- Email: [email protected]
The influence of vertical dimension of occlusion changes on the electroencephalograms of complete denture wearers
journal of prosthodontic research 58 (2014) 121–126
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/jpor
Original article
The influence of vertical dimension of occlusion changes on the electroencephalograms of complete denture wearers Risa Matsuda DMD*, Yoshikazu Yoneyama DMD, PhD, Masakazu Morokuma DMD, PhD, Chikahiro Ohkubo DMD, PhD Department of Removable Prosthodontics, Tsurumi University of Dental Medicine, 2-1-3 Tsurumi, Tsurumi-ku, Yokohama, Kanagawa 230-8501, Japan
article info
abstract
Article history:
Purpose: The present study was conducted to identify how changes in the vertical dimen-
Received 14 November 2013
sion of occlusion (VDO) affect the sensory perception and activity of the brain in complete
Received in revised form
denture wearers using an electroencephalogram (EEG).
21 January 2014
Methods: Subjects were 21 individuals wearing complete dentures who regularly visited the
Accepted 25 January 2014
Division of Prosthodontics at Tsurumi University Dental Hospital for checkups (12 males
Available online 16 April 2014
and 9 females, average age: 76.6). Based on their original dentures, two duplicate dentures with different VDO ( 3 mm and +5 mm) were fabricated. EEG activity and occlusal force
Keywords:
were measured before and after gum chewing with each denture in all subjects. Negative
Vertical dimension of occlusion
indicator scores for psychological conditions and stable neuronal activity (Da) were calcu-
Complete denture
lated using EEG data. Statistical analysis was performed using the Wilcoxon test to compare
Edentulous
changes in the sensory perception, activity of the brain, and occlusal force (a = 0.05).
Electroencephalogram
Results: After gum chewing with the +5-mm denture, a significant increase was observed in the negative indicator score ( p < 0.05). No significant difference was found in the Da values before and after gum chewing with any of the dentures ( p > 0.05). A significant decrease was observed in the occlusal force between the original denture and the
3-mm denture
( p < 0.05). Conclusion: Psychological condition and occlusal force were influenced by immediate changes in the VDO of the complete denture. # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
1.
Introduction
Long-term use of a complete denture can result in jaw displacement due to abrasion of the artificial teeth. This can not only lead to aesthetic impairment but can also cause
reduced masticatory performance and create abnormal stresses during chewing and biting. The vertical dimension of occlusion (VDO) affects the occlusal force, which may influence the stimulation of the central nervous system via the trigeminal nerve [1]. Also, previous studies have shown that occlusal disharmony caused by reduced masticatory
* Corresponding author. Tel.: +81 45 580 8421; fax: +81 45 573 9599. E-mail address: [email protected] (R. Matsuda). 1883-1958/$ – see front matter # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. http://dx.doi.org/10.1016/j.jpor.2014.01.003
122
journal of prosthodontic research 58 (2014) 121–126
Table 1 – Subjects’ characteristics. No.
Age
Gender
Type of denture base
Type of artificial teeth
Duration of denture use
Occlusal force (N) Original
1 2 3 4 5 6 7 8
76 80 77 85 76 84 84 83
M M F F M F M F
Resin Metal Metal Resin Resin Metal Resin Metal
9
86
F
Metal
10 11 12 13 14 15 16 17 18 19 20 21
70 76 84 67 83 69 67 76 67 58 82 66
M F F M M F M M M M M F
Resin Resin Resin Resin Resin Resin Resin Resin Resin Resin Resin Metal
Hard resin Hard resin Hard resin Hard resin Hard resin Metal Hard resin Metal (upper) Hard resin (lower) Metal (upper) Hard resin (lower) Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin
performance produces chronic stress, which, if prolonged, can cause a decrease in learning ability [2]. Animal experiments have shown that changes in the VDO or presence of occlusal interference can cause changes in the serum corticosterone levels as well as an increase in the amount of dopamine released into the brain. Indeed, previous research has shown that occlusal interference and occlusal disharmony can cause stress on higher brain functions as well as on the whole body, and that occlusal disharmony can have a serious effect on the immune system and the nervous system [3]. Rhythmic masticatory muscle activity is coordinated by voluntary as well as reflexive control exerted by the upper central nervous system, including the motor area of the cerebral cortex and the hypothalamic-amygdala pathway, as well as by the reflex arc whose central connections are located in the midbrain, the pons, the medulla oblongata, and the upper cervical segments of the spinal cord [4]. In addition to receiving input from the upper central nervous system, the masticatory function processes peripheral sensory feedback from the teeth, jaws, masticatory muscles, and temporomandibular joints, whereas the central nervous system issues motor commands to muscles. The act of mastication is known to promote and maintain certain cognitive functions such as learning and memory [5]. Previous research has demonstrated that loss of the periodontal membrane due to the loss of teeth causes decreased stimulation to the hippocampus, which increases the risk of Alzheimer’s disease [6]. According to a previous animal experiment, a partial loss of trigeminal mesencephalic neurons regulating periodontal mechanoreceptors occurs in guinea pigs that had had their teeth removed, which triggers remodeling of the central neural circuits that control masticatory muscle activity [7,8]. However, there is no existing evidence as to how changes in the VDO can affect the sensory perception and activity of
2M 6Y 6Y 9Y 1Y 4Y 2M 5Y
3 mm
+5 mm
311 224 154 119 305 368 299 174
307 230 171 102 286 225 191 161
416 273 255 427 334 249 135 216
12 Y
177
116
144
2M 1Y 9M 2M 3Y 4M 2M 7Y 16 Y 5M 7Y 6M 1Y 3M 4M
299 88 197 480 221 469 230 338 69 172 465 208
134 90 200 467 106 276 357 227 126 220 358 121
253 97 267 348 332 493 667 193 153 225 671 242
7M 5M 7M 7M 11 M
the brain in complete denture wearers. In view of this, the present study was conducted to discover how changes in the VDO affect the sensory perception and activity of the brain as measured by an electroencephalogram (EEG) in patients with complete dentures. Occlusal force was also measured because it also has the potential to affect brain activity.
2.
Materials and methods
2.1.
Subjects
Subjects were 21 individuals wearing maxillary and mandibular complete dentures who regularly visited the Division of Prosthodontics at Tsurumi University Dental Hospital for checkups (12 males and 9 females, aged 58–86 years with the average age being 76.6 7.8 years) (Table 1). There was wide variation in the duration of denture use, but all of the dentures had been maintained by prosthodontic specialists. Severe abrasion of the artificial teeth was never observed, and the VDO of all the dentures was appropriate. No subjects had histories of brain disease, such as cerebral infarction, or had been diagnosed with dementia, such as Alzheimer’s disease. All subjects were fully informed of and consented to the research methods, which had been approved by the ethics committee of Tsurumi University School of Dental Medicine (approval number: 305, August 31, 2005).
2.2.
Fabrication of duplicate dentures
To observe the brain response to the alteration of the vertical dimension of occlusion, the 2 types of duplicate dentures for
journal of prosthodontic research 58 (2014) 121–126
123
(Xylitol, Lotte, Tokyo, Japan) wearing three types of dentures. Our setting of EEG measurement is as follows: - EEG A: EEG obtained before gum chewing with the wearing the original denture - EEG B: EEG obtained after gum chewing with the wearing the original denture - EEG C: EEG obtained before gum chewing with the wearing 3-mm denture - EEG D: EEG obtained after gum chewing with the wearing 3-mm denture - EEG E: EEG obtained before gum chewing with the wearing +5-mm denture - EEG F: EEG obtained after gum chewing with the wearing +5-mm denture
subject subject subject subject subject subject
A 30-min interval (30-min break) was allowed between measurements with different dentures because a preliminary study showed the effect of stimulation fades in 30 min. Measurement of the occlusal force with each denture was performed after the EEG recording to prevent the effects of biting on the EEG readings (Fig. 2).
2.4.
Fig. 1 – EEG measurements were performed while the subjects were seated in a resting position with their eyes closed.
EEG measurements were performed inside a semi-anechoic room using ESA-pro (Brain Functions Laboratory Inc., Kanagawa, Japan), with each subject wearing a helmet with pasteless electrodes (Brain Functions Laboratory Inc.) mounted on it, in accordance with the previous study [10].
2.4.1. each subject were fabricated. To fabricate the duplicate dentures, an impression of the original denture was taken using a silicon impression material. Autopolymerized denture base resin (PalaXpress, Heraeus Kulzer, Germany) was then poured into the impression according to the manufacturer’s instructions, and was then pressurized to 2 atmospheres at 608 for 30 min. Each subject was asked to try on the duplicate dentures for adjustments. Because we had to ensure that the duplicate dentures used in the present study were identical in all aspects except for the VDO, we used a semi-adjustable articulator (Pro Arch IIIEG, SHOFU, Kyoto) with a face-bow transfer to recreate a 3-dimensional orientation of the maxillary dental arch relative to a subject’s skull. The occlusion of contact balance of the duplicate denture was adjusted according to Nakazawa’s measurements [9]. Two types of duplicate dentures were fabricated with different VDO: a denture adjusted to occlude with the incisal guide pin lowered by 3 mm (hereafter the 3-mm denture) and a denture adjusted to occlude with the incisal guide pin raised by 5 mm (hereafter the +5-mm denture).
2.3.
Measurement procedure
An EEG measured each subject at rest for 3 min so that the measurement could be performed under stable conditions (Fig. 1). With each subject, an EEG was performed for 3 min immediately before and after a minute of chewing gum
EEG recording
ESAM analysis
The Emotion Spectrum Analysis Method (ESAM) was performed to quantify changes in a subject’s psychological condition over time based on the phase positions of brain wave frequencies relative to one another. EEG measurement bands were narrowed down from 5 Hz to 20 Hz. They were divided into three frequency bands of 5–8 Hz (u wave), 8–13 Hz (a wave), and 13–20 Hz (b wave). ESAM analysis was calculated in the theta, alpha, and beta frequency bands every 5.12 s, from which the four emotional states (i.e., stress, depression, joy, and relaxation) were separated and their levels evaluated. In the present study, a negative indicator score was calculated to estimate the emotional change. The higher the negative indicator score, the greater the discomfort [11].
2.4.2.
DIMENSION analysis
The recorded EEG data was stored in a dedicated personal computer, which was then transferred to the Electroencephalogram Analysis Center of Brain Functions Laboratory Inc., where the Diagnosis Method of Neuronal Dysfunction (DIMENSION) was performed. DIMENSION analysis quantitatively estimates synaptic/neuronal dysfunction based on the smoothness of the EEG frequency distribution. In general, when neuronal activity in the cerebral cortex is stable, the scalp potentials are distributed smoothly from high to low. In DIMENSION analysis, an ideal potential distribution of an a wave, which indicates stable neuronal activity, is defined as Da = 1. The Da value decreases as cognitive function of the brain deteriorates [12].
124
journal of prosthodontic research 58 (2014) 121–126
Fig. 2 – Flowchart of measurement.
2.6.
Statistical analysis
Statistical analysis was performed using the Wilcoxon test (SPSS software version 17.0J, SPSS Inc., Tokyo, Japan) to evaluate changes in the sensory perception and activity of the brain before and after gum chewing with the original denture, the 3-mm denture, and the +5-mm denture (a = 0.05). The occlusal forces of the 3-mm denture and the +5-mm denture were compared to the original denture using the Wilcoxon test (a = 0.05).
Fig. 3 – Comparison of the negative indicator score before and after gum chewing (n = 21).
3.
Results
3.1.
ESAM analysis
Sensory evaluation revealed no significant difference in the negative indicator score before and after gum chewing with the original denture and the 3-mm denture ( p > 0.05, EEG A and EEG B: p = 0.848; EEG C and EEG D: p = 0.063). After gum chewing with the +5-mm denture (EEG E and EEG F), a significant increase was observed in the negative indicator score ( p < 0.05, p = 0.009) (Fig. 3).
3.2.
DIMENSION analysis
The Da values of 11 subjects were confirmed at the baseline to be lower than 0.952, which means the subject possibly suffered from Alzheimer’s dementia [14]. No significant difference was found in the Da values before and after gum chewing with any of the dentures (EEG A and EEG B: p = 0.835; EEG C and EEG D: p = 0.509; EEG E and EEG F: p = 0.085) (Fig. 4).
3.3.
Fig. 4 – Comparison of brain function index before and after gum chewing (n = 21).
2.5.
Occlusal force
The occlusal forces of each subject were measured using an Occluzer 709 (GC Corporation, Tokyo, Japan) and Dental Prescale (without wax) (GC Corporation, Tokyo, Japan). The Frankfurt plane of the subject’s head was first aligned parallel to the floor. The operator pulled the subject’s lips away from the teeth and had the subject bite down to check whether the subject could bite with ease. The subject was subsequently instructed to bite as hard as possible in central occlusion for 3 s [13].
Occlusal force
The average occlusal force for the original dentures was 255 120 N, that for the 3-mm denture was 213 100 N, and that for the +5-mm denture was 304 157 N. A significant difference was observed in the occlusal force between the original dentures and the 3-mm denture ( p < 0.05, p = 0.034) (Fig. 5). There was no significant difference between the original and the +5-mm dentures.
4.
Discussion
The present study used an EEG to examine how VDO can affect brain function in complete denture wearers. In edentulous individuals, periodontal mechanoreceptors are lost due to the loss of teeth. Sensory and motor information related to dental
journal of prosthodontic research 58 (2014) 121–126
Fig. 5 – Comparison of the occlusal force (n = 21).
occlusion is, therefore, believed to be transmitted to the brain via receptors in the periosteum, temporomandibular joint, masticatory muscles, and oral mucosa [15]. The problem with a complete denture is that, over time, abrasion of the artificial teeth leads to decreased VDO, reduced masticatory efficiency, alteration of the pathways that drive mastication, fatigue in the masticatory muscles, mandibular displacement, and aesthetic impairment [16,17]. The highlight of the present study is that, because subjects were wearers of complete dentures, it was possible to specify an arbitrary VDO, which allowed us to examine how changes in the VDO could influence brain function. Generally, altering a VDO by using a bite-elevating appliance, such as an occlusal splint, is known to increase occlusal force in healthy dentulous individuals. In the present study, however, although patients wearing the +5-mm denture tended to exert increased occlusal force, this difference was not statistically significant. This may be due to the fact that, even though the +5-mm denture was carefully adjusted to occlude with the VDO raised 5 mm above the proper vertical position, some patients found it much easier to bite by using their original dentures. On the other hand, a significant reduction in the occlusal force was observed with the 3-mm denture that was adjusted to occlude with the VDO lowered 3 mm below the proper vertical position. This finding is in line with the general notion that a decreased VDO leads to a reduced occlusal force [18]. Discomfort associated with the alteration of the VDO might have also influenced the occlusal force in 3-mm denture wearers. In the previous study, it was suggested that objective assessment of dental therapy is possible based on physiological indicators using EEG [10,19,20]. According to a previous study, experimental occlusal interference causes discomfort, and the resulting emotional change is reflected in an EEG, which allows us to detect the impairment of masticatory ability through EEG analysis [21]. Kikuchi demonstrated through EEG measurements that discomfort in subjects with palatal dentures increased due to a change in the oral environment [22]. Nishiyama et al. examined gum chewing with and without an occlusal interference device that
125
stimulated a negative change in the oral sensation in four subjects using EEG and ESAM [11]. Furthermore, a previous animal experiment showed that occlusal disharmony in rats and monkeys that underwent the bite-raising procedure induced a stress reaction caused by increased plasma corticosterone and urine cortisol levels [23,24]. Koshino et al. claimed that complete denture wearers’ satisfaction with the dentures and ease of mastication can influence their physiological and psychological aspects of Quality of life [25]. In the present study, sensory evaluation revealed that the usage of the +5-mm denture caused a significant increase in the negative indicator score, indicating that increasing a VDO by 5 mm can induce stress. This finding is in line with a previous study’s report that an increase in VDO often leads to psychological distress [26]. Removal of pain and a considerable improvement in masticatory capabilities, including the attainment of a normal molar occlusion, brought about by denture treatment positively affect brain function [10]. In the present study, the effects of both increased and decreased of VDO on Da values were not significant. On the other hand, this study found that there were 11 patients with low Da values (Da < 0.952). Musha et al. reported that a person whose Da was lower than 0.952 had a higher possibility of suffering from Alzheimer’s dementia [14]. Considering that we intended to include subjects with no history of brain disease confirmed by clinical interview and examination, this finding might suggests heterogeneity of edentulous patient in terms of brain function as measured by Da value. This study experimentally changed the vertical dimension of the complete dentures in 21 edentulous patients and measured the EEGs of these patients. To the best of our knowledge, this is the first study that evaluated the brain function in such a large number of edentulous patients under conditions where 3 different occlusal dimensions were precisely simulated. However, one of the limitations of this study was that we only tested the immediate effects of the change in the vertical dimension, while a long-term effect is more clinically relevant. Another limitation was that, although the brain function may potentially be affected by many factors and highly variable and therefore the effects of potential confounding factors and the reproducibility of the measurements should be taken into considerations, we conducted only a single measurement of a limited number of variables. Actually, while a significant change in the negative indicator scores before and after gum chewing with the increased VDO was found, no significant difference was found for scores with and without the VDO change after gum chewing, which suggests that the study result is not conclusive and further studies are warranted. We selected this study design because of its technical complexity and mainly ethical considerations; our study should clearly be considered an exploratory investigation into whether change in the vertical dimension is related to brain function. Confirmatory studies that investigate carefully selected variables for a longer term should be done only after several studies like ours find and suggest a significant relationship. Given this general cautionary note and based on the data collected, we suggest that psychological conditions and occlusal force might be influenced by changes in the VDO of complete dentures.
126
5.
journal of prosthodontic research 58 (2014) 121–126
Conclusion
This study changed the vertical dimension of the experimental complete dentures in 21 edentulous patients and measured the occlusal forces and the EEGs of these patients. Within the limitations of this study, the following conclusions were obtained: (1) The occlusal force was significantly decreased by the denture with a lower vertical dimension; (2) ESAM analysis revealed a significant increase in psychological distress after gum chewing with the denture with a higher vertical dimension; and (3) DIMENSION analysis found no consistent effect of the vertical dimension, and half of the subjects indicated low Da values.
Acknowledgments The authors thank Drs. Haruyama Matsuzaki, Yohei Kobayashi, and Toshimitsu Musha (Brain Functions Lab., Inc., Kanagawa, Japan) who provided support and advice in this study.
references
[1] Williams WN, Levin AC, LaPointe LL, Comell CE. Bite force discrimination by individuals with complete dentures. J Prosthet Dent 1985;54:146–50. [2] Kim JJ, Diamond DM. The stressed hippocampus, synaptic plasticity and lost memories. Nat Rev Neurosci 2002;3: 453–62. [3] Kubo KY, Yamada Y, Iinuma M, Iwaku F, Tamura Y, Watanabe K, et al. Occlusal disharmony induces spatial memory impairment and hippocampal neuron degeneration via stress in SAMP8 mice. Neurosci Lett 2007;414:188–91. [4] Westberg KG, Kolta A. The trigeminal circuits responsible for chewing. Int Rev Neurobiol 2011;97:77–98. [5] Ono Y, Yamamoto T, Kubo K, Onozuka M. Occlusion and brain function: mastication as a prevention of cognitive dysfunction. J Oral Rehabil 2010;37:624–40. [6] Okamoto N, Morikawa M, Okamoto K, Habu N, Hazaki K, Harano A, et al. Tooth loss is associated with mild memory impairment in the elderly: the Fujiwara-kyo study. Brain Res 2010;1349:68–75. [7] Kimoto A, Enomoto S, Kobayashi S, Ohashi I, Nakamura Y. Responses of trigeminal mesencephalic neurons to teeth extraction in the guinea pig. In: Morimoto T, Matsuya T, Takada K, editors. Brain and oral functions – oral motor function and dysfunction. Amsterdam: Elsevier; 1995. p. 49– 58. [8] Hu JM, Dostrovsky JO, Lenz GJ, Sessle BJ. Tooth pulp deafferentation is associated with functional alterations in the properties of neurons in the trigeminal spinal tract nucleus. J Neurophysiol 1986;56:1650–68.
[9] Nakazawa I. Zenbusyougishigaku. 8th ed. Tokyo: Nagasuesyoten; 1979: 137–8 [article in Japanese]. [10] Morokuma M. Influence of the functional improvement of complete dentures on brain activity. J Jpn Prosthodont Soc 2008;52:194–9. [11] Nishiyama Y, Ohnuki M, Kikuchi S, Suzuki K, Ohkubo C. Evaluation of psychological effect of prosthetic treatment using Emotion Spectrum Analysis Method (ESAM). J Prosthodont Res 2011;55:82–8. [12] Hara J, Shankle WR, Musha T. Cortical atrophy in Alzheimer’s disease unmasks electrically silent sulci and lowers EEG dipolarity. IEEE Trans Biomed Eng 1999; 46:905–10. [13] Suzuki T, Kumagai H, Watanabe T, Uchida T, Nagao M. Clinical evaluation of measuring system of occlusal force. Kokubyo Gakkai Zasshi 1994;61:437–45 [article in Japanese]. [14] Musha T, Asada T, Yamashita F, Kinoshita T, Chen Z, Matsuda H, et al. A new EEG method for estimating cortical neuronal impairment that is sensitive to early stage Alzheimer’s disease. Clin Neurophysiol 2002;113:1052–8. [15] George AZ. Biomechanics of the edentulous state. In: George AZ, Charles LB, editors. Prosthodontic treatment for edentulous patients: complete dentures and implantsupported prostheses. 12th ed. Philadelphia: Elsevier; 2004. p. 6–23. [16] Hirano S, May KB, Wagner WC, Hacker CH. In vitro wear of resin denture teeth. J Prosthet Dent 1998;79:152–5. [17] Ekfeldt A, Karlsson S. Changes of masticatory movement characteristics after prosthodontics rehabilitation of individuals with extensive tooth wear. Int J Prosthodont 1996;9:539–46. [18] Prombonas A, Vlissidis D, Molyvdas P. The effect of altering the vertical dimension of occlusion on biting force. J Prosthet Dent 1994;71:139–43. [19] Okamoto N. Effect of occlusal support by implant prostheses on brain function. J Prothodont Res 2011; 55:206–13. [20] Shibuya N. Influence of dentures in partial denture wearers on brain function. Ann Jpn Prothodont Soc 2009;1:148–56 [article in Japanese]. [21] Ichikawa D. The effect of experimental occlusal interference on electroencephalograms by comparing psychophysiological response and masticatory muscle activity. J Jpn Prosthodont Soc 2001;45:305–14 [article in Japanese]. [22] Kikuchi S. Evaluation of the psychological effect of changes in the oral environment. Prosthodont Res Pract 2005; 4:84–93. [23] Yoshihara T, Taneichi R, Yawaka Y. Occlusal disharmony increases stress response in rats. Neurosci Lett 2009;452:181–4. [24] Budtz-Jørgensen E. Occlusal dysfunction and stress. An experimental study in macaque monkeys. J Oral Rehabil 1981;8:1–9. [25] Koshino H, Hirai T, Ishijima T, Tsukagoshi H, Ishigami T, Tanaka Y. Quality of life and masticatory function in denture wearers. Oral Rehabil 2006;33:323–9. [26] Otsuka T, Watanabe K, Hirano Y, Kubo K, Miyake S, Sato S, et al. Effects of mandibular deviation on brain activation during clenching: an fMRI preliminary study. Cranio 2009;27:88–93.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/jpor
Original article
The influence of vertical dimension of occlusion changes on the electroencephalograms of complete denture wearers Risa Matsuda DMD*, Yoshikazu Yoneyama DMD, PhD, Masakazu Morokuma DMD, PhD, Chikahiro Ohkubo DMD, PhD Department of Removable Prosthodontics, Tsurumi University of Dental Medicine, 2-1-3 Tsurumi, Tsurumi-ku, Yokohama, Kanagawa 230-8501, Japan
article info
abstract
Article history:
Purpose: The present study was conducted to identify how changes in the vertical dimen-
Received 14 November 2013
sion of occlusion (VDO) affect the sensory perception and activity of the brain in complete
Received in revised form
denture wearers using an electroencephalogram (EEG).
21 January 2014
Methods: Subjects were 21 individuals wearing complete dentures who regularly visited the
Accepted 25 January 2014
Division of Prosthodontics at Tsurumi University Dental Hospital for checkups (12 males
Available online 16 April 2014
and 9 females, average age: 76.6). Based on their original dentures, two duplicate dentures with different VDO ( 3 mm and +5 mm) were fabricated. EEG activity and occlusal force
Keywords:
were measured before and after gum chewing with each denture in all subjects. Negative
Vertical dimension of occlusion
indicator scores for psychological conditions and stable neuronal activity (Da) were calcu-
Complete denture
lated using EEG data. Statistical analysis was performed using the Wilcoxon test to compare
Edentulous
changes in the sensory perception, activity of the brain, and occlusal force (a = 0.05).
Electroencephalogram
Results: After gum chewing with the +5-mm denture, a significant increase was observed in the negative indicator score ( p < 0.05). No significant difference was found in the Da values before and after gum chewing with any of the dentures ( p > 0.05). A significant decrease was observed in the occlusal force between the original denture and the
3-mm denture
( p < 0.05). Conclusion: Psychological condition and occlusal force were influenced by immediate changes in the VDO of the complete denture. # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
1.
Introduction
Long-term use of a complete denture can result in jaw displacement due to abrasion of the artificial teeth. This can not only lead to aesthetic impairment but can also cause
reduced masticatory performance and create abnormal stresses during chewing and biting. The vertical dimension of occlusion (VDO) affects the occlusal force, which may influence the stimulation of the central nervous system via the trigeminal nerve [1]. Also, previous studies have shown that occlusal disharmony caused by reduced masticatory
* Corresponding author. Tel.: +81 45 580 8421; fax: +81 45 573 9599. E-mail address: [email protected] (R. Matsuda). 1883-1958/$ – see front matter # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. http://dx.doi.org/10.1016/j.jpor.2014.01.003
122
journal of prosthodontic research 58 (2014) 121–126
Table 1 – Subjects’ characteristics. No.
Age
Gender
Type of denture base
Type of artificial teeth
Duration of denture use
Occlusal force (N) Original
1 2 3 4 5 6 7 8
76 80 77 85 76 84 84 83
M M F F M F M F
Resin Metal Metal Resin Resin Metal Resin Metal
9
86
F
Metal
10 11 12 13 14 15 16 17 18 19 20 21
70 76 84 67 83 69 67 76 67 58 82 66
M F F M M F M M M M M F
Resin Resin Resin Resin Resin Resin Resin Resin Resin Resin Resin Metal
Hard resin Hard resin Hard resin Hard resin Hard resin Metal Hard resin Metal (upper) Hard resin (lower) Metal (upper) Hard resin (lower) Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin Hard resin
performance produces chronic stress, which, if prolonged, can cause a decrease in learning ability [2]. Animal experiments have shown that changes in the VDO or presence of occlusal interference can cause changes in the serum corticosterone levels as well as an increase in the amount of dopamine released into the brain. Indeed, previous research has shown that occlusal interference and occlusal disharmony can cause stress on higher brain functions as well as on the whole body, and that occlusal disharmony can have a serious effect on the immune system and the nervous system [3]. Rhythmic masticatory muscle activity is coordinated by voluntary as well as reflexive control exerted by the upper central nervous system, including the motor area of the cerebral cortex and the hypothalamic-amygdala pathway, as well as by the reflex arc whose central connections are located in the midbrain, the pons, the medulla oblongata, and the upper cervical segments of the spinal cord [4]. In addition to receiving input from the upper central nervous system, the masticatory function processes peripheral sensory feedback from the teeth, jaws, masticatory muscles, and temporomandibular joints, whereas the central nervous system issues motor commands to muscles. The act of mastication is known to promote and maintain certain cognitive functions such as learning and memory [5]. Previous research has demonstrated that loss of the periodontal membrane due to the loss of teeth causes decreased stimulation to the hippocampus, which increases the risk of Alzheimer’s disease [6]. According to a previous animal experiment, a partial loss of trigeminal mesencephalic neurons regulating periodontal mechanoreceptors occurs in guinea pigs that had had their teeth removed, which triggers remodeling of the central neural circuits that control masticatory muscle activity [7,8]. However, there is no existing evidence as to how changes in the VDO can affect the sensory perception and activity of
2M 6Y 6Y 9Y 1Y 4Y 2M 5Y
3 mm
+5 mm
311 224 154 119 305 368 299 174
307 230 171 102 286 225 191 161
416 273 255 427 334 249 135 216
12 Y
177
116
144
2M 1Y 9M 2M 3Y 4M 2M 7Y 16 Y 5M 7Y 6M 1Y 3M 4M
299 88 197 480 221 469 230 338 69 172 465 208
134 90 200 467 106 276 357 227 126 220 358 121
253 97 267 348 332 493 667 193 153 225 671 242
7M 5M 7M 7M 11 M
the brain in complete denture wearers. In view of this, the present study was conducted to discover how changes in the VDO affect the sensory perception and activity of the brain as measured by an electroencephalogram (EEG) in patients with complete dentures. Occlusal force was also measured because it also has the potential to affect brain activity.
2.
Materials and methods
2.1.
Subjects
Subjects were 21 individuals wearing maxillary and mandibular complete dentures who regularly visited the Division of Prosthodontics at Tsurumi University Dental Hospital for checkups (12 males and 9 females, aged 58–86 years with the average age being 76.6 7.8 years) (Table 1). There was wide variation in the duration of denture use, but all of the dentures had been maintained by prosthodontic specialists. Severe abrasion of the artificial teeth was never observed, and the VDO of all the dentures was appropriate. No subjects had histories of brain disease, such as cerebral infarction, or had been diagnosed with dementia, such as Alzheimer’s disease. All subjects were fully informed of and consented to the research methods, which had been approved by the ethics committee of Tsurumi University School of Dental Medicine (approval number: 305, August 31, 2005).
2.2.
Fabrication of duplicate dentures
To observe the brain response to the alteration of the vertical dimension of occlusion, the 2 types of duplicate dentures for
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(Xylitol, Lotte, Tokyo, Japan) wearing three types of dentures. Our setting of EEG measurement is as follows: - EEG A: EEG obtained before gum chewing with the wearing the original denture - EEG B: EEG obtained after gum chewing with the wearing the original denture - EEG C: EEG obtained before gum chewing with the wearing 3-mm denture - EEG D: EEG obtained after gum chewing with the wearing 3-mm denture - EEG E: EEG obtained before gum chewing with the wearing +5-mm denture - EEG F: EEG obtained after gum chewing with the wearing +5-mm denture
subject subject subject subject subject subject
A 30-min interval (30-min break) was allowed between measurements with different dentures because a preliminary study showed the effect of stimulation fades in 30 min. Measurement of the occlusal force with each denture was performed after the EEG recording to prevent the effects of biting on the EEG readings (Fig. 2).
2.4.
Fig. 1 – EEG measurements were performed while the subjects were seated in a resting position with their eyes closed.
EEG measurements were performed inside a semi-anechoic room using ESA-pro (Brain Functions Laboratory Inc., Kanagawa, Japan), with each subject wearing a helmet with pasteless electrodes (Brain Functions Laboratory Inc.) mounted on it, in accordance with the previous study [10].
2.4.1. each subject were fabricated. To fabricate the duplicate dentures, an impression of the original denture was taken using a silicon impression material. Autopolymerized denture base resin (PalaXpress, Heraeus Kulzer, Germany) was then poured into the impression according to the manufacturer’s instructions, and was then pressurized to 2 atmospheres at 608 for 30 min. Each subject was asked to try on the duplicate dentures for adjustments. Because we had to ensure that the duplicate dentures used in the present study were identical in all aspects except for the VDO, we used a semi-adjustable articulator (Pro Arch IIIEG, SHOFU, Kyoto) with a face-bow transfer to recreate a 3-dimensional orientation of the maxillary dental arch relative to a subject’s skull. The occlusion of contact balance of the duplicate denture was adjusted according to Nakazawa’s measurements [9]. Two types of duplicate dentures were fabricated with different VDO: a denture adjusted to occlude with the incisal guide pin lowered by 3 mm (hereafter the 3-mm denture) and a denture adjusted to occlude with the incisal guide pin raised by 5 mm (hereafter the +5-mm denture).
2.3.
Measurement procedure
An EEG measured each subject at rest for 3 min so that the measurement could be performed under stable conditions (Fig. 1). With each subject, an EEG was performed for 3 min immediately before and after a minute of chewing gum
EEG recording
ESAM analysis
The Emotion Spectrum Analysis Method (ESAM) was performed to quantify changes in a subject’s psychological condition over time based on the phase positions of brain wave frequencies relative to one another. EEG measurement bands were narrowed down from 5 Hz to 20 Hz. They were divided into three frequency bands of 5–8 Hz (u wave), 8–13 Hz (a wave), and 13–20 Hz (b wave). ESAM analysis was calculated in the theta, alpha, and beta frequency bands every 5.12 s, from which the four emotional states (i.e., stress, depression, joy, and relaxation) were separated and their levels evaluated. In the present study, a negative indicator score was calculated to estimate the emotional change. The higher the negative indicator score, the greater the discomfort [11].
2.4.2.
DIMENSION analysis
The recorded EEG data was stored in a dedicated personal computer, which was then transferred to the Electroencephalogram Analysis Center of Brain Functions Laboratory Inc., where the Diagnosis Method of Neuronal Dysfunction (DIMENSION) was performed. DIMENSION analysis quantitatively estimates synaptic/neuronal dysfunction based on the smoothness of the EEG frequency distribution. In general, when neuronal activity in the cerebral cortex is stable, the scalp potentials are distributed smoothly from high to low. In DIMENSION analysis, an ideal potential distribution of an a wave, which indicates stable neuronal activity, is defined as Da = 1. The Da value decreases as cognitive function of the brain deteriorates [12].
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Fig. 2 – Flowchart of measurement.
2.6.
Statistical analysis
Statistical analysis was performed using the Wilcoxon test (SPSS software version 17.0J, SPSS Inc., Tokyo, Japan) to evaluate changes in the sensory perception and activity of the brain before and after gum chewing with the original denture, the 3-mm denture, and the +5-mm denture (a = 0.05). The occlusal forces of the 3-mm denture and the +5-mm denture were compared to the original denture using the Wilcoxon test (a = 0.05).
Fig. 3 – Comparison of the negative indicator score before and after gum chewing (n = 21).
3.
Results
3.1.
ESAM analysis
Sensory evaluation revealed no significant difference in the negative indicator score before and after gum chewing with the original denture and the 3-mm denture ( p > 0.05, EEG A and EEG B: p = 0.848; EEG C and EEG D: p = 0.063). After gum chewing with the +5-mm denture (EEG E and EEG F), a significant increase was observed in the negative indicator score ( p < 0.05, p = 0.009) (Fig. 3).
3.2.
DIMENSION analysis
The Da values of 11 subjects were confirmed at the baseline to be lower than 0.952, which means the subject possibly suffered from Alzheimer’s dementia [14]. No significant difference was found in the Da values before and after gum chewing with any of the dentures (EEG A and EEG B: p = 0.835; EEG C and EEG D: p = 0.509; EEG E and EEG F: p = 0.085) (Fig. 4).
3.3.
Fig. 4 – Comparison of brain function index before and after gum chewing (n = 21).
2.5.
Occlusal force
The occlusal forces of each subject were measured using an Occluzer 709 (GC Corporation, Tokyo, Japan) and Dental Prescale (without wax) (GC Corporation, Tokyo, Japan). The Frankfurt plane of the subject’s head was first aligned parallel to the floor. The operator pulled the subject’s lips away from the teeth and had the subject bite down to check whether the subject could bite with ease. The subject was subsequently instructed to bite as hard as possible in central occlusion for 3 s [13].
Occlusal force
The average occlusal force for the original dentures was 255 120 N, that for the 3-mm denture was 213 100 N, and that for the +5-mm denture was 304 157 N. A significant difference was observed in the occlusal force between the original dentures and the 3-mm denture ( p < 0.05, p = 0.034) (Fig. 5). There was no significant difference between the original and the +5-mm dentures.
4.
Discussion
The present study used an EEG to examine how VDO can affect brain function in complete denture wearers. In edentulous individuals, periodontal mechanoreceptors are lost due to the loss of teeth. Sensory and motor information related to dental
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Fig. 5 – Comparison of the occlusal force (n = 21).
occlusion is, therefore, believed to be transmitted to the brain via receptors in the periosteum, temporomandibular joint, masticatory muscles, and oral mucosa [15]. The problem with a complete denture is that, over time, abrasion of the artificial teeth leads to decreased VDO, reduced masticatory efficiency, alteration of the pathways that drive mastication, fatigue in the masticatory muscles, mandibular displacement, and aesthetic impairment [16,17]. The highlight of the present study is that, because subjects were wearers of complete dentures, it was possible to specify an arbitrary VDO, which allowed us to examine how changes in the VDO could influence brain function. Generally, altering a VDO by using a bite-elevating appliance, such as an occlusal splint, is known to increase occlusal force in healthy dentulous individuals. In the present study, however, although patients wearing the +5-mm denture tended to exert increased occlusal force, this difference was not statistically significant. This may be due to the fact that, even though the +5-mm denture was carefully adjusted to occlude with the VDO raised 5 mm above the proper vertical position, some patients found it much easier to bite by using their original dentures. On the other hand, a significant reduction in the occlusal force was observed with the 3-mm denture that was adjusted to occlude with the VDO lowered 3 mm below the proper vertical position. This finding is in line with the general notion that a decreased VDO leads to a reduced occlusal force [18]. Discomfort associated with the alteration of the VDO might have also influenced the occlusal force in 3-mm denture wearers. In the previous study, it was suggested that objective assessment of dental therapy is possible based on physiological indicators using EEG [10,19,20]. According to a previous study, experimental occlusal interference causes discomfort, and the resulting emotional change is reflected in an EEG, which allows us to detect the impairment of masticatory ability through EEG analysis [21]. Kikuchi demonstrated through EEG measurements that discomfort in subjects with palatal dentures increased due to a change in the oral environment [22]. Nishiyama et al. examined gum chewing with and without an occlusal interference device that
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stimulated a negative change in the oral sensation in four subjects using EEG and ESAM [11]. Furthermore, a previous animal experiment showed that occlusal disharmony in rats and monkeys that underwent the bite-raising procedure induced a stress reaction caused by increased plasma corticosterone and urine cortisol levels [23,24]. Koshino et al. claimed that complete denture wearers’ satisfaction with the dentures and ease of mastication can influence their physiological and psychological aspects of Quality of life [25]. In the present study, sensory evaluation revealed that the usage of the +5-mm denture caused a significant increase in the negative indicator score, indicating that increasing a VDO by 5 mm can induce stress. This finding is in line with a previous study’s report that an increase in VDO often leads to psychological distress [26]. Removal of pain and a considerable improvement in masticatory capabilities, including the attainment of a normal molar occlusion, brought about by denture treatment positively affect brain function [10]. In the present study, the effects of both increased and decreased of VDO on Da values were not significant. On the other hand, this study found that there were 11 patients with low Da values (Da < 0.952). Musha et al. reported that a person whose Da was lower than 0.952 had a higher possibility of suffering from Alzheimer’s dementia [14]. Considering that we intended to include subjects with no history of brain disease confirmed by clinical interview and examination, this finding might suggests heterogeneity of edentulous patient in terms of brain function as measured by Da value. This study experimentally changed the vertical dimension of the complete dentures in 21 edentulous patients and measured the EEGs of these patients. To the best of our knowledge, this is the first study that evaluated the brain function in such a large number of edentulous patients under conditions where 3 different occlusal dimensions were precisely simulated. However, one of the limitations of this study was that we only tested the immediate effects of the change in the vertical dimension, while a long-term effect is more clinically relevant. Another limitation was that, although the brain function may potentially be affected by many factors and highly variable and therefore the effects of potential confounding factors and the reproducibility of the measurements should be taken into considerations, we conducted only a single measurement of a limited number of variables. Actually, while a significant change in the negative indicator scores before and after gum chewing with the increased VDO was found, no significant difference was found for scores with and without the VDO change after gum chewing, which suggests that the study result is not conclusive and further studies are warranted. We selected this study design because of its technical complexity and mainly ethical considerations; our study should clearly be considered an exploratory investigation into whether change in the vertical dimension is related to brain function. Confirmatory studies that investigate carefully selected variables for a longer term should be done only after several studies like ours find and suggest a significant relationship. Given this general cautionary note and based on the data collected, we suggest that psychological conditions and occlusal force might be influenced by changes in the VDO of complete dentures.
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5.
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Conclusion
This study changed the vertical dimension of the experimental complete dentures in 21 edentulous patients and measured the occlusal forces and the EEGs of these patients. Within the limitations of this study, the following conclusions were obtained: (1) The occlusal force was significantly decreased by the denture with a lower vertical dimension; (2) ESAM analysis revealed a significant increase in psychological distress after gum chewing with the denture with a higher vertical dimension; and (3) DIMENSION analysis found no consistent effect of the vertical dimension, and half of the subjects indicated low Da values.
Acknowledgments The authors thank Drs. Haruyama Matsuzaki, Yohei Kobayashi, and Toshimitsu Musha (Brain Functions Lab., Inc., Kanagawa, Japan) who provided support and advice in this study.
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