Nonprecious alloys for use in fixed prosthodontics: A literature review

Nonprecious alloys for use in fixed prosthodontics: A literature review J. Robert Kelly, D.D.S.,* and Thomas C. Rose, D.D.S. Naval Denial Research Institute. Great Lakes, Ill., and Solana Beach, Calif.

N

onprecious alloys are now commonly used for ceramometal dental restorations. This article reviews five topics of interest regarding their use: (1) constituents, (2) physical properties, (3) biocompatibility, (4) porcelain bonding, and (5) corrosion.

CONSTITUENTS Nonprecious, or base metal, alloys contain no gold, silver, platinum, or palladium. The four basic groups of nonprecious dental alloys are listed in Table I along with commercial examples that fall within these major metal groupings. Nickel and cobalt are the primary metals in most commercially available alloys, with chromium being the next most predominant metal. Chromium increases a nickel or cobalt alloy’s resistance to oxidation and also assists in solid solution hardening.] Minor elements, presented in Table II, have more effect on the physical properties of an alloy than does the relative nickel-cobalt-chromium concentration.’ Generally these elements are used to improve casting and handling characteristics, porcelain bonding ability, and corrosion resistance. Silicon is one of the more important additions, imparting good casting properties to a nickel alloy and increasing its ductility.’ Beryllium is also added to nickel alloys to enhance casting of the metal to high tolerances.’ Beryllium may act by helping to keep the metals alloyed during casting and also may provide oxides involved in porcelain bonding.? Manganese, molybdenum, tungsten, and iridium may be present to aid in corrosion resistance.‘,3 Manganese in combination with silicon can double the corrosion resistance of nickel wire at elevated temperatures.4

‘I‘hr opinions expressed herein se those of the authors and cannot be conrlrued as rellecGng the views of the Navy Department or the Na\al Swvicc at Ixge. 7TheUSCof commercially available products dors no~ imply endorsement of these products or preference 10 other similar products on the market. *l’oscgraduate Research Fellow, Dental Materials. ttlearnes, I3. Personal communication, 1980.

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Carbon affects hardness, strength, and ductility of nickel and cobalt alloys. The exact concentration of carbon is the most critical concentration of the minor elements to maintain.’ Excessive carbon, introduced in either manufacturing or casting, produces a brittle alloy.* Most other elements are involved in providing oxides for porcelain bonding and adjusting physical properties such (ashardness, strength, and coefficient of thermal expansion as listed in Table 11

PHYSICAL PROPERTIES Certain physical properties of nonprecious and gold alloys are compared in Table III. The following discussion is presented to elaborate on how differences in these physical properties can affect the handling of nonprecious alloys for fixed prosthodontics.

Elevated fusion range The fusion range of nonprecious alloys is generally 100” to 260” C: above that for gold-containing porcelain alloys. Because they are cast at higher temperatures, the contraction on cooling of nonprecious alloys can be much greater than that for gold alloys.5,” According to Weiss,’ an investment must expand 3.4% for nickel-chromium alloys to compensate for this shrinkage. Combined setting and thermal expansion of phosphate- and silicate-bonded investments is limited to 1.5% to 2.4%.” Expansion approaching 3.4% can be achieved only hygroscopically with currently available investments.5 Consequently, new investments that will have the requisite expansion within their dimensional limits need to be developed. Due to their higher fusion temperatures, nonprecious alloys may have a greater sag resistance than precious metal alloys at the temperatures required for porcelain firing.’ Gold alloys experience some flow, or creep, at temperatures above 980” C.’ Because porcelain matures at approximately 816” to 982” C,’ some chance of distortion is present during the porcelain firing of gold al10ys.‘~ Less marginal distortion during firing cycles, however, may not be provided by higher fusing alloys. 363

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Table I. Basic classification

of nonprecious

II. III. IV.

Nickel, 75% to 80%” Chromium, 11% to 15% Cobalt, 40% to 70% Chromium, 20% to 30% Iron, 59% Chromium, 26% Titanium Copper

ROSE

alloys by major constituents

Type I.

AND

Examples

Pentillium, Verabond, Beta, Bio-bond C + B, Permabond, Litecast B, Phenix Metal, Unibond, Neobond II, Ticonium Biocast, Formula CS, No-Nickel Dentillium CB

Witon-S

Experimental

Data from NIH Conference Proceedings: Alternatives to gold alloys in dentistry, 1977; Phillips, R. W.: Science of Dental Materials. Philadelphia, 1973, W. B. Saunders Co.; and Everhart, J. L.: Engineering Properties of Nickel and Nickel Alloys. New York, 1971, Plenum Press, Inc. *Concentrations may vary widely. The exact formulation of individual alloys is guarded by manufacturers for proprietary reasons.

Buchanan et aL6 report less marginal distortion with a precious alloy than a nonprecious ceramometal. Bertolotti and Moffa” suggest that the yet incompletely understood mechanisms involving sag and marginal opening involve not only alloy composition but also past thermal history and activation energy as well.

Lower thermal conductivity Thermal conductivity in nickel is approximately four times less than in gold.’ Therefore, even at the temperatures required to melt these alloys, nonprecious ingots will not exchange fusion energy with the ease of gold alloys. As a result all nonprecious ingots need to be heated at about the same rate to produce a uniform melt. A large, multiorifice gas torch is preferred for melting high casting temperature alloys.” A small, highly oxidized flame (acetylene) may simply spot burn the alloy before all ingots reach liquidus conditions.* Such a large flame provides heat volume as opposed to heat intensity. Induction casting may also be a preferred method.‘O

Lower specific gravity Because these nonprecious alloys have nearly half the specific gravity of gold alloys,7,‘2 casting machines must generate a higher initial thrust to develop equivalent intramold pressures.‘* This phenomenon may be further aggravated by the insufficient porosity of some investments, thus impeding rapid escape of trapped gases, and by the faster freezing nature of nonprecious alloys. ” Some induction casting machines do not develop sufficient thrust to cast thin margins.” Wight et al.14 found that the use of vents and sprues of adequate size eliminated the majority of casting problems they *Hearnes, E.: Personal communication, 1980.

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encountered. The work of Preston and Berger’O demonstrates the additional importance of higher mold and casting temperatures for complete mold fill.

Increased hardness and strength Many nonprecious restorations are more difficult to finish because of their increased hardness.12 Once polished, however, the finish is reported to outlast that of a gold alloy in the mouth.15 In addition, finer margins seem less likely to be lost during finishing of a nonprecious alloy.15 Nonprecious alloys provide a stiffer structural base for porcelain due to the metal’s increased strength. Similarly, cast connections between units may be less bulky without sacrificing strength. Weiss5 believes that coping thickness beneath porcelain may be thinned to as little as 0.1 to 0.2 mm should esthetic considerations require this. This reduction, however, may be counteracted by coating agents added prior to the opaquing of certain metals.

Greater chemical reactivity Most elements in nonprecious alloys will undergo chemical reactions under casting conditions. Molten nickel alloys are especially sensitive toward carbon, nickel carbides forming above 1,200” C, and beryllium carbides above 500” to 750” C.4 All other elements in nonprecious alloys are suspected of forming carbides and/or nitrides under casting conditions.*,‘* Thus an acetylene flame is again contraindicated, its being both heavily contaminated with carbon combustion products as well as being too intensely hot.13 Oxidation/reduction reactions will also proceed rapidly at elevated temperatures. Silicon becomes highly volatile above 1,300” C and is reduced to silicon monoxide.4 Both aluminum and nickel form oxides

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above 400” CL4 Such oxidation/reduction reactions also may measurably change the alloy’s mechanical properties.‘” A film, or slag, of reaction products always forms around nonprecious ingots during casting.” This slag keeps the ingots apart when molten, further decreasing heat transmission between ingots. The ingots will retain much of their original shape, inside the slag shells, even when molten and ready to cast.’ This slag residue, being much lighter than the alloy, primarily remains in the crucible during casting. A short melting time (10 to 15 seconds) will help to minimize these chemical changes.* Nickel is attacked by sulfuric acid of pickling solutions so castings should be cleaned with air abrasives or “live steam.“5, ‘(’

Table II. Minor

elements in nonprecious

dental alloys ____-Element Beryllium

____-.-..--~__-_____ Concentration l-Z%

Iron

0.2-2.5’;:

Iridium

0.2-l%

Tin

0.2-l%

Manganese

0.5-6%

Volatility Table IV lists the volatility of seven minor alloy elements relative to nickel and gold (the volatility of which is about equal at their respective casting temperatures).” Depending on their vapor pressure, all alloy constituents will vaporize to a certain degree during casting. Prolonged or intense heating can create a new alloy by evaporation of important minor elements.” Nonprecious alloys are much more sensitive to small changes in constituents than are gold alloys.‘, 4,”

Heated body radiation

BIOCOMPATIBILITY Many metals may be biologically active in one or all of three chemically distinct states: (1) the pure metal as an ingot or dust (many metals are vastly more reactive as dust [for example, nickel becomes flammable]), (2) organometallic and metallic salt compounds, and (3) alloys.” However, not all chemical states of a certain metal appear to be equally hazardous.‘* Nonprecious metal use may conceivably expose dental personnel to a metal in all three of these chemical states. Pure metal vapor evolved during casting may undergo chemical reactions and/or condense as dust.3 Organometallic compounds and metal salts form during corrosion, both in the mouth and ‘I tr;irnes. E.: Personal communication. 1980.

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0.5-3.5s

Aluminum

l.l-6% 0.02-l%

Titanium

All metals glow when heated.9 At a given temperature gold and nonprecious alloys will emit at different wavelengths and intensities of light.’ Because some induction casting machines use optical pyrometers to measure a molten metal’s temperature, they may have to be recalibrated for a nongold alloy.’

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Silicon

DENTISTRY

Boron

0.5::

Molybdenum

2-12s

Tungsten

6-7%

Properties Allows casting to high tolerances, imprcoves mailtabrlity and ductiirty, and reduces fusion temperature; flux to kr*:p elements alloy.:d Hardrrs alloy and 0xidc.s for porcelain bond Soluttott hardentng; c>xi&i for bonding Solution hardenrng; oxtdvs for bonding Corrosion resistance tdeoxidtzeri; solutron hardening Atds castability and increases ductility; solution ltardening, oxides for bonding Solution hardening Soluttc~n hardening; oxid:~ tor bondir1g Wtdrn-. melting range; corrosion resistance tdeoGdrzer1: so!u?ron hardttnrng Coefficient of thermal expansion; corn lston resistance; oxrdes tar bonding Coefhc rent of thermal expansion;

corrosion Iridium

0.15%,

Carbon

0.050.4%

reri%;tance Corrosion resistance; modulus ot elasticity Strength, hardness, and ductility

Daub from Phillips, R. W.: Science of Dcnrai ?\~l.ctcr~& Philadelphid, 1973, W. B. Saunders Co.; NIH C!onfrrvnct~ proceedings: .\lternativer to gold alloys in dentistry, 1977, ant: Everhart, .J L.: Enqineering Properties of Nickel and Nickel ~\ilov~ New York, 1971, Plenum Press. Inc.

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Table V. Toxicity

Table III. Comparison of basic physical properties of gold and nonprecious ceramoalloys Property Fusion range Thermal conductivity (gold vs. nickel) Specific gravity Hardness (Brinell) Ultimate tensile strength Elongation Modulus of elasticity

Beryllium

935-1,200” c 3.15 W/cm “C

1,150-1,400” c 0.89 W/cm “C

Nickel

18.3 gm/cc 140-200 kg/mm’

7-9 gm/cc 240-400 kg/mm*

5.098 kg/cm’

6,500-10,000 kg/cm’

Chromium

5% 95,000 mn /m2

Tin

Boron Molybdenum

9-12% 195,000 mn/m*

Iron Indium

Table IV. Vapor pressure of elements at casting temperatures Element Nickel Gold Iron Beryllium Tin Chromium Aluminum Manganese Iridium

Manganese Vapor pressure (mm Hg)

Tungsten

1 1 1 10 40 10 100 100 300

Data from Weast, R. C., editor: Handbook of Chemistry Physics, ed 55. Cleveland, 1974, Chemical Rubber Co.

Iridium Silicon Aluminum Titanium

and

during casting. I7 The alloyed metal may exist as both an ingot (in the mouth) and as dust (in the laboratory). Table V lists known toxicity data for many elements found in nonprecious metals. Of these listed, only nickel and beryllium are positive animal carcinogens.18,19These are only known to be carcinogenic as pure elements and in certain compounds such as nickel carbonyl, nickel oxide, and beryllium oxide.‘* The following discussion focuses primarily on nickel and beryllium toxicity as related to the dental environment. Elemental nickel and many nickel compounds (notably nickel carbonyl) are extremely effective in producing rhabdomyosarcomas. 20*2’Workers in nickel refineries are reported to suffer increases in both lung and nasopharyngeal carcinomas.22~23Drinking water containing 5 ppm of nickel was reported to decrease litter size and increase mortality in rats.24 Most nickel 366

Toxicity

Element

Nonprecious ceramoalloy

Gold ceramoalloy

of elements in nonprecious

alloys data’**

Carcinogen: animal positive Highly toxic as dust OSHA standard: 0.002 mg/m) air Carcinogen: animal positive Occupational exposure: 0.015 mg/m’ air Carcinogen: animal suspected OSHA standard: 1 mg/m3 air All organic compounds toxic Elemental tin low toxicity Tolerance: 0.1 mg/m3 air (elemental) Halogen compounds highly toxic Elements nontoxic Compounds low toxicity Tolerance: 5 mg/m) air (trioxide) Essentially nontoxic Tolerance: 10 mg/m’ air (oxide fumes) Low toxicity Threshold limit: 0.1 mg/m’ air Low toxicity for element and compounds OSHA standard: 5 mg/m) air (all) Low toxicity NTIS occupational exposure: 5 mg/m’ air Probably low toxicity Low toxicity Tolerance: 10 mg/m3 air Essentially nontoxic Essentially nontoxic

Data from Hawley, G. G., editor: The Condensed Chemical Dictionary. New York, 1977, Von Nostrand Reinhold Co.; and NIOSH: Registry of Toxic Effects of Chemical Substances. DHEW (NIOSH) Publ. No. 79-100. Cincinnati, 1978.

compounds may not be shipped on passenger aircraft and are considered inhalation hazards in concentrations of approximately 0.001 to 0.1 mg/m3 of air.3 According to Pedersen et a1.,25the average time interval between exposure to nickel vapors and the appearance of nasal tumors was 31.6 years. In only one case was the appearance in less than 20 years. The absence of epidemiologic data on nickel-related health problems of laboratory technicians should, therefore, be interpreted with caution. The more immediate biocompatibility risk with nickel alloys seems to be allergic contact dermatitis. Nickel produces more contact dermatitis than all other metals combined.26-28Partial denture frameworks containing as little as 1.5% nickel have been reported to cause contact dermatitis.29j30 Reactions to nickel have occurred in the mouth adjacent to the metal,“*3’ at extraoral sites in the vicinity of chromium-plated or stainless steel jewelry,” and at locations unrelated to MARCH 1983

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direct metal exposure.‘?, ” Reactions noted include edema of eyelids; swollen, fissured lips; and chronic eczema of cheeks and palms3” In perspective, allergies to nickel are expected in only 5% to 8% of the population.* Most people live safely with nickel-containing alloys while cooking and wearing eyeglasses or watches. Nickel is part of many common alloys including brass and one type of stainless steel.“’ According to Huget’” the routine use, handling, and wearing of nickel-containing items have never been implicated as a cause of cancer. Beryllium has come under scrutiny in the dental literature due to the hazard of inhaling beryllium and beryllium compounds as dust.” Beryllium is considered to be toxic in the air at 0.002 mg/m3,3 is a carcinogen, and is carcinogenic in certain compounds (for example, BeO.,).” Some beryllium will be vaporized during casting, can undergo reactions and/or condense as dust, and may pose a threat in poorly ventilated casting areas. Moffa et al.” sampled air from the breathing zones of laboratory workers while they ground and polished beryllium-containing alloys. They were unable to find beryllium in any of the samples obtained while proper ventilation and exhaust equipment was operating. Methods for the safe handling of beryllium-containing alloys have been published by the American Dental Association (ADA)“’ and the National Institute for Occupational Safety and Health (NIOSH).“’ Toxicologic research has been performed with nickel-chromium implants in animals and in tissue cultures. Moffa et al.” showed that subcutaneous implants of nickel-chromium alloys were just as well tolerated as gold implants in rabbits. Woody et al.” subjected tissue cultures to nickel-chromium powders obtained by wetmilling and to whole castings. Cultures exposed to whole castings showed no adverse cellular changes. Cultures containing nickel-chromium powders did show zones of lysis and cell alteration. Piliero et al.?” studied implants of various gold alloys and nonprecious metal alloys in hamsters. The hamsters exhibited no weight or behavioral changes. Gross examinations revealed no inflammation, surface corrosion, or histopathologic reactions. PORCELAIN

BONDING

The porcelain-metal bond is thought to result from both chemical and mechanical forces.“’ Numerous researchers believe that retention is related to the formation of metal oxides,““.“’ which can act by facilitating wettin$’ and by dissolving into and interacting

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chemically with the porcelain.‘” ” Mechanical forces are thought to play a subsidiary, yet real, role in porcelain-metal coupling,46 -I-especially those compressive forces developed due to the purposeful mismatching of porcelain and metal coefhciems of thermal expansion.’ Both the chemical and metallurgic complexity of nonprecious alloys vastly complicates any uniform understanding of this bond. Besides known elemental differences among nonprecious alloys, as seen in Tables I and II, FairhursP discovered that microstructural variations also occur. Each of 15 different alloys he tested had a different metallurgic mrarostructure. Microstructural differences are known IO affect properties such as ductility and strength and may also play a role in porcelain bonding and corrosion. Data relating chromium oxide formation to bond strengths further add to the confusion. 1,ubovich and Goodkind“’ and McLean and Scedi” report that chromium oxides might weaken the porcelain bond. I’airburst’” suggests ,just the opposite, and ?p,fofl’a”’ states that restricting speculation to the formation ot chromium oxides “would appear to be an oversimplification of a rather complex phenomenon.” Bonding agents add variables to the puzzle by producing a variety of new adherence i;one reaction products.” Depending on the specific alloy-porcelain system studied, they may either broaden or suppress the width of metal-ceramic interactions.’ i:arter et al.“’ reported that adherence was increased only indirectly by the bonding agent used since additional oxides were formed during its application. Both Anrhony et al.? and Goeller et al.” found that bonding mav actually be weakened in the presence of surface agtnts. In addition: specific ceramic-metal combinations appear to perform significantly better than others in laboratory tests.” “’ No uniformly accepted method for determining bond strengths seems to have evolved. Three basic types of tests are reported. Two methods use the hending of flat, porcelainized metal strips: one bent in a transverse flexure”’ and one bent by twisting the strip through its long axis.;’ The third method, the Shel!-Nielson pullthrough test? involves pushing or pulling a metal rod through a doughnut of porcelain vitrified to the rod.“” 11 combination of four tests has recently been suggested for determining porcelain-alloy compatibility by the ADA Council on Dental hlaterials. Instruments and Equipmen.‘“: (1) thermal expansion, (2) thermal shock, (3) three-point loading or flcxure, and (4) multiple porcelain firings of long-span fixed partial dentures. It is hoped that this combination will prove to be predictive of clinical successesor failjircs of porce367

KELLY

lain-alloy systems. These tests are now required as part of the Council’s Acceptance Program for Alloys for Cast Dental Restorative and Prosthetic Devices.56 Proper metal surface roughness may be more important to bond strengths with nonprecious metals than with gold alloys. 57Carpenter and Goodkind5’ suggest that greater wetability of the nonprecious metal by porcelain may account for this observation. Another explanation is offered by Carter et a1.,53who found that gritblasting samples improved adhesion. They speculate that the resultant increase in surface area produced more oxides per unit area as well as a greater area for mechanical interlocking. Wight et al. 59found that opaquing temperature and degassing time affected bond strengths with the nonprecious alloy they tested. By prolonging degassing times, they decreased the bond strength by almost 20%. They also reported that neither the degassing temperature nor the direction of surface preparation had any effect on bond strength. COBROSION Four basic mechanisms are recognized for corrosion of metals in the oral cavity? (1) uniform attack, (2) crevice attack (where oxygen circulation is poor), (3) pitting attack (establishing self-perpetuating electrolytic cells), and (4) galvanic attack. All four of these mechanisms may act in concert, vastly complicating a detailed understanding of their relative importance. These new alloys are complex both chemically and metallurgically.’ By themselves chromium and cobalt are easily corroded.4 When they are combined properly, these elements can passivate an alloy (allowing a thin oxide film to form), decreasing its rate of corrosion.4 Hodges60reported that an alloy rich in molybdenum was particularly sensitive to chloride and that beryllium may decrease the ability of nickel and cobalt to passivate. Many metals are chemically reactive toward free ions such as chloride and sulfide present in saliva or foods.” Nickel alloys have been found to passivate at low chloride ion concentrations yet are subject to pitting at higher concentrations.6’ This pitting may not be a problem at the chloride concentrations found in saliva.60 Oxygen p 1ay s a role in chloride attack of gold-based alloys in saliva-equivalent solutions but has no such role with chloride attack of base metal alloys.@ These isolated ion studies are important but do not yet extrapolate to clinical situations. Even less is known about the nonprecious alloys’ activity toward proteins, enzymes, organic acids, and other constituents of plaque.60 Galvanic currents between dissimilar metals may 368

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cause tissue damage and corrosion.63 According to Marek64 nonprecious alloys used for fixed prostheses, however, may be less galvanically active than gold. Marek found that anodic destruction of low-copper amalgams in vitro was less severe with nonprecious alloys than either gold alloys or high-copper amalgams. Laboratory tests are not yet sophisticated enough to predict the clinical behavior of these alloys. The few controlled clinical studies available speak encouragingly of tarnish resistance within their limited observation times of 4 years or less.65-67The American Dental Association’s criteria for acceptance now require this type of study. CLINICAL

DISCUSSION

Adopting a new and complex technology is probably never accomplished in a rational, lock-step manner. Precipitous increases in gold prices hastened the transition to nonprecious metals and created more confusion about these alloys’ suitability than may have been warranted. Many nonprecious alloys are being specifically designed for ceramometal restorations. In certain ways these new alloys are superior to traditional gold metals adapted for porcelain use. However, we have also seen launched a large, uncomrolled experiment the results of which may not be completely understood for years. The following discussion briefly reviews the major positive and negative aspects of nonprecious alloy clinical use. Nonprecious alloys can be cast with acceptable accuracy.” In addition, increased strength and fusion properties of nonprecious alloys may make them the preferred metal for more rigid, more thermally stable porcelain substructures, especially for multiple-unit restorations.5 Porcelain frameworks cast in nonprecious metals can also be less bulky interproximally than gold alloys.5 Patients may be more comfortable with these alloys due to their decreased thermal transmission.’ Their color provides a neutral background for esthetic porcelain application.* Some of these new alloys may be less active galvanically than gold.64 The few controlled clinical studies available speak well of nonprecious alloys’ corrosion resistance.65V67Once polished, the finish on nonprecious restorations can outlast that on gold units. I5 Porcelain bonding ability of nonprecious alloys probably equals that of gold alloys on properly designed restorations. 36 Dentists have inserted thousands of units of nonprecious restorations with consistently good success.5,68 *Hearnes, E.: Personal communication, 1980

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Obviously the total nonprecious alloy picture involves some unknowns and qualifications. These alloys are complex chemically and metallurgically.‘~4 Their laboratory procedures require more precision than gold alloys, especially during casting.‘O In fact, a dentist’s choice of laboratory may be more important than the choice of alloy. Currently available investments are just able to provide proper expansion.5 In addition, the handling characteristics of one alloy may be very different from those of another.’ Elemental beryllium and nickel dust and certain of their compounds are serious health hazards in air concentrations as low as 0.002 mg/m3.3~‘8 More research needs to be performed to determine the concentrations of nickel and beryllium compounds in casting and finishing areas of dental laboratories. Nickel causes more allergic contact dermatitis than any other meta1.‘5-” Patients should be questioned about nickel sensitivity. Further, such sensitivity should be included in any differential diagnosis following soft tissue changes after crown placement.

6.

8. 9. IO. I I.

12.

13.

14.

1.5.

SUMMARY The physical properties of nonprecious alloys can differ significantly from those of alloys containing a high percent of gold. Relationships among constituents, physical properties, and handling characteristics of base metal alloys were surveyed. Toxicity of nickel, beryllium, and their compounds was discussed with attention given to the dental environment. Allergic contact dermatitis appears to be a health risk to certain patients from nickel-containing prostheses. Beryllium dust is apparently not a hazard in properly ventilated and exhausted grinding and polishing areas. Lack of data on nickel-related health problems in dental laboratory workers should be interpreted with caution. This article also reviewed research on porcelain bonding and corrosion of nonprecious alloys. Although this research cannot yet predict an alloy’s porcelain bonding behavior in mouths, little or no porcelain bond problems have been reported. A few controlled clinical studies report little corrosion in up to 4 years. REFERENCES

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2.5.

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29.

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Reprint requests lo: DR. J. ROBERT KELLY NAVAL DENTAL RESEARCH INSTITUTE BLDG. 1-H. GREAT LAKES, IL 60088

MARCH 1983

VOLUME 49

NUMBER 3