Influence of axiopulpal line angle and proximal retention on fracture strength of amal restcwatisns

Adilson Amorim, C.D.,* Maria Fidela de Lima Navarro, C.D., M.S.D.,* * JosC Mondelli, CD., M.S.D., Ph.D.,*** and Eymar Sampaio Lopes, C.D., M.S.D.*** * Faculdade de Odontologia de Bauru, Universidade de SPo Paulo, Estado de So Pauio, Brazil

T he axiopulpal line angle in Class II cavity preparations may be sharp, as recommended by Black,’ beveled, as suggested by Simon2 or rounded. On the other hand Nadal and associates3 and Ireland4 tested changes in the pulpal wall designed to increase the bulk of amalgam and thus increase the strength of the restoration.

The influence of the axiobuccal and axiolingual grooves on mechanical retention remains a contro- versial subject. Markley,” Simon,’ and Strickland” advocated the preparation of grooves in the axiobuc- cal and axiolingual line angles. Recent investigations by Mondelli and associates’ and Crockett and asso- ciates’ demonstrated the effectiveness of this proce- dure. However, clinical research by Terkla and Mahler” established that proximal retentive grooves did not prevent clinical fracture of the restoration.

The purpose of this investigation was to determine the influence of four types of axiopulpal line angles and proximal grooves on the fracture strength of amalgam restorations. Another objective was to determine if chromium-cobalt dies are as effective as extracted human teeth for testing cavity forms.

MATERIALS AND METHODS

Chromium-cobalt dies with four different MO Class II cavities were employed in the first part of this study.’ The cavities were prepared with different axiopulpal line angles or pulpal walls. In die A the axiopulpal line angle was sharp (Fig. 1 [Ml, [w); in die B the axiopulpal line angle was rounded (Fig. 1 [ 01, [PI); in die C the pulpal wall contained a groove and sloped toward the axiopulpal line angle (Fig. 1

*Senior student and Fellow of Fundac$o de Amparo a Pesquisa do Estado de Sfio Paula.

**Assistant Professor, Department of Operative Dentistry. ***Professor and Chairman, Department of Operative Dentis-

try. ****Assistant Professor, Department of Social Dentistry.

[RI, [S]); and in die D the pulpal wall curved buccolingually and sloped toward the axiopulpal line angle (Fig. 1 [Xl, [YJ).

All buccal and lingual walls of the proximal portion converged toward the occl~sal.~ Retentive grooves were prepared in the axiobuccal and axio- lingual line angles in the original acrylic resin dies with a slowly revolving 69 L bur. This standardized the width and depth of the grooves in the chromi- um-cobalt dies.

All cavities were restored with dental amalgam. A matrix assured proximal standardization, The occlu- sal surface of the restoration was flat, and thus the shape and the thickness of the restorations were constant. All restorations were polished.

The mercury/alloy* ratio was 1: 1. After 20 seconds of mechanical triturationt the amalgam was condensed, first with a 1.5 mechanical condenser$ at the maximal frequency. Final condensation was made with a 3 mm hand condenser. The procedure was completed in approximately 5 minutes, and the specimens were stored in a humid environment at 37°C until tested. The compressive strength was recorded for 10 restorations in each of the four dies after 1,24, and 168 hours of storage. There was, then, a total of 120 tests.

The grooves at the axiobuccal and axialingual line angles were then tilled with zinc phosphate cement. The dies were restored with amalgam and the compressive strength recorded on 120 specimens without proximal retention.

The second part of the study was made on extracted human teeth. The occlusal surface was flattened and the roots reduced so that they could be embedded in self-curing acrylic resin to adapt them

*Fine-cut alloy, The L. D. Caulk Co., Miiford, Del. tDentomat, Degussa, Frankfort, West Germany. $Electro Mallet, Roberts McShirley, Glendale, Calif.

W22-3913/78/O-240-0169600.50/00 1978 The C. V. Mosby Co. THE JOURNAL OF PRO!STHETIC DENTISTRY 169

AMORIM ET AL

for test procedures. Ten groups of four teeth, similar in form and volume, were selected. Each group contained the four cavity forms previously described. The initial preparations did not include mechanical retention at the axiobuccal and axiolingual line angles. After restoration the teeth were stored in a humid environment at 37” C for 24 hours and then tested. Axiobuccal and axiolingual grooves were then prepared in the 40 teeth. New restorations were inserted and tested.

The restorations in human teeth were stored for only 24 hours, since this is a period intermediate between 1 and 168 hours. According to the statistical method and based on preliminary tests, a 24 hour period would suffice for making comparisons with the results obtained with metal dies.

All specimens were tested in a universal testing machine* at a speed of 0.5 mm/min.

Fig. 1. Schematic drawing [M) and chromium-cobalt die (N) of the cavity with a sharp axiopulpal line angle; schematic drawing (0) and chromium-cobalt die (P) of the cavity with a rounded axiopulpal line angle; schematic drawing (R) and chromium-cobalt die (5) of the cavity with sloping pulpal wall; schematic drawing (X) and chromi- um-cobalt die (Y) of the cavity with rounded and sloping pulpal wall.

RESULTS AND DISCUSSION

Table I presents the means and respective stan- dard deviation of all the tests with chromium-cobalt dies. It clearly shows that restorations inserted in cavities with proximal retentive grooves presented greater fracture strength than cavities without prox- imal mechanical retention. This occurred with all types of cavities in all three time periods.

The analysis of variance (Tables II, III, and IV) indicates significant differences for both sources of variations-with and without retentive grooves and among preparations. For the 168 hour period the interaction between the two factors was also signifi- cant, demonstrating that in this period one factor affected the other.

For the 1 hour period only the type A cavity (Fig. 1 [A.!], [q) showed statistically significant contrasts when compared with the other types of cavities. For 24 hours and 168 hours there was a significant difference between type A cavities with sharp axio- pulpal line angles and type B with rounded axio- pulpal line angles. A similar difference occurred between type C cavities, which had a sloping pulpal wall, and type D cavities, with curved sloping walls.

The increased fracture strength of cavities with proximal retention was a meaningful finding. Table I shows a highly significant increase in strength for all types of cavities having proximal retention.

These results agree with those obtained by Mondelli and asssociates’ and Crockett and asso-

*LosenhauSenwerk, Dusseldorf, West Germany.

170 AUGUST 1978 VOLUME 40 NUMBER 2

AXIOPULPAL LINE ANGLE, RETENTION, AND FRACTURE STRENGTH

Table I. Fracture strength (in kgf) of amalgam restorations made in chromium-cobalt dies (means of 10 tests)

Die type

D (rounded

A (sharp) B (rounded) C (sloped) and sloped)

Storagetime No P.R.* P.R.t No P.R. P.R. No P.R. P.R. No P.R. P.R.

1 hour

Mean S.D.1

24 hours Mean S.D.

168 hours Mean S.D.

27.3 41.1 28.1 43.0 28.0 43.5 28.9 43.4 0.856 1.983 1.487 1.764 1.291 1.225 1.542 1.912

59.1 90.4 59.0 91.4 60.8 93.3 61.2 92.6 1.329 1.787 2.362 1.279 1.029 0.537 1.208 1.547

68.5 92.9 68.6 93.1 70.9 93.6 70.8 93.9 1.ooo 0.497 0.394 0.738 0.699 0.762 0.715 0.438

*Without proximal retention. tWith proximal retention. $Standard deviation.

Table II. Analysis of variance of fracture Table III. Analysis of variance of fracture strength data obtained with amalgam restorations strength data obtained with amalgam restorations made in chromium-cobalt dies (storage time: made in chromium-cobalt dies (storage time: 1 hour) 24 hours)

Source of variakion SS df MS F* Source of variation SS df MS F*

Among types Without/with

retention Interaction

Residual Total

42.9375 3 14.3125 5.97t 4307.1125 1 4307.1125 1795.681

7.6375 3 2.5458 l.O6$ 172.7000 72 2.3986

4530.3875 79

*F.,, (1,60) = 4.00; F.,, (3,60) = 2.76. ‘F.,, (l&O) = 4.00; F.,, (3,60) = 2.76. ‘$ignificant at the .05 level (p < .05). $%gnificant at the .05 level (p < .05). $Not significant. $Not significant.

ciateqn and with the laboratory tests of Terkla and Mahler.Y However, the results do not agree with the clinical observations of Terkla and Mahler,Y who did not detect differences between restorations with and without proximal retention. Perhaps the lack of a clinical difference could be due to the morphology and location of the teeth.

Another aspect which emphasizes the merit of proximal retention is that the increased strength at 1 hour is close to that recorded for restorations without proximal retention at 24 hours. Also, values obtained for cavities with retention at 24 hours were always greater than those obtained at the 168 hour period without retention. From a clinical point of view these data are important because at 1 hour the amalgam is still in the initial phase of crystallization. This is a critical period, since fracture at the isthmus or

Among types Without/with

retention Interaction

Residual Total

82.4125 3 27.4703 12.65t 20352.2000 I 20352.2ooO 9372.42t

5.5250 3 1.8417 0.85$ 156.3500 72 2.1715

20596.4875 79

marginal ridge may occur at this time. Obviously proximal retentive grooves should be included in Class II amalgam preparations.

Actually the four cavities with different internal designs did present different results. Table V shows statistically significant differences among cavity types, although all means appear similar in terms of absolute values. From an exclusively statistical point of view it could be assumed that the cavities being tested were different because individual comparisons of the results indicated differences among them. These results, based on statistical differences, point out the superiority of one type of cavity over another.

The small variability and standard deviations show that the tests were well controlled and carefully conducted. For this reason only small differences

THE JOURNAL OF PROSTHETIC DENTISTRY 171

AMORIM ET AL

Table IV. Analysis of variance of fracture strength data obtained with amalgam restorations made in chromium-cobalt dies (storage time: 168 hours)

Source of varktion SS df MS F”

Among types Without/with

retention

Interaction Residual Total

46.1344 3 15.3781 33.08f 11221.9531 1 11221.9531 24138.427

13.0344 3 4.3448 9.35t 33.4750 72 0.4649

11314.5969 79

*F.,, (l&O) = 4.00; F.,, (3,60) = 2.76. @ignificant at the .05 level @ < .05).

Table V. Contrasts (Tukey’s test) among means (chromium-cobalt dies)

Storage time: 1 hour Critical difference = 1.2952

A X B = 35.55 - 34.20 = 1.35* A X C = 35.75 - 34.20 = 1.55* AxD = 36.15 - 34.20 = 1.95* B x C = 35.75 -35.55 = 0120 B X D = 36.15 - 35.55 = 0.60 C X D = 36.15 - 35.75 = 0.40

Storage time: 24 hours Critical difference = I.2324

AxB = 75.25 - 74.77 = 0.48 A X C = 77.07 - 74.77 = 2.30* AxD = 76.95 - 74.77 = 2.18* B X C = 77.07 - 75.25 = 1.82* B X D = 76.95 - 75.25 = 1.70" C X D = 76.95 - 77.07 = 0.12

Storage time: 168 hours Critical difference = 0.5702

AxB = 80.85 - 80.72 = 0.13 A X C = 82.22- 80.72 = MO* A X D = 82.37- 80.72 = 1.65* B X C = 82.22 - 80.85 = 1.37* B X D = 82.37 - 80.85 = 1.52* C X D = 82.37 - 82.22 = 0.15

*Significant at the .05 level (p < .05)

could be detected through the analysis of variance and Tukey’s contrast technique.‘” These differences, a!though statistically significant, mean little from a clinical point of view, and the values can be consid- ered similar. However, the retentive factor defihitely influenced strength, not only statistically but in a way that could have clinical implications.

Variations in the pulpal wall alone did not contribute to increased strength. They did when there were proximal retentive grooves. This interpre-

Table VI. Fracture strength (in kgf) of amalgam

restorations made in human teeth after 24 hours of storage (means of 10 tests)

Die type

B C D (rounded Retention (sh*up) (rounded) (sloped) and sloped)

With Mean S.D.*

Without Mean S.D.

79.2 79.6 82.8 83.2 0.856 0.883 0.747 0.537

w.7 52.6 54.4 54.3 2.214 2.135 1.792 3.010

*Standard deviation.

Table VII. Analysis of variance of fracture strength obtained with amalgam restorations in human teeth without proximal retention

Source of variation SS df MS F”

-. Amonggroups 111.3813 9 12.3757 3.947 Among types 89.6188 3 29.8729 9.58OOt

Residual 84.1938 27 3.1183 Total 285.1939

*F.,, (3.27) = 2.96. tsignificant at the .05 level ($ < .05).

tation agrees with that of Nadal and associates,” who suggested that increasing bulk at the axiopulpal line angle was a superfluous detail.

The tests with human teeth attempted to deter- mine if the results on metallic dies were realistic. Table VI presents the means and standard devia- tions obtained with restorations inserted in human teeth. The results show numerical differences in the means, which were always greater for restorations in the chromium-cobalt dies. When comparing the contrasts made for data obtained after 24 hours with metallic dies (Table V) and human teeth (Tabie IX), the same statistical significance is observed for both. Therefore the comments on chromium-cobalt die are applicable to human teeth in terms of both cavity type and proximal retention.

The analysis of variance (Table VII) shows greater variability for cavities without proximal retention. This variability significantly decreased when the same cavities were prepared with proximai retention (Table VIII). These data again confirm the effective- ness of proximal retention. The proximal grooves not only increased the strength of the restorations but also decreased the variability observed in cavities without proximal retention.

The greater variation observed with human teeth

172 AUGUST1978 VOLUME&I NUMBER2

AXIOPULPAL LINE ANGLE, RETENTION, AND FRACTURE STRENGTH

Table VIII. Analysis of variance of fracture strength obtained with amalgam restorations in human teeth with proximal retention

Table IX. Contrasts (Tukey’s test) among means (human teeth)

Source of variation SS df MS

Among groups 11.2250 9 1.2472 Among types 116.8500 3 38.9500

Residual 10.0250 27 0.3713 Total 138.1000

*F.,, (3,27) = 2.96.

F*

3.3590 104.9017t

I-Without plv3&4 retmtions Critical diffkemcb = ~1778

A X B = 52.6 - 50.7 = 1.9 A x C = 54.4 - 50.7 = 3.7” A X D = 54.3 - 50.7 = 3.6* B X C = 54.4 - 52.6 = 1.8 B X D = 54.3 - 52.6 = 1.7 C x D = 54.3 - 54.4 = 0.1

tsignificant at the .05 level (p < .05).

is in agreement with the report of Mahler and associates,” which advocated use of metallic dies because they present less variation than human teeth, and therefore differences in cavity form can be detected more easily. Considering the consistency of the data obtained with chromium-cobalt dies and the difficulties in selecting extracted human teeth with similar morphology, the chromium-cobalt dies command preference. These results are not in accordance with the conclusions of Farah and asso- ciates,‘” who suggested that chromium-cobalt dies are not realistic because their modulus of elasticity is superior to that of dentin. However, the results obtained show that chromium-cobalt dies are as effective as human teeth for detecting differences in the fracture strength of amalgam restorations. Metallic dies are more advantageous, since they would require less replications.

II-With pmximal retention critical &&nmce = 0,751s

A X B = 79.6 - 79.2 = 0.4 A X C = 82.8 - 79.2 = 3.6* A X D = 83.2 - 79.3 = 3.9* B X C = 82.8 - 79.6 = 3.2* B X D = 83.2 - 79.6 = 3.6* C X D = 83.2 - 82.8 = 0.4

*Significant at the .05 level (p < .05).

6.

7.

8.

CONCLUSIONS

Proximal retentive grooves significantly increase the strength of amalgam restorations in Class II cavities.

There were no remarkable differences that could have clinical significance between sharp or rounded axiopulpal line angles and rounded or rounded and sloped pulpal walls.

Chromium-cobalt dies are effective for tests of fracture strength of amalgam restorations.

REFERENCES

9.

10.

11.

12.

13.

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Strickland, W. D.: Amalgam restorations for Class II cavity preparations. In Sturdevant, C. M., Barton, R, E., Brautr., J. C., and Harrison, M. L.: The Art and Science of Qxrative Dentistry. New York, 1968, McGraw-Hill Book Compady, Inc., pp 235-259. Mondelli, J., Ikhikiriama, A., Lima Navarm, M. F., Galan Junior, J., and Coradazzi, J, L.: Fracture strength of amalgam restorations in modern Class II prtpar4ttions with proximal retentive gmovcs. J PROSTHET DENT 32:564, 1974. Crockett, W. Ib., Shepard, F. E., Moon, P. C., and Creal, A. F.: The influence of proximal retention grwves on the retention and resista&e bf Class II preparations for amal- gams. J Am Dent Assoc 91:1053, 1975. Terkla, L. G., and I&&x, D. B.: Clinical evaluation of inter-proximal retention Ives in Class II amalgam cavity design. J PROS-TWIT DENT 17:596, 1967. Scheffb, H.: The Analysis of Variance. New York, 1959, John Wiley & Sons, Inc., pp 76, 77.

Nadal, R., Phillips, R. W., and Swartz, M. L.: Clinical investigation on the relatioq of mercury to the amalgam restoration. II. J Am Dent Assoe 63:488, 1961. Mahler, D. B., Terkla, L. G., and Johnson, L. N.: Evaluation bf technics for analyzing cavity design for amalgam restvra- tions. J Dent Res 40:497, 1961. Farah, J. W., Powers, J. M., Dennison, J. B., Craig, R. G., and Spencer, J.: Effects of cement bases on the stread and deflections in composite restorations. J Dent Res 55:115, 1976.

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investigation on the relation of mercury to the amalgam DR. MARIA FIDELA DE LIMA &V.&RR0 restoration, I. J Am Dent Assoc 63:8, 1961. UNNERSIDADE DE SAO PAULO

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THE JOURNAL OF PROSTHETIC DENTISTRY 173