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Occlusal Force and Condylar Motion in Patients with Anterior Open Bite

Department of Orthodontics and Dentofacial Orthopedics, Okayama University Graduate School of Medicine and Dentistry, 2-5-1, Shikata-cho, Okayama, 700-8525, Japan;

^* corresponding author, t_yamamo{at}md.okayama-u.ac.jp

	ABSTRACT

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Patients with open bite often show a weak occlusal force and temporomandibular disorders (TMDs). If these are the main cause of open bite, it may be hypothesized that both pre-pubertal and adult open-bite patients would show a weak occlusal force and abnormal condylar motion. The purpose of this study was to test this hypothesis. Test group subjects consisted of 13 consecutive pre-pubertal and 13 adult patients with anterior open bite. They were compared with age-matched normal subjects. The adult open-bite group showed a weaker occlusal force and a shorter range of condylar motion compared with the control subjects. In the pre-pubertal subjects, however, there were no significant differences in the occlusal force and range of condylar motion between the open-bite and control groups. Therefore, these results suggest that a weak occlusal force or TMDs may not be the main cause of open bite.

KEY WORDS: anterior open bite • condylar motion • occlusal force • orthodontic patients • temporomandibular joint (TMJ)

	INTRODUCTION

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Open bite is defined as a state in which the upper and lower teeth are separated when the jaw is closed completely, and generally refers to an anterior open bite in which the upper and lower anterior teeth do not occlude at a centric occlusion (CO) position (Mizrahi, 1978). It has been reported that the incidences of open bite in the general population whose ages are 8–11, 12–17, and 18–50 yrs are 3.6, 3.5, and 3.3%, respectively (Proffit and Fields, 2000).

It has been reported that patients with open bite often show a weak occlusal force and/or long face, and that individuals with long face show weaker occlusal force when compared with those with short face (Proffit et al., 1983; Proffit and Fields, 1983; Bakke and Michler, 1991). Therefore, it can be speculated that a weak occlusal force may often cause open bite (Kiliaridis et al., 1989). If this speculation is correct, both pre-pubertal and adult orthodontic patients with open bite, and more than 8 yrs old, would be expected to show weaker occlusal force than would normal subjects.

Recent epidemiological studies have showed that open-bite patients often suffer from temporomandibular disorders (TMDs) and temporomandibular joint internal derangement (TMJID; Henrikson et al., 1997; Thilander et al., 2002) and suggested that osteoarthrosis (OA) of the temporomandibular joint (TMJ) might cause open bite (Pullinger et al., 1993; Yamada et al., 2001). It is known that the number of patients with TMDs increases particularly during the pubertal period, and that the incidence of TMDs then is around 10 to 20% in adults (Matsuka et al., 1996; Thilander et al., 2002). Furthermore, it is known that OA of the TMJ often causes TMJID (Westesson and Rohlin, 1984; Dijkgraaf et al., 1999). If OA of the TMJ often causes open bite, we may speculate that both pre-pubertal and adult orthodontic patients with open bite, and more than 8 yrs old, would show an abnormal condylar motion, because patients with TMDs or TMJID often show an abnormal condylar motion (Kenworthy et al., 1997; Gsellmann et al., 1998; Miyawaki et al., 2001b). However, there have been few studies in which condylar motion was examined in patients with open bite.

The purpose of this study was to test the hypothesis that both pre-pubertal and adult orthodontic patients with open bite show a weak occlusal force and/or abnormal condylar motion compared with normal individuals.

	MATERIALS & METHODS

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Subjects
All test-group subjects consisted of pre-treatment patients whose chief complaint was anterior open bite. They were selected from among consecutive initial outpatients who had visited an orthodontic clinic of the university hospital during the preceding 2 yrs. The outpatients with anteroposterior or transverse skeletal problems were excluded. Thirteen pre-pubertal patients, who were from 8 to 12 yrs of age (three boys and 10 girls; mean age, 9.9 yrs [SD 1.7 yrs]), and 13 adult female patients, whose age ranged from 18 to 30 yrs (mean age, 21.6 yrs [SD 4.5 yrs]), were chosen as the test groups. All adult patients had showed anterior open bite from childhood.

Fourteen pre-pubertal (five boys and nine girls; mean age, 10.3 yrs [SD 1.5 yrs]) and 14 adult female control group subjects (mean age, 23.9 yrs [SD 1.2 yrs]) whose overjet and overbite were normal, and whose age and sex matched those of the test groups, were also chosen as the respective control groups. They had no skeletal problems. All adult subjects and parents of pre-pubertal subjects gave informed consent after having received a full explanation of the goals and structure of the present study. The Ethical Review Board of the university hospital approved this study.

For this study, we used lateral cephalometric radiographs taken for orthodontic diagnosis in only test-group subjects, to reveal the characteristics of their dentofacial morphology (Fig.). The pre-pubertal open-bite patients (three boys and 10 girls) showed a high mandibular plane angle, mainly due to upper molar extrusion, when compared with 10-year-old normal Japanese females with good occlusion (Wada, 1977). The reason female control data were used for comparison with mixed data from pre-pubertal patients was that there were no significant differences in cephalometric measurements between 10-year-old boys and girls (Wada, 1977). The adult open-bite patients (all females) also showed a high mandibular plane angle, mainly due to upper and lower molar extrusion, when compared with the Japanese adult control females with good occlusion (Wada, 1977). The severity of anterior open bite in the adult patients was greater than that of the pre-pubertal patients.

View larger version (36K): [in this window] [in a new window]   Figure. Mean profilograms and cephalometric measurements in the pre-pubertal and adult open-bite groups. Solid lines represent mean profilograms of patients, and dotted lines indicate mean profilograms of Japanese normal females according to the corresponding age (Wada, 1977). PP-U6 means a distance between U6 (upper first molar) and PP (palatal plane), and PP-U1 indicates a distance between U1 (upper central incisor) and PP. Mp-L6 means a distance between L6 (lower first molar) and Mp (mandibular plane), and Mp-L1 indicates a distance between L1 (lower central incisor) and Mp. These profilograms were superimposed on the SN plane, registered at S. SNA angle means an angle of N-A to S-N, and SNB angle indicates an angle of N-B to S-N. The ANB angle means an angle of S-N to palatal plane. Occl pl angle means an angle of S-N to occlusal plane, and Mp-SN angle indicates an angle of S-N to mandibular plane.

Recording and Analysis of Jaw Movement
For the recording of jaw movement, we used an optoelectric jaw-tracking system with 6 degrees of freedom, and consisting of a head frame, face bow, light-emitting diodes (LEDs), CCD cameras, and a personal computer (Gnathohexagraph system Ver. 1.31, OnoSocki Ltd., Kanagawa, Japan). The sampling frequency was 89.3 Hz. The mean measurement error of the system was 150 µm (SD 10 µm).

Each subject was seated in an upright but relaxed position with the head unsupported and naturally oriented. A head frame and a face bow, each with 3 LEDs, were attached securely to the head and the dental clutch, which was bonded to the labial surface of the lower incisors. The clutch was bent to ensure that the movement of the mandible and lip was inhibited as little as possible (Miyawaki et al., 2000, 2001a,b). Two CCD cameras were placed approximately 1.2 meters in front of the subject. The lower central incisor point and bilateral condylar points on the skin—located 13 mm anterior and 5 mm inferior from the tragus to the lateral ocular angle on the right and left sides, i.e., the mean condylar point on the skin—were recorded with the use of a pointer with 2 LEDs. We calculated the hinge axis point on the sagittal plane mathematically by solving the most minimal point of the moving distance around the condylar point on the skin on each side, when minor jaw-opening and -closing tapping movements were performed for 10 sec. We used the hinge axis points 20 mm medial from the skin (Gibbs and Lundeen, 1982) as condylar points on the right and left sides of each subject (Miyawaki et al., 2000, 2001a,b).

Each subject performed voluntary maximum jaw-opening and -closing movements without pain or discomfort for 10 sec at a frequency that he/she felt to be natural and comfortable. Each subject also performed voluntary maximum tooth-clenching at the CO position for 2 sec. We calculated the maximum displacements at the bilateral condylar and lower incisor points when each subject performed the aforementioned test movements from the CO position (Miyawaki et al., 2000, 2001a,b).

Recording of Maximum Occlusal Force and Contact Area
Using an occlusal force recording system (Dental Prescale & Occluzer, Fuji Film, Tokyo, Japan), we recorded the occlusal force and contact area when each subject performed maximal voluntary clenching on a 0.1-mm-thick pressure-sensitive sheet. These variables were found to be stable within a given subject (Hidaka et al. 1999).

Statistical Analyses
For each measurement, we used the unpaired t test or Mann-Whitney U test to analyze the difference between the test and control groups according to the data distribution. The observed significance level of each test, i.e., probability (P), was calculated for each comparison. Probability levels of P < 0.05 were considered statistically significant. These tests were performed with the use of conventional statistical analysis software (Statview, SPSS, Chicago, IL, USA).

	RESULTS

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Twelve of the 13 adult open-bite patients showed TMJID (bilateral, n = 8; unilateral, n = 4), and six of the 13 patients showed OA of the TMJ (bilateral, n = 4; unilateral, n = 2). However, only three of the 14 control subjects showed TMJID, which was unilateral, and they did not suffer from OA at all (Table 1).

There were no significant differences in the maximum gapes at the bilateral condylar and lower incisor points between the pre-pubertal open-bite and control groups (Table 2). However, those in the adult open-bite group were significantly shorter than those in the adult control group (Table 3).

As for the maximum displacement at each measurement point during maximum clenching from the CO position, there were no significant differences between the pre-pubertal or adult open-bite group and their respective control group (Tables 2, 3).

	DISCUSSION

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MRIs are often used for the diagnosis of OA of the TMJ and/or TMJID (Orsini et al., 1999; Brandlmaier et al., 2003). According to previous studies, some children suffered from TMJID, although the incidence was low (Matsuka et al., 1996; Thilander et al., 2002). With regard to the MRIs of TMJ in children, children sometimes move during the taking of MRIs, the outline of cortical bone is sometimes unclear due to immature cortical bone, and adequate acquisition of TMJ imaging is difficult due to the small size of the condyles (Thilander et al., 1976). Furthermore, the sensitivity and specificity of diagnosis of TMJID by MRI range from 40 to 100%, even in adults (Hansson et al., 1989). Therefore, an accurate diagnosis of TMJID based on MRI data may be difficult in many pre-pubertal patients, in contrast to adults.

The range of condylar motion is considered to be a good index for assessment of the state of the TMJ in relatively young individuals, including young adults, because such patients with TMJID often show a limited condylar translation (Harper and Schneiderman, 1996; Kenworthy et al., 1997; Gsellmann et al., 1998; Miyawaki et al., 2001b). Furthermore, by using a six-degrees-of-freedom jaw-tracking system with high accuracy, we revealed that young adult patients with TMJID showed a shorter condylar translation during maximal jaw opening when compared with normal subjects (Miyawaki et al., 2001b). Therefore, we examined the range of condylar motion during maximal jaw opening in both pre-pubertals and adults, to reveal the state of the TMJ. As a result, in adult subjects, the state of TMJ diagnosed by MRIs reflected the condylar motion. Also, we used a reliable bite-force recording system (Hidaka et al., 1999). Therefore, the measurement systems used in the present study were reliable.

With regard to maximum occlusal force, pre-pubertal open-bite patients showed a normal value, whereas adult patients showed a value lower than normal. The results for pre-pubertal open-bite patients almost coincided with previous data (Rentes et al., 2002), as did those for adult open-bite patients (Bakke and Michler, 1991). In addition, the severity of open bite in the adult patients was greater than that for the pre-pubertal ones. Therefore, these results suggest that a weak occlusal force may be a promoting factor for, rather than the cause of, open bite.

With respect to the range of condylar motion during maximal jaw opening, pre-pubertal open-bite patients showed a normal range of condylar motion. This means that pre-pubertal patients had a normal TMJ, because the range of condylar motion is a good index for assessment of the state of the TMJ (Harper and Schneiderman, 1996; Kenworthy et al., 1997; Miyawaki et al., 2001b). Adult patients with open bite, which had continued from childhood, showed a shorter condylar translation and a higher incidence of OA of the TMJ and/or TMJID than did normal adults. However, the incidence of OA of the TMJ was less than 50%. These findings on condylar motion in open-bite patients are new. Therefore, the hypothesis that both pre-pubertal and adult patients with open bite show a limited condylar translation was rejected. This means that OA of the TMJ or TMJID may not be the main cause of open bite clinically, despite the fact that OA of the TMJ can theoretically cause open bite. Probably, OA of the TMJ and/or TMJID may often occur as a result of open bite.

In the present study, both pre-pubertal and adult patients with open bite showed a normal range of condylar motion during maximal biting. Furthermore, almost all adult open-bite patients showed TMJID, i.e., no disc between the condyle and mandibular fossa at the CO position. Therefore, we may speculate that TMJID leads to more condylar translation in the superior direction during maximal biting, probably leading to a strong load on the TMJ (Isberg et al., 1985). The sensory receptors in the TMJ and other tissues, such as the periodontal ligament, continuously feed back information, and noxious stimuli are reflexively avoided, so that jaw movement can occur with minimal injury to these tissues (Okeson, 1998). Therefore, the condylar motion in the superior direction in adult open-bite patients might be limited reflexively, to prevent the TMJ from being strongly pressed during maximal biting, thus leading to weaker bite-force endurance (Stegenga et al., 1992).

	ACKNOWLEDGMENTS

 
This study was supported by grants-in-aid for scientific research (B, #15390634; C, #15592162; and C, #20322232) from the Japan Society for the Promotion of Science and by a grant from the Suzuken Memorial Foundation. The kind assistance of the clinical staff of the Department of Orthodontics and Dentofacial Orthopedics, Okayama University, Graduate School of Medicine and Dentistry, is gratefully acknowledged.

Received September 8, 2003; Last revision October 25, 2004; Accepted November 22, 2004

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