The load factor in complete denture intolerance

Completedentures

The load

factor

in complete

denture

intolerance

J. C. Thomson, M.D.S., H.D.D., L.D.S.* University

of Glasgow, Glasgow, Scotland

T

he vertical load transmitted to the supporting tissues from the occluding surfaces of the teeth is a major factor in the environment of complete dentures. Support from the basal seat resists this load, as retention resists vertical dislodging forces, and as occlusal balance and muscular balance provide lateral stability. Failure of support causes intolerance of complete dentures when the load exceeds the supporting capability of the denture-bearing (basal seat) tissues.

CLENCHED, MASTICATORY,

AND NONFUNCTIONAL

LOADS

Identification of the forces involved is the first step toward achieving balance between the loads which dentures impose and the support which the tissues can comfortably provide. Clenched loads between dentures have been recorded by the use of gnathodynamometers with widely varying results, apparently influenced by the design of the apparatus and the pain tolerance of the subjects, as well as by the power of the muscu1ature.l Performance is improved when the supporting tissues are anesthetized and when the loads imposed by complete dentures during experiments are more uniformly distributed over the denture-bearing mucosa. So consciously applied clenched loads are limited by the tolerance of the patient. Masticator-y loads are much smaller than those produced by conscious effort. Maximum forces of 6 to 8 Kg. during chewing have been recorded with complete dentures, but the average loads are much less than these maxima.2 The forces required vary with the type of food being chewed; patients may therefore limit the loading of supporting tissues by selecting food which does not demand masticatory effort exceeding tissue tolerance. Nonfunctional habits involving repeated or sustained occlusion of the teeth, not associated with mastication or deglutition, cause additional load upon denture-bearing tissues which is worthy of consideration. When the incidence of tooth contacts during Based on part of a thesis submitted of Glasgow. *Senior

4

Lecturer

in Dental

for the degree of Master

Prosthetics.

of Dental

Surgery,

University

Load factor in complete denture intolerance

5

sleep was recorded by Brewer3 by use of radio telemetry, one of his subjects showed a total duration of tooth contact of 21/2 hours in the course of one night. The forces associated with these contacts were not recorded. However, the biologic reaction of denture-bearing tissues to loading by nonfunctional jaw activity, both during sleep and when awake, is a product of the frequency as well as the magnitude of the forces involved. Since the patient is usually not aware of such habits, conscious control of the consequent tissue irritation is ineffective.

SUPPORTING CAPABILITY OF THE TISSUES The unsuitability of the tissues supporting complete dentures for the function of load-bearing must be recognized. In normal function, no heavy load is placed upon the mucous membrane, but in complete denture prosthodontics, it is made to serve the same purpose as the periodontal membrane which is adapted to provide support for natural teeth. The area of mucosa available to receive the load from complete dentures is deficient also. Support areas have been estimated in both the upper and lower jaws of 20 edentulous patients selected at random .* In considering vertical loading, it is appropriate to use a vertical projection of the denture base to calculate the area of distribution. Wax baseplates extending to the normal limits of complete upper and complete lower dentures were made on master casts of each subject, projections of the peripheral limits were traced, and the area within each outline was then computed (Fig. 1). The mean denture-bearing areas in this sample were 22.96 sq. cm. in the edentulous maxillae and 12.25 sq. cm. in the mandible. The corresponding areas of support available for a complete natural dentition, as obtained by adding together the areas of the periodontal membranes of natural teeth quoted

Fig. 1. The method of assessing vertical

support areas of denture

bases.

6

J. Prosth. Dent. January, 1971

Thomson

by Watt and and one half bearing area in the region offers another

associates5 are approximately 45 sq. cm. in each jaw-more than three times the average complete lower denture-bearing area. The loadof the sole of a foot in one selected subject, which was found to be of 64.5 sq. cm. and adapted to bear a load of approximately 75 Kg., interesting comparison.4

HABITUAL NONFUNCTIONAL

OCCLUSION OF THE TEETH

The terms “bruxism,” “ bruxomania,” and “occlusal habit neurosis” seem to signify rather rare instances of persistent gnashing of the teeth associated with gross attrition and severe psychologic disturbance. Less obvious habits of occlusion are a relatively common accompaniment of modern life, however, and for this the less dramatic term “habitual nonfunctional occlusion” seems to be more appropriate. The initial discomfort associated with new dentures is known to evoke unusual patterns of behavior in the surrounding musculature. For instance, facial mannerisms are at times responsible for lip discomfort. Complaints of irritation of the tongue are sometimes due to a habit of thrusting the tongue against the denture, and usually the patient is unaware of the causal relationship between the painful tongue and its contact with the teeth. This habitual activity is not limited by tissue intolerance. Similarly, patients tend to occlude the teeth of new dentures frequently at first-perhaps to strengthen confidence in retention until the surrounding muscles accommodate, or because some accommodation in chewing pattern is usually required and experimental closure of the teeth is part of the process of adaptation. These tentative tooth contacts may trigger a persistent habit, so in prosthodontic patients there is greater possibility of habitual nonfunctional occlusion developing. The significance of the load on tissues caused by such habits has been investigated. Attrition of teeth is a consequence of work done during mastication and during habitual nonfunctional activity. It can be observed on clinical examination and, when it is seen on the teeth of a denture, the time span over which it occurred may be known, which presents a possibility of assessing the load on the teeth of an individual if the conditions in which attrition is produced can be defined. Attrition of the occluding surfaces of acrylic resin teeth can be caused by friction of food on the acrylic resin or by actual contact of tooth against tooth. The two types can be distinguished, and so the predominant cause can be identified.6 When friction of food causes attrition of acrylic resin teeth, there is a reduction in cusp height which eventually progresses to an irregular obliteration of the occlusal contours (Fig, 2). The surface polish deteriorates so that staining occurs, and rough surfaces with inadequate occlusion may result. This wear by food is most often seen in persons who frequently eat salads containing uncooked vegetables, such as lettuce, carrots, etc., which require positive and vigorous chewing. A second distinct pattern of wear is of special interest. This type of attrition varies in degree, but, whether it is confined to small facets on occluding cusps or is so extensive as to cause flattening of the whole occlusal and incisal surfaces of the teeth, the areas affected are highly polished and in close contact when the upper and lower teeth are suitably occluded. This contrasts with the rough surfaces and loss of close occlusion which characterizes attrition of acrylic resin teeth caused by food. The teeth appear to be accurately ground into perfect occlusion between flat

ic%z ‘1”

Load

Fig. ‘2. Acrylic

resin teeth worn by the friction

Fig.

on acrylic

3. Wear

resin

teeth

factor

in complete

of food during

of a patient

who

denture

intolerance

7

mastication.

habitually

grinds

his teeth

together.

contacting areas which are well defined, and one can visualize that no food particles could have been interposed when the wear occurred (Fig. 3). This condition is found in lesser degree on porcelain teeth. Among patients who seek advice about denture difficulties arising from intolerance by the denture-bearing tissues, this type of tooth wear is frequently seen. It occurred in 20 per cent of an unselected group of patients recalled after they had worn acrylic resin dentures for four years. In all instances, it was associated

8 Thomson

Fig. 4. The apparatus designed teeth under known conditions.

J. Prosth. Dent. January. 1971

to simulate

attrition

of complete

sets of acrylic

resin posterior

with habitual nonfunctional occlusion of the teeth, and in most patients there was discomfort in the basal seat mucosa which seemed to indicate overloading. To investigate the conditions in which the attrition in question could be produced, apparatus was developed to simulate attrition in complete sets of acrylic resin posterior teeth (Fig. 4) / A series of tests showed that, to produce attrition in the laboratory equivalent to that observed in patients who habitually grind the teeth, a prolonged vigorous grinding action under loads similar to the maximum forces used during chewing with dentures was required. Fig. 5 shows an experimental specimen compared to a clinical denture, and Fig. 6 is typical of the results obtained. These experiments demonstrated that attrition of acrylic resin teeth of the type produced by actual tooth contact represents frequent or sustained forceful activity, and it may be concluded that habitual nonfunctional occlusion of the teeth imposes an additional load upon denture-bearing mucosa. This might be expected to be a cause of discomfort and intolerance of complete dentures by the patient. CONTROL OF THE LOAD FACTOR In the treatment of problems of support of complete dentures, the capability of the supporting tissues may be improved. The hard and soft tissue foundation should be prepared. Minor surgical procedures will eliminate irregular residual alveolar

Volume 25 Number 1

Load factor in complete denture intolerance

9

Fig. 5. Comparison of a specimen (left) after simulated attrition for 240 hours under a load of 4.5 Kg. (10 lb.) followed by 240 hours under a load of 9 Kg. (20 lb.) with an acq rlic resin denture which had been worn for 15 months by a patient who habitually grin ds his teeth.

bone surfaces and areas inadequatily healed. The health of the mucosa is promoted techniques may be by hygienic and therapeutic measures, and tissue-conditioning applied when appropriate. The load imposed upon the basal seat tissues may be controlled. Full extension of complete denture bases within morphologic limits can effect a considerable reduction in load per unit area of mucosa. An increase in vertical support area of 50 per cent has been achieved in complete dentures with underextended bases.* Resilient denture linings promote a more uniform distribution of load and may supplement selective impression techniques or relief chambers to compensate for variation in the thickness of the investing mucosa. The load during mastication can be decreased by reducing the area of the occlusal table of dentures so that less food is involved in each chewing action and less work is required.

DIFFERENTIAL DIAGNOSIS Evidence of habitual nonfunctional occlusion should be sought. Sometimes the habit may be noted in the course of the interview with the patient, but it is not usual for the patient to be aware of his “tooth-grinding” or clenching when first asked. He may confirm the suggestion on a later visit after his attention has been directed to the possibility. So recognition of the existence of a habit may not be easy for either patient or dentist, and its relation to denture discomfort may not be apparent. A complaint of pressure on the denture-bearing mucosa, sometimes expressed as a feeling of “tightness” of the upper denture, is an indication of overloading. The denture may be inserted each morning with reasonable comfort but must be

10

J. Prosth. Dent. January, 1971

Thomson

20

Fig. 6. Wear of acrylic resin teeth under varying loads. removed after a few hours, or on returning home in the evening, because of intolerable discomfort. Fatigue of the muscles of mastication, reported as an ache in the temporal and masseter regions, is another tfpical symptom. A history of intermittent instability of dentures which appear to be well balanced for normal functional purposes also provides evidence of nonfunctional habits. Perfectly balanced occlusion is especially desirable in complete dentures for patients subject to habitual nonfunctional occlusion. The distinct type of attrition associated with forceful tooth contact is a useful diagnostic sign worth seeking. When the habit is one of clenching or clamping the teeth, and no grinding movement is used, the attrition facets may be small and difficult to detect although considerable loads may be involved. When the dentures have not been worn continuously, due to discomfort, the signs of attrition may be slight. Careful scrutiny of the occlusal surfaces of the teeth of dentures under a good light is a useful measure during examination of patients. Attrition facets may vary in angulation and be seen only when light strikes them from appropriate directions. CAUSES OF DESTRUCTIVE

HABITUAL

NONFUNCTIONAL

OCCLUSION

When habitual nonfunctional occlusion is detected as a factor causing excessive load, predisposing causes should be sought. First establish that an adequate interocclusal distance exists at rest. An excessive occlusal vertical dimension of complete dentures will cause habitual nonfunctional occlusion. Local irritating factors, such as inadequate retention and occlusal imbalance, should be eradicated. Stress-motivated habits, when forceful occlusal contacts occur during work or sport, or during periods of anxiety, physical effort, or pain, are less surely eliminated. Stress-motivated habits have been found to be associated with intense concentration on work, such

Volume 25 Number 1

Load

factor

in complete

denture

intolerance

11

as steel erection, metal casting, transport driving, cash accounting, personnel management, and sales promotion. Difficult domestic situations, anxiety, and bereavement have been disclosed as causes of tension, or the relevance of these circumstances has become apparent although the patient was not prepared to admit them to discussion. Such situations may not be readily resolved but, if the patient becomes conscious of their relationship to his tooth-clenching habit and to his denture discomfort, there is a possibility of the habit being rejected. An explanation of the existence and importance of the interocclusal distance in the rest position of the mandible is a useful approach to treatment. Patients are interested in the information that there should be a space between the teeth in the normal relaxed posture, and their co-operation is secured. They are invited to note now and again each day whether in fact the teeth are apart. If the opposing teeth are found to be in contact, it is explained, this is wrong, and the jaw should be allowed to drop. In this way patients discover their habit, accept their responsibility for the consequence, and are motivated to try to correct their error. The explanation is reasonable, and the advice-to relax the jaw-is positive and more acceptable than mere exhortations to persevere in discomfort. The identification of factors affecting the load upon supporting tissues, including recognition of the role of habitual nonfunctional occlusion of the teeth, is important in dealing with intolerance of complete dentures, Facilities for laboratory experiments in simulation of attrition were made available by Professor R. M. Kenedi, Head of the Bioengineering Unit of the University of Strathclyde. Mr. Ian Stevens, Chief Technician, developed and constructed the apparatus. References

1. Lim, K. A.: Biting Forces in Edentulous Patients, Malaysian Dent, J. 6: 18-31, 1966. 2. Yurkstas, B. S., and Curby, W. A.: Force Analysis of Prosthetic Appliances, J. PROSTH. DENT. 3: 82-87, 1953. 3. Brewer, A. A.: Prosthodontic Research in Progress at the School of Aerospace Medicine, J. PROSTH. DENT. 12: 49-69, 1963. 4. Thomson, J. C.: M.D.S. Thesis, University of Glasgow, 1967. 5. Watt, D. M., MacGregor, A. R., Geddes, M., Cockburn, A., and Boyd, J. L.: A Preliminary Investigation of the Support of Partial Dentures and its Relationship to Vertical Loads, Dent. Pratt. 9: 2-15, 1958. 6. Thomson, J. C.: Non-functional Muscle Activity in Prosthetic Cases; Brit. Dent. J. 114: 326-328, 1963. GLASGOW DENTAL HOSPITAL, 211 RENFREW STREET GLASGOW C.3., SCOTLAND