Effect of denture thickness and curing cycle on the dimensional stability of acrylic resin denture bases

J. C. Chen, W. R. Lacefield, D. J. Castleberry School of Dentistry, University of Alabama at Birmingham, U.S.A.

Chen JC, Lacefield WR, Castleberry DJ. Effect of denture thickness and curing cycle on the dimensional stability of acrylic resin denture bases. Dent Mater 1988: 4: 20-24.

Abstract - This study evaluated the relationship between denture thickness and the dimensional stability of acrylic resin denture bases as a result of the compression molding process. Forty-eight maxillary complete dentures were fabricated in 3 different thicknesses using 4 curing and cooling cycle combinations. After processing, the dentures were replaced on the original cast and both the molar-to-molar linear shrinkage and the dimensional change in the posterior palatal area were measured using an optical comparator. The different curing cycles tested (e.g., 165~ for 9 h as opposed to 165~ for 1 h followed by 212~ for 30 min) had little effect on the processing shrinkage. However, significantly more distortion (p = 0.05) was observed for the dentures quenched in water as compared to those specimens which were benched cooled. The thicker dentures had less molar-to-molar linear shrinkage, but more dimensional change in the posterior palatal area as compared to the thinner dentures.

Key words: dental acrylic resin, maxillary complete dentures, denture processing, denture fit.

W. R. Lacefield, Department of Dental Materials, Box 49, SDB, University of Alabama at Birmingham, Birmingham, AL 35294, U.S.A.

Received September 15, 1986; accepted April 21, 1987.

Since 1937 (1) poly (methyl methacry- late) acrylic resin has been the most widely accepted denture base material. Most acrylic resin dentures are fabri- cated by the heat-cured compression mQlding technique. Other popular al- ternatives include the use of autopoly- merizing resins or fabrication of the denture by the injection molding tech- nique (2). Regardless of the curing technique or type of acrylic resin se- lected, the processing deformation that is apparent when the cured dentures are removed from the cast is universally recognized as a major disadvantage of this material (3-5).

In addition to continuing the search for more accurate and dimensionally stable materials for denture bases, it is also necessary to identify those factors which may contribute to dimensional change during fabrication. The thick- ness of acrylic resin dentures is believed to be a significant factor in determining the magnitude of the shrinkage which occurs during curing (6-9). Previous studies have focused on the relation-

ship between the denture thickness and molar-to-molar linear shrinkage rather than the dimensional change which oc- curs in the posterior palatal region of the denture (8,9). In general it has been

shown that the thicker the denture base, the lesser the linear dimensional change in the molar-to-molar dimen- sion. However, curing shrinkage which occurs across the posterior palatal por-

Fig. 1. Stone mold and metal cast for duplicating 1 mm wax denture bases.

Denture thickness and curing cycle efJects 21

tion of the denture is known to be of significantly greater magnitude than the shrinkage in any other area. This occurrence is clinically significant be- cause integrity of the contact of the denture surface to the palatal mucosa is fundamental to retention of the den- ture. Dimensional changes which effect a separation of the palatal denture base from intimate contact with the mucosa cause a decrease in retention.

The ability of a practitioner or lab- oratory to rapidly process acrylic resin dentures offers potential economy of both time and expense. It is important to ascertain whether sufficient accuracy can be obtained by the use of either a short time curing cycle or a cooling technique which includes quenching.

The purpose of this study was to eval- uate the relationship between denture thickness and the dimensional change of acrylic resin dentures which occurs as a result of processing, with primary concern being the dimension across the posterior palatal portion. Another ob- jective was to determine whether either a short-time curing technique or rapid cooling by quenching is an acceptable alternative to the standard denture processing procedure, based on a mea- surement of the resulting dimensional change.

Material and methods

For this study, ADA standard edentu- lous maxillary metal casts of 1.5 ram, 3 mm and 5 mm thicknesses were se- lected. In order to repeatedly construct wax denture bases of the same thick- ness (+ 0.05 mm), a stone mold with a metal cast was centrally placed in a standard denture flask, as illustrated in Fig. 1. Wax denture bases of the same thickness were reproduced by pouring fluid wax into the mold. For duplicating the arrangement of teeth, a stone index for making subsequent identical tooth arrangements was obtained from a pre- liminary wax-up on a representative wax denture base. Identical molds of acrylic resin monoplane posterior teeth were selected. The compression mold- ing technique recommended by Rudd (10) was employed to trial pack and process all dentures. After deflasking, each denture was lightly finished with an acrylic bur, polished with pumice and then placed back on its original cast to be measured.

Forty-eight complete dentures in 3 different thicknesses (1.5, 3 and 5 mm)

were made of Lucitone* and Hircoe t heat-cured acrylic resins following the manufacturers' recommendation. The four different curing/cooling cycles that were used included: 1) short time cure, bench cool; 2) short time cure, quench- ing in water; 3) regular cure, quench in water; and, 4) regular cure, bench cool. The cycle times and temperatures were the same as recommended by the man- ufacturers except that in some cases a rapid quenching in water was em- ployed.

After processing the dentures were inspected and carefully replaced on the original cast without pressure. To de- termine the posterior palatal dimen- sion, points were placed at the midline

* Lucitone. L. D. Caulk Company, Milford, DE. tHircoe. Coe Laboratories, Inc., Chicago, IL.

of the denture and at equidistant points on both sides of the midlinel Each point was scribed with a Bard-Parker blade and marked with indelible dye. To facilitate molar-to-molar measure- ments, index marks were placed on the distal marginal ridges of the second mo- lars before processing. After process- ing, the distance between the two marks was remeasured and the percent- age change was calculated. The den- tures were removed from the casts, stored in deionized water for one month, and then measured a second time. All measurements were made us- ing an optical comparator (traveling microscope) to a accuracy of 0.001 mm.

In order to determine the accuracy of the measurement technique and the ability of the observer, a "t" test was run on measurements made on the same denture by two different observ- ers. An analysis of variance was used to

Fig. 2a. 1.5 mm thick denture on cast showing less dimensional change in posterior palatal region. 2b. 5 mm thick denture on cast.

22 Chen et al.

Dimensional changes at posterior palatal area

0 . 6 0 -

A E E v 0 . 5 0 -

s

- ~. o.~o i

0 .20-

0.10 L Short t i m e c u r e

B e n c h cool

M a t e r i a l s

L u c l t o n e

H l r c o e [ I

T T h i c k n e s s 1 . S m m 3 m m 5 r a m

m mN-1

NI ~ N . Regular c u r e

Bench cool N, Short time cure Ill. RIF elm cure

Quenching Ouenchklg

T e c h n i q u e s

I 30 days in delonlzed water 74F2~ PH 5.0-5.5

Fig. 3. Dimensional change of acrylic resin denture base materials at posterior palatal area.

determine if significant differences ex- isted among the various denture thick- nesses, denture materials or curing/ cooling cycles.

Results The discrepancy between cast and den- ture for a typical 1.5 mm thick speci- men and a 5 mm thick specimen are shown in Fig. 2a and b, respectively. The average dimensional change in the posterior palatal area for each of the test denture groups is given in Fig. 3. In all 4 cases the mean value of the dimen- sional change for the 5 mm thick den- tures was greater than that for the 1.5 mm thick specimens (at p = 0.05). The 3 mm thick dentures were not signif- icantly different from the 1.5 mm thick specimens in Cycles I, III, and IV for either of the 2 denture materials. The 3 mm thick Hircoe specimens were not significantly different from the 5 mm dentures in either Cycle II or Cycle IV. The dimensional change in the 5 mm Lucitone dentures was significantly greater than the 3 mm specimens in all cases. The analysis of variance revealed that the differences between the den- ture bases of various thicknesses were significant.

Regardless of which combination of curing cycle and cooling method was employed, the dentures processed of Hircoe and Lucitone exhibited signif- icant gaps when replaced on the casts. For example, in Fig. 3 it is apparent that the discrepancy between the cast and the denture at the posterior palatal

creased relative to the original mea- surement. The magnitude of the differ- ence was smaller in the thicker dentures than in the thinner dentures, which is contrary to the results ob- tained in the posterior palatal measure- ments. In the posterior palatal area the larger discrepancies were in the thicker dentures. In general, quench cooling increased the molar-to-molar dimen- sional change, with the exception of the 3 mm thick denture made of Lucitone. The curing method was shown to have little influence on either molar-to-mo- lar dimensional changes or on changes in the space between denture and cast in the posterior palatal portion. Follow- ing 30 day storage in de-ionized water, the gap in the posterior palatal area increased for most denture specimens.

areas is generally larger in the thicker denture groups than in the thinner groups. Quenching immediately in wa- ter resulted in dentures with a larger discrepancy at the posterior palatal ar- ea than those which were slowly bench cooled. These results also indicated that with either curing method, no con- stant relationship existed between the dentures cured in the short time cycle and those which were cured by the standard long heat cure method.

The changes in molar-to-molar dis- tance before and after processing (cal- culated in percentages) are given in Fig. 4. It was found that after proc- essing, the molar-to-molar distance de-

Discussion The processing shrinkage of dental acrylic resin has been accepted as an inevitable shortcoming of this material. Polymerization shrinkage and distor- tion from induced strain have been identified as occurring during the proc- essing of a denture. The slight expan- sion which occurs during heat curing compensates for a portion of the poly- merization shrinkage. The differential in thermal contraction between the mold and the acrylic resin is believed to be the cause of residual strain in the processed denture, and is also consid- ered to be the main contributor to the strain release which occurs when the denture is separated from the cast.

A space beneath the posterior palatal

Dimensional changes st molar to molar - 0.60

-0.50 A

i - 0.40 -0.30

-0.20

-0.10

- 0.00 +0.10

IHI 111 1 Mater ia ls

Luc i tone Hirooe I J Thickness

1 .~m ~ m ~ _ m . ]

I. Short time cure II. Short time cure IN. Reguinr cure IV. Regular cure Bench COOl Quenching Quenching Bench cool

Techniques

3o days In deionlzad water 74"-2"1 = PH G.0-S.5 Fig. 4. Dimensional change of acrylic resin denture base materials at molar to molar region.

Denture thickness and curing cycle effects 23

Table 1. The manufacturers' recommended curing and cooling cycles for two brands of acrylic resin denture base materials.

Lucitone Hircoe

Cure time Regular cure & 6h

Cure temperature 163~ Quick cure

1.5 h 164~ &

0.5 h 212~

Cooling Bench cool in air for 30 min

Tap water for 15 min

Regular cure 9h 165~

Quick cure 1.5 h 164~

& 0.5 h 212~

Bench cool in air for 30 min

Tap water for 15 min

region resulting from processing shrinkage has previously been ob- served, but there is a lack of agreement as to its cause (4,9,11-15). Based on the results of this study, the most likely explanation is that the strain release in the maxillary denture tends to draw the flange inward, and the resulting prema- ture contact of the denture with the mold in these regions causes the palate to be elevated.

Molar-to-molar distance changes or measurements made between points marked on ridge crests have been widely employed to study linear dimen- sional changes of dental acrylic resin (8,9, 16-20). The negative difference between two measurements is recorded as linear shrinkage. The molar-to-mo- lar linear dimensional shrinkage in the present study ranged from 0.16% to 0.49% as shown in Table 5, and is simi- lar to the results reported by Mowery et al. (18) and Mirza (17). Such shrinkage causes tensile stress in the palatal re- gion, and eventually the denture base is pulled away from the palate with the posterior teeth on opposite sides of the arch being pulled closer together.

This study showed that soaking in water for 30 days decreased molar-to- molar shrinkage but increased dimen- sional change in the palatal area. A study by Anthony et al. (11) on heat- cured acrylic resin dentures which had been soaked in water for 8 months showed an improvement in fit at some areas of the dentures but not in others. Although dimensional changes were relatively small in their study, a worse fit after water storage was measured in the flange areas and the palatally in- clined surface of the alveolar ridge. Part of the explanation of the results of both studies is that an unequal stress distribution is built into the denture during the compression molding proc- ess. Subsequent storage in water causes

variable expansion and unequal release of stresses in different parts of the den- ture.

The results of this study and others (9,21) clearly indicate that the molar- to-molar dimensional change in thin dentures is significantly more than in thick dentures. Wolfel et al. (9) sug- gested that the dentures with thicker cross section were stiff enough to pre- vent release of some of the strain when the cured dentures were removed from the casts. However, in their study the effect of strain release in the posterior palatal area was not measured. The re- sults of our study showed that greater dimensional change occurred in the posterior palatal area of the thicker dentures, but this was not the case for measurements made in the molar-to- molar area. The greater volumetric shrinkage which occurs during the poly- merization of the thicker dentures may have more effect on distortion of the posterior palatal area than of the mo- lar-to-molar area.

Conflicting results have been re- ported on the effect of heat processing on the shrinkage of acrylic resin. The present study indicates that there is no significant difference in the amount of shrinkage which occurs in processed dentures using any of the curing cycles studied. Such findings are in agreement with those of other studies (12,14,18,21).

The wider gap between denture and cast in the posterior palatal region which occured in the water quenched group indicates that the rate of cooling of the flask has a definite effect upon the observed shrinkage of dentures. Both bench cooling and quench cooling take the denture from the high proc- essing temperature down to room tem- perature. Thus, if one considers the high temperature alone, it fails to ex- plain these findings. The most reason-

able explanation is that during the sud- den change in the temperature of the denture, caused by quenching in water, unequal thermal contraction in various areas occurs, thus inducing a greater amount of warpage.

Summary and conclusions

1. Processing shrinkage occurred in all 48 dentures which were fabricated using 2 different brands of dental acrylic resin and 4 different curing/ cooling cycles. When the processed dentures were placed back on the original casts, a visual discrepancy was observed between each cast and denture in the posterior palatal re- gion, as well as on the palatal side of the ridge crest.

2. The width of the misfit between the denture and the cast in the posterior palatal region ranged from 0.23 mm to 0.58 mm, with the thicker denture bases exhibiting wider gaps. Since remounting is a routine clinical pro- cedure, molar-to-molar changes can be corrected by occlusal adjustment. The posterior palatal region, howev- er, is a critical area relative to reten- tion. Elevated posterior seals can compensate for part of the discre- pancy, but large discrepancies in this area cannot be easily corrected after processing. Thus, clinically it ap- pears prudent that the palatal wax pattern be as limited in thickness as possible.

3. The amount of molar-to-molar di- mensional shrinkage ranged from 0.16% to 0.49% among specimens of the 2 acrylic materials tested. In general, the thinner the denture, the greater was the shrinkage in the mo- lar-to-molar dimension.

4. The type of curing cycle seems to have little effect on the processing shrinkage. This study, however, found a significant difference in shrinkage between those dentures bench cooled and those quenched in water. Thus, quenching in water is not an acceptable method for cool- ing heat-cured acrylic resin denture bases if the highest accuracy is to be obtained.

5. The dimensional changes observed are a result of data obtained from a static laboratory model. Thus, changes related to variables in soft tissue may not be deduced directly from this study.

6. For the curing and cooling cycles utilized, no significant difference in

24 C h e n et al.

dimens iona l stabil i ty was n o t e d be- tween the 2 b rands of acrylic mater i - als tes ted.

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