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Biomaterials & Bioengineering |
Occlusal and TMJ Loads in Subjects with Experimentally Shortened Dental Arches
Y. Hattori1,*,
C. Satoh1,
S. Seki1,
Y. Watanabe2,
Y. Ogino1 and
M. Watanabe1
1 Department of Aging and Geriatric Dentistry, Tohoku University Graduate School of Dentistry, 4-1 Seiryo-machi, Aoba-ku, Sendai, 980-8575, Japan; and
2 Kansei Fukushi Research Center, Tohoku Fukushi University, 6-149-1 Kunimigaoka, Aoba-ku, Sendai, 989-3201, Japan;
Correspondence: *corresponding author, yhattori{at}mail.cc.tohoku.ac.jp
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ABSTRACT
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To determine whether shortened dental arches (SDAs) cause functional overloading of the teeth and the temporomandibular joints, which has been implicated in periodontal diseases and temporomandibular disorders, we investigated the influences of SDA on occlusal and joint loads. Bite force and masticatory muscle electromyograms were recorded in five dentate subjects who clenched maximally on intra-oral appliances, creating symmetrical SDAs experimentally. Muscular forces estimated from the recorded electromyograms were fed into a finite element jaw model for calculating bite forces and joint loads. Comparison between the measured and the calculated bite forces ensured that the joint loads were representative. The bite force on each tooth increased with missing molar occlusions, while joint loads decreased. The bite force per root surface area was always greatest on the most posterior tooth, and these values were most constant. The findings provide no evidence that SDA causes overloading of the joints and the teeth, which suggests that neuromuscular regulatory systems are controlling maximum clenching strength under various occlusal conditions.
Key Words: shortened dental arch • bite force • TMJ load • maximum voluntary clenching
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INTRODUCTION
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Shortened dental arch (SDA)" is a term describing a dental arch missing posterior teeth (Käyser, 1981) and also represents a treatment philosophy of non-replacement of all missing teeth in the dental arch in this condition (Käyser, 1989). This philosophy is mainly supported by epidemiological findings indicating that occlusal changes resulting from missing molar teeth are self-limiting, and that neither occlusal instability nor temporomandibular disorders (TMD) are an inevitable consequence of not replacing all missing teeth (Witter et al., 1994a,b, 2001). However, there is little direct evidence to contradict previous assumptions that a SDA will increase the risk of TMD (Christensen and Ziebert, 1986) and periodontal breakdown in the residual teeth, both of which are thought to be the consequences of excessive mechanical loading.
Because no non-invasive method permits the direct measurement of temporomandibular joint (TMJ) loads, mathematical modeling methods have been used for their estimation. Such estimations could be reliable, provided a suitable model were built and adequate muscular forces were input. However, present models still cannot precisely reflect the complex architecture and inhomogeneous material properties of the jaw components, or the contraction forces of functionally heterogeneous jaw muscles. Thus, the model simplification indispensable in model studies may cause discrepancies between model predictions and living structures.
In the present study, effects of missing molar occlusions on tooth and TMJ loads during maximum voluntary clenching were studied both experimentally and mathematically. We compared the calculated bite force on each residual tooth in the model with the experimentally obtained results to verify the reliability of the simulation.
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MATERIALS & METHODS
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In vivo Experimental Study
Subjects
Five healthy males, aged 25 to 32 yrs (mean, 27.0), participated in this study. All had Angle’s Class I complete natural dentitions, except for the third molars, and were free from existing or previous signs and symptoms of TMD. Informed consent was obtained from the subjects according to the guidelines of the Ethics Committee, Tohoku University Graduate School of Dentistry.
Intra-oral Appliances
Stabilization splint-like full-arch mandibular intra-oral appliances were fabricated with acrylic resin. The occlusal surfaces were adjusted so that simultaneous and evenly distributed contact of teeth would be obtained when the subjects closed their jaws. The appliances were then divided into premolar and molar parts covering the individual lower posterior teeth (Fig. 1 ).
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Figure 1. Experimental set-ups. Intra-oral appliances used in the in vivo study (A) and its occlusal condition visualized with silicone check-bite (B). The lower figure (C) shows the three-dimensional FE model of the human mandible used in the in vitro study, which consisted of 6 materials (enamel, dentin, periodontium, cortical and cancellous bone, and soft tissues within TMJs). Arrows at the nodes on the upper surface of the glenoid fossa and the middle node of each occlusal surface indicate three-dimensional restraints of these nodes. The reaction forces on these nodes were regarded as the TMJ loads and bite forces. Missing occlusal contacts on each tooth were modeled by not restraining the node of the occlusal surface of the tooth concerned.
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Four symmetric occlusal conditions simulating complete dentition (M2-M2) and three bilateral SDAs (M1-M1, P2-P2, and P1-P1, omitting occlusal contacts on bilateral second molars, both bilateral molars, and bilateral second premolars plus both molars, respectively) were represented by these appliances. The second molars did not come into contact when the subjects, wearing appliances that covered the anterior teeth between the first premolars, clenched as hard as they could. The mean thickness of the appliances was 1.0 + 0.2 mm at the first molars.
Bite Force
A pressure-indicating film (Dental Prescale 50H, type R, Fuji Photo Film Co., Tokyo, Japan), designed to register bite force during forceful clenching (Watanabe et al., 1995), was used to record bite force. We measured the bite force on each lower tooth by scanning the colored film with a pre-calibrated device (Occluzer, FPD-705, Fiji Photo Film Co., Tokyo, Japan). We used bite records, taken with silicone impression material (Flexicon, injection type, GC Co., Tokyo, Japan) from the subjects wearing the entire appliances at their habitual closure position, to identify colored spots on the film as representing occlusal contact areas on the lower arches.
Electromyography
Electromyographic (EMG) activities of bilateral superficial masseter, anterior, and posterior temporalis muscles were recorded with two 10-mm Ag/AgCl surface electrodes, their centers 20 mm apart, placed over and parallel with the longitudinal axes of the muscles, and a common ground electrode placed over the fifth cervical vertebra. The skin was cleaned with 70% ethanol prior to electrode application. The EMG signals were differentially amplified with AC amplifiers (6R12, NEC San-ei Co., Tokyo, Japan) at a gain of 1000 and with a bandwidth of 50 to 1000 Hz, then fed into a computer (IBM PC/AT-compatible) via a 12-bit analog-to-digital converter with a sampling frequency of 2560 Hz.
Tasks
Each subject was seated in a chair with his head leaning against a headrest so that the Frankfort horizontal plane was parallel to the floor.
First, the subject was asked to produce a maximum EMG signal for each of the recorded muscles by trying to clench in different ways. Subsequently, 6 clenching tasks were performed, while EMGs and bite forces were recorded, in the following order: maximum voluntary clench (MVC) in the intercuspal position without appliances, MVC with appliances M2-M2, MVC with appliances M1-M1, MVC with appliances P2-P2, MVC with appliances P1-P1, and a repeat of the second task, i.e., MVC with appliances M2-M2.
To avoid muscle fatigue, the duration of clenching was restricted to 2 sec, and the tasks were interrupted by four-minute rest periods.
Data Analysis and Statistics
All measured bite forces for the same lower teeth were pooled and averaged for the same tasks. The load borne by the periodontal tissues (periodontally borne load [PBL]), expressed by the averaged bite force divided by the normal root surface area, was also calculated for each kind of tooth. Root surface areas were obtained from previously published data (Hujoel, 1994).
Recorded EMG signals were rectified and integrated (running integration, window 0.5 sec) for all the recording periods, and the highest value of integrated EMG for each muscle was found. The EMG signals during tasks were integrated for a period of 0.5 sec in the central 2 sec of the clenching periods, and normalized as a percentage of the largest integrated EMG obtained, i.e., an activation ratio. All activation ratios of each muscle for the same tasks were pooled and averaged for the simulation study.
We used Bartlett’s test to determine whether the data had equal variances, then a one-way repeated-measure analysis of variance (ANOVA) to test whether bite forces or muscle activation ratios differed among the occlusal conditions. The differences between the control (M2-M2) and other conditions were tested with Dunnett’s test (multiple comparison) or a paired t test.
In vitro Mathematical Study
Finite Element Jaw Model
A previously developed three-dimensional finite element (FE) model of a human mandible, including TMJs and lower dental arch (Watanabe, 2000), was used (Fig. 1 ). It had 7724 nodes and 6224 eight-noded hexahedronal isoparametric elements, and consisted of 6 different materials (dental enamel and dentin, periodontium, cortical and cancellous bone, and soft tissues of the TMJ). Material properties of these components were based on previous studies (Korioth and Hannam, 1994; Tanaka et al., 1994). FE analysis was done with MARC K7 and its pre-/post-processor MENTAT 3 (MSC Software Co., Palo Alto, CA, USA).
The central node of the occlusal surface of each lower tooth and the nodes on the upper surface of the glenoid fossa were restrained three-dimensionally so that the reaction forces acting on these nodes could be measured. Reaction forces acting on the occlusal surfaces and on the glenoid fossa were regarded as bite forces and TMJ loads, respectively.
Estimation of Muscular Forces
The magnitude of muscle contraction force (N) was estimated with the product of the cross-sectional area of the muscle (cm2), the averaged activation ratio (no unit) specific for the task, and a constant (40 N/cm2) (Pruim et al., 1980; Weijs and Hillen, 1985a). The activation ratio for medial pterygoid muscle was assumed to be identical to that of the masseter. The cross-sectional area as well as the orientation of each muscle was based on previous studies (Weijs and Hillen, 1984, 1985b; Hannam and Wood, 1989; Koolstra et al., 1990).
Calculation of Bite Forces and TMJ Loads
Four different clenching tasks with the occlusal conditions for three symmetric bilaterally shortened dental arches (M1-M1, P2-P2, and P1-P1) and a complete dental arch (M2-M2) were simulated. Corresponding muscular forces were input for each occlusal condition. Missing occlusal contacts on each tooth were modeled by not restraining the middle node of the occlusal surface of the tooth.
We compared the calculated bite forces acting on the lower teeth, as well as their distribution on the lower dental arch, with the averaged experimental values to verify the reliability of the simulation. We then recalculated TMJ loads and muscular contraction forces by multiplying the ratio of the measured to the calculated bite forces acting on the entire dental arch.
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RESULTS
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In vivo Study
There were no significant differences in bite forces and muscular activities between MVC without appliances and MVC with M2-M2 or between the two (the second and the sixth) trials of the MVCs with M2-M2. Thus, the results of the second trial were used to represent experimental values for MVC with M2-M2.
The influences of the absence of molar occlusion on muscular activities varied among muscles. The activation ratios of P1-P1, however, were significantly smaller than those of M2-M2 for all muscles (p < 0.05) (Table). Conversely, bite forces acting on all residual teeth tended to increase (Table, Fig. 2). Bite force, as well as the PBL, was always greatest in the most posterior tooth of the dental arch. The PBLs were more invariable than any other factors in this study (Fig. 3).
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Table. Influences of Experimentally Shortened Dental Arch on the Bite Forces Acting on Each Tooth and the Activation Ratios of Each Jaw Muscle
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Figure 2. The measured (in vivo) and the calculated (in vitro) bite force acting on each lower tooth in complete arches and 3 bilateral SDAs. The 4 occlusal conditions—M2-M2, M1-M1, P2-P2, and P1-P1—represent complete dentitions and bilateral SDAs omitting occlusal contacts on bilateral second molars, both bilateral molars, and bilateral second premolars plus both molars, respectively. Abbreviations: M2, the second molar; M1, the first molar; P2, the second premolar; P1, the first premolar; C, canine; and I, incisor.
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Figure 3. The sum of estimated unilateral muscular forces, the estimated unilateral TMJ load, and the periodontally borne load (PBL) on the most posterior tooth with occlusal contact (M2 for M2-M2, M1 for M1-M1, P2 for P2-P2, and P1 for P1-P1). PBL is expressed by the recorded and averaged bite force divided by the normal root surface area for each kind of tooth. For other abbreviations, see the legend to Fig. 2.
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In vitro Study
Total muscular force increased slightly when bilateral second molars were lost, then decreased significantly following the loss of bilateral first molars and even more with the loss of second premolars.
Although the calculated bite forces were smaller than the measured values, the distributions of bite forces were similar between these two results (Fig. 2). The ratios of calculated to measured total bite forces ranged from 0.53 to 0.70. After the recalculation of muscular and joint forces based on these ratios, the absolute difference of bite force acting on each tooth ranged from 3 to 62 N (average, 20 N). The percentages of these values relative to total bite forces ranged from 0 to 16%, and were less than 6% for all conditions except P1-P1.
Following the loss of both molars, the TMJ load decreased (Fig. 3) until the value of P1-P1 was 63% that of M2-M2. The ratio of TMJ load to the total muscular force increased monotonously, suggesting a loss of muscular efficiency.
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DISCUSSION
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As stated by Mah et al.(1997), a model is no better than the data and assumptions used in its development. Oversimplification and the use of inadequate assumptions should be avoided whenever possible by verifying the results of the model with comparable in vivo data. Bite force is the sole directly measurable parameter in the stomatognathic system, and is therefore an optimum way to obtain in vivo data for the verification of models. Thus, in this study, bite forces were experimentally recorded and compared with the results of the model simulation. The results showed consistent patterns of bite force distribution, improving the reliability of the model and providing plausible grounds for estimating TMJ loads.
On the other hand, there were some wide discrepancies in the magnitudes of bite forces. This may have been due to forces from muscles other than the masseter, temporalis, and medial pterygoid, or because the activation ratios of medial pterygoid muscles were hypothesized to be the same as those of the masseter. Other reasons for the discrepancies may be due to oversimplification of the muscle force estimation. Jaw muscles are heterogeneous both morphologically and functionally (Herring et al., 1979; Blanksma et al., 1992; Blanksma and van Eijden, 1995). Thus, the activation ratio derived from surface EMG may not reflect the activation level of a whole muscle. In addition, the relation between electromyographic activity and contraction force, which was assumed to be linear, may be non-linear (Hagberg et al., 1985; Haraldson et al., 1985; Mao et al., 1996).
In this study, muscular forces and TMJ loads were recalculated based on the ratios of the measured to the calculated bite forces because there were reasons to doubt the reliability of the estimated muscular forces. Also, the FE model used in this study caused the resultant forces to change linearly with the muscular input.
The TMJ loads during MVC were less in SDAs than in the complete dentitions (Fig. 3), and therefore SDA never caused overloading in the TMJ, which has been implicated in degenerative diseases of this joint (Zarb and Carlsson, 1994). Here, the increased ratio of TMJ load to muscular force was compensated for by the reduction of muscular force. Thus, our study provides no evidence to support the idea that lack of posterior occlusal support predisposes patients to overloaded TMJs. Our result is in line with previous epidemiological studies on SDA subjects which emphasize that SDA is not a risk factor for TMD (Witter et al., 1994b, 2001) and also with a clinical, tomographic, and arthroscopic study in age-matched cases which found no statistically significant difference in the TMJs between the subjects with complete dentitions and those with reduced molar occlusions (Holmlund and Axelsson, 1994).
A possible explanation for these findings, which deny a relation between TMD and SDA, is that neuromuscular regulatory mechanisms protect the joints from overloading. However, the finding that TMJ load was not the most invariable factor suggests that muscle activation patterns are designed to control some factor other than TMJ loads. Because the sensory innervation of this joint is limited mainly to the joint capsule, retrodiscal area, and the posterior band of the disk, neither the mechanoreceptors nor the nociceptors in the joint are well-suited for detecting excessive load during clenching. Our speculation is consistent with that of Throckmorton et al.(1990), who concluded that the muscle activity patterns did not maintain equal joint forces, nor did muscles respond to joint forces exceeding critical limits.
The neuromuscular regulatory system thus seems designed to control the clenching strength so as not to exceed the critical limit of the load-bearing capacity of the periodontal tissues, as shown by the fact that PBL was the most stable among all the variables in our study. The fact that the periodontal ligament and the periosteum of the alveolar bone are rich in both mechanoreceptors and nociceptors would support this idea.
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ACKNOWLEDGMENTS
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The authors express their gratitude to Prof. Alan G. Hannam at the University of British Columbia, Vancouver, BC, Canada, for proofreading our manuscript. This work was supported by Grants-in-Aid from the Ministry of Education of Japan (Nos. 11771206 and 13672020).
Received for publication August 13, 2002.
Revision received February 18, 2003.
Accepted for publication April 1, 2003.
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Journal of Dental Research, Vol. 82, No. 7,
532-536 (2003)
DOI: 10.1177/154405910308200709
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ABSTRACT
INTRODUCTION