The present cross-sectional study was conducted at the Department of Maxillofacial Prosthetics, Dental Hospital, Tokyo Medical and Dental University, Tokyo, Japan. A clinical difference (δ) of five in a* value between the two groups was considered clinically significant when the standard deviation within each group (σ) was also five. With exacts method based on the T distribution and the power and sample size calculations software (version 3.1.2, 2014), a sample of 17 was required in each group to provide a power of 0.8. The Type I error probability (α) associated with test of this null hypothesis was 0.05. Twenty fully dentate subjects (control group) and Twenty patients who had undergone marginal mandibulectomy (marginal mandibulectomy group) due to head and neck lesion and who were satisfied with their dento-maxillary prosthesis (i.e., did not require adjustment of the device at enrolment), and had worn the prosthesis for at least six months were included in the study. The segmental mandibulectomy, maxillectomy as well as glossectomy patients were excluded. All the subjects were instructed to chew colour-changeable chewing gum continuously for 100 strokes. The subjects were asked to rest for a sufficient time between each trial. The study protocol was approved by the Ethics Committee of Tokyo Medical and Dental University (No.865). All study participants received a written and verbal description of the study and gave their written informed consent prior to participation.

Intraoral photographs and clinical examination details from the patients’ medical records were gathered for age, sex, mandibulectomy characteristics, reconstruction type, pathological diagnosis, and number of mandibular teeth, dento-maxillary prosthesis type {removable partial denture (clasp partial denture), overdenture and complete denture} [Table/Fig-1].

Patient perception of chewing ability was rated using a food intake questionnaire consisting of 35 food items list. Previously reported list by Hisashi Koshino was used to develop a new food intake questionnaire method [Table/Fig-2] [7].

The participants rated their ability to chew each of the 35 food items using the following scale: 0- cannot eat; 1- can eat with difficulty; and 2- can eat easily. An additional two categories of “do not eat because of aversion” and “have not eaten since starting to wear dentures” were scored as 0. To represent the ability of mastication using the questionnaire, masticatory score was calculated as follows, after the sum for the five categories had been calculated by multiplying the total points by the difficulty rate of each category, the percentage of this value of the maximum possible total was represented as MS. Thirty-five foods classified into five grades as A, B, C, D and E of masticatory difficulty [4]. The maximum point in calculating method (MS) was 111.4 point. The sum of the difficulty ratio was 7.96 (1.00+1.14+1.30+1.52+3.00), the total point value in calculating method (MS) was 14 point (Total Point=Sum of the points for 7 foods in each grade: Full point=7 foods x 2 point = 14). The points for each grade were summed and the MS was calculated as follows [7]:

Objective masticatory function was measured using a colour-changeable chewing gum (masticatory performance evaluating gum Xylitol, 70 mm×20 mm×1 mm, 3.0 g; Lotte Co., Ltd., Saitama, Japan). Participants were instructed to chew the gum continuously for 100 strokes on their unaffected side for mandibulectomy patients, and normal dentate participants were instructed to chew the gum continuously for 100 strokes on their habitual chewing side. The chewed gum was collected immediately after chewing, wrapped in polyethylene film, and compressed to a thickness of 1.5 mm between glass plates. Using a colourimeter (CR-13; Konica-Minolta, Tokyo, Japan) colour was read at five sites on the gum specimen: at the center and approximately 3 mm above, below, and to the right and left of the center. The colour of the gum was assessed in the CIELAB colour space and the a* value, denoted by redness, was measured as an indicator of food mixing ability (a higher score indicating greater mixing ability). From each mean values of L*, a* and b*(with dimensions L* for lightness and a* and b* for the colour-opponent dimensions, the red/green opponent colours were represented along the a* axis, with green at negative a* values and red at positive a* values. The yellow/blue opponent colours were represented along the b* axis, with blue at negative b* values and yellow at positive b* values). The difference between two colours (before and after chewing) in the CIELAB colour space (∆E) was calculated for 100 strokes using the following equation [8]:

The CIEDE2000 colour distance formula (∆E00) was used to calculate the colour difference as follows [9]:

calculation formula, weighting Functions: SL, SC, SH, RT, Neutral. Positional corrections to the lack of uniformity of CIELAB. Compensation for lightness (SL), Compensation for chroma (SC), Compensation for hue (SH). Parametric factors: kL, kC, kH. Corrections accounting for the influence of experimental viewing conditions where kC and kH are usually both unity and the weighting factors kL depend on the application. L*, a*, and b* before chewing were measured as 72.3, -14.9, and 33.0 [10], respectively. Details of the calculation formula and calculation system are shown on the "ColourMine.org" website [11].

Wilcoxon signed-rank test was applied for the comparison between normal dentate subjects and marginal mandibulectomy patients. Relationships between the scores in each group were examined using Spearman’s rank correlation coefficient. The statistical analysis was performed using SPSS version 21.0 software (SPSS Japan Inc., Tokyo, Japan), with the level of significance set at p<0.05.

Compared to normal dentate subject’s data, related details of the masticatory function in marginal mandibulectomy patients are presented. The Wilcoxon signed-rank test revealed a significant difference between normal dentate and marginal mandibulectomy groups (p<0.01) [Table/Fig-3].

A correlation was found between perceived chewing ability (Masticatory score {MS}) and objective mixing ability (index of the masticatory function {∆E}) in marginal mandibulectomy patient. (∆E00 = 0.481, a* = 0.587, ∆Eab = 0.668) p<0.05 [Table/Fig-4].

In this study, the null hypothesis between perceived chewing ability (MS) and the objective masticatory function (∆E) in marginal mandibulectomy patients due to head and neck lesion was rejected, as shown in [Table/Fig-4], a correlation was found between ∆Eab and MS was 0.668, ∆E00 and MS was 0.481. This study established that there were significant correlations between perceived chewing ability (MS) and the objective masticatory function (∆E) in marginal mandibulectomy patients.

As for the comparisons of masticatory function between normal dentate subjects data and marginal mandibulectomy patients, differences in colour difference values determined by the two formulas (CIELAB and CIEDE2000) were also observed. Such differences might arise from the weighting functions (St, Sc, and Sl) introduced in the CIEDE2000 colour difference formula and also from the change of the a* axis of CIELab, affecting mainly colours with low chroma (neutral colours) [Table/Fig-3].

The relationships of a*, ∆Eab, and ∆E00 with the outcome, MS, were different in patients with a dento-maxillary prosthesis after marginal mandibulectomy. ∆Eab was more strongly correlated with MS than was a*or ∆E00. The ∆Eab appears to be a better indicator of masticatory function. Of note, Komagamine Y et al., reported that the difference between two colours before and after chewing is more accurately represented by ∆E than by a* [5]. However, CIE standard is to define procedures for calculating the coordinates of the CIE 1976 Lab (CIELAB) colour space and the Euclidean colour difference values based on these coordinates. The standard does not cover more sophisticated colour difference formulae based on CIELAB, such as the CMC formula, the CIE94 formula, the DIN99 formula, and the CIEDE2000 formula nor does it cover the alternative uniform colour space, CIELUV [12]. CIE has developed a series of formulae to improve the consistency of measurement and visual assessment of colour differences. Thus, ∆E00 calculated using the CIEDE2000 colour-difference formula is better than ∆Eab calculated using the CIELAB colour-difference formula. This finding may be attributed to the discrepancies in visible colour tolerance at different regions in the CIELAB colour space. When investigating the colour difference between before and after, ∆E00 rather than ∆Eab is recommended by CIE [6]. The formula is an extension of the CIELAB colour-difference formula with corrections for variation in colour-difference perception dependent on lightness, chroma, hue and chroma-hue interaction. However, the standard of the reliability of using the formulas CIELAB determinate ∆Eab values to represent the average visual colour assessments needs to be further discussed in the studies for evaluating masticatory function using colour-changeable chewing gum. The advantage of using a* is that a colour change is easily detected. If the chewed gum turns red, this is visible to the eye at the chair side and the redness can be evaluated by a colourimeter. However, a disadvantage of using a* is that a* may not be able to capture the difference in colour of the gum between before and after it is chewed. In our experience, when two pieces of chewed gum are compared, the same a* values are obtained but the b* and L* values can be different, meaning that the two ∆E values (the difference in colour of the gum between before and after it is chewed) in the CIELAB colour space are different. The advantage of using ∆E00 is that it can calculate the colour change of the gum in the CIELAB colour space after it is chewed. However, the disadvantage of using ∆E00 is the difficulty of calculations using L*, a* and b* data. Some colourimeters do not include a ∆E00 calculation function, meaning that a computer system or smartphone application is needed to help calculating the ∆E00.

Although, the literature contains numerous reports on the perceptibility and/or acceptability of colour-changeable chewing gum, these reports seldom mention the separate contributions of lightness, chroma, and hue changes to the total calculated colour difference. Therefore, we cannot continue to use a* just because it is the easiest way of evaluating the masticatory function and we need to explore other methods. A standardized colourimetric measurement approach would be needed to provide more detailed data for both dental treatments of individual patients and in studies with small sample sizes.

The study is limited due to the small number of the patient, further studies should enroll more number of subjects, and also different groups based on different reconstruction type should be evaluated in detail. In this study, we focused on the assessment of ∆E calculation, future studies are warranted to assess the significance in the evaluation of colour differences of colour-changeable chewing gum, as well as the relative significance of each correction term introduced in CIEDE2000. There is also the possibility that the calculation formula could change, recording all data from investigations of the relationship between colour changes of colour-changeable chewing gum and masticatory function is imperative.

A correlation was found between perceived chewing ability (MS) and objective mixing ability (∆E) in marginal mandibulectomy patients, it is reasonable to assess masticatory function from the aspects of questionnaires and colour-changeable chewing gum. When investigating the colour difference between before and after, ∆E00 rather than ∆Eab is recommended by CIE. Thus, in the present study of colour-difference formula CIEDE2000 (∆E00) was considered to be a useful modality for evaluating masticatory performance after mandibulectomy in patients with head and neck lesions.