Bruxism is a parafunctional activity. Although definitions vary [1
], it is divided into two types: “sleep bruxism” (nocturnal) and “awake bruxism” (diurnal). The American Academy of Sleeping Medicine recommends that these two types be separated because of different aetiologies and presumed risk factors [4
]. Awake bruxism is defined as awareness of jaw clenching [6
]. The prevalence of bruxism is around 10% and declines gradually with age [6
], and the prevalence of awake bruxism (8–34%) [9
] is higher than that of sleep bruxism (9.7–15.9%) [9
Bruxism can seriously affect quality of life, and induce problems such as pain, temporomandibular disorders and failure of prosthetic treatments [8
]. The large forces involved in bruxism can have detrimental effects on the components of the masticatory system [18
]. A load of more than 20 g over periods of 2.5 s per clenching might be imposed on a tooth, exceeding normal functional stresses [18
]. Such force can induce tooth movement and contribute to malocclusion [21
In a previous cross-sectional study, the prevalence of malocclusion (crowding) was significantly associated with awareness of clenching in university students [22
]. Since malocclusion provides functional and aesthetic disturbances, and may lead to psychological stress [23
], prevention or control are important for dental clinicians. Confirmation of causal relationships in a prospective cohort study is therefore necessary. Based on our previous study involving young adults [22
], we hypothesized that clenching during the daytime as awake bruxism would be a risk factor for malocclusion. We configured the null hypothesis in which clenching does not induce malocclusion. The purpose of this prospective cohort study was to investigate the association between the onset of malocclusion and awareness of clenching during the daytime in young adults. We could clarify a part of association between clenching and malocclusion.
A previous cross-sectional study showed a significant association between prevalence of malocclusion (crowding) and awareness of clenching in Japanese university students [22
]. To the best of our knowledge, the present study is the first prospective cohort study to investigate whether awareness of clenching during the daytime is a risk factor for malocclusion in young adults. In this study, the results showed that awareness of clenching during the daytime was associated with the onset of crowding (adjusted OR, 3.63; 95% CI, 1.08–12.17). These findings may support our hypothesis that awareness of clenching during the daytime is a risk factor for crowding.
Previous studies have suggested that clenching forces contribute to tooth movement [21
]. This force can induce tooth movement and contribute to malocclusion [21
]. The mean clenching force is 720 N (162 lb) with a range of 244–1243 N (55–280 lb) [36
]. The bite force needed to contribute to displacement is approximately 100 N [18
]. Because the force of clenching is higher than the threshold of tooth displacement, clenching during the daytime may represent a risk factor for crowding through tooth movement.
Underweight (BMI < 18.5 kg/m2
) was associated with the onset of crowding. Underweight is related to skeletal maturation [39
], and delayed maturation might affect the onset of malocclusion [40
]. Kataoka et al. [22
] showed a significant association between underweight and the prevalence of malocclusion in a cross-sectional study. In the present study, mean height in students with normal weight (18.5 ≤ BMI < 25 kg/m2
) at baseline increased significantly during the 3-year study period (164.6 cm [standard deviation (SD), 8.3 cm] at baseline vs. 164.9 cm [SD, 8.5 cm] at follow-up; paired t
-test, [SD of the difference, 0.7 cm], P
< 0.001). However, mean height in students with underweight (BMI < 18.5 kg/m2
) was not significantly increased (164.0 cm [SD, 8.8 cm] at baseline vs. 164.2 cm [SD, 9.2 cm] at follow-up; paired t
-test, [SD of the difference, 0.8 cm], P
> 0.05). These results suggest that underweight students have less skeletal maturation or less maxillary and/or mandibular growth. Taken together, underweight could affect the onset of malocclusion through reduced skeletal maturation.
Oral habits, including biting fingernails/pens/pencils, biting the mucosa of the cheeks/lips, and gum chewing [28
], were not significantly associated with the onset of malocclusion (chi-square test; P
>0.05). Previous studies have shown that bad oral habits are related to the prevalence of malocclusion [41
]. Some inconsistencies are apparent between our study and previous studies. The reasons for this are unclear, but may involve differences in age (young adults vs. children) and dentition (permanent vs. primary).
The results of the present study might be clinically relevant. Treatment for awake bruxism is based on behaviour modification and habit reversal. Behaviour modification has the potential to stop or reduce awake bruxism [2
]. Treatment for awake bruxism could therefore prevent the onset of malocclusion. When clinicians encounter younger patients who are aware of clenching during the daytime, increased efforts may be needed to prevent malocclusion. The present work was only an observational study. Further studies are therefore needed to clarify whether clinical interventions can help prevent the onset of malocclusion.
The prevalence of malocclusion was higher than that of the previous studies. In the present study, the rate of onset of malocclusion was 53.8% (Table 2
). In previous research using IOTN, the prevalence of malocclusion varied widely among subjects (21–44.9%) [23
]. As participant age, country, sample size, and study design differed between the present and other investigations, caution is warranted in regard to the generalizability of the results.
The participants were not considered an unusual sample based on the two aspects as below. The prevalence of awake bruxism was 8.0% in the present work, which is within the 8–34% range reported in previous studies [9
]. Moreover, the distributions of underweight, normal weight, and overweight based on BMI classifications established by the World Health Organization were 17.2%, 75.6% and 7.1%, respectively. As a reference, the 2013 Japan National Health and Nutrition Survey showed distributions of underweight, normal weight, and overweight of 22.3%, 70.2%, and 7.4%, respectively (age range, 15–19 years) [47
]. The distribution of BMI categories in the present study did not differ significantly from that in the Japanese national survey (chi-square test; P
No relationship was observed between sex and the onset of malocclusion. Previous studies have also reported that sex was not significantly associated with the prevalence of malocclusion based on the IOTN [48
]. Those studies support our results.
Several limitations of the present study must be considered when interpreting the results. First, these findings to young adults in general. Second, we did not investigate bite force or clenching force; these are difficult to measure in general oral examinations because they require special instruments [51
]. Third, there may have been a selection bias, given the low follow-up rate. In the present study, analysed students (n = 238) comprised 21.8% of all eligible students (n = 1,092). However, no significant differences were seen in the ratios of bruxism, oral habits, or BMI between the analysed and non-analysed students (238 vs. 854 students, chi-square test; P
> 0.05), with the exception of sex (chi-square test; P
< 0.05). Any effects of a selection bias would have therefore been negligible. Forth, we could not investigate malocclusion in the participants’ parents. We have to pay attention to deal with the data of parents’ malocclusion based on the questionnaire because of bias. Finally, we did not investigate tongue thrust, which might affect malocclusion, based on a recent case report [52