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Improved fit of maxillary complete dentures processed on high expansion stone casts
Improved fit of maxillary complete dentures processed on high expansion stone casts Oskar Sykora, CDT, DDS, PhD," and Elliott J. Sutow, BS, PhD, MEd b Oalhousie University, Faculty of Dentistry, Halifax, N. S., Canada P u r p o s e . Although relatively well-fitting dentures are produced at low cost and with easy-to-manipulate material, current denture base materials are not ideal. Because acrylic resin complete and removable partial dentures change dimensionally as a result of polymerization and thermal contractions, a high expansion stone was tested in this study to determine its ability to compensate for some of the dimensional change. Material a n d M e t h o d s . Maxillary complete dentures were processed on type III dental stone and high expansion stone casts. The accuracy of fit along the posterior palatal border was measured and compared after the prostheses were trimmed and polished and after their immersion in water at 23 ° C at 1-day and 1week intervals. Results. Statistical analysis of the data revealed that at every measurement interval the maxillary complete dentures processed on high expansion stone had posterior palatal border openings that were significantly smaller when compared with type III dental stone (p <0.001). (J Prosthet Dent 1997;77:205-8.)
Acrylic polymers were introduced to the dental profession in the late 1930s. By 1940, 95% o f all denture bases were made o f methyl methacrylate polymers and copolymers.1 However, acrylic resin shrinks substantially on curing and cooling, thereby inducing stresses that are released over time. M t h o u g h relatively well-fitting dentures are produced at low cost and with easy manipulation, current denture base materials are not ideal in every respect. Thus, there is need for further research and development. 2 This was acknowledged over 50 years ago by Sldnner and Cooper, 3 who suggested that a certain lack o f dimensional stability must bc accepted as one o f the disadvantages o f acrylic resin dentures. The increase in the dimension o f the denture during immersion in water or saliva does not compensate for the processing shrinkage. 4 Some o f the dimensional changes in the acrylic resin are caused by the inherent properties o f the material itself. Efforts have been made to improve the physical p r o p e r t i e s o f t h e material by modifying the acrylic polymer, for example, through copolymerization, s Investing and processing procedures were developed, such as direct and trial-pack techniques, dry and wet curing, pour techniques, and injection techniques. 6q8 As early as 1963, Peyton and Anthony 19 questioned many o f the claims o f superior properties, improved accuracy o f fit, and greater dimensional stability in seraProfessor, Department of Dental Clinical Sciences. bprofessor, Department of Applied Oral Sciences. FEBRUARY 1997
vice. They believed that the full significance o f these claims about the function and performance o f the denture in service were not always made clear. In 1989, Takamata and Setcos 2° reviewed investigations o f the accuracy o f acrylic resins for making dentures and concluded that irrespective o f the processing techniques or acrylic resin chosen, three-dimensional changes o f the internal surface o f the denture will occur. Shrinkage is particularly noticeable in the posterior palatal border region, where the retentive seal and stability of the prostheses can become compromised. This problem has been studied by many investigators and various techniques have been advocatedY 23 The first unavoidable dimensional change in every acrylic resin prosthesis is shrinkage that occurs during processing and finishing. The second change, expansion, occurs when the dentures are either stored in a water bath or inserted in the m o u t h and then absorb oral fluids?4 28 Since it has been determined that a typical acrylic resin complete denture may require a period o f almost 17 days to become fully saturated when immersed in water at r o o m temperature, 29 it is safe to assume that the majority o f prostheses are delivered to patients before this state is achieved. One o f the clinical criteria for a successful prosthesis is its accurate adaptation to the denture-bearing area. Little research has been conducted on the influence o f the type o f dental stone used to fabricate the cast. A previous study demonstrated that the high expansion stone produced smaller posterior palatal border openTHE JOURNAL OF PROSTHETIC DENTISTRY 205
THE J O U R N A L O F P R O S T H E T I C D E N T I S T R Y
SYKORA AND SUTOW
hAiHlin~
E:::k o3 i,,m
(!)
o
Fig. 1. Posterior view of cast shows five measurement locations.
ings3°; however, that study examined acrylic resin bases, not complete dentures. The purpose o f this study was to investigate whether the use o f a high expansion stone could achieve greater accuracy in a complete denture prosthesis.
0 IIB
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O
T r i m m e d a n d Polished (23°C) 9 9 % C o n f i d e n c e Intervals 900 I 800/ /
700~/
60o I -
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High Expansion Stone
I
]-
I
400t; oo
"r
:
1000
i 10
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i Midline
i 5
110
I
M e a s u r e m e n t Location, mm left and right of midline
MATERIAL
AND METHODS
Thirty edentulous maxillary master casts were made from a master mold that represented the patient. The mold material was unvulcanized dimethyl siloxane filler mixture with the addition o f silicone, a synthetic rubber with a linear contraction o f 0.08% (Deguform, Degussa Co., South Plainfield, N. J.). The casts were poured according to the manufacturer's directions in type III dental stone (Labstone Buff, M o d e r n Materials, Columbus Dental, St. Louis, Mo.). Thirty additional edentulous maxillary casts were made from the same master mold and were used for processing the complete dentures. Fifteen casts were poured in type III dental stone (Labstone Buff, M o d e r n Materials, Columbus Dental) as recommended by the manufacturer. The remaining 15 casts were made from a high expansion stone material (Model 1 material, Ivoclar A.G., Schaan, Liechtenstein). M1 o f the casts were poured from vacuum-spatulated stone by one examiner. A 2 mm thick metal transferable shim (Vitallium, Austenal Inc., Chicago, Ill.) was constructed to control thickness o f the denture bases. A modified Hanau remount jig was used to m o u n t the casts on an articulator (Teledyne Hanau, Buffalo, N. Y.). Acrylic resin posterior anatomical teeth (mold T4, shade 3A) and anterior teeth (mold A42, shade 3A) (Ivoclar A. G.) were set in a transferable remount stone index and sealed to the base. The maxillary dentures were then invested, tasked, processed with acrylic resin (Lucitone 199, Dentsply, York, Pa.), and cured for 9 hours at 73 ° C. After deflasking and decasting, each denture was trimmed and polished. Because the cast, which represented an elderly edentulous situation, had no undercuts, each prosthesis was then carefully seated by one examiner with a gentle pressure on the type III dental stone master cast, which was m o u n t e d on a tilt-top table o f a dental surveyor (Ney Co., Hartford, Conn.). An acrylic resin preformed cast holder with a cast survey level allowed consistent orientation. Adaptation between the inner (palatal) surface o f the 206
Fig. 2. Simultaneous 99% confidence intervals for posterior palatal border opening means at trimmed-and-polished measurement interval. maxillary denture and the master cast was measured perpendicular to the cast along the posterior palatal border with a traveling microscope (0.001 mm resolution, Be& Mfr., ). Measurements were made (23 ° C) at the midline and at 5 and 10 m m on each side o f the midline after trimming and polishing. The midline was determined by following a line from the incisal papilla along the median palatine raphe to the posterior border of the cast (Fig. 1). The casts were then immersed in water at 23 ° C for 24 hours and 1 week. All measurements were made by one examiner. An estimate o f the precision o f the cast dimensions was made by measuring eight casts o f each stone along the posterior palatal base area and calculating the coefficient o f variation. RESULTS The coefficient o f variation associated with producing type III dental stone and high expansion stone casts from the master mold was 0.2% for both materials. The amount o f posterior border opening at each measurement location for the various measurement times for both groups o f complete dentures is illustrated in Figures 2 through 4. The data were analyzed by use o f the ranks o f the observations, and the resulting mean vectors were compared by the T 2 test for two independent multivariate populations. 31 Statistical analysis revealed that at each measurement period the mean posterior palatal border openings were significantly different for the dentures produced on high expansion stone versus type III dental stone (p <0.001). Figures 2 through 4 show the simultaneous 99% confidence intervals for the means over the three measurement periods at the five measurement locations. The confidence intervals were produced by a Bonferroni correction. Figures 2 through 4 also demonstrate that none o f the confidence intervals overlap. V O L U M E 77
NUMBER2
SYKORA AND SUTOW
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THE J O U R N A L O F P R O S T H E T I C D E N T I S T R Y
24 H o u r s I m m e r s i o n in W a t e r (23°C) 9 9 % C o n f i d e n c e Intervals -m
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? H
High Expansion Stone H
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. i
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5
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Measurement Location, mm left and right of midline
"'
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i 5
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I 5
l J0
Measurement Location, mm left and right of midline
Fig. 3. Simultaneous 99% confidence intervals for posterior palatal border opening means at 24-hour immersion measurement interval.
Fig. 4. Simultaneous 99% confidence intervals for posterior palatal border opening means at 1-week immersion measurement interval.
At each measurement location the mean openings for the high expansion stone were consistently at least 50% smaller than those for the type 1II dental stone. A Student-Newman-Keuls multiple comparison test was used to determine whether immersion in water had an effect on the posterior palatal border opening over time. 32 Analysis o f the five measurement locations showed no statistical differences caused by immersion for the dentures processed on either stone (p = 0.05).
type III dental stone are 80 x 10-6/°C and 10 x 10-6/ °C, respectively. According to Marx, who assumed a temperature change from 100 ° to 20 ° C, the contraction of the acrylic resin base is approximately 0.32 mm, but only 0.04 mm for the type III dental stone. Thus, the difference in thermal contraction between these two materials would be 0.28 mm. as Although use o f the high expansion stone improved the fit o f the complete dentures along the posterior palatal border compared with type III dental stone, immersion in water for up to 1 week indicated no statistically significant effect. Theoretically, water sorption can help compensate for processing shrinkage by expanding the denture. Nevertheless, observations in this study were in agreement with other studies that showed no significant dimensional change for 1-week immersion in water for dentures produced by the trial-pack technique 1*and for storage in water up to 30 days. 13 With the high expansion stone, it is theoretically possible that it could result in a complete maxillary denture that is too tight in some other area; however, this theory requires further investigation.
DISCUSSION According to Anthony and Peyton, a3 the simplest method o f evaluating the fit o f dentures is to place them lightly on a master cast and observe the fit. The greatest discrepancy for a maxillary complete denture will be seen in the central portion o f the posterior maxillary border. 33 Analysis o f the data revealed that the maxillary dentures processed on high expansion stone casts produced openings along the posterior palatal border o f the prostheses that were substantially smaller compared with the dentures processed on type III dental stone. One o f the principal factors in denture stability and retention is a minimum space between the supporting structures and the prosthesis. Historically, the physical property o f dental stone setting expansion was kept to a •minimum to accurately capture the oral structures that were registered by an impression material. Thus, a typical type i i i dental stone would have a setting expansion o f only 0.12%. In contrast, the high expansion stone tested has a setting expansion o f 0.55% after 2 hours and 0.59% after 24 hours. 34 This study demonstrated that expansion o f the stone can help compensate for shrinkage that occurs as a result o f thermal contraction o f the acrylic resin material. Marx as used a hypothetical cast that has 50 m m between the alveolar crests; the coefficient o f thermal expansion o f the acrylic resin and FEBRUARY 1997
CONCLUSIONS It was concluded that the high expansion stone compensated to a substantial degree for the shrinkage that occurs as a rcsult o f the processing o f the acrylic resin material. Compared with the prostheses that were processed on type III dental stone casts, the use o f the high expansion stone significantly (p <0.001) reduced the size of the posterior palatal border openings o f the maxillary dentures by at least 50%. REFERENCES 1. Peyton FA. History of resins in dentistry• Dent Clin North Am 1975;9:21122. 207
THE J O U R N A L OF PROSTHETIC DENTISTRY
2. Mutlu G, Harrison A, Huggett R. A history of denture base materials. Quintessence Dent Tech 1989;13:145-50. 3. Skinner EW, Cooper EN. Physical properties of denture resins: part I. Curing shrinkage and water sorption. J Am Dent Assoc 1943;30:1845-52. 4. Skinner EW. Acrylic denture base materials: their physical properties and manipulation. J Prosthet Dent 1951 ;I :161-7. 5. El Ghazaii S, Glantz PO, Randow K. On the clinical deformation of maxillary complete dentures, influence of the processing techniques of acrylate-based polymers. Acta Odontol Scand 1988;46:287-95. 6. Woelfel JB, Paffenbarger GC, Sweeney WT. Dimensional changes occurring in dentures during processing. J Am Dent Assoc 1960;16:413-30. 7. Mirza FD. Dimensional stability of acrylic resin dentures: clinical evalua tion. J Prosthet Dent 1961 ;11:848-57. 8. Paffenbarger GC, Woelfel JB, Sweeney WT. Resins and technics used in constructing dentures. Washington, D.C.: National Bureau of Standards, 1964:251-62. 9. Becket CM, Smith DE, Nicholls JI. The comparison of denture-base processing techniques. Part 11. Dimensional changes due to processing. J Prosthet Dent 1977;37:450-9. 10. Bessing C, Nilsson B, Bergman M. SR 3/60 and SR-Ivocap. A comparison between two heat cured denture base resins with dissimilar processings. Swed Dent J 1979;3:221-8. 11. Knott B, Randall D, Bell G, Satgurnathan R, Bates IF, Huggett R. Are present denture base materials and standards satisfactory? Br Dent,i 1988;165:198200. 12. Huggett R, Bates JF, Knott NJ. A comparison of some properties of den ture base acrylic resin polymerized by dry and wet curing systems. Quintessence Dent Technol 1987;11:265-9. 13. Anderson GC, Schulte JK, Arnold TG. Dimensional stability of injection and conventional processing of denture base acrylic resin. J Prosthet Dent
1988;60:394-8. 14. 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;4:20-4. 15. Jackson AD, Grisius RJ, Fenster RK, Lang BR. Dimensional accuracy of two denture base processing methods. Int,i Prosthodont 1989;2:421-8. 16. Latta GH Jr, Bowles WF Ill, Conkin JE. Three-dimensional stability of new denture base resin systems. J Prosthet Dent 1990;63:65&61. 17. Sykora O, Sutow EJ.Comparison of the dimensional stability of two waxes and two acrylic resin processing techniques in the production of complete dentures. J Oral Rehabil 1990;17:219-27. 18. Sykora O, Sutow EJ. Posterior palatal seal adaptation: influence of processing technique, palate shape and immersion. J Oral Rehabi11993;20:1931. 19. Peyton FA, Anthony DH. Evolution of dentures processed by different techniques. J Prosthet Dent 1963;13:269-82.
208
SYKORA A N D S U T O W
20. Takamata T, Setcos JC. Resin denture bases: review of accuracy and methods of polymerization. Int J Prosthodont 1989;2:555-62. 21. Naylor WP, Rempala JD. The posterior palatal seal--its forms and functions. (11) -Design and cast preparation. Quintessence Dent Technol 1986;10:489-92. 22. Johnson DL, Duncanson MG Jr. The plastic postpalatal denture seal. Quintessence Int 1987;18:457-62. 23. Jow J. Mechanical undercuts as a means of decreasing shrinkage in the postpalatal sea[ region of the maxillary denture. J Prosthet Dent 1989;62:110-5. 24. Woelfel Jg, Paffenbarger GC, Sweeney WT. Changes in dentures during storage in water and in service. J Am Dent Assoc 1961 ;62:643-57. 25. Woelfel ,IB, Paffenbarger GC, Sweeney WT. Dimensional changes in complete dentures on drying, wetting and heating in water. J Am Dent Assoc 1962;65:495-505. 26. Braden M. The absorption of water by acrylic resins and other materials. J Prosthet Dent 1964;14:307-16. 27. Stafford GD, Braden M. Water absorption of some denture base polymers. J Dent Res 1968;47:341. 28. Patel MP, Braden M. Heterocyc[ic methacry]ates for clinical applications. III. Water absorption characteristics. Biomaterials 1991 ;12:653-7. 29. Ristic B, Carr L. Water sorption by denture acrylic resin and consequent changes in vertical dimension. J Prosthet Dent 1987;58:689-93. 30. Sykora O, Sutow EJ. Posterior palatal seal adaptation: influence of a high expansion stone. J Oral Rehabi11996;23:342-5. 31. Johnson RA, Wickern DW. Applied multivariate statistical analyses. Englewood Cliffs, N,I: Prentice Hall, 1993. 32. Woolf CM. Principles of biometry. New York: D. Van Nostrand Co Inc, 1968. 33. Anthony DH, Peyton FA. Dimensional accuracy of various denture-base materials. J Prosthet Dent 1962;12:67-81. 34. Ivoclar AG. Product data report. Schaan, Liechtenstein: July 6, 1985. 35. Marx H. Herste[lungverfahren und Passform von Kunstoffvo[[prosthesen. Dent Labor 1975;23:591-5.
Reprint requests to: DR. O. SYKORA DALHOUStEUNIVERSITY FACULTYOF DENTISTRY HALIFAX,NOVASCOTIA B3H 3J4 CANADA Copyright © 1997 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/97/$5.00+0. 10/1/78114
VOLUME 77 NUMBER2
Acrylic polymers were introduced to the dental profession in the late 1930s. By 1940, 95% o f all denture bases were made o f methyl methacrylate polymers and copolymers.1 However, acrylic resin shrinks substantially on curing and cooling, thereby inducing stresses that are released over time. M t h o u g h relatively well-fitting dentures are produced at low cost and with easy manipulation, current denture base materials are not ideal in every respect. Thus, there is need for further research and development. 2 This was acknowledged over 50 years ago by Sldnner and Cooper, 3 who suggested that a certain lack o f dimensional stability must bc accepted as one o f the disadvantages o f acrylic resin dentures. The increase in the dimension o f the denture during immersion in water or saliva does not compensate for the processing shrinkage. 4 Some o f the dimensional changes in the acrylic resin are caused by the inherent properties o f the material itself. Efforts have been made to improve the physical p r o p e r t i e s o f t h e material by modifying the acrylic polymer, for example, through copolymerization, s Investing and processing procedures were developed, such as direct and trial-pack techniques, dry and wet curing, pour techniques, and injection techniques. 6q8 As early as 1963, Peyton and Anthony 19 questioned many o f the claims o f superior properties, improved accuracy o f fit, and greater dimensional stability in seraProfessor, Department of Dental Clinical Sciences. bprofessor, Department of Applied Oral Sciences. FEBRUARY 1997
vice. They believed that the full significance o f these claims about the function and performance o f the denture in service were not always made clear. In 1989, Takamata and Setcos 2° reviewed investigations o f the accuracy o f acrylic resins for making dentures and concluded that irrespective o f the processing techniques or acrylic resin chosen, three-dimensional changes o f the internal surface o f the denture will occur. Shrinkage is particularly noticeable in the posterior palatal border region, where the retentive seal and stability of the prostheses can become compromised. This problem has been studied by many investigators and various techniques have been advocatedY 23 The first unavoidable dimensional change in every acrylic resin prosthesis is shrinkage that occurs during processing and finishing. The second change, expansion, occurs when the dentures are either stored in a water bath or inserted in the m o u t h and then absorb oral fluids?4 28 Since it has been determined that a typical acrylic resin complete denture may require a period o f almost 17 days to become fully saturated when immersed in water at r o o m temperature, 29 it is safe to assume that the majority o f prostheses are delivered to patients before this state is achieved. One o f the clinical criteria for a successful prosthesis is its accurate adaptation to the denture-bearing area. Little research has been conducted on the influence o f the type o f dental stone used to fabricate the cast. A previous study demonstrated that the high expansion stone produced smaller posterior palatal border openTHE JOURNAL OF PROSTHETIC DENTISTRY 205
THE J O U R N A L O F P R O S T H E T I C D E N T I S T R Y
SYKORA AND SUTOW
hAiHlin~
E:::k o3 i,,m
(!)
o
Fig. 1. Posterior view of cast shows five measurement locations.
ings3°; however, that study examined acrylic resin bases, not complete dentures. The purpose o f this study was to investigate whether the use o f a high expansion stone could achieve greater accuracy in a complete denture prosthesis.
0 IIB
Q. .m
O
T r i m m e d a n d Polished (23°C) 9 9 % C o n f i d e n c e Intervals 900 I 800/ /
700~/
60o I -
500
•
Type III Dental Stone
•
High Expansion Stone
I
]-
I
400t; oo
"r
:
1000
i 10
~ 5
i Midline
i 5
110
I
M e a s u r e m e n t Location, mm left and right of midline
MATERIAL
AND METHODS
Thirty edentulous maxillary master casts were made from a master mold that represented the patient. The mold material was unvulcanized dimethyl siloxane filler mixture with the addition o f silicone, a synthetic rubber with a linear contraction o f 0.08% (Deguform, Degussa Co., South Plainfield, N. J.). The casts were poured according to the manufacturer's directions in type III dental stone (Labstone Buff, M o d e r n Materials, Columbus Dental, St. Louis, Mo.). Thirty additional edentulous maxillary casts were made from the same master mold and were used for processing the complete dentures. Fifteen casts were poured in type III dental stone (Labstone Buff, M o d e r n Materials, Columbus Dental) as recommended by the manufacturer. The remaining 15 casts were made from a high expansion stone material (Model 1 material, Ivoclar A.G., Schaan, Liechtenstein). M1 o f the casts were poured from vacuum-spatulated stone by one examiner. A 2 mm thick metal transferable shim (Vitallium, Austenal Inc., Chicago, Ill.) was constructed to control thickness o f the denture bases. A modified Hanau remount jig was used to m o u n t the casts on an articulator (Teledyne Hanau, Buffalo, N. Y.). Acrylic resin posterior anatomical teeth (mold T4, shade 3A) and anterior teeth (mold A42, shade 3A) (Ivoclar A. G.) were set in a transferable remount stone index and sealed to the base. The maxillary dentures were then invested, tasked, processed with acrylic resin (Lucitone 199, Dentsply, York, Pa.), and cured for 9 hours at 73 ° C. After deflasking and decasting, each denture was trimmed and polished. Because the cast, which represented an elderly edentulous situation, had no undercuts, each prosthesis was then carefully seated by one examiner with a gentle pressure on the type III dental stone master cast, which was m o u n t e d on a tilt-top table o f a dental surveyor (Ney Co., Hartford, Conn.). An acrylic resin preformed cast holder with a cast survey level allowed consistent orientation. Adaptation between the inner (palatal) surface o f the 206
Fig. 2. Simultaneous 99% confidence intervals for posterior palatal border opening means at trimmed-and-polished measurement interval. maxillary denture and the master cast was measured perpendicular to the cast along the posterior palatal border with a traveling microscope (0.001 mm resolution, Be& Mfr., ). Measurements were made (23 ° C) at the midline and at 5 and 10 m m on each side o f the midline after trimming and polishing. The midline was determined by following a line from the incisal papilla along the median palatine raphe to the posterior border of the cast (Fig. 1). The casts were then immersed in water at 23 ° C for 24 hours and 1 week. All measurements were made by one examiner. An estimate o f the precision o f the cast dimensions was made by measuring eight casts o f each stone along the posterior palatal base area and calculating the coefficient o f variation. RESULTS The coefficient o f variation associated with producing type III dental stone and high expansion stone casts from the master mold was 0.2% for both materials. The amount o f posterior border opening at each measurement location for the various measurement times for both groups o f complete dentures is illustrated in Figures 2 through 4. The data were analyzed by use o f the ranks o f the observations, and the resulting mean vectors were compared by the T 2 test for two independent multivariate populations. 31 Statistical analysis revealed that at each measurement period the mean posterior palatal border openings were significantly different for the dentures produced on high expansion stone versus type III dental stone (p <0.001). Figures 2 through 4 show the simultaneous 99% confidence intervals for the means over the three measurement periods at the five measurement locations. The confidence intervals were produced by a Bonferroni correction. Figures 2 through 4 also demonstrate that none o f the confidence intervals overlap. V O L U M E 77
NUMBER2
SYKORA AND SUTOW
E ¢¢-
9o0
THE J O U R N A L O F P R O S T H E T I C D E N T I S T R Y
24 H o u r s I m m e r s i o n in W a t e r (23°C) 9 9 % C o n f i d e n c e Intervals -m
80O 700
0
o lZl
•
Type III Dental Stone
H
? H
High Expansion Stone H
600
500
. i
L,.
o
a.
O n e W e e k I m m e r s i o n in W a t e r (23°C) 9 9 % C o n f i d e n c e Intervals
:=L
90o
"
800-
•
Type Ill Dental Stone
700
•
High Expansion Stone
e-
• O
6oo5oo-
o
400-
400
300 D.
E
300-
t~
200
a.
200-
100
"=
1O0
o
0
0
I
I
I
I
I
10
5
Midline
5
10
Measurement Location, mm left and right of midline
"'
ii
110
i 5
~ Midline
I 5
l J0
Measurement Location, mm left and right of midline
Fig. 3. Simultaneous 99% confidence intervals for posterior palatal border opening means at 24-hour immersion measurement interval.
Fig. 4. Simultaneous 99% confidence intervals for posterior palatal border opening means at 1-week immersion measurement interval.
At each measurement location the mean openings for the high expansion stone were consistently at least 50% smaller than those for the type 1II dental stone. A Student-Newman-Keuls multiple comparison test was used to determine whether immersion in water had an effect on the posterior palatal border opening over time. 32 Analysis o f the five measurement locations showed no statistical differences caused by immersion for the dentures processed on either stone (p = 0.05).
type III dental stone are 80 x 10-6/°C and 10 x 10-6/ °C, respectively. According to Marx, who assumed a temperature change from 100 ° to 20 ° C, the contraction of the acrylic resin base is approximately 0.32 mm, but only 0.04 mm for the type III dental stone. Thus, the difference in thermal contraction between these two materials would be 0.28 mm. as Although use o f the high expansion stone improved the fit o f the complete dentures along the posterior palatal border compared with type III dental stone, immersion in water for up to 1 week indicated no statistically significant effect. Theoretically, water sorption can help compensate for processing shrinkage by expanding the denture. Nevertheless, observations in this study were in agreement with other studies that showed no significant dimensional change for 1-week immersion in water for dentures produced by the trial-pack technique 1*and for storage in water up to 30 days. 13 With the high expansion stone, it is theoretically possible that it could result in a complete maxillary denture that is too tight in some other area; however, this theory requires further investigation.
DISCUSSION According to Anthony and Peyton, a3 the simplest method o f evaluating the fit o f dentures is to place them lightly on a master cast and observe the fit. The greatest discrepancy for a maxillary complete denture will be seen in the central portion o f the posterior maxillary border. 33 Analysis o f the data revealed that the maxillary dentures processed on high expansion stone casts produced openings along the posterior palatal border o f the prostheses that were substantially smaller compared with the dentures processed on type III dental stone. One o f the principal factors in denture stability and retention is a minimum space between the supporting structures and the prosthesis. Historically, the physical property o f dental stone setting expansion was kept to a •minimum to accurately capture the oral structures that were registered by an impression material. Thus, a typical type i i i dental stone would have a setting expansion o f only 0.12%. In contrast, the high expansion stone tested has a setting expansion o f 0.55% after 2 hours and 0.59% after 24 hours. 34 This study demonstrated that expansion o f the stone can help compensate for shrinkage that occurs as a result o f thermal contraction o f the acrylic resin material. Marx as used a hypothetical cast that has 50 m m between the alveolar crests; the coefficient o f thermal expansion o f the acrylic resin and FEBRUARY 1997
CONCLUSIONS It was concluded that the high expansion stone compensated to a substantial degree for the shrinkage that occurs as a rcsult o f the processing o f the acrylic resin material. Compared with the prostheses that were processed on type III dental stone casts, the use o f the high expansion stone significantly (p <0.001) reduced the size of the posterior palatal border openings o f the maxillary dentures by at least 50%. REFERENCES 1. Peyton FA. History of resins in dentistry• Dent Clin North Am 1975;9:21122. 207
THE J O U R N A L OF PROSTHETIC DENTISTRY
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