This study aimed to evaluate the changes in MP in adult patients who had completed orthodontic fixed appliance treatment with extraction or non-extraction during a 1-year retention period. Furthermore, it aimed to compare the MP after orthodontic treatment with that associated with normal occlusion.
Mastication is the first step to digest food and involves breaking down and comminuting, mixing, kneading, and transporting the food bolus into the oropharynx [16, 17]. A variety of methods have been used to clinically evaluate the MP. The most conventional method is based on a comminution test, that is, grinding of a test material, which helps evaluate the distribution of particle sizes after a given number of chewing cycles; other methods place emphasis on the bolus formation preparatory to swallowing and evaluate bolus mixing [15, 18–21]. However, the comminution test using the sieving method is messy, time-consuming, and inconvenient in clinical practice; therefore, a variety of other MA tests have been used recently [15, 21–25]. In this study, the two-color chewing gum MA test using ViewGum software, which can be easily applied in clinical practice, was used to evaluate the MP. It evaluates the degree of mixing of the bolus by measuring the SDHue of the specimen using ViewGum software that quantifies HIS color space (hue, saturation, intensity). Previous studies have reported that this method has good sensitivity and reliability, although its validity should be established [15, 21, 23, 24, 26]
To minimize the influence of sex, the same number of male and female participants comprised the experimental and normal occlusion groups. Three specimens were obtained from each participant by asking the participants to chew the gum for 5, 10, and 20 cycles. The SDHue at 5, 10, and 20 chewing cycles was analyzed in a total of 148 samples to assess whether this digital image analysis method showed differences on the basis of the number of chewing cycles. In this study, the SDHue value was found to be larger than that reported in other studies [15, 26]. According to previous studies, the pink color Hubba-Bubba Tape gum was discontinued [26], but we were able to purchase it through Coupang apps. It was speculated that the SDHue values obtained in the present study were greater than those obtained in the previous studies because the pink color of the Seriously Strawberry Hubba-Bubba gum used in the current study was darker. The SDHue showed a significant decrease as the number of chewing cycles increased; thus, this digital image analysis method showed differences in the SDHue based on the number of chewing cycles. However, in the two experimental groups, the SDHue value showed no significant change over time at 5 and 10 chewing cycles and was not significantly different from that of the normal occlusion group. On the other hand, the SDHue showed significant changes over time in the two experimental groups, with the greatest difference at 20 chewing cycles. This was in line with the findings of previous studies, according to which 20 chewing cycles were sufficient for extracting valid conclusions for chewing efficiency [15, 26]. Based on the above results, the MA was evaluated using the specimen obtained after 20 cycles. If the SDHue was large, the MA was considered low, and if the SDHue was small, the MA was considered high.
The MA immediately after the removal of fixed appliances in the two experimental groups was lower than that in the normal occlusion group. In the extraction and non-extraction groups, MA showed a significant increase over time, and at 1 month post-treatment, it had improved to a level not significantly different from that in the normal occlusion group. Furthermore, the mean value of MA at 1 year post-treatment was similar to that in the normal occlusion group. Thus, the first null hypothesis was rejected.
The low MA values immediately after removal of the fixed appliance may be attributed to the fatigue of the jaw-opening and closing muscles during removal of the orthodontic appliance and bonding of the fixed retainer. Gameiro et al. [6] reported that non-extraction orthodontic treatment helped improve the MP to the normal occlusion level. Makino et al. [28] reported that the occlusal force and OCA were lower than those before the orthodontic treatment, but the subjective masticatory ability improved in all patients post-treatment. In this study, MA at 1 month post-treatment showed no significant difference between the two experimental and normal occlusion groups, which was in line with the above studies [6, 28].
On the other hand, we observed no significant difference in the MA between the non-extraction and extraction groups. The post-canine functional tooth units (FTUs) in the non-extraction and extraction groups were 12 and 10, respectively. Hatch et al. [31] stated that the number of FTUs and bite force influenced MP, and that the number of FTUs was the most crucial factor. Many studies have suggested that > 20 teeth and > 8 FTUs are sufficient to maintain the masticatory function. The results of our study matched with those of Hatch et al. [31]; it was shown that the FTU of a premolar was 0.5, and that premolar extraction did not affect the MP after orthodontic treatment. Thus, the second null hypothesis in this study was accepted.
The MBF and OCA showed low values in the non-extraction and extraction groups immediately after removal of the fixed appliance and a significant increase 1 year post-treatment (P < 0.001); however, even at 1 year post-treatment, they remained lower than the corresponding values in the normal occlusion group. Based on previous studies, it is expected that two more years would be necessary for the MBF and OCA values to reach the normal occlusion group levels [11]. The results of this study showed that at 1 month post-treatment, unlike the MBF and OCA, the MP improved to a level not significantly different from that in the normal occlusion group.
In this study, the correlation between MP, MBF, and OCA was examined by evaluating the changes in MP immediately and at 1 month, 6 months, and 1 year after active orthodontic treatment. The Pearson’s correlation coefficient for the association between the MA and MBF and between the MA and OCA was 0.382 and 0.350, respectively (P < 0.01).
Previous studies reported that various factors, such as occlusal force, OCAs, jaw movement, and characteristics of muscle activity, among others, affect MP [29–35]. Although there is a controversy regarding the correlation between MP and occlusal force, OCA, jaw movement, and muscle activity, several studies have reported that the MBF and OCA are the most important factors, and that the jaw movement and salivary flow rate play important roles [29, 30, 36]. Okiyama et al. [37] examined the relationship between MP and MBF in dentate subjects using test foods of varying hardness, and found a positive correlation between the two variables, with a lower positive correlation coefficient with soft gummy jelly than with harder ones. In this study, which used gums of a hardness lower than that of jelly, the correlation between the MA and bite force was less than that found in other studies. This result suggests that the function of mixing and kneading foods might not require a high bite force. According to Yoshida et al. [38], MA assessed using the two-color chewing gum method is expected to correlate more with the mandibular movement parameters than with the bite force. Further research should be conducted to identify the factor that significantly influences the MA.
This study has a few limitations. The MP was not evaluated before treatment and after more than 20 chewing cycles. Moreover, the MP was not evaluated for the number of chewing cycles that reached saturation. Further research in this regard is therefore necessary.