Predictability of retentive values of dental cements

Predictability

of retentive

values

of dental

W. A. Richter, D.M.D., M.S.,* J. C. Mitchem, D.M.D., J. D. Brown, D.M.D.*** Uniuersity of Oregon Dental School, Portland, Ore.

cements

M.S.,**

and

T If n ormation derived from laboratory tests has been relied upon extensively to predict clinical behavior. Previous studies, utilizing metal dies, have shown the relationship between compressive strength and retention of cast restorations. Jorgensen and Holstl reported an almost proportional increase in retention with increasing compressive strength when using zinc phosphate and conventional zinc oxide and eugenol (ZOE) cements. Lorey and Myers’ partially concurred with these results when using ZOE, ethoxybenzoic acid (EBA) reinforced ZOE, and zinc phosphate cements; however, a reversal in this trend was illustrated in their data for the three-quarter crown preparations. Various studies, employing extracted teeth or ivory dies, have shown the relationship of cement types, strength, and retention. Grieve” reported a similarity in retentive or bond strengths between zinc phosphate and polycarboxylate cements while, at the same time, he demonstrated that the former was stronger by one third in compressive strength. Phillips and co-workers,4 as well as Coleman and Kirk,5 illustrated a similar relationship between zinc phosphate cement and an EBA reinforced ZOE cement, with the former possessing twice the compressive strength. Williams, Swartz, and Phillips6 reported that, in most instances, there was a direct relationship between retention of orthodontic bands and compressive strength when using EBA reinforced ZOE and zinc phosphate cements. However, the reported retention values for a silicophosphate and the’ zinc phosphate cement were the same despite the fact that the former possesses approximately twice the compressive strength. From the latter five reports, 2-6 there appears to be a question as to the validity of using compressive strength as a predictor of retentive ability in cast restorations. Because of this question, the purpose of this study was to determine the relationship of laboratory tests to the retention of full crown castings when using permanent cements that are chemically different. *Professor **Associate ***Assistant

298

and Chairman, Professor, Professor,

Department

Department Department

of Crown

of Dental of Crown

and Bridge.

Materials

Science.

and Bridge.

Volume 24 Number 3

Predictability

EXPERIMENTAL

of

retentiue

ualues of cements

299

DESIGN

Retention tests were performed using full crown preparations prepared on extracted teeth. Each cast crown was cemented twice with four different cements using standardized and clinically acceptable techniques. The values obtained from these tests were then compared to the laboratory tests of compressive and tensile strength. In addition, the extent of seating and/or cement film thickness produced by the nonvented crowns was compared to the conventional laboratory test for film thickness. METHODS

AND

MATERIALS

Four permanent cementing agents were used in this study. These were (1) zinc phosphate (Fleck’s), (2) hydrophosphate (Calmix), (3) carboxylate (Duralon) ? and (4) alumina reinforced EBA-ZOE (Opotow) . The liquid: powder ratios of the first three cements (0.5 C.C. per 1.0 Gm., 0.3 CL. per 1.5 Gm., and 1.0 C.C. per 1.5 Gm., respectively) complied with the consistency test of A.D.A. Specification No. 8. The manufacturer’s suggested ratio of two scoops per eight drops was used for the Opotow cement. Compressive and diametral tensile strength samples were made and stored following the procedure outline in the A.D.A. specification for zinc phosphate cement. The samples were tested at 48 hours using an Instron testing machine at a loading

I ILbS. Fig. 1

Fig. 1. The tooth preparation, casting, and cementation Fig. 2. The assembly used to remove the cemented’crowns.

Fig. 2

force

used in this study,

rate of 500 pounds per minute fol colnprcssivr strrn@ and nt a head rate of 0.X inch per minute for tensilr str~n~tli. The film thickness of each cclnrxnt 1~1s tlrterrninc~tl following the l)rocednr~A ;r? ADA. Specification Ko. 8 for zinc phosphate cc>mc:nt. F~lll crown preparations ~v~‘rc prepared on freshly extracted posterior tact ir. Prior to preparation, each tooth was mounted in a plastic ring with cold-cur&g acrylic resin. The mounted teeth \\-ere then placed in a lathe and were turned doM.11 under a water spray with a cylindrical, fapered diamond bur until they were thcx shape of a cone having a slope of 7 de,grecs. The occlusal surfaces were then flattened perpendicular to the ion, cr axis of the tooth producing a preparation shaped like the frustum of a cone. A shallow “V” groove was then cut circumfcrentially to outline a gingival finish line. The prepared surfaces were not polished. Was patterns were prepared directly on the teeth and were cast in a Type III gold. The “U” type sprues remained attached to facilitate the removal of the crowns following cementation. The crolsns and cervical region of the teeth were scribed with vertical lines so that the castings could be properly oriented during cemcntation. Fi;:. 1 schematically depicts the tooth preparation, casting, and mounting rillx. Cementation was performed with a force of 11 pounds, which follows the optimum force determined by Jorgensem7 Prior to each cementation, the crolvns were seated on the preparations with 11 pounds of force, and a fiducial measurcment was made for determining the rstent of the seating of the casting. This measurement was made between the top of a small cone placed at the center of the occlusal surface and the gingival edge of the “V” groove. h second measurement was made after cementation. All cemented crowns were stored at 37’ C. and 100 per cent relative humidity for 48 hours prior to testing for retention. Fig. 2 illustrates the removal assembly. In order to insure alignment, a multiple-rin,g device was used in mounting the cemented crown in an Instron testing machine. The crowns were removed with a head rate of 0.05 inch per minute. Ten crown preparations were used in this study, and each crown was cemented twice with c~ch of tl!(~ four cements. This procedure rcquircd careful rlcanin, (r of the teeth and crowns between tests. In order to establish that the cleaning did not affect the surface characteristics of the preparations or crowns, the first and last cementations were done with the same cement. The retcntive values for thrse two sets of data were then compared.

RESULTS AND DISCUSSION Fig. 3 presents the results of the retentive tests as well as the laboratory tests of compressive and tensile strengths. The values for the retentive tests represent the rnean of 20 cementations (10 castings cemented twice on each preparation). The retention parameter was derived by dividing the load to induce failure by tht: area of the crown preparation. Values for the strength tests represent the mean of eight samples. Bars connected by vertical lines are statistically equal according to analysis of variance and Duncan’s multiple range tests. A statistical evaluation by t-test of the first and last series of cementings made with the same cement indicated that the repeated cleaning and testing had no apparent effect on retention. When comparing the tensile strengths of the four cements, the zinc phosphate, hydrophosphate, and ZOE cements are equal, and the carboxylate cement is at least

Predictability

?%E ” ‘3”

of

retentive values of cements

301

CROWN RETENTION

TENSILE STRENGTH

psi I 100

COMPRESSIVE STRENGTH

2

4

6

6

IO

I2

p*i I 1000

Fig. 3. Comparison

of crown

retention

to tensile

and compressive

ADA TEST (FILM

THICKNESS

CARBOXYLATE

ZINC

30

f

-

strengths.

CROWN MICRONSI

IINCOMPLETE

1.5

PHOSPHATE

[ 31 2 1.5

HYDROPHOSPHATE

EBA

of laboratory

SEATIN

-107f

127

43 f 3.0

-163 f

152

25 f 2.0

- 69 f 152

and crown

- MICRONS)

-112 2 I27

VALUES

Fig. 4. Comparison

CEMENTATl0t.j

cementation

ARE

I( t S

film thicknesses.

one third stronger. In the compression test, it was shown that all the strength values with carboxylate having the lowest, zinc-phosphate having the highest, unequal and hydrophosphate and ZOE had “in between” values. On the other hand, when evaluating retentive ability, the carboxylate, zinc phosphate, and hydrophosphate cements are equal, and the ZOE cement is approximately one half as retentive. In the light of these results, it appears that some mechanism other than strength must be involved when considering the retention of a cast restoration to tooth structure. When examining the surfaces of the preparations and castings following

302

Richter,

Mitchem,

and Brozvn

failure, it was quite apparent, as illustrated by othersj3s S that the ZOE cement failed at the cement-dentin interface since the castings always contained the cement which left the tooth surface relatively clean. On the other hand, portions of the other three cements remained attached to the surface of the dentin. So, it appears that compressive and tensile strengths are not good predictors when comparin? cements with differing potentials for reaction with or on the tooth surface. These tests may be predictive when comparing a single cement with varying strengths or when comparing different cements when one or both do not have a potential for affecting the tooth surface. The results of this study and that of Grieve3 differ from those of Phillips and co-workers4 and Coleman and Kirk5 in which they reported equal retention values for both zinc phosphate and ERA reinformed ZOE cements. Fig. 4 contains the results of the film thickness determinations. The values for the laboratory tests represent the means of three samples and those of 20 crown-cementation samples. Connected figures are statistically equal according to analysis of variance and Duncan’s multiple range tests. There appears to be a connection between the A.D.A. laboratory test and the mean values for crown cementation; however, the extremely high variances encountered in the latter prevent specific conclusions. Although the values recorded for incomplete seating are large in comparison to the laboratory tests, the actual cement thickness on the axial walls approaches the laboratory value when the degree of taper and roughness of the crown preparation is considered. There is little doubt that film thickness is an important consideration in the seating of a cast restoration. The cementation of the crowns, which were nonvented in this study, indicated ;t surprisingly high degree of nonseating as well a~ high variances. These findings appear to be clinically significant considering the fact that the crowns in this study were cemented under rather ideal laboratory mnditions.

SUMMARY Four types of permanent cements were used to determine whether the laboratory tests of compressive and tensile strength were predictive of their retentive abilities when retaining cast gold crowns to natural teeth. The standard laboratory test for film thickness also was compared to the degree of nonseating of these cast restorations. The laboratory tests of compressive and tensile strength are not predictive of retentive ability when considering cements which have differing potentials for reacting with or on the tooth surface. The calculated film thicknesses on the axial walls of the cemented restorations in this study were of the same order of magnitude as those values in the laboratory tests. However, the degree and variance of nonseating of the clinical crowns were considerably greater.

REFERENCES

1. Jorgensen, K. D., and Holst, K.: The Relationship Between the Retention of Cemented Veneer Crowns and the Crushing Strength of the Cements, Acta odont. seandinav. 25: 355-359, 1967. 2. Lerey, R. E., and Myers, G. E.: Comparison of the Retention Qualities of Bridge Re-

Volume 24 Number 3

3. 4.

5. 6. 7. 8.

Predictability

of

retentive values of cements

303

tainers Cemented With Zinc Oxide Eugenol Cements and Zinc Phosphate Cements, Washington, 1967, I.A.D.R., Abstract No. 457. Grieve, A. R.: A Study of Dental Cements, Brit. D. J. 127: 405-410, 1969. Phillips, R. W., Swartz, M. L., Norman, R. D., Schnell, R. J., and Niblack, B. F.: Zinc Oxide and Eugenol Cements for Permanent Cementation, J. PROS. DENT. 19: 144-150, 1968. Coleman, J. M., and Kirk, E. E. J.: An Assessment of a Modified Zinc Oxide/Eugenol Cement, Brit. D. J. 118: 482-487, 1965. Williams, J. D., Swartz, M. L., and Phillips, R. W.: Retention of Orthodontic Bands as Influenced by the Cementing Media, Angle Orthodontist 35: 278-285, 1965. Jorgensen, K. D.: Factors Affecting the Film Thickness of Zinc Phosphate Cements, Acta odont. scandinav. 18: 479-490, 1960. Richter, W. A.: Crown Cementation Retention Factors, Washington, 1967, I.A.D.R., Abstract No. 462. 611 S. W. CAMPUS DR. PORTLAND, ORE. 97201