Sequencing Platinum FoilMatrix Removal in

Postceramic Solderingof the Collarless

Veneered Retainer

Stephen M. Hannon, ODS'

Ranald 8. Cunderson. ODS"

Lewis Lorton, DDS. MSD'"

Theodore Zislis, DDS, MS""

Steven O. Hondrum, DDS, MS'

VS Army Dental Corps

Postceramic soldering of collarless veneered retainers inwhich the porcelain margins were formed with a platinumfoil matrix technique was investigated. The purpose of thestudy was to determine if removing the platinum foil matrixbefore or after indexing and soldering procedures wouldaffect the porcelain margin integrity in fixed partial dentures.Prostheses were fabricated on a nickel-chromium laboratorymodel using both sequences. Six test cycles wereperformed. Each cycle included one soldering with matrixsupport and one without the matrix, for a total of 12solderings. The fixed partial dentures were compared fordegree of marginal seating with a measuring microscope andfor configuration changes at the porcelain margins withscanning electron micrographs. In the microscopic analysisof marginal closure, soldering without foil matrix wasstatistically equal to soldering with foil in place. Removal ofthe platinum foil matrices prior to indexing provided for nofurther seating of the metal ceramic retainers. Evaluation byscanning electron microscopy demonstrated distinctconfiguration changes in the porcelain margins for thespecimens soldered without matrix support. Int] Prosthodont1991:4:457-464.

T he platinum foil matrix technique is a well-established method for constructing metal

ceramic restorations with porcelain facial margins.A sheet of platinum foil is secured temporarily to

the facial veneer of a cast coping. Conventionalfeldspathic porcelain is condensed and fired. Themargin is then completed against the foil matrixand the platinum foil is removed prior to luting.

'Senior Resident. US Army Advanced Prosthodontics Res-idency. Walter Reed Army Medical Center. Washington,DC. Currentiy Chief. Fixed Prosthodontics Service. USArmy Dental Activity, Presidio of San Francisco. California.

"Director. Advanced Prosthodontic^ Residency, WalterReed Army Medical Center. Washington. DC, CurrentlyClinical Associate Professor, Department of Fixed Restor-ative Derttistry, University of Maryland Dental School. Bal-timore. Maryland.

"Chief, Bioengineering, US Army Institute of DentalResearch. Port Meade, Maryland. Currently Senior ClinicalResearch Associate for HIV Research, fienry M. /acksonFoundation. Rockville. Maryland.

"Chief. Pathology. US Army Institute of Dental Research,

Port Meade. Maryland. Currently Deputy Commander forCiinicai Services. lOth US Army Medical Laborafory. Land-stuhl. Germany.

"'"Chief, Dental Materials. US Army Institute of DentalResearch. Fort Meade. Maryland.

Reprint requests; MAJ Stephen M. Hannon. DC, Chief. FixedProsthodontics Service. US Army Dental Activity. Presidio ofSan Francisco, California 94129-6700.

The v/eivs expressed in this artide are those of the authorsand do not reflect the official policy or position of the Depart-ment of the Army, the Department of Defense, or the USGovernment.

umber S, 1991 4 5 7 The International Journal of Frosthodontii

Sequencing Platinum Foil Matrix Rerroval

Fig 1 Ni ekel-Chrom i um master model.

Fig 2 Sketch of model defines ttie nine points of marginassessment.

Restorations fabricated using this method dem-onstrate reasonably accurate marginal adaptationand sharpness; excellent esthetic quality and trans-lucence; and smooth, well-condensed porcelainadjacent to gingival tissues.̂

This porcelain margin technique may also beused for fixed partial dentures, although fabricatingsuch prostheses from one-piece castings can betechnically challenging. Making individual collar-less metal ceramic retainers and then joining therestorations by postceramic soldering facilitates theprocess.

The recommended sequence for completing thesingle collarless restoration includes removing theplatinum matrix after the glazing procedure.^ Thefoil serves to support and protect the establishedporcelain edge from thermal alteration, which tanoccur at temperatures used for autoglazing.' Refir-ing sintered porcelain to glazing temperatures pro-duces dimensional and surface character changes

that are related to the glass transition temperatureof the ceramic material.-''' However, postceramicsoldering is completed at lower temperatures thanthose used for autogiazing, and the effects of sol-dering temperatures on unsupported porcelainmargins are not widely known.

Incomplete seating (or "lifting") of a coping mustbe considered a potential source of error whenplacing a tasting over 0.001-inch platinum foiladapted to a die.'' ' The actual amount of lifting isnot known but could be a factor affecting the accu-racy of the indexing procedure. The purpose ofthis investigation was to determine whetherremoving the platinum foil matrix either before orafter indexing and soldering procedures wouldaffect either the adaptation or the form of the por-celain labial margin.

Materials and Methods

A model was constructed in a nickel-chromiumalloy (Ticonium Co, Albany, NY) to simulate molarand premolar fixed partial denture abutments (Figs1 and 2). The molar was prepared for a completemetal veneer retainer, and the premolar was pre-pared for a metal ceramic retainer with a facialporcelain margin. Nine reference dimples, four onthe molar and five on the premolar, were placed2 mm apical to the prepared finish lines. Scribelines were then plated from the dimples to themargins to standardize the locations to be micro-scopically assessed for margin closure. Six meas-uring points were on metal margins and three onporcelain. The base of the model was notched andan acrylic resin block (Plastodent Inc, Bronx, NY)was formed to position the model during indexingprocedures. A nickel-chromium indexing supportframe was cast to fit over the resin block and modelin a spetific, repeatable orientation (Fig 3).

A fixed partial denture framework cast using agold-palladium alloy (Olympia, )F |elenko Co,Armonk, NY) was finished for porcelain applicationand soldering. The framework was cast prior toplacing the scribe lines on the model. The con-nector area for soldering was located between themolar and the pontic. The experiment wasdesigned so that one set of castings could be usedwith repeated porcelain veneer applications foreach of six test cycles. An initial soldering proce-dure was performed to establish a layer of residualsolder in the connector area and to determine theworking melting range of the selected solder forthis set of test conditions. That range was 1,600°Fto 1,620"?.

The porcelain margins for the premolar retainer

The Inlernational lournal of Proslhodonlits 4 5 8 Volurre 4, Njmber 5

Sequercing Platinum Foil Matrix Re

were established using a conventional platinum foilmethod. After blocking out the faciocervical under-cut, six sfone dies (Silky Rock, Whip-Mix Corp,Louisville, Kyi were consfrucfed by duplicating themetal die. Platinum foil 0.001 inch thick (EngelhardCorp. Carteref, N)) was adapfed to the facial sur-face of fhe stone die and extended 2 to 3 mmcervical fo the finish line. The metal casting wasseated over the foil, and the foil was spot welded(Dial-A-Weld, iîocky Mountain Orthodontics,Denver, Colo) to the casting. The casting waspreoxidized and opaque porcelain was applied. Alayer of varnish (Unitek Corp, Monrovia, Calif) wasapplied to the platinum foil on the shoulder areato allow the initial firing of body porcelain (VitaVMK 68, Videnf, Baldwin Park, Calif) fo shrinkwifhout disforting fhe mafrix.'" The narrow, well-defined gap was fhen filled with subsequenf bakes.The porcelain veneers were contoured and fin-ished prior to autoglazing.

The connector gap widfh was sfandardized witha 0.45-mm feeler gauge. Six fesf cycles were per-formed wifh fwo different soldering procedures foreach cycle. The first soldering procedure was com-pleted retaining the foil mafrix, and the secondafter removing fhe matrix for the indexing and sol-dering procedures.

In fhe indexing procedure, the castings weresecured to the model using sticky wax (MoycoIndustries Inc, Philadelphia, Pa) and fhe connecforarea was filled with wax. Impression plasfer (Plas-togum, |FI Bosworth Co, Skokie, 111) was mixed andapplied to the index support frame. The frameworkwas then seated over fhe model in the acrylic resinorientation block and allowed to set undisturbedfor 30 minutes (Fig 3). The index, with the castings,was then separated from the model, waxed, boxed,and poured using a soldering investment (Ther-movest, Kerr, Romulus, Mich). After 1 hour, thesoldering block was separated and wax was elim-inated using boiling water.

The soldering block was placed at the muffledoor of a porcelain furnace QM Ney Co, Bloom-field, Conn). The connector area was fluxed (NeyPink Flux, |M Ney Co), and a pre-cut, 4-mm-iongstrip of 650 fine gold solder (WF Mowrey Co, StPaul, Minn) was placed. The block was then setinto fhe muffle af 400°F and was heafed accordingfo a time and temperature regimen suggested byWeinstein.'" Temperature was elevated af approx-imately 100° F per minute under 28 Ib vacuum. Thesolder flowed at a temperature range of 1,600°Fto 1,620''F, after which the vacuum was released.The connector was visually assessed, and the sol-dered specimen was allowed to cool fo room tem-

Fig 3 Solder indexing assembly.

perature. The fixed partial donfure was thendivesfed, cleaned, and placed back on the mastermodel for evaluafion of marginal adapfafion. Thesoldered connector was sectioned, and fhe index-ing and soldering procedures were repeated with-ouf the foil matrix in place. After each test cycle,the connector was sectioned, the porcelain veneerwas removed from the casting, and the next testcycle was performed.

Marginal adaptation was determined by meas-uring gap widths using a measuring microscope(Nikon Measurescope Model II, Nippon KogakuKK, Tokyo, |apan). To ensure precision of mea-surement with the microscope at each session, thewidth of a piece of standard dental wire wasrecorded for a calibration check. All measurementswere performed by one operator. Gap widfhs wererecorded at each reference dimple, from fhe poinfwhere the scribe line crossed fhe prepared marginon the metal preparation fo fhe marginal edge ofthe fixed partial denfure. Five readings were madeaf each locafion for statistical averaging. Initialbaseline measurements of the finished metal cast-ings served as controi data for the study. The metalcollar on the premolar casting was nof yefremoved.

Four sets of measurements were made in eachtest cycle. Set 1 was prior fo soldering, and readingswere made at the six reference poinfs relafed focasfing margins. The platinum foil was retained atthe porcelain margin. Set 1 data were used fo verifymefal margin infegrity from cycle to cycle. As con-trol data, they were used to quantify any metalbreakdown in the porcelain stripping and reappli-cation procedures. Set 2 measurements were madeon the soldered specimens of each cycle that hadbeen indexed and soldered wifh the matricesremaining. Matrices were removed prior to meas-

•4. Number 5, 199t 459 The International journal of Prosthodortic

Sequencing Platinum

Table 1 Cast Margin

Data sel

1 (Prior toporcelainapplioation)

2 (indexedand soideredwith foil)

3 (Connectorseciioneai

4 (Indexedand solderedwithout foii)

Fuii Mairl» Removal Han

Measurements*

1

7.4±1.9

7.4±2.9

5.8±1.2

7.0±1.5

2 3

81 9.6±1.5 11.6

8.5 10.6±1.8 ± 1.5

9.7 10.8±2.4 It 2.6

8.1 10.4± 1 8 ± 2 . 2

non et al

Position

4

8.4± 3 0

8.6±2.6

9.1±1.8

9.5±2.9

5

2^.5±13.7

19.8±12.1

18.0±15.1

18.3±13.9

6

29.5±7.1

31.5±10.7

23.4±100

29.6±16.0

Mean

14.1

14.4

13.0

13.8

"Means in tun {+ standard deviations) tor ail six test cycles at eac« of ttie six matai margin positions.

Table 2 Porcelain Margin Measurements*

Data set

2

3

4

(indeseOandsolderedwith foii)

(Conneotorsectioned)

(Indexedandsolderedwithout foil)

7

75.7±39.4

68.6±29.0

70.6±32.3

Position

8

90.6±41.7

89.1±45.5

93 5±30.5

9

62.8±38.4

56.6±33.8

63.9±33.1

Mean

76.4

71.4

76.0

•ti^eans in iim (* standard deiiiations) for att six test cycles at each atttie three porcelain margin positions.

uring at the six metal and three porcelain margins.For set 3, the connectors were sectioneiH and theretainers seated individually for reassessment at allnine reference locations. Set 4 readings were madeon the soldered specimens for which the indexingand soldering procedures had been performedwith the platinum foil matrices removed. Nine ref-erence locations were again measured.

The margin junction of the three-unit fixed par-tial denture was measured at the nine positionsdepicted in Fig 2. All measurements were made byone investigator. The gap at each location wasmeasured five times and an average value was cal-culated. A two-way analysis of variance model withrepeated measures on the data sets and positionswas used to analyze the cast metal and the por-celain positions separately. All tests were done atthe .05 level of significance (P value].

Results

Cast margin scores displayed in Table 1 weredetermined by averaging the mean scores at eachof positions 1 through 6 for all six test cycles of thestudy. The accumulated porcelain scores shown inTable 2 were similarly calculated for positions 7, 8,and 9 in all test cycles. Mean scores for each dataset are listed in the final column of both tables.

Analysis of the cast metal margin data indicatesthat the soldering procedures related the retainersto each other with very nearly equal precisionthroughout tbe six test cycles. A P value of .4651indicated no significant differences.

Porcelain margin data demonstrated substantialvariation from cycle to cycle. This variation was notcontrollable and was probably tbe result of thetechnique sensitivity involved in tbe formation of

The International lournal of Prosthodontic 460 Voiume4, Number 5. 1991

Sequentmg Platinum Foil Mötrn Removal

porcelain margins. Still, no significant differencesbetween data sets 2 and 4 for the six test cyclesof ihe iaboralory study were revealed. The two-way analysis of variance tests comparing the datasets yield a P value of .4321 and indicate no sig-nificant difference. Regarding gap width measure-ments, indexing and soldering with foil matrixsupport did not show a difference compared toindexing and soldering without the matrix in place.

Representative specimens were selected forqualitative scanning electron microscopic evalua-t ion. Distinct configurational changes wereobserved in the porcelain margins for the speci-mens soldered without platinum foil matrix sup-port. The "sharpness" of the margin observed in

the matrix-supported soldering procedures was notseen when soldering was accomplished withoutthe matrices (Figs 4a and 4bl.

Each connector was visually inspected prior tocooling. On two separate soldering events, onewith foil and one without foil, the connectors wereevaluated as being inadequate in strength and size.The soldering blocks were returned to the oven,the temperature was raised an extra 20°F, andadditional solder was applied to the connectorarea. These two specimens also seated accuratelyon the model with no detectable rocking and weretherefore included in the data analyzed, it wasobserved qualitatively that the additional thermalcycle caused further alteration of the marginal por-

Fig 4a Specimen indexed andsoldered with platinum foiimatrix.

Fig 4b Soidering performedwithout matrix.

Nurrber 5, 1991 461 alionai lournal of Prosthodorti«

SequencinR Plaiinum loil Moinn Removal Hsnnon et al

Figs 5a to 5c Test Cycle 4.

Fig 5a Specimen indexed andsoldered with piatinum foiimatrix.

Fig 5b Soldering performedwithout matrix.

Fig 5c Effects of additionalau tog laze cycle.

The International lournal of Prosthodontics 462 Volume 4, Number 5, 1991

Sequencing Plalinum Foil Malrix Removal

celain in the unsupported specimen (Figs 5a to 5c¡.The margin opened somewhat, perhaps as theresult of extra jostling at the oven door, theincreased temperature, and the time exposed atthat temperature.

An additional test was performed to furtherinvestigate thermal effects on an unsupported por-celain margin. The fixed partial denture from thesame test cycle pictured in Figs 5a to 5c was sec-tioned to allow the premolar and pontic to be sub-jected to an autoglaze tycle the with porcelainunsupported. The retainer and pontic were held at1,760°F in air for 1 minute. The retainers werereseated on the model, held with finger pressurewhile a connettor was formed in autopolymerizingacrylic resin (Duralay, Reliance Dental Mfg Co,Worth, Ill¡, and viewed under the scanning elec-tron microscope. Figure 5c shows the margin withessentially the same degree of opening as in Fig5b, but with more evident configurational changeresulting from the autoglaze firing.

Discussion

For the six test cycles performed in this exper-iment there was no significant difference in fitwhen soldering the collarless veneered retainerwith or without the platinum foil matrices. The0.001-inch platinum foil matrices did not create ameasurable "lifting" effett, given the parametersof this study, and did not interfere with proper seat-ing of the cast retainer. Margin gap measurementsin set 2 captured the premolar retainer in the liftedrelationship, and set 3 data reflected the com-pletely seated casting. There was no significant dif-ference in the measured margin adaptation of thesetwo data sets.

Although the variation in the porcelain marginmeasurements makes it difficult to derive specificconclusions, it is possible to speculate aboutcauses for trends noted in tbe final column of Table2. The means in that column indicate a tendencytoward improved seating on the order of 5 /¿m,comparing the sectioned specimens to the samplessoldered with foil in place. However, this 5-^lmdifference would seem to result more from the var-iables involved in the soldering process thanincomplete seating taused by platinum foil. A netmarginal discrepancy of similar magnitude (5 ßm)resulted when the samples without foil wereindexed and soldered (set 3 compared to set 4),Again, this discrepancy could have resulted fromthe variables of the soldering process or the con-figurational changes in the unsupported porcelainmargins interpreted as widened gaps. The cumu-

lative mean for the samples soldered without foilwas very nearly the same as the mean for the with-foil soldering procedures. Such an observationlends support to the thought that the casting wasnot in actual physical contact with the metal diein any area where foil was adapted, or that thecasting was able to further compress the foil inareas of physical contact. The space between cast-ing and die must approach the thickness of theburnisbed foil, even over the margins. Leaving thefoil in place for indexing procedures produced nosignificant error resulting from lifting of the retainer.It is recommended that the foil be maintained inplace until just prior to cementation of the fixedpartial denture.

Standardization of reading margin openingsunder a microscope was challenging. The preparedmargin on the Ticonium model appeared distinctto the unaided eye but looked somewhat roundedwhen viewed using the microscope. The gap mea-surement vaiues were not the key information pro-vided in this study. The important differences werein the averaged values when data sets 1 through4 were compared. Throughout this experiment,major emphasis was given tc standardization of themeasurement technique. Measurements weremade from the porcelain or metal margins of therestoration to the first visible illuminated line onthe metal of the Ticonium model. The visible linebecame the margin for purposes of standardiza-tion, sinte it was the most clearly readable land-mark and was adequately away from the darkcrevice.

Previous studies have described various meansof assessing marginalfit.'-*"'-^ West et aP examinedembedded, sectioned spetimens and found muchbetter adaptation of a porcelain shoulder in a spe-tific area well within the marginal crevice. Theexperimental regimen of the present study did notlend itself to embedding and sectioning samples.Other in vivo and in vitro studies"-'^ promote rep-lication of samples in epoxy resin for scanning elec-tron microscopic evaluation. Some previouslyreported studies have yielded porcelain margindata that were obtained from measurements on astone die rather than using a metal model. A metalsystem eliminates the possibility of improved seat-ing as the result of abrasion or wear in manipula-tion. Gap values reported in the present study arelarge tompared to others mentioned in the liter-ature,'-'^"'^ but the objective was to collect datain a facilitated, critical, and repeatable method.

Evaluation by scanning electron microscopydemonstrated configuration changes in the por-celain margins as the result of thermal effects of

4. Number 5, 1991 4 6 3 The Internatior I of Prosthodortics

Sequencing Plaiinum Foil Matrix Remov

Fig 6 Residual solder subiecteO to porcelain fifing cycle.

Conclusions

Indexing and soldering collarless veneeredretainers with and without platinum foil matricesresulted in no significant difference in marginal fitbetween the two groups. Removing the platinumfoil matrices prior to indexing did not allow furtherseating of the restoration or closure of fhe con-ventional feldspafhic body porcelain margins.Qualitative evaluation with scanning electronmicroscopy revealed changes in the configurationof these unsupported porcelain margins subjectedto soldering (1,600°F) and glazing (l,760°F) tem-peratures.

Acknowledgments

The authors express special thanks to Dr Virginia Gilrien-gorin, Letterman Army Institute of Research, ior her assistancein statistical evaluation.

oven soldering at temperatures as low as 1,600°F¡Figs 4a and 4b). These changes did nof result inobservable opening of the margin, and so no clin-ical relevance was substantiated.

The presence of residual solder in fhe connectorarea was an unquantifiable variable in this exper-iment. There was no practical means of quantifyingthe amount of solder remaining after each testcycle: therefore, the tesf condifions were estab-lished so that there was a layer of residual solderpresent at each soldering procedure, beginningwifh fhe first cycle. Each time a connector wassectioned and the castings refinjshed for soldering,an aftempt was made to minimize the amount ofsolder left on the pontic surface because the cast-ing was to undergo porcelain thermal cycles (Fig6). This residual solder creafed no significant hin-drance in completion of the experiment. It mayhave had some underlying effects on fhe parentalloy. Establishing a standardized gap width foreach soldering, however, was never a problem.

It should be emphasized that the experimentwas performed with a conventional feldspathicbody and incisai porcelain, which are well adaptedto a platinum foil margin technique. Some marginporcelains do nof compare favorably with regardto translucence and surface finish, but were devel-oped to facilitate fhe direct-Iiit margin techniques.Conclusions derived from this study apply specif-ically to the tested feldspathic porcelain productsand the platinum foil margin technique.

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lourral ol Prosthoduntii 464 Volume 4, Number 5,