Clinical studies of strain behaviour of complete dentures

CLINICAL STUDIES OF STRAIN BEHAVIOUR OF COMPLO-E DENTURES A. A. Obeid.* G. D. Stafford

and J. F. Bates.

ABSTRACT Thi investigation examines the variation in strain on maxillary dentures worn by five subjects in clenching. Strain gauges were attached to each denture in four places. The strain was measured with the posterior teeth in their normal position and then again when these teeth had been moved buccally and palatably. Keyworda

The strain levels varied when the poste7a’or teeth were moved palatally or buccally, but the tension or compression response of the gauges was consCtent. The strain levels agreed generally with those reported by other workers.

Dentures; strain gauges; tooth prosthesis; bite.

INTRODUCTION The literature shows that littIe work has been carried out to investigate the strain on complete maxillary dentures during function. Brittle lacquer techniques for strain measurement have been used by Wain to demonstrate the direction of the principal strains when loading complete dentures but this system did not measure the amount of strain involved. Lambrecht and Kydd2 used strain gauges to formulate the pattern of strain behaviour and to measure the strain. They placed twelve gauges on each of the maxillary dentures examined and were able to select from sites that had shown maximum strain for later study. Johnson3 also used brittle lacquer to indicate the direction of strain and selected the sites of maximum strain for the positioning of strain gauges. He placed gauges in three sites on the maxillary denture and found that, when tested on a laboratory rig, the anterior palatal area showed the greatest tensile strain, and that this strain decreased when the posterior teeth were cuspless and positioned inside the line of the crest of the ridge. Stafford and Griffiths4 showed clinically that strain decreased in magnitude during clenching and swallowing movements when the width of the occlusal table was reduced. Their calculated data of rosette gauge recording showed that the direction of the minimum plane of the stress at the gauges positioned mid-way along the posterior border of the denture on the polished surface was antero-posteriorly and the maximum plane ran across the posterior aspect of the denture.

Five subjects were selected for these experiments. The strain was recorded using four linear foil gauges cemented to selected sites of each maxillary denture. A technique was devised to modify the positions of the posterior teeth and observations were made of the variations of the strain magnitude in three tooth positions. METHOD Four linear foil strain gauges (Tokyo Sokki, Kenkyujo Co., gauges type FLG-02-03 lot No. 340011) of gauge length 0.2 mm, gauge resistance of 120 _+0.3 Q and a gauge factor of 2.14 were cemented to the sites shown in Figure 1. The gauges were cemented with a cyano-acrylate adhesive M-Bond 200*. After cementation, leadwires (336 FTE)* of identical length were soldered to the gauge leads and these fine wires were then soldered to additional thicker leads (three lead conductor flat cable 7/O 12)” which were then connected to the strain gauge conditioner and amplifier (Vishay 2 100 System)*. After completing the installation of the strain gauge, the resistance was checked by a strain gauge tester*, and the gauge and soldered joints on the denture then covered with a silicone water proofing material (Dow Coming 3140 RTV Coating. Dow Coming Corpn., Midland, Michigan, U.S.A.).

PROCEDURE This investigation was carried out to investigate the variation in strain magnitude during clenching, and with the posterior teeth in the approximate positions previously occupied by the natural teeth. This tooth position was then varied by moving the posterior teeth buccally (outwardly) and palatally (inwardly). Figure

1 Sites chosen

for strain gauges.

* Dental Centre, Behind Amirie Hospital, Kuwait. i’ Department of Restorative Dentistry, Dental School, Heath, Cardiff CF4 4XY, UK.

0141-5426/82/010049-06 $03.00 0 1982 Butterworth & Co. (Publishers) Ltd

* Welwyn Strain Measurements Ltd., Armstrong Road, Basingstoke, Hants.

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Strain of dentures: A.A. Obeid et al.

The amplified output from the Vishay 2100 was recorded on a pen recorder (Bryans Pen Recorder, Model BS316, W.W. Electronic, Scientific Instruments, CH-4002, Basle, Switzerland). The experimental arrangement is shown in Figure 2. Five male subjects were selected for this study, and are designated A, B, C, D and E. The average age was 60 years. The experience of wearing complete dentures ranged between 15 and 20 years. They all had moderately resorbed ridges covered by a firm healthy mucosa and a normal jaw relationship. All the experimental dentures were processed in polymethylmethacrylate* with anterior and posterior acrylic teeth. All posterior teeth were of the same mould (Cosmopolitan, mould 29N”). A copy technique’ was used to prepare the complete dentures for A and B who had recently been supplied with their dentures, and C who had used his dentures for two years. The experimental dentures of subjects D and E were prepared at the same time as the dentures for their own use. Gauge 1 (Figure 1) was located at the point of intersection of two lines, one representing the midline of the palate and the second joining the two canines. Gauge 2 was located at the point of intersection of the midline with the line joining the two second molars, 5 mm in front of the posterior border of the denture. Gauge 3 was located opposite gauge 1, when the shape of the fitting surface permitted, otherwise it was located on the rise of the palate. Gauge 4 was located on the labial flange of the denture just below the notch of the labial fraenum. A jig (Figure 3) was constructed for the precise location of gauges at the various sites. The use of clear acrylic resin denture bases and the coloured backing material of the strain gauges facilitated this. The jig was important to enable accurate positioning of a replacement gauge in the event of damage or failure of a gauge in a test. A jig was also constructed

to locate

Figure 3 Jig for the location and orientation of the strain gauges in place

the three positions of the posterior teeth, when they were moved from the normal tooth ridge relationship (ABN) (Figure 4) to a more buccal (outer) position (ABo) or to a palatal (inner) position (ABr). Lines were scribed on the jigs (f+igure 5) to act as guides for the tooth movement, These lines radiated from the centre of the messial margin of the first premolar, running backwards to touch the second molar at three points. The distance CD was 7 mm, representing the bucco-palatal width of the molar distally. The total movement of this tooth was 3.5 mm whether moved bucally or palatally. Lines AB, AC and AD coincided with the central line of the posterior teeth when the positions ABo, ABN and A& were used. The posterior teeth were removed as a block from the maxillary and mandibular dentures, using carborundum cutting discs, and carefully repositioned using the control jig and plaster dies to control the lateral movement and to nreserve the iaw relationships. The teeth blocks w&e reattachead to the denture bases using autopolymerizing resin.

and orientate

L Figure 2 Strain gauge bridge system with subject. * TREVALONB,

Amalgamated

Dental Co., London,

50 J. Biomed. Engng. 1982, Vol. 4, January

England.

Figure 4 Diagrammatic representation to show the positions of the maxillary teeth

Strain of dentures:

A.A. Obeid et al.

denture of Subject A were then replaced and on the third day the same procedure adopted. The statistical comparison shows that when the experiments of day 1 and day 2 were compared there was no significant difference in the variability of gauges 1, 2, 3 and 4. The variations between the two series were reproducible with a coefficient of variation of *12 per cent.

Figure

5

Diagram

of location

jig for prosterior

When the experiments of the three days were compared, gauge 1 results were reproducible with a coefficient of variation of about +12 per cent. There was no significant difference in gauge 2 and 4 results, but for gauge 3 there was a significant difference. Thus, if gauge 3 has to be replaced there may be significantly different results from the new gauge.

teeth

Readings were taken from each gauge simultaneously. The test dentures were immersed in warm water at 37°C before being inserted into the mouth and a period of 10 minutes was allowed for temperature equilibration of the denture in the mouth before beginning the test. A record of the base line was made before and after each experiment with the mouth closed and the mandible in the rest position so as to ensure that any drift of the signal could be detected and to provide a base line. Calibration was carried out by applying the fixed calibration signal (1000 pe) of the Vishay 2 100 unit. The subjects carried out ten maximum clenches for each series with a 2 second period of relaxation between clenches. Ten series of 10 maximum clenches were recorded for each gauge in each experiment. Rest periods of 2 minutes were allowed between successive series and 15 minutes between the 5th and 6th series.

Clinical strain gauge measurements Three experiments were completed on each of the five subjects as follows:- Readings were recorded (1) when the posterior teeth were in the ABN position, (2) when the posterior teeth were in the ABo position and (3) when the posterior teeth were in the ABI position. Tables 1, 2,3,4 and 5 refer to subjects A, B, C, D and E respectively and show the experimental final means for the three positions determined for each gauge and the analysis by students ‘t’ test. DISCUSSION Before discussing the results in detail it is appropriate to make some general comments concerning intra-oral strain gauge experiments:(1) Intra-oral strain gauge measurements involve personal factors such as general health, muscular power, fatigue, discomfort and varying sensitivity and pain thresholds which could cause variations in clenching pattern.

RESULTS Reproducibility An experiment of 100 clenches (in a series of 10 repeated 10 times with the rest periods indicated) were completed on two different days with Subject A. These results were analysed for variance using Snedecor’s F test. The gauges from the Table 1 Subject A. The mean strain values (microstrain) statistical results. (All the ‘t’ values are highly significant) Gauge Gauge Response

Every effort was made to standardize the procedure to ensure that the subjects were comfortable and completely relaxed. Adequate rest periods were

for each gauge obtained

as the teeth positions

1

2

3

Tension

Tension

Compression

were altered

with

4 Compression

Experiment

2

1

3

2

1

3

2

1

3

2

1

3

Teeth position

ABo

ABN

ABI

ABo

ABN

ABI

ABo

ABN

ABI

ABo

ABN

ABr

Final experimental means.

1031

933

706

742

1056

380

284

255

429

557

270

176

Difference between means. Standard deviation of means. ‘t’ values.

98 24.57

227 68.15

4.29

314 28

9.74

54

676 67

11.5

29 24

29.84

11

174

287

17 4.60

39

42

12.98

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Strain of dentures: A.A. Obeid et al.

Table 2 Subject B. The mean strain values (microstrain) for each gauge obtained as the teeth positions were altered, with statistical results. (All the ‘t’ values are highly signifkant except where indicated:-- * ‘t’ value not significant, ** ‘t’ value is significant) 1

G2MllZ.Z

2

Tension

GaugeResponse

4

3

Tension

Compression

Compression

Exneriment

2

1

3

2

1

3

2

1

3

2

1

Teeth position

*Bo

*RN

*BI

*Bo

ABN

*BI

*Bo

ABN

A%

‘ABO

*BN . ABT

1415

1226

1191

884

1066

649

756

951

563

940

741

Final experimental means.

Difference between means.

189

Standarddeviation of means.

35

118

‘t’ values.

182

58 4.55

48

417

80

*1.51

195

62 5.67

74

388

76

199

87

13.59

3

39

57

45

5.35 12.87

684

36

48

10.92 **3.05

Table 3 Subject C. The mean strain values (microstrain) for each gauge obtained as the teeth positions were altered with statistical results. (All the ‘t’ values are highly significant except where indicated:- ** ‘t’ value is significant) 1

Gauae

Gauge Response

2

Tension

3

Tension

4

Compression

Compression

Experiment

2

1

3

2

1

3

2

1

3

2

1

3

Teeth position

ABO

*RN

*BI

*Bo

ABN

M

*Bo

*BN

ABI

*Bo

*BN

ABI

Final experimental means.

1189

1089

453

602

730

227

643

862

389

381

286

105

Difference between means.

100

Standarddeviation of means.

52

636 117

**2.48

‘t’ values.

128 22

26

16.93

503

219

91 4.29

16

23

17.27

473

95

71

19

181

17

9.27 20.32

39 7.01

13 13.91

Table 4 Subject D. The mean strain values (microstrain) for each gauge obtained as the teeth positions were altered, with statistical results. (All the ‘t’ values are highly significant except where indicated:- * ‘t’ value is not significant) Gauge

1

Gauge Response

2

Tension

3

Tension

4

Compression

Compression

Experiment

2

1

3

2

1

3

2

1

3

2

1

3

Teeth position

*Bo

*BN

*BI

*Bo

*BN

*BI

*Bo

*BN

*BI

*Bo

*EN

ABI

Finalexperimental means.

2089

1707

1649

481

680

297

229

204

144

888

1025

692

Difference between means. Standarddeviation of means. ‘t’ values.

382 131

58 143

6.23

199 72

71

*1.14

63 6.64

included in the routine to obviate fatigue and to prevent soreness of the mucosa. (2) Waterproof coating is an important feature of intra-oral measurements. It must be applied in thin layers to cover the gauges and the solder-joint so that its bulk will not interfere with the function of the gauge. (3) Other factors are also important; such as the variation in the size of the maxillary ridges, the palatal shape, the size of the denture bearing area, the presence of a torus palatinus (midline bony palatal prominence), the size of the tuberosities

52 J. Biomed. Engng. 1982, Vol. 4, January

383

25 26

17.86

66

60 25

*1.13

137 18

6.34

33

333 82

4.93

27 12.29

(posterior convexities of the upper jaw in which the molar teeth were attached) and the form and thickness of the soft tissues of the palate. All of these could affect the degree of bending of the dentures when loaded. Therefore the strain IeveI at the selected sites would vary from one person to another. Normal tooth position Gauge 1 and 2 strain on the outer palatal surface were positive in all the results in all subjects. Thus, the midline of the polished surface of the denture is subjected to tension during clenching.

Strain of dentures: A.A. Obeid et al.

Table 5 Subject E. The mean strain values (microstrain) for each gauge obtained as the teeth positions were altered with statistical results. (All the ‘t’ values are highly significant except where indicated:- ** ‘t’ value is significant)

Gauge Response

4

3

1

2

Tension

Tension

Gauge

Compression

Compression

Experiment

2

1

3

2

1

3

2

1

3

2

1

3

Teeth position

*Bo

*BN

*RI

ARo

-N

-4’31

*Bo

*BN

*BI

*Bo

*BN

@

Final experimental means.

2150

2568

1991

410

602

349

128

782

139

610

436

361

Difference between means. Standard deviation of means.

418 147

400 **3.10

‘t’ values

577

192 182

4.14

36 11.38

The general findings of this study agree with the strain pattern shown qualitatively in the brittle lacquer clinical and laboratory studies of Wain’ and Johnson3. The results also agree with the clinical and uantitative investigation of Stafford and Griffiths 94. Gauge 3 strain on the fitting surface of the denture was negative or compressive in the results of subjects A, B and C. This finding agreed with Stafford and Griffiths4. The compressive strain in this area of the denture was expected. However, in subjects D and E, gauge 3 registered a positive or tensile strain. Subject D has a wide denture bearing area and a marked torus platinus. Gauge 4 recorded a negative or compressive strain in all experiments. This is due to further compression of the tissues overlying the crest of the alveolar ridge and to the final tendency of the denture to scissor outwards from the tuberosity areas, thus moving outwards and forwards. in posterior

654

40

This can be related to the compressibility of the tissues and the bending of the denture. In the resting state, the denture is separated from the underlying bone by layers of soft tissues which are thinner in the mid-palatal area. On the application of the load bilaterally, the denture becomes more firmly seated in the mid-palatal area. When the limit of compressibility is attained, the denture begins to bend outwards laterally, thus imposing tension in gauges 1 and 2.

Variations

253

tooth position

Comparison between the strain levels in the outer and inner positions of posterior teeth as compared to those of the normal position. Gauge 1. The results of the investigations, with aI1 the subjects except one, show that the anterior palatal area is the region which is subjected to the maximum tensile strain, when the posterior teeth are in any of the three chosen positions. In Subject A the anterior palatal area was subjected to the greatest strain when the posterior teeth were in outer positions. The strain was a little less when

31 15.92

21

643 136

14.99

174 19

44

14.77

75 64

7.1

30 **3.38

the teeth were in the normal position on this gauge, when compared with gauge 2. Gauge 2. In all five subjects studied, the tension in gauge 2 during clenching was consistently the highest when the posteriors were in the normal position. It was somewhat less in the outer position and substantially less in the inner positions. Gauge 3. In three subjects, the anterior fitting surface of the denture was in a state of compression during clenching. However, the compressive strain monitored was much less than the tensile strain on the outer palatal surface, i.e., the strains were not balanced. When an idealized cylindrical structure is subjected to bending strains, the tension on the outer surface is balanced by a compressive strain on the inner surface of the curve. The centre of the structure is the region of the least strain. A denture, however, is much more complex in shape and the strains are unevenly distributed through the structure, possibly partly due to uneven section. The difference between the high strain levels in gauge 1 and the low strain levels of gauge 3 is impor tant. The complex pattern of strains, so produced, could be partially responsible for this area being a common site for crack production and failure in complete maxillary dentures. In two subjects (D and E) the anterior fitting surface of the denture registered a tensile strain during clenching. In subject D, the values of this strain in the three positions of the posterior teeth were very low compared with the high tension in the outer anterior palatal area. This could be related to the depression in fitting surface of the denture, owing to the presence of a torus palatinus, i.e., there is a secondary stress bearing area which could account for the exceptional pattern of tension rather than compression recorded by gauge 3. On the other hand, subject E possessed the smallest denture bearing area with a high U shaped palatal vault and the vertical dimension of the inter-arch space was low when compared to the other subjects of this study. Moreover, this subject had strong muscular power which greatly increased the strain

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Strain of dentures: A.A. Obeid et al.

level recorded by gauge 1 (Table 5). The high levels of strain during clenching recorded by gauge 1 and the tension registered by gauge 3 could be related to the personal factors above. Gauge 4. The tension and lateral extension in both gauge 1 and gauge 2 was accompanied by a compressive strain in gauge 4. With one exception, the compressive strain in gauge 4 was greatest when the posterior teeth were in the outer position and was much reduced when the posterior teeth were in the inner position. CONCLUSION This study reports the strain levels recorded by four gauges located in the midline of complete auxillary dentures made from PMMA. Five subjects co-operated in the study and recordings were made when their dentures were loaded during maximum clenching. This work is a continuation of previous work and is compared with earlier studies. (1) Though the strain levels varied when the posterior teeth were displaced either to the outer or inner positions, the tension or compression response of the gauges was consistent. (2) With one exception, the tensile strain level of gauge 1 increased when the posterior teeth were displaced outwards. The exceptional subject had a high U-shaped palate.

(4) Variable results were obtained for the strain levels in gauge 3 when the posterior teeth were displaced outwards. Of the three subjects who registered compression in this area, two showed a decrease in strain levels with the teeth in the outer position. The remaining subjects registered tension, and in one with the high palatal vault there was a marked decrease of tension with the teeth in the outer position. The high strain levels in gauge 1, with the concomitant low levels on the opposing fitting surface, could be an important factor in the incidence of fatigue in this area and could explain the crack propagation proceeding from the outer to inner fitting surface. (5) The strain levels registered in gauge 4 showed a marked increase in all subjects with one exception when the posterior teeth were displaced outwards. The decrease of the strain level in the remaining subject could be related to the wider bearing area, a low palatal vault and the presence of a torus platinus. On the other hand, the compressive strain levels in gauge 4 showed a decrease in all subjects when the posterior teeth were displaced inwards. ACKNOWLEDGEMENTS The authors wish to acknowledge the assistance of Mr. J.E. Scott, Senior Instructor in the Department of Restorative Dentistry, Dental School, Cardiff, for his painstaking technical support. REFERENCES

These clinical results, in general, agree with the laboratory work of Johnson3. (3) The tensile strain levels of gauge 2 decreased during clenching when the posterior teeth were displaced either outwards or inwards. The effect was more pronounced when the posterior teeth were displaced inwards.

54 J. Biomed. Engng. 1982, Vol. 4, January

1 2 3 4 5

Wain, E.A., Dent. Practit. 1957 8,37 Lambrecht, J.R. and Kydd, W.L., J. Pros. Dent. 1962 12,865 Johnson, W., Dent. Practit. 1965 15,374 Stafford, G.D. and Griffiths, D.W. J. Oral Rehab. 1979 6,241 Stafford, G.D. and Huggett, R. Dent. Practit. 1971 22,119