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Keywords:

  • torus mandibularis;
  • occlusal force;
  • elderly;
  • genetic factors

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Objective

To examine the prevalence of torus mandibularis (TM) and identify clinical factors associated with its presence.

Materials and methods

A total of 664 Japanese adults over age 60 were studied by means of a questionnaire and clinical examination.

Results

In all, 197 subjects (29.7%) had TM. We found a significant association between the presence of TM and torus palatinus (TP; 45.1% vs. 26.5%, p < 0.001). Moreover, occlusal force with TM was significantly higher than without TM (p < 0.05). Logistic regression adjusted for age and occlusal force revealed a significant relationship between TM and occlusal force and TP [p = 0.005, odds ratio (OR) = 2.44; p < 0.001, OR = 2.66, respectively].

Conclusion

This study suggests that there is a relationship between TM and factors related to occlusal factor.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Torus mandibularis (TM) is a non-pathological bony growth on the inside (lingual) surface of the lower jawbone or mandible, above the mylohyoid line. It is located in the premolar segment and is frequently symmetrical. In edentulous patients, oral tori can make reconstruction with dentures difficult. According to some reports, TM can also interfere with denture fittings[1] as well as—infrequently—leading to obstructive sleep apnea without tongue pushed backward by large TM[2] and difficulties with endotracheal intubation[3].

Data on the prevalence of TM are inconsistent and controversial. Ihunwo and Phukubye[4] and Jainkittivong et al.[5] report that TM is more frequent in men, although others report a higher frequency among women[6-8]. Additional studies indicate an insignificant difference in prevalence between genders[9-11]. A wide variety of prevalence rates have been reported in different racial populations[4, 5, 12, 13], although few studies have addressed the incidence of TM in a Japanese population[6, 9].

The aetiology of TM is also controversial. Alvesalo et al.[14] have reported that TM is linked to the sex chromosomes. It has also been associated with parafunctional habits[15-17] and masticatory stress[8, 18]. Other reports suggest that TM may be associated with more adequately developed marginal alveolar bone[19] or number of teeth at the mandibular level[20]. However, the prevalence and potential causes of TM remain unclear, especially among Japanese elders.

Development of torus palatinus (TP) may be related to genetic factors[21], although evidence suggests that the presence of TM is related to occlusal force or support[8, 18, 20]. The primary aim of this study was to determine the prevalence of TM in Japanese elders, and factors associated with them.

Material and methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

A total of 664 volunteers (294 men and 370 women; age range, 60–82 years) participated in the study. All lived independently in the community and attended weekly lectures at the Senior Citizens' College in Osaka Prefecture in 2007–2008.

The questionnaire and study protocol were approved by the Institutional Review Board of Osaka University Graduate School of Dentistry (No. H14-7). The study was carried out in accordance with the Declaration of Helsinki for Human Experimentation. All volunteers provided written informed consent.

We used two questions on a self-administered questionnaire: (i) ‘Have you been aware of clenching or grinding of the teeth when awake?’; and (ii) ‘Have you or your cohabitant been aware of clenching or grinding of the teeth when sleeping?’ to solicit information about dental clenching and grinding[16]. Participants were asked to respond with yes, no or ‘don't know’.

In this study, the clinical examination assessed dental status, temporomandibular joint symptoms and occlusal force according to previous studies[22]. Oral examinations were performed by five dentists instructed on the research project and calibrated prior to the investigation. Participants were grouped according to the Eichner index[23-26] based on the number of occlusal contacts in existing natural teeth or fixed partial dentures in the pre-molar and molar regions of both jaws. Class A had contacts in four support areas; Class B in one to three areas or in the anterior area only. Class C, with few if any remaining teeth, had no contact in any area. The groups represented the functional value of each dentition based on the type, number and arrangement of the teeth. Occlusal support in the premolar or molar areas was examined in a similar fashion.

TP and TM were assessed as present when a painless bony swelling was seen or palpated in the middle of the hard palate or in the lingual area of the mandible[5, 16] (Fig. 1). When a bony swelling was not distinguished clearly by inspection and palpation, it was recorded as absent.

image

Figure 1. Torus mandibularis.

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Occlusal force was measured using a pressure-sensitive sheet (Dental Prescale, 50 H type R, 97 μm thick; Fuji Photo, Tokyo, Japan) and apparatus for analysis (Occluzer; Fuji Photo). With the sheet placed between the maxillary and mandibular dental arches, participants were indicated to occlude on the Dental Prescale with maximum force for 3-s to release microcapsules containing colour-forming and colour-developing materials in the pressure-sensitive sheets. Occlusal force in newtons was calculated from the depth of colour released (Fig. 2)[27-30]. The measurements of the system are all based on an elementary unit of an average bite pressure which is calculated from the degree of colouring in a square of 0.25 mm[30] The uses, limitations, validity and reliability of this method have been described[27-30].

image

Figure 2. A measurement of occlusal force with Occluzer (Fuji Photo) in a typical case.

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The jaw joints were examined for temporomandibular disorders (TMDs), such as clicking, crepitus or pain on opening the mouth to 40 mm or more.

Chi-square and Mann–Whitney U-tests were performed to examine factors related to the presence of TM. A multiple logistic regression with forced entry determined whether an independent variable remained statistically significant after controlling for other confounding variables. All of the statistical significance was identified at p < 0.05.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The mean age of participants was 66.5 ± 4.2 years (range, 60–82 years). The mean ages for men and women were 63.1 ± 4.2 years and 68.1 ± 3.3 years, respectively. In all, 197 (29.7%) participants had TM (Table 1). There was a significant difference between men and women with TM but not with TP (Table 2). A significant difference was evident between men and women having TP with TM and men and women having TP without TM (Table 2).

Table 1. Associations between the presence of torus mandibularis (TM), and various age, gender and occlusal variables
Variables n TMDifference
Prevalence%p-valuea
  1. a

    A chi-squared test was used.

Overall66429.7 
Gender
Men29432.00.142
Women37027.8
Age
60–6952731.10.082
70+13724.1
Self-reported Diurnal bruxism
Absent59030.30.086
Present4233.3
Questionable3212.5
Self-reported Nocturnal bruxism
Absent49729.00.024
Present10239.2
Questionable6520.0
Eichner index
Class A41734.50.001
Class B21522.3
Class C3215.6
Occlusal support on premolars
Absent12014.2<0.001
Present54433.1
Occlusal support on molars
Absent24220.7<0.001
Present42234.8
Torus Palatinus
Absent55126.5<0.001
Present11345.1
Temporomandibular disorders
Absent50729.00.278
Present15731.8
Table 2. The prevalence of torus mandibularis and torus palatinus in 664 subjects
Presence (+) or absence (−) of tours mandibularis (TM) & torus palatinus (TP)Men (n = 294) n (%)Women (n = 370) n (%)Total (n = 664) n (%)p-valuea
  1. a

    A chi-squared test was used.

TP(+)TM(+)11 (3.7)40 (10.8)51 (7.7)<0.001
TP(+)TM(−)11 (3.7)51 (13.8)62 (9.3)<0.001
TP(−)TM(+)94 (32.0)52 (14.1)146 (22.0)<0.001
TP(−)TM(−)178 (60.5)227 (61.4)405 (61.0)0.447

The presence of torus mandibularis was associated significantly with torus palatinus, the Eichner index (Class A vs. Class B and Class A vs. Class C), occlusal supports and self-reported nocturnal bruxism (Table 1). The occlusal force in subjects with TM was significantly higher than without TM (743.1 ± 335.1 N vs. 616.2 ± 325.9 N, p < 0.05) (Fig. 3). Logistic regression analysis adjusted for age, and occlusal force showed a significant relationship between TM and occlusal force and TP (Table 3).

Table 3. Contributions from multiple regression analyses of significant variables to the presence of torus mandibularis
VariablesContributions
Significance (p–value)Odds- ratio95% confidence interval
Significant explanatory variables
Occlusal Force (×1000N)0.0052.441.32–4.52
The presence of TP<0.0012.661.65–4.30
Insignificant explanatory variables
Age0.0670.630.39–1.03
Gender (men)0.0831.420.95–2.13
Diurnal bruxism0.8571.080.49–2.38
Nocturnal bruxism0.2780.760.46–1.25
Eicher index0.5981.230.56–2.70
TMD symptom0.8590.960.62–1.50
Occlusal support at premolar area0.0791.360.92–3.97
Occlusal support at molar area0.4741.040.57–3.22
image

Figure 3. Associations between occlusal force and the presence of torus mandibularis. A Mann–Whitney U-test was used. *p <0.05.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This report examined the prevalence of TM and TP among Japanese elders, and some of the factors associated with TM. Previous studies described the prevalence of TM, but few have assessed it in Japanese populations[6, 9] or investigated factors associated with its occurrence[5, 14-17, 19, 20, 31]. The present study demonstrates that approximately one-third of the people we examined have tori and that factors related to oromaxillofacial function might have a role in creating them. The prevalence of tori varies among different populations[4, 5, 12, 13]. In Asian populations like ours, the prevalence seems to fall somewhere around one-third[5, 9, 10, 13], which is consistent with our findings. We also noted approximated prevalence in men and women (32.7% vs. 27.8%)—an outcome that is similar to Ohno's report[9] (29.4% vs. 28.1%) but differs from Suzuki's report[32] (38.4% vs. 51.9%). Although the reasons for the difference have yet to be determined, gender[14, 32] as well as other factors[15-17, 20] have been implicated in TM. These include parafunctional habits[16] and the number of present teeth[20].

Data from previous studies indicate that TM might predict the risk of TMD[15-17]. The present study found no relationship between TM and self-reported diurnal bruxism or temporomandibular disorders. However, in our study, there was no significant association between TM and them. Rather, it was suggested that there were significant associations between TM and self-reported nocturnal bruxism, Eichner index, occlusal support at the premolar and molar areas, occlusal force, and the presence of TP. Eggen et al.[20] suggested that TM may be associated with number of teeth at the mandibular level; however, we found that TM was associated with mechanical stimulation from occlusal contacts. Alvesalo[31] suggested that different effects of the X and Y chromosomes on growth explain the expression of TM, but our result indicate that the participants with TP tend to increase bone mass.

Wolff's law[33, 34] supports that a loading force on bone prompts remodelling over time, thereby strengthening the bone. Perhaps, therefore, occlusal load via the periodontal ligament increases the bone mass and density and causes bony ridges on the lingual surface of the mandibular corpus. Schepdael et al.[35] described how bone formation took place in a tension zone, whereas Christen et al.[36] used a murine model and suggested that the history of bone loading can be estimated from its morphology. Long-term stress from nocturnal bruxism increases the load on the premolar or molar areas, increasing the bone mass and strengthening the occlusal force. It also gives TM an unusual character in the incisal areas[5], which supports our finding that occlusal force is stronger in the presence of TM than in its absence. However, further studies are needed to confirm the significance of this observation.

Overall, our results suggested that the prevalence of TM in Japanese elders is associated with oromaxillofacial functions rather than genetic factors.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We are grateful to Drs. Ken-ichi Matsuda, Ryosuke Kagawa, Taiji Ogawa, Kaori Enoki, Ken Ishida, Sayaka Tada, Syunsuke Murai, Yoko Mizuno and Tadashi Okada for data collection. This research was supported by a Grant-in-Aid for Scientific Research (No.19390496) from the Japan Society for the Promotion of Science.

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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