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Risk factors for low molar bite force in adult orthodontic patients

Malene Krogh Andersen, Liselotte Sonnesen
DOI: http://dx.doi.org/10.1093/ejo/cjs003 421-426 First published online: 30 January 2012

Abstract

The aim was to analyse which parameters in a standard orthodontic material are most important for identifying factors for low bite force. Such analyses have not previously been reported in adult orthodontic patients. The sample comprised 95 adults (67 females and 28 males) aged 18–55 years sequentially admitted for conventional orthodontic treatment. All subjects had moderate to severe malocclusions. Bite force was measured by a pressure transducer, craniofacial dimensions and head posture were measured on profile radiographs, number of teeth in contact were evaluated with a plastic strip in intercuspidal position, and symptoms and signs of temporomandibular disorders (TMD) were evaluated by TMD screening. Associations were assessed by Spearman correlations, Wilcoxon signed-rank sum test, and multiple stepwise regression analyses. Associations were found between bite force and craniofacial dimensions as mandibular prognathia (S–N–Pg, P < 0.05; S–N–sm, P < 0.05), sagittal jaw relationship (SS–N–Pg, P < 0.05), mandibular inclination (NSL/ML, P < 0.05), and mandibular plane angle (ML/RL, P < 0.01) and between bite force and TMD symptoms (P < 0.05) and TMD signs (P < 0.05). Multiple regression analysis showed that gender (P < 0.001), TMD symptoms (P < 0.01), and mandibular plane angle (P < 0.001) were the most important factors for the magnitude of the bite force in adult orthodontic patients (R 2 = 0.32). The results showed that particularly women with TMD symptoms and an increased mandibular plane angle are at risk of having low bite force. This may prove valuable in the clinic, especially in orthodontic cases with an increased need for vertical anchorage during treatment.

Introduction

Bite force represents the strength in the mandibular elevator muscles (e.g. Bakke, 1993) and is associated with many factors such as craniofacial morphology, morphological occlusion, number of erupted teeth, teeth in occlusal contact, temporomandibular disorders (TMD), age, and gender. Previously, it has been shown that women had a significantly lower bite force compared to men (Bakke et al., 1990; Ingervall and Minder, 1997; Usui et al., 2007) and that the bite force level increased until the age of 25 years in both genders (Bakke et al., 1990). In women, the bite force level decreased after the age of 25 years, and in men, the bite force level was constant between the age of 25 and 45 years and then decreased (Bakke et al., 1990). Studies have agreed that there is a clear association between low maximum molar bite force and long-face morphology in adults, whereas no definite conclusion could be drawn in children (Proffit and Fields, 1983; Braun et al., 1995; Ingervall and Minder, 1997; Sonnesen and Bakke, 2005; Usui et al., 2007). With regards to morphological occlusion, it was found that low bite force was associated with crossbite and anterior open bite (Bakke and Michler, 1991; Sonnesen et al., 2001a; Sonnesen and Bakke, 2007), whereas no firm conclusion could be drawn between bite force, deep bite, and the sagittal morphological occlusion (Pereira et al., 2007; Sonnesen and Svensson, 2008). Many studies have shown that low bite force was associated with few erupted teeth and few teeth in occlusal contact (Bakke et al., 1989, 1990; Bakke and Michler, 1991; Ingervall and Minder, 1997; Miyaura et al., 1999; Sonnesen and Bakke, 2005; Bakke, 2006). It has also been shown that subjects with low bite force have more tenderness of the jaw elevator muscles and more signs of TMD in terms of the Helkimo’s Clinical Dysfunction Index (Sonnesen et al., 2001b; Ahlberg et al., 2003; Hansdottir and Bakke, 2004; Duarte Gavião et al., 2006; Pizolato et al., 2007; Pereira et al., 2007).

Prior to orthodontic treatment, many of the factors associated with low bite force are already obtained and analysed. Study casts and orthopantomograms are obtained for evaluation of the dentition and occlusion, profile radiographs for evaluation of the craniofacial morphology and head posture, and temporomandibular screening for the evaluation of the masticatory muscles and temporomandibular joints. Only a single study has analysed which parameters are most important for the magnitude of the bite force in children based on a standard material taken prior to orthodontic treatment (Sonnesen and Bakke, 2005). This study concluded that the vertical jaw relationship and number of erupted teeth were the most important factors for the magnitude of bite force in boys. In girls, the most important factor was number of erupted teeth.

As bite force is not currently measured regularly in the orthodontic clinic, it is relevant to examine which parameters are most important for the magnitude of bite force based on already existing diagnostic material.

The present study focuses on bite force in relation to craniofacial morphology, head posture, morphological occlusion, number of teeth, teeth in occlusal contact, and TMD in adult patients before orthodontic treatment. The aim of the study was to analyse which parameters from an orthodontic standard diagnostic material were most important for the magnitude of bite force.

Materials and methods

The sample comprised 95 adults, 67 females and 28 males, aged 18–55 years (mean 28.9 years) sequentially admitted for conventional orthodontic treatment at the Department of Orthodontics, University of Copenhagen, Denmark. All subjects had moderate to severe malocclusions. The inclusion criteria were 1. patients older than 18 years of age, 2. no prior history of orthodontic treatment, and 3. a minimum of 24 erupted teeth. The exclusion criteria were 1. patients with craniofacial anomalies, 2. systemic muscle or joint disorders, 3. patients with large amalgam restorations, and 4. infractions of first permanent molars. TMD patients were excluded originally and only orthodontic patients with symptoms and signs of temporomandibular disorders who did not need TMD treatment were included in the study.

The study was based on four types of examination: recordings of the maximum unilateral bite force, cephalometric analysis including head posture based on profile radiographs, assessment of occlusion on casts of the dental arches, and a functional examination.

Molar bite force

The maximum unilateral bite force was measured at the first mandibular molars on each side by means of a miniature pressure transducer (Flöystrand et al., 1982). With the transducer placed on mandibular first molars, bite force was measured unilaterally two times in the right side and then two times in the left side as stored peak values during maximal effort. The peak value during 1–2 seconds of maximum clenching was registered and the bite force was determined as the average of the four measurements (e.g. Bakke et al., 1989, 1990; Bakke, 1993).

Cephalometry

The craniofacial dimensions and head posture were measured on profile radiographs taken for treatment planning. The profile radiographs were taken in a natural head posture (mirror position) with the teeth in occlusion as described by Siersbæk-Nielsen and Solow (1982). The radiographs were taken in a Phillips Valmet BR 2002 cephalostat and a Phillips MEDIO 30 CP cephalostat. Both cephalostats had a film-to-focus distance of 180 cm and a film-to-median plane distance of 10 cm. Correction was made for the constant linear enlargement of 5.6 per cent. An aluminium wedge placed between the cassette and the patient’s face and a movable grid were used to increase the sharpness of the image. Eighteen radiographs were not taken in the natural head posture and therefore, those patients were excluded from the head posture examination. The reference points were marked and digitized directly on the radiographs (Figures 1 and 2). Twenty-two variables describing the craniofacial morphology and head posture were calculated by means of Tiops 2000 and analysed by means of Tiops 2005 (Total Interactive Orthodontics Planning System, Tiops, 2000, 2005 Version 2.7.0 Version 2.12.4).

Occlusion

Overjet and overbite were registered on the profile radiographs. Angle classification was assessed on plaster casts taken for treatment planning and registered sagittally and transversally (Björk et al., 1964). Class l occlusion was defined from ¼ cusp distally to ¼ cusp mesially, Class II occlusion more than ¼ cusp distally, and Class III occlusion more than ¼ cusp mesially. When occlusion differed between sides the occlusion was assessed in the more deviating side.

Number of teeth, permanent and persistent primary, was counted on plaster casts and number of teeth in contact in the intercuspal position was assessed in the mouth from the ability to hold a plastic strip, 0.05 mm thick and 6 mm wide (Hawe Transparent Strips No. 690, straight), between the teeth against a strong pull when the patient’s teeth were firmly closed (Bakke et al., 1990).

Functional examination

The interview to evaluate the subjective TMD symptoms consisted of standardized questions relating to functional disorders (difficulties in jaw opening, biting, and chewing) and pain (facial pain and headache) related to the masticatory system. Symptoms had to occur at least once a week to be registered. If a patient had one or more subjective symptoms, the evaluation of the subjective TMD symptoms was classified as positive (Sonnesen et al., 1998).

The clinical examination to evaluate the objective TMD signs comprised a registration of maximal mouth opening, an evaluation of the TMJs and masticatory muscles. The maximal opening was measured in millimetres making allowance for the overbite. Joint sounds were classified as clicking or grating sounds directly audible or noticeable as irregularities when palpated. Only joint sounds that occurred at least two of three times were recorded (Dworkin and LeResche, 1992). Tenderness was assessed for the anterior temporal and superficial masseter muscles and for the TMJs. Tenderness was assessed on each side by unilateral palpation with a pressure of 1 kg (Dworkin and LeResche, 1992) exerted by one or two fingers and by a gradation of the response. Only tenderness that triggered reflex blinking or flinching was recorded (e.g. Bakke and Michler, 1991). If a patient had one or more clinical signs, the evaluation of the objective TMD signs was classified as positive (Sonnesen et al., 1998).

Reliability

The reliability of the bite force measurements was determined on 12 randomly selected adults not included in the study. These adults underwent bite force measurements at intervals of 14 days, using the same method as in the present study (Sonnesen et al., 1998). There was no significant difference between the two sets of measurements, and the method error (Dahlberg, 1940) of the individual measurements was s(i) = 5 per cent.

The reliability of the cephalometric measurements was assessed by remeasurement of 20 lateral radiographs (Sonnesen et al., 2001b). No significant differences were observed between the two sets of recordings. The method errors ranged from 0.14 to 1.04 degrees or millimetres (Dahlberg, 1940) and the reliability coefficients from 0.98 to 1.0 (Houston, 1983).

Statistical methods

The statistical analyses were performed by means of SAS (SAS Institute Inc., Cary, NC, USA). Results were considered significant at P-values below 0.05. The normality of the distributions was assessed by the parameters of skewness and kurtosis and by Shapiro–Wilks W-test. In order to identify the independent variables for the multiple regression analysis, associations between maximum bite force and the continuous variables were assessed by Spearman Rank order correlation coefficients and Wilcoxon Rank sum test was used to assess the correlations between the bite force and the categorical variables. In order to test which parameters were most important for the magnitude of the bite force, a multiple linear regression analysis with stepwise backwards elimination was performed, with bite force as the dependent variable and the variables significantly associated with bite force as independent variables.

Results

The morphological occlusion according to Angle’s classification were 33.7 per cent Class I, 55.8 per cent Class II, and 10.5 per cent Class III. Furthermore, 21.1 per cent had subjective TMD symptoms and 30.5 per cent had objective TMD signs. The mean values for bite force, the craniofacial morphology, head posture, and number of teeth and teeth in occlusal contact are presented in Table 1.

View this table:
Table 1

Overview of continuous variables in the total group (N = 95).

VariableMeanSD
Bite force (N)483.1122.4
Craniofacial morphology (°)
    Cranial base angle
        N–S–Ba132.25.47
    Maxillary prognathia
        S–N–SS80.84.35
    Mandibular prognathia
        S–N–Pg78.53.80
        S–N–Sm77.33.86
    Sagittal jaw relation
        SS–N–Pg2.33.25
    Maxillary inclination
        NSL/NL7.62.96
    Mandibular inclination
        NSL/ML30.87.29
    Vertical jaw relation
        NL/ML23.17.40
        Jaw angle
        RL/ML119.57.75
    Maxillary incisor inclination
        ILs/NL110.810.99
    Mandibular incisor inclination
        Ili/ML99.57.97
Head posture (°)
    Craniovertical
        NSL/VER99.94.11
        NL/VER92.54.40
    Craniocervical
        NSL/OPT98.67.61
        NSL/CVT104.27.14
        NL/OPT91.07.96
        NL/CVT96.67.53
    Cervicohorizontal
        CVT/HOR86.16.29
        OPT/HOR91.46.87
    Cervical curvature
        OPT/CVT5.62.61
Number of teeth and teeth in contact
    Number of teeth291.7
    Teeth in contact20.84.3

The maximum bite force was significantly lower in women than in men (P < 0.0001, Table 2). No significant difference was found between bite force, age, number of teeth present, and number of teeth in occlusal contact.

View this table:
Table 2

Significant correlations between bite force and categorical variables. TMD, temporomandibular disorders.

Variables P
TMD subjectively*
TMD objectively*
Gender***
  • Variables with no significant correlations have been deleted from the table.

  • *P < 0.05, ***P < 0.001.

The results from the correlation analysis between bite force and craniofacial morphology, head posture, morphological occlusion, and TMD were generally low to moderate, the numerical values ranging from 0.23 to 0.28, and there were no significant correlations between bite force, head posture, and morphological occlusion. The bite force was significantly positively associated with the mandibular prognathia (S–N–Pg, P < 0.05; S–N–Sm, P < 0.05) and significantly negatively associated with the sagittal jaw relationship (SS–N–Pg, P < 0.05), mandibular inclination (NSL/ML, P < 0.05), and mandibular plane angle (ML/RL, P < 0.01; Table 3). Furthermore, the bite force was significantly negatively associated with subjective TMD symptoms (P < 0.05) and objective TMD signs (P < 0.05; Table 2).

View this table:
Table 3

Significant correlations between bite force and continuous variables.

VariablesCorrelations
RL/ML−0.28**
S–N–Pg0.24*
S–N–Sm0.23*
SS–N–Pg−0.26*
NLS/ML−0.23*
  • Variables with no significant correlations have been deleted from the table.

  • *P < 0.05, **P < 0.01.

Multiple regression analysis showed that gender (P < 0.001), TMD symptoms (P < 0.01), and mandibular plane angle (P < 0.001) were the most important factors for the magnitude of the bite force in adult orthodontic patients (R 2= 0.32; Table 4).

View this table:
Table 4

Significant factors most important for the magnitude of the bite force. TMD, temporomandibular disorders.

Variables P-value
Gender<0.0001
TMD0.009
RL/ML0.0002
  • Variables with no significant associations in the multiple regression analysis have been deleted from the table.

Discussion

The aim of the present study was to analyse which parameters were most important for the magnitude of the bite force in adult orthodontic patients. This has not previously been examined on an adult orthodontic standard material obtained before orthodontic treatment.

The present study found that gender, craniofacial morphology in terms of the mandibular plane angle, and TMD symptoms were the most important factors for the magnitude of bite force in adult preorthodontic patients. The association between the three factors and bite force are previously confirmed, although as single factors.

In agreement with previous studies, the bite force in the present study was significantly lower in women than in men (Bakke et al., 1990; Ingervall and Minder, 1997; Usui et al., 2007). Meanwhile, no significant differences were found in the present study between bite force, age, and occlusal contact, probably due to the distribution of age and occlusal contact.

A clear association was found between bite force and craniofacial morphology in the vertical and sagittal plane. In agreement with previous studies in both children and adults, the bite force was significantly negatively associated with the mandibular inclination and mandibular plane angle (Ringqvist, 1973; Ingervall and Helkimo, 1978; Proffit et al., 1983; Braun et al., 1995, Ingervall and Minder, 1997; Raadsheer et al., 1999; Sonnesen and Bakke, 2005; Usui et al., 2007). In the present study, a significantly positive association was found in the sagittal plane between bite force and the prognathia of the mandible and a significantly negative association between bite force and the sagittal jaw relationship. A plausible explanation for the association in the sagittal plane could be that the masticatory muscles have better working conditions when the mandibular prognathia is large and the sagittal jaw relation is small. Thus, the masticatory muscles gain more strength (Throckmorton et al., 1980, 1995). Also, in surgical patients, a similar association has been found between maximum bite force and mandibular prognathia (Throckmorton et al., 1995; Ellis et al., 1996; Iwase et al., 1998; van der Braber et al., 2004).

With regards to TMD symptoms, this study confirms what previous studies have found in both children and adults, i.e. that there is a significant association between TMD symptoms both subjectively and objectively and reduced bite force (Wenneberg et al., 1995; Sonnesen et al., 2001b; Ahlberg et al., 2003; Hansdottir and Bakke, 2004; Duarte Gavião et al., 2006; Pereira et al., 2007). An explanation for these findings could be that the patients feel pain when chewing because of tenderness of the temporomandibular joints and masticatory muscles and thereby avoid loading of the muscles (Harper et al., 2000; Pereira et al., 2007). The reason for a low bite force could also be that patients with TMD symptoms chew less than subjects without TMD symptoms and thereby develop weaker masticatory muscles because of lack of muscle training (Kiliaridis, 1995).

During orthodontic treatment, undesirable extrusion of teeth often occurs in the preliminary levelling phase as a low force is required for extrusion of teeth (Proffit and Fields, 2000). The extruding force can be counteracted using the bite force of the patient as the intruding power of the bite force counteracts the levelling power of extrusion (Burstone, 1977; Melsen and Burstone, 1996). Therefore, orthodontic patients with low bite force may have an increased need for vertical anchorage during treatment.

The present study showed that particularly women with TMD symptoms and an increased mandibular plane angle are at risk of having low bite force. The results may prove valuable in the clinic, especially in orthodontic cases with an increased need for vertical anchorage during treatment.

Funding

The Danish Dental Association Foundation (no. 64585).

Acknowledgments

Ib Jarle Christensen, Associate Professor at Copenhagen Biocenter, is acknowledged for statistical advice and analyses. The Danish Dental Association Foundation (no. 64585) is acknowledged for funding. Colleagues and co-students are acknowledged for collaboration in connection with patient management. Maria Kvetny, MA, is acknowledged for linguistic support and manuscript preparation.

References

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