Scanning Microscopy

Volume 3 Number 4 Article 5

12-5-1989

Properties of a New Dental Porcelain

T. K. Vaidyanathan University of Medicine and Dentistry of New Jersey

J. Vaidyanathan University of Medicine and Dentistry of New Jersey

A. Prasad Jeneric Pentron, Inc.

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Recommended Citation Vaidyanathan, T. K.; Vaidyanathan, J.; and Prasad, A. (1989) "Properties of a New Dental Porcelain," Scanning Microscopy: Vol. 3 : No. 4 , Article 5. Available at: https://digitalcommons.usu.edu/microscopy/vol3/iss4/5

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PROPERTIES OF A NEW DENTAL PORCELAIN

T.K. Vaidyanathan 1 , J. Vaidyanathan 1

and A. Prasad 2 1 Departrnent of Prosthodontics and Biornaterials University of Medicine and Dentistry of New Jersey New Jersey Dental School 110 Bergen Street, Newark, NJ 07103-2425 2J ener1c' Pentron, Inc., Wallington, CT.

(Received for publication September 23, 1989, and in revised form December 05, 1989)

Abstract Introduction

A high strength dental porcelain A natural tooth reflects, disperses OPTEC, HSP™ has recently been intro­ and transmits light through enamel and duced for use in the fabrication of dentin, imparting an appearance of depth crowns, three-unit anterior bridges with and vitality. To create the illusion of a single pontic, inlays, on lays, nature in a restoration, the selection veneers, etc., without a metal sub­ of material to build up the restorations strate. This investigation reports the becomes the most demanding challenge. results of our research involving the Dental porcelain with its characteristic characterization of the structure translucency, is probably the material properties and thermally induced trans~ of choice to duplicate tooth in its sub­ tletj._es of color, texture and formations of this porcelain. It has 5 3 9 4 been found that the material possesses form. (l, , , ,l ) However, traditional superior strength and other properties porcelain used in dentistry lacks the relative to conventional porcelains. necessary tensile strength for use by The improved properties appear to result itself in stress bearing restorative ap­ from the presence of a very fine and plications. A metal substrate is used relatively dense distribution of leucite for porcelain build up so as to combine crystals. Scanning electron microscopy the benefits of metallic tensile and indicates, however, that the new product flexural properties with the trans­ needs additional optimization to improve lucency, color and aesthetics of por­ uniformity of crystallite distribution. celain. Even though such porcelain­ Scanning electron microscopy and X-ray fused to-metal ( PFM) restorations have diffraction studies also reveal well enjoyed a high reputation in fixed pros­ defined structural and crystallographic thodontics, a certain degree of com­ transformations during thermal treatment promise is involved in the use of a me­ at selected temperature ranges. The tal substrate. The 1 irni tations stern primary transformation appears to be from the following reasons: crystallization of metastable sanidine 1. Loss of translucency due to the use from the glass matrix. The crystal­ of metal substrate. This loss of lization of sanidine leads to a trans­ translucency is particularly a short­ lucent to opaque transition. The trans­ coming at the gingival margin. formations appear to follow the phase 2. Visible metal margin. transitions expected from the phase 3. Discoloration of porcelains due to diagrams. the presence of silver in some al­ loys. 4. Decreased 1 ight transmission due to KEY WORDS: Dental Ceramic, High the metal substrate. Strength, Properties, Scanning electron 5. Potential corrosion of metal sub­ microscopy, X-ray diffraction, Leucite, strate and health hazard in some of Sanidine. the popular base metal alloys used as porcelain substrates. The long term Address for Correspondence: effects of corrosion and release of T.K. Vaidyanathan cationic species into the biological Department of Prosthodontics and system are not known. Some known al­ Biornaterials lergenic and toxic elements like Ni University of Medicine and Dentistry of and Be are used in popular PFM al­ New Jersey loys. However, the presence of an 110 Bergen St., Newark, N.J. 07103 intra-oral nickel appliance has been Telephone No. (201) 456-6250 reported to reduce the likelihood of

1023 T.K. Vaidyanathan, J. Vaidyanathan and A. Prasad

developing nickel allergy, (l 3 ) and (X-ray diffraction) studies were carried the concentrations of beryllium used out in a Philips or Rigaku X-ray dif­ in dental alloys (less than 2%) have fractometer (Copper, Koc , 4 5kV, 3 5mA) . been demonstr 9ted to be non-toxic in Thermal processing of the disk samples cell culture. \lO) included isothermal treatment by holding A further drawback is the labor in­ the fused porcelain disk for 1 hr. at tensive nature of waxing, casting and 65o 0 c, 705°c, 76o 0 c, 815°c, 87o 0 c, 925°c finishing to obtain the alloy sub­ and 1/2 hr. at 98 o 0 c. The thermally strates, thus increasing the cost and processed samples were visually examined time for the fabrication of restora­ and characterized by scanning electron tions. The above limitations of PFM microscopy and X-ray diffraction. restorations have undoubtedly created searches for alternatives to optimize Experimental Results and Discussion aesthetics, strength, biocompatibility and cost effectiveness. The initial Properties of OPTEC, HSP™: outcome of these searches is the intro­ Table 2 lists the mechanical and duction of a first generation of high physical properties measured for OPTEC, strength ceramic systems such as Dicor HSP™ (Dentsply Inc., York, PA) 2 Cerapear Generally, the mechanical proper­ (Kyo Ce ram Inc., San Diego, CA) , 1 ties (compressive strength and modulus aluminous core materials (Vita of rupture) were found to be superior to Zahnfabrik, Bad Sackingen, W.G.), OPTEC, the reported properties for conventional HSP™ (Jeneric Pentron Inc., Wal­ porcelains. For example, the reported lingford, CT), etc. While each of these modulus of rupture of conventional por­ systems has its merits, the focus of celaf£i/s significantly lower at 55-75 this investigation is an in vitro MPa. Impact strength values were, evaluation of the leucite-strengthened however, nearly equal at around 5 kg. cm. porcelain called OPTEC, HSP™. Since The high thermal expansion value of the system uses conventional fabrication O. 899% at 5oo 0 c observed for OPTEC, procedures in the dental laboratories, HSP™ sample is attributed to arise from there is a distinct advantage of accept­ the presence of a large volume fraction ance by the dental lal.loratories, as no of leuctie in the structure. Theim­ added capital outlay is involved. provement in the mechanical properties In this study, OP'l'EC, HSPT M was can be explained by the strengthening evaluated for mechanical properties, effect associated with a fine dispersion microstructures and thermally induced of a second phase. It is believed that transformations. the large thermal expans~on mismatch be­ tween leucite (20-25xl0- ·; 0 ci 4 , 7 and the Materials and Methods glass matrix (8xlo- 6; 6 c) result in stress fields within and around leucite Porcelain frit supplied with OPTEC, particles during cooling. The stress HSP™ kit from Jeneric Pentron Inc. was field created by a higher expanding par­ moistened, packed in appropriate molds ticle ( leuci te) in a lower expanding and compacted with a Carver Press under matrix (glass) is compressive in the 1500 kg load . The compacted samples tangential direction of the particle­ were fused in an automatic porcelain matrix interface and tensile in the furnace (Ney Sunfire) by raisin% the radial direction. Such stress field is temperature at 55°C/min from 650 c to believed to strengthen the glass ceramic 1035°c in 720mm vacuum. The samples system, resulting in the observed im­ were held for 2 minutes at 1035°c and provement in selected properties. bench cooled. They were tested for the different properties using the specimens Frit Characterization and instruments indicated in table 1. Porcelain was characterized by Figure 1 is a scanning electron scanning electron microscopy and X-ray micrograph of OPTEC, HSPT M porcelain diffraction. Disk samples were also frit supplied by the manufacturer in the prepared as described previously for original kit. Figure 2 is a segment of the XRD profile of the above frit in the scanning electron microscopy, X-ray dif­ 0 fraction studies and thermal processing o to 37°, 20 range. XRD profile of a studies. Scanning electron microscopy conventional feldspathic porcelain frit was carried out in a Hitachi SEM (Pencraft), obtained under identical (Scanning Electron Microscope) model s- scan range and conditions is given in 2500 interfaced to a Kevex EDS (Energy figure 3. Search and match software Dispersive Spectroscopy) and advanced identified the XRD spectral peaks (of imaging system. The surfaces were both the OPTEC and PENCRAFT frits) to be slightly etched with a 1% HF acid solu­ that of leuci te. In addition, the tion for 20 sec and sputtercoated with presence of the amorphous glass phase is gold prior to SEM evaluation. The XRD also apparent from the difuse back­ ground. Table 3 lists the ratios of

1024 T.K. Vaidyanathan, J. Vaidyanathan and A. Prasad

2<50

1aee

Fig. 2: XRD Profile of OPTEC, HSP Frit

<050

32 00

2< 50

Fig. 1: Microstructure of OPTEC, HSP 1800

Frit 12 50 selected leucite XRD spectral peaks in th e conventional frit to that in OPTEC frit . It is apparent from the values of int e nsity ratio I (conv.) /I ( optec) in t able 3 that OPTEC, HSP™ porcelain frit co ntains approximately 60 % more leucite th a n the conventional frit.

Microstructures and XRD Study of Fused Fig. 3 : XRD Profile of PENCRAFT Frit Porcelain frit is prepared by mixing potash Figures 4 and 5 show the scanning feldspar with KN03 , K20 : Si0 2 , ~i 2co 3 , caco , Mgco and cerium oxide in electron micrographs of selected regions 3 3 of a fused porcelain disk . A second specified ratios and raising the tem­ phase dispersion in a matrix phase is perature of the mix to a specific time clearly visible in the microstructures. interval at that temperature. The The second phase particle sizes range soaked mass is then cooled to an inter­ from submicron size to 20µm or more. mediate temperature, quenched in water, However, most of the second phase par­ dried, pulverized and ball milled. Al­ ticles appear to be in the 1 - lOµm though the product contains about 60% range and a considerable fraction in the more leucite than the conventional 1 - 5µm range. The particle dispersion feldspathic dental porcelain frit, the also ranges from dense dispersion in manufacturing process is not presently some micro-regions to lighter dispersion optimized to disperse the second phase with larger interparticle separation in uniformly, densely and in optimal par­ other micro-regions (see Figures 6a to ticle size distributions. 6d). Analysis by Kevex feature analysis Figure 8 shows an XRD profile and software showed that the particle volume clearly identifies the second phase to fraction ranged from 20 to 55% in dif­ be leucite. The four peaks at 25.9, ferent imaging areas. Thus, the second 27.2, 30.6 and 31.5° 20 values cor­ phase distribution appears to be non­ respond to the expected high intensity uniform and clearly needs improvement. peaks for leucite in the scanned range. In addition, fracture 1 ines were ob­ A spot spectrum by CARD (Composition served in some part of the microstruc­ Analysis by Robinson Detector) and iden­ tures (Figure 7). Such non-uniform dis­ tification by effective atomic number persion and appearance of fracture lines confirmed the crystalline phase to be probably result from the specific leucite (see Figure 9}. manufacturing process used. Porcelain

1025 A NEW DENTAL PORCELAIN

TABLE 1 : Specimen and Experime ntal Setup Details

s. #I Property Specimen Size and Geometry I Sample Test Setup I I size(#)

I x 3mm thick) 1 Thermal Expansion Disks(6mm dia ' 3 Dupont Thermomechanical Analyzer I 2 Impact Strength 6mm x 6mm x 50mm bars 10 lzod Test Setup (Testing 'i ! (V notch) M/ C Inc.) I I 3 Modulus of Rupture 4mm x 6mm x 50mm bars 10 Three point bending in an I ( All sides ground flat on lnstron Universal Testing I ' emery paper, three sides Machine given a wash of glaze) 4 Compressive Strength 4mm dia x 8mm thick Cylinders 10 lnstron Universal Testing Machine

! 5 Vicker's Hardness 25mm dia x 6mm thick disks 10 Clark Microhardness Tester I Number (VHN )

TABLE 2: Propert ies of OPTEC, HSP

5.No. Property Mean(SD) Units

1 Thermal Expansion ( 30 - 500°C) 0.899(0.015) % 2 Modulus of Rupture 145.9(16.56) MPa 3 Compressive Strength 965.9(54.52) MPa 4 Impact Strength 4.95(0.3) Kg.Cm. 5 VHN(Vicker's Hardness Number) 402 ( 14)

TABLE 3: Ratios of Intensities of Selected Leucite XRD Peaks of Conventional Frit vs. Optec Frit

I I I Remarks I 5. No. I 20 I ' 1conv ' lo'ritec lconv. I (deg.) (~) i (count s) (co nts) (calculated increase i loptec I in leucite) I ! ! 1 16.45 5.389 581 942 0.62 61% I 2 25.93 3.436 1,444 2, 421 0.60 67% i i 3 ! 27.21 3.277 1,875 3, 192 0.59 69% ! 4 ! 31. 43 2.846 630 1,109 0.57 75% i 5 ' 31. 79 2.815 380 605 0.63 59% I 6 37.95 2.371 408 666 I 0.61 64% I i I crysta lline phase detected in the opaque Thermal Processing Effects samples revealed an interpenetrating rod-like morphological appearance. X­ Visual examination of the thermally ray diffraction confirmed that leucite processed disks revealed that thermal was the only detectable crystalline processing of the samples at 65o 0 c and 0 phase in samples processed at 650°c and 9ao c showed no changes in translucency. 9ao 0 c. However, samples processed at 705°c, However, thermal treatment at 705°c, 76o 0 c, 815°c, 87o 0 c and 925°c all 0 925°c resulted 76o c, 815°c, a10°c and produced XRD peaks typical of a second in a loss of translucency and the crystalline phase in addition to that of samples became opaque. SEM analysis leuci te. Figures 17 and 18 show XRD revelaed that while the leucite dis­ profiles of samples thermally processed persed structure was retained after at 98o 0 c and 76o 0 c, respectively. The thermal processing at temperatures of 0 0 XRD profiles at 980°c indicate peaks at 65o c and 98o c, an additional crystal­ 25.9, 27.2 and 30.4°, 20 values and can line phase was detected at 705°c through readily be identified to correspond to 925°c. Figures 10-16 show repre­ the d values of 3.436, 3.277 and 2.92A sentative microstructures observed for leucite from JCPDS file #25-47. On across the translucent-opaque-trans­ the other hand, the thermally processed lucent transition temperatures. The new samp le at 760°c shows three additional

1026 A NEW DENTAL PORCELAIN

Fig. 4: Microstructure of fused OPTEC, Fig. 5: Higher magnification micro­ HSP structure of a selected region

Fig. 6a: Illustration of a dense distri­ Fig. 6c: Illustration of sparse, coarse bution of leucite crystals in a distribution of leucite in some selected region region Fig. 6b: Marked region in 6(a) magnified Fig. 6d: Marked region in 6(c) magnified five times five times peaks not observed in the sample sanidine. Thus, it is reasonable to processed at 980°c. These peaks occur conclude that the translucent-opaque­ at 28 values of 26.9°, 27.6° and 29.8°, translucent transition is associated respectively and corres__pond to d values with the crystallization of metastable of 3. 27, 3. 26 and 2. 98A, the three ex­ sanidine phase and the temperature range pected high intensity lines for across which this phase is found to be

1027 T.K. Vaidyanathan, J. Vaidyanathan and A. Pras a d

3SE~I 2sssl I 2 2 ,;(.)~ ,J... ::J A JI\\ ::f ./\ I /\ t, mry ,.. \..;'~r ~J!'¾1~,rl V I 1~<+ I ,sf I 2~. '------'----. 0 26.0

Fig. 8: XRD Profile of fused OPTEC, HSP

FSC 4K SPT : HSC1124 AUSOff DNAOff

CAL!BIATIVI OPerato, CURSOR-2.2 COUHTS 1 CO!POU!t- " ' Fig. 7a: Illustration of fracture lines I Lucite-::.: in OPTEC, HSP microstructure ! 4 Fig. 7b; Marked fracture region in 7(a) 5 6 magnified five times 7 8 stable. The ratios of the sanidine peak ------" !!9 (27.6° and 29.8°, 29) intensities to ATORICIIMI FICTII leucite peak (25.9°, 21.2° and 30.4° 2()) intensities vs. firing temperature~ - are shown in Figure 19. The formation and disappearance of the sanidine phase across the translucent-opaque transition Fi g. 9: Spot spectrum from CARD. Note range of temperatures of 650°C-980°c is the superposition of the spec­ clearly evident. In addition, the maxi­ tral peak with the cursor posi­ mum sanidine appears to form in the 760- tion at 12.2 corresponding to 87o0c temperature range with the highest the atomic number factor of transformation probably occuring at leucite 760°C. Moreover, the relative absence of change in the intensity levels of leucite peaks across the translucency­ at the lower temperature range where ad­ to-opaque transition range of tempera­ ditives used in the formulation have tures indicated that sanidine formation probably shifted the range of metas­ does not occur at the expense of leucite tability to higher values of tempera­ phase, but rather through crystal­ tures of >650°c. lization of sanidine from the glass The identification of a translucent matrix. This is, however, to be ex­ to opaque optical transformation in the pected because the glass and sanidine thermal processing also needs some ex­ compositions are stoichiometrically amination. In spite of the higher volume fraction of leucite phase in OP­ ~earl~ identical to (Na,K) 2O•Al 2o 3 ·6Sio 2 in this system. The formation and dis­ TEC, HSP™ (as compared to conventional appearance of sanidine phase across a dental porcelains), the product appears temperature range observed in this study to retain its translucency. This is can be readily explained by an examina­ readily attributed to the closeness of tion of the Albite (Na o·A1 o ·6sio) - the refractive index of leucite 2 2 3 6 Orth?r~fse (K2o:Al 2~3 ·6sio 2 ) phase ~ia­ (n=l. 509) ( ) to that of glass matrix gram shown in Figure 20. It can be (n=l. 505+0. 004) . However, formation of ~een that sanidine is a metastable phase sanidine-in the 705°-925°C range results in the temperature range of about 5oo O - in loss of translucency. 925°c. This range nearly coincides with the temperatures where sanidine forma­ tion is observed in this study, except

1028 A NEW DENTAL PORCELAIN

Fig. 11: Microstructure after 1 hr. at 705°c Fig. l0a: Microstructure of OPTEC, HSP after 1 hr. at 65o 0 c Fig. 10b: Marked region in l0(a) mag­ nified five times

Fig. 13: Microstructure after 1 hr. at 815°C

crease in strength is probably the result of a dispersed second phase of Fig. 12: Microstructure after 1 hr. at leucite crystals over a wide spectrum of 760°C particle sizes and distribution. OPTEC, HSP™ system undergoes optical, crystal­ Conclusions lographic and morphological changes in its structure in the 650-98o 0 c tempera­ The mechanical properties of OPTEC, ture range. The primary transformation HSP™ appear to be superior to those of occurs through the change of glass conventional dental porcelain. The in- matrix to sanidine phase.

1029 T.K. Vaidyanathan, J. Vaidyanathan and A. Prasad

Microstructure after 1 hr. at Fig. 15a: Microstructure after 1 hr. at Fig. 14: 925°c 870°C Fig. 15b: Marked region in 15a magnified five times

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Fig. 17: XRD Profile after 1 / 2 hr. at 98o 0 c tessence Publishing Co., Inc., Chicago, 371-414. 2 . Grossman DG (1985) Cast glass ceramics. Dental Clinics of North America . 29 , 725-739. Fig. 16: Microstructure after 1/2 hr. 3. Hobo S, Iwata T (1985) Castable at 980°C apatite ceramics as a new biocom­ pa tible restorative material. References Quintessence, Intl., 3, 207-215. 4 . Hoshikawa T, Agaki S (1972) Forma­ 1. Anusavice KT (1983) Screening tests tion of leucite and thermal expan­ for metal-ceramic systems. In: sion in Sio 2 -Al 2 o 3 -K 2 O-Na 2 o Dental Ceramics: Proceedings of glasses. Yogyo-Kyuokai-Shi, 80, the First International Symposium 42-48. on ceramics, McLean JW (ed). Quin- 5. Jones DW (1983) The strength and

1030 A NEW DENTAL PORCELAIN

3500 Ratio of Sanidine to Leucite Lines

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25.0 2£:;.e. 27. 0 29.0 30.0 699 754 810 ffi5 921 976 IQJ2 Fig. 18: XRD Profile after 1/2 hr. at Frrq l "'l' 1llo:jC l 76o 0 c Fig. 19: Variation of the ratios of sanidine to leucite intensi­ tites of selected peaks as a function of temperature

tional state of the Art Conference on Restorative Dental Materials, Clinical Center, National In­ stitutes of Health, Bethesda, Md., 5-18 . 10. Nakamura M (1984) In vitro cell response to Co-Cr-Mo alloy contain­ ing Be. J Prosth Dent, 51, 790- 796. 11. Phillips, RW (1982) In: Skinner's Low Albite + Orthoc lose Science of Dental Materials, W.B . Saunder's Co. , Philadelphia, Chap­ 40 60 Orlhoclose ter 31, 516-519. 12. Smith JV (1960) Intern. Geol. Cong. 21s Copenhagen, p 190, In Rept. Fig . 20: Albite - orthoclase Phase Session, Norden. Diagram 13 . Vreeburg KJ, de Groot K, von Bloom­ berg M, Scheper RJ (1984) Introduc­ tion of immunological tolerance by strengthening mechanisms of dental oral administration of nickel and ceramics. In: Dental Ceramics: chromium. J Dent Res, 63, 124-128. Proceedings of the First Interna­ 14. Weiss P (1983) State of the art in tional Symposium on Ceramics, metal ceramics. In: Dental McLean JW (ed). Quintessence Pub­ Ceramics, Proceedings of the First lishing Co., Inc., Chicago, 83-141. International Symposium on Ceram­ 6. Mackert Jr. JR, Butts MB, Beadreau ics. McLean JW (ed), Quintessence GM, Fairhurst CW, Beauchamp RH Publishing Co., Inc., Chicago, 231- (1985) Ultra thinning of dental 255. porcelain for transmitted light microscopy. J Dent Res, 64, 1170- 1175. Discussion with Reviewers 7. Mackert Jr. JR, Butts MB, Fairhurst CW (1986) The effect of the low­ Reviewer 1: The authors use ratios of high leucite transformation on den­ peak height intensities to derive infor­ tal porcelain expansion. Dent mation about relative concentrations of Mater, 2, 32-36. leuci te in OPTEC versus a conventional 8. McLean JW (1985) Current status and porcelain. While peak heights are future of ceramics in dentistry. proportional to concentrations, peak In: Ceramic Engineering and Science broadening caused by particle size ef­ Proceedings. The American Ceramic fects and other factors will cause the Society, 6, 1-2, pp. 1-9. peak height to be smaller even when the 9. Moffa JP (1986) Porcelain materi­ concentration is the same. The authors als. Presented at the Interna- should use the integrated intensities

1031 T.K. Vaidyanathan, J. Vaidyanathan and A. Prasad with background subtracted out to com­ ratio is determined, identical marginal pare the relative concentrations of integrity can be obtained. leucite in the two porcelains. Authors: The total integrated area of Reviewer 2: The modulus of rupture five leucite peaks in the 21 - 28° 28 (MOR) of the OPTEC HSP specimens cannot range, after correction for background, be determined with a "wash of glaze" ap­ was determined to be 54,051 counts for plied. Why are the MOR values not OPTEC. The corresponding integrated clearly associated in the Discussion area for PENCRAFT was 29,081. The ratio sect~on for altered (glazed) OPTEC, HSP of IPENCR to IoPTEC was. 0.54 ~as~d ~n specimens? these measurements. While this indi­ Authors: The three point bending test cates 80% more leucite in OPTEC than in methodology used for the MOR determina ­ PENCRAFT, our data in the text indicate tion has not been standardized and a conservative estimate to a reasonable universally accepted at the present degree of accuracy sought by us. More time. A discussion of the various accurate determination can be made by processing procedures for the test have statistical and quantitative techniques, been summarized in a report (Feb 4, but our objective was more limited in 1988) by the R & D department, Ivoclar­ this investigation. Vivadent. Apparently, different inves­ tigators have used different methods for Reviewer 2: The shrinkage of OPTEC por­ the sample preparation. We recognize celain compromises the fit of restora­ the potential variability due to the tions at marginal areas compared with a method of sample preparation. According metal margin. Firing shrinkage of this to the ISO procedures recommended for product requires one or more extra jacket crown ceramic, grooves produced firings to achieve equivalent marginal in the grinding process are smoothed fit. Since this deficiency requires ex­ with a glaze firing (DIN- Vorlage tra laboratory time, the labor intensive 0013925, January 1987: Dental-Keramische process of metal preparation of PFM res­ Mas sen; International Standards Or­ torations may not increase the cost ap­ ganization, standard 6872, 1984: Dental preciably compared with the OPTEC cost. Ceramic) . The wash of glaze in our test This economy should be substantiated. wa~ done at a temperature of 98o 0 c, Another benefit of PFM crowns is their which is 5o 0 c lower than the firing tem­ success rates, i.e., sucess rates of 97% per ature of OPTEC. We have carried out for crowns and bridges over a 7.5 year additional tests according to the ISO period (Coornaert). OPTEC HSP has no standard and obtained a mean MOR value such track record of longevity. o f 133. 44 (22.0 3 ) MPa using a sample Authors: There is no question in our s i z e of n=7. minds that PFM restorations are the standards against which all-ceramic res­ Reviewer 2: The implication of streng­ torations have to be clinically judged, thening due to fine dispersion of a but the jury on this judgment is still s econd phase has not been proven in this out until clinical performance evalua­ study. Why should a second phase neces­ tions of the product are completed. Our sarily strengthen the ceramic? What is studies, collaborated by dental the influence of other factors such as laboratory evaluations, indicate a mini­ stresses or microcracks around the par­ mum of 15 - 20% savings in the fabrica­ ticles due to large thermal contraction tion time for equivalent marginal fit. differences? The shrinkage of OPTEC is only slightly Authors: We have demonstrated that higher than that of porcelain for PFM. there is a high volume fraction and If a thick layer of opaque porcelain is small particle distribution of leucite fired onto a metal substrate, even the crystals in the OPTEC structure. Both conventional porcelain will have the SEM and XRD studies support this conclu­ tendency to delaminate or lift up at the sion. We show that there is at least a margins. Similarly, if a thick layer of 60 % higher leucite content in OPTEC than OPTEC porcelain is applied on to a in the conventional porcelain control. refractory die substrate and not fired Davidge and Green (Davidge RW, Green TJ properly, a tendency for lift up is to (1968) The strength of two phase ceramic be expected. However, this problem is materials. J Mater Sci, 3, 629-634.) circumvented by the application of a have discussed the strength of two phase very thin band at the margin in the ini­ ceramic materials and indicated that the tial bake. The marginal integrity of important factors influencing the OPTEC is also dependent on the proper strength are: refractory powder to 1 iquid ratio. 1. The expansion coefficient of the two Similarly, the marginal integrity of a phases metal substrate in PFM is dependent on 2. The elastic properties of the two the proper ratio of investment to liquid phases ratio. In both systems, once a proper

1032 A NEW DENTAL PORCELAIN

3. The volume fraction of the crystal­ metric composition. line phase 4. The particle size of the crystalline phase While the relative contributions of these factors is not yet known, it is only reasonable to conclude at this stage that these factors are important in the strengthening of OPTEC porcelain. Clearly, additional systematic work is needed to establish the interactive ef­ fects of these factors in the strengthening of this system.

Reviewer 2: Has a quantitative analysis been performed to calculate the particle size, volume fraction and particle size distribution? Authors: We plan to carry out a sys­ tematic investigation on leucite volume fraction, particle size and distribution and their influence on properties in fu­ ture.

Reviewer 2: Has translucency been measured? Visual examination is not ac­ ceptable since the observers were not calibrated. Authors: It is important to realize that this study was not focused on the topic of translucency. Measurement and observer calibration are important only when a range of translucency values are involved. In our opinion, differenta­ tion of opaque and translucent specimens do not require detailed measurements. Qualitative differentiation of opaque samples from translucent sampleswas not complex.

H.J. Mueller: Is it likely to manufac­ ture porcelains with even higher con­ centrations of leucite than in OPTEC, HSP™? Authors: Yes, but there will be an in­ crease in the fusion range and it is not known what effect such increase in volume fraction will have on the trans­ lucency.

H.J. Mueller: If potash feldspar is mixed with other ingredients, where does the sodium come from? Authors: Potash feldspar usually con­ tains some albite.

H.J. Mueller: Are there chemical or structural differences between leucite and sanidine? Authors: Our studies indicate an inter­ pentrating rod-like morphology for sanidine. From the phase diagram in figure 20, it is also clear that the chemical composition of sanidine may vary. The crystal structure observed for sanidine is monoclinic. Leucite can crystallize in the tetragonal or cubic polymorphs and has a definite stoichio-

1033