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Dimensional stability of dentures processed in boilable acrylic resins: A comparative study
Dimensional stability of dentures processed in boilable acrylic resins: A comparative study Gregory L. Polyzois, D.D.S., Dr.Odont.,* Hercules C. Karkazis, D.D.S., Dr.Odont.,* Alcibiades J. Zissis, D.D.S., Dr.Odont.,* and Pyrros P. Demetriou, D.D.S., Ph.D.** University of Athens, School of Dentistry, Athens, Greece
H
eat-cured poly(methy1 methacrylate) (PMMA) coupled with the compression molding technique, remains the denture base material of choice, although various polymers have been developed and introduced in dentistry to overcome the deficiencies of PMMA. Polymers such as epoxy resin, polysterene, vinyl acrylic, nylon, and polycarbonate have been used. In the past few years acrylic resin polymers have also been modified not only to improve physical and mechanical properties but also the working properties that facilitate laboratory techniques of denture construction. Two major innovations in denture base polymers were the acrylic rubber reinforced polymers, called highimpact resins, and the fast boilable (20 to 25 min) denture acrylic resins. In evaluating new denture base materials and processing techniques, one important comparison that can be made concerns dimensional stability of the denture base and changes in the positions of artificial teeth.’ The objective of this laboratory investigation was to examine and compare the linear dimensional changes of three boilable denture resins with a conventional and a high-impact heat-cured resin.
MATERIAL
AND METHODS
Three fa.st boilable denture resins were selected to be compared with a conventional and a high-impact denture base resin. The materials, manufacturers, proportions of powder to liquid, and the curing conditions recommended by the manufacturer are shown in Table I.
Construction
technique
A rubber mold was made of a nonundercut edentulous maxillary cast that had a M-inch border. This mold was used to prepare 25 identical stone casts. Dental stone was measured and mixed with the recommended amount of water.
A master
complete
maxillary
denture
was
*Lecturer, Department of Prosthodontics, Division of Removable Prosthodontics. **Associate Professor, Department of Prosthodontics, Division of Removable Prosthodontics. THE JOURNAL
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Fig. 1. Complete maxillary denture wax-up indicating location of reference points F, D, and E. A, Frontal view; B, right lateral view; C, left lateral view.
waxed up with acrylic resin denture teeth. In this wax-up four points of reference were made for the purpose of obtaining measurements on denture bases. The reference points F, D, and E were small dimples (1 mm in diameter), and point G was a larger dimple (4 mm diameter) to accommodate the stylus end of the depth gauge (Figs. 1 and 2). The wax-up immediately 639
I’OLYZOIS
Table I. Materials
used in this investigation
Resin material
Code
SR 3160 Rapid QC-20
(C’) CD’)
Acron Rapid (Boiled) SR 3160 (Conventional) SR 3/60 Plus (High-impact)
(E’)
Table II. Dimensional
Powder/liquid ratio
Manufacturer
(A’)
Ivoclar AG, Amalgamated England Howmedica England Ivoclar AG
(B’)
Ivoclar
Boiling Boiling
water water
25 min 20 min
Int.
3.5:1
Boiling
water
20 min
3:l
8 Hours
at 70” C
3:l
8 Hours
at 70”
Ltd.,
DF
EF
DE
Palatal
height
A’ = SR 3;‘60; water
B’ = SR 3/60
B’
C’
D’
0.78 1.04 0.84 0.66 0.88 0.58 0.76
?I k zt 2 * + +
0.13 0.12 0.13 0.11 0.09 0.15 0.15
0.64 0.83 0.60 0.52 0.71 0.49 0.53
k 2 zt 2 * 2 k
0.11 0.15 0.16 0.13 0.10 0.14 0.12
1.08 1.29 0.86 0.68 0.97 0.59 0.71
zk 0.36 + 0.34 of- 0.45 ri 0.13 * 0.21 2 0.12 * 0.14
B C A B C
1.00 0.65 1.48 2.53 1.49
k 2 * k zk
0.20 0.25 0.41 0.67 0.50
0.73 0.54 0.34 0.17 -0.08
zk 2 r k k
0.18 0.28 0.21 0.14 0.19
0.99 0.84 0.77 0.29 0.19
* i + 2 It
C’ = SR 3/60
Rapid;
D’
= QC-20;
behind the second molars (tuberosity region) was flattened to facilitate the placement of the depth gauge (Fig. 3). To ensure uniformity and permit comparisons, a combined mold from silicone rubber with a stone occlusal index to immobilize teeth was made from the master denture. With the same mold of teeth, the denture wax-ups were duplicated by melting and pouring wax into the mold that held the acrylic resin teeth and stone casts in correct relationship to each other. Twenty-five denture wax-ups (five for each denture resin) including the reference points were made. The wax-ups were identical in thickness, height, and contour because they were prepared from the same mold. Small holes were drilled into the central incisors and the second molars to be used as reference points for measuring distances. The holes were marked with black indelible paint to make them more visible (Fig. 2). All of the dentures were prepared by a standarized compression molding technique according to the manufacturers’ processing directions for each denture resin. The dentures were processed in a water-bath curing unit and bench cooled before deflasking. 640
base materials
A B C A B C A
Plus;
C
bases (% + SD)
A’
Phase
cycle
3:l 3.3:1
AG
changes of denture
Curing
Liechtenstein Dental Co.,
Denture Distance
ET AL
E’ = Arron
0.40 0.56 0.29 0.38 0.49 0.36 0.46 0.61 0.44 0.48 0.23 -0.11
0.22 0.22 0.33 0.33 0.35
Rapid;
A = processing;
E’
k ii -+ * +k i t 2 k +
0.11 0.12 0.16 0.07 0.14 0.14 0.12 0.11 0.22 0.28 0.15 0.24
0.42 0.95 0.47 0.50 0.84 0.55 0.44 0.72 0.52 0.52 0.32 0.09
B = &casting;
It * 2 k * ?I + -+ * t t lir
0.17 0.10 0.16 0.16 0.12 0.15 0.04 0.12 0.07 0.15 0.12 0.17
C: = storage
in
Measurements Four distances (FD, FE, DE, and palatal height) on denture bases and three (AB, AC, and BC) on artificial teeth were designated to be measured (Figs. 3 and 4). Measurements were made at the wax-up stage before investing. These were used as zero readings; all values were calculated with these measurements as the starting point. Additional measurements were made (1) after processing and before decasting, (2) after decasting, and (3) after the dentures had been stored in distilled water at room temperature (20” ~fr1” C) for 1 weeek. All measurements on denture bases and artificial teeth were made with a dial caliper and recorded to the nearest 0.05 mm. Palatal height measurements on the midpalatal posterior region of dentures were made with a depth gauge reading to 0.01 mm. Measurements were made by the same operator and the average of five readings was used as the measurement. Statistical
analysis
A one-way analysis of variance (ANOVA) on the data from this investigation was followed by a post hoc Duncan’s new multiple range test to determine whether MAY
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Fig. 2. Complete maxillary denture wax-up indicating measuring points (A, B, C) on teeth midpalatal area (G), and right and left distal buccal flange (D and E).
Fig. 3. Placement of depth gauge immediately behind second molars measuring
palatal height
of denture.
statistically significant differences existed among the groups. Statistical analysis was conducted at the 95’% level of confidence (a = 0.05). RESULTS AND
DISCUSSION
The mean values, standard deviations of percent change for the FD, FE, DE, and palatal height distances are presented in Table II and Figs. 5 through 8. The data in Table II were statistically evaluated and the results of ANOVAs and Duncan’s tests are presented in Table III. The shrinkage or expansion of measured distances are presented as positive or negative values in all tables and graphs. The three distances DF, EF, and DE on each denture base showed a shrinkage after processing. After decasting of complete maxillary dentures, the same distances showed additional shrinkage until immersion and storage of the dentures in water. After storage in room temperature water for 1 week, the dentures showed an expansion that failed to compensate for the initial processing shrinkage. The processing shrinkages in the distances before decasting of the dentures were the same (p > .05) as were the final shrinkages after expansion due to the sorption of water for all of the denture resins investigated and distances DF, EF, and DE, with the exception of SR 3/60 Rapid resin in DF distance. Paired t-tests performed for these comparisons confirmed previous findings related to the dimensional changes of denture bases due to processing and storage in water.‘-” The five denture resins studied presented a similar pattern of shrinkage in the distances DF, EF, and DE. The SR 3/60 Rapid resin showed the maximum and the QC-20 resin the minimum shrinkage fairly consistently in the three denture base distances. The Acron Rapid, SR 3/60 Plus, and SR 3/60 resins showed an intermediate amount of shrinkage. THE JOURNAL
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Fig. 4. Measurement
of molar-to-molar
distance with a
dial caliper.
Differences among the five denture resins were identified with Duncan’s test and presented in Table III. It can be seen (Table III) that there are no significant differences (is > .05) among the SR 3/60 Plus, QC-20, and Acron Rapid denture resins. The same is also true for the SR 3/60 and SR 3/60 Rapid resins. The final amount of shrinkage in the distances DF, EF, and DE ranged from 0.29% to 0.86% (Table II). The flangeto-flange cross arch linear distance (DE) showed a shrinkage ranging from 0.44% (QC-20) to 1% (SR 3/60) between the five denture resins. After storage in water for 1 week, the shrinkage of flange-to-flange distance ranged from 0.44% (QC-20) to 0.84% (SR 3/60 Rapid). Flange-to-flange final linear shrinkage for all the denture resins investigated ranged 641
POLYZOIS
ET AL
Ei Fig. 5. Histogram of percent dimensional decasting; C, storage in water.
Table III. Statistical analyses (ANOVAS and Duncan’s artificial teeth between the five denture resins.
change
in DE distance.
A, Processing;
tests) of distances on denture
B,
bases and
Phase Distance
A
B
C
Denture bases DF
D’ E’ B’ A’ C’
D’ B’ E’ A’ C’
D’ E’ B’ A’ C’
EF
D’ E’ B’ A’ C’
D’ B’ E’ A’ C’
NS
DE
E’ D’ B’ C’ A’
D’ E’ B’ C’ A’
NS
Palatal height
B’ D’ E’ C’ A’
B’ D’ C’ E’ A’
D’ B’ E’ C’ A’
Artificial teeth AB
B’ D’ E’ C’ A’
8’ D’ E’ C’ A’
B’ E’ D’ A’ C’
AC
B’ D’ E’ C’ A’
B’ D’ E’ C’ A’
B’ D’ E’ A’ C’
BC
B’ D’ E’ C’ A’
B’ D’ E’ C’ A’
B’ D’ E’ A’ C’
Letters conne<~tedby horizontal lines are not significantly different at a = 0.05. NS = ANOVA revealed no significant dilference between means at a = 0.05. A’ = SR 31’60; B’ = SR 3/60 Plus; C’ = SR 3/60 Rapid; D’ = QC-20; E’ = Acron Rapid; A = processing; B = dccasting: C = storage in water.
from 0.26 to 0.42 mm. It should be noted, however, that the maximum shrinkage across the posterior section of complete maxillary dentures was less than 1% or 0.5 mm and unlikely to be of clinical significance.8*9,‘2,‘3 Dimensional changes in complete maxillary dentures during or after processing affect their shape. It has been shown that warpage is three dimensional and does not 642
occur in only one plane.3,6,‘4 Kern3 in 1941 attempted to evaluate the warpage of complete maxillary denture bases by measuring the flange-to-flange, molar-tomolar, and the depth of the vault distances, and concluded that these measurements were most likely to affect the fit of a denture to the greatest degree. More recently de Gee et a1.6and Becker et a1.14used MAY
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f:HAtJGE:S
I T-i EF-D
I ::iTkPi!T:E
Fig. 6. Histograms of percent dimensional changes in EF distance. A, Processing; B, decasting; C, storage in water.
I Fig. 7. Histograms of percent dimensional changes in DF distance. A, Processing; B, decasting; C, storage in water.
micrometer readings on the impression surface of dentures to check the overall warpage of campi ete maxillary dentures. They included the palatal height of dentures in their measurements and suggested that this measurement offers information about the fit in the palatal area. The palatal height of dentures as measured in this investigation showed a shrinkage until storage in water. All of the materials expanded during the storage and, in the case of SR 3/60 Plus and QC-20 resins, the water uptake was sufficient to cause an expansion greater than THE JOURNAL
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the initial shrinkage and thus a final slight expansion occurred (Table II). This finding is in agreement with Kern3 and de Gee et a1.6The final shrinkage of palatal height ranged from 0.09% (Acron Rapid) to 1.49% (SR 3/60). ANOVA and Duncan’s test revealed that the group of fast boilable resins showed a similar pattern of change in palatal height throughout the measurements. There were no differences (ir, > .05) between the three boilable resins in each step of measurements. The final slight 643
POLYZOIS ET AL
CHFINGE
w 5:
2.4 1.8
: I”
1.2 0.6
(21
0 SR3/68 I SR3160P RSR3/60R IIBIACRON-R
m 0 ; -0.6 iii
z -1.2 a $ -1.8 w -2.4 C”QN%ES
IN
P:‘ATAL
Hk&lT
Fig. 8. Histograms of percent dimensional changes in palatal height. A, Processing; B, decasting; C, storage in water. Table IV. Dimensional
changes of artificial
teeth (% + SD) Denture
Distance
Phase
AB
A B C A B C A B C
AC
BC
A’ 0.46 0.75 0.29 0.47 0.76 0.34 0.46 0.77 0.42
+ 0.07 ?z 0.08 + 0.18 + 0.15 k 0.15 + 0.16 -+ 0.15 i 0.17 f 0.14
base materials
B’ 0.01 -0.07 -0.17 0.02 0.05 -0.12 0.09 0.14 0.11
2 ? * Ik + 2 + 2 i
C’ 0.01 0.11 0.14 0.05 0.11 0.24 0.15 0.20 0.19
0.43 0.60 0.31 0.59 0.68 0.49 0.45 0.70 0.43
* + f + t A t I i
D’ 0.15 0.15 0.09 0.18 0.15 0.12 0.13 0.14 0.15
0.22 0.17 0.17 0.17 0.10 0.07 0.21 0.34 0.30
E’
k 0.05 2 0.06 +- 0.06 2 0.07 2 0.10 + 0.07 -t 0.05 t 0.11 k 0.13
0.29 0.22 0.05 0.42 0.27 0.22 0.39 0.39 0.39
t 2 f + + + i 2 It
0.07 0.22 0.16 0.17 0.20 0.24 0.19 0.19 0.19
A’ = SR 3/60; B’ = SR 3/60 Plus; C’ = SR 3/60 Rapid; D’ = QC-20; E’ = Acron Rapid; A = processing; B = decasting; (: = storage in water
shrinkage or expansion of palatal heights for the three boilable resins and the high impact resin compared with the initial processing shrinkage were significantly different (paired t-tests; p < .05) whereas the opposite was true for the conventional resin. It is evident that decasting and immersion in water caused a gradual expansion and a respective reduction of shrinkage for the palatal heights of maxillary complete dentures made from the three fast boilable resins and the high-impact resin. It is notable that the palatal height of dentures made from the three boilable resins contracted significantly less @ < .05) than the palatal height of dentures processed in the conventional heat-cured resin (Table III). This finding can be coupled with another reported by Firtell et a1.15that a boilable denture resin produced significantly less distortion and better fit in the midpal644
atal posterior area of a denture base than a conventional heat-cured acrylic resin. It can also be seen from Table III that all of the denture resins studied showed a similar pattern of dimensional behavior during the storage in water for one week. In addition, dimensional changes of denture bases during and after processing produce associated three-dimensional changes in the position of artificial teeth.14 In this investigation linear distances between certain teeth were measured. Table IV and Figs. 9 through 11 represent quantitatively and graphically the percent changes in distances AB, AC, and BC among teeth. Statistical analyses are summarized and depicted in Table III. It can seen from Table IV and Figs. 9 to 11 that the five denture resins showed a shrinkage after processing in the distances between teeth. This finding MAY 1987
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CHANGE
(2s)
0.8 ;
0.6
;
0.4
USR3/60 m SR3160P R SR3160R
E 0.2 m 0 z-0.2 0
S-0.4 u E-0.6 w -0.8 A CHANGES Fig. 9. Histograms of percent decasting; C, storage in water.
CHANGE
0.4
K ii
0.2
A, Processing;
B,
T - -.
0.5.
Q Y z
change in AB distance.
I ;=I
0.8 W Q
dimensional
# SR3/60R I::::1lYJc:-z’G1 UIIIIACRON-R
0
Fig. 10. Histograms of percent decasting; C, storage in water.
dimensional
confirms previous results reported by Grunewald et a1.4 McCartney,” Becker et a1.,14Vieira,16 Zani and Vieira,” Woelfel I8 and Garfunkel.”
The AB and AC distances after storage of dentures in water expanded and reached the initial shrinkages; only the high-impact resin showed a final expansion in distance AB. The Acron Rapid resin failed to reach the initial processing contraction of AB and AC distances (paired t-tests; j6 < .OS). The behavior of each denture THE JOURNAL
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change in AC distance. A, Processing;
B,
resin in AB and AC distances between teeth during immersion in water was quite different as ANOVAs and Duncan’s test revealed (Table III). The removal of dentures from their master casts revealed a different behavior among the five resins studied. The SR 3/60 and SR 3/60 Rapid resins had a similar behavior (p > -05) and showed a further shrinkage of distances between teeth after decasting (Tables III and IV). ANOVA and Duncan’s test showed a more complex 645
POLYZOIS
ET AL
T T
A Fig. 11. Histograms of percent decasting; C, storage in water.
P dimensional
of SR 3/60 Plus, QC-20, and Acron Rapid resins for the AB and AC distances after decasting (Table III). These distances expanded slightly, compensating in part the initial contraction. The molar-to-molar distance (BC) contracted further for all the denture resins after decasting, but returned to the initial processing contraction after water immersion (paired t-test; p > .05). The behavior of each resin in molar-to-molar distance (BC) was similar for SR 3/60 and SR 3/60 Rapid resins 0, > .OS) but more complex for the SR 3/60 Plus, QC-20, and Acron Rapid resins (Table III). Our results are similar to those of Grunewald et a1.,4 Woelfel,‘8 and Garfunkel.” The final molar-to-molar shrinkage is a critical measurement across the posterior section of a complete maxillary denture. In this investigation final dimensional changes of molar-to-molar span had a range of 0.05 mm (SR 3/60 Plus) to 0.19 mm (SR 3/60 Rapid). Our findings are in agreement with those of McCartney,‘O Woelfel and Paffenbarger,” Woelfel et al.,‘” Woelfel,18 and Mowery et al.” Changes of this amount are difficult if not impossible to detect clinically. *,“, 13.lR,” Although the dimensional changes reported in this laboratory investigation concern in vitro dentures stored in water, predictions of the approximate in service behavior of clinical dentures can be made according to Woelfel et alI3 However, the clinical significance of laboratory tests is difficult to interpret.9,21-23Clinical controlled studies will be necessary to evaluate the behavior of denture base materials in an oral environment. Finally, a combination of laboratory and clinical testing is of utmost importance in the evaluation of new denture base materials. behavior
646
1;
change in BC distance. A, Processing;
SUMMARY
B,
AND CONCLUSIONS
An investigation was conducted to evaluate and compare the dimensional stability of three fast boilable denture resins with a conventional and a high-impact denture resin processed with a long-curing cycle. Although the boilable denture resins have been introduced to the dental profession and offer merits such as faster processing, saving time for the dentist, patient, and technician, reduction of energy costs, and dramatic increase in denture laboratory production without adding personnel or purchasing new equipment, the relevant dental literature is sparse. The results of this investigation indicate that all five denture resins produced dentures that shrink. Also measurements between certain teeth showed shrinkages. Linear shrinkages of denture bases and teeth distances were less than 1%. Flange-to-flange and molar-to-molar changes were less than 0.5 mm and 0.2 mm respectively. Maxillary complete dentures processed in boilable resins presented less distortion in the midpalatal area across the posterior section than dentures processed in the conventional heat-cured resin. Although linear changes reported in this investigation were thought to be clinically insignificant, clinical studies should be conducted to establish correlation with laboratory findings. We thank Ivoclar AC, Howmedica International Ltd., De Trey Division-Dentsply Ltd., for supplying the denture resins. We gratefully acknowledge the assistance of the following people: Mr. P. Wollwage, Research and Development of Ivoclar AG, Mr. J. Foster of Howmedica International Ltd. for information concerning boilable MAY
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denture resins and Dr. 1’. E. Lagouvardos for the statistical tion and analy!sis.
consulta-
REFERENCES Woelfel JB, Paffenbarger GC, Sweeney WT. Dimensional changes occurring in dentures during processing. J Am Dent Assoc 1960;61:413-30. Woelfel JB, Paffenbarger GC. Dimensional changes occurring in artificial dentures. Int Dent J 1959;9:451-60. .3 Kern WR. Possible dimensional changes in denture base materials. J Am Dent Assoc 1941;28:1952-8. ‘I. Grunewald AH, Paffenbarger GC, Dickson G. The eff’ect of molding processes on some properties of denture resins. J Am Dent Asr.oc 1952;44:269-84. 5. Skinner EW’. .Jones PM. Dimensional stability of self-curing denture base acrylic resin. J Am Dent Assoc 1955;51:426-31. 6. dr <;ee .2J. ten Harkel EC, Davidson CL. Measuring procedure for the determination of the three-dimensional shape of dentures. J PKOS.I.H~~.DENY 1979;42:149-53. Stalford GL). Batcc JF, Huggett R, Handley RW. A review of the properttes ol \ome denture base polymers. J Dent 1980; 8:292-306. Rlurphy \VM. Bates JF. Huggett R, Bright R. A comparative study 01 tlhree denture base materials. Br Dent J 1982; I i2:2?3. klurphy Whl. liuggett R. Handley RW. A laboratory and clinical SIudy of Trevalon denture base material. J Oral Rehabil 1982;9:401. hfc(:arrney .JW. FI,anLe;e adaption discrepancy, palatal base distortion, and induced malocclusion caused by processing acrylic resin maxillarv complete dentures. J PROS.~HE.I. DENT 19tm4; 1.
52:545-53
,Ilirza I’ll. IXmensional stability of arrylic resin dentures: clinical evaluation J PROSTHE.I. DENT 1961:11:848-57. W’oelfel ~JB. Pall’enbarger GC. Method of evaluating the clinical effect of warping a denture: report of a case. J Am Dent Assoc 1959:59:?il-60
Accuracy
Woelfel JB, Paffenbarger GC, Sweeney WT. Changes in dentures during storage in water and in service. ~JAm Dent Assoc 1961;62:643-57. 14. Becker CM, Smith DE, Nicholls JI. The comparison of denturebase processing techniques. Part II. Dimensional changes due to processing. J PROSTHET DENT 1977;37:450-9. 15. Firtell DN, Green AJ, Elahi JM. Posterior peripheral seal distortion related to processing temperature. J PROSTHET DENT 1981;45:598-601. 16. Vieira DF. Ch anges in the relative position of teeth in the construction of denture bases. J Dent Res 1962;41:1450-60. 17. Zani D, Vieira DF. A comparative study of silicone as a separating medium for denture processing. J PROFTHET DENT 13.
1979;42:386-91.
18. Woelfel JB. Processing complete denture\;. Dent Clin North Am 1977;21:329-38. 19. Garfunkel E. Evaluation of dimenstonal changes in complete dentures processed by injection-pressing and the pack-and-press technique. J PROXHET DENT 1983;50:757-61. 20. Mowery WE, Burns CL, Dickson G, Sweeney WT. Dimensional stability of denture base resins. J Am Dent Assoc 1958;57:34553. 21. Bates JF, Stafford GD. Polymeric denture base specifications. Biomed Eng 1978;8:288. 22. Woelfel JB, Paffenharger GC, Sweeney WT. Some physical properties of organic denture base materials. J Am Dent Assoc 1963;67:489-504. 23. Staflord GD, Smith DC. Polycarhonates. A preliminary report on the use of polycarhonates as a denture base material. Dent Pratt Dent Ret 1967;17:217-23. Ke~mn/ rrcp3I lot DR. GREGORY L. POLYZOIS DEPhRThlENT OF RESTORATIVE DENTISTRY DENT.U. SCHOU. & HOSPITAL ~~E.z.I.11PARK CARIIIFF CF4 4X\ EN(;I.\ND
of zeroing the articulator
T. P. Nowlin, D.D.S., M.A.,* J. 0. Bailey, Jr., D.D.S., M.S.,* and T. R. Taubert,** University of Texas Health ScienceCenter, Dental School. San Antonio, Tex.
T
he accurate use of a semiadjustable articulator is in part contingent upon the dentist’s ability to consistently zero the instrument before mounting casts. Zeroing an articulator is the process of standardizing it to a reproducible starting point.’ In the case of the Hanau 130-28
*Associate Professor, Department of Restorative Dentistry. **Dental Ceramist Technician, Department of Restorative Dent.stry. THE JOURNAL
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semiadjustable articulator (Teledyne-Hanau Division, Buffalo, N.Y.) several criteria must be met for the articulator to be zeroed correctly. The incisal pin must strike the incisal table in the center of the table both anteroposteriorly and right to left. The offset pin, if used, should be set on the fifth mark. There should be no side play or side-to-side movement between the upper and lower members when the centric locks are secured.’ Some articulators come equipped from the manufacturer with zeroing devices designed to facilitate proper 647
H
eat-cured poly(methy1 methacrylate) (PMMA) coupled with the compression molding technique, remains the denture base material of choice, although various polymers have been developed and introduced in dentistry to overcome the deficiencies of PMMA. Polymers such as epoxy resin, polysterene, vinyl acrylic, nylon, and polycarbonate have been used. In the past few years acrylic resin polymers have also been modified not only to improve physical and mechanical properties but also the working properties that facilitate laboratory techniques of denture construction. Two major innovations in denture base polymers were the acrylic rubber reinforced polymers, called highimpact resins, and the fast boilable (20 to 25 min) denture acrylic resins. In evaluating new denture base materials and processing techniques, one important comparison that can be made concerns dimensional stability of the denture base and changes in the positions of artificial teeth.’ The objective of this laboratory investigation was to examine and compare the linear dimensional changes of three boilable denture resins with a conventional and a high-impact heat-cured resin.
MATERIAL
AND METHODS
Three fa.st boilable denture resins were selected to be compared with a conventional and a high-impact denture base resin. The materials, manufacturers, proportions of powder to liquid, and the curing conditions recommended by the manufacturer are shown in Table I.
Construction
technique
A rubber mold was made of a nonundercut edentulous maxillary cast that had a M-inch border. This mold was used to prepare 25 identical stone casts. Dental stone was measured and mixed with the recommended amount of water.
A master
complete
maxillary
denture
was
*Lecturer, Department of Prosthodontics, Division of Removable Prosthodontics. **Associate Professor, Department of Prosthodontics, Division of Removable Prosthodontics. THE JOURNAL
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Fig. 1. Complete maxillary denture wax-up indicating location of reference points F, D, and E. A, Frontal view; B, right lateral view; C, left lateral view.
waxed up with acrylic resin denture teeth. In this wax-up four points of reference were made for the purpose of obtaining measurements on denture bases. The reference points F, D, and E were small dimples (1 mm in diameter), and point G was a larger dimple (4 mm diameter) to accommodate the stylus end of the depth gauge (Figs. 1 and 2). The wax-up immediately 639
I’OLYZOIS
Table I. Materials
used in this investigation
Resin material
Code
SR 3160 Rapid QC-20
(C’) CD’)
Acron Rapid (Boiled) SR 3160 (Conventional) SR 3/60 Plus (High-impact)
(E’)
Table II. Dimensional
Powder/liquid ratio
Manufacturer
(A’)
Ivoclar AG, Amalgamated England Howmedica England Ivoclar AG
(B’)
Ivoclar
Boiling Boiling
water water
25 min 20 min
Int.
3.5:1
Boiling
water
20 min
3:l
8 Hours
at 70” C
3:l
8 Hours
at 70”
Ltd.,
DF
EF
DE
Palatal
height
A’ = SR 3;‘60; water
B’ = SR 3/60
B’
C’
D’
0.78 1.04 0.84 0.66 0.88 0.58 0.76
?I k zt 2 * + +
0.13 0.12 0.13 0.11 0.09 0.15 0.15
0.64 0.83 0.60 0.52 0.71 0.49 0.53
k 2 zt 2 * 2 k
0.11 0.15 0.16 0.13 0.10 0.14 0.12
1.08 1.29 0.86 0.68 0.97 0.59 0.71
zk 0.36 + 0.34 of- 0.45 ri 0.13 * 0.21 2 0.12 * 0.14
B C A B C
1.00 0.65 1.48 2.53 1.49
k 2 * k zk
0.20 0.25 0.41 0.67 0.50
0.73 0.54 0.34 0.17 -0.08
zk 2 r k k
0.18 0.28 0.21 0.14 0.19
0.99 0.84 0.77 0.29 0.19
* i + 2 It
C’ = SR 3/60
Rapid;
D’
= QC-20;
behind the second molars (tuberosity region) was flattened to facilitate the placement of the depth gauge (Fig. 3). To ensure uniformity and permit comparisons, a combined mold from silicone rubber with a stone occlusal index to immobilize teeth was made from the master denture. With the same mold of teeth, the denture wax-ups were duplicated by melting and pouring wax into the mold that held the acrylic resin teeth and stone casts in correct relationship to each other. Twenty-five denture wax-ups (five for each denture resin) including the reference points were made. The wax-ups were identical in thickness, height, and contour because they were prepared from the same mold. Small holes were drilled into the central incisors and the second molars to be used as reference points for measuring distances. The holes were marked with black indelible paint to make them more visible (Fig. 2). All of the dentures were prepared by a standarized compression molding technique according to the manufacturers’ processing directions for each denture resin. The dentures were processed in a water-bath curing unit and bench cooled before deflasking. 640
base materials
A B C A B C A
Plus;
C
bases (% + SD)
A’
Phase
cycle
3:l 3.3:1
AG
changes of denture
Curing
Liechtenstein Dental Co.,
Denture Distance
ET AL
E’ = Arron
0.40 0.56 0.29 0.38 0.49 0.36 0.46 0.61 0.44 0.48 0.23 -0.11
0.22 0.22 0.33 0.33 0.35
Rapid;
A = processing;
E’
k ii -+ * +k i t 2 k +
0.11 0.12 0.16 0.07 0.14 0.14 0.12 0.11 0.22 0.28 0.15 0.24
0.42 0.95 0.47 0.50 0.84 0.55 0.44 0.72 0.52 0.52 0.32 0.09
B = &casting;
It * 2 k * ?I + -+ * t t lir
0.17 0.10 0.16 0.16 0.12 0.15 0.04 0.12 0.07 0.15 0.12 0.17
C: = storage
in
Measurements Four distances (FD, FE, DE, and palatal height) on denture bases and three (AB, AC, and BC) on artificial teeth were designated to be measured (Figs. 3 and 4). Measurements were made at the wax-up stage before investing. These were used as zero readings; all values were calculated with these measurements as the starting point. Additional measurements were made (1) after processing and before decasting, (2) after decasting, and (3) after the dentures had been stored in distilled water at room temperature (20” ~fr1” C) for 1 weeek. All measurements on denture bases and artificial teeth were made with a dial caliper and recorded to the nearest 0.05 mm. Palatal height measurements on the midpalatal posterior region of dentures were made with a depth gauge reading to 0.01 mm. Measurements were made by the same operator and the average of five readings was used as the measurement. Statistical
analysis
A one-way analysis of variance (ANOVA) on the data from this investigation was followed by a post hoc Duncan’s new multiple range test to determine whether MAY
1987
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57
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5
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STABILITY
OF DENTURES
Fig. 2. Complete maxillary denture wax-up indicating measuring points (A, B, C) on teeth midpalatal area (G), and right and left distal buccal flange (D and E).
Fig. 3. Placement of depth gauge immediately behind second molars measuring
palatal height
of denture.
statistically significant differences existed among the groups. Statistical analysis was conducted at the 95’% level of confidence (a = 0.05). RESULTS AND
DISCUSSION
The mean values, standard deviations of percent change for the FD, FE, DE, and palatal height distances are presented in Table II and Figs. 5 through 8. The data in Table II were statistically evaluated and the results of ANOVAs and Duncan’s tests are presented in Table III. The shrinkage or expansion of measured distances are presented as positive or negative values in all tables and graphs. The three distances DF, EF, and DE on each denture base showed a shrinkage after processing. After decasting of complete maxillary dentures, the same distances showed additional shrinkage until immersion and storage of the dentures in water. After storage in room temperature water for 1 week, the dentures showed an expansion that failed to compensate for the initial processing shrinkage. The processing shrinkages in the distances before decasting of the dentures were the same (p > .05) as were the final shrinkages after expansion due to the sorption of water for all of the denture resins investigated and distances DF, EF, and DE, with the exception of SR 3/60 Rapid resin in DF distance. Paired t-tests performed for these comparisons confirmed previous findings related to the dimensional changes of denture bases due to processing and storage in water.‘-” The five denture resins studied presented a similar pattern of shrinkage in the distances DF, EF, and DE. The SR 3/60 Rapid resin showed the maximum and the QC-20 resin the minimum shrinkage fairly consistently in the three denture base distances. The Acron Rapid, SR 3/60 Plus, and SR 3/60 resins showed an intermediate amount of shrinkage. THE JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 4. Measurement
of molar-to-molar
distance with a
dial caliper.
Differences among the five denture resins were identified with Duncan’s test and presented in Table III. It can be seen (Table III) that there are no significant differences (is > .05) among the SR 3/60 Plus, QC-20, and Acron Rapid denture resins. The same is also true for the SR 3/60 and SR 3/60 Rapid resins. The final amount of shrinkage in the distances DF, EF, and DE ranged from 0.29% to 0.86% (Table II). The flangeto-flange cross arch linear distance (DE) showed a shrinkage ranging from 0.44% (QC-20) to 1% (SR 3/60) between the five denture resins. After storage in water for 1 week, the shrinkage of flange-to-flange distance ranged from 0.44% (QC-20) to 0.84% (SR 3/60 Rapid). Flange-to-flange final linear shrinkage for all the denture resins investigated ranged 641
POLYZOIS
ET AL
Ei Fig. 5. Histogram of percent dimensional decasting; C, storage in water.
Table III. Statistical analyses (ANOVAS and Duncan’s artificial teeth between the five denture resins.
change
in DE distance.
A, Processing;
tests) of distances on denture
B,
bases and
Phase Distance
A
B
C
Denture bases DF
D’ E’ B’ A’ C’
D’ B’ E’ A’ C’
D’ E’ B’ A’ C’
EF
D’ E’ B’ A’ C’
D’ B’ E’ A’ C’
NS
DE
E’ D’ B’ C’ A’
D’ E’ B’ C’ A’
NS
Palatal height
B’ D’ E’ C’ A’
B’ D’ C’ E’ A’
D’ B’ E’ C’ A’
Artificial teeth AB
B’ D’ E’ C’ A’
8’ D’ E’ C’ A’
B’ E’ D’ A’ C’
AC
B’ D’ E’ C’ A’
B’ D’ E’ C’ A’
B’ D’ E’ A’ C’
BC
B’ D’ E’ C’ A’
B’ D’ E’ C’ A’
B’ D’ E’ A’ C’
Letters conne<~tedby horizontal lines are not significantly different at a = 0.05. NS = ANOVA revealed no significant dilference between means at a = 0.05. A’ = SR 31’60; B’ = SR 3/60 Plus; C’ = SR 3/60 Rapid; D’ = QC-20; E’ = Acron Rapid; A = processing; B = dccasting: C = storage in water.
from 0.26 to 0.42 mm. It should be noted, however, that the maximum shrinkage across the posterior section of complete maxillary dentures was less than 1% or 0.5 mm and unlikely to be of clinical significance.8*9,‘2,‘3 Dimensional changes in complete maxillary dentures during or after processing affect their shape. It has been shown that warpage is three dimensional and does not 642
occur in only one plane.3,6,‘4 Kern3 in 1941 attempted to evaluate the warpage of complete maxillary denture bases by measuring the flange-to-flange, molar-tomolar, and the depth of the vault distances, and concluded that these measurements were most likely to affect the fit of a denture to the greatest degree. More recently de Gee et a1.6and Becker et a1.14used MAY
1987
VOLUME
57
NUMBER
5
DIMENSIONAL
STABILITY
OF DENTURES
f:HAtJGE:S
I T-i EF-D
I ::iTkPi!T:E
Fig. 6. Histograms of percent dimensional changes in EF distance. A, Processing; B, decasting; C, storage in water.
I Fig. 7. Histograms of percent dimensional changes in DF distance. A, Processing; B, decasting; C, storage in water.
micrometer readings on the impression surface of dentures to check the overall warpage of campi ete maxillary dentures. They included the palatal height of dentures in their measurements and suggested that this measurement offers information about the fit in the palatal area. The palatal height of dentures as measured in this investigation showed a shrinkage until storage in water. All of the materials expanded during the storage and, in the case of SR 3/60 Plus and QC-20 resins, the water uptake was sufficient to cause an expansion greater than THE JOURNAL
OF PROSTHETIC
DENTISTRY
the initial shrinkage and thus a final slight expansion occurred (Table II). This finding is in agreement with Kern3 and de Gee et a1.6The final shrinkage of palatal height ranged from 0.09% (Acron Rapid) to 1.49% (SR 3/60). ANOVA and Duncan’s test revealed that the group of fast boilable resins showed a similar pattern of change in palatal height throughout the measurements. There were no differences (ir, > .05) between the three boilable resins in each step of measurements. The final slight 643
POLYZOIS ET AL
CHFINGE
w 5:
2.4 1.8
: I”
1.2 0.6
(21
0 SR3/68 I SR3160P RSR3/60R IIBIACRON-R
m 0 ; -0.6 iii
z -1.2 a $ -1.8 w -2.4 C”QN%ES
IN
P:‘ATAL
Hk&lT
Fig. 8. Histograms of percent dimensional changes in palatal height. A, Processing; B, decasting; C, storage in water. Table IV. Dimensional
changes of artificial
teeth (% + SD) Denture
Distance
Phase
AB
A B C A B C A B C
AC
BC
A’ 0.46 0.75 0.29 0.47 0.76 0.34 0.46 0.77 0.42
+ 0.07 ?z 0.08 + 0.18 + 0.15 k 0.15 + 0.16 -+ 0.15 i 0.17 f 0.14
base materials
B’ 0.01 -0.07 -0.17 0.02 0.05 -0.12 0.09 0.14 0.11
2 ? * Ik + 2 + 2 i
C’ 0.01 0.11 0.14 0.05 0.11 0.24 0.15 0.20 0.19
0.43 0.60 0.31 0.59 0.68 0.49 0.45 0.70 0.43
* + f + t A t I i
D’ 0.15 0.15 0.09 0.18 0.15 0.12 0.13 0.14 0.15
0.22 0.17 0.17 0.17 0.10 0.07 0.21 0.34 0.30
E’
k 0.05 2 0.06 +- 0.06 2 0.07 2 0.10 + 0.07 -t 0.05 t 0.11 k 0.13
0.29 0.22 0.05 0.42 0.27 0.22 0.39 0.39 0.39
t 2 f + + + i 2 It
0.07 0.22 0.16 0.17 0.20 0.24 0.19 0.19 0.19
A’ = SR 3/60; B’ = SR 3/60 Plus; C’ = SR 3/60 Rapid; D’ = QC-20; E’ = Acron Rapid; A = processing; B = decasting; (: = storage in water
shrinkage or expansion of palatal heights for the three boilable resins and the high impact resin compared with the initial processing shrinkage were significantly different (paired t-tests; p < .05) whereas the opposite was true for the conventional resin. It is evident that decasting and immersion in water caused a gradual expansion and a respective reduction of shrinkage for the palatal heights of maxillary complete dentures made from the three fast boilable resins and the high-impact resin. It is notable that the palatal height of dentures made from the three boilable resins contracted significantly less @ < .05) than the palatal height of dentures processed in the conventional heat-cured resin (Table III). This finding can be coupled with another reported by Firtell et a1.15that a boilable denture resin produced significantly less distortion and better fit in the midpal644
atal posterior area of a denture base than a conventional heat-cured acrylic resin. It can also be seen from Table III that all of the denture resins studied showed a similar pattern of dimensional behavior during the storage in water for one week. In addition, dimensional changes of denture bases during and after processing produce associated three-dimensional changes in the position of artificial teeth.14 In this investigation linear distances between certain teeth were measured. Table IV and Figs. 9 through 11 represent quantitatively and graphically the percent changes in distances AB, AC, and BC among teeth. Statistical analyses are summarized and depicted in Table III. It can seen from Table IV and Figs. 9 to 11 that the five denture resins showed a shrinkage after processing in the distances between teeth. This finding MAY 1987
VOLUME 57
NUMBER 5
DIMENSIONAL
STABILITY
OF DENTURES
CHANGE
(2s)
0.8 ;
0.6
;
0.4
USR3/60 m SR3160P R SR3160R
E 0.2 m 0 z-0.2 0
S-0.4 u E-0.6 w -0.8 A CHANGES Fig. 9. Histograms of percent decasting; C, storage in water.
CHANGE
0.4
K ii
0.2
A, Processing;
B,
T - -.
0.5.
Q Y z
change in AB distance.
I ;=I
0.8 W Q
dimensional
# SR3/60R I::::1lYJc:-z’G1 UIIIIACRON-R
0
Fig. 10. Histograms of percent decasting; C, storage in water.
dimensional
confirms previous results reported by Grunewald et a1.4 McCartney,” Becker et a1.,14Vieira,16 Zani and Vieira,” Woelfel I8 and Garfunkel.”
The AB and AC distances after storage of dentures in water expanded and reached the initial shrinkages; only the high-impact resin showed a final expansion in distance AB. The Acron Rapid resin failed to reach the initial processing contraction of AB and AC distances (paired t-tests; j6 < .OS). The behavior of each denture THE JOURNAL
OF PROSTHETIC
DENTISTRY
change in AC distance. A, Processing;
B,
resin in AB and AC distances between teeth during immersion in water was quite different as ANOVAs and Duncan’s test revealed (Table III). The removal of dentures from their master casts revealed a different behavior among the five resins studied. The SR 3/60 and SR 3/60 Rapid resins had a similar behavior (p > -05) and showed a further shrinkage of distances between teeth after decasting (Tables III and IV). ANOVA and Duncan’s test showed a more complex 645
POLYZOIS
ET AL
T T
A Fig. 11. Histograms of percent decasting; C, storage in water.
P dimensional
of SR 3/60 Plus, QC-20, and Acron Rapid resins for the AB and AC distances after decasting (Table III). These distances expanded slightly, compensating in part the initial contraction. The molar-to-molar distance (BC) contracted further for all the denture resins after decasting, but returned to the initial processing contraction after water immersion (paired t-test; p > .05). The behavior of each resin in molar-to-molar distance (BC) was similar for SR 3/60 and SR 3/60 Rapid resins 0, > .OS) but more complex for the SR 3/60 Plus, QC-20, and Acron Rapid resins (Table III). Our results are similar to those of Grunewald et a1.,4 Woelfel,‘8 and Garfunkel.” The final molar-to-molar shrinkage is a critical measurement across the posterior section of a complete maxillary denture. In this investigation final dimensional changes of molar-to-molar span had a range of 0.05 mm (SR 3/60 Plus) to 0.19 mm (SR 3/60 Rapid). Our findings are in agreement with those of McCartney,‘O Woelfel and Paffenbarger,” Woelfel et al.,‘” Woelfel,18 and Mowery et al.” Changes of this amount are difficult if not impossible to detect clinically. *,“, 13.lR,” Although the dimensional changes reported in this laboratory investigation concern in vitro dentures stored in water, predictions of the approximate in service behavior of clinical dentures can be made according to Woelfel et alI3 However, the clinical significance of laboratory tests is difficult to interpret.9,21-23Clinical controlled studies will be necessary to evaluate the behavior of denture base materials in an oral environment. Finally, a combination of laboratory and clinical testing is of utmost importance in the evaluation of new denture base materials. behavior
646
1;
change in BC distance. A, Processing;
SUMMARY
B,
AND CONCLUSIONS
An investigation was conducted to evaluate and compare the dimensional stability of three fast boilable denture resins with a conventional and a high-impact denture resin processed with a long-curing cycle. Although the boilable denture resins have been introduced to the dental profession and offer merits such as faster processing, saving time for the dentist, patient, and technician, reduction of energy costs, and dramatic increase in denture laboratory production without adding personnel or purchasing new equipment, the relevant dental literature is sparse. The results of this investigation indicate that all five denture resins produced dentures that shrink. Also measurements between certain teeth showed shrinkages. Linear shrinkages of denture bases and teeth distances were less than 1%. Flange-to-flange and molar-to-molar changes were less than 0.5 mm and 0.2 mm respectively. Maxillary complete dentures processed in boilable resins presented less distortion in the midpalatal area across the posterior section than dentures processed in the conventional heat-cured resin. Although linear changes reported in this investigation were thought to be clinically insignificant, clinical studies should be conducted to establish correlation with laboratory findings. We thank Ivoclar AC, Howmedica International Ltd., De Trey Division-Dentsply Ltd., for supplying the denture resins. We gratefully acknowledge the assistance of the following people: Mr. P. Wollwage, Research and Development of Ivoclar AG, Mr. J. Foster of Howmedica International Ltd. for information concerning boilable MAY
1987
VOLUME
57
NUMBER
5
DIMENSIONAL
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OF DENTURES
denture resins and Dr. 1’. E. Lagouvardos for the statistical tion and analy!sis.
consulta-
REFERENCES Woelfel JB, Paffenbarger GC, Sweeney WT. Dimensional changes occurring in dentures during processing. J Am Dent Assoc 1960;61:413-30. Woelfel JB, Paffenbarger GC. Dimensional changes occurring in artificial dentures. Int Dent J 1959;9:451-60. .3 Kern WR. Possible dimensional changes in denture base materials. J Am Dent Assoc 1941;28:1952-8. ‘I. Grunewald AH, Paffenbarger GC, Dickson G. The eff’ect of molding processes on some properties of denture resins. J Am Dent Asr.oc 1952;44:269-84. 5. Skinner EW’. .Jones PM. Dimensional stability of self-curing denture base acrylic resin. J Am Dent Assoc 1955;51:426-31. 6. dr <;ee .2J. ten Harkel EC, Davidson CL. Measuring procedure for the determination of the three-dimensional shape of dentures. J PKOS.I.H~~.DENY 1979;42:149-53. Stalford GL). Batcc JF, Huggett R, Handley RW. A review of the properttes ol \ome denture base polymers. J Dent 1980; 8:292-306. Rlurphy \VM. Bates JF. Huggett R, Bright R. A comparative study 01 tlhree denture base materials. Br Dent J 1982; I i2:2?3. klurphy Whl. liuggett R. Handley RW. A laboratory and clinical SIudy of Trevalon denture base material. J Oral Rehabil 1982;9:401. hfc(:arrney .JW. FI,anLe;e adaption discrepancy, palatal base distortion, and induced malocclusion caused by processing acrylic resin maxillarv complete dentures. J PROS.~HE.I. DENT 19tm4; 1.
52:545-53
,Ilirza I’ll. IXmensional stability of arrylic resin dentures: clinical evaluation J PROSTHE.I. DENT 1961:11:848-57. W’oelfel ~JB. Pall’enbarger GC. Method of evaluating the clinical effect of warping a denture: report of a case. J Am Dent Assoc 1959:59:?il-60
Accuracy
Woelfel JB, Paffenbarger GC, Sweeney WT. Changes in dentures during storage in water and in service. ~JAm Dent Assoc 1961;62:643-57. 14. Becker CM, Smith DE, Nicholls JI. The comparison of denturebase processing techniques. Part II. Dimensional changes due to processing. J PROSTHET DENT 1977;37:450-9. 15. Firtell DN, Green AJ, Elahi JM. Posterior peripheral seal distortion related to processing temperature. J PROSTHET DENT 1981;45:598-601. 16. Vieira DF. Ch anges in the relative position of teeth in the construction of denture bases. J Dent Res 1962;41:1450-60. 17. Zani D, Vieira DF. A comparative study of silicone as a separating medium for denture processing. J PROFTHET DENT 13.
1979;42:386-91.
18. Woelfel JB. Processing complete denture\;. Dent Clin North Am 1977;21:329-38. 19. Garfunkel E. Evaluation of dimenstonal changes in complete dentures processed by injection-pressing and the pack-and-press technique. J PROXHET DENT 1983;50:757-61. 20. Mowery WE, Burns CL, Dickson G, Sweeney WT. Dimensional stability of denture base resins. J Am Dent Assoc 1958;57:34553. 21. Bates JF, Stafford GD. Polymeric denture base specifications. Biomed Eng 1978;8:288. 22. Woelfel JB, Paffenharger GC, Sweeney WT. Some physical properties of organic denture base materials. J Am Dent Assoc 1963;67:489-504. 23. Staflord GD, Smith DC. Polycarhonates. A preliminary report on the use of polycarhonates as a denture base material. Dent Pratt Dent Ret 1967;17:217-23. Ke~mn/ rrcp3I lot DR. GREGORY L. POLYZOIS DEPhRThlENT OF RESTORATIVE DENTISTRY DENT.U. SCHOU. & HOSPITAL ~~E.z.I.11PARK CARIIIFF CF4 4X\ EN(;I.\ND
of zeroing the articulator
T. P. Nowlin, D.D.S., M.A.,* J. 0. Bailey, Jr., D.D.S., M.S.,* and T. R. Taubert,** University of Texas Health ScienceCenter, Dental School. San Antonio, Tex.
T
he accurate use of a semiadjustable articulator is in part contingent upon the dentist’s ability to consistently zero the instrument before mounting casts. Zeroing an articulator is the process of standardizing it to a reproducible starting point.’ In the case of the Hanau 130-28
*Associate Professor, Department of Restorative Dentistry. **Dental Ceramist Technician, Department of Restorative Dent.stry. THE JOURNAL
OF PROSTHETIC
DENTISTRY
semiadjustable articulator (Teledyne-Hanau Division, Buffalo, N.Y.) several criteria must be met for the articulator to be zeroed correctly. The incisal pin must strike the incisal table in the center of the table both anteroposteriorly and right to left. The offset pin, if used, should be set on the fifth mark. There should be no side play or side-to-side movement between the upper and lower members when the centric locks are secured.’ Some articulators come equipped from the manufacturer with zeroing devices designed to facilitate proper 647