Fatigue Life of PorcelainRepair Systems

A. Uobeit, DDS, MSD'

I. I. Nicholis, PhD"

I. C. Kois, DMD, MSD'"

C. H. Daiy, PhD'"'

Private Practice and University of Washington

Eight intraoral porcelain repair systems, ie, Oral Ceram-Etch(Gresco], Scotchprime (3M), Rocatec (ESPE), CommandUltrafine (Kerr), Silistor (Kulzer), Clearfil Porcelain Bond (JMorita), All-Bond (Bisco), and Monobond S (Vivadent) wereused in this study. The control specimens consisted ofunetched porcelain surfaces onto which a resin compositewas polymerized without the use of an adhesive. Loadfatigue was used as the testing method to simulate therepetitive action of mastication. The peak stress applied toeach test specimen was 1500 psi (10.34 MPa), and an upperlimit on the number of load cycles applied to any specimenwas set at 2,000,000 cycles. Statistical analysis revealed twosignificant subsets. Only Clearfil Porcelain Bond and All-Bond did not fail before reaching the 2,000,000-cycIe upperlimit. Int J Prosthodont 1992;5;205-213.

M any factors may cause intraoral porcelain frac-ture including impact load, fatigue load,

improper design, and microdefects within thematerial,'-f' Fracture of a porcelain restoration isoften considered an emergency treatment and rep-resents a challenge for the dentist.

Because of the nature of porcelain processing,new porcelain cannot be added to an existing res-toration intraorally. As an altemative, resin com-posites have been used for repair.'-"' Adhesivemonomers and polymeric materials developed to

'Private practice, Valencia, Spain."Professor, Department of Restorative Dentistry, Universi

of Washington.of Washington."Lecturer, Department of Restorative Denti

of Washington.'Professor, Department of Mechanical Engisity of Washington.

ity

istry, University

gineering, Univer-

Reprint requests: Or I. i. Nicholis, Department of RestorativeDentistry, Schooi of Dentistry, University of Washington, O-767 Heaith Sciences Building SM-56. Seattle, Washington98195.

obtain a bond between porcelain denture teethand acrylic resin denture base materials havebeen used for the mtraoral repair of fractured por-celain,'-'^ Also, practitioners have relied onmechanical macroretention such as grooves orundercuts to retain the resin materials. This typeof repair has been considered an interim proce-dure,"-'^

Advances in the manufacturing of resin com-posite matenals and improvement in the tech-niques used for resin-bonded porcelain veneershave provided longer lasting results."'^ The avail-able literature on porcelain repair proceduresusing resin composites, organosilanes, and etch-ing agents presents differences of opinion.

When testing the intraoral porcelain repair sys-tems researchers have focused on the effect ofload, water storage, or a combination of both. Thedifferent repair systems usually have been com-pared by either shear or tensile tests. There is agreat variability in the recommended steps andthe time required to perform them."-'^- '^"

Studies evaluating the effect of thermocyclinghave revealed that thermal fatigue significantly

Volume 5, Number 3, 1992 205 The International journal of Prosthodoniics

Faligue Life ol Porcelain Repair Syste

decreases the mean bond strength for all systems

The effecl of water storage and the differencebetween testing dry or wet samples has also beenstudied,"'-'•••^' Silanaled interfaces appear to beunstable in humid conditions and may lead tostress c o r r o s i o n and subc r i t i ca l crackgrowth, ' " " '''^"-"'

The sttidies related to the effect of etching agentsand organosilanes have shown that the use of sil-anes increases the bond strength after etching. Thecombined effect of etching and silanating is moreimportant than is etching or silanating alone,"^'"•""'-

Fig 1 Dimensions of cast rods.

Table 1 Product List

Product name Manufacturer

Porceiain repair systemsEnamelbond ¡Control)Oral Ceram-EtchAll-BondClearfil Porcelain BondScotch primeRoca tecCommand Ultrafine Porcelain

Repair Bonding SystemSilistorMonobond S

3M, St Paul, MinnGresco, Stafford, TexBISCO, Downers Grove, illJ Morita, Tustin, Calif3M, St Paul, MinnESPE, Norriston, Pa

Kerr, Romulus, MichKulzer, Irvine, CalifVivadent, Amherst, NY

Resin compositesSiluxBisfilVisio-FilHerculiie HRMultifil VSHelio-Molar

3M, St Paul, MinnBISCO, Downers Grove, IIIESPE, Norriston, PaKerr, Romulus, MichKulzer, Irvine, CalifVivadent, Amherst, NY

MisceitaneousWaves Wax #3DeguvestLite cast BVITA VMK 68Horico Diaflex Diamand

Disk 347/190 (86x)

The Intern

Maves, Cleveland, OhioDegussa, Long Island, NYWilliams, Buffalo, NYVident, Baldwin Park, Calif

Pfingst, South Plainfieid,NJ

ario nal lournal of Proslhodontics 206

Most of the research on porcelain repair mate-rials has either discussed shear''-^'^-"'^''"' ' |"'fensile load testing,!''''"^''^ Some studies use* atbree-pomt loading test,'"" " and othci s combinedshear load with thermal fatigue.^"'""''^''^ ^°^'^fatigue testing results have been published on den-ture base polymers but not on porcelain repan" sys-tems,"""' ' Load fatigue may be defined as aphenomenon whereby a sample that has beenrepeatedly subjected to a load well below the levelthat causes fracture in static tensile tests eventuallyfails after being subjected to this comparativelysmall cyclic load. This fatigue failure is precededby a combination of crack initiation and crackpropagation. Finally catastrophic failure occurs inthe form of fracture.""""

Fatigue is of considerable importance for certaintypes of dental restorations subjected to small alter-nating forces during mastication. Although thesefailures are not frequent, some restorations failentirely as a result of fatigue without any othercause,"'' In view of the type of loading inducedclinically from mastication and parafunction, whereintraoral occiusal forces create repetitive dynamicloading that may be resolved as a combination ofcompressive, tensile, and shear stresses and strains,perhaps fatigue would be a more significant testthan static shear or tensile tests.

The purpose of this study was to use load fatiguetesting to compare the efficacy of eight differentcommercial systems available and recommendedto repair clinical porcelain fractures. The test var-iable was the number of load cycles required tocreate catastrophic failure,

Malerials and Methods

The experimental design is based on dynamictesting and the products used in this study are listedin Table 1,

Specimen Design

A methylmethacrylate rod was machined in alathe to the two separate pieces shown in Fig 1,Five Maves wax patterns were injection moldedand retrieved from an addition silicone mold {3MFxpress putty, St Paul, Minn) of these machinedrods. These wax patterns were sprued, invested inDeguvesl phosphate investment, torch (MétauxPrécieux SA Metalor, Neuchatel, Switzerland)melted, and cast in Litecast B alloy using a Kerrcentrifugal casting machine (Kerr, Romulus, Mich).

The end surface ß of casting A-B {see Fig l ) wasground flat using a carborundum separating disk

•5, Number Î , 1992

Fatigue Lile of Porcelain Repair Systems

Table 2 Firing Cycles

Initialtemp (

Finaltemp (X ¡

Heatingrate i°C/min)

Hoid|min)

First opaqueSecond opaqueFirst body bakeSecond body bakeGlaze

600600600600600

9S0960910900900

5555555555

11111

YYYYN

(IF lelenko and Co, Armonk, NY) and polishedusing a brown rubber wheel {Shofu Dental Corp,Menio Park, Calif), This procedure was accom-plished with a Titan slow-speed handpiece {Star.Valley Forge, Pa) attached to a Unimat engineeringlathe (Emco, Austria).

Porcelain (Vita VMK 68, Vident Corp, BaldwinPark, Calif) was baked on the end B of the five 0.5-inch castings A-B {see Fig 1), Manufacturers' sug-gested firing cycles were followed (Table 2), Afterthe glaze cycle, the porcelain was ground on theengineering lathe using a green stone (Shofu DentalCorp) to obtain a 1,2-mm porcelain thickness withthe same diameter as the cast rod. These metal-ceramic rods were steam cleaned and stored dry.

To obtain additional microretention the ends Cof the five C-D castings (see Fig 1) were abradedusing 50 ^im aluminum oxide under a pressure of40 psi for 10 seconds. These castings were steamcleaned and stored dry.

Specimen Fabrication

To form a complete test specimen, a customalignment vee-jig was fabricated. This allowedalignment of the two cast cylinders A-B and C-D

leaving a 1-mm space between the porcelain sur-face of A-B and the retention knob of C-D (Fig 2),

After treating the porcelain surface according toone of the eight treatment categories, one cast rodof each type was secured in the alignment jig, andthe gap between the rods was filled with resin com-posite. The unpolymerized resin was held in placeusing a clear gelatin capsule (Fli-Lilly, Indianapolis,Ind], Initial light polymerization was performedwith a light-polymerizing unit (Optilux 400, Deme-tron, Danbury, Conn) for 60 seconds.

Because of the thickness of resin, complete resinpolymerization was performed under vacuum for15 minutes using a Beta light unit (ESPE, Norriston,Pa],'''' This procedure eliminated the need for lay-ering the resin and assured standard polymeriza-tion for all specimens.

Resin composite flash was removed using aScotchbrite wheel 13M, St Paul, Minn) mounted ona Titan slow-speed handpiece attached to a Unimatengineering lathe as shown in Fig 3, The resin com-posite and porcelain had the same diameter at thejoint. Specimens were checked under 20X mag-nification to confirm that there were no defects orresin flash left at the joint periphery. Differentcolors of porcelain and resin composites allowed

Fig 2 Gelatin capsule surrounding the two specimen halves. Fig 3 Scotchbrite wheei trimming the resin flash. Specimenis mounted in the engineering iathe.

•5, Number 3, 7992 207 lournai of Prosth odonl u

Fatigue Life of Porcelain Repair Syste

for accurate inspection and easy removal of resinflash (Fig 3). Each completed specimen was placedin a water bath at 37''C for 5 days prior to beingtested.^f' The effect of thermocycling was notincluded and is the subject of a separate investi-gation. A total of 60 specimens were prepared,according to the manufacturers' directions, using12 differenf treatment categories (Table 3). Thecontrol specimens were repaired with unfilledresin/filled resin composite without the use of anyporcelain repair system.

Load Fatigue Testing

Following specimen fabrication and prior toactual testing, two dimensions (Fig 4) were deter-mined:1, The diameter d of the porcelain/resin inter-

face was measured with a Vernier caliper(Mitutoyo Instrument Center, Paramus, NJ)having an accuracy of 0.005 mm and a readingerror of ± 0.01 mm.

2. The moment arm m between the applied loadand the porcelain/resin interface was mea-sured with a measurescope (Measurescope20, Nikon, Tokyo, Japan) having an accuracyof 0.001 mm and a reading error of ± 0.005mm. The required load needed to create agiven stress al the test interface was computedfrom fhe formula;^"

applied load =

(applied stress X (section modulus of interface)

(movement arm)

where section modulus of interface is equal io(3.142 X dV32 , applied stress = 1500 psi (10.- 'MPa), and cyclic rate = 30 Hz.'"''''=^i-"

Figure 5 shows fhe fatigue machine used to testthe specimens. End A of each specimen wasclamped in the three-jaw chuck of the fatiguemachine. End D had a roller-bearing assemblyattached. This roller bearing allowed specimens torotate with the three-jaw chuck, while the loadremained vertical (acting downward). As eachspecimen rotated, the applied load created a sinus-oidally varying stress at the porcelain/resin inter-face. At any point on the joint periphery thisapplied stress ranged between 1500 psi (tensile)and 1500 psi (compressive). A water pump applieda constant stream of 37°C water to the resin duringthis testing (see Fig 5). This water stream was main-tained throughout the total time of testing. A rotat-ing mirror allowed reflection of an infrared beamthat tnpped a counter at every revolution. At cat-astrophic failure, the applied load dropped onto amicroswitch, thus terminating the test.

Because fatigue testing requires considerabletime, an upper limit was set on the number ofcycles. Peyton'-' suggested 100,000 cycles, but, toshow a separation between the test categories, a2,000,Q00-cycle limit was set for this study. All fail-ure sites were examined under 70X magnificationto determine the mode of failure.

Procedures

Manufacturer's directions were specifically fol-lowed for each porcelain repair system used.

Table 3 Summary of the 12 Treatment Categories

Repair System" Symbol Category

C Enamel bon d/Silux

CE/S 9.5% HF/Prilane/Scotchbond 2 Adhesive/Si lux

CE/ES 9.5% HF/Prilane/Enamelbond/SiluxAB/S 8% HF/Porcelain primer/Dentin-Enamel Bonding Resin/Silux

AB/B 8% HF/Porcelain primer/Dentin-Enamel Bonding resin/Bisfil

CL/S 40% H3P0,/Clearfil/SilLjx

CL/H 40% H3P0„/Clearfil/Herculite XRSP/S 37% H3PO„/Scotctiprime/Scotchbond 2 Adhesive/SiluxFl/V Sandblasting with Rocatec-Pre/Rocatec-Plus/Rocatec-Sil/Visio-Bond/Visio-Fil

CU/H 37% HjPO,/Porcelain primer/Command Bonding re s rn/Herculite XRSK/M K1 bur treatment/Silicer/Silibond/Multifil VSMS/HM 37% H3PO,/Monobond S/Heliobond/Heliomolar

'Five specimens ir aach category."Sandblasting recommended as an altarnative to H etching.

ControlOral

Cera m-EtchOral

Ceram-EtchAll-Bond"All-Bond"Cleartil Porcelain

BondClearfil Porcelain

BondScotch primeR o cateeCommand UltrafinePorcelain RepairBonding systemSiiistorMon o bond S

I of Prosthodonti. 208 Volume 5, Number 3, 1992

Analysis of Data

All data were analyzed by means of a siatisticalsoftware package (SPSS-X Rel 3.0 for DECstation3100 Digital Equip Corp, Maynard, Mass), Thenumber of load cycles was the independent vari-able. Each category tested had five load-fatiguecycle values. These results were stibjected to aone-way analysis of variance and tested for signif-icance with the Sludent-Newman-Keuls intervaltest. Significance was defined at the 95'/Ü level.

Fatigue Life of Porcelain Repair Syslems

Results

Eatigue testing of polymers is influenced by theirviscoelastic nature, resulting in widely varyingfatigue lives and scattering of the dala. In this study,although large standard deviations were present insome of the categories with the lowest fatigue val-ues, a significant grouping of the data wasobtained.

Table 4 presents the number of cycles to failurefor each specimen tested along with the means andstandard deviations, A total of 12 test categories

Table 4 Fatigue Life For Each Category

Category

CCE/SCE/ESAB/SAB/8CL/SCL/HSP/SR/VCU/HSK/MMS/HM

1

113648

1231460572030

*1540960

'1291

9781687787

1061836033

2

4139

-1564042

•

789221•

2269130705

•

112788891

Specimen number

3

3266

87966•

1680464

92031677898920

9235100

4

53110

628640

1916550

2618714569

17276538

89145

5

62740

867258

1416992

34911280108

3072720

53415

Mean

251,6260,6

>1846412703987,2

>20000001468837,4

>200000081696,847995,2

> 533348,870,2

560516,8

SD

459.1279.5

—534699.7

—422788,4

—149828,538419.1

—47.5

780144,6

•Specimens that did not tail prior to 2,000,000 cyciBS |no taiiura specimens).

TTiree-jaw chucK

•v _ J Joint diameter d

1 \ , Hesin composite

- \ ^

J Porcelain-^

MoiTiftni arm m

Beanng

Applied road

Fig 4 Definitions of moment arm m and joint diameter d.

Fig 5 Specimen clamped in three-jaw chuck,showing water running over resin compositedunng test and reinforcing resin compositelayer used only with ttie microfilled resins.

Volume 5, Number 3, 1992 209 I of Prosttiodontics

and 60 specimens were used in this study. Figures6a and 6b present these same data in the form ofa bar graph.

As shown in Figs 6a and 6b and Table 4, all Clear-fil Porcelain Bond/Herculite (CL/H) specimens, allAll-Bond/Bisfil (AB/B) specimens, four Ceram-Etch/Enamelbond/Silux (CE/ES) specimens, andone Command Ultrafine/Herculite (CU/H) spec-imen reached the upper limit of 2,000,000 cycleswithout failing.

Two subsets are defined to facilitate statisticalanalysis. Subset 1 contains CE/ES, AB/B, CL/S, andCL/HA, Subset 2 contains C, CE/S. AB/S, SP/S, R/V, CU/H, SK/M, and MS/HM, A significant dif-ference (P < ,05) was found between subsets 1and 2. To analyze the load-fatigue data, those spec-imens that did not fail were assigned values ofeither 2,000,100 or 2.000,200 cycles. Two valueswere used to avoid a zero standard deviation-

Table 5 is a tabulation of the mode of failure forthe specimens tested. Failure surfaces were closelyexamined at 70X and divided into four groups: (11adhesive failure at the resin composite/porcelaininterface; (2) cohesive failure within the resin com-posite; (3) no failure; and (4) dislodgment (failureat load application = zero cycles].

All specimens in categories Control (C), Ceram-Etch/Scotchbond 2 Adhesive/Silux (CE/S),Scotchprime/Silux (SP/S], Rocatec/Visiofil (R/V),and Monobond S/Heliomolar ¡MS/HM] failedadhesively. Four out of five specimens in cate-gories Silistor/Multifil (SK/Ml, All-Bond/Silux (AB/S), and CU/H exhibited adhesive failure. All spec-imens in category Clearfil Porcelain Bond/Silux(CL/S) failed cohesively even with reinforcedmicrofilled resin composite. Category CE/FS hadone adhesive failure. Categories AB/B and CL/Hdid not fail.

Table 5 Failure Mode of TeslSpeciiTieris

Category

CCE/SSP/SR/VMS/HMSK/WAB/SCU/HCL/SCE/ESAB/BCL/H

Adhesive

555554440100

Cohesive f

000000105000

——000000010455

—00000

n

nu0Û00

Discussion

Altemating forces of mastication result in a com-bination of cyclic stresses and strains in restora-tions. As a consequence, fatigue testing is moreclinically relevant than are static shear or tensilestress tests. Typically, stress levels in fatigue testsare 40% to 60% of the tensile limit,^' In this studyall specimens were tested with a maximum cyclicstress of 1500 psi. This level was chosen as 40%of the average tensile strength for resin compositebonding to porcelain from previous studies,^^"These studies obtained high tensile bond strengthsof 2078 psi and 5427.5 psi, respectively.

Maintaining a 37°C water stream throughout thetotal time of testing kept the periphery of the por-celain/resin interface under controlled tempera-ture ¡Fig 5),

Time becomes a factor in any test involvingfatigue. Ideally fatigue tests should use the speedof flexure that materials undergo clinically.Although chewing rates have been measured as1,25 Hz," fatigue tests upon dental resins haveused rates from 0-5 to 37,5 Hz,"''^ In this study thecyclic rate was of 30 Hz- This rate allowed a rea-

lions

(mde

s

o0.5

0

T MTl

- ^ W-\^ " IIBBBÎ ^

^H • • ^1

H

H H 1 •^ ^ ^ F""]"

CL/3 CE/ES AB/a CL/H

Test category

•M 1 5

"ES 1yO

Fig 6a Fatigue iite vaiues; bar graph of statistical subset 1 Fig 6b Faligue life values: bar graph of statistical subset 2.

210 • 5, Number 3, 1992

Fatigue Life of Porcelain Repair Systems

sonable testing time and met the requirementsimposed by the ASTM Standard D671 - 7 1 , " Spec-imens that did not fai! at 2,000,000 cycles (18,5 hr)were expected to continue for longer periods ofcyclic loading. This 2,000,000-cycle limit is consid-erably higher than the 100,000-cycle limit pro-posed by Peyton," but is less than the 10,000,000-cycle value quoted in engineering literature,^' Con-sidering the data obtained in this study, this com-bination of cyclic rate and applied stress hasresulted in a clear separation between the treat-ment categories.

Two of the recommended treatment categories,AB/B and CL/FH, consistently went to the upperlimit of 2,000,000 cycles. The number of cycles andmode of failure of the specimens in all other cat-egories was variable. This may be the result of theability or inability of the chemistry to form a con-tinuous layer of the chemistry without voids orinclusions and/or the time-dependent water deg-radation of resin composites,""^

Further points of interest here are the contactangle or wettability of the cement on the porcelainand resin composites. Small discontinuities such asair bubbles in the cement film, especially towardthe outer edge of the joint, will reduce the fatiguelife because such discontinuities create stress con-centrations that can initiate crack formation morerapidly under fatigue loading,^' Because the sizeand location of such discontinuities cannot he pre-dicted prior to specimen fabrication, the fatiguedata can be quite variable. For example, a bubblein the adhesive layer, at the center of the circularbonding area, would have little or no effect on thefatigue life because this location is one of zerostress. !f this same bubble were at the circumfer-ence, the fatigue life would be reduced becausethis circumferential location is one of high tensilestress under the applied loading, Similady, thecloser the bubble is to the center, the less will bethe effect on the fatigue life. Because there can beno control placed on such a bubble location, largedata variation must be expected if this bubble loca-tion is random. Of course, the goal is to have nobubbles. Thus, a product that wets the bondingsurface completely is the most desirable.

Because of the large standard deviations thatsuggest a nonnormally distributed population, per-haps a Kruskal-Wallis test would have been moreappropriate than the ANOVA, The obvious supe-riority of the CF/ES, AB/B, CL/S, and CL/H groupswould not change, however, because these datain general do not represent failure conditions. Theproblem of running a statistical analysis under theconditions imposed here, where samples do not

fail, is not easily solved. Perhaps it would havebeen more prudent not to run any statistical analy-sis. This would have left the question as to whetherthe samples were really different. To answer thisquestion,arbitraryvalues (2,000,100 and 2,000,200cycles) were assigned to each category where nofailure conditions were found. In this way, statisticalsignificance could be assigned. This significancewas determined on the basis of very low incre-ments to the cyclic failure values over the presetlimit of 2,000,000 cycles. Had no significant dif-ferences been found, additional load cycles wouldhave been necessary for the samples that did notfail. Because significance was found, there was noneed to continue the testing, and the sample sizeused was deemed acceptable.

Rocatec treatment of the specimens gave lowload-fatigue values and had a detrimental effect onthe porcelain test surfaces. Because of severe dam-age to the surface, a significant amount of porcelainhad to be removed on the engineering lathe toobtain a new flat test surface. Intraoral air-abrasivespraying procedures may damage the remainingglazed-porcelain surfaces, even with light pressure,unless careful isolation is added to the recom-mended guidelines.

Manufacturers recommendations for the All-Bond system suggest the use of an intraoral microb-lasting unit or the use of their 8% F1F acid. Becauseof the results obtained with the Rocatec system, itwas decided to use the hydrofluoric acid proce-dure.

This study demonstrated a lack of compatibilitybetween systems and resins. For example, the AB/B combination provided a fatigue life in excess of2,000,000 cycles for all five specimens. For thecombination AB/S, all five specimens failed below2,000,000 cycles. One of these exhibited a cohe-sive failure and the other four exhibited adhesivefailure.

The results of the CL/H combination showed afatigue life in excess of 2,000,000 cycles. Howeverthe CL/S combinations all failed below this limit.All five specimens exhibited cohesive failure in theresin composite. This failure occurred in spite ofthe additional resin composite reinforcement pro-vided (Fig 5), This finding agrees with the work ofDrummond," who stated that hybrid resin com-posites are more resistant to fracture than aremicrofilled resin composites.

Clinicat Significance

It is recommended by manufacturers and com-mon practice to use microfilled resins for Class IV

Volumes, Number 3, 1992 211 The Inter national Journal of Prosltiodontics

F.nligue Life of Porcelain Repair Systt

restorations. Within the confines of this load-fatigue study it was found that wben Ihe repairsystem allowed a greater number of cycles, mitro-filled resin composites required reinforcement toavoid fracture, while hybrid resin composites didnot exbibit cobesive failures. A comparison of theresults shown in Table 4 reveals that microfiliedresin composites tend to fatigue earlier than dohybrid resin composites, Perbaps the use of hybridresins should be recommended in situations wherefatigue loading is a consideration,

Scotchbond 2 Adhesive was the only variablebetween categories CF/S and CE/ES, Results inTable 5 show a significant difference in their behav-ior, with Enamelbond unfilled resin providing alonger fatigue life. When the SP/S was used in theScotchprime/Silux category, it also provided lowtest values.

Comparing the different systems by the numberof steps and specific times involved in each repairprocedure, Clearfil Porcelain Bond undoubtedlyappears to be the simples! system to use. This sys-tem provided consistently superior load-fatigueresults and does not require the use of hydrofluoricacid.

Conclusions

This study compared 12 resin material combi-nations that are used to repair porcelain fractures.Only porcelain fractures were evaluated (not thebonding of tbe resin to metal], A 1500 psi (10.34MPa) sinusoidally varying stress was applied to arotating cantilever beam specimen that was rotat-ing at 30 Hz,

From this study, the following conclusions maybe drawn:

1, Only two groups did not fail before reachingthe upper limit of 2,000,000 cycles: ClearfilPorcelain Bond/Herculite and All-Bond/Bisfil,

2. Four additional categories showed signifi-cantly higher fatigue lives: All-Bond/Bisfll,Clearfil Porcelain Bond/Herculite, ClearfilPorcelain Bond/Silux, and Ceram-Ftch/Ena-melbond/Silux,

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Literature Abstraat-

Usporedba Polzaja Nasiona i Orbitale na Mekím Tkivmai Kostanim Strukturama Glave u Artikulacijskoj Tehnici(A Comparison of the Soft Tissue and SkeletalPositions of the Nasion and Orbitale in Relation to theAccuracy of Mounting Casts in an Articulator)

Cephalometric tracings were used to compare the skin and skeletal positions of the nasion andorbitale in 22 subjects. Ttie vertical nasion-orbitale skin distance averaged 27.9 mm compared to theskeletal value of 23.52 mm. The average discrepancy betv^een skin and skeletal points ot the nasionamounted to 1 mm and of the orbitale amounted to 1.4 mm. Ali of these differences are statisticallysignificant. The discrepancies between the skin points and skeletal landmarks of the nasion andorbitale result in inaccurate mounting of casts in an articuiator.

Seifert D, Stan ko V, Zellmir M, Panduric J./cía Slomato/Croat 1991:25:9-102. Refererces: 25. Reprints: Dr DavorSeifert, z'avod za mobilnu protetiku, Stomatoioski takuitet Univerütet Zagreb. Gurüuliceva 5, 4100 Zagreb,Yugoslavia-Moily Stiapiro. UP Abstract Editor /Non-English Language)

^Volumes, Number 3, 1992 213 The International Journal of Prosthodortics