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Dental ceramic materials exhibit many desirablematerial properties, including biocompatibility,

esthetics, diminished plaque accumulation, low ther-mal conductivity, abrasion resistance, and color sta-bility.1–3However, brittleness and low tensile strengthare weak points of ceramic materials. Therefore, theclinical success of all-ceramic fixed partial dentures(FPD) has been disappointing, especially for posterior

FPDs when compared with metal-ceramic restora-

tions.4–7 Although metal frameworks have inherentdisadvantages, metal-ceramic restorations are cur-rently the most widely and successfully used for FPDs.Because metal-ceramic FPDs are the standard of carein practice, their clinical survival rate should be usedas the criterion for new all-ceramic systems.5,7

Unfortunately, only a few reliable studies are con-cerned with the survival rate and average service

times of metal-ceramic FPDs in clinical practice.8–13

Available data suggest that conventional FPDs showa survival rate of approximately 90% at 10 years.14

Until recently, only small FPDs made of glass-in-filtrated alumina porcelain were recommended forthe anterior area. Available data from clinical stud-ies on all-ceramic anterior FPDs indicate a successrate of 93% to 100% after 3 years.7,15–17However, thesmall number of restorations investigated in thesestudies as well as the short observation period limitany further conclusion from the results. To achieveall-ceramic FPDs with appropriate fracture strength,

The International Journal of ProsthodonticsVolume 14, Number 3, 2001 231

Purpose: The purpose of this study was to determine the fracture resistance of three-unit

fixed partial dentures (FPD) made of new core ceramics. Materials and Methods:A base

metal three-unit master FPD model with a maxillary premolar and molar abutment was

made. Tooth preparation showed 0.8-mm circumferential and 1.5-mm occlusal reduction

and a chamfer margin design. FPDs were constructed with a uniform 0.8-mm-thick core

ceramic and a porcelain veneer layer. In-Ceram Alumina, In-Ceram Zirconia, and DC-

Zirkon core ceramics were machined by a computer-aided design/manufacturing system,

whereas IPS Empress 2 core ceramic was indirectly built up using the fabrication

technology of waxing and heat pressing. FPDs of IPS Empress were heat pressed as

complete restorations without core material. To ensure standardized dimensions, the

FPDs were controlled at different points. All FPDs were cemented with ZnPO 4 on the

master model and loaded on a universal testing machine until failure. The failure load

and mode of failure were recorded. Results: The highest failure loads, exceeding 2,000

N, were associated with FPDs of DC-Zirkon. FPDs of IPS Empress and In-Ceram Alumina

showed the lowest failure loads, below 1,000 N, whereas intermediate values were

observed for FPDs of IPS Empress 2 and In-Ceram Zirconia. Differences in mean values

were statistically significant. Conclusion: The high fracture resistance evaluated for FPDs

made of DC-Zirkon underscores the remarkable mechanical properties of high-

performance ceramic, which could be useful for highly loaded all-ceramic restorations,

especially in the molar region. Int J Prosthodont 2001;14:231–238.

a Assistant Professor, Department of Prosthodontics, University of

Aachen, Germany.bDental Technician, Dental Laboratory Gerd Natt, Köln, Germany.c Associate Professor, Department of Prosthodontics, University of

Aachen, Germany.dProfessor and Chairman, Department of Prosthodontics,

University of Aachen, Germany.

Reprint requests: Dr J. Tinschert, Department of Prosthodontics,

University of Aachen, Pauwelsstrasse 30, D-52074 Aachen,

Germany. Fax: + 49 2418888410. e-mail: [email protected]

Fracture Resistance of LithiumDisilicate–, Alumina-, and Zirconia-

Based Three-Unit Fixed PartialDentures: A Laboratory Study

Joachim Tinschert, Dr Med Dent a

Gerd Natt, MDT b

Walter Mautsch, Dr Med Dent, MScc

Michael Augthun, Dr Med Dent, PhDc

Hubertus Spiekermann, Dr Med,Dr Med Dent, PhDd

COPYRIGHT © 2001 BY QUINTESSENCE

PUBLISHING CO, INC. PRINTING OF THIS

DOCUMENT IS RESTRICTED TO PERSONAL

USE ONLY. NO PART OF THIS ARTICLE MAY

BE REPRODUCED OR TRANSMITTED IN ANY

FORM WITHOUT WRITTEN PERMISSION

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new ceramic core materials were recently introducedinto the dental market. Apart from lithium disilicateglass ceramic,18,19 new zirconia- and alumina-basedceramic materials are now available.20–22 The me-chanical properties of high-performance alumina andzirconia ceramics make them interesting as potentialcandidates for all-ceramic restorations in highstress–bearing areas.23–27 These ceramics are manu-

factured under optimized industrial conditions, andthey are designed to be processed by computer-aideddesign/manufacturing (CAD/CAM) technologies.28,29

The purpose of the present in vitro study was to testthe fracture resistance of three-unit FPDs made of newcore ceramics.

Materials and Methods

The maxilla of a phantom model (OK-16, Kavo) wasused to create the clinical situation of a three-unit FPDreplacing the first molar. The second premolar and the

second molar were prepared with chamfer marginsfor complete crowns. Tooth preparations were stan-dardized with (1)a total convergence angle of 10 to12 degrees, (2) chamfer margins of 30 degrees cir-cumferentially, and (3)occlusal reduction of 1.5 mm.All line angles were rounded.

The prepared teeth were placed in their correct po-sitions in the maxillary arch, and a half-arch impres-sion was made of the prepared teeth with a siliconeimpression material (President Coltène, Coltène). Theimpression was poured in a resin material (Palavit 55,Heraeus-Kulzer) to create a preparation model that

was invested, burned out, and cast in a nickelchromium alloy (Wiron 99, Bego). In this way, anickel chromium model of a three-unit FPD withfixed dies was fabricated. The surfaces of the dieswere smoothed with rubber polishers, first withcoarser, then with finer rubber cusps and points(Shofu Dental). The metal master model was used asa working cast to fabricate the FPDs and to evaluatetheir fracture strength in an axial load test.

Various three-unit FPDs with substructures made of lithium disilicate–, alumina-, and zirconia-based coreceramics were tested in this study. In addition, three-

unit FPDs constructed of leucite-reinforced porcelainwithout core ceramic were also investigated. All ce-ramic materials used for the fabrication of the FPDsare listed in Table 1. FPDs that consisted of a sub-structure and a veneering porcelain layer were con-structed with a uniform 0.8-mm-thick core ceramic.

The connector areas of the substructures between theabutment retainers and the pontics were modeled

with an occlusogingival height of 4.0 mm. Beforeand after veneering, all FPDs were controlled at dif-ferent measurement points using a digital micrometerto ensure standardized dimensions as shown in Fig 1.

The core ceramics ICA, ICZ, and DZ were ma-chined by the Precident DCS system (DigitizingComputer System, Production). Therefore, the toothpreparation and the position of the dies were mea-sured and digitized by a mechanical scanning in-strument (Digitizer). The data were transmitted to acomputer to design and calculate the substructures of the FPDs. Afterward, the resulting control and milling

data of the substructures were forwarded to a millingmachine (Precimill). The substructures were groundfrom various ceramic blanks manufactured underoptimized industrial conditions. After grinding, thesubstructures of group ICZ ceramic were additionallyinfiltrated with a low-viscosity infiltration glass (In-Ceram Zirconia glass powder) according to the rec-ommendations of the manufacturer. Finally, the sub-structures were veneered with their respectiveveneering porcelains.

Restorations of groups IE and IE2 ceramic were in-directly fabricated using the fabrication technology of

waxing and heat pressing. FPDs of group IE ceramicwere obtained directly from the wax pattern using thesurface-coloration technique, whereas FPDs of groupIE2 ceramic were produced with the layering tech-nique. After wax elimination, the appropriate ceramicingots were pressed into the preheated muffles. Atemperature of 1,075°C was required for pressing thecomplete form, and a temperature of 920°C was usedfor the layering technique. Both techniques required5 kPa of pressure for 30 to 40 minutes. When the lay-ering technique was used, the substructures were ul-timately covered with veneering porcelain followed

Volume 14, Number 3, 2001 The International Journal of Prosthodontics 232

Tinschert et alFracture Resistance of FPDs Made with Different Core Ceramics

Table 1 Ceramic Materials Used for the Fabrication of the Three-Unit FPDs

Product Code Core ceramic Veneering porcelain Manufacturer

IPS Empress IE Leucite-reinforced porcelain — Ivoclar-VivadentIPS Empress 2 IE2 Lithium disilicate glass ceramic Fluorapatite glass ceramic Ivoclar-VivadentIn-Ceram Alumina ICA Glass-infiltrated alumina porcelain Feldspathic porcelain VitaIn-Ceram Zirconia ICZ Zirconia-reinforced glass-infiltrated Feldspathic porcelain Vita

alumina porcelain

DC-Zirkon DZ Partially stabilized zirconia ceramic Feldspathic porcelain DCS Dental/Vita







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by a glazing cycle. During the manufacturing process,

various jigs were used to standardize the dimensionsof the substructures and the final veneering porcelainlayer as mentioned before.

A total of five veneered FPDs as well as five sub-structures without a veneering porcelain layer werestored and tested dry in an axial load test for each groupof restoration. Additionally, five FPDs of group IE ce-ramic were included as all-ceramic restorations with-out core material. Before testing, the restorations werecemented on their respective metal master modelswith zinc phosphate cement. Each FPD was loaded oc-clusally in the midpontic region at a cross-head speed

of 0.5 mm/min by using a universal testing machine(Zwick). For each FPD, a diagram of the load at initialfracture (Fig 2) and the mode of failure were recorded(Fig 3). Afterward, the mean failure loads were calcu-lated. Two-way analysis of variance (ANOVA) was ap-plied to determine statistically significant differences.

The significance level was established at a P value <.05. Differences between groups were identified withthe SchefféF multiple comparison test.

Results

Two-way ANOVA revealed that there were significantdifferences between the mean failure loads of the in-vestigated FPDs (P < .001). The highest failure load,exceeding 2,000 N, was found for FPDs of group DZ–veneered ceramic (Fig 4). Generally, FPDs of groupDZ ceramic revealed fracture values that were greaterthan those of all other FPDs. FPDs of group IE and ICAceramics exhibited the lowest mean failure loads,below 1,000 N, whereas intermediate values wereobserved for FPDs of group ICZ and IE2 ceramics.Except for FPDs of group IE and ICA ceramics, SchefféF multiple comparison analysis indicated that the

differences in the mean failure loads were statisticallysignificant (P < .05).

Veneered FPDs were always associated with higher

failure loads than those evaluated for the pure sub-structures without veneering porcelain. The statisti-cal analysis yielded significantly different mean val-ues (P < .05). In comparison with the mean failureloads of the pure substructures and veneered FPDs of group IE2, ICA, ICZ, and DZ ceramics, FPDs of groupIE ceramic showed also statistically significant dif-ferences (P < .05).

During the load test, initial crack formation oc-curred adjacent to the load points. The initial fractureoriginated from the locally induced stresses of theload application, and further crack propagation was

Fracture Resistance of FPDs Made with Different Core Ceramics Tinschert et al


2.3 (0.18)

5.4 (0.22) 5.4 (0.26)

2.2 (0.22)

1.5(0.13)

1.0(0.09)

2.5(0.20) 1.6

(0.11)

1.5(0.12)

1.0(0.09)

Cross-head path length (mm)

L o a d

( N )

0.0 0.1 0.2 0.3 0.4 0.5

5000

3750

2500

1250

0

Fig 1 Measurement points and dimensions of the veneered three-unit FPDs. Measurements

Fig 2 Typical load-strain diagram of a veneered FPD made ofgroup DZ ceramic.







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observed along the plane of maximum tensile stressfrom the load point to the mucosal side of the con-nectors (Fig 3). Sometimes fractures of the abutmentcrowns were also localized at the gingival margin ad-

jacent to the connector areas.

Discussion

New core ceramics may offer a unique alternative toconventional restorations, but are they strong enoughfor the use of all-ceramic FPDs? Maximal bite forceshave to be considered first. Numerous investigatorshave been interested in the maximal bite forces usedduring mastication.30–32 Apart from individualanatomic and physiologic characteristics, it has beenshown that bite force varies with the region in the oralcavity. The greatest bite force was found in the first

molar region, whereas at the incisors it decreased toonly about one third to one fourth that in the molar re-gion. In these previous studies, mean values for themaximal force level have varied from 216 to 847N.33–38 For the incisal region, smaller values ranging

from 108 to 299 N have been reported.33,35,36,38 Menoften achieve significantly greater bite forces thanwomen.37,38From the results of several studies, Körberand Ludwig39surmised that posterior FPDs should bestrong enough to withstand a mean load of 500 N.Additionally, cyclic fatigue loading and stress corro-sion fatigue caused by the oral environment must beconsidered. In contrast to crowns or FPDs of cast-metal alloys, these are important factors that can con-siderably weaken the fracture resistance of all-ceramicrestorations.40,41 Under the conditions of the oral en-vironment, the inherent flaws of ceramic materials act



Figs 3a and 3b Examples of a fractured substructure and veneered FPD made of group DZceramic. The propagation of the critical cracks is indicated by arrows.

IPS Empress IPS Empress 2 In-Ceram In-Ceram DC-ZirkonAlumina Zirconia

FPD withoutsubstructure

Substructure

VeneeredFPD

F a i l u r e l o a d ( N )

Fig 4 Mean failure loads and standard deviation bars of the tested restorations.

2500

2000

1500

1000

500

0







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as the origin of crack propagation and can grow to crit-ical size.42–45 Catastrophic failures ultimately resultfrom a final loading cycle that exceeds the mechani-cal capacity of the remaining sound portion of the ce-ramic material. This is different from metal-ceramicrestorations that have an inherent stress-absorbingmechanism in the metal substructure that limits crack

propagation.4,40As a rule of thumb, the endurance limitfor fatigue cycling that can be applied to dental ce-ramics is approximately 50% of the maximal fracturestrength.46–48 Therefore, it is reasonable to assume thatan initial fracture resistance within a safety range of 1,000 N should be required for a favorable clinicalprognosis of all-ceramic FPDs. Nevertheless, further invivo studies must ensure that this claim is transferableto clinical situations.

FPDs of group IE ceramic exhibited the lowest fail-ure loads of all restorations tested. The mean valuesimply that a possible clinical failure may occur even

at very small loads. The low mechanical properties of group IE ceramic are obviously not adequate for three-unit FPDs, particularly not for the posterior tooth area.

This may be why this kind of restoration has not beenrecommended by the manufacturer. However, thelow strength of this material does not negate its indi-cation for dental ceramic crowns. In several clinicalstudies, it was found that single-crown restorations of group IE ceramic have favorable clinical long-term sta-bility, especially in the anterior dentition, when theprocedures outlined are carefully followed.49,50

Compared with restorations of group IE ceramic,

higher failure loads were found for FPDs of group ICAceramic. But the mean values of the investigated FPDsdid not reach the initial fracture strength of 1,000 N.

These findings challenge the use of glass-infiltratedalumina porcelain for posterior FPDs and agree withclinical results reported by Sorensen et al.7 In a studyon 61 three-unit FPDs, there was no failure of ante-rior restorations, but a 35% failure rate of posteriorFPDs was recorded at the 3-year recall. In reviewingthe seven failed FPDs, crack propagation was alwaysobserved through the connector.51 The majority of fail-ures had already occurred within the first year after ce-

mentation. Therefore, it was concluded that glass-in-filtrated alumina porcelain cannot be reliably used forposterior FPDs as advised by the manufacturer.

In contrast to the fracture results of group IE and ICAceramics, three-unit FPDs of group IE2 ceramic re-vealed mean failure loads of 1,000 N. However, itmust be considered that this value was not observedfor all FPDs tested in this investigation, particularly notfor substructures without a veneering porcelain layer.For this reason, FPDs of group IE2 ceramic should beused only in the premolar region, as recommended bythe manufacturer, not in high stress–bearing areas

like the molar region. In a clinical study on 60 three-unit FPDs fabricated with lithium disilicate glass ce-ramic, Sorensen et al52 determined a failure rate of 7%.Within a period of 6 to 12 months, four FPDs suffereda catastrophic failure, defined as a fracture through thecore material. Three failures occurred in the premo-lar region, whereas only one failure was observed in

the anterior region. Thus, those authors also suggestcaution in the use of group IE2 ceramic for premolarFPDs unless an occlusogingival connector height of 5.0 mm can be achieved.

The second highest failure loads in this investiga-tion were found for zirconia-reinforced FPDs of groupICZ ceramic. The mean values of these FPDs were al-most twice as high as those made of conventionalglass-infiltrated alumina porcelain of group ICA ce-ramic. This demonstrates that the fracture strength of glass-infiltrated alumina porcelain can be significantlyincreased by the addition of partially stabilized zir-

conia.20,26 It is likely that the so-called transformation-toughening mechanism contributes to the improvedfracture strength of group ICZ ceramic.24,25 Underunrestrained conditions, zirconia undergoes a high-to-low-temperature phase transformation from thetetragonal state into the monoclinic phase, which in-volves a 3% to 5% volume increase.23 In the case of group ICZ ceramic, the tetragonal phase of the zirco-nia grains is constrained at room temperature by theaddition of a stabilizing oxide (33% ZrO2 stabilizedby 16% CeO2). A propagating crack, however, can re-lease the stresses on the zirconia grains, which then

transform again from the metastable state into themonoclinic phase. The transformed phase occupies agreater volume in the bulk material, resulting in com-pressive stresses that tend to counteract or shield anyadvancing crack propagation. Thus, in contrast toFPDs of group IE2 ceramic, the higher failure loads of zirconia-reinforced restorations may allow the clini-cal use of all-ceramic FPDs also in the molar region.

FPDs of group DZ ceramic revealed the highest fail-ure loads of all restorations tested in this study.Particularly, the veneered FPDs achieved fracture val-ues comparable to metal-ceramic restorations. Mean

values in the range of 2,000 to 2,500 N were reportedfor metal-ceramic three-unit FPDs with dimensionssimilar to the all-ceramic FPDs used in this study.53–55

However, it must be considered that the metal-ce-ramic FPDs showed cracks only in the ceramic layer,whereas the all-ceramic FPDs of the present study un-derwent global fracture. Nevertheless, the high meanfailure loads of FPDs of group DZ ceramic underscorethe potential of CAD/CAM-manufactured high-per-formance ceramic restorations, which makes them of great interest for restorative dentistry. In contrast to thezirconia/alumina-containing material ICZ, group DZ









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ceramic consists of pure partially stabilized zirconiaparticles (95% ZrO2 stabilized by 5% Y2O3) with amean grain size of 0.4 µm. These particles are denselysintered under industrial conditions, resulting in afinal microstructure in which voids, flaws, and cracksare reduced to a minimum.23For this reason, and be-cause of the transformation-toughening mechanism,

FPDs of group DZ ceramic offered remarkable me-chanical properties that are useful for highly loadedall-ceramic restorations in the posterior dentition. Incomparison with FPDs fabricated with new core ce-ramics of groups IE2 and ICZ, the fracture strengths of zirconia FPDs were significantly higher and almostthree times as high as those made of the conventionalceramic materials of group IE and ICA ceramics.

After veneering, the failure loads of all FPDs in-creased even further. The mean failure load values of the veneered FPDs were significantly different from thefailure loads of the pure substructures. This suggests

that there was a stable bond between the veneeringporcelain layers and the core ceramics. However, themean failure loads of the restorations tested in thisstudy demonstrate that FPDs made of core ceramicsgain their high strength from the core material. Froma clinical point of view, these types of FPDs do not re-quire adhesive cementation techniques to strengthenthe restoration,7,52,56,57 whereas acid etching and ce-mentation with adhesive resins are recommended forother all-ceramic crown systems like group IE ce-ramic.49,50,58 Several studies showed that an apparentincrease of the fracture resistance can be expected

when glass-ceramic or leucite-reinforced porcelaincrowns are bonded to the underlying dentin of thecrown preparation.59–61 Although FPDs of group IE2,ICA, ICZ, and DZ ceramics can also be used with ad-hesive resins,62,63all FPDs tested in this study were ce-mented with zinc phosphate cement because this ce-mentation technique is less time consuming andtechnique sensitive under clinical conditions.

However, caution must be exercised when ex-trapolating laboratory data to clinical situations be-cause many in vivo variables are excluded from acontrolled laboratory study. Although the model

used resembled clinical conditions, it did not simu-late the movement of the abutment teeth within theperiodontal ligament.4 Considering this fact, the clin-ician who is confronted with the indication of an all-ceramic FPD should also take the mobility of thedental abutments into account. When loaded withan occlusal force, teeth undergo deflection becauseof the compression of the Sharpey’s fibers. The mag-nitude of this effect cannot be estimated, but the rigidmetal model used in this study probably increasedthe load resistance of the tested FPDs.51,64 Moreover,in the oral environment, the forces applied on

dental restorations are more likely to be of a cyclicnature.65 Therefore, cycling loading would simulatemore accurately the mastication forces under clini-cal conditions than the static strength used in this in-vestigation.

The fracture mode observed in this investigation,however, was consistent with clinical data of previ-

ous in vivo studies.7,52 After analyzing the clinicallyfailed restorations, it was detected that most of the all-ceramic FPDs failures occurred in the occlusoproxi-mal line angles of the abutments adjacent to the pon-tic.51 Consequently, crack propagation to theconnector areas was also expected in this in vitrostudy, because these regions commonly receive max-imum stress concentrations.66–68 Therefore, the oc-clusogingival connector dimension or vertical con-nector height of all-ceramic FPDs should bemaximized as far as possible. Unfortunately, the con-nector areas of FPDs are generally limited in posterior

regions. Under clinical conditions, the occlusal con-tact and the gingival tissue define the limits of the con-nector dimensions, and the vertical connector heightof 5.4 mm used for the FPDs in this study cannot beachieved in every situation. Additionally, a gingivalembrasure must be maintained for oral hygiene accessand avoidance of iatrogenic periodontal disease. If theminimum vertical dimension is not available, the clin-ician may consider performing electrosurgery to re-move the soft tissue to gain space for the connectorheight, although the extent of tissue removal is limitedand biologic width must be respected.52Because the

core ceramic is significantly stronger than the ve-neering porcelain, it is sometimes recommended thatlittle or no veneering porcelain be applied at the tis-sue side of the connectors.52,57 This will also maximizethe strength conferred by the core material.

Conclusions

A laboratory investigation of the fracture strength of three-unit all-ceramic FPDs was performed. The fol-lowing conclusions can be drawn:

1. The mean failure loads of the investigated FPDswere significantly different. All-ceramic FPDs madeof partially stabilized zirconia ceramic revealed thehighest failure loads of all FPDs tested in this in-vestigation. In comparison with FPDs consisting of conventional dental ceramic materials, the meanfailure loads were almost three times as high.

2. After veneering, the fracture resistance of theFPDs increased even further. The mean failureloads of pure substructures were significantlylower than those evaluated for the veneered FPDs.However, the fracture results demonstrate that









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FPDs made of core ceramics gain their highstrength from the core material.

3. Caution must be exercised when extrapolating thelaboratory data to clinical situations becausemany in vivo variables, such as cyclic loading orstress corrosion, were excluded from the presentstudy. Therefore, further clinical studies must be

performed to ensure that the in vitro results aretransferable to clinical situations.

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Literature Abstract

Evaluation of speech in patients rehabilitated with various oral implant-

supported prostheses.

Speech function was assessed in 138 subjects: 113 patients with various implant-supported pros-

theses and 25 subjects with natural dentitions constituting a control group. There were four sub-

groups of the patients wearing implant-supported prostheses: (1) complete maxillary denture +

complete-arch mandibular fixed prosthesis on implants (n = 22); (2) maxillary complete-arch fixed

prosthesis on implants + natural dentition in the mandible (n = 27); (3) maxillary complete denture

+ mandibular overdenture on two implants (n = 49); and (4) complete-arch fixed prostheses on

implants in both jaws (n = 15). A standard clinical evaluation of the speech was carried out by a

speech pathologist. One or more pronunciation difficulties were noted for 84% of the patients withimplant-supported prostheses, compared to 52% in the control group (P < 0.05). The pronuncia-

tion of s-z sounds and/or t-d sounds was significantly different in the experimental groups com-

pared to the control group (P < 0.001). The difference was more pronounced for the subjects hav-

ing a fixed implant-supported prosthesis in the maxilla. Subjects with fixed implant-supported

prostheses in the maxilla seemed to experience more problems with s-z sounds, and those with

fixed prostheses in the mandible had more problems with t-d sounds. However, no clear relation-

ships could be established between different oral prosthetic factors and speech performance.

Jacobs R, Manders E, Van Looy C, Lambrechts D, Naert I, van Steenberghe D. Clin Oral Implants Res

2001;12:167–175. References: 38. Reprints: Dr Reinhilde Jacobs, Departmant of Periodontology, Catholic

University of Leuven, Kapucijnenvoer 7, B-3000 Leuven, Belgium. e-mail:

[email protected]—SP







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