Button versus buttonless castings for removable partial denture frameworks

Button versus buttonless castings for removable partial denture frameworks

uttonless arks PhQa casting Moustafa A. Ekss:a College of Dentistry, Ming Saud University, Riyadh, Saudi Arahia Casting removable partial dentures ...

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uttonless arks PhQa

casting Moustafa

A. Ekss:a

College of Dentistry, Ming Saud University, Riyadh, Saudi Arahia Casting removable partial dentures (RPDs) without completely filfing the sgrue channels and generating casting buttons would result in saving metal and rnak~~~ more defect-free castings. This investigation tested whether a complete and sound RPD casting can be obtained when a minimal amount of metal is used. A factorial exp~~~rne~ta~ design, three spruing methods, two metal feeding directions, and two erent weigbts of metal were used to cast 60 Kennedy class IT, mo RPDs. The metal used to cast each framework was either enough to result in a full button or in no button. Visual and radiographic examinations and counting of defects were made by two independent operators who were unaware of the spruing method, feeding direction, or amount of metal used to make the frameworks Tbe compBeteness of the casting and the presence of porosities were evaluated clasps, major connectors, and meshworks. The use of minimal metal to cast was equally as successful as using enough for a full button, provided that the a~~r~pr~~te spruing arrangement and metal feeding direction were chosen. Indirect metal feeding for maxillary RPDs was successful witb the proper spruing arrangement. (J BROSTHET DENT 1994;‘72:433-44.)

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he amount of base-metal alloy used to cast one removable partial denture (KPD) framework is 32 to 40 grams. The weigh.t of the framework is only 12 to 18 grams. Can half the amount of metal be used with equal results? The sprue must supply molten metal continuously to force gases out of the mold cavity and compensates for the shrinkage of the casting as it so1idifies.l However, no studies have demonstrated that the sprue channel must be completely filled with meta: to serve such a function. Young et aL2 obtained complete castings of three-unit bridges whether a full amount or half amount of metal was used. Naylor” discussed buttonless casting of crowns. Another reason for completely filling the sprue channel with casting metal is to assure more metal bulk near the sprue buttoh than near the castmg. Such an arrangement is supposed ?,oassure that the sprue, acting as a reservoir, will remain liquid and capable of feeding the casting as it solid&s and shrinks.4 However, in centrifugal casting, metal freezing depends also on h,ydrostatic pressure.” On completion of t,he casting, a hydrostatic pressure gradient highest at the castiing tip and lowest at the sprue button surface is in action. This gradient also causesa gradient in the heat transfer rate, assuring that the tip of the casting freezes first.

Trofessor and Chairman, Department of Prosthetic Dental Sciences. “Assistant Professor, Department of Prosthetic Dental Sciences. Copyright I@? 1991 by ?‘he Editorial Council of THE JOIXKAL OF ~I?O‘?l'THFTlC i7lW~KiX\-. . ‘ 0022~3913/94/$X00 -i 3. 1.0/1!56261

Since the tip Of the casting will free.ze fksr beea.~se of 3s higher hydrostatic pressure, feeding the ca.~mg as lt freezes seems to require only a quantity of :iqu-Zd metal in the sprue channel a8 small as the amouni. d” ~~~~~~~~~(CZ2% j.. sprumg, 6-12 investing,l*v 11,I:i b~tno~~~ pPOC&Ure),fi. &?3.14 alloy meltifig,6,& 9, 13 casting,7s 1%I5 ix s~)~~dj~~at~.~llof the

casting.” When these variables are ~~~~~~~~~~e~ in accordance with reported studies, a full amount of metd may not be necessary. Even though it has been stated that maxillary RPDs may be sprued indirectly,L’” as in the case of mandibular RPDs, it has been the tradition co spree them directly. Direct spruing requires even more castiag metal, and indirect spsuiag of maxiiiary RPDs cec3~tIig proved to be as efficient as direct spruing.l’ The purpose of this i~v~st~~~~~~)~ W&S “to deterrr,ine

whether half the metal can be used in ::as’Gng a sound maxillary RPD with a variety of spruing ~~~~a~g~rn~~~t~ and two directions of :iq:Jid metal feeding. ET

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To study the inlluence of the three variabies and ihe levels of each variable, a multivarianr fac~.oGJ ~~~~r~~~l~~ta~ design was adopted for this study /Fig. Ii. The fa~t5~ (variabies) studied were: metal feedk~g
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FI

S

ST 11 SC 11 SB 1

Factors

SC

Se

Fx

FI

FI

ST

SC

Se

WC

WC

WC

Fr

FI

FI

ST ST

SC SC

Se Se

Wn Wn

WI-I WI-I

W” W”

Levels

F : Metal Feeding S : Spruing Method W : Weight of Metal

Fig. 1. Multivariant

A dental cast that required a Kennedy class II modification 1 RPD was selected to serve as the master cast. The master cast was duplicated to produce 60 identical refractory casts. Thirty of the refractory casts were prepared for indirect metal feeding (holes were placed in their bases). Wax patterns were identically made with Bego pre-formed wax (Bego, Bremen, Germany). The 60 waxed refractory casts were sprued, either from about (direct feeding) or through (indirect feeding) the cast (Figs. 2 and 3). Ten waxed casts intended for direct feeding and 10 waxed casts intended for indirect feeding were sprued with one spruing arrangement.ls The tree sprue design (ST) (Figs. 2 and 3) consisted of four wax sprues, each 3 mm in diameter. The circular sprue design (SC) (Figs. 2 and 3) consisted of a circular main feeder 3 mm in diameter and six auxiliary sprues, each 2 mm in diameter, fastened to the waxed framework. The circular feeder was attached to the sprue button by four 434

ST

FD : FI : ST : Sc : Se : wc : WH :

Direct Feed Indirect Feed Tree Sprue Circular Sprue Ball Sprue Complete (Full) Weight Half Weight of Metal

factorial design.

sprues, each 3 mm in diameter (Figs. 2 and 3). The ball sprue design (Se) (Figs. 2 and 3) consisted of four feeder sprues 3 mm in diameter with a 5 mm reservoir on each that was 5 mm away from the wax pattern. Twenty waxed refractory casts were sprued by each of the three spruing arrangements (10 for the FD and 10 for the F r metal feeding groups). All wax patterns were invested with the same batch of Wirovest investment material (Bego) and were left to bench set for 2 hours. The invested patterns were placed in the furnace at room temperature. The furnace was then heated to 300” C (572’ F) and rings were rotated. The furnace temperature was raised to 975” C (1787’ F) and the rings were heat-soaked for 45 minutes before casting. A Formax centrifugal induction casting machine (Model 35 EM, Bego) was used and the metal was cast at 1200° C (2192OF) with Wironit cobalt-chromium alloy (Bego). The centrifugal force used was 9 bars.

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Fig. 2. Cast made with full amount of metal and direct spruing; ‘A) tree sprues, :Z?,l ball sprues, (‘C} circular sprues. B, Casting made with half amount of meta? and d&z”; spraing; {A) ball sprues, (B) tree sprues, (C) circular sprues.

Ten samples of each group of 20 frameworks for one of the sprue designs were cast with the full amount of metal (‘25to 35 gm); five were cast with the direct metal feeding and the other five with the indirect approach. The other 10 samples were cast with the minimal amount of metal (14 to 2%gm); live were cast with direct metal feeding and five were cast with the indirect approach. The approximate weight for each spruing and feeding combination was determined experimentally jn a pilot study. After the cast RPD frameworks were photographed and each marked with a random number, all sprues were cut off,

ail frameworks were finished, and each one VW ylaced II? an envelope. Examples of the castings ob:ained are shown in Figs. 2 and 3. To fully assessthe cast RPD frzmewo:~ks, two ciinicio.ns and two evaluation procedures were used. Each clinician examined the 60 frameworks visua!i~ wkih ~5 magnification. Each clinician, working i~de~~~~di?~~~~~ reported his findings to a third chnician. Each e~.am!.nerwas asked to examine the three clasps, the major connector, and the two sections of the meshwork of each casting for porosity or smaZ casting deficiencies and report rhe iindings as scores.

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3. A, Casting made with full amount of metal and indirect spruing; (A) circular sprues, fB) ball sprues, CC’)tree sprues. B, Castings made with half amount of metal and indirect spruing: (A) circular sprues, (B) ball sprues, (C) tree sprues.

Fig.

Each framework component was scored from 0 to 5, based on the number of visible defects. No defect in a given component was scored as 0, one defect as 1, two defects in a given component or one defect in each component of the same type (clasps or meshworks) was scored as 2, and so on. By use of occlusal films, radiographs were made for all 60 RPD castings (90 kV [peak] and 15mA)1g-21(Heliodent 70-Model, Siemens Medical Eng., Inc., Benshein, Germany). The 60 radiographs were then evaluated by the two

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clinicians independently. A light box and ~5 magnification were used to aid vision in evaluating the radiographs. In such a multivariant experiment, statistical analysis capable of delineating the influence of each variable on the experimental outcome for an expanded sample was necessary. Frequency histograms clearly summarized the data. To study the effect of each variable on the occurrence of defects, a multiway analysis of variance (ANOVA) was conducted. The unit of analysis was the individual RPD

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Source of variation

Sum of squaxes

^I

dP

X2

a .-m-I__

Between speekens Weight Feeding Sprning

Weight x feeding Weight X spruing Feeding x spruing -Weight X feeding X spruing Error specimens within cells Within specimens ~Methods of observations(M) Weight x methods Feeding X methods Spruing x methods Weight x feeding x M ilTie&t X spruing X ?ii Feeding X spruing X M Weight x feeding x spruing by M Error specimens within cells by M Components examined (C) Weight x components Feeding X components Spruing X components Weight X feeding x C Weight x spruing x c E’eedixg X spruing X C Weight X feeding X spruing by C Error speeimsns within

cells by C

Methods x components (M x C) Weight x methods x C Feeding x methods x C Spruing X methods X C Weight x: feeding x methods x C Weight x spruing x methods X C Feeding x spruing x methods X C Weight x feeding x spruing X M X C Error specimens within

cells by ?(/I by C

185487.27 Totalmeililsqimrssfor heewaen specimens ____59

59 1 1

485,487.27 6,051.60 I9,301.38 18,940.15 532.90 56J39.86 8,236.23 43,407.oo 332,878.15

2 I 2 2 2 48

2,022,616.73 0.90 2,151,11 344.18 4,479.03 2,689.60 1:417.63 3,224.56 1,790.x? 38,958.82

300 I 1 1 2 1 2 2 48 -

353,864.08 32,7X03 102,575.27 120,267.10 12 1,020.88 71,209.92 25,097.73 546,029.15 496,637.57

2 2 2 4 2 4 4 4 96

6,314.OO 860.68 641.91 1,956.98 2,554.99 3,356.82 3,585.59 5,716.00 73,240.M

2 2 2 42 4 4 4 4 96

= 8228.598. Total mean squares for with

framework. Because the distribntion of the scores (ratings) proved to be a Poisson distribution (not normal), a nonparametric ANOVA was used.2” MI 360 observations were listed in ascending order (all OS first, then ail Is, all 2s, and so on). Each observation was assigned a statistical rank, for example, each of the 252 zero scores were assigned the statistical rank of 126.5 if252 47 I) i 21. The 360 statistical ranks replaced the actual observation scores. The statistical program was then rnn as parametric to determine the sums of squares. The total of the mean squares (TMS) was determined by dividing the total sum of the squares for each variable by the corresponding degree of freedom (DF). The H statistic was

0.7354 2 I”“$56 I 2.3017 0.0648 6.8225 1.0009 5.2761

-

52.48SO 4.8547 1%21‘s% 17.8383 17.9601 IO.5620 3.7226 80.9884 0.3365 0.1277 0.0952 0.2903 0.3788 0.4976 0.5318 0.8478

2022616.73 specimens c~_ = 6742.C6 300

obtained by dividing the sum of square of each factor by the total mean square. For ~0~~~~~~~~~~~ data, the 4 values are approximated as &i-square (X2:, vaI*+res.lii With the respective degrees of freedom, the critical valuea to determine significance of a factor or ~a^acsas-~nB,eractionwere obtained from a &i-square table.

The muitivariant factorial experirnen~;a! design of this study is illustrated in Fig. I, whi& contains 12 blocks herdered by double lines. Each block is one ~~~@~~~~~~~ that combines one level of each of the three fact dependent variables). As an exampie, 9’p&

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11

SC

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WC 25.2 WC 35.0 WC 35.0

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30.1 FI

35.1

ST

30.0 SC 35.0 se

Fx 35.0 35.0

WC 30.1 WC 35.0 WC 35.0 35.0

22.2

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20.2 F.

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20.7

21.6

20.4

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20.4

20.9

4. Weight of metal in grams used for each combination..

casting made by direct feeding with a tree spruing arrangement and a full metal weight. In each of the five small blocks in this particular combination, the mean dependent variable (occurrence of one defect or more) is reported as a categorical value (assigned ratings of 0, 1, 2 . . . ). Because all of the scores by the two observers were identical, all categorical values were single-digit integers. The weight of metal used in each of the five castings for each combination of independent variables is shown in Fig. 4. The top half of Fig. 4 (above the triple line) reports weights of the 30 RPD framework castings that used enough metal (level F of factor W) to fill the mold cavity, the sprue channels, and a part of the crucible former to form the sprue button. One would expect to have 30 defect-free RPD castings in these six combinations because adequate metal was ‘used. The bottom half of Fig. 4 reports the weights of the 30 RPD framework castings that used enough metal to fill only the mold cavity and a small part of the sprue channels (level H of factor W). One would expect to have RPD castings with defects of various ratings because the metal used was significantly less than what is conventionally used to cast an RPD framework. The mean visual clasp ratings (counts of defects) are reported in Fig. 5. One clasp showed one defect even though the metal charge weight was maximal (W,). When the metal weight was minimal (Wn), nine clasp defects were visually detected. The mean visual major connector ratings are reported in Fig. 6. When the metal weight was complete, 24 defects were observed with as many as three defects in one major 438

35.0

20.6

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TALIC

35.0

30.0

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35.0

35.0

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Fig.

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HASSABALLA,

connector. When the metal weight was minimal, only 19 major connector defects were detected. However, these latter defects occurred under experimental conditions (sprue and direction) different from those that occurred when maximum metal weight was used. The mean visual meshwork rankings are reported in Fig. 7. When the metal weight was maximum, 12 casting defects were observed. As many as four defects occurred in the two meshwork portions of one RPD. When the metal weight was minimal, only 10 meshwork defects occurred. The maximum number of meshwork defects in an RPD cast with minimal metal weight was only two. The radiographic clasp ratings, (Fig. 8) were in agreement with the visual ones under all experimental conditions except in the FrSoWn conditions, where visual clasp ratings showed higher defect incidence. The radiographic major connector rankings (Fig. 9) were also in general agreement with the visual ones under all experimental conditions except in the FISTWC conditions, where radiographic meshwork ratings (Fig. 10) were in lesser agreement with their visual ratings than were clasps and major connectors. Radiographic meshwork rankings demonstrated a higher defect incidence rate under the experimental conditions FnSoWo. Visual meshwork ratings demonstrated a higher defect incidence rate under the experimental conditions FnSoWn and FrScWn. Because 60 RPDs were cast and each of them was rated in three components, and each ranking was done by one of two methods, the number of mean observations (average for two examiners) was 360 (60 X 3 X 2). Each mean observation is expressed as a categorical value or rating (mean VOLUME

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Fig.

5. Mean visual clasp ratings.

-I -8 -0t -0 zIzzzz r-0 -11 _ Fig.

defects)

courts OS ranging from 0 to 5. The wbich each rating occurred is expressed quency histogram in Fig. IL The histogram observations had no defect (zero rating), OCTOBER1994

6. Mean visual major connector ratings.

frequency with as a defect freshows that 252 80 observations

had one ciefect, 16 0 servatio?ls bad ‘;wo ciefects, ?.Oobservations had three defects, one observation had four, and another had dive defects. summary oftbe results of nonparame~ rie ANOVA si8-

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I

S ST

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0

ss-T-llTl SC 11 Se 1

Se 0

1

0 F,z, 2 FD 0 0 SC 2 Se 0

0

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0

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0 Fo 0 F,, 1 0 SC 0 Se 1

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0

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7. Mean visual meshwork

ratings.

F FD

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SC 0

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8. Radiographic

tistics is depicted in Table I. The table is reported in four sections because the statistical analysis was divided to show the influence of members of each of the four groups of factors on the experimental outcome (defect occur-

FI

clasp ratings.

rence). These four groups are (1) the between-specimen factors (main experimental variables of metal weight, feeding direction, and spruing arrangement) and their four interactions with each other; (2) one within-specimens facVOLUME

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Fig. 9. Radiographic major connector ratings.

tor (method of dei’ect observation “M”) and its interaction with ali factors in the first group; (3) another within-specimens factor (component examined “C”) and its interactions wit.b ah the factors in the first group; and (4) interaction between the two within-specimen factors (M x C) and the interactions of the latter with all factors in the first group. As Table I shows, the -three between-specimen factors (experimental variables) proved to have no significant effect on the dependent variable (occurrence of defects). Three of the four interactions between these factors also bad no significant elect on the occurrence of a defect. Howeverl the interaction between weight and spruing had a significant~ effect (p < 0.05). In other words, there was variation in the number of defects (dependent variable) observed as a result of the specific combinations of spruing arrangement and weight of the metal used. Not all spruing arrangements used in this study are suitable for either weight of metal. Maximum weight of metal could be used, but casting defects would still occur as a result of the inappropriate spruing arrangement. The method of observation (M) and its interactions with the seven factors of the first group had no significant effect on the occurrence of casting defects. The RPD component (C) examined had a significant influence (p < 0.001) on the occurrence of casting defects. Except for the interactions of C with weight and with feeding and spruing, which had no significant effects, its five other interactions had significant eFects (p < 0.05 to < 0.001) on the occurrence of casting defects. These significant levels mean that the design of an RPD component (such as clasps, major connector, or QCTOBEBEse4

) by itself wili cause the LWLIEWX of cast@ defects. A casting defect will also occurs iis a fk~tion of component interactions With disecrkm cf feeding (p < O.OOP),specific ~ornb~~a~~o~~OS’wei&t, feed.ing and spruing. However, the design of the REJ ~orn~(~~~~~wih not cause a casting defect as a function ;J its ~~~~~~~~tjo~s with metal weight alone or with feeding and spruing combinations used in this study. The interaction between the method of observation and the component exam&red, and this keraction with the seven factors of the first group had co s~~~~~~~~~; e%ect on the occurrence of casting defects. ~~~~o~~~~~~~~~ among the three between-specimens factors (~~~~e~~~~~~~~ variables), none had a sign2ican.t influence on zhe occurrence of a defect (dependent variable). Among the l;;:~ intera,ctions of the between-specimens factory, oniy one
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S

-

-

WC

v

= -

ST

SC

0

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0

0 Fo 2 FD

0

0

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0

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0 .----. .----. _----. I

1

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I

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0

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0 FD

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0

0

-

I 0

-

-

-

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0

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Fig.

0 = -7 0 0 0 0

= -

-

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0

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0 I

0 0 0 0

0 =: --c 0 0 0 0 -

10. Radiographic meshwork ratings.

feet on the experimental outcome (dependent variable). None of the seven sources was an experimental variable and only one of them was the result of interaction between two experimental variables.

DISCUSSION The visual clasp rankings in Fig. 5 show that only one clasp of a possible 90 (30 RPD X 3 clasps) showed a porosity when the metal weight was maximum. These results suggest that when maximum metal weight is used, sound clasp castings could be achieved with any of the three spruing arrangements tested, whether the metal feeding was direct or indirect. It appears that sound clasp castings are not dependent on spruing arrangement or liquid metal feeding direction. Fig. 5 also shows that when minimum metal weight was fed directly, sound clasps were also obtained; however, when minimal metal weight and indirect feed were used, clasps with various degress of porosities were obtained. In the latter situation, the spruing arrangement seemed to have an effect; tree and circular sprue designs were more effective than the ball sprue design. It appears that indirect metal feeding and minimum metal weight caused some porosities in the clasps. These observations suggest that full metal weight is important for casting sound clasps. If a minimal weight of metal is to be used, direct metal feeding is necessary. The visual major connector ratings of Fig. 6 show more defects when the maximum metal weight was used (24 defects) than when a minimum metal weight was used (19

442

FI ST

0

1

1 WH 0 WH

-

-

= -

0 WH

Se I - ST-

defects). The largest number of defects was obtained with the maximum metal weight, indirect metal feeding, and circular or ball spruing. This observation may suggest that, in the presence of a large liquid metal volume, a more sound major connector is likely to be obtained with direct metal feeding. On the other hand, when the minimum metal weight was used, sound major connectors were obtained with both direct and indirect metal feeding. These two observations combined may suggest that the weight of the maximum metal can be detrimental to proper fluid dynamics during the casting d a major connector. When metal was used in a minimal amount, the number of feeding/spruing/direction combinations that yielded a sound major connector was equal to the number of sound major connectors made when the maximum metal was used. This observation may indicate that sound major connectors are likely, irrespective of the amount of metal used. The visual meshwork rankings of Fig. 7 show that when the maximum metal was used, the poorest meshworks were obtained with direct feeding and circular spruing. It appears that meshwork casting requires a more direct metal access through thicker sprues. When the minimum metal was used, the number of feeding/spruing/direction combinations that yielded sound meshworks was greater than the number made when the maximum amount of metal was used. The maximum amount of metal was valuable for sound cast clasps, although not as important for sound major connectors or meshworks.

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Fig. 11. Defect frequency b~s~og~a~~.

Sound R,PD components could be obtained whether the marrimnm or minimum metal was used, provided metal feeding was direct. Radiographs can show more defects in clasps and major connectors than seen by the naked eye (Figs. 8 and 9) because some of these defects are completely enclosed within the RPD components. Wnen the RPD component was rather thin in cross-section, such as in meshworks, radiograpk.ie ratings (Fig. 10) were less capable of detecting defects observed visually (Fig. 7). Because it was difficult to obtain radiographs of the two complete meshworks in the same plane, some of the defects observed visually were not detected radiographicahy. The defect frequency histogram in Fig. 11 indicates that among 360 observations, 252 specimens were free of defects Because most of the observations were ranked 0, it is suggested that most of feeding/spruing/metal weight combinations are likely to produce sound RPD components. The nonparametric multiway analysis of variance in Table 1 shows that the main variables-weight of the metal, metal feeding direction, and spruing arrangement-had no significant effect on the occurrence of a defect. The type of RPD component examined demonstrated a significant effect 0x1the occurrence of defects. This relationship is expected because various RPD components examined had different configurations and thicknesses. It is much easier, for example, for the liquid metal to flow freely in the mold space of an RPD major connector than in the mold space of a clasp or meshwork component. The interaction between feeding direction and the component examined as welt as the spruing arrangement and the component examined showed significant effects @ < 0.001 andp C 0.005, respectively) on the occurrence of

defects. This means that the compoaeni (2 the RFD examined may or may not have casting def~ts, which component it is and what metal feeding Sire&ion was used or what spruing arrangemen-t ‘was usecl. A given RFD ~~rn~one~~ may or may not contain ‘L casting defect, depending on metal feeding dire&on cx apsuiag arrangement. In a similar manner, the ~~~~~~~~~~~~~~ between metal of weight and spruing arrangement cau5es the ~~~~~~~~~~e defects That is, for a given metal W&:X, certain apruing arrangements are not suitable 5x producing defect-free castings.

The following conclusions are m&e 5xn indications of this study. 1. Minimal metal weight cou% be liar&. s~~~~~s~~~~l~ to produce sound RPD frameworks. 2. The direction of metal feeding, the apruing arrangements used in this study, and the weig-ht of the castir?g metal had no infiuence on the souodness of the rea~~~~~~ RFD framework casting when adequate -metal reached the mold cavity and the risers. 9. Notwithstanding the second eonclasion, when mixrima1 meta: was used, it seemed.better CoG333the iiqGid metal directly for sound clasp castings,

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metal volume and sprue design to porosity in nonprecious castings. Quint Dent Tech 1987;11:399-404. Naylor WP. Spruing, investing and casting techniques. Section 4. Nongold base dental casting alloys: volume II. Porcelain-fused-to-metal alloys. Washington, DC: US Government USAFSA-TR-5 19867599. Phillips RW. Skinner’s science of dental materials. gth ed. Philadelphia: WB Saunders, 1991:4X Vaidyanathan TK, Schulman A, Nielsen JB, Shalita S. Correlation between macroscopic porosity location and liquid metal pressure in centrifugal casting technique. J Dent Res 1981;60:59-66. Ryge G, Kozak SF, Fairhurst GW. Porosities in dental gold castings. J Am Dent Assoc 1957;54:746-53. Asgar K, Peyton FA. Pits on inner surfaces of cast gold crowns. J PROSTHET DENT 1959;8:448-53. Kelly GP. Study of porosity and voids in dental gold castings. J Dent Res (Suppl) 1970;49:986. Nielson JP. Suck-back porosity. Quint Dent Tech 1976;1:161-7. Phillips RW. Studies on the density of castings as related to their position in the ring. J Am Dent Assoc 1947;35:329-33. Brumfield RC. How to vent full cast crown molds to avoid gas porosity. Thermatrol Technician, March 1950. Dewald E. The relationship of pattern position to the flow of gold and casting completeness. J PROSTHET DENT 1979;41:531-4. Strickland WD, Sturdevant CM. Porosity in the full cast crown. J Am Dent Assoc 1959;58:69-73. Leinfelder KF, Fairhurst CW, Ryge G. Porosities in dental gold castings II. Effects of mold temperature, sprue size, and dimension of wax pattern. J Am Dent Assoc 1963;67:816-21.

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15. Vincent PF, Stevens L, Basford KE. A comparison of the casting ability of precious and nonprecious alloys for porcelain veneering. J PROSTHET DENT 1977;37:527-36. 16. Stananought D. Laboratory procedures for full and partial dentures. London: Blackwell Scientific Publications Ltd 1978. 17. Hassaballa MA. The feasibility of indirect casting of maxillary removable partial denture. Saudi Dent J 1993;5:114-20. 18. Talic YF. Influence of sprue design on soundness of RPD castings. Saudi Dent J 1994;6:88-93. 19. Elarbi EA, Ismail YH, Azarbal M, Saini TS. Radiographic detection of porosities in removable partial denture castings. J PROSTHET DENT 1985;54:674-7. 20. Wictorin L, Julin P, Mollersten L. Roentgenological detection of casting defects in cobalt-chromium alloy frameworks. J Oral Rehabil 1979;6:137-46. 21. Lewis AJ. Radiographic evaluation of porosities in removable partial denture castings. J PROSTHET DENT 1978;39:278-81. 22. Zar JH. Biostatistical analysis. 2nd ed. New York: Prentice Hall, 1984249-52. Reprint requests to: DR. HAMDI MOHAMMED-AL TAHAWI DEPARTMENT OF REMOVABLE PROSTHODONTIC DENTAL SCIENCES COLLEGE OF DENTISTRY KING SAUD UNIVERSITY PO Box 60169 RIYADH 11545 KINGDOM OF SAUDI ARABIA

Terms, Sixth Edition

The Glossa y of Pvosthodontic Terms is the ultimate resource for the professional. This document, a collection of words/terms and their special connotation in the art and science of prosthodontics, was created to provide a standard lexicon for the profession. The sixth edition of the Glossary (printed in the January 1994 issue of The Joourn~~lof Prosthetic Dentistry) is now available from Mosby in your choice of formats: l

Reprints of the 72-page Glossary are available for $12 each. (For quantities of 25 or more, cost is $10 each.)

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For $32, you may purchase the Glossary on disk. The disk can be used on any IBM or compatible computer, with a minimum of 640K RAM and DOS 3.1 or later.

The Glossary, which includes more than 2,500 entries, was prepared by The Academy of Prosthodontics under the auspices of Dr. Clifford W. VanBlarcom, chairman of the Academy’s Nomenclature Committee. A total of 18 organizations participated in the development of the Glossary. Order your copies

of the Glossary

today! (Orders accepted

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VOLUME

72

NUMBER

4