIE L*a*b* compontents with those of Lehman, our dE was slightly higher and therefore got on the upper level of clinically visible difference obvious only to a trained eye (23-27).
On the other hand, in the in vitro model in this study we found statistically significant difference between two measurements for L* component, but at the same time the difference between the measurements was almost invisible, even for the trained eye (mean dE was 1.25) and ICCs were nearly perfect (0.992 to 0.994; Table 2). This finding may be explained by uniformity of prefabricated plastic teeth and difficulties we had in assessing color on the shade-tabs. According to the manufacturer's instructions for the in vivo model we used the recomended basic shade measurement mode aimed to measure natural teeth. But for the in vitro model (measurement of the shade tabs) it would be better to use „training mode“aimed at measuring artifical shade tabs, and we did so for the accuracy evaluation. There is the problem with this mode because it cannot be used for CIE L*a*b* measurements and we had to use „basic shade measurement“ in order to calculate the differences within and between models and their dE. The differences in this model may be caused by inadequate mode. Nevertheless, Browning et al. reported the same issue, explaining that Vita tabs have a thickness of approximately 3.5-4.0 mm in the middle-third, enough to measure them the same way the natural teeth are measured (28).
On the other hand, wider variety of the measurements in the in vivo model (mean dE was 3.51) can be attributed to the unique enamel prismatic and dentin tubular structure of the natural teeth and the importance of the correct positioning of the device tip during the measurement. In the case of the in vitro model, the teeth on the tabs have different, more uniform structure with no real prismatic or tubular simulation, prefabricated in the same way and therefore the differences in the measurements were more condensed.
For the accuracy evaluation, the null hypothesis that accuracy of the device tested in this study was higher than 90% was proved as it was 93.75%.
Accuracy measurement enables an evaluation of the validity of the shade output when measuring a shade tab of known colour and it is important because instrument must provide the measurements of the true values of color parameters without substential deviation. In Kim-Pusateri study, the accuracy of four electronic shade-matching devices evaluated in the in vitro model and ranged between 66.8% and 92.6%, whereas VITA Easyshade demonstrated the greatest accuracy and in our study it was even slightly higher (1). In some in vivo studies the same shade-matching device also revealed excellent accuracy (29, 30).
The limitation of this study is the small sample which has to be increased in the further studies and the main goal has to be in vivo measurement.
This study showed that measuring repeteability was nearly perfect for both in vitro and in vivo models tested (ICCs=0.992-0.994; ICCs=0.858-0.971; respectively). The colour differences for in vitro and in vivo models were high, but at a clinically acceptable esthetic level (1.25-3.51 dE units respectively). Accuracy of the shade-matching device was very high. Within the limitations of the study, VITA Easyshade® Advance 4.0 dental shade-matching device enabled reliable and accurate measurement. It can be a valuable tool for the determination of tooth colours.