Relationship between oral frailty and locomotive syndrome in working-age individuals: a cross-sectional survey of workers in Japan

As the population ages, increasing healthy life expectancy is a goal in Japan. As oral health is important in maintaining overall health [1], age-related oral frailty and its prevention have received considerable attention. In 2013, oral frailty was defined by the Japanese Dental Association (JDA) as a set of phenomena and processes in which age-related changes in various oral conditions (number of teeth, oral hygiene, oral function, etc.), combined with decreased oral health awareness and physical and mental reserve capacity, lead to increased oral frailty and eating dysfunction, resulting in a decline in physical and mental function [2].

There is a lack of interest in oral health in the early stages of oral frailty, and the risk of tooth loss appears. In the advanced stages, a decline in the masticatory ability as well as the development of sarcopenia, a loss of muscle mass throughout the body, and locomotive syndrome (LS), a loss of mobility functions such as standing and walking due to disorders of the locomotor system (bones, joints and discs, muscles and nervous system), are believed to occur.

The concept of LS was proposed by the Japanese Orthopaedic Association (JOA) in 2007: it is caused by disorders of the locomotor system, with symptoms such as pain, limited joint range of motion, joint deformity, reduced balance ability, and slow walking speed, which increases with age [3, 4]. Musculoskeletal conditions, such as falls, fractures, and joint disease, account for approximately 20% of the care needs of older people, and approximately 67% of general people have LS [5]. LS stage 1, the early stage of LS, indicates that muscle strength and balance are beginning to decline, whereas LS stage 3 is almost similar to physical frailty, with a severe decline in mobility that progresses to a state wherein it interferes with social participation.

Factors contributing to LS include female sex, older age, low body weight, obesity, low physical activity, lack of regular exercise, low frequency of current smoking, and low frequency of current drinking habits. The biological relationship between physical frailty and oral frailty in elderly people has been extensively studied, and the evidence base for relevant oral indicators is growing. Physical frailty and oral frailty in older people are most frequently associated with the number of remaining teeth, followed by masticatory function [6]. Furthermore, falls have also been reported to be associated with the number of remaining teeth. Additionally, the relationship between LS, as assessed by a questionnaire, and oral frailty in the elderly has been reported to be affected by the number of remaining teeth, low masticatory ability, and the presence of subjective mastication and swallowing symptoms in patients with LS [7‐10].

LS has traditionally been considered a problem for the elderly population, but it has recently been shown that a considerable number of people in the working-age population are affected by LS [11‐14]. In the latest report on the prevalence of LS in the working-age population, it was reported that LS stage 1 was present in 21.7% and 25.0% of males and females younger than 40 years of age, 31.6% and 41.1% of those aged 40–49 years, and 41.5% and 42.1% of those aged 50–59 years, respectively [5].

LS has been identified in the working-age population, which means that signs of oral frailty, believed to occur before LS, can be detected, as physical frailty and oral frailty are associated, but few studies have investigated this. The hypothesis of this study was that the working-age population with LS would show signs of oral frailty. Therefore, this study aimed to determine the relationship between LS and the number of remaining teeth and masticatory ability, which are closely related to oral frailty in the working-age population.

In the present study, we investigated the relationship of the number of remaining teeth and masticatory ability with LS in the working-age population. The results showed that according to the stand-up test result, the group with ≤ 19 remaining teeth had a higher rate of LS than the group with ≥ 20 remaining teeth, with an odds ratio of 5.99.

The participants had a median age of 43 years. The age range of the participants was 20–49 years, with 80% of the population being 20–49 years and a small percentage being 50 years or older. The BMI was in the normal range for both males and females.

Regarding the number of remaining teeth, as of 2019, the average percentage of Japanese aged 20–59 years with 19 or fewer teeth was approximately 5%, and the number of teeth decreased with age [17]. The results of the present study showed similar results; the number of remaining teeth of 19 or fewer was 2%. Masticatory ability was comparable to the reference values for males and females. Approximately 6% of participants were classified in the group with low masticatory ability. Overall, those with signs of oral frailty were rare. It is assumed that this was a reasonable result since the target population was the working-age population with an average age of 43 years. However, the lack of regular dental check-ups is a cause for concern, with only approximately 40% of people having regular dental check-ups. Japanese statistics showed that the rate of consultation among the working-age population is lower than that of the elderly population, with the rate of consultation among those 49 years of age and younger being less than 50% [26]. Therefore, the goal is to have 60% of the population receiving annual dental check-ups. We believe this relates to the dental healthcare system in the Japanese workplace. All workers in Japan are required to undergo a medical check-up by a physician but are not obligated to undergo dental examinations, except for those engaged in special work involving the handling of acids. Thereby, outside of working hours, the working-age population must voluntarily engage in self-care through dental check-ups and dental visits conducted by the local government that has jurisdiction over their area of residence for early detection and early treatment of tooth decay and periodontal disease. However, it has been noted that many people find it difficult to do so due to their busy work schedules and time constraints [27]. For this reason, half of the study participants were also considered unable to attend regular dental appointments.

As mentioned earlier, regular dental check-ups are fundamental and very important to prevent oral frailty since poor oral health literacy is considered in the early stage of oral frailty and increases the risk of tooth loss due to tooth decay and periodontal disease [2]. The working-age population's access to dental health services needs to be improved.

As mentioned earlier, in working-age males and females, it was reported that LS stage 1 was present in 21.7% and 25.0% of those younger than 40 years of age, 31.6% and 41.1% of those aged 40–49 years, and 41.5% and 42.1% of those aged 50–59 years, respectively [5]. This indicates a high incidence of LS stage 1 in the working-age population, approximately over 30%. The prevalence of LS based on the criteria of the two-step test was approximately 1% [11, 12], with a very low incidence in the working-age population, which is consistent with the results of the present study. The prevalences of LS based on GLFS-25 criteria were reported to be 17% and 25% for males and females, respectively, by Otsuki et al. and 15.2% and 22.8% for males and females, respectively, by Nishimura et al., which are in agreement with the results of this study [11, 12]. Additionally, Akahane et al. reported a prevalence of LS of 5.3%; the LS cut-off value they used was ≥ 16 points, corresponding to LS stages 2 and 3 in this study, and the results were consistent with ours [7]. The prevalences of LS based on the criteria of the stand-up test have been reported to be 6.5% and 12.6% [11] and 3.1% and 8.8% [12] in males and females, respectively. The results of the present study showed that the prevalence of LS in males was in line with those of previous studies, whereas the prevalence in females was very high at 35%. LS is more prevalent in females compared to males [28]. In the present study, females were also 4.8 times more likely than males to have the condition. Ogata et al. found that from the age of 20 to 49 years, females scored significantly worse than males in the stand-up test [29]. Narumi et al. found that knee extension muscle strength, as assessed by the stand-up test, was significantly lower in females than in males in each age group from 20 to 89 years [30].

The results of the present study further supported the findings of previous studies. The female participants in this study were characterised by the fact that approximately 60% of the participants were aged 40–59 years. Yoshimura et al. reported that more than 40% of women aged 40–59 had LS [5]. Another characteristic of the females was that 80% them had low physical activity levels, with a UCLA score of ≤ 4 points or less. Those with low physical activity were 2.3 times more likely to have LS than those with high physical activity; those with low physical activity (UCLA score of ≤ 4 points) had a higher prevalence of LS, and Ohtsuki et al. reported that approximately 80% of those with LS had a UCLA score of ≤ 4 points [11, 12]. These factors were predicted to affect the high prevalence of LS among females in the present study.

Regarding the effect of age, the odds ratio was slightly lower, at 0.96 for age, in contrast to results of previous studies [11‐14]. This could be because more younger participants (20–30 years) had worse scores.

In the stand-up test, the group with ≤ 19 remaining teeth had a higher LS rate than the group with ≥ 20 remaining teeth, and the results of the logistic regression analysis showed that the group with ≤ 19 remaining teeth had a 5.99-times higher LS onset. In the stand-up test, participants with LS may not have sufficient knee extensor strength to stand on one leg from a seat height of 40 cm. Knee extensor strength is significantly increased by clenching through a phenomenon called simultaneous activation enhancement [31, 32]. Clenching also increases muscle activity in the sternocleidomastoid and trapezius muscles, which are involved in maintaining head position when standing [33]. It also stimulates the H-reflex in the tibialis anterior and soleus muscles, which aids postural control associated with the forward and upward shift of the centre of gravity during standing [34‐36]. In areas of tooth loss, maximum occlusal force is reduced [37, 38]. In other words, it has been speculated that reduced clenching force may have resulted in a lack of afferent sensory input from periodontal ligament receptors [39], affecting knee extensor strength and postural control. The effects of clenching during the standing test should be investigated in the future.

Previously, Akahane et al. and Tominaga et al. reported significant associations between GLFS-25 scores and the number of remaining teeth in populations with mean ages of 58 and 69.9 years, respectively [7, 9]. Nagilla et al. reported a significant association between Loco-Check LS scores and the number of remaining teeth in a population with a mean age of 59 years [8]. The participants in the previous study were older than those in the present study, and the previous study used a rating index derived from the participants' subjectivity. The present study found a significant association between the number of remaining teeth and presence of LS according to the stand-up test, a dynamic and objective assessment measure. To the best of our knowledge, this is the first study to examine the relationship between the presence of LS according to the stand-up test and the number of remaining teeth in a working-age population.

As mentioned earlier, LS stage 1 occurs before physical frailty, whereas LS stage 3 is almost identical to physical frailty. Tanaka et al. reported a significant increase in physical frailty two years later in those with one or two signs of oral frailty compared with those with no signs of oral frailty [16]. Our study identified those in the pre-stage of physical weakness based on the stand-up test result for LS and found that standing movements were associated with the number of remaining teeth, supporting Tanaka et al.'s findings [16]. In conjunction with the LS test assessment, the number of remaining teeth may be an effective tool for identifying those at risk for physical frailty at an earlier stage. The purpose of LS examinations in our working-age population was to prevent occupational accidents such as falls at work. Nishimura et al. stated that to prevent LS, it is necessary to develop exercise habits early and create an environment where the working-age population can exercise at work [40]. A decrease in the number of remaining teeth was associated with difficulty in standing movements in the present study, which suggests that oral frailty prevention measures should be used to prevent LS in the working-age population.

This study has some limitations. First, this study was limited to four regional companies, and the number of participants was limited; additionally, as workplace surveys tend to include generally healthy people, there may have been a selection bias. Therefore, it is important to remember that the results reflect the workplace setting, necessitating the expansion to geographical areas to generalise the study’s findings. Secondly, as the target population was of working age, there were fewer people with < 19 remaining teeth. Thus, this association should be interpreted with caution. Larger studies are needed to estimate the effect more precisely. Thirdly, the number of remaining teeth and masticatory ability were assessed, but bite and clenching were not, so a more detailed assessment of occlusion is needed to confirm the relationship with LS. Finally, because this is a cross-sectional study, the causal relationship between the decrease in the number of remaining teeth and LS is unclear. Longitudinal studies are needed to clarify these causal relationships.

The present study investigated the relationship between oral frailty and LS in a working-age population. Signs of oral frailty, such as tooth loss and masticatory ability, were identified, and a higher proportion of LS was found in the group with ≤ 19 remaining teeth.