Effect of increasing vertical dimension on the masticatory system in subjects with natural teeth

Effect of increasing vertical dimension on the masticatory system in subjects with natural teeth Gunnar E. Carlsson, L.D.S., Odont.Dr.,* Giilumser Kocak, D.M.D.** * Faculty

of Odontology,

University

of Giiteborg-,

Bengt Ingervall,

M

Presented at the 10th Store Kro Conference, Spiez, Switzerland. *Professor and Chairman, Department of Stomatognathic Physi~%Yy. **Associate Professor, Department of Orthodontics, Director of the EMG laboratory. ***Guest researcher, Department of Stomatognathic Physiology; Associate Professor. Department of Prosthodontics, University of Istanbul, Turkey.

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any studies show that the postural (rest) position of the mandible changes subsequent to dental attrition, loss of teeth, and prosthetic treatment.’ ’ Immediate changes of the mandibular posture may also be achieved in many ways, e.g., by changing the occlusal vertical dimension both in subjects with natural teeth and in those with complete dentures, by placing or removing dentures in edentulous subjects, by muscular fatigue,“-” or simply by changing the position of the head. In spite of these findings, which demonstrate an adaptability of the mandibular posture to changes in occlusal vertical dimension, the concept of constancy of the rest position has not been abandoned by all authors. Increasing the vertical dimension of occlusion is therefore often held to be a hazardous procedure in prosthetic treatment.“-” Results of experiments that increased vertical dimension both in subjects with natural teeth and with complete dentures have been interpreted as deranging the function of muscles and joints of the masticatory system.’ Clinical experience, however, supports the opinion that the masticatory system adapts well to moderate changes in the occlusal vertical dimension, “’ The present study was designed to test the effect of increasing the vertical dimension of occlusion by a modification of the method used by Christensen.’ Electromyography was added to the methods of

284

L.D.S., Odont.Dr.,*

investigation studied.

so that

muscular

MATERIALS

AND METHODS

reactions

could

be

The subjects were six healthy persons, two women and four men, with a mean age of 35 years (age range 23 to 46 years). The subjects had a complete or almost complete set of natural teeth and reported no subjective symptoms of pain or dysfunction of the masticatory system. Assessment of postural position. The postural position of the mandible was used in the radiographic and electromyographic examinations. The subjects were told to relax, to look straight forward, and to not close their teeth to assume the postural position.

Procedure for increasing the vertical dimension. Casts of the maxillary and mandibular arches were made from alginate (irreversible hydrocolloid) impressions and mounted on a Dentatus articulator* using a wax record made in the terminal hinge axis with an increase of the occlusal vertical dimension of about 4 mm in the incisor region. Clear acrylic resin splints were constructed to cover the mandibular canines: premolars, and molars (Fig. 1). After occlusal adjustment for balanced occlusion (stability in the retruded contact position and in habitual closure slightly anterior of that position plus smooth lateral and protrusive excursions) the acrylic resin splints were cemented with a temporary cement and were used for 7 days. In two subjects recementation with phosphate cement was necessary during the course of the investigation. The experimental design is shown in Fig. 2. Clinical examination. A functional examination *Dentatus Ltd.. IGigersten, Sweden

0022-3913/79/wl“84

+ 06$00.60:0~E 19i9 The c. V. Mushy

co.

INCREASING

VERTICAL

DIMENSION

Fig. 2. Experimental MM 6t

Dil 0-O

design

MORPOLOGlC REST t INSERTION + REMOVAL

FACE WElGHT

OF BITE RAISING 1~ ‘.

SPLlNTS ”

Fig. 3. Mean changes in morphologic and resting face height in six subjects while increasing the vertical dimension of occlusion.

Fig. 1. A, Clear acrylic resin splints for increasing vertical dimension of occlusion. B, The splints c,emented in the lower jaw of one of the subjects.

the are

of the masticatory system was made before and at the end of the experimental period. Routine methods including palpation of muscles and joints and assessment of mandibular mobility were used.” The subjects were asked to record their subjective symptoms in a diary. Radiographic cephalometry. Eight radiographs were made of each subject using a cephalostat. Radiographs were made with the teeth in occlusion in the postural position before and immediately after insertion of the splints and at the end of the experimental period before and immediately after removal of the splints. The distances between nasion and gnation (n-gn) and articulare and pogonion (ar-pgn) were measured directly on the films using a ruler graduated in 0.5 mm and read by estimation to 0.1 mm. All measurements were made twice, and the average value was used for the statistical calculations. No correction was made for the enlargement, i.e., 6% in the sagittal plane.

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Electromyographic (EMG) recording. The muscle activity was recorded with a DISR eiectromyograph (DISA 14 A 30)* with four direct channels and two double mean voltage units (DISA 14 C 20) connected to a six-channel ink-.jetwriter (Mingograph 800).1 Th e writer (freqency (! IO 1200 Hz) was run with a paper speed of 5 cm.;sec. Activity below 20 Hz was filtered off. The EMG activity was recorded bilaterally from the anterior and posterior parts of the temporal muscle and from the masseter muscle. Bipolar hookelectrodes (platinum wire 0.2 mm in diameter) were used.‘” The placement of the electrodes, fastened intracutaneously, has been described earlier. *’ In this investigation the placement of the masseter electrodes was, however, modified to a 5 mm more superior position. During the recordings the subject was sitting upright in a dental chatr with neck support. The following recordings were made: 1. Muscle activity in postural position of mandible, calibration 50 pV/cm. 2. Maximal tooth contact in intercuspal position, calibration 200 pVlcm. 3. Swallowing of 5 ml of water, repeated four times, calibration 100 pV/cm. *DIM Electronik Ltd.. Herler, Denmark tSiemens-Elema, Solna. Sweden.

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CARLSSON,

Table I. Symptoms

Subject A B C D E F

reported

Headache

by subjects

Tiredness in muscle joints

during

Tenderness in muscles

opening

Tenderness in teeth

Clenching of teeth

++ +

+

++

AND

KOCAK

of occlusion

Cheek biting

Problems with chewing

++ ++

+ ++ ++ +

Problems with phonetics

+ 4-

+++ +

+

+ + + = Moderate

reaction,

+ + + = Severe reaction.

4. Muscle activity in a mandibular position where the mandible had been lowered at a 7 mm distance between the upper and lower incisal edges) calibration 50 pV/cm. 5. New recording of muscle activity in the postural position. Before insertion of the splints to increase the vertical dimension of occlusion, the EMG recordings were made on two occasions with a Z-day interval. One EMG recording was made immediately upon insertion of the splints, and a final recording was made with the splints after 7 days use. EMG recording No. 4 was not made on the two occasions when the subject used the splints. The second recording without splints and the EMG recording immediately after placement of the splints were made in one sitting with unchanged electrodes. Analysis of the electromyograms consisted of measurement, to the closest 0.5 mm, of the characteristic mean voItage amplitude in postural position, maximal occlusion (centric occlusion), and lowered mandible position (recording No. 4). The muscle activity during swallowing was evaluated by measuring the maximal mean voltage amplitude. Statistical methods. Differences between means of different observations were examined with the t-test and the Mann-Whitney U-test.

cephalometry

Vertical dimension of occlusion. The freeway space (interocclusal distance at the postural position) before the start of the experiment was on the average 2.2 mm (range, 0.3 to 3.8 mm). The placement of the occlusal splints led to a mean increase of the occlusal vertical dimension by 3.9 mm (range, 3.0 to 4.2 mm). Immediately after the placement of the splints, which for all subjects meant a vertical opening exceeding the original value of the freeway space, a new interocclusal distance w-as established. During

286

dimension

+ + ++ +

Note. + = Slight reaction;

RESULTS Radiographic

of the vertical

INGERVALL,

the experimental period the interocclusal distance increased an average 0.5 mm (nonsignificant difference). After the removal of the splints the resting face height diminished, but the interocclusal distance remained greater than before the start of the experiment (Fig. 3). Horizontal dimension. The movements of the mandible in an anteroposterior direction were evaluated by measurements of the distance ar-pgn. Ko statistically significant changes of the ar-pgn distance were found, following either the placement or the removal of the splints.

Subjective symptoms A great variety of symptoms were reported by the subjects who initially in general found the splints uncomfortable (Table I). Most of the symptoms diminished in severity after the first I or 2 days. One subject (Ej, however, could not adapt to the splints and felt a severe discomfort and psychic stress during the whole experimental period, although the only symptom he reported was clenching of the teeth. ‘4nother subject (B) reported phonetic difficulties during the entire experimental period. In contrast to these negative symptoms, subject C reported that after some irritation during the first hours he found wearing the splint to be an agreeable experience and he asked for a permanent restoration to increase the vertical dimension of occlusion. The clinical examination at the end of the experiment did not reveal an increase of tenderness to palpation of the masticatory muscles and ‘I’MJ or changes in mandibular movement capacity as compared with the initial data. No remaining symptoms were reported after the end of the experi ment.

Electromyographic

observations

For comparison of the muscle activity with and without splints, the recordings from the muscles on

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Table II. Mean and median voltage amplitude (pV) in postural position, lowered mandible por;ition recording (No. 41, and maximal closure, and maximal mean and median voltage amplitude swallowing Anterior Activity,

Posterior temporal muscle

A*

Bt

CS

A

1.5 1.5

0.6 0.6

0.8 0.6

7.8 7.6

Postural Mean Medidn Lowered mandible

0.9

.h'kdn

Median Maximal btte Mean Median Swallowing

temporal muscle

B

12.6 12.4

0.6 183.0 176.0

171.0

202.0

134.0

172.0

207.0

119.0

Medn

18.9

12.3

10.1

7.8

Median

11.4

7.3

4.1

2.5

c

_II-.-. A

_

0.2

duri:lg

Masseter muscle .- ___. -._.__II_ B c

0.i

125.0 704.0

150.0 140.0

137.0 130.0

130 c! I3P 17

14q,o In7 0

*Without splints. tlmmediately after placement of splints. $.4ftrr i days’ use of splints

the right and left sides were pooled. The EMG recordings from the two occasions without splints as well as the repeated recordings of the postural and swallowing activity were also pooled. The postuml activity in the anterior part of the temporal muscle decreased when the splints were placed and was, after 7 days use, lower than before the insertion (Table II). A significant difference (.Ol

The postural activity in the posterior part of the temporal muscle did not change significantly upon placement of the splints, but after 7 days of use the activity was lower than before placement (difference between A and C, .Ol

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lowered position was highest in the posterior part of the temporal muscle, followed by that in the anterior part, and was lowest in the masseter muscle. The amplitude at maximal closure in all three muscles examined was numerically higher after 7 days of wearing of the splints compared with the activity before and immediately after placement of the splints (Table II). Statistically significant differences were, however, not found for any of the muscles. The amplitude during maximal closure was always highest in the anterior part of 1he temporal muscle and lowest in the posterior part. with the activity in the masseter muscle in between. The maximal mean voltage amplitude during swallowing did not differ significantly with or without splints in the mouth for any of the muscles. In the anterior part of the temporal musck the median value decreased numerically during the study. while it was unchanged in the posterior part. In the masseter muscle the median value was the same at the end as at the start of the investigation, while it was higher immediately upon placement of the splints.

DISCUSSION These experiments showed that the procedure for increasing the vertical dimension of occlusion for all participants exceeded the previous interocclusal distance and created a new postural position of the mandible. This adaptability of the masticatory system to vertical changes of occlusicjn has been

287

CARLSSON,

shown in previous studies, but such changes have been thought to be hazardous for the health of muscles and joints. Some support for such an assumption was given by the symptoms reported during the experimental period. The clinical examination of muscles and joints, however, did not expose any increased tenderness to palpation, and the electromyographic recording showed a decreased postural muscle activity with the splints inserted. Most of the reported symptoms could be related to discomfort with the temporary restorations that were occlusal splints rather than fixed partial dentures. The material, shape, and size of the restoration made chewing and speech difficult and led to cheek biting in three of the six subjects. In Christensen’s investigation,’ both subjective symptoms and clinical findings seemed to be more severe than in our study. One explanation for this difference might be that he altered the occlusion by covering only the lower molars. while our study provided occlusal contacts anteriorly to the canines. This probably resulted in differences in occlusal stability between the two types of procedures. To have occlusal contacts only on the molars is usually considered unfavorable for the health of the masticatory system. Christensen’s findings might therefore be interpreted more as a consequence of occlusal instability than of increasing the vertical dimension of occlusion. The electromyographically recorded postural muscle activity was lower with than without splints. The increase in occlusal vertical dimension thus seems not to induce increased muscle activity to restore the original vertical dimension of occlusion. The results of Thompson”’ have often been interpreted in this way. The lower postural activity with splints and in the anterior temporal muscle also in the lowered mandibular position is in accordance with results from many studies demonstrating that a lowering of the mandible leads to decreased muscle activity.” “I The decrease has been explained by inhibitory impulses from tendon organs.“i’ Contrary to the findings in the anterior temporal and masseter muscles, the activity in the posterior part of the temporal muscle was higher in the 7 mm lowered mandibular position than the postural activity without splints. This increased activity in the posterior part can probably be explained as muscle activity balancing the activity in the opening muscles to stabilize the mandible at the predetermined degree of opening. No significant differences in muscle activity with

288

INGERVALL,

ANIl

KOCAK

and without splints were found during maximai closure and swallowing. A tendency to an inr:r,easetl amplitude in maximal closure was, however. lour~ti when the subjects had adapted to the splints ‘I’his increased activity in maximal closure at a vcrticai level above the original occlusal vertical dimension agrees with results from bite force measurements ‘Ihus Ann”’ found an increase in closing force. Gth increasing separation of the jaws, and others have found the closing force to be higher at a vertical levei above the rest face height. The lack of clectromyographic findings oi disturbed muscular activity after increasing the vertical dimension of occlusion in our study, and the clinical experience that most patients easily adapt to changes in vertical dimension, support the vie\v that the determination of the height of occlusion might not be such a critical procedure as has often iieen stated. Again, it was verified that the postural position of the mandible varies according to conditions and thus is not entirely acceptable as a basis for determining the occlusal vertical dimension. This has been shown to be true also for other current methods.’ ’ Instead, vertical relation of occlusion should be estimated from esthetic, phonetic, and functional aspects. and from the standpoint of patient comfort.“. “’ The “comfortable zone” of occlusal vertical relation has also been suggested to be an interesting method, and its reliability has been studied by many investigators in the last fc~ years.” 2’1

SUMMARY

AND CONCLUSION

The effect of a temporary increase in the vertical dimension of occlusion was studied in six adult persons with complete natural dentitions. The increase in occlusal vertical dimension was accomplished by 7 days’ use of cemented acrylic resin splints covering the lower canines, premolars, and molars. The splints increased the vertical dimension beyond the original rest face height, which resulted in establishment of a new postural position of the mandible and a new interocclusal distance. ‘The subjects experienced moderate symptoms of discomfort upon placement of the splints. These symptoms decreased toward the end of the experimental period. No clinically demonstrable symptoms were found at the end of the experiment. Electromyographic recordings of the activity in the anterior and posterior parts of the temporal muscle and in the masseter muscle were made before

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and after placement of the splints. The splints brought about a reduction of the postural activity but did not change the activity during maximal closing or swallowing. No signs of increased muscle activitv to restore the original vertical dimension of occlusion were found. A moderate increase in the vertical dimension of occlusion does not seem to be a hazardous procedure, provided that occlusal stability is established. REFERENCES I

2.

:1.

t.

i

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12. 1.3

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