Dimensional accuracy of castable apatite ceramic crowns: he influence of heat treatment on dimensional changes and distortion of crowns Hiroshi Taizo

Ishida,

DDS,”

Hamada,

DDS,

Yukinori

Nah’ara,

DDS,

PhD,b

and

PhDc

Hiroshima University School of Dentistry, Hiroshima, Japan Castable apatite ceramic crowns are subjected to heat treatment during crystallizing and during coloring after casting. The dimensional accuracy of the beat-treated crowns was investigated with respect to its influence on dimensional changes in castable apatite ceramic crowns and on distortion of the interior of the crowns, using two investment techniques and a three-coordinate measuring machine. Dimensional changes were not influenced by the number of coloring firings. Distortion of the interior of crowns was slightly outward at the gingival plane. Shrinkage because of crystallizing was approximately 20 pm and the shrinkage of the gingival plane was equal to that of the occlusal plane. (J PROSTHET DENT 1992;68:279-83.)

etal-ceramic restorations are the most commonly used esthetic restorations. They require fabrication of a cast metal coping and a multistage buildup of dental porcelain. Castable ceramic crowns can be fabricated using the lost wax process and relatively simple techniques.1-4 Castable apatite ceramic crowns are subjected to heat treatment during crystallizing and coloring. Therefore the

dimensional accuracy should be determined. dimensional changes and distortion of the gated.

MATERIAL

AND

of castable apatite ceramic crowns The influence of heat treatment on in castable apatite ceramic crowns interior of the crowns was investi-

METHODS

Die aInstructor, Department of Prosthetic Dentistry. bAssistant Professor, Department of Prosthetic Dentistry, Section of Dentistry for the Handicapped. cProfessor and Chairman, Department of Prosthetic Dentistry. 1011137423

A hardened stainless steel die with dimensions approximating a lower posterior complete crown preparation was used for wax pattern construction (Fig. 1). The die taper of 4 degrees (total convergence angle of 8 degrees) was considered within the accepted range recommended for resistance, retention,5 and stress concentration6 of porcelain castings.

Cap

I-

2.0

i-11.04

unit : mm

unit : mm Fig.

THE

1.

JOURNAL

Die assembly

OF

PROSTHETIC

for wax pattern

DENTISTRY

construction.

Fig.

2. Position

of wax pattern.

279

ISHIDA,

(A)

NAHARA,

AND

HAMADA

(6)

CROWN

Dimensional

change [ !I”,:$:

Fig. 3. Measurement

of dimensional change. A, Determination of the reference plane (shoulder of the crown). B, RI is radius of a circle formed by the plane at a distance of X millimeters from the reference plane and the interior of the crown. HI is height of the interior of the crown. C, R2 is radius of a circle formed by the plane at a distance of X millimeters from the reference plane (shoulder of the die) and the die. He is height of the die.

-L

-20

o

20

I

-T -I--

0: As-cast crown 0: Glass-ceramic

crown

40 60 80 Dimensional change (pm)

Fig. 4. Dimensional changes at R for ring liner specimens. Wax patterns Complete crown patterns were formed with wax (Blue Inlay Wax, G-C Dental Industrial Corp., Tokyo, Japan) at approximately 80’ C by incrementally building, compressing (15 kg/cm2), and holding for 5 minutes. The pattern was stored at room temperature for at least 2 hours to allow the release of residual strain in the wax. A 2.5 mm sprue was luted to the center of the occlusal surface and the wax pattern was removed from the die, mounted on the sprue former, and was immediately invested (Fig. 2).

Investment

techniques

Two techniques were compared: (1) A stainless steel ring lined with one layer of 1.2 mm thick special liner was pre280

pared. The surface of the liner was coated with petroleum jelly to prevent liquid sorption by the investment mix. (2) A wax ring was used for making a ringless investment mold. The wax ring was separated from the investment after setting. The wax pattern was invested using a special phosphatebonded investment. The investment was mechanically spatulated using a vacuum mixer (Vat-U-Vestor, WhipMix Corp., Louisville, Ky.) and decanted into the ring containing the wax pattern according to the manufacturer’s instructions. A total of 10 wax patterns (five for the ring liner and five for the ringless) were fabricated and invested at 23’ C (F 1” C) and at 55 % ( t 10 % ) humidity. The investment molds were allowed to bench set overAUGUST

199%

VOLUME

68

NUMBER

2

DIMENSIONAL

CHANGES

IN APATITE

CROWNS



‘.-

E a 3.52= g 3.0i g 2.5-

=: Glass-ceramic

2 F 2.0a 1.5 -

-k-

-L -A-

1.0 0.5

I b

-20 Fig.

Table

I. Dimensional

I

0: As-cast crown

0a

-!-

20

40 I 60 80 Dimensional change ( ,Um )

5. Dimensional

changes at R for ringless

Distance reference

from

plane

As-cast

(mm)

(one

layer) Coloring

Coloring

crclwn

(3 firings)

(6 firings) 31 (4)

4.0

47 (5)

28 (5)

31 (6)

3.5

50 (6)

30 (7)

32 (8)

32 (6)

3.0

54 (7)

33 (8)

34 (9)

33 (8)

2.5

54 (5)

33 (9)

34 (8)

33 (7)

2.0

55 (6)

34 (8)

1.5

56 (7)

32 (7) 36 (7)

34 (7) 35 (7)

1.0

56 (6)

36 (7)

36 (8) 36(7)

0.5

54 (6)

34 (7)

33 (7)

Values are in microns (pm). Numbers in parentheses are standard deviations. R, Radius of circle formed by a plane X millimeters

(18)

from reference

104

plane and interior

night. They were then heated to 300” C, heat-soaked for 30 minutes, raised to 800” C, and heat-soaked for 1 hour. The investment molds were then cooled to 600’ C and held for 30 minutes. The castings were completed automatically by a motor-driven centrifugal casting machine. A castable apatite ceramic materia12a3 (Cerapearl, Kyocera Corp., Kyoto, Japan) was placed in a disposable crucible, melted at 1520° C, and cast into each mold. Each mold was then heated to 750’ C and heat-soaked for 10 minutes to allow the release of residual strain for annealing. The molds were cooled to room temperature, and the investment was cleaned from the castings by an air abrasive unit under low pressure with 50 pm aluminum oxide. The “as-cast” crowns were cleaned ultrasonically in distilled water. The “as-cast” crowns were embedded in an investment material and placed in a crystallizing furnace. They were heated to 885” C, heat-soaked for 60 minutes, and cooIed OF PROSTHETIC

liner

Glass-ceramic

Clown

120

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specimens.

changes for ring liner specimens. Ring

THE

crown

DENTISTRY

(22)

104

of crown; H, height of interior

(22)

36 (6) 33 (6) 100

(21)

of crown; see text for explanation.

to room temperature to fabricate the glass-ceramic apatite crown. The glass-ceramic apatite crowns were then blasted with 50 wrn aluminum oxide and cleaned ultrasonically in distilled water.

Coloring After crystallizing, glass-ceramic apatite crowns are usually fired three to six times during coloring. The 10 glass-ceramic apatite crowns in the experiment were fired three times for 1 minute each (three firings), measured, and were then fired three times for 1 minute each (six firings) in air at 820” C.

Measurement

of dimensional

change

The method of measurement is shown in Fig. 3. The interior surfaces of the crowns were measured using a threecoordinate measuring machine (TST GOO-FC, Nikon Corp., 281

ISHIDA,

Table

II. Dimensional

changes for ringless

NAHARA,

AND

HAMADA

specimens Ringless

Distance reference

from plane

As-cast crown

(mm)

4.0 3.5 3.0 2.5 2.0 1.5

R

43 47 50 52 50 53 52 50 115

1.0 0.5 H Values are in microns (pm). Numbers in parentheses are standard Abbreviations as in Table I.

Table

III.

Shrinkage

(3) (2) (2) (3) (3) (4) (5) (7) (32)

of crowns caused by crystallizing

R

from plane

(mm)

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

H Values are in microns (em). Numbers in parentheses are standard Abbreviations as in Table I.

Ring liner (lone layer)

19 (1)

20 (2) ‘21(2) 21 (5) 23 (1) 20 (1)

20 (2) 20 (5) 16 (7)

Ringless

19 22 19 21 18 19 20 17 21

(3) (3) (3) (5) (4) (3) (4) (4) (10)

deviations.

Tokyo, Japan) graduated to 0.5 pm with a ball stylus measuring 1 mm in diameter. The shoulder of the crown was measured at 12 points with the stylus, and the reference plane was determined using the method of least squares. The axial wall, at a distance of X mm from the reference plane, was measured at 12 points with the stylus, and the radius of the circle formed by the plane at a distance of X mm from the reference plane anid the interior of the crown was determined (RI). The occlusal surface was then measured at 12 points with the stylus and the height of the interior of the crown (HI) was determined. Measurements were made after annealing (“as-cast” condition), crystallizing (glass-ceramic crown), and coloring (glass/ceramic apatite crown with three and six firings). The dimensional change of the die was measured and the difference between the crown and die dimensions was recorded as dimensional change (R and H). The crowns and die were measured at 20° C (&lo C) and at 50% (*5%) humidity.

282

24 25 30 30 32 34 33 33 94

(5) (3) (4) (5) (5) (6) (8) (8) (28)

Coloring (3 firings)

25 26 30 31 31 33 32 30 95

Coloring (6 firings)

(4) (4) (5) (5) (5) (6) (7) (8) (23)

24 25 28 31 30 32 31 30 93

(7) (5) (5) (5) (5) (7) (7) (9) (26)

deviations.

Shrinkage Distance reference

Glass-ceramic crown

RESULTS The results of this investigation are shown in Tables I and II. Statistical analysis was performed using the Student’s t test. Although no significant differences existed (p > 0.01) between the values of the ring liner specimens and those of the ringless specimens for annealing, crystallizing, and coloring, the values of the ring liner specimens were larger than those of the ringless specimens at each site. Statistical evaluation of the values at R and H showed no significant differences (p > 0.01) among the glass-ceramic crowns fired three and six times. The dimensional changes at R are shown in Figs. 4 and 5. For “as-cast” crowns, the expansion difference between the gingival and the occlusal plane was observed and the nature of distortion was investigated. The values of the gingival plane were slightly larger than those of the occlusal plane, and the distortion was slightly outward at the gingival plane. The shrinkage of the crowns caused by crystallizing of the glass-ceramic crown is shown in Figs. 4 and 5 and in Table III. The values of the ring liner specimens were much the same as those of the ringless specimens and the shrinkage of the crowns was approximately 20 pm. The shrinkage of the gingival plane was the same as that of the occlusal plane.

DISCUSSION Since setting and thermal expansion of the investment are affected by restrictive stresses,7, 8 investment techniques employing two layers of ring liner? or ringless techniques3 were used. In the Cerapearl system, a 1.2 mm thick resilient ring liner was used. Therefore the influence of such a ring liner on dimensional changes was investigated and was compared with ringless techniques to evaluate the efficacy of the liner. Although the values of the ring liner specimens were not statistically different from those of

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CROWNS

ringless specimens, the former were larger than the latter. This ring lining material minimized restraint and provided mold expansion as great as ringless investment techniques. After crystallizing, castable apatite ceramic crowns are repeatedly fired in an oven for coloring. The influence of these coloring firings on dimensional changes in crowns was investigated. Statistical analysis showed no significant difference between glass-ceramic crowns fired three and six times. The distortion of the interior of “as-cast" crowns was also investigated, and it was shown that the distortion was slightly outward at the gingival plane. It was also observed that shrinkage caused by crystallizing was approximately 20 pm, and shrinkage of the gingival plane was equal to that of the occlusal plane by three-coordinate measurement. Although dimensional changes between crowns and dies can be measured with cross-sectional techniqueslo the crowns are necessarily destroyed. If a three-coordinate measuring machine is used, the dimensional changes can be measured nondestructively. Because the crowns are not destroyed, this measuring system is useful in studying castable ceramic or metal-ceramic restorations subjected to repeated firings.

SUMMARY 1. Special ring liners provided mold expansion equal to ringless investment techniques. 2. Dimensional changes in castable apatite ceramic crowns were not influenced by the number of coloring firings. 3. Distortion of the interior of “as-cast” crowns was slightly outward at the gingival plane. 4. Shrinkage caused by crystallizing was approximately 20 pLmand shrinkage of the gingiva1 plane was equal to that of the occlusal plane. 5. Nondestructive measurements using a three-coordi-

Availability

nate measuring system were capable of determining the nature of distortion and the influence of several factors on dimensional changes in crowns. This study seems to provide further insights into crown fabrication (for example, investing, coloring, and use of paint-on die spacer) to improve the fit of castable apatite ceramic crowns. REFERENCES 1. Adair PJ, Grossman DG. The eastable ceramic crown, Int J Periodont Restor Dent 1984;2:33-45. 2. Hobo S, Iwata T. Castable apatite ceramics as a new biocompatible restorative material. I. Theoretical considerations. Quintessence Int 1985; 2:135-41. 3. Hobo S, Iwata T. Castable apatite

ceramics as a new biocompatible restorative material. II. Fabrication of the restoration. Quintessence Int 1985;3:207-16. 4. Malament KA, Grossman DG. The cast glass-ceramic restoration. J PROSTHET

DENT

1987;57:674-83.

5. J$rgensen KD. The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Stand 1955;13:35-40. 6. El-Ebrashi MK, Craig RG, Peyton FA. Experimental stress analysis of dental restorations. Part IV. The concept of parallelism of axial walls. J PROSTHET DENT 1969;22:346-53. 7. Mahler DB, Ady AB. The influence of various factors on the effective setting expansion of casting investments. J PROSTHET DENT 1963;13: 365-73.

8. Jones DW, Wilson HJ. Variables affecting the thermal expansion of refractory investments. Br Dent J 1968;125:249-55. 9. Eden GT, Franklin OM, Powell JM, Ohta Y, Dickson G. Fit of porcelain fused-to-metal crown and bridge castings. J Dent Res 1979;58: 2360-S.

10. Davis DR. Limiting J PROSTHET DENT Reprint

requests

wax pattern

distortion

caused by setting expansion.

1987;58:229-34.

to:

DR. HIROSHI ISHIDA HIROSHIMA UNIVERSITY SCHOOL OF DENTISTRY KASUMI l-2-3, MINAMI-KU, JAPAN

HIROSHIMA

134

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Dimensional accuracy of castable apatite ceramic crowns: the influence of heat treatment on dimensional changes and distortion of crowns.

Castable apatite ceramic crowns are subjected to heat treatment during crystallizing and during coloring after casting. The dimensional accuracy of th...
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