Leucite CrystallizationDuring Processing of a

Heat-PressedDental Ceramic

I. Rodway Muckert, ¡r, DMO, PhD'

CarlM. Russell, DMD, PhD**

Medical College of GeorgiaAugusta, Georgia

This investigation was conducted in an effort to elucidate the mechanism ofthe strengthening that occurs during processing of a heal-pressecl dentalceramic (IPS Empress!, The effect of processing on ieucite content wasdetermined via quantitative x^ray diffraction, A significant effect ofprocessing was reveaied by anaiysis of variance iP< ,0001 ¡. Post-hoccomparisons by Student-Neuman-Keuls and Duncati's Multiple Range testsrevealed no difference in ieucite content after pressing, but the leucitecontent was significantly higher (P< ,05) after a single surface colorant firing,A second colorant firing produced a further significant increase in leucitecontent (P< ,05), but no further increase ¡n ieucite content was observedfollowing the remaining colorant and glaze firings. The increase in leuciteafter firing is consistent with the increase in flexural strength following firingobserved by Dong et al in 1992. Int j Prosthodont 1996,9:261-265.

Leucite (K,O-AI,O3-4Siü^] is a mineral formed bythe incongruent melting of feldspar (K,O-Ai,O3-

ôSiOi), and it is used in the manufacturing of manydental porcelains, Leucite-based frits have beenused since the early 196(5s in the manufacture ofmaterials for metal ceramic applications. The high-expanding leucite raises the bulk porcelain thermalexpansion to a level where it is compatible with themetal substrate. More recently, leucite has beenused in all-ceramic materials, not for thermal com-patibility', but as a reinforcing material,

IPS Empress (Ivoclar Williams, Amherst, NY) is aleucite-reinforced all-ceramic material tbat is formedinto the desired restoration using a heat-pressing(high-temperature injection molding) technique.

'Professor, Section of Dental Materials, Department of OraiRehabiiitation, Schooi of Dentistry.

"Associate Professor, Office of Bioslatistics.

Reprint requests: Or I. Rodway Mackert, )r. Section of DentaiMaterials, Department of Oral Rehabilitation. Schooi of Denlislry.Medical College of Georgia. Augusta, Georgia 30912-1260.

Presented at the 24lh Annual Session of the American Associationfor Dental Research. March 3-12. 1995. San Antonio. Texas.

Dong et aP reported that the flexural strength of thismaterial increased during the heat-pressing and fur-ther increased with the firing treatments required toproduce the final restoration. They showed that theheat-pressing treatment produced better dispersion ofthe leucite particles in the glassy matrix and attrib-uted the improved strength following pressing to thisbetter leucite dispersion. There was no satisfactoryexplanation for the observed increase in flexuralstrength following firing, however, Leucite-reinforcedall-ceramic matenals have a higher leucite contentthan conventional feldspathic porcelains that are de-signed for fusing to metal. The higher leucite contentis thought to be responsible for increased strength.Since many of the processing heat treatments used inthe fabrication of metal ceramic restorations areknown to induce crystallization of additional leucitein conventional feldspathic porcelains, it was hypo-thesized that the heat treatments associated with theprocessing of IPS Fmpress could induce additionalleucite crystallization. The purpose of this study wasto determine whether the various processing stepsin the fabrication of a restoration from this leucite-reinforced ali-ceramic material could produce anincreased leucite content.

Volume y. Number 3, ] 261 Tlie Irrernational Inurnal of Prosthodontics

LoutileCryçtalli^ationola Heal-Preaed Centai Ce

Materials and Methods

Preparation of Leucite Standards

Standards containing 07o, 25'>i,, 50%, 75%, and100% leucite were prepared for quantitative x-raydiffraction analysis,^ An internal standard, Al ,Oj,was used to correct for potential absorption differ-ences between leucite and the glass matrix. Naturalleucite crystals (Ward's Natural ScienceEstablishment, Rochester, NY) weighing approxi-mately 3 g were crushed in a percussion mortar andground to a fine powder with an agate mortar andpestle, A strong magnet was passed over the powderto remove any possible iron contamination from thetool-steel percussion mortar. Approximately 3 g of aglass powder, prepared according to the Weinsteinet al ' patent "Component No, 3" composition, wasground to a fine powder with an agate mortar andpestle. Twenty standards—four of each leucite con-centration—were prepared by blending weighedportions of leucite and glass to yield the desiredleucite weight fractions. The internal standard,AljO^, was added in the amount of 25% by weightof the total mass of leucite plus glass. The standardswere prepared in random sequence to avoid system-atic bias resulting from preparation variables.

Each powder specimen was divided into two por-tions (designated A and B), and each portion waspacked into the well of a glass x-ray powder diffrac-tion sample holder. Diffraction patterns wererecorded of the leucite peaks in the 2fi range 29,6 to32,3 degrees and the alumina peaks in the 26 range34,6 to 35.5 degrees (D/Max 1 B, Rigaku USA,Danvers, MA), Copper K̂ radiation and a curvedgraphite monochromatur were used. The patternswere recorded with a step size of 0,05 degrees anda dwell time of 6 seconds per step. The mean of theintegrated intensities for the A and ß portions of agiven standard specimen was taken as tbe measuredvalue for that specimen. For each standard speci-men, a corrected intensity ratio, R, was calculatedfrom the weight fraction of the alumina internalstandard, X ,̂ the mean integrated intensity of tbeleucite peaks, 1̂ , and the mean integrated intensityof the alumina peaks, L, as follows:''

(eq 1)R =(1 - Xs) Is

The corrected intensity ratio values for each ofthe 20 standard specimens were plotted againstthe weight fraction of leucite, and a nearly linearplot was obtained (Fig 1 ), Although the plot of cor-rected intensity ratio versus leucile content is theo-retically linear,- tbe actual plot had a slight negative

deviation from linearity. Therefore, a srcond-orderpolynomial was fit through the data (Fig U, and anexcellent multiple correlation was obtained (r =0,996), The regression value of tbe coirccted inten-sity ratio, R, is obtained using the equation;

R = af, ^ -h bf, +c (eq2)

where f,̂ is the weight fraction of leucite in thestandard, and a, b, and c are tbe regression coeffi-cients 0,20503, 0,79786, and 0,02206, respec-tively. The solution of this regression equation forfi yields a relation for determining the leuciteweight fractions of the unknown specimens, giventhe corrected intensity ratios measured via quanti-tative x-ray diffraction:

- b ± - 4a(c - R)2a (eq3)

Preparation of Heat-Pressed Ceramic Specimens

There are two different techniques in the Empresssystem. In the "staining technique," a full-contourcore is pressed from a colorless ceramic ingot, andlayers of a surface colorant are applied and fired.In the "layering technique," a full-contour core ispressed from a dentin-shade ingot, but is cut backto allow the application of an incisai characteriza-tion and layering firing. The present study exam-ined the "staining technique" material. The manu-facturer states that a minimum of 3 stain firings(designated "colorant firings" in the present paper)and two glaze firings are necessary for the "stain-ing technique,"

Eighteen molar crowns were prepared from the"OH" shade and were randomly assigned to sixtreatment groups:

1. Pressed only2. One colorant firing3. Two colorant firings4, Three colorant firings5, Three colorant firings and one glaze firing6, Tbree colorant firings and two glaze firings

The parameters for the surface colorant and glazefiring procedures are listed in Table 1, A randomassignment schedule was developed for the fabrica-tion of the 18 crowns. After the completion of thecrowns according to this schedule, each crown andeach of three unpressed pellets was crushed in apercussion mortar and ground to a fine powderusing an agate mortar and pestle. Weighed portionsof each crown and pellet were blended witb 25

The Intemalional 262 Volume 9, Number 3, 199b

' - -kertand Hu

Fig 1 Calibration curve relatingthe known leucite weight fraclionot the standards to their cor-rected x-ray intensity ratios(leucite to the AI^O^ internaistandard]. Ttie ieucite weighttractions ol the iPS-Empressspecimens were determinedtrom this calibration curve usingequation 2,

Leucite Crysiallizaiitm o(,i He.i¡-Pressed Denial Cer,

o

s

tens

itec

ted

iC

or

1,20

1.00

0.80

0.60

0.40

0.20

0.00 '

(-

0.00 0.25

r̂ = 0,996

0.50

Leucite content (%)

0.022

0.75

%

1.00

wt% Ai^Oj as an internal standard. Blending wasaccomplished by grinding the powders togetherwith an agate mortar and pestle. The blend wasground until caking occurred; this consistency en-sured suitable packing of the powder into the wellof the x-ray diffraction powder sample holder,

Fach crown or pellet powder specimen was di-vided into two portions, A and B, and packed intothe well of the x-ray diffraction sample holder.Integrated intensities of the leucite peaks in the 26range 29,6 to 32,3 degrees, and integrated intensi-ties of the alumina peaks in the 2G range 34,6 to35,5 degrees, were determined for each crownand pellet specimen in the same manner as theieucite standards. The corrected intensity ratios foreach specimen were calculated according toequation 1, and these corrected intensity ratioswere used to calculate the weight percent ofleucite in each specimen using equation 3,Whether processing treatments had a significanteffect on leucite content was determined byANOVA; post-hoc comparisons to identify specificprocessing steps with a significant effect on leucitewere performed using Student-Neuman-Keuls andDuncan's Multiple Range tests.

Table 1 Parameters Employed tor the Firing of IPSEmpress CrownsParameter

Standby lempeiatureTemperature increase per minuteStand time (dry)Holding timeUpper temperatureVacuum-on temperatureVacuum-otf temperature

Coiorant tiring Glaze tiring

4OO'C6O'C/min

4 min2 mm85O'C580 C849'C

4OO'C60"C/min

6 min3 min87O'C58O'Ce65"C

Restjits

The leucite content of the leucite-reinforced a l l -ceramic material as a function of the processing stepare shown in Fig 2. The leucite contents (mean w t%± SE) were as follows; pellet, 41,7% ± 0,3%; pressedonly, 41,37û ± 0,7%: 1-colorant, 43,4% ± 0,5%; 2-colorant, 45,4% ± 0.6%; 3-colorant, 4 5 , 1 % ± 0,3%;3-co I ora nt-1-glaze, 46,0% ± 0,2%; and 3-colorant-2-glaze, 45,4% ± 0,5%, A significant effect of treat-ment group was revealed by ANOVA [P < ,0001),Post-hoc comparisons by Student-Neuman-Keulsand Duncan's Mult iple Range tests revealed no dif-ference in leucite content after pressing, but the

Voiiinie9, Number 3, 1996 263 The inernationai ]ournai or" Proslhodoniii

Leucite Cryslallii.ilion ota H.MI-Pressed Denial Ce

45

4n

a 3(1a.

igh

tte

W6

_i 1 '"

11

s

_

•

-

-

-

-

—

-

-

Pellet Prsssed 1 coloianl 2 colorantfinng firings

liiimiH

culûrditirings

- -

—

-

—

t 3 colortir.t,1 glazetirings

-

3 colorant.2 giazetirings

Fig 2 Shaded vertical bars indi-cate the mean leucite content {inweight percent) ot the Empresspellets and crowns at variousstages during processing. Errorbars indicate ± 1 standard errorcf Ihe mean.

leucite content was significantly higher (P < -05)after a single colorant firing. A second surface col-orant firing produced a further significant increase inleucite content (P < .05), but no further increase inleucite content was observed following the remain-ing color and giaze firings.

Discussion

The result5 of this study indicate that certain proce-dures within the processing protocol for this all-ceramic material result in significant increases inthe leucite content, while other procedures do not.Heat-pressing does not increase the leucite contentof this material, so the observed increase in strengthof the material following heat-pressing is not attrib-utable to increased leucite in the ceramic. This find-ing is indirect evidence that the leucite dispersionmechanism proposed by Dong et aP may indeed beresponsible for the strength increase after pressing.The increase in leucite after firing, however, is con-sislent with the increase in flexural strength follow-ing firing observed by Dong et al.

Changes in leucite content of feldspathic dentalporcelains designed for porcelain-fused-fo-metal(PFM) applications have been observed to occurunder a variety of thermal treatments.'''^ In PFMporcelains, changes in leucite content are poten-tially detrimental, because leucite exerts a stronginfluence on the thermal expansion behavior ofthe porcelain.^ Changes in leucite content wil l

produce corresponding changes in the bulk ther-mal expansion coefficient of the porcelain andupset the thermal compatibility balance with themetal. In leucite-reinforced all-ceramic materials,however, the thermal expansion of the material isnot a critical property, because thermal compati-bility with a metallic substrate is not a considera-tion. Therefore, the crystallization of additionalleucite during the processing of an all-ceramiccrown is not detrimental, and appears to have abeneficial strengthening effect.

Conclusions

1. The pressing operation did not cause any in-crease in leucite content relative to the as-received pellets.

2. The first surface colorant firing caused a signifi-cant increase in leucife content relative to thepressed crowns.

3. The second surface colorant firing caused anadditional significant increase in leucite con-tunt relative to tbe crowns fired only once-

4. Subsequent firings beyond the second surfacecolorant firing did not produce further increasesin leucite content beyond the 45-4 wt% leuciteattained during the second colorant firing.

5- The increase in flexural strength following firingobserved by Dong et al' is consistent with theobservation of leucite crystallization during thefirst two colorant firings-

The International Journal o( Prosiliodoniii 264 •;% Number 3, 1

Leucilo Crystólliíiiliun of a Heat-Pr(

Acknowledgment

Th¡5 worl< was supported in part by NIDR grant No.DEO7eO6. The authors gratefully acknowledge this support.

References

1. Dong |K, Lulhy H, Wohlwend A, Scharer P. Heat pressed ce-ramics: Technology and strength. Int | Proslhodont 1992;S;9-16.

2. Klug HP, Alexander LE. X-Ray Diffraction Procedures forPolycrystalline and Amorphous Materials. New York: lohnWiiey & Sons, 1974:531-538.

.1. Weinstein M, Kau S, Weinstein AB |inventors|. PermadentManufacturing Corporation, assignee. Fused porcelam-to-metalteeth. US Patent 3,052,982. 11 Sept 1962.

4. Azaroff LV. Eiements ol X-Ray Crystallography. New York:McCraw-Hill, 1968:518-519.

5. Mackert JR Jr, Evans AL. Quantitative s-ray diffraction determi-nation of leucite thermal instability in dental porcelain 1 Am

6. Mackert IRJr, Evans AL. Effect of cooling rate on leucite volumefraction in dental porcelains. | Dent Res 199l;70:137-139.

7. Mackert JR |r. Butts MB, Fairhursl CW. The effect of the leucitetransformation on dental porcelain expansion. Denl Mater1986:2:32-36.

Literature Abstract

Gingival recession related to removable patiial dentures in older patients

In previous studies of a seiected older population attending a general dental practice inEngland, removable partiai denture (RPD] patients demonstrated a greater number of ex-posed root surfaces per person than non-RPO patients, and tooth surfaces adjacent to theRPDs demonstrated a greater prevalence ot gingival recession tban other tooth surfaces.The purposes of this study were to investigate whether patient and denture characteristicsinfluence the levei of gingival recession and whether there is a difference in the extent ofgingivai recession between individuals with RPDs and those without RPDs, A tctai of 127patients (88 women and 39 men with a mean age of 67.9 years) were examined on twooccasions, 3 years apart, to determine relationships among RPDs, their design, and gingivalrecession. Muitiple regression techniques were used to obtain adjusted estimates of the ef-fects of different predictor variabies on mean gingivai recession. Of tne 146 patients exam-ined at the initial appointment, 80(54.8%) patients presented with 111 RPDs The majority(77.5%¡ of the RPDs were mucosa-supported, and ali were resin dentures. Whiie this studyconfirmed the association of RPDs with increased gingiyai recession, none of the tactors ex-amined in the multipie regression modei (age, gender, presence ot maxillary and/ormandibuiar RPDs, materiais used for the RPDs, type of support, and number of tooth sur-faces associated with a iinguai piate) were able to predict the effect of RPDs on gingival re-cession for linguai plate-oovered teeth and gingiva. The authors concluded that patient cbar-acteristios and RPO design factors may be less important than the maintenance of goodorai hygiene fcr the prevention of periodontal disease in patients with RPDs.

Wriglil PS HeWyer PH. J Prosthel Denr 1995;74 602-607. References: 13. Reprints: Paul S. Wright,Departmen't of Prosthelic Dentistry, The London Hospitai Medicai Coiiege, Dental School, Turner Street,London E1 2AD, United Kingdom.-RicrtardR Seals. Jr, DDS. MEd MS. Department of Prostnodontics,The University ot Texas Health Science Center st San Antonio, San Amonio, Texas

Volume?, Numbers, 1996 265 The Inlernational Journal of Prosthodontii