FACIALLY

BUTTED

PORCELAIN

MARGINS

2. The density and strength of porcelain-wax and porcelain-resin combinations were significantly lower than those of unmodified porcelain. 3. Low density and strength limit the rational use of modified porcelains to final bakes for correcting small, troublesome labiogingival discrepancies. REFERENCES 1. West AJ, Goodacre CJ, Moore BK, Dykema RW. A comparison of four techniques for fabricating collarless metal-ceramic crowns. J PROSTHET DENT 1985;54:636-42. 2. Vickery RC, Badinelli LA, Waltke RW. The direct fabrication of restorations without foil on a refractory die. J PROSTHET DENT 1969;21:22734. 3. Vryonis P. A simplified approach to the complete porcelain margin. J PROSTHET DENT 1979;42:592-3. 4. Onar R. Scanning electron microscopy of the marginal fit of ceramometal restorations with facially butted porcelain margins. J PROSTHET DENT

1987;58:13-8.

5. Prince J, Donovan

TE, Presswood

RG. The all-porcelain

for ceramometal

restorations:

a new concept.

J PROSTHET

DENT

1983;50:793-6.

6. Schrader JA, Duke ES, Haney SJ, Herbold ET. Volumetric shrinkage of a porcelain suspended in wax technique. J PROSTHET DENT 1986;55:302-4.

7. Wiley MG, Huff TL, Trebilcock C, C&van TB. Esthetic porcelain margins: a modified porcelain-wax-technique. J PROSTHET DENT 1986;56:527-30.

porcelain margins: a 8. Pinnell DC, Latta GH, Jr, Evan JB. Light-cured new technique. J PROSTHET DENT 1987;58:50-2. 9. Edge MJ, Maccarone T. An alternate method for establishing porcelain margins. J PROSTHET DENT 1987;57:276-7. 10. Young RK, Veldman DJ. Introductory statistics for the behavioral sciences. 2nd ed. New York: Holt, Rinehart & Winston, Inc, 1972;300-12. 11. Daniel WW. Biostatistics: a foundation for analysis in the health sciences. 3rd ed. New York: John Wiley & Sons, 1983;224-6. Reprint

requests

to:

DR. EUGENE F. HUCET COLLEGE OF DENTISTRY UNIVERSITY OF TENNESSEE MEMPHIS, TN 38163

labial margin

Effects of the temperature of cooling water during high-speed and ultrahigh-speed tooth preparation H.-Ch. Lauer, Dr.Med.Dent., Dr.Med.Dent.Habil.,* E. Kraft, Dr.Med.Dent.,** W. Rothlauf, Dr.Med.Dent.,*** and Th. Zwingers, Dipl.Ing.**** Ludwig-Maximilians

University, School of Dentistry, Munich, Fed. Rep. Germany

In vitro measurements of heat production in the pulp chamber during tooth preparation were performed on intact third molars. The experiments were designed to simulate physiologic temperature conditions in the tooth and oral cavity and to standardize parameters of tooth preparation. Two drive systems, the turbine and the high-speed angle, were compared by using two ranges of cooling water temperature. The critical temperature of 41’ C to 42’ C that is irreversibly harmful to pulpal tissue was not reached with a cooling water temperature of 30’ C to 34” C. Because the temperature elevation during turbine preparation was dependent on the diminishing thickness of remaining dentin, in preparing teeth close to the pulp, a high-speed angle was advantageous. (J PROSTIIET DENT 1990;63:407-14.)

K

erschbaumand Vossi found that 10 years after crown placement 9.1% of these teeth were not vital and 5.5% recorded only an equivocal reaction. These results have been confirmed in similar studies.2,3 Heat production during tooth preparation and mechan-

Presented before the European Prosthodontic Association, Dresden, GDR. *Professor, Department of Prosthetic Dentistry. **Professor and Director, Department of Prosthetic Dentistry. ***Research Assistant, Department of Prosthetic Dentistry. ****Statistician, Biometric Center for Therapeutic Studies. 10/1/17003

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ical damageare major sourcesof trauma.4-6In vivo studies have demonstrated irreversible tissue damagewhen the temperature in the pulp chamber reached 42.P C.7 Schubert! regarded41.5” C asthe critical temperature for causingtissue irritation. Technical advancements have produced a constant stream of improvements in dental drive systems.With the rotational speedand power of the turbine handpiece, the dentist is capableof contact grinding for tooth preparation that was previously the exclusive domain of the micromotor.g This in vitro study measuredheat production in the pulp chamber during high-speed and ultrahigh-speed tooth preparation and the effects of the cooling water.

407

LAUER

ET AL

PRESSURIZED WATER AIR VOLTAGE l-l-lPRESSURE

DRIVING

RATE OF DRIVING COOLING

AIR

COOLING SUPPLY

AIR

AIR

WATER VOLTAGE

Fig. 3. Plexiglass housing with hot-air blower.

Fig. 1. Block diagram of test setup.

Fig. 2. Device for extraoral preparation of teeth for replant&ion with mechanical holding apparatus, recipient for nutrient medium and heating coil.

MATERIAL

AND

METHODS

!l’urbine and high-speed handpiece. The study used a Super-Torque LUX 630B turbine and an INTRA K 186B micromotor with an INTRAmatic LUX 24L high-speed angle and INTRA LUX 2304LD head (Kaltenbach und Vogt, Biberach, FRG). The rotational speed of both drive systems was determined with an electronic “rev” counter (Moviport D711, Industrie Elektronik, Stuttgart, FRG). The turbine recorded a free-running speed of 335,000 rpm, corresponding to a speed of 220,000 to 260,000 rpm during preparation.‘O The free-running speed of the high-speed 408

angle (160,000 to 165,000 rpm) was maintained during grinding, because the drive is electronically regulated so that the rotational speed was independent of the load. The contact force during preparation with the turbine was 1 Ngv l1 and with the micromotor 2 N.‘Opl2 Both drive systems were provided with a three-jet cooling systemi similar in design. To achieve efficient spray cooling, the flow rate of the cooling air was 0.1 ft3/min and the cooling water was maintained at 50 ml/min.12 Preliminary experiments were conducted by using a microthermocouple at the entrance of the instrument shaft to measure the temperature of the cooling water. Although an initial cooling water temperature of 20” C or room temperature was used, temperatures of more than 40’ C were recorded during continuous operation of the turbine. The peak temperatures were approximately 45’ C after 8 to 10 minutes intermittently operating a high-speed angle at cycles of 30 seconds followed by a 30-second pause. Therefore, two different temperatures were selected for the investigation, 32’ C recommended by the manufacturer and 42’ C for extended periods of operation. Burs. A cylindrical diamond bur of medium grain size (No. 837014, Komet, Lemgo, FRG) was used to maintain constant speeds. A bur length of 8 mm and a diameter of 1.4 mm were chosen to create optimal cooling by the spray jet.i4 The burs were replaced after three trials with a total grinding time of 2 to 3 minutes. Experimental

design

The block diagram in Fig. 1 presents an overview of the experimental design. The equipment (Fig. 2) devised for reimplantation was assembled in a closed Plexiglass housing (Fig. 3) to simulate the physiologic temperature conditions in the tooth and the exchange of supplied heat with that of the environment.15r l6 The tooth was clamped in the upper third of the root with this apparatus to facilitate preparation in the region of the crown. The lower two thirds of the root was embedded in semisolid 2% agar in physi-

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THERMALLY REFERENCE FOR THERM0

Fig.

Fig.

4. Block diagram of temperature measurement.

5. Air-supported slide.

ologic saline at 37.5OC. An electronic thermostat and a hot-air blower maintained the intraoral temperature in the housingat 34’ + 0.5’ C.17,laThe initial temperature in the pulp chamber was 37” + 0.4” C. Two encasedNiCrNi thermocouples (Philips, Kassel, FRG)lS of a 0.25mm crosssectionwereusedto measurethe temperature in the pulp chamber.The thermocoupleswere connected to two digital temperature meters (Therm 2220-4 and Therm 2220-5, Ahlborn Mess und Regeltechnik, Holzkirchen, FRG) that permitted continuous monitoring. An additional temperature meter (Therm 2220-7) recorded the temperature of the cooling water at the entrance of the instrument shaft. This measurementwas accomplishedwith a NiCrNi thermocouple (DIN 43710, Kaltenbach und Vogt, Biberach, FRG). The plug connections of the thermocouplesformed the reference points for the temperature measurements(Fig. 4). The reference points were additionally insulated with foam in plastic containers to prevent temperature fluctuations due to warm-air convection in the Plexiglasshous-

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aTHERM / ITHERMO

COUPLE COUPLE

1 2

Fig. 6. Temperature responsein pulp chamber during preparation by turbine handpiece. Cooling water temperature 31.8” C; T,, temperature in pulp chamber (“C); t, time (s); Si, Ss, Ss:grinding intervals.

ing. A water circuit parallel to the circuit, controlling the temperature of the semisolidmedium and supplied with a thermostat-controlled pump (Colora, Larch, FRG), regulated the temperature of the referencepoints. The temperatures were recordedin graphic form with an XT recorder (Riken Denshii Marketing Company: OmniRay AG, Zurich, Switzerland) connected to an integrated operational amplifier.

LAUER

ET AL

35.0.

57.5.

q THERM0 aTHERM

aTHERM

COUPLE

1

I iTHERM

COUPLE

2

iii;; 0

Fig. 7. Temperature

response in pulp chamber during preparation by high-speed angle. Cooling water temperature 31.6” C; Tp, temperature in pulp chamber (“C); t, time (8); S1, Se, Sa, grinding intervals.

COUPLE COUPLE

2 1

i 30

so

50

130

tM

.

Fig. 9. Temperature

response in pulp chamber during preparation by high -speed angle. Cooling water temperature 43.6’ C; T,, temperature in pulp chamber (“C); t, time (s); SI, SZ, Ss, grinding intervals. A rapid-tensioning device with a special mount on an air-supported slide (Kaltenbach und Vogt, Biberach, FRG) was assembled (Fig. 5) to accurately measure and reproduce the contact force, the angle. of the bur to the tooth, and the thickness of the layer to be removed. A pan containing weights to adjust the contact force was connected to the slide by a thread. The thickness of hard tooth structure removed at each step was controlled with a micrometer screw, because the drive system was attached with a swivelling arm to a tensioning device. A second micrometer screw was used to adjust the height of this bearing arm so that the bur was suitably directed to the tooth during reduction. The Plexiglass housing was also provided with a suction apparatus to simulate intraoral tooth preparation.

Experimental

q THERM0 ATHERMO

Fig. 8. Temperature

COUPLE COUPLE

2 1

response in pulp chamber during preparation by turbine handpiece. Cooling water temperature 43.2” C; Tv, temperature in pulp chamber (“C); t, time (8); Sr, Se, Sa, grinding intervals.

410

protocol

Tooth preparation was performed on intact thiid molar teeth stored in isotonic saline solution during the maximum S-hour interval between extraction and the commencement of the experiment.20 The teeth were approximately the same dimensions to provide similar wall thicknesses of hard tooth structure and ensure equal distances from the pulp to the surface of the tooth.21 The thermocouples were introduced into the pulp chamber through two mesiodistally adjacent and anatomically suitable root canals. The positioning of the thermocouples was then checked radiographically.

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0

w-

0 m

2,0-

:

%5

AT*--

- -to-

+-.--.*-.--

--

--

E :: m

W0 1 -0.5

I -. I,0

i

AT-;; - --.--,--*--I--.--. . $

-I *.. -.2.5 i

:

3,o

Fig. 10. Relative change of temperature in pulp chamber during preparation by turbine handpiece. *, Measuring results 01 to 16; cooling water temperature 29.P to 31.8OC; 0, measuringresults 17 to 32; cooling water temperature 42.2Oto 44.4’ C; AT, relative changeof temperature in pulp chamber(“C); ATl, meanvalue AT of measuringresults01 to 16; AFz, mean value AT of measuringresults 17 to 32.

Fig. 11. Relative change of ~m~erature in pulp chamber during preparation by high-speed angle. er Measuring results 01 to 16; cooling water temperature 31.4” to 33.7’ C; 0, measuringresults 17 to 32; cooling water temperature 42.1” to 43.6’ C; AT, relative changeof temperature in pulp chamber (“C); ATl, meanvalue AT of measuringresults01 to 16; ATs, meanvalue A?.’ of measuringresults 17 to 32.

The pressureand flow rates of the driving air, including the cooling air and water, were controlled with the assistance of a specialsupply unit (Kaltenbach und Vogt, Biberach, FRG). Four different test serieswere conducted for a group of eight teeth. The temperature in the pulp chamber during tooth preparation, both with the turbine and with the high-speedangle, wasdetermined for two different ranges of cooling water temperature. The depth from the three grinding intervals was 0.8 mm, 0.5 mm, and 0.5 mm, and two synchronized thermocouples recorded temperature changesduring each test.

C to 3.5OC in relation to the referencevalue. The recorded curves of the two thermocouplesdisplayed a difference, reflecting the various times required by the thermal “front” generatedby the bur to reach the sensortips of the thermocouplesduring preparation. In the lower cooling water temperatures, the initial valuesfor high-speedangle were approximately lo C to 2’ C higher (31.4’ C to 33.7’ C). This wasattributed to the preliminary warming of the cooling water in the instrument shaft resulting from the high frictional heat generated by the transmissionbearings.The initial temperature reduction was consequently smaller (Fig. 7). After each of the grindings there was a substantial rise in temperature that reached the initial value of 37” C. The relative drop in temperature in relation to the third grinding wasbetween 0” C and 2.7” C. Sincethe contact force with the high-speed anglewasapproximately 1 N greater, the associatedgrinding times were shorter than with turbine prep~at~on. Turbine preparations using cooling water temperatures of 42.2’ to 44.4’ C (Fig. 8) created increasesin temperature that reached their peaksduring the pauses.A slight drop in temperature at the beginning of the first grinding step was followed by a temperature increasethat reached the highest value after the third grinding step. In this temperature range of cooling water, the relative increasein tem-

RESULTS There was a continual drop in temperature in the pulp chamber after the turbine preparation commencedwith coolingwater temperaturesbetween29.8’ and 31.8” C (Fig. 6). After the initial grinding, the temperature decreased slowly. After the second grinding, the temperature remained relatively constant or increasedslightly. After the third grinding, there was a substantial elevation in temperature causedby abrasion of material by the bur. This rise in temperature wasmore marked as the thickness of remaining dentin wasreduced.After the third grinding; the temperature in the pulp chamber had diminished by 0.7’

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411

LAUERETAL

r,,-

--.--.--

--.o

O.--.-~-.--O--.--.--.--

43.0-

--

-_.--.-

I I 0

I I

0

42.5-

0 0

I I I I

0 0

42.00

I 0,s

I ! L

1,o

. 1.5

2.5

2.0

AT [Oc]

AT

Fig. 12. Turbine handpiece. Response of relative temperature increase in pulp chamber to cooling water temperature. Grinding time 24 to 36 seconds; residual thickness of dentin 0.9 to 2.5 mm; AT, relative increase of temperature in pulp chamber (“C); A'T, mean value of AT, TCT, cooling water temperature (“C); ?ic~, mean value of TCT.

I I

to-

0

I I I I I

05-

-I 0

I 095

lx)

1

1,5

290

.

25 AT [oc]

AT

Fig. 13. Turbine handpiece. Response of relative temperature increase in pulp chamber to residual thickness of dentin. Grinding time 24 to 36 seconds; cooling water temperature 42.2’ to 44.4’ C; AT, relative increase of temperature in pulp chamber (“C); AT, mean value of AT; d, residual thickness of dentin (mm); 3, mean value of d.

perature was 0.4O C to 3” C. The peaks in the temperature curves were more pronounced compared with preparation with lower cooling water temperatures. This was apparent after the first two grinding steps. The high-speed angle created an elevation in temperature in the pulp chamber with cooling water temperatures of 42.1’ C to 43.6’ C (Fig. 9). After three grinding steps, this temperature was between 0.1” C and 2.8’ C above the initial value. The temperature curve revealed less-pronounced peaks after the first two grinding steps, but the tempera412

ture increase was greater after each successive grinding. However, times were less than with turbine preparation.

Statistical systems increase

comparison of the two drive in terms of relative temperature in the pulp chamber

Sixteen measurements were recorded for each test series after the third grinding step. Figs. 10 and 11 illustrate the recorded values in relation to the respective arithmetic means. The drop in temperature was greater with turbine APRIL

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preparation (0.7’ to 3.5’ C) than with the high-speed angle (0’ to 2.7’ C) with lower cooling water temperatures. The arithmetic mean of the temperature reduction was also significantly lower with turbine preparation (-1.9” C) than with the high-speed angle (-0.9’ C). With higher temperatures of cooling water, there were no differences between the arithmetic means of the relative temperature increase in the pulp chamber (turbine 1.3” C; high-speed angle 1.4’ C) or between the individual values (turbine O.4Oto 3’ C; high-speed angle 0.1“ to 2.8’ C) of the two systems. With turbine preparation, a relationship between the cooling water temperature and the thickness of the remaining dentin was observed (Figs. 12 and 13). Higher temperatures of cooling water and reduction in dentin thickness increased the relative temperatures in the pulp chamber. Conversely, the relative temperature elevation in the pulp chamber with the high-speed angle was increased only by higher cooling water temperatures.

DISCUSSION The introduction of high-speed and ultrahigh-speed drive systems has necessitated specific precautions during tooth preparation to protect the vitality of restored teeth.1*4 This study introduced an in vitro experimental design that included investigation of relevant parameters for tooth preparation. The results demonstrated that preparation with cooling water temperatures of 29.8’ to 33.7O C with the turbine or the high-speed angle did not produce an increase in temperature in the pulp chamber after three grinding steps. The three-jet, cooling spray system counteracted the heating of the tooth that was not naturally dissipated. Therefore, damaging irritation of the pulp does not occur at these temperatures. Continuous operation, which includes fiberoptic turbines and high-speed angles, elevates cooling water temperatures above 44“ C at the entrance of the instrument shaft. The admixture of cooling air reduces this temperature approximately 2’ C in the spray.22 Both drive systems produced temperature increases in the pulp chamber, but the elevation was higher with the turbine than with the high-speed angle. The results of this investigation were similar to other in vivo studies.gx 23The individual temperature increases, which were apparent after the first grinding step, became greater with subsequent grinding. Particular attention should be directed to the temperature of the cooling water during tooth preparation. At temperatures above 40’ C there was a clear relationship between the relative increase in the temperature of the pulp chamber and the temperature of the cooling water. An increase in cooling water temperature of lo C leads to an increase of lo C in temperature in the pulp chamber. The temperature elevation with turbine preparation was also dependent on the diminishing thickness of remaining dentin. In the vicinity of the pulp, tooth preparation with the

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high-speed angle is preferable. When either system is used, preparation should be performed with intervals to avoid raising the temperature of the cooling water. The two systems can, therefore, be regarded as complementary instead of competitive when perceptively deployed.

SUMMARY The heat developed in the pulp chamber during highspeed and ultrahigh-speed grinding was measured by use of an in vitro experimental design. Specific attention was directed to determine the influence of the type of drive and the temperature of the cooling water. In vivo conditions were simulated by providing a pulp temperature of 37” C and a temperature of 34’ C in the experimental oral cavity. The experimental settings for the drive, the cooling system, the contact force, and the thickness of the hard tooth structure removed were established from previous investigations. The results emphasized the importance of the cooling water temperature, which should not exceed 35’ C. After prolonged periods of tooth preparation, increases of approximately 3” C in the temperature of the pulp chamber resulted with both the turbine and the high-speed angle systems when water temperatures were between 42.1” and 44.4O C. During protracted preparations, these temperatures may be exceeded and thermal damage to the pulp tissue can be anticipated. Conversely, the temperature of the pulp chamber diminished with cooling water temperatures between 29.8’ and 33.7’ C.

REFERENCES

4. 5.

9. 10.

11.

12. 13.

Kerschbaum Th, Voss R. Zum Risiko der ijberkronung. Dtsch Zahnarztl z 1979;34:740-3. Kerschbaum Th, Voss R. Die praktische Bewiihrung van Krone und Inlay. Dtach Zahnarztl Z 1981;36:243-9. Internationales Institut fti zahniintliche Praxisfiihrung. Priiparationstrauma durch zu geringe Spraywassermenge. Schweiz Monataschr Zahnheilkd 1983;93:192. Klatzer WT. Die traumatische Schiidigung der Pulpa bei der ijberkronung. Dtsch Zahnarztl Z 1984;39:791-4. Polman-Moy AC. Temperaturmessungen in Zahnhartsubstanzen beim normal-, hoch- und hdchsttourigen Schleifen. D&h Zahnarztl 2 1963;18:130-5. Musil R. Rationelle Verfahren der Kronenprlparation. Stomatol DDR 1977;27:552-5. Zach L, Cohen G. Pulp response to externally applied heat. Oral Surg 1965;19:515-30. Schubert L. Temperaturmessungen im Zahn w&rend des Schleif-und Bohrvorgangs mittels des Lichtatrichgalvonometers. Zahnarztl Welt 1957;58:768-72. Simon U. Vergleichende Messungen van handelsiiblichen Bohimaschinen und Turbinen. Dtach Zahnarztl 2 1979;34:768-72. Martin HH. VergleichendeUntersuchung iiber die Temperaturentwicklung und die Schleifleistung van Turbine und Mikromotor mit Schnellaufwinkelstiick bei der Kronenstumpfpriiparation. Med Diss; Freiburg, 1983. Eifinger FF, Schulz R-P. Temperaturmessungen im Pulpakavum w&rend der Onlay- und Inlayprtiparation. Schweiz Monatsschr Zahnheilkd 1979;89:1239-49. Bleicher P. Bohren und Schleifen 1981. Quintessenz 1981;32:1033-44. Klaiber B, Eibofner E, Gleinser A, Lingenhale B. Der Kiihleffekt verschiedener Spraysysteme bei Turbine und Schnellaufwinkelstiick. Dtsch Zahnarztl Z 1985;40:1194-7.

413

14. Nentwig

15. 16.

17.

18. 19. 20.

GH. Die Kronenpriiparation unter Berticksichtigung der zur Zeit erhiiltlichen Praparationssiitse. Med Diss; Koln, 1978. Lauer H-Ch. Eine Vorrichtung zur extraoralen Praparation von Ziihnen ftir die Replantation. Dtach Zahnantl 2 1985;40:850-2. Lauer H-Ch. Experiment&e Untersuchungen zur WjiImeentwickhmg im Pulpakavum durch Kunststoffprovisorien. Dtsch Zahnarztl 2 1986,41:468-72. Fuhr K. Vergleichende Untersuchung tiber die Temperaturverhiiltnisse beim zahniirztlichen Bohren und Schleifen. Dtsch Zahnarztl Z 1963; l&986-91. Maeda R, Stolze K, User A, Kroone H, Brill N. Oral temperatures in young and old people. J Oral Rehabil 1979;6:159-65. Lenz P, Gilde H. Temperaturverlauf im Pulpakavum bei Schmelzversiegelung mit Laserstrahlen. D&h Zabnarztl Z 1978;33:623-8. Klijtser WT. Tierexperimentelle Prtifung von Mate&lien und Methoden der Kronen- und Brtickenprothetik. Med Habilschr, Ttibingen, 1971.

Effect of opaque silver-palladium D. G. Jochen, Wadsworth Dentistry,

D.D.S.,*

porcelain alloys A. A. Caputo,

Veterans Administration Los Angeles, Calif.

Medical

21. Stambaugh RV, Wittrock JW. The relationship of the pulp chamber to the external surface of the tooth. J PR~STHET DENT 1977;37:537-46. 22. Klos H, Rottke R. Experimentelle Untersuchungen zum Temperaturproblem beim hiichsttourigen Bohren und Schleifen mit Turbinenger&ten. Dtech Zahnarztl Z 1960;15:1659-79. 23. Stiiben J, Hoppe WF. Experimentelle Untersuchungen fiber die Veriinderungen der Pulpa nach normal- und hijchsttourigem Schleifen und Bohren im histologischen Bild. Dtech Zahnarztl 2 1964,19:601-11. Reprint

application Ph.D.,** and Center,

requests

to:

DR. HANS-CHRIS~PH LAUER LUDWIG-MAXIMILIANS UNNEWITY SCHOOL OF DENTISTRY GOETHJNXASSE 70 8000 MUNICH 2, FED. REP. GERMANV

J.

and University

on strength

of bond to

Matyas, A.S.D.+** of California,

School

of

The opaque porcelain layer for porcelain-fused-to-metal restorations is critical for success. This investigation examined opaque porcelain application using one- and two-layer techniques with respect to their effect on the strength of bond to three silver-palladium alloys. There were no significant differences in the flexural bond strengths of the three alloys for the one- and two-layer opaque application techniques. With respect to bond strength, these results afford flexibility in the choice of the one- or two-layer techniques. (J PROSTHET DENT 1990;63:414-18.)

T

he porcelain opaquelayer for porcelain-fused-tometal (PFM) restorations is critical for successbecauseit isthe first to be placedover the treated alloy. The interface ofthe metal and opaquedeterminesthe bond strength. The opaque layer also masks the metal so that appropriate tooth shadesare developed. The intimacy with which the opaquelayer contacts the alloy surface is directly related to the bond strength. This considerationis onereasonfor applying the opaqueporcelain in at least two layers on PFM systems.The first thin layer acts as a wetting layer, and the subsequentlayers fill in irregularities to mask the metaLl*2 The application of opaque in one layer has been traditional for ceramists becauseadditional layers of opaque

*St&Prosthodontist, Wadsworth Veterans Administration ical Center. **Profeaaor and Chairman, Biomateriala Science Section, sity of California, School of Dentistry.

MedUniver-

***SeniorResearch Associate, Biomateriala ScienceSection,University

of California,

1011117136

414

School

of Dentistry.

require more firing cycles, reducing the number of units produced for a given period. While multilayer opaqueapplication hasgainedin popularity, the relationship of bond strength to opaque layering has not been resolved. This investigation examined one- and two-layer opaque porcelain applications to determine the effect on the bond strength of three silver-palladium alloys.

MATERIAL

AND METHODS

Three silver-palladium metals were evaluated; W-l (Williams Gold, Bufl’alo, N.Y.), Rx-91 (Jeneric Gold Co., Wallingford, Conn.) and JP-5 (Jensen Industries, Inc., North Haven, Conn.). The nominal compositionsof these alloys are presented in Table I. The four-point flexural bond strength test was used to test specimensthat were made from wax patterns of dimensions0.5 X 8 X 25 mm, All of the alloys were centrifugally cast by using gas and oxygen with a multiorifice torch. Twelve specimensof each alloy were quenched after casting, a method suggestedfrom a previous study.3 Each specimenreceived a surface treatment of blasting with 50 pm aluminum oxide particles without degasaingp

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