.4rch. oral Bid. Vol.7, pp.7-16.1962. Pergamon Press Ltd. Printed in cit. Britain.

TOOTH MOVEMENT IN EXPERIMENTAL MALOCCLUSION

D. J. ANDERSON

Physiological Laboratory, Guy’s Hospital Medical School, London Bndge, London S.E.1

Abstract-In tive human subjects, the occlusal surface of the right lower first permanent molar was raised by means of a removable metal cap approximately 0.5 mm thick. In every case the cap was an anatomical replica of the occlusal surface of the chosen tooth and was worn continuously without discomfort for 2341 days. Immediately after insertion of the cap the subjects were unable to make contact anywhere with the jaws clenched except between the capped tooth and antagonists. Within a short period whole-arch centric occlusal contact became possible, and measurements between reference points on the capped tooth and antagonist and between other pairs of teeth show that adjustments in tooth position took place.

INTRODUCTION

WHEN a tooth is extracted. the unopposed antagonist usually erupts further to occupy

part of the space previously filled by the lost tooth. Likewise, if occlusal contact between opposing teeth is lost as a result of the removal of tooth substance in the

preparation of a crown restoration for example, the teeth frequently re-establish occlusal contact in the period elapsing before the restoration is inserted, unless the prepared tooth is covered by a temporary filling. From these common observations, it seems that even in adults, the teeth have a latent eruptive potential normally held in check by contact with their antagonists in the opposing arch, although this contact is intermittent. Yet there are many examples of arches in which teeth do not make contact with their opponents, although apparently free to do so. and the result must be that the masticatory loads are distributed over a smaller area than normal, with the possibility of trauma to the supporting tissues. Tt is of interest in the study of periodontal disease to investigate factors which influence the inter-occlusal relation- ship of teeth, and this paper provides data on tooth movement during experimental disturbance of this relationship.

ANDERSON and PICTON (1957) described acute experiments in which masticatory forces were measured in a tooth of which the occlusal surface had been raised by a metal cap approximately 0.5 mm thick. With the cap in position, this tooth, a lower tirst molar, was the only one in the arch which made contact with the upper dentition, yet the forces recorded did not differ greatly from those recorded without the cap. Furthermore, the subjects were able to wear the cap without discomfort. In the experiments to be reported here, five subjects wore a bite-raising cap on the lower right first molar for periods of up to 41 days, during which measurements were made to follow any alterations in the relative positions of the capped tooth and its opponent,

7

8 D. J. ANDERSON

and in the relative positions of pairs of opposing teeth elsewhere in the arch. The method of making the bite-raising cap differed from that described previously; this new method and a method of determining the distance between reference points on opposing teeth will be described.

METHODS

1. The preparation of the bite-raising cap

It was considered essential to reproduce in the bite-raising cap the configuration of the occlusal surface of the chosen teeth, so that the contact area and the distribution of force between the opposing teeth in occlusion would be unaffected by the presence of the cap. Two rubber-base impressions were taken of the chosen tooth; one impression was cast in stone, the other in casting investment. A “reverse” in casting investment was poured on the stone model and allowed to overlap the occlusal surface on to the buccal and lingual surfaces as far as the greatest curvature. It was removed when set and a sprue-hole drilled through its upper surface. It was then placed in position on the investment model and the two were mounted on a platform which rested on the upper surface of a micrometer screw-gauge spindle (see Fig. 1).

Anvil

Investment reverse

Investment model

Spindle

Fxo. 1. Diagram of micrometer screw-gauge showing the investment model and reverse in position.

As the gauge was screwed up and down the platform rose and fell with the spindle, but was prevented from rotating by two extensions which embraced the micrometer frame. The model was attached to the platform with wax and then elevated until the reverse just made contact with the micrometer anvil, to which it was then attached with wax. Now by unscrewing the micrometer gauge the model and investment reverse could be separated by any required distance. The periphery of the space

TOOTH MOVEMENT IN EXPERIMENTAL MALOCCLUSION 9

between the model and reverse was filled with casting wax and after following the usual investment and casting technique it was possible to make a hard gold cap, the upper and lower surfaces of which were replicas of the occlusal surface of the original tooth. The cap thickness chosen for these experiments was approximately 0.5 mm; buccal and lingual clasps retained it in position on the tooth.

2. Recording the distance between Jixed points on opposing teeth It did not seem possible to find a reliable fixed point in or around the mouth

with which to relate tooth position, and so all determinations were of the position of one tooth relative to its opponent in the other arch, with the jaws firmly clenched in centric occlusal contact. If the distance between two opposing teeth increased, the technique did not allow the observer to distinguish between the possibilities that (a) the opposing teeth had intruded into their sockets, (b) the surrounding teeth had extruded from their sockets and (c) the teeth had tilted so that they made contact at different positions on the cuspal inclined planes. If only one pair of teeth were used in the measurements, apparent separation of these might be due merely to incomplete closure of the jaws. Rather than add to the complications of the technique by trying to ensure reproducible closure by standardizing the force of jaw clenching. records were also made of the distances between reference points on other pairs of teeth which were not expected to move and these were used as control measurements. Taking the lower right first molar (61) for bite-raising, control measurements were made on the first premolars (41 and 41) or upper first premolar and lower second premolar of the same side (41 and 5;) depending on suitability, and on suitable premolars and the first molars on the left side.

The reference points were small cuts of approximately 0.1 mm in depth on the buccal surface of the enamel. The first stage in making measurements of the distance between cuts on opposing teeth was to take a rubber-base impression of the buccal surfaces of the cheek teeth with the jaws clenched in centric occlusion and one hand pressed firmly upwards against the chin. The rubber base was inserted on dental compound roughly adapted to the teeth and strengthened with a shellac backing. Three impressions of each side, all extending from canine to second molar, were taken on the first occasion; subsequently only two were needed. One of the first set of three impressions was used to make a master model to be employed throughout the investigation to ensure constant alignment of the impression during measurements. This model was cast so that its buccal surface was vertical and the line joining buccal cusps of adjacent teeth was approximately horizontal. For measurement, an im- pression was placed against the master model which rested on a plane mirror. A large mass of modelling clay on the mirror was moulded against the back of the impression to hold it in place after removal of the model. The impression could then be viewed with a travelling microscope adjustable in the vertical plane. Viewing of the reference marks was facilitated by an oblique light source and the reflected light from the mirror below. Two readings were made on each tooth and from the two impressions on each side there were thus four readings from which to calculate a mean value for the distance between the reference marks.

10 D. J. ANDERSON

Before the bite-raising cap was inserted, readings were made on three or four occasions during a week to provide control values, the means of which provided control baselines. After insertion of the cap, impressions were taken with the cap temporarily removed, at intervals of a few days or a week. When possible, a record was obtained the day after inserting the cap and the day after its removal. In Subject 5 it was possible to study the 8 hr period immediately following insertion, at 2 hourly intervals, and in Subjects 4 and 5 the 8 hr following removal of the cap were similarly studied. In all subjects measurements were continued after removal of the cap until the base line positions had been reached, and in Subjects 4 and 5 the cap was inserted a second time for a short period. The duration of each experi- ment depended on the patience of the subjects and the experiments were terminated quite arbitrarily to suit their convenience.

RESULTS

The results are shown in Figs. 2-6. In every record time in days is shown on the abscissa; the heavy horizontal line and vertical dotted lines demarcate the period during which the bite-raising cap was in position, preceded by a control period and followed by a recovery period. In Figs. 5 and 6 (Subjects 4 and 5) the time scale has been expanded in a total of five places to allow the display of results obtained at 2 hourly intervals over 8 hr periods. These periods are shown as shaded areas. In all the records, the ordinates show the distance in millimetres between reference points on pairs of opposing teeth. The mean values obtained in the control period are represented by horizontal lines drawn throughout the records, and the shaded area around this line indicates twice the standard deviations of the control means.

. . . .

;t x6.0 --Ft94

7.9 4

I ’ 6 ” k i4 ’ 3’2 ’ i0 ’ 48 * ‘i6 ’ ;4 ’

TIME IN DAYS

FIG. 2. Graph of interdental distances in millimetres plotted against time in days. The period during which the bite-raising cap was worn on 61 is shown by the heavy horizontal line between vertical dotted lines, preceded by a control period and followed by a recovery period. The mean control values are shown by thin horizontal lines throughout the records and 2 x standard deviations of these means are shown by shaded areas in the control period.

TOOTH MOVEMENT IN EXPERIMENTAL MALOCCLUSION II

6.9r I

49 --_u_LLL,__L- 0 4 8 12 16 20 24 28 32 36 40 44

TIME IN DAYS

FIG. 3. Graph of interdental distances in millimetres plotted against time in days. The period during which the bite-raising cap was worn on $1 is shown by the heavy horizontal line between vertical dotted lines, preceded by a control period and followed by a recovery period. The mean control values are shown by thin horizontal lines throughout the records and 2x standard deviations of these means are shown by shaded areas in the control period.

Some individual features of the records require explanation. From Subject 1 (Fig. 2) records are available from only two pairs of control teeth, since the 1% was missing and the I? was not sufficiently accessible to be of use. The retaining clasps

broke shortly after the bite-raising cap was inserted, with the result that the subject could wear it only between meals. The cap was taken for repair after 4 days in the

mouth on day I I and reinserted on day 13. This 4 day period of intermittent wear

is indicated by breaks in the heavy horizontal line on the record.

On account of a misunderstanding, Subject 3 (Fig. 4) removed his bite-raising

cap for about 16 hr from day 29 to 30. To indicate the break, the lines joining the

readings on day 23 to the next readings on day 34 are dotted. The first two control readings in Subject 5 were taken some weeks prior to the remainder, because the

subject was not available in the interim.

In all subjects the capped tooth and its opponent appear to separate. The rate of separation is most rapid in Subject 3 who was aged 19 years and the youngest

of the group. With Subjects 2, 3, 4 and 5 it was possible to make measurements during or after the lapse of 24 hr following the insertion of the cap and in all except Subject 5 substantial change was shown during this short period. Likewise, in the 24 hr following removal of the cap there is a marked reverse movement and the return to the control level is more rapid than the initial separation. Tn all except

Il. .I. ANDERSON

a-=-=-% R'/5

0 4 8 12 16 20 24 28 32 36 40-6

TIME IN DAYS

b’rti. 4. Graph of interdental distances in millimetres plotted against time in days. The period during which the bite-raising cap was worn on 61 is shown by the heavy horizontal line between vertical dotted lines, preceded by a control period and followed by a recovery period. The mean control values are shown by thin horizontal lines throughout the records and 2 x standard deviations of these means are shown by shaded areas in the control period. The cap was removed for 16 hr from day 29 to 30. To indicate the break the lines joining readings on day 23 to readings on day 34 are broken.

Subject 5 the inter-premolar distance on the right side remains constant, but there is

apparent separation of 16 and F in Subject 2, and also 14 and 13, and 16 and 16 in - - - Subject 4.

In Subjects 4 and 5 the cap was reinserted after its removal and the return to

control values. It can be seen in Figs. 5 and 6 that there is a rapid separation to very nearly the previous level, with approximately the same time course as the

recovery. The difference between the first and second 24 hr periods after insertion of the cap is particularly striking in Subject 5, who showed hardly any separation in the first 24 hr period.

The subjective reports are of interest. None of the subjects was able to make contact between opposing arches (except between the capped tooth and opponents) immediately following insertion of the cap. However, with an effort, contact on the

other side of the mouth could be achieved on the following day, and was more easily achieved from day to day until the presence of the cap went unnoticed, At no time did any subject experience pain with the cap in position,

TOOTH MOVEMhNT IN LXPEIUMENTAL MALO~‘(‘I.LlSION I.3

su6JEcr 4

0 8 I6 24 32

TIME IN DAYS

FIG. 5. Graph of interdental distances in millimetres plotted against time in days. The period during which the bite-raising cap was worn on 61 is shown by the heavy horizontal line between vertical dotted lines, preceded by a control period and followed by a recovery period. The mean control values are shown by thin horizontal lines throughout the records and 2 x standard deviations of these means are shown by shaded areas in the control period. Measurements were taken at 2 hourly intervals at 8 hr periods following removal and reinsertion of the cap. These periods are shown by shaded columns.

SUBJECT 5

TIME IN DAYS

FIG. 6. Graph of interdental distances in millimetres plotted against tune in days. The period during which the bite-raising cap was worn on 61 is shown by the heavy horizontal line between vertical dotted lines, preceded by a control period and followed by a recovery period. The mean control values are shown by thin horizontal lines throughout the records and 2 x standard deviations of these means are shown by shaded areas in thecontrol period. Measurements were taken at 2 hourly intervals for 8 hr periods following the first insertion, the removal and reinsertion of the cap. These periods are shown by shaded columns.

14 D. J . ANDERSON

DISCUSSION

In five subjects within the age range 19-40 years, the presence of a bite-raising cap on -81 has been tolerated without pain for periods of as long as 41 days. In Subjects 1 and 3 (Figs. 2 and 4) the distances between reference points on all the control teeth remained constant, whereas, between the capped tooth and its opponent the distance increased. The only explanation for this and for the fact that 24 hr after insertion of the cap the subjects were able to make contact over the whole arch, must be that the capped tooth and opponent actually separated, allowing the cap to be accommodated in the occlusal plane. As stated earlier in this paper, the method is unable to provide evidence as to how this separation is achieved, but it must have been by intrusion of the capped tooth and its opponents into their sockets, eruption of the other teeth, or a combination of both possibilities.

The distances recorded between pairs of control teeth are not so constant in Subjects 2. 4 and 5 as in Subjects I and 3. There is a slight separation of 16 and 16 in Subject 2 throughout the experimental period, but the more obvious separations are between 14 and 14 and 16 and js in Subject 4 and 41 and 41 in Subject 5. An explanation for these changes is hard to find. At first sight it would appear that they might be due to carelessness in clenching the teeth during impression taking. If this were so, then the apparent separation would be expected as an occasional anomaly as on day 8 between 14 and 14 and 16 and 16 m Subject 2. However the subjects were confident that their technique in closure and impression taking did not alter. The consistency of the separation and its limitation to the period of wearing the bite-raising cap strongly suggest that it is not an experimental error. However, if in spite of subjective evidence, the left separation in Subject 4 is due to consistent incomplete closure on that side during the experimental period, this in no way invalidates the result obtained on the right side where the 41 and 5/ distance is constant while the 61 and 61 distance increases.

In Subject 5, ho&ever, both experimental and control teeth on the right appear to separate, and if this is due to incomplete closure then the results obtained on the 61 and 61 cannot be claimed to indicate actual separation of these teeth. However, &complete closure on the right side would be expected to affect both pairs of teeth to much the same extent, yet 61 and 61 have separated by O-165 mm before any separation of 4_ and $1 begins,&d the maximum separation of almost 0.30 mm shown by 61 and 61 is twice the maximum separation of 41 and 41. Furthermore, the secondinsertion of the cap resulted in re-separation of%/ and 61 to the previous level with hardly any alteration in the 41 and ;;il measurement.

In view of the fact that the subjects were eventually able to bring their teeth into occlusal contact with the cap in position, the increased distance between reference points on the capped tooth and opponent is taken to indicate actual separation of these teeth with the appearance of an inter-occlusal space. This is not necessarily the case with the apparent separation of some control teeth. The increased distances between reference points on these teeth may mean long-axis movement and separation with the appearance of an inter-occlusal space, but it seems much more likely that these teeth make contact when the jaws are clenched, but at different positions on

TOOTH MOVEMENT IN IYPtRlMENrAL MALOCCLUSION IS

their cuspal inclined planes, thereby increasing the distance between reference points

without the appearance of an inter-occlusal space. No evidence is at present available to throw further light on these possibilities.

It appears that the tooth movements involving the capped tooth and its opponent

consist of a rapid component followed by a slower phase. PARFITT (1960) reported axial displacement of teeth under intermittent and continuous force and it is possible that the initial separation recorded in our experiments is due to an intrusion of the

capped tooth and opponent and extrusion of the other teeth, resulting from fluid

movement in the peridontal tissues, as suggested by PARFITT.

Although the slow component of the separation may be accompanied by bony

changes, the fast component and the recovery are most certainly not. If the recovery is due to adjustments in the soft supporting tissues, then it should be possible to

separate the teeth once again and more quickly immediately after recovery than on the first occasion. This has been done in Subjects 4 and 5.

These experiments have demonstrated that large disturbances in the intrr-

vcclusal relationship of teeth can be tolerated without discomfort. They have also shown tooth movements which appear to have the effect of restoring inter-occlusal

contact.

Ackno~l~lr~l~emrrzts-I am greatly indebted to the subjects of this experiment foi

their patience and to Mr. P. KING of Guy’s Hospital Dental School for technical

assistance, and to the Nuffield Foundation for a grant which has supported this work.

REFERENCES ANDERSON, D. J. and PICTON, D. C. A. 1957. Masticatory stresses in normal and modified occlusion.

J. dent. Res. 37, 312-317. PARFITT, G. J. 1960. Measurement of physiological mobility of individual teeth in an axial

direction. J. dent. Res. 39, 608-618.