FIXED PROSTHODONTICS OPERATIVE DENTISTRY HENRY

E. EBEL, SAMUEL

The effects gold

E. GUYER,

of etching

WILLIAM

LEFKOWITZ,

on the margins

Section editors

of cast

restorations

John F. Farne, D.D.S.,* and Everett T. Nealey, Dental Activity MEDDAC, Fort Dix, N. J.

D.&D.**

A

n extremely accurate cast gold restoration can be produced if meticulous attention is paid to all the details of waxing, sprueing, investing, and casting. However, this very precision will often present difficulties in seating and cementing the restoration (Fig. 1, A). To compensate for these difficulties, it has become common practice to make adjustments in the various laboratory procedures that cause the final casting to expand. This expansion creates space between the inside of the casting and the prepared surface of the tooth to allow for ease of seating and space for the cementing medium (Fig. 1, B) . When casting inlay restorations, the reverse process is often used-the casting is contracted to create the desired space. In both instances, the most critical part of any dental restoration is ignored-the margins. In either process, the margins are expanded or contracted along with the remainder of the restoration and therefore fit with the precision and finesse, for an ideal restoration is impossible. Therefore, it becomes obvious that dental castings should always be made with all the precision and accuracy possible. To create the necessary space for seating and cementing, a small amount of metal must be removed from the tooth-contacting surfaces of the casting, with the exception of the margins and possibly an occlusal stop (Fig. 1, C) . Currently there are four possible methods used to remove this excess metal. These are (1) grinding with stones and/or burs, (2) sandblasting, (3 ) etching with aqua regia,, and (4) electrochemical milling (stripping) . This investigation was limited to the !atter two techniques as, in the experience of the authors, they produce the most consistent results. It has been stated in the literature that both etching with aqua regia and electrochemical milling have little or no undesirable effect on the margins within the time frames required for the process.1’ ’ A search of the literature failed to show definitive evidence to support this claim. The *Major, DC, USA; Senior Resident. **Colonel, DC, USA; Professional Services. 273

274

Fame

J. Prosthet. Dent. March, 1976

and Nealey

A

B

C

Fig. 1. Diagram of (A) an accurate casting, (B) an expanded casting, and (C) an accurate casting after etching.

present study was undertaken to determine the margins of cast restorations. REVIEW

the effects, if any, of these processes on

OF THE LITERATURE

Etching before cementation was advocated by Hollenback3 prior to 1920. He believed internal surface relief was a fundamental aid in the seating and sealing of full crown restorations. Relief space made on the internal surface of cast gold restorations reduces both the resistance of the cement (hydraulic pressure created during cementation) and the cement thickness at the margins, thereby permitting more accurate seating and sealing. Fusayama, Ide, and Hosoda* found that cast gold restorations cemented without internal relief had a cement thickness of 90 p on the occlusal floor, whereas those with relief had a cement thickness of 46 I*, approximately one half that of the former. They also showed cement exposure to be greatest on the shoulder-type preparation, much less on the chamfer type, and least on the shoulderless preparation. Fusayama5 previously recommended the chamfer type of preparation crowns to produce cervical margins at 45 degrees in wax patterns. There appears to be considerable controversy in the literature over electrochemical milling. Weiss1 and Bassett and Stauts2 advocated stripping without protection of margins since this action took place on the bulkier areas of the casting. Pate&’ Lorencki,7 and Merrill and Welk8 advocated stripping with marginal protection since they found margins to be affected. McEweng found that, even with what appeared to be adequate care, the margins were often involved. Since the loss of gold at the margins is critical, there is a tendency to avoid the use of both electrochemical removal of gold and etching with aqua regia. The specific purpose of this investigation was to find the effects, if any, of etching with aqua regia and electrochemical milling on the margins of cast gold restorations. MATERIALS

AND

METHODS

Test samples. Two types of dental casting gold were selected for this study: “C” bridge gold* (a dental casting gold alloy) and Ceramcot (a gold-based porcelain substrate alloy). Both of these are Type III hard fine-grain gold alloys with casting

l J. Aderer, Inc., New York, N. Y. +Ceramco, Inc., Long Island City, N. Y.

Effects

of etching

on gold restoration

margins

275

Fig. 2. Cast test sample.

temperatures of 1,065O C. and 1,260° C., respectively. These golds were cast into test samples approximately 10 by 6 by 1.5 mm. with a 45 degree bevel along the 6 mm. edge (Fig. 2). They were cast by routine techniques used for fabrication of restorations for patients. This was accomplished to eliminate the possibility of different grain structures affecting the results. After casting, a portion of the larger surface, opposite the bevel, was highly polished with gold rouge to simulate the highly polished exterior surface of a restoration. The beveled surface was refined to obtain an exact angle of 45 degrees with as smooth and sharp a marginal edge as possible. The remaining surfaces were left as cast to simulate the inside of a casting. Reference lines were scribed on the polished surface as follows: one parallel to and 5 mm. from the margin and three lines vertical to the margin, with one centered and one on each side of the center. During the tests, one half of each test sample was protected from the effects of the etching. For the aqua regia etching, protection was provided by inlay wax, and for the electrochemical milling, protection was provided by fingernail polish. The protective medium was placed on one half of the polished surface to the scribed center line, carried over the margin, across the beveled surface, and up the cast surface on the other side. All test samples were coded as to the type of gold and the test to be accomplished. Aqua regia etching. For the chemical etching portion of the study, aqua regia was freshly mixed using one part USP nitric acid and three parts IJSP hydrochloric acid.‘O The acid was maintained at room temperature (2 1O C.) throughout the tests. An alligator clip, attached to the end of a vertically adjustable rod, was used to hold the test sample, beveled edge down. A glass vial containing 10 C.C. of a fresh solution of aqua regia was placed on the platform below the rod and the test sample lowered into the aqua regia to a depth of 1 mm. above the horizontal reference line. Timing was accomplished with a stopwatch. Immediately upon removal from the aqua regia, the excess acid on the test sample was neutralized in a sodium bicarbonate solution and then thoroughly washed in running water. The bulk of the protective layer of inlay wax was removed with a blunt waxing instrument, and residual wax was removed with chloroform. Electrochemical milling. The electrochemical milling was accomplished using a

276

Fame

Table

J. Prosthet. Dent. March, 1976

and Nealey

I. Length

of gold removed

by aqua regia etching

“C” bridge alloy Time 5 sec. 10 sec. 30sec. 1 min. 2 min.

4 min.

Table

Protected 0 0 0 0 0 0 0

10 min.

II. Length

Ceramco

Unprotected (I.4 0 12 22 32 78 182

of gold removed by electrochemical

Protected 0 0 0 0 0 0 0

0 0 1 I 12 15 40

milling

“C” bridge alloy

Ceramco

Unprotected Time 10 sec. 30sec.

1 min. 2 min. 5 min. 20 min. 25 min.

Unprotected (Pi

Unprotected

Protected

ft.4

Protected

IF)

0 0 0 0 0 0 0

6 13 20 26 41 89 143

0 0 0 0 0 0 0

0 5 8 13 20 44 68

Lektro-Dip stripping unit. * This equipment consists of a power source providing 7.5 v. of direct current and a bottle warmer to provide a heat source for the stainless steel cup containing the stripping solution. The stripping solution was mixed according to the manufacturer’s instructions from Howmedica No. 6440-8 stripping sa1ts.t A fresh 75 C.C. portion of the solution was used for each test. An alligator clip, attached to the end of a vertically adjustable rod, held the test sample, beveled edge down. The cathode of the Lektro-Dip was attached to the stainless steel cup containing the stripping solution, and the anode was attached to the clip holding the test sample. A glass laboratory thermometer was suspended in the stripping solution so that it would not contact the cup. The bottle warmer stabilized the temperature of the stripping solution at 93O C., so this temperature was selected as the starting point for each test. The adjustable rod, with the test sample attached, was lowered until the test sample was immersed in the stripping solution to a depth of 1 mm. above the horizontal reference line. The current was turned on and timed with a stopwatch. The test sample was washed in running water immediately after removal from the solution. The protective coating of fingernail polish was removed by use of acetone in an ultrasonic cleaner. *T-Flex Inc., New York, N. Y. tEngelhardt Products, Los Angeles, Calif.

Vohne 35 Number 3

Etects

of etching

on gold restoration

margins

277

Measurements. Measurements were made with a binocular microscope equipped with an ocular micrometer and calibrated with a stage micrometer to 1 p.* The test samples were placed on the stage and aligned using the reference marks previously described, and the protected edge was placed on zero. The difference between the protected edge on zero and the etched edge was read using the ocular micrometer markings. RESULTS Table sample at Table sample at

I indicates the length of gold the various times listed, using II indicates the length of gold the various times listed using

removed from the beveled edge of each test the aqua regia technique. removed from the beveled edge of each test the electrochemical milling technique.

DISCUSSION The primary purpose of both electrochemical milling and aqua regia etching is to provide relief space on the internal surfaces of cast gold restorations. This relief reduces both the hydraulic pressure during cementation and the cement thickness at the margins, thereby permitting more accurate seating and sealing of margins. With protection of the margins, the present experiment has shown that relief may be obtained using either technique. The study clearly indicated that aqua regia etching is more time-consuming than electrochemical milling no matter which of the alloys was tested. The unprotected margins were affected when the test samples were etched and also when they were milled. However, there was no visible effect on the margins protected from electrochemical milling by fingernail polish or protected from aqua regia by Type 2 inlay wax. The Ceramco alloy was more resistant to the action of aqua regia and electrochemical milling than the Type III “C” alloy used in this study. It was found that it took 10 seconds to remove 12 ,U of “C” bridge gold and 120 seconds to remove 12 p of Ceramco through stripping. It took 30 seconds to remove 13 p of “C” bridge gold and 120 seconds to remove 13 ,U of Ceramco gold through etching. CONCLUSIONS 1. Any etching, either chemical or electrical, sufficient to remove an adequate bulk of metal from the tissue surface of a casting will have a measurable and, hence, adverse effect on the margins of the restoration. 2. During any etching process, all margins and contact and occluding surfaces should be protected. 3. For aqua regia etching, Type 2 inlay wax appeared to provide sufficient protection. For electrochemical milling, fingernail polish appeared to provide sufficient protection. 4. Ceramco gold exhibited more resistance to the action of both electrochemical milling and aqua regia etching than did the “C” bridge alloy. 5. Aqua regia etching required more time than electrochemical milling on the two types of gold tested. “Bausch

and Lomb, Rochester,

N. Y.

270

Fame

J. Pro&et. Dent. March, 1976

and Nealey

References 1. Weiss, M. B.: Stripping Gold Castings, Microfilm, Int. Assoc. Dent. Res. Abst. No. M-24, March, 1964. 2. Bassett, R. W., and Stauts, B. M.: Evaluation of Electra-chemical Milling (Stripping) Versus Etching With Aqua Regia, J. South. Calif. Dent. Assoc. 34: 478-483, 1966. 3. Hollenback, G. M.: A Practical Contribution to the Standardization of Casting Technique, J. Am. Dent. Assoc. 10: 5-11, 1928. 4. Fusayama, T., Ide, K., and Hosoda, H.: Relief of Resistance of Cement of Full Cast Crowns, J. PROSTHET. DENT. 14: 95-106, 1964. 5. Fusayama, T.: Chamfered Preparation of Veneer Three-Quarter Crown, Shikaitenbo (Nippon Dent. Rev.) 5: 17-24, 1958. 6. Patel, F. G.: The Effect of Electrochemical Milling on Margins of MOD Inlays, J. PROSTHET. DENT. 30: 66-73, 1973. 7. Lorencki, S. F.: A Rationale for Electra-deplating a Cast Restoration, Dent. Dig. 74: 249257, 1968. 8. Merrill, 0. M., and Welk, D. A.: Electrochemical Etching of Gold Castings, Microfilm, Int. Assoc. Dent. Res. Abst. No. 195, March, 1966. 9. McEwen, R. A.: Efficient Restorative Procedures, Dent. Clin. North Am., July, 1965, pp. 343-354. 10. Hollenback, G. M. P.: Science and Technic of the Cast Restoration, St. Louis, 1964, The C. V. Mosby Company, p. 73. MAJOR FARNE WHITSIDE DENTAL CLINIC DENTAL ACTIVITIES FORT RILEY, KAN. 66442

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The effects of etching on the margins of cast gold restorations.

FIXED PROSTHODONTICS OPERATIVE DENTISTRY HENRY E. EBEL, SAMUEL The effects gold E. GUYER, of etching WILLIAM LEFKOWITZ, on the margins Section...
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