Evaluation of heat-cured resin bases following denture teeth using a second heat cure Kenneth M. Polukoshko, DDS, MSD,* James S. Brudvik, Jack I. Nicholls, PhD,C and Dale E. Smith, DDS, MSDd

the addition

of

DDS,b

University of Washington School of Dentistry, Seattle, Wash., and University of Alberta, Edmonton, Alberta, Canada This study compared heat-cured acrylic resin denture baseplate distortions following a second heat cure used to add the denture teeth. The second heat cure was done with three ditTerent water-bath curing temperatures. The distortions were evaluated in three planes by use of a measuring microscope. Recorded distortions were not clinically significant. (J PROSTHET DENT 1992;67:556-62.)

T

he accuracy of jaw relation records and the final occlusal relationships are directly related to the accuracy of the bases on which the records are made. Elder1 listed the requirements for ideal record bases. They should (1) have a border form and adaptation similar to the base of the completed denture, (2) be sufficiently rigid to resist distortion, (3) be dimensionally stable, (4) be able to be made inexpensively, easily, and quickly, (5) have no undesirable color, and (6) be useful as a base for arranging teeth. Graser2 advocated a procedure for making heat-cured resin bases that are used for recording the jaw relation, arranging the teeth for a trial fitting, and processing the teeth for the completed denture. The temperature of the base cure was 16.5’ F for 9 hours. For the second cure to attach the teeth, a temperature of 138’ for 12 hours was advocated. No data were reported to support this temperature and time for the second processing. An article by Brewer3 in 1963 presented a procedure that used a second cure of the completed base to attach the denture teeth and claimed that very little dimensional change occurs in the denture bases. Brewer3 stated that this

second cure was done at 140° F. There was no mention of the processing time. The dental literature has long indicated that when a denture is relined or repaired with a heat-cured resin, Project supported in part by a grant from the Education and Research Foundation of Prosthodontics. Submitted in partial fulfillment of the requirements for the Master of Science in Dentistry degree, Department of Prosthodontics, University of Washington, Seattle, Washington. aAssociate Professor of Prosthodontics, University of Alberta. bProfessor of Prosthodontics, University of Washington. CProfessor of Restorative Dentistry, University of Washington. Qrofessor of Prosthodontics, University of Washington. 10/l/35079

556

3.5 mm

3.5 mm

I I

I I

@ I_

-

,0.75 mm

I

3.25 mrh

lateral view

top view

Fig. 1. Schematic drawing of the brass measuring inserts.

stresses inherent in the acrylic resin will be released, causing dimensional changes in the denture bases.4-6 McCabe7 described glass transition temperatures of amorphous polymers such as acrylic resin. All acrylic resins have their own specific glass transition temperature. When cured acrylic resin reaches this temperature and undergoes several changes in its physical properties, the material may undergo a drastic reduction in its modulus of elasticity, becoming more flexible. During processing, the heat-cured resins are heated beyond their glass transition temperatures by a combination of the temperature within the processing bath and the exothermic heat of reaction. On completion of curing, when the resin begins to cool it passes through its glass transition temperature. Little stress is set up within the resin on cooling from the maximum temperature to the glass transition temperature because the resin is relatively flexible. However, below the glass transition temperature the material becomes more rigid and as it cools toward room temperature, stresses are induced by the differential rates of contraction within the resin and the dental cast. It is reasonable to assume that the greater the amount of acrylic resin cured in a second cure of the den-

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Fig. 2. Cast with measuring inserts in place.

ture teeth, the greater the chance for exceeding the glass transition temperature. The temperature at which the second cure is processed is also a potential factor in introducing residual stresses. Brewer3 and Graser2 used temperatures of 140” F and 138’ F, respectively, in the second cure of dentures in an attempt to reduce distortion due to stress release. At these temperatures there is a possibility that the complete cure of the acrylic resin will not take place and residual monomer may be retained in the acrylic resin. The presence of residual monomer in the cured resin will change the physical properties of the resin. As the residual monomer increases, the modulus of elasticity, yield strength and the indentation hardness decrease. If, however, the monomer content is below 1% , the mechanical properties are not substantially reduced.8 The residual monomer may also result in an inflammatory response in the oral mucosa. Austin and Bashkarg reported three patient histories of mucosal irritation caused by a high content of residual monomer that ranged from 1.7 % to 3.2 % . Incomplete polymerization may be detected by measuring the mechanical and physical properties of the resin and by chemical detection of the extracted monomer.‘0 Smith and Bainsll described a simple test that can be used for detecting residual monomer qualitatively. Hardness of the resin could be affected by an incomplete polymerization. Craig,6 used the Knoop hardness test to evaluate the surface hardness of cured resin. Barco5 found that greater distortion of the base occurs when it is heat cured with denture teeth present than when a base alone is processed. deGee et a1.12speculated that a procedure for measuring three-dimensional values of dentures after processing will show quite different results for a denture base alone than for a denture base with teeth. It has been documented that dimensional change occurs

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in a complete denture base when the denture teeth and associated gingival contours are added with heat cured acrylic resin.5 The purpose of this study was to evaluate the following: 1. The variation in dimensional change between two commonly used acrylic resin dentures 2. The variation in dimensional change relative to the second curing temperatures 3. The effect of the volume of resin added in the second cure 4. The hardness of the added resin after processing for all the above variables 5. The presence of residual monomer in the added resin

METHODS

AND MATERIAL

Sixty identical maxillary casts were made of type III dental stone (Dentstone, Columbus Dental, St. Louis, MO.) using the manufacturer’s recommended water/powder ratio. These casts had normal topography (such as rugae and incisive papilla). Parallel holes 5 mm in diameter were drilled with a straight handpiece into four predetermined places on the cast. Parallelism was maintained with the use of a dental surveyor (J. M. Ney Company, Hartford, Conn.). One hole was at the incisive papilla, one at the posterior surface of the palate, and one in each second molar position. The cast used in this study was small; therefore, it simulated a small denture. Machined brass inserts were cemented into these holes with a soft mix of dental stone (Fig. 1). The inserts were placed level with the tissue surface of the cast (Fig. 2). The cast with its four inserts in place was placed on the measuring table of a measuring microscope (Fig. 3). The position of each insert was measured in the x, y and z coordinates as baseline measurements (Fig. 4). The measurement mean was determined and recorded. Each x1 position,

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Fig.

x’

ET AL

3. Cast on measuring microscope table.

x2

x’

Y’

x2

Y’

2

z

Y2

Y2

#

@ Position 1

Position 3

Position 4

x’

x2 outside of insert -inside

of insert

Position 2 Fig.

4. Schematic of the four measured positions.

x2 position, and y’ and y2 positions for each imbeded brass insert was measured three times. The z coordinate was also measured three times for each of the four insert positions. Three measurements of each coordinate were averaged to reduce operator error. Wax bases were made for each cast with two thicknesses of Kerr No. 10 baseplate wax 1 mm thick (Sybron/Kerr, Emeryville, Calif.). The wax baseswere flasked in maxillary flasks (Hanau Engineering, Buffalo, N.Y.) with type III dental stone. The flasks were boiled for 5 minutes, opened, and the wax removed. Separator was placed on each side of the flask

558

(Ivoclar AG, Schaan, Lichtenstein). The separator was removed carefully from the inserts to obtain a good replication of the metal inserts. Thirty bases were processed with Lucitone 199 (L.D. Caulk Co., Milford, Del.) acrylic resin and 30 with Microlon (The Hygenic Corp., Akron, Ohio) acrylic resin. The acrylic resins were measured and mixed according to the manufacturer’s recommendations. One trial pack was made and the flash removed. The final closure incorporated a pressure of 3200 psi. All of the bases were cured for 9 hours at 165’ F. When the flasks had bench cooled to room temperature they were opened and the bases recovered. The rough flash

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5. Acrylic resin denture base with measuring insert negative reproductions.

Fig.

Table I. Acrylic resins, volumes, and curing temperatures used in study Curing temperature (“F) 165 152 140

Lucitone 6 ml resin N=5 N=5 N=5

199

12 ml resin N=5 N=5 N=5

N=5 N=5 N=5

6. Silicone mould with denture teeth in place.

Table II. Mean original distortions in millimeters from master cast to denture base Acrylic

Microlon 6 ml resin

Fig.

12 ml resin N=5 N=5 N=5

resin

Lucitone 199 Microlon

Alto2

-o.u30* -0.035 -0.146* -0.029

Alto3

SD SD

-0.148* -0.030 -0.131* -0.025

Alto4

-0.089

SD

-0.039 SD

SD

-0.08 -0.019

SD

*Statistically significant p = < 0.05.

was removed and the bases were placed in room temperature water for 24 hours. The bases were placed on the measuring microscope table one at a time. Each of the four insert negative replications was measured relative to position 1 (Fig. 5). These negative replications were measured in the same manner as the brass inserts. A silicone material (Dent-Kote, Dentsply International, York, Pa.) was placed over each acrylic resin replication to protect it from the second flasking and deflasking. Plastic New Hue teeth (Dentsply International) were arranged on one of the completed bases. To control the volume of wax used on each base, a silicone mold was made with Silastic ERTU silicone rubber (Dow Corning Corp., Midland, Mich.). The teeth were placed in the mold and a given volume of wax was poured into the mold and allowed to cool (Fig. 6). Volume 1 was set at 6 ml of wax, and volume 2 at 12 ml. The teeth with wax gingival contours were attached to the base as a single unit and the flasking was completed. The flasks were boiled for 5 minutes, opened, the wax removed, and cleaned with low-sudsing detergent. A separating liquid was placed on each side of the warm flasks. The two acrylic resins in the study were mixed according to the manufacturer’s recommendation. The flasks were packed with one trial pack, the flask removed and a final closure using 3200 psi. Monomer was painted on the baseplate before closure. The 60 bases were treated under the conditions listed in Table I.

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Following the curing and bench cooling of each flask, the dentures were recovered. The rough flash was removed and the dentures were placed in room temperature water. After a 24-hour immersion in the water, the dentures were placed on the microscope measuring table and the changes in position of the negative contours of the insert replicas were measured and recorded, again relative to point 1 (Fig. 5). Following the measurement of the completed dentures, all of the recorded data were entered in a computer and the amount of distortion that had occurred in (1) the original base and (2) the finished denture was determined by using a program developed by Nicholls.13 Five samples with dimensions of 50 mm x 20 mm x 4 mm of both acrylic resins were flasked and cured according to the three temperatures used in this study. Following deflasking, each sample was placed in a container of distilled water holding 10 ml of liquid. At 12 hours each container of liquid was tested for free monomer. This was done by placing 1 ml of the liquid in a test tube, adding 1 ml of dilute, neutral aqueous potassium permanganate 0.001 N solution to it, and noting the color change.ll After these tests the samples were washed in new distilled water and left immersed in 10 ml of fresh distilled water for 48 hours, after which the monomer determinations were made again, Surface hardness tests were conducted on both resins by using rectangular samples 50mm x 20mm x 4mm that had

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Table

III.

ET AL

Analysis of variance for distortion point 1 to point 2 Sum of

Source of variation

squares

Main effects Resin Volume Temperature 2-Way interactions Resin-volume Resin-temp Volume-temp 3-Way interactions Resin-vol-temp

4681.692 497.952 685.126 3498.614 1296.372 822.955

DF

Mean square

F

Signif ofF

4 1 1 2 5 1 2 2 2 2

1170.423 497.952 685.126 1749.307 259.274 822.955 30.102 206.607 69.649 69.649

7.207 3.066 4.219 10.772 1.597 5.068 0.185 1.272 0.429 0.429

0.000 0.086 0.045* 0.001* 0.179 0.029* 0.831 0.289 0.654 0.654

60.203 413.214

139.297 139.297

*Significant at p < 0.05.

Table

IV.

Analysis of variance for distortion point 1 to point 3

Source of variation

Sum of squares

DF

Mean square

F

Signif ofF

Main effects Resin Volume Temperature P-Wayinteractions Resin-volume Resin-temp Volume-temp 3-Way interactions Resin-vol-temp

4798.519 205.054 412.598 4180.867 971.442 112.504 240.087 618.851 287.365 287.265

4 1 1 2 5 1 2 2 2 2

1199.630 205.054 412.598 2090.434 194.288 112.504 120.043 309.425 143.682 143.682

6.536 1.117 2.248 111.390 1.059 0.613 0.654 1.686 0.783 0.783

0.000 0.296 0.140 0.001* 0.395 0.438 0.524 0.196 0.463 0.463

*Significant at p < 0.05.

been immersed 24 hours in room temperature water. The Rockwell superficial hardness test (Wilson Mechanical Instrument Co. Inc., New York, N.Y.) was done on each of the five samples for each acrylic resin and for each temperature.

RESULTS In every sample the distances measured were less than the distance taken from the marker cast, indicating polymerization shrinkage for all points. Table II gives the mean shrinkage values of Lucitone 199 and Microlon acrylic resins for the first cure at 165’ F for 9 hours in the water bath. The total shrinkage recorded was in the range of 0.2 to 0.3 mm for points 1 to 2 and points 1 to 3. The shrinkage for points 1 to 4 was approximately 0.1 mm (roughly half of the other measurements). The mean shrinkage values for the second cure were consistently smaller than for the initial cure. For example, the initial mean shrinkage from point 1 to point 3 for one group of 5 Lucitone 199 samples processed at 165’ F was -0.176 mm, whereas the second cure shrinkage of 6 ml of additional resin processed at 152’ was -0.059 mm. The raw data indicating the dimensional change of the

denture base was converted to a percentage change by the formula; (1st processingdistortion - 2nd processingdistortion) x loo~ 0 1st processingdistortion The converted percentage values, representing the changes that resulted from the second cure, were subjected to an analysis of variance for each of the measured points. A p value of

Evaluation of heat-cured resin bases following the addition of denture teeth using a second heat cure.

This study compared heat-cured acrylic resin denture baseplate distortions following a second heat cure used to add the denture teeth. The second heat...
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