IDimensional energy
accuracy
of denture
Phillip W. Wallace, DDS, Gerald N. Graser, DDS, Michael I,. Myers, DlMD, and Howard M. Proskin, Easttran Dental Center, Rochester, N.Y.
resin
cured
by microwave
MS, PhD
Microwave processing has several time-saving advantages over conventional processing of denture base resins. Little is known about the adaptation of bases processed by microwave radiation to the cast and the mouth. Comparisons were made in five regions of the palate and ridge. The microwave-processed denture bases had equal or better dimensional accuracy than conventionally processed bases. (J F'ROSTHETDEXT 1991;66:403-9.)
A
cryhc polymers were first introduced as denture base materials in 1937.’ Poly(methy1 methacrylate) has been the resin most commonly used to make removable complete and partial denture.3. The polymerization of this resin is an additional reaction that requires the activation (of an initiator, such as benzoyl peroxide, to produce free radicals, The polymerization process occurs as the free radicals open the double bonds of the methyl methacrylate, (creating a chain reaction in which the monomer at,taches to the polymer free radicals. The conventional curing of denture resin is usually ,accomplished in a waterbath by bringing the temperature to at least I65’ k‘ and holding it there for a period of time. ‘The dimensional change that occurs during the curing ‘of the pol:y(methyl methacrylate) resin is the result of processing shrinkage and has been well documented.2-” Phillips9 stated that the distortion occurring upon sepa-rating the cast from the denture is much greater than any -that will ta.ke place in subsequent clinical service. Curing denture resin with microwave energy was first reported by Nishii in 196% o Conventional heat-curing resins were polymerrzed in a perforated metal flask. The microwave energy, generated in a magnetron oscillator, was transferred to a heating chamber and the flask with resin was irradiated. During irradiation, the microwave energy is .absorbed by the object irradiated and is instantly changed to heat (dielectric heating). l‘he difference between ordinary conduction heating and dielectric heating is that with the dielectric method, the inside and outside of the lsubstance *are heated equally,, and the temperature rises much more quickly. NishiilO concluded that the physical properties of resins cured with microwave irradiation were
.10/l/20551
‘THE
JOURNAI.
OF
PROSTHETIC
DENTISTRY
as satisfactory as those of resins cured with conventional heat-curing techniques. Kimura et al.lls r2 studied the adaptability of heat-curing denture resins cured by microwave energy and those cured by water bath heating. The conventional water bath method consisted of processing the flasked denture bases for 40 minutes at 65” C (149” F) and then boiling them for 30 minutes, rather than processing them at 165” F for 9 hours. The space discrepancy between the microwavecured resin denture bases and the stone casts was smaller than that of bases cured in a water bath, indicating better adaptability. No information was given as to the number of samples included in the study and comparative measurements of linear dimensional accuracy were not made following removal of the denture bases from the casts. Other advantages reported by Kimura et al.,ll but not substantiated, were a shortened dough-forming time, a more homogeneous dough, a shorter curing time, and minimal color changes in the resin. Reitz et a1.l3 compared some of the physical properties of 25 x 12 X 2.5 mm resin strips cured by microwave energy to those cured by conventional water bath heating. A special flask made of fiber-reinforced plastic and polycarhon bolts was used. No statistically significant difference in the hardness and transverse strengths was found. Porosity was discovered in thicker sections of the resin, but was greatly reduced by lowering the wattage, increasing the curing time, and using a rotating table. Bafile et a1.14 compared the total porosity of denture resin cured by microwave energy with denture resin cured by the conventional heat method. They found no significant differences in mean total porosity between the control group and the experimental groups processed with microwave energy and a special liquid (Micro Liquid, H.D. Justi Co., Oxnard, Calif.). When the special liquid was not used,
403
WALLACE
Fig. 1. Reference point locations for resin base deformation measurements.
significant porosity occurred in 10 mm thick sections. Bafile et a1.14concluded that the curing of denture resin by microwave energy could be accomplished without porosity when the special liquid and the appropriate wattage and time combination were used. The conventional hot water bath technique has been considered the best means of processing heat-cured denture resins; however, it does have disadvantages. The method is inefficient because of the relatively long time needed to properly cure the material, and it is not a particularly clean procedure. Microwave processing of denture bases is cleaner and more time efficient, but little information is available about dimensional changes of the bases following separation from the casts, which is the time when the greatest amount of distortion occurs.g This study compares the dimensional accuracy of denture bases cured by microwave energy to bases cured using conventional processing methods.
MATERIAL
AND METHODS
The control group consisted of resin bases polymerized in a Hanau (Hanau Mfg. Co. Inc., Buffalo, N.Y.) curing tank. The four experimental groups consisted of resin bases cured by microwave energy using various wattage/time combinations and the liquids, Micro Liquid. A resin edentulous maxilla with seven reference points served as the master model. Reference point Al was located in the anterior midline of the edentulous ridge, A2 and A3 in the right and left posterior midlines of the edentulous ridge in the second molar regions, Bl and B2 in the midpalatal and posterior palatal midlines, and Cl and C2 in the right and left hamular notches (Fig. 1). Measurements were made from one point on the edentulous ridge to another, from one point in the palate to the other, and from hamular notch to hamular notch. The five dimensions evaluated were: Al-A2, Al-A3, A2-A3, Bl-B2, and Cl-C2. Custom acrylic resin impression trays were made on the
ET AL
master model, using two thicknesses of baseplate wax for relief. Stops were placed in three locations away from the reference points to provide for the accurate seating of the loaded impression tray. Impregum impression material (Premier Dental Products Co., Norristown, Pa.), mixed according to the manufacturer’s recommendations, was used for making impressions of the master model. A syringe was used to inject impression material into the reference points. The impressions were immediately poured with improved dental stone (Indic Die Stone, Columbus Dental, St. Louis, MO.). The stone was vacuum-mixed using distilled water at the recommended water/powder ratio and was allowed to set for 1 hour prior to separation. A total of 50 casts was made. Each was carefully inspected to ensure that the reference points had been duplicated in the stone surface (Fig. 2). Ten casts were made for each of the five test groups, and the dimensions between reference points were measured and recorded. The casts were numbered so that comparative measurements could be made between each cast and its respective resin base. A uniform denture base pattern was made on each cast with a single thickness of baseplate wax.i5 A small rectangular wax extension beyond the posterior border was adapted to the back of the cast to reduce some of the expected shrinkage of the denture base during processing.i6 The casts with wax patterns were flasked in dental stone (Modern Materials, Columbus Dental), with each flasking pour allowed to set for 45 minutes. A thin film of petroleum jelly was applied to the stone between pours. The flasks were placed in boiling water for 5 minutes to soften the wax. The flasks were separated, the wax was eliminated, and the stone was thoroughly cleaned with liquid soap and boiling water. Two coats of Al-tote liquid (The L.D. Caulk Co., Milford, Del.) were applied to the dental stone and allowed to dry. The denture resin (Caulk Ch Lucitone, Characterized, Dentsply International, Inc., York, Pa.) was measured to the ratio of 30 cm3 of powder to 10 ml of liquid and was mixed thoroughly in a clean glass jar. The resin was covered and allowed to set for 20 minutes. It was then trial packed twice in the denture flask and before processing. For the control group (group l), standard metal flasks were placed in a curing tank (Hanau Model JC-1, Hanau Manufacturing Co. Inc.) and cured at 165’ F for 9 hours. For the four microwave experimental groups, the casts were flasked in fiber-reinforced plastic flasks (Miracle Flask, H.D. Justi Co.) (Fig. 3) and polymerized in a microwave oven (Daytron Model DMR-604, Daewoo Electronics Corp. of America, Carlstadt, N.J.) with a built-in turntable and a variable power setting ranging from 86 to 500 W at 2450 MHz. Each flask was placed flat while being polymerized (Fig. 4). The microwave groups were processed as follows: group II-86 W for 13 minutes, then 448 W for 2 minutes; group III-86 W for 6% minutes on each side, then 448 W for 1 minute on each side; group IV-241 W for 10 minutes; and group V-397 W for 2% minutes on each side. Both con-
.mcuRAcYoFnfIci~ow~~~-CURED
Fig. Fig. Fig. Fig.
RESINBASES
2. 3. 4. 5.
Stone cast with reference points reproduced on surface. Fiber-reinforced flasks for microwave processing. Experimental groups were cured on rotating table in microwave Resin base with reference points.
trol and experimlental groups were bench-cured for 1 hour prior to processing and then tlench-cooled for 1 hour after processing ,and prior to deflasking. The processed bases were deflasked, trimmed, and stored in distilled water at roc,m temperature for 1 month. The bases, with positive reference points reproduced in resin (Fig. ,5), were measured in the same manner as the stone casts. Measurements were made under 10 power magnificatilon using an optical comparator (Deltronic DH14 Profile Projector, Deltronic Corp., Santa Ana, Calif.) (Fig. 6) with a traveling stage calibrated to 0.002 mm and a fiberoptic light source that provided surface illumination for the magnifiled reference points. on the casts and resin bases (Fig. 7). Th’e refe:rence points ‘were aligned with the screen grid (Fig. 8) and the comparator automatically measured the straight line distance between the reference points for dimensions being evaluated. Five measuremenls were made for each of the five dimensions and a mean value was calculated. A total of 2500 individual measurements were made on the 50 casts a.nd the 50 bases. The reliability of the method was determined by repeated measures of the same sample on two separate occasions. Based on the statistical analysis of the data, it was determined that each study measurement THEJOURNALOFI'ROWTHETIC
DENTISTRY
oven.
would differ from the true measurement by no more than 0.0034 mm (0.0001 inch), with a 95”( certainty. The measurements made on each resin base were compared only with those made on the corresponding cast from which the base had been made.
METHODS USED EVALUATION
FOR STATISTICAL
Dimensional accuracy was determined by considering the magnitude (absolute value) of the differences between the measurements of the casts and resin bases. The higher values indicated less accuracy. Analysis of variance (ANOVA) was used to investigate differences between the curing methods with respect to this parameter. PostANOVA multiple comparison methods (least significant difference tests) were used to determine which methods differed from others when the ANOVA indicated that differences existed. A level of significance of (p < 0.05) was used in all tests.
RESULTS Significant differences were found between the control group and several of the microwave-processed groups in two of the five dimensions. In dimension Al-A2 (anterior
WALLACE
ET AL
Fig. 6. Optical comparator used for making measurements on casts and resin bases. Fig. 7. Optical comparator with resin base secured on traveling stage.
ing casts. The percentages did vary among the four groups cured with microwave energy; group II, 52% ; group III, 54 % ; group IV, 66 % ; and group V, 44 % .
DISCUSSION
Fig. 8. Reference point on cast under 10 power magnification on screen grid, using fiberoptic illumination with 0.002 mm accuracy.
midline to second molar), microwave-processed groups III and V showed significantly greater dimensional accuracy than the control group. In dimension Cl-C2 (along the posterior palate), microwave-processed groups II, IV, and V showed significantly greater dimensional accuracy than the control group and microwave-processed group III (Table I). No statistically significant differences were found between the control group and the four groups cured with microwave energy in dimensions Al-A3, A2-A3 (second molar to second molar), and Bl-B2 (midpalatal and posterior palatal midlines). There was a general trend toward shrinkage of the 10 denture bases processed by the conventional method, in which 72% of the measurements were smaller than those of the corresponding casts. However, on the 40 denture bases processed with microwave energy, only 54% of the measurements were smaller than those of the correspond-
Two of the microwave-processed groups showed particularly interesting results. Group III samples were irradiated at 86 W for 6% minutes on each side, then at 448 W for 1 minute on each side. They showed significantly greater dimensional accuracy than the control group samples in dimension Al-A2 (from anterior midline to the second molar area), yet they showed significantly less dimensional accuracy than the other three microwave-processed group samples in dimension Cl-C2 (from hamular notch to hamular notch). Perhaps the fact that the microwave oven had to be turned off several times for the flask to be turned over before continuing the polymerization process allowed inconsistencies in the polymerization of the samples from this group. Like those from group III, samples from microwave group V, which were irradiated at 397 W for 21/2minutes on each side, also showed significantly greater dimensional accuracy than the control group samples in dimension AlA2. In fact, group V samples showed some of the lowest dimensional changes measured, being the most accurate in dimensions Bl-B2 and Cl-C2. Group V samples did, however, show the greatest discrepancy among all groups in dimension A2-A3, although this difference was not statistically significant. Questions have been raised concerning the effect of microwave energy on the gypsum investment material and the fiber-reinforced plastic flasks. De Clerckl’ suggested that the gypsum mold within the microwave flask be dessicated to reduce its volume, presumably to allow for volumetric expansion of the gypsum within the flask. Hayden18 expressed concern for the possible expansion created within the mold during the processing of denture resin by micro-
SEPTEMBER
1991
VOLUME
66
NUMBER
3
ACCVRACY
OF MICHOW::VE-CURED
RESIN
BASES
wave energy. He noted that the fiber-reinforced flasks had no expansion-compensating feature, and fractures in the middle part of the three-part flask were observed after only a few curings. This was believed to have resulted from expansion of the resin dough during polymerization and I o a lesser extent from expansion of the gypsum matrix in the flask. Levin et al.lg noted that the flasks break down after processing only several dentul,es. No such fractures in the flasks were observed in this siudy. Kimura et al.“, ‘* SIbserved no significant expansion of the gypsum mold following microwave irradiation. When comparing conventional and microwave heating methods, they noted that ‘differences in the temperature gradient within the gypsum mold existed between the two methods. While a lar,ge discrepancy existed between the temperatures in the center and at the edge of the gypsum investment for the water bath-cured resin samples, no such difference existed within the gypsum mold of the microwave-cured samples. These investigators thought that the small temperature gradient in the gypsum investment resulted in good adaptability of the resins cured by microwave energy to the stone casts. An interesting observation from this study was the relative accuracy of all of the microwave-cured samples, regardless of trea’;ment methods. This could indicate that tlhere may be a range of wattage/time combinations that will be successful, and that the method used could be modified according to the prosthodontic restoration being proaessed, relative to thickness and type of resin used. For example, Bafile et al.‘” f,)und that resin samples polymerized initially at 90 W for i3 minutes followed by 450 W for 2 minutes had the least, porosity for thick resin sections. The use of cast metal frameworks may also require modifications as to the method used. This study shows that the processing of denture resin can be readily accom,plished by microwave energy, and that this curing method provides for excellent dimensional accuracy. Further studies are necessary concerning the effects of microwave energy on the fracture resistance of denture res:.ns, on cast meta. frameworks, on different types of res.ins, on porcelain and resin denture teeth, on previously processed resin, and on residual monomer content.
CLINICALL
SIGNIFICANCE
Because microwavr energy is independent of thermal conductivity, it is a more efficient and convenient method of heating nonthermally conductive materials such as denture resins, which polymerize rapidly. With the microwave technique, it is now possible to consistently process resins of various thicknesses in a much shorter period of time and to be confident of the dimensional accuracy of the prosthesis. With the reductic~n of the time needed for laboratory procedures, some services, such as relining and rebasing, can be done within a matter of several hours. In conclusion: (1) Microwave-processed denture bases
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
I. Absolute value of difference between base and cast for each measurement dimension
Table
Measurement dimension
Treatment
Al-A”
I
group
Nean
+ SE
(mm)
II III‘ IV \.*
.42-A:?
1 II III I\ v
Bl-B2
I II III IV
Cl-c?
I 11t III IV+ vt
0.126 i0.1143 2 0.$&9 ‘Ir.r)h:! _i !).043
0.016 0.013 0.013 0.018
? 0.015
‘These two treatment groups demonstrated greater dimensiorial accurary than did group I (control). tThese treatment groups demonstrated greater riimen>~rnal accuracy than did group I or group III.
had equal or better dimensional accuracy than conventionally processed bases. (2) No specific microwave-processed group showed superior dimensional accuracy compared with any other microwave-processed group. (3) Microwave processing of denture resin is cleaner and more time-efficient than the conventional technique and provides excellent dimensional accuracy. This method should be considered for clinical applications in prosthodontics. REFERENCES 1. Craig RG. Restorative dental materials. irrl rd
accuracy
of denture
Phillip W. Wallace, DDS, Gerald N. Graser, DDS, Michael I,. Myers, DlMD, and Howard M. Proskin, Easttran Dental Center, Rochester, N.Y.
resin
cured
by microwave
MS, PhD
Microwave processing has several time-saving advantages over conventional processing of denture base resins. Little is known about the adaptation of bases processed by microwave radiation to the cast and the mouth. Comparisons were made in five regions of the palate and ridge. The microwave-processed denture bases had equal or better dimensional accuracy than conventionally processed bases. (J F'ROSTHETDEXT 1991;66:403-9.)
A
cryhc polymers were first introduced as denture base materials in 1937.’ Poly(methy1 methacrylate) has been the resin most commonly used to make removable complete and partial denture.3. The polymerization of this resin is an additional reaction that requires the activation (of an initiator, such as benzoyl peroxide, to produce free radicals, The polymerization process occurs as the free radicals open the double bonds of the methyl methacrylate, (creating a chain reaction in which the monomer at,taches to the polymer free radicals. The conventional curing of denture resin is usually ,accomplished in a waterbath by bringing the temperature to at least I65’ k‘ and holding it there for a period of time. ‘The dimensional change that occurs during the curing ‘of the pol:y(methyl methacrylate) resin is the result of processing shrinkage and has been well documented.2-” Phillips9 stated that the distortion occurring upon sepa-rating the cast from the denture is much greater than any -that will ta.ke place in subsequent clinical service. Curing denture resin with microwave energy was first reported by Nishii in 196% o Conventional heat-curing resins were polymerrzed in a perforated metal flask. The microwave energy, generated in a magnetron oscillator, was transferred to a heating chamber and the flask with resin was irradiated. During irradiation, the microwave energy is .absorbed by the object irradiated and is instantly changed to heat (dielectric heating). l‘he difference between ordinary conduction heating and dielectric heating is that with the dielectric method, the inside and outside of the lsubstance *are heated equally,, and the temperature rises much more quickly. NishiilO concluded that the physical properties of resins cured with microwave irradiation were
.10/l/20551
‘THE
JOURNAI.
OF
PROSTHETIC
DENTISTRY
as satisfactory as those of resins cured with conventional heat-curing techniques. Kimura et al.lls r2 studied the adaptability of heat-curing denture resins cured by microwave energy and those cured by water bath heating. The conventional water bath method consisted of processing the flasked denture bases for 40 minutes at 65” C (149” F) and then boiling them for 30 minutes, rather than processing them at 165” F for 9 hours. The space discrepancy between the microwavecured resin denture bases and the stone casts was smaller than that of bases cured in a water bath, indicating better adaptability. No information was given as to the number of samples included in the study and comparative measurements of linear dimensional accuracy were not made following removal of the denture bases from the casts. Other advantages reported by Kimura et al.,ll but not substantiated, were a shortened dough-forming time, a more homogeneous dough, a shorter curing time, and minimal color changes in the resin. Reitz et a1.l3 compared some of the physical properties of 25 x 12 X 2.5 mm resin strips cured by microwave energy to those cured by conventional water bath heating. A special flask made of fiber-reinforced plastic and polycarhon bolts was used. No statistically significant difference in the hardness and transverse strengths was found. Porosity was discovered in thicker sections of the resin, but was greatly reduced by lowering the wattage, increasing the curing time, and using a rotating table. Bafile et a1.14 compared the total porosity of denture resin cured by microwave energy with denture resin cured by the conventional heat method. They found no significant differences in mean total porosity between the control group and the experimental groups processed with microwave energy and a special liquid (Micro Liquid, H.D. Justi Co., Oxnard, Calif.). When the special liquid was not used,
403
WALLACE
Fig. 1. Reference point locations for resin base deformation measurements.
significant porosity occurred in 10 mm thick sections. Bafile et a1.14concluded that the curing of denture resin by microwave energy could be accomplished without porosity when the special liquid and the appropriate wattage and time combination were used. The conventional hot water bath technique has been considered the best means of processing heat-cured denture resins; however, it does have disadvantages. The method is inefficient because of the relatively long time needed to properly cure the material, and it is not a particularly clean procedure. Microwave processing of denture bases is cleaner and more time efficient, but little information is available about dimensional changes of the bases following separation from the casts, which is the time when the greatest amount of distortion occurs.g This study compares the dimensional accuracy of denture bases cured by microwave energy to bases cured using conventional processing methods.
MATERIAL
AND METHODS
The control group consisted of resin bases polymerized in a Hanau (Hanau Mfg. Co. Inc., Buffalo, N.Y.) curing tank. The four experimental groups consisted of resin bases cured by microwave energy using various wattage/time combinations and the liquids, Micro Liquid. A resin edentulous maxilla with seven reference points served as the master model. Reference point Al was located in the anterior midline of the edentulous ridge, A2 and A3 in the right and left posterior midlines of the edentulous ridge in the second molar regions, Bl and B2 in the midpalatal and posterior palatal midlines, and Cl and C2 in the right and left hamular notches (Fig. 1). Measurements were made from one point on the edentulous ridge to another, from one point in the palate to the other, and from hamular notch to hamular notch. The five dimensions evaluated were: Al-A2, Al-A3, A2-A3, Bl-B2, and Cl-C2. Custom acrylic resin impression trays were made on the
ET AL
master model, using two thicknesses of baseplate wax for relief. Stops were placed in three locations away from the reference points to provide for the accurate seating of the loaded impression tray. Impregum impression material (Premier Dental Products Co., Norristown, Pa.), mixed according to the manufacturer’s recommendations, was used for making impressions of the master model. A syringe was used to inject impression material into the reference points. The impressions were immediately poured with improved dental stone (Indic Die Stone, Columbus Dental, St. Louis, MO.). The stone was vacuum-mixed using distilled water at the recommended water/powder ratio and was allowed to set for 1 hour prior to separation. A total of 50 casts was made. Each was carefully inspected to ensure that the reference points had been duplicated in the stone surface (Fig. 2). Ten casts were made for each of the five test groups, and the dimensions between reference points were measured and recorded. The casts were numbered so that comparative measurements could be made between each cast and its respective resin base. A uniform denture base pattern was made on each cast with a single thickness of baseplate wax.i5 A small rectangular wax extension beyond the posterior border was adapted to the back of the cast to reduce some of the expected shrinkage of the denture base during processing.i6 The casts with wax patterns were flasked in dental stone (Modern Materials, Columbus Dental), with each flasking pour allowed to set for 45 minutes. A thin film of petroleum jelly was applied to the stone between pours. The flasks were placed in boiling water for 5 minutes to soften the wax. The flasks were separated, the wax was eliminated, and the stone was thoroughly cleaned with liquid soap and boiling water. Two coats of Al-tote liquid (The L.D. Caulk Co., Milford, Del.) were applied to the dental stone and allowed to dry. The denture resin (Caulk Ch Lucitone, Characterized, Dentsply International, Inc., York, Pa.) was measured to the ratio of 30 cm3 of powder to 10 ml of liquid and was mixed thoroughly in a clean glass jar. The resin was covered and allowed to set for 20 minutes. It was then trial packed twice in the denture flask and before processing. For the control group (group l), standard metal flasks were placed in a curing tank (Hanau Model JC-1, Hanau Manufacturing Co. Inc.) and cured at 165’ F for 9 hours. For the four microwave experimental groups, the casts were flasked in fiber-reinforced plastic flasks (Miracle Flask, H.D. Justi Co.) (Fig. 3) and polymerized in a microwave oven (Daytron Model DMR-604, Daewoo Electronics Corp. of America, Carlstadt, N.J.) with a built-in turntable and a variable power setting ranging from 86 to 500 W at 2450 MHz. Each flask was placed flat while being polymerized (Fig. 4). The microwave groups were processed as follows: group II-86 W for 13 minutes, then 448 W for 2 minutes; group III-86 W for 6% minutes on each side, then 448 W for 1 minute on each side; group IV-241 W for 10 minutes; and group V-397 W for 2% minutes on each side. Both con-
.mcuRAcYoFnfIci~ow~~~-CURED
Fig. Fig. Fig. Fig.
RESINBASES
2. 3. 4. 5.
Stone cast with reference points reproduced on surface. Fiber-reinforced flasks for microwave processing. Experimental groups were cured on rotating table in microwave Resin base with reference points.
trol and experimlental groups were bench-cured for 1 hour prior to processing and then tlench-cooled for 1 hour after processing ,and prior to deflasking. The processed bases were deflasked, trimmed, and stored in distilled water at roc,m temperature for 1 month. The bases, with positive reference points reproduced in resin (Fig. ,5), were measured in the same manner as the stone casts. Measurements were made under 10 power magnificatilon using an optical comparator (Deltronic DH14 Profile Projector, Deltronic Corp., Santa Ana, Calif.) (Fig. 6) with a traveling stage calibrated to 0.002 mm and a fiberoptic light source that provided surface illumination for the magnifiled reference points. on the casts and resin bases (Fig. 7). Th’e refe:rence points ‘were aligned with the screen grid (Fig. 8) and the comparator automatically measured the straight line distance between the reference points for dimensions being evaluated. Five measuremenls were made for each of the five dimensions and a mean value was calculated. A total of 2500 individual measurements were made on the 50 casts a.nd the 50 bases. The reliability of the method was determined by repeated measures of the same sample on two separate occasions. Based on the statistical analysis of the data, it was determined that each study measurement THEJOURNALOFI'ROWTHETIC
DENTISTRY
oven.
would differ from the true measurement by no more than 0.0034 mm (0.0001 inch), with a 95”( certainty. The measurements made on each resin base were compared only with those made on the corresponding cast from which the base had been made.
METHODS USED EVALUATION
FOR STATISTICAL
Dimensional accuracy was determined by considering the magnitude (absolute value) of the differences between the measurements of the casts and resin bases. The higher values indicated less accuracy. Analysis of variance (ANOVA) was used to investigate differences between the curing methods with respect to this parameter. PostANOVA multiple comparison methods (least significant difference tests) were used to determine which methods differed from others when the ANOVA indicated that differences existed. A level of significance of (p < 0.05) was used in all tests.
RESULTS Significant differences were found between the control group and several of the microwave-processed groups in two of the five dimensions. In dimension Al-A2 (anterior
WALLACE
ET AL
Fig. 6. Optical comparator used for making measurements on casts and resin bases. Fig. 7. Optical comparator with resin base secured on traveling stage.
ing casts. The percentages did vary among the four groups cured with microwave energy; group II, 52% ; group III, 54 % ; group IV, 66 % ; and group V, 44 % .
DISCUSSION
Fig. 8. Reference point on cast under 10 power magnification on screen grid, using fiberoptic illumination with 0.002 mm accuracy.
midline to second molar), microwave-processed groups III and V showed significantly greater dimensional accuracy than the control group. In dimension Cl-C2 (along the posterior palate), microwave-processed groups II, IV, and V showed significantly greater dimensional accuracy than the control group and microwave-processed group III (Table I). No statistically significant differences were found between the control group and the four groups cured with microwave energy in dimensions Al-A3, A2-A3 (second molar to second molar), and Bl-B2 (midpalatal and posterior palatal midlines). There was a general trend toward shrinkage of the 10 denture bases processed by the conventional method, in which 72% of the measurements were smaller than those of the corresponding casts. However, on the 40 denture bases processed with microwave energy, only 54% of the measurements were smaller than those of the correspond-
Two of the microwave-processed groups showed particularly interesting results. Group III samples were irradiated at 86 W for 6% minutes on each side, then at 448 W for 1 minute on each side. They showed significantly greater dimensional accuracy than the control group samples in dimension Al-A2 (from anterior midline to the second molar area), yet they showed significantly less dimensional accuracy than the other three microwave-processed group samples in dimension Cl-C2 (from hamular notch to hamular notch). Perhaps the fact that the microwave oven had to be turned off several times for the flask to be turned over before continuing the polymerization process allowed inconsistencies in the polymerization of the samples from this group. Like those from group III, samples from microwave group V, which were irradiated at 397 W for 21/2minutes on each side, also showed significantly greater dimensional accuracy than the control group samples in dimension AlA2. In fact, group V samples showed some of the lowest dimensional changes measured, being the most accurate in dimensions Bl-B2 and Cl-C2. Group V samples did, however, show the greatest discrepancy among all groups in dimension A2-A3, although this difference was not statistically significant. Questions have been raised concerning the effect of microwave energy on the gypsum investment material and the fiber-reinforced plastic flasks. De Clerckl’ suggested that the gypsum mold within the microwave flask be dessicated to reduce its volume, presumably to allow for volumetric expansion of the gypsum within the flask. Hayden18 expressed concern for the possible expansion created within the mold during the processing of denture resin by micro-
SEPTEMBER
1991
VOLUME
66
NUMBER
3
ACCVRACY
OF MICHOW::VE-CURED
RESIN
BASES
wave energy. He noted that the fiber-reinforced flasks had no expansion-compensating feature, and fractures in the middle part of the three-part flask were observed after only a few curings. This was believed to have resulted from expansion of the resin dough during polymerization and I o a lesser extent from expansion of the gypsum matrix in the flask. Levin et al.lg noted that the flasks break down after processing only several dentul,es. No such fractures in the flasks were observed in this siudy. Kimura et al.“, ‘* SIbserved no significant expansion of the gypsum mold following microwave irradiation. When comparing conventional and microwave heating methods, they noted that ‘differences in the temperature gradient within the gypsum mold existed between the two methods. While a lar,ge discrepancy existed between the temperatures in the center and at the edge of the gypsum investment for the water bath-cured resin samples, no such difference existed within the gypsum mold of the microwave-cured samples. These investigators thought that the small temperature gradient in the gypsum investment resulted in good adaptability of the resins cured by microwave energy to the stone casts. An interesting observation from this study was the relative accuracy of all of the microwave-cured samples, regardless of trea’;ment methods. This could indicate that tlhere may be a range of wattage/time combinations that will be successful, and that the method used could be modified according to the prosthodontic restoration being proaessed, relative to thickness and type of resin used. For example, Bafile et al.‘” f,)und that resin samples polymerized initially at 90 W for i3 minutes followed by 450 W for 2 minutes had the least, porosity for thick resin sections. The use of cast metal frameworks may also require modifications as to the method used. This study shows that the processing of denture resin can be readily accom,plished by microwave energy, and that this curing method provides for excellent dimensional accuracy. Further studies are necessary concerning the effects of microwave energy on the fracture resistance of denture res:.ns, on cast meta. frameworks, on different types of res.ins, on porcelain and resin denture teeth, on previously processed resin, and on residual monomer content.
CLINICALL
SIGNIFICANCE
Because microwavr energy is independent of thermal conductivity, it is a more efficient and convenient method of heating nonthermally conductive materials such as denture resins, which polymerize rapidly. With the microwave technique, it is now possible to consistently process resins of various thicknesses in a much shorter period of time and to be confident of the dimensional accuracy of the prosthesis. With the reductic~n of the time needed for laboratory procedures, some services, such as relining and rebasing, can be done within a matter of several hours. In conclusion: (1) Microwave-processed denture bases
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
I. Absolute value of difference between base and cast for each measurement dimension
Table
Measurement dimension
Treatment
Al-A”
I
group
Nean
+ SE
(mm)
II III‘ IV \.*
.42-A:?
1 II III I\ v
Bl-B2
I II III IV
Cl-c?
I 11t III IV+ vt
0.126 i0.1143 2 0.$&9 ‘Ir.r)h:! _i !).043
0.016 0.013 0.013 0.018
? 0.015
‘These two treatment groups demonstrated greater dimensiorial accurary than did group I (control). tThese treatment groups demonstrated greater riimen>~rnal accuracy than did group I or group III.
had equal or better dimensional accuracy than conventionally processed bases. (2) No specific microwave-processed group showed superior dimensional accuracy compared with any other microwave-processed group. (3) Microwave processing of denture resin is cleaner and more time-efficient than the conventional technique and provides excellent dimensional accuracy. This method should be considered for clinical applications in prosthodontics. REFERENCES 1. Craig RG. Restorative dental materials. irrl rd
