DEGRADABILITY OF DENTAL CERAMICS K. J. ANUSAVICE
Department of Dental Biomaterials College of Dentistry University of Florida Gainesville, Florida 32610-0446 Adv Dent Res 6:82-89, September, 1992
Abstract—The degradation of dental ceramics generally occurs because of mechanical forces or chemical attack. The possible physiological side-effects of ceramics are their tendency to abrade opposing dental structures, the emission of radiation from radioactive components, the roughening of their surfaces by chemical attack with a corresponding increase in plaque retention, and the release of potentially unsafe concentrations of elements as a result of abrasion and dissolution. The chemical durability of dental ceramics is excellent. With the exception of the excessive exposure to acidulated fluoride, ammonium bifluoride, or hydrofluoric acid, there is little risk of surface degradation of virtually all current dental ceramics. Extensive exposure to acidulated fluoride is a possible problem for individuals with head and/or neck cancer who have received large doses of radiation. Such fluoride treatment is necessary to minimize tooth demineralization when saliva flow rates have been reduced because of radiation exposure to salivary glands. Porcelain surface stains are also lost occasionally when abraded by prophylaxis pastes and/or acidulated fluoride. In each case, the solutes are usually not ingested. Further research that uses standardized testing procedures is needed on the chemical durability of dental ceramics. Accelerated durability tests are desirable to minimize the time required for such measurements. The influence of chemical durability on surface roughness and the subsequent effect of roughness on wear of the ceramic restorations as well as of opposing structures should also be explored on a standardized basis.
This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.
82
D
ental ceramics are generally used to restore damaged, diseased, or fractured teeth because of their excellent esthetics, wear resistance, chemical inertness, low thermal conductivity, and low thermal diffusivity. In addition, they match the characteristics of tooth structure fairly well. Ceramic restorations represent one of few esthetic choices for treatment of small to large areas of posterior teeth. Compared with glass-ionomer cement, dental ceramics are more durable, less technique-sensitive, more predictable from an esthetic viewpoint. However, they are more costly by a factor of from 5 to 10. Compared with office-produced composites (direct) and lab-processed composites (indirect), ceramics are more color-stable, higher in flexural strength, more abrasion-resistant, potentially more abrasive to opposing enamel, and from five to 10 times more costly. Compared with all-metal or metal-ceramic restorations, ceramic restorations are more esthetic. However, they have a shorter life expectancy. The degradation of dental ceramics in the oral environment generally occurs because of mechanical forces, chemical attack, or a combination of these effects. The possible physiological side-effects of ceramic degradation include the increased abrasion of opposing dental structures, the release of radioactive components, increased plaque adhesion, and the release of potentially toxic species as a result of wear and chemical attack.
CHEMICAL DURABILITY OF DENTAL CERAMICS Chemical durability is a principal property requirement of ceramics for intra-oral use, since dental prostheses must resist degradation in the presence of a wide range of solutions of variable pH at temperatures above ambient conditions. Durability is defined as the resistance to the attack of glass by water and aqueous solutions (Newton, 1985). The first stage of glass corrosion was previously represented by the ion exchange between alkali ions in the glass and hydrogen ions in the water (Charles, 1958). However, Ernsberger( 1980) suggests that chemical attack proceeds by the inward diffusion of water molecules which react with nonbridging oxygen atoms to form hydroxyl ions that diffuse out with the alkali ions to maintain electrical neutrality. The relative incidence of biological side-effects of dental ceramics compared with other restorative materials is considered to be low. In general, conventional dental ceramics are considered to be the most inert of all dental materials used for dental restorations. However, in the presence of acidulated fluoride gels or solutions, the surfaces of dental porcelains can exhibit surface deterioration. No data are available to analyze the clinical significance of these corrosion interactions with acidulated fluoride. Feldspathic Porcelain
In 1962, Weinstein et al. (1962) and Weinstein and Weinstein (1962) were awarded US patents for high-expansion porcelains
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TABLE 1 COMPOSITION OF FELDSPATHIC PORCELAINS (wt%) ACCORDING TO WEINSTEIN et al (1962) SiO2
A12O3
CaO
MgO
Na 2 O
K2O
Li 2 O
57.8-73.0
11.1-17.1
0.1-2.6
0.1-1.8
1.9-9.6
6.4-19.3
0-5.0
and thermally compatible alloy formulations. These patents have contributed significantly to the clinical success of metalceramic restorations. Because of these patents, metal-ceramic restorations have become more successful than previously. For single-unit restorations, crowns made from porcelainfused-to-metal (PFM) systems have withstood the test of time during the past 30 years. These restorations are now used for over 60% of crown and bridge restorations. The properties of feldspathic porcelains depend on their composition, microstructure, residual stress state after being processed, and surface finish. Porcelains for metal-ceramic restorations are generally composed of a majority of silica (SiO2) and commonly contain oxides of Na, K, Ca, Al, B, and Zn. Colorant oxides of Fe, Ni, Cu, Ti, Mn, and Co may also be added in addition to Sn, Zr, and Ti, which are included as opacifiers. The compositional range of porcelains given in the patent of Weinstein et al. (1962) is shown in Table 1. Typical compositions of commercial feldspathic porcelains are given in Tables 2 and 3. As the concentration of alkali ions increases, disruption of -Si-O-Si- bonds occurs, with a concomitant increase in thermal expansion coefficient and a reduction in softening temperature and chemical durability. The thermal expansion coefficient is further increased by the formation of leucite crystals (K 2 0Al 2 0 3 -4Si0 2 ). The reduction in chemical durability is of importance, since an increased susceptibility to chemical attack may release ions of the above elements, which, in certain circumstances, could be considered as undesirable from a biocompatibility perspective. In addition, surface roughening could cause a significant weakening of the structure as well as an increased susceptibility to pellicle formation and plaque accumulation. It is generally believed that two dominant mechanisms are responsible for the aqueous corrosion of alkali-silicate glasses:
(1) the selective leaching of alkali ions and (2) dissolution of the glass network. At a pH of 9 or less, selective leaching of alkali ions will be the dominant mechanism (Charles, 1958; Newton, 1985). This mechanism is controlled by the diffusion of H + or H 3 O + ions from an aqueous solution into the glass and the loss of alkali ions from the glass surface. In general, alkali metal ions from glass are much less stable in the glass phase than in the crystalline phase and thus will be leached more rapidly. Further research is needed for the corrosion process in dental ceramics to be characterized. Little information is available on the corrosion of dental ceramics. Since most of these consist of glass-crystalline structures, the above mechanisms are assumed to be valid. Except in rare and usually contra-indicated uses of acidulated fluoride or HF on porcelain surfaces in the oral cavity, the chemical durability of dental porcelains and most other dental ceramics is generally considered to be superior to that of most restorative materials. DeRijk et al (1985) concluded that the dissolution of two dental ceramics and some experimental porcelains after one year occurred by slow dissolution of the glazed silica-rich outer surface rather than by the selective removal of a given phase. The solubility rates at 80°C in 4% glacial acetic acid were 0.92 mg/cm 2 /day for a nepheline syenite-based porcelain (Ney) and 0.57 x 10 2 mg/cm 2 /day for a conventional feldspathic porcelain (Ceramco). No explanation was given for the high solubility of the nepheline syenite-based porcelain. They concluded that immersion in artificial saliva at 22°C for 22 years would be required to produce the same degree of surface dissolution as an exposure to 4% acetic acid at 80°C for one week. The rank order of porcelain solubility for artificial saliva and acetic acid is the same. These values are low. For example, a 12-ounce serving of Coca Cola contains 778 times as much sodium as is released from 32 Ceramco
TABLE 2 MAIN CONSTITUENTS OF OPAQUE PORCELAIN (wt%) ACCORDING TO MEYER (1971) AND OKAMOTO AND HORIBE (1984)
TABLE 3 MAIN CONSTITUENTS OF DENTIN PORCELAINS (wt%) ACCORDING TO MEYER (1971) AND OKAMOTO AND HORIBE (1984)
Component
Biodent Cera 8 Ceramco Ivoclar
Vita
Component
Biodent Cera 8 Ceramco Ivoclar
Vita
SiO,
52.0
37.1
55.0
50.0
52.4
SiO 2
56.9
57.5
62.2
60.9
56.8
A1,O,
13.6
9.1
11.7
17.0
15.2
A12O3
11.8
14.2
13.4
14.4
16.3
Na,O
5.3
10.4
4.8
5.2
6.7
Na 2 O
5.4
14.4
5.4
4.0
8.6
K,0
11.1
5.3
9.6
11.1
9.9
K2O
10.0
8.8
11.3
14.0
10.3
TiO,
3.0
0.1
0.3
2.6
TiO 2
0.6
0.1
—
0.3
0.3
ZrO2
3.2
27.1
0.2
—
5.2
ZrO 2
1.5
0.2
0.3
0.1
1.2
SnO.
6.4
5.4
15.0
18.0
4.9
SnO.
0.5
0.5
—
—
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ADV DENT RES SEPTEMBER 1992
ANUSAVICE
TABLE 4 COMPOSITIONAL RANGE OF DICOR® GLASSCERAMIC (ADAIR, 1984) Weight Percent Component MgO
14-19
SiO2
55-65
ALO3
0-2
ZrO 2
0-7
F
4-9
porcelain crowns per day at 80°C in 4% HAc (0.09 mg). A12ounce can of tomato juice contains 13,333 times as much sodium (1200 mg) and 6666 times as much potassium (600 mg) as is released on a daily basis by the 32 crowns under the same conditions (0.09 mg). The chemical durability of glasses made with potash (K2O) is approximately half that of glasses made with soda (Na^), presumably because of the difference in ionic size (El-Shamy, 1973). A mixed alkali effect occurs when both Na and K are present (Hench and Clark, 1978), such as is often the case with feldspathic porcelains. This means that a glass which contains 3 mol% K2O and 12 mol% Na2O will be half as susceptible to chemical attack (twice as durable) as a glass which contains 15 mol% Na2O. The reduction in durability of glass by alkali ions can be offset by the addition of divalent network modifiers such as Ca, Mg, Sr, Zn, and Ba. Zirconia and alumina are also effective in improving the chemical durability of glass. The trivalent Al ion acts to immobilize alkali ions and therefore reduces ion exchange in the pH range of 4 to 9 (Paul and Zaman, 1978). FeO and MnO can be substituted for CaO in glasses containing Na2O and 70 mol% SiO2 to improve the durability in the pH range of 9 to 12 by immobilizing the alkali ions (Paul and Youssefi, 1978a). However, in the pH range of 0 to 2 the chemical durability decreases when these substitutions are made.
The use of acidulated fluoride is known to attack the surface of porcelain chemically (Gau and Krause, 1973; Copps et al., 1984; Rawson et a/., 1984; Sposetti et a/., 1986; Wunderlich and Yaman, 1986). Such chemical degradation can: (1) increase the abrasiveness of the ceramic against opposing teeth and other restorative materials, (2) change the appearance of the ceramic, (3) increase the retention of plaque, and (4) increase the susceptibility of the ceramic to future chemical attack. A neutral fluoride formulation should eliminate this chemical attack of dental porcelains. The use of HF in the mouth is sometimes advocated for repair of fractured porcelain surfaces. However, because of the serious danger to tissues, this chemical agent should be avoided for in vivo situations. Neither the fluoride nor the abrasive in toothpaste is expected to have a significant effect on either the integrity of surface stain or the unstained surface of dental porcelain. Bativala et al. (1987) used a simulated toothbrushing device to demonstrate that a 20- to 30-pm-thick extrinsic stain layer on porcelain was not affected significantly after 8.5 years (120,000 strokes) of being brushed with a fluoridated toothpaste. After 11.4 years (160,000 strokes) of being brushed, the glazed surface was removed, but no marked loss of stain was evident. Under normal conditions, the chemical durability of porcelain and other ceramic surfaces should be superior to that of dental amalgam. Normal conditions can be defined as: (1) a temperature range of 0°C to 100°C, (2) a wide variety of organic solutions, (3) inorganic solutions ranging in pH from 1 to 13, (4) a diet which is relatively free of highly abrasive substances, and (5) applied vertical forces of 1000 N (225 lbs) or less.
Chemical Attack by Organic Agents Certain organic acids, such as EDTA (sodium salt of ethylene diamine tetra-acetic acid) and citric acid, are relatively corrosive toward glasses because of their chelating effects, which lead to the formation of soluble complexes (Newton, 1985). Any agents which have a chelating effect, including sucrose and
TABLE 5 COMPOSITIONAL RANGE AND ACID DURABILITY OF MACHINABLE GLASS-CERAMICS IN 5% HCl at 95°C (GROSSMAN, 1973) Weight Percent A Component B E F D C MgO
13.8
16.2
12.9
12.0
11.2
11.6
MgF2
10.6
10.6
9.9
9.3
8.6
9.0
SiO2
61.8
64.2
57.5
53.4
50.1
51.8
K2O
13.8
9.0
7.5 11.4
10.6
BaO
12.2
SrO
7.4
Rb2O
13.9
Cs2O Acid Durability (mg/day)
5.1
3.1 /
12.0
13.0
19.5
20.2
12.0
9.2
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ethanol, can degrade glasses (Paul and Youssefi, 1978b). The influence of these agents on dental ceramics is not known.
TABLE 6 COMPOSITIONAL RANGE OF OPTEC HSP (KATZ, 1989)
GLASS CERAMICS
Glass ceramics are polycrystalline solids which are prepared by the controlled crystallization of glasses. The most widely used glass ceramic in the US today is sold under the trade name of DICOR® (Dentsply International, York, PA). A machineable glass ceramic which can be melted and cast into the form of dental inlays, onlays, and crowns was introduced by Adair (1984) and Grossman (1985). This glass-ceramic system is based on the growth of fluorine-containing, tetrasilicic mica crystals, first reported in a patent by Grossman (1973). Compositions enriched in K2O and SiO2 can be melted from 1350°C to 1400°C and then crystallized to yield a tetrasilicic fluormica glass ceramic (KMg2 5Si4O10F2). Addition of ZrO2 in amounts up to 7 wt% is believed to improve chemical durability and enhance translucency. Like other glass ceramics, this product must be stained on the external surface with shading porcelain for acceptable esthetics to be achieved. If too thin a layer is applied, the entire colorant layer may be lost during routine dental prophylaxis, because of the abrasiveness of the prophylaxis paste. The daily use of acidulated fluoride gels may cause the dissolution of the shading porcelain. Boudrias and Buithieu (1988) reported no alteration of the surface of shading porcelain after 64 min of immersion in neutral NaF (0.05% solution and 2.0% gel). Etching of DICOR® shading porcelain surfaces was observed after exposure to acidulated phosphate fluoride for only four min, and a loss of glaze occurred after an exposure time of 16 min. The application of two coats of Copalite varnish before exposure to the acidulated fluoride preserved the integrity of the porcelain surface. So that it can withstand the chemical environment and stresses which develop under intra-oral conditions, DICOR® glass ceramic is prepared within the compositional range shown in Table 4. For optimum durability, the formulation should contain at least 0.5 wt% A12O3 and 2 wt% ZrO2 (Adair, 1984). However, the original formulations identified by Grossman (1973) for DICOR® contained no ZrO2 or A12O3 and seem to exhibit acceptable durability in 5% HC1 at 95°C for 24 h. As shown in Table 5, the acid durability of the six possible formulations of tetrasilicic fluormica glass ceramics ranges from 3.1 to 13.0 mg/cm2. These conditions are quite severe and represent at least two years of exposure time at 37°C. Grossman and Walters (1984) reported that the durability of a tetrasilicic fluormica glass ceramic in 4% acetic acid at 80°C was only 4.2 x 10 3 mg/cm2/day. Corresponding durability values were 16.5 x 10 3mg/cm2/day for Vitadur porcelain, 20.0 x 103 mg/cm2/day for Vitadur aluminous core porcelain, and 9.5 x 103 mg/cm2/day for Ceramco feldspathic porcelain. These conditions are equivalent to a two-year exposure to the acid at a temperature of 37°C. It is unlikely that these low concentrations are of any clinical or toxicologic consequence. The maximum release of ions from 32 full-ceramic crowns exposed to HA at 80°C would amount to approximately 0.1 mg/day. The estimated depth of attack after one year was only 0.3 urn for the glass ceramic and 1.4 pm for Vitadur N opaque
General Composition (weight percent)
Preferred Composition (weight percent)
SiO2 A1
55-70
60-64
16-20
16-19
K2O
12.5-22.5
12.5-14.5
CaO
0.5-5.0
0.5-2.0
MgO
0.5-5.0
0.5-1.5
Li 2 O
1.0-5.0
1.0-3.0
Na 2 O
2.0-5.0
2.0-4.0
CeA
0.0-1.0
0.0-0.15
Component
A
porcelain.
Hydroxyapatite Glass Ceramics Hobo and Iwata (1985a) introduced a castable apatite ceramic in which crystallization of oxyapatite occurs after a heat treatment at 870°C for one h. Upon exposure to water, the crystals convert to hydroxylapatite. The components of this product consist of SiO2, CaO, P2O5, and MgO. The glass ceramic produced has a relatively low flexure strength (150 MPa) and appears to be susceptible to dissolution (Hobo and Iwata, 1985b). Lithium-containing Glass Ceramics
A glass-ceramic product, Olympus Castable Ceramic (OCC), which was recently introduced in Japan, contains SiO2, A12O3, Li2O, MgO, ZnO, TiO2, Na2SiF6, and small concentrations of oxide stains (Uryuef aL, 1989). The crystallization treatment produces mica crystals [NaMg3(Si3AlO10)F2] and beta spodumene crystals (Li2OAl2O3-4SiO2). No information on chemical durability is available on this material. Gelenberg etal. (1989) recommend that serum levels of Li+ not exceed 1.0 mmol/L. Such low doses are used in medicine to prevent or attenuate manic and depressive episodes in patients with bipolar disorder. This dose does not lead to serious nephrotoxicity (Gelenberg etal., 1987), although sideeffects such as tremor, diarrhea, urinary frequency, weight gain, and a metallic taste have been reported (Gelenberg et ai, 1989). Since 75 to 96% of a one-time lithium ingestion is excreted within 72 h, the risk of lithium ingestion from dental ceramics should be negligible. However, research is needed to determine the chemical durability of lithium-based glass ceramics and the release rates of lithium from dental prostheses. In the early stages of corrosion of a Li,O-2SiO2 glass ceramic in a neutral aqueous environment, Li+ is selectively leached from the glass surface by ion exchange with H+ or H3O+ from the solution. For a pH of 9 or higher, the corrosion mechanism consists of dissolution of the glass network by OHattack on the Si-0 bonds (Charles, 1958; McCracken et ai,
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ANUSAVICE
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1982). In general, acid exposure caused the least surface damage, and basic solutions caused the most extensive surface change in glass. A higher-strength feldspathic porcelain (Optec HSP, Jeneric Industries, Wallingford, CT) which has recently been introduced is believed to contain between 1.0 and 5.0 wt% of Li2O (Katz, 1989). Strengthening of conventional porcelains is attributed to an increase in crystalline content by the addition of up to 45 wt% crystalline leucite (K 2 OAl 2 0 3 4Si0 2 ). The composition range of this material is given in Table 6. Calcium oxide, which is converted from calcium carbonate when fired, strengthens the glass phase and, in the presence of a high potassium oxide content, reduces the solubility of the glass phase. Pigments— in the form of chromates, vanadates, and manganates—are also added in amounts ranging from 0.5 to 1.0 wt%. Lithium oxide is added to control the firing range and viscosity to favor the formation and growth of leucite crystals. Such a component would remain in the glass phase, which would be more susceptible to chemical attack. No data are available on the chemical durability of this material. Because of a higher crystal content, one would expect a decreased solubility rate compared with that of conventional feldspathic porcelain.
ABRASIVITY AND WEAR RESISTANCE OF DENTAL CERAMICS Chemical attack of ceramics can: (1) increase the abrasiveness of the ceramic against opposing teeth and other restorative materials, (2) increase the retention of plaque, (3) weaken the structure by creating critical surface flaws, (4) strengthen the structure by reducing the influence of surface flaws, and (5) increase the susceptibility of the ceramic to future chemical attack. One of the main disadvantages of inlays, onlays, crowns, and fixed partial dentures made of ceramic used as an alternative to small or large amalgam restorations is the high abrasive potential of the ceramic surface relative to opposing natural teeth or other dental materials. Mahalick et al. (1971) reported high volumetric wear rates of porcelain against porcelain (loss of 90 volume units/unit time), enamel against enamel (80), and enamel against porcelain (60) specimens compared with lower rates for porcelainacrylic (26), enamel-acrylic (25), acrylic-acrylic (8), porcelaingold (5.8), enamel-gold (3.6), acrylic-gold (2.3), and goldgold (0.88). These results suggest that the wear rate for porcelain occluding against another porcelain surface is similar to that for enamel against enamel. The in vitro results of Mahalick et al. (1971) suggest a 100-fold increase in wear of porcelain-porcelain compared with gold-gold specimens. Ekfeldt and Qilo (1990) reported a volume loss per unit time for porcelain-porcelain in an in vivo study to be only 5 and 10 times greater than that of Type III gold-Type III gold and Type IV gold-Type IV gold, respectively. The influence of abrasive food particles may explain the difference between the two studies. The ingestion of wear particles from porcelain should be considered as a minor biological concern. If ingestion of wear fragments represents any biological risk at all, the highest risk group would be individuals who show evidence of bruxism, a condition which would enhance the two-body wear situation which was
ADV DENT RES SEPTEMBER 1992
simulated in the study of Mahalick etal (1971). The presence of an abrasive substance such as sand in food is also an important factor in the wear of dental ceramics. Harrison (1978) reported that the presence of 600-grit carborundum abrasive caused the highest wear (1.2 mm) of a glazed porcelain pin which was abraded by a glazed porcelain disk compared with acrylic resin against acrylic resin (0.45 mm), acrylic resin against glazed porcelain (0.62 mm), and acrylic resin against an abrasive-blasted porcelain pin (0.62 mm). DeLong et al. (1986) reported a high coefficient of friction between enamel and dental porcelain and concluded that the wear of porcelain appears to be one order of magnitude greater than that of dental amalgam. Monasky and Taylor (1971) showed that, while the wear of tooth enamel opposed by porcelain increased nearly linearly with time, the porcelain surface wears rapidly initially until a "polish" of the porcelain surface is obtained. It is not known if this localized smoothing effect causes a reduction in subsequent wear rates.
FRACTURE RESISTANCE OF CERAMIC INLAYS The bulk fracture of ceramic prostheses may also be considered an effect of degradation. Christensen et al. (1991) reported the following failure levels after two years for the following ceramic and resin inlays: Cerapearl-hydroxyapatite glass ceramic (39%), Cerinate porcelain (26%), Mirage fiberreinforced porcelain (12%), Brilliant directly-processed resin (10%), Estilux CVS indirectly-processed resin (7%), and DICOR® glass ceramic (6%). The two resin systems caused virtually no wear of opposing teeth. Significant wear of opposing teeth was observed in 45% and 32% of the cases for DICOR® and Mirage, respectively. The implication of these results is that fragments of fractured ceramic inlays can be swallowed and exposed to hydrochloric acid in the stomach. Subsequent dissolution and absorption of ionic species under this condition has not been previously studied.
TOXICITY OF LEACHED COMPONENTS The toxicity potential of dental ceramics is believed to be negligible because of their excellent chemical durability and low wear rates of opposing materials during oral function. However, the potential toxicity of the components of ceramics should be considered to be aware of the potential consequences of unexpectedly higher release rates under unusual circumstances. Of greatest concern is the ingestion of ceramic fragments from fractured inlays, onlays, or crowns. The toxicity of the components of dental ceramics, although highly unlikely because of low release rates, is worth a brief review. The principal elements of dental ceramics and potential toxic effects are summarized below. Aluminum: Few cases of aluminum toxicity have been reported in humans except for those with impaired renal function. Absorption is relatively low in healthy individuals. Controversial evidence exists to suggest that aluminum may be involved in the pathogenesis of Alzheimer's disease, an insidious neurodegenerative disorder of unknown etiology (Bertholf^a/., 1988). Arsenic: The dominant source of arsenic in the human body is from seafood and other food substances. It does not normally
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accumulate to toxic levels. Arsenic is included as a component of some glasses to minimize the evolution of gases during melting and to refine the crystal size in glass ceramics. It is not believed that any current dental glass ceramics contain arsenic. Calcium: Although calcium compounds are relatively harmless, certain calcium compounds (such as the oxides) may be hazardous as dusts because of the mechanical effects on the integrity of lung tissue and the potential for induction of lung disease (Birch, 1988a). Cerium: Elements of the lanthanide series are of low toxicity on the Hodge-Sterner classification scale. Cesium: Salts of Cs are virtually non-toxic. Fluorine: The threshold limit value (time-weighted average) in the US is 2 mg/m3. However, the daily fluoride intake from food consumption varies from 0.3 mg for individuals in nonfluoridated areas to 2 mg for those in fluoridated areas. A level of 1 mg/kg in food and drinking water is not believed to produce adverse effects. The maximum amount of fluorine released from 32 DICOR® crowns exposed to 4% HAc at 80°C is estimated to be 0.1 mg/day. Lithium: Salts of Li have been used to treat recurrent affective disorders (manic-depressive psychoses). Some patients have been receiving lithium on a regular basis for over 20 years, and this is an indication of the safe use of Li over an extended period (Birch, 1988b). Toxicity can result from plasma concentrations of 2 mmol/L, and a concentration of 4 mmol/L may be fatal. For psychotherapy, a maintenance plasma level of 0.4-0.8 mmol/L is considered typical. Magnesium: There is virtually no danger of Mg toxicity except from certain compounds. Otherwise, only very high doses in the presence of kidney failure will produce symptoms of toxicity. Potassium: The K ion itself is not toxic, although the anions of some salts are toxic. The risk for toxicity from oxides is nil. Sodium: It is non-toxic except when its associated anion may be toxic, e.g., the cyanide ion. Forms included in ceramics are considered to be harmless. Tin: Metallic and inorganic tin compounds are usually nontoxic. There is only limited absorption of soluble tin salts after oral administration. Titanium: Because of its low absorption rate, titanium dioxide is considered physiologically inert when assimilated via ingestion and inhalation as well as via dermal and subcutaneous pathways. Titanium metal and salts are relatively non-toxic except for titanic acid. Zinc: Zinc toxicity is uncommon. Symptoms of hematologic, hepatic, or renal toxicity have not been observed even in individuals ingesting up to 12 g of elemental zinc over a twoday period. Zirconium: Zirconium and its salts are generally of low systemic toxicity, and their tolerance limits are fairly high. RADIOACTIVE COMPONENTS IN CERAMICS Another effect of the chemical and physical degradation of dental porcelains is the release of radioactive components to other organs of the body. Binns( 1983) reported that fluorescing agents such as uranium oxide (UO2) and cerium oxide (CeO2), when added to glass in equal amounts of 1000 ppm, were
87
capable of producing a reasonably good match for natural tooth fluorescence. When the potential hazard of radioactive uranium was recognized, attempts were made in 1978 and 1979 to investigate the fluorescing potential of rare-earth oxides. A mixture of cerium oxide and terbium oxide can produce a fluorescent effect ranging from bluish-white and neutral-white to greenish yellow. Warmer color tones can be developed by additions of dysprosium oxide and samarium oxide (Binns, 1983). Since radiation effects on biological tissues are dose- and time-dependent, the main effects of naturally occurring radioactive compounds in dental porcelains would be expected in the oral cavity. O'Riordan and Hunt (1974) calculated that the annual dose to epithelial tissue from a uranium-containing porcelain crown was 2.7 rem. This value exceeds the limit established by the International Commission on Radiological Protection (ICRP) for unspecified tissues (1.5 rem/year). However, to minimize the fear of ingestion of radioactive porcelain, O'Riordan and Hunt (1974) estimated that a patient could swallow 60 pulverized crowns every week without exceeding the limit. Likewise, a dental laboratory technician could swallow up to 300 g of porcelain powder per week without exceeding the ICRP limit. The international community has taken steps to minimize the use of naturally occurring radioactive materials. The International Standards Organization and the American Dental Association, in conjunction with the American National Standards Institute, have specified in their standards and guidelines for acceptance that radioactive substances shall not be added intentionally to the porcelain formulations. RECOMMENDATIONS FOR RESEARCH ON DENTAL CERAMICS The chemical durability of dental ceramics appears to be excellent overall. However, few studies have been performed to measure the effects of chemical attack over the entire pH range. Furthermore, no study has investigated the combined effect of chemical attack and abrasive degradation of dental ceramics and the possible cumulative uptake of ceramic components from the gastro-intestinal tract. The apparent high volume loss of ceramic in functional contact with opposing ceramic restorations has received little attention from a biocompatibility viewpoint. Research is needed to investigate the effects of chemical attack by HC1 and the risk from components leached from ceramic particles which have been released by abrasion or by fracture of a restoration. Such studies should analyze the effects of ceramic microstructure and surface preparation on the abrasion and abrasivity of dental ceramics. With the exception of the excessive exposure to acidulated fluoride or hydrofluoric acid, there appears to be little risk of being poisoned by the majority of current dental ceramics. Extensive exposure to acidulated fluoride is a possible problem for individuals who have received large doses of radiation, with associated damage to salivary glands. Such fluoride treatment is necessary for tooth demineralization to be minimized when saliva flow rates have been reduced because of radiation effects. Porcelain surface stains are also lost
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ANUSAVICE
occasionally when abraded by prophylaxis pastes and/or acidulated fluoride. In these cases, the solutes are not usually ingested, but surface roughness may accelerate wear and further chemical breakdown. Research is needed on the development of more durable surface stains. The overall safety and effectiveness of dental ceramics have been well-accepted by the profession because of the freedom from adverse effects of feldspathic porcelains during this century. However, safety cannot be inferred or extrapolated from measurements of one ceramic formulation to other compositions or conditions. A greater emphasis is recommended for future research on safety aspects of glasses, porcelains, glass ceramics, coloring agents, and stains. These materials have received much attention recently from an esthetic viewpoint, but they have not been adequately characterized from a biocompatibility perspective. The research community should focus on the establishment of standardized test conditions for the measurement of chemical durability of dental ceramics. Acknowledgment
Preparation of this paper was supported by NIDR Grants DE09307 and DE06672.
REFERENCES Adair PJ (1984). Glass-ceramic dental products. US patent 4,431,420. Bativala F, Weiner S, Berendsen P, Vincent GR, Ianzano J, Harris WT (1987). The microscopic appearance and effect of toothbrushing on extrinsically stained metal-ceramic restorations. / Prosthet Dent 57:47-52. Bertholf RL, Wills MR, Savory J (1988). Aluminum. In: Seiler HG, Sigel H, Sigel A, editors. Handbook on toxicity of inorganic compounds. New York (NY): Marcel Dekker, 59-64. Binns D (1983). The chemical and physical properties of dental porcelain. In: McLean JW, editor. Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago (IL): Quintessence Publishing Co., Inc., 41-82. Birch NJ (1988a). Calcium. In: Seiler HG, Sigel H? Sigel A, editors. Handbook on toxicity of inorganic compounds. New York (NY): Marcel Dekker, 175-179. Birch NJ (1988b). Lithium. In: Seiler HG, Sigel H, Sigel A, editors. Handbook on toxicity of inorganic compounds, New York (NY): Marcel Dekker, 383-393. Boudrias P, Buithieu H (1988). L'effet, in vitro, de differentes preparations fluorees sur la porcelaine Dicor. J Dent Que 25:535-541. Charles RJ (1958). Static fatigue of glass. 1. / Appl Phys 29:1549-1553. Christensen R, Christensen G, Vogl S, Bangerter V (1991). 2year clinical evaluation of 6 inlay systems (abstract). JDent Res 70:561. Copps DP, Lacy AM, Curtis T, Carman JE (1984). Effects of topical fluorides on five low-fusing dental porcelains. / Prosthet Dent 52:340-343. DeLong R, Douglas WH, Sakaguchi RL, Pintado MR (1986).
ADV DENT RES SEPTEMBER 1992
The wear of dental porcelain in an artificial mouth. Dent Mater 2:214-219. DeRijk WG, Jennings KA, Menis DL (1985). A comparison of chemical durability test solutions for dental porcelains. In: Sauer BW, editor. Biomedical engineering—recent developments. Proceedings, Southern Biomedical Engineering Conference. New York (NY): Pergamon, 152-155. Ekfeldt A, 0ilo G (1990). Wear of prosthodontic materials— an in vivo study. / Oral Rehabil 17:117-129. El-Shamy TM (1973). The chemical durability of K2O-CaOMgO-SiO2 glasses. Physics Chem Glasses 14:1-5. Ernsberger FM (1980). The role of molecular water in the diffusive transport of protons in glasses. Physics Chem Glasses 21:146-149. Gau DJ, Krause EA (1973). Etching effect of topical fluoride on dental porcelains: a preliminary study. JCanDentAssoc 39:410-415. Gelenberg AJ, Kane JM, Keller MB, Lavori P, Rosenbaum JF, Cole K, LaVelle MS W (1989). Comparison of standard and low serum levels of lithium for maintenance treatment of bipolar disorder. NEnglJMed 321:1489-1493. Gelenberg AJ, Wojcik JD, Falk WE, Coggins CH, Brotman AW, Rosenbaum JP et al. (1987). Effects of lithium on the kidney. Acta Psychiatr Scand 75:29-34. Grossman DG (1973). Tetrasilicic mica glass-ceramic method. US patent 3,732,087. May 8. Grossman DG (1985). Cast glass ceramics. In: O'Brien WJ, editor. The dental clinics of North America. Philadelphia (PA): W.B. Saunders Company, 725-739. Grossman DG, Walters HV (1984). The chemical durability of dental ceramics (abstract). J Dent Res 63:234. Harrison A (1978). Wear of combinations of acrylic resin and porcelain on an abrasion testing machine. / Oral Rehabil 4:111-115. Hench LL, Clark DE (1978). Physical chemistry of glass surfaces. JNon-Cryst Solids 28:83-105. Hobo S, Iwata T (1985a). Castable apatite ceramic as a new biocompatible restorative material. I. Theoretical considerations. Quint Int 16:135-141. Hobo S, Iwata T (1985b). A new laminate veneer technique using a castable apatite ceramic material. I. Theoretical considerations. Quint Int 16:451-458. Katz S (1989). High strength feldspathic dental porcelains containing crystalline leucite. US patent 4,798,536. Jan 17. Mahalick JA, Knap FJ, Weiter EJ (1971). Occlusal wear in prosthodontics. J Am Dent Assoc 82:154-159. McCracken WJ, Clark DE, Hench LL (1982). Aqueous durability of lithium disilicate glass-ceramics. Ceram Bull 61:1218-1223. Meyer J-M (1971). Contribution a l'etude de la liaison ceramo-metallique des porcelaines cuites sur alliage en prothpse dentaire (dissertation). Geneva (Switzerland): Univprsjte Geneve. Monasky GE, Taylor DT (1971). Studies on the wear of porcelain, enamel, and gold. J Prosthet Dent 25:299-306. Newton RG (1985). The durability of glass—a review. Glass Technol 26:21-38.
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Okamoto Y, Horibe T (1984). Physical properties and color analysis of dental porcelain. Dent Mater J 3:148-162. O'Riordan MC, Hunt GJ (1974). Radioactive fluorescers in dental porcelains. Harwell (UK): Report Nat. Rad. Prot. Board (NRBB R25). Paul A, Youssefi A (1978a). Alkaline durability of some silicate glasses containing CaO, FeO, and MnO. / Mater Sci 13:97-107. Paul A, Youssefi A (1978b). Influence of complexing agents and nature of buffer solution on the chemical durability of glass. 2. Effect of EDTA, ethyl-alcohol, and sugar in the leach solution. Glass Technol 19:166-170. Paul A, Zaman MS (1978). The relative influences of Al2O3and Fe2O3 on the chemical durability of silicate glasses at different pH values. J Mater Sci 13:1399.
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Rawson RD, Bowers M, Beauregard D, Noorda J, Peressini M (1984). SEM and reflectometric evaluation of the effects of fluoride on porcelain. Dental Hygiene 58:442-445. Sposetti VJ, Shen C, Levin AC (1986). The effect of fluoride application on porcelain restorations. / Prosthet Dent 55:677-682. Uryu Y, Suzuki H, Ijima H, Kurokawa H, Watanabe F, Hata Y, Sakauchi H, Hanamura T, Seki Y (1989). Clinical use of castable ceramics (OCC) crown. Shikagu 77:1485-1495. Weinstein M, Katz S, Weinstein AB (1962). Fused porcelainto-metal teeth. US patent 3,052,982. Sept 11. Weinstein M, Weinstein AB (1962). Porcelain-covered metalreinforced teeth. US patent 3,052,983. Sept 11. Wunderlich RC, Yaman P (1986). The in vitro effect of topical fluoride on dental porcelain. J Prosthet Dent 55:385-388.
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Department of Dental Biomaterials College of Dentistry University of Florida Gainesville, Florida 32610-0446 Adv Dent Res 6:82-89, September, 1992
Abstract—The degradation of dental ceramics generally occurs because of mechanical forces or chemical attack. The possible physiological side-effects of ceramics are their tendency to abrade opposing dental structures, the emission of radiation from radioactive components, the roughening of their surfaces by chemical attack with a corresponding increase in plaque retention, and the release of potentially unsafe concentrations of elements as a result of abrasion and dissolution. The chemical durability of dental ceramics is excellent. With the exception of the excessive exposure to acidulated fluoride, ammonium bifluoride, or hydrofluoric acid, there is little risk of surface degradation of virtually all current dental ceramics. Extensive exposure to acidulated fluoride is a possible problem for individuals with head and/or neck cancer who have received large doses of radiation. Such fluoride treatment is necessary to minimize tooth demineralization when saliva flow rates have been reduced because of radiation exposure to salivary glands. Porcelain surface stains are also lost occasionally when abraded by prophylaxis pastes and/or acidulated fluoride. In each case, the solutes are usually not ingested. Further research that uses standardized testing procedures is needed on the chemical durability of dental ceramics. Accelerated durability tests are desirable to minimize the time required for such measurements. The influence of chemical durability on surface roughness and the subsequent effect of roughness on wear of the ceramic restorations as well as of opposing structures should also be explored on a standardized basis.
This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.
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D
ental ceramics are generally used to restore damaged, diseased, or fractured teeth because of their excellent esthetics, wear resistance, chemical inertness, low thermal conductivity, and low thermal diffusivity. In addition, they match the characteristics of tooth structure fairly well. Ceramic restorations represent one of few esthetic choices for treatment of small to large areas of posterior teeth. Compared with glass-ionomer cement, dental ceramics are more durable, less technique-sensitive, more predictable from an esthetic viewpoint. However, they are more costly by a factor of from 5 to 10. Compared with office-produced composites (direct) and lab-processed composites (indirect), ceramics are more color-stable, higher in flexural strength, more abrasion-resistant, potentially more abrasive to opposing enamel, and from five to 10 times more costly. Compared with all-metal or metal-ceramic restorations, ceramic restorations are more esthetic. However, they have a shorter life expectancy. The degradation of dental ceramics in the oral environment generally occurs because of mechanical forces, chemical attack, or a combination of these effects. The possible physiological side-effects of ceramic degradation include the increased abrasion of opposing dental structures, the release of radioactive components, increased plaque adhesion, and the release of potentially toxic species as a result of wear and chemical attack.
CHEMICAL DURABILITY OF DENTAL CERAMICS Chemical durability is a principal property requirement of ceramics for intra-oral use, since dental prostheses must resist degradation in the presence of a wide range of solutions of variable pH at temperatures above ambient conditions. Durability is defined as the resistance to the attack of glass by water and aqueous solutions (Newton, 1985). The first stage of glass corrosion was previously represented by the ion exchange between alkali ions in the glass and hydrogen ions in the water (Charles, 1958). However, Ernsberger( 1980) suggests that chemical attack proceeds by the inward diffusion of water molecules which react with nonbridging oxygen atoms to form hydroxyl ions that diffuse out with the alkali ions to maintain electrical neutrality. The relative incidence of biological side-effects of dental ceramics compared with other restorative materials is considered to be low. In general, conventional dental ceramics are considered to be the most inert of all dental materials used for dental restorations. However, in the presence of acidulated fluoride gels or solutions, the surfaces of dental porcelains can exhibit surface deterioration. No data are available to analyze the clinical significance of these corrosion interactions with acidulated fluoride. Feldspathic Porcelain
In 1962, Weinstein et al. (1962) and Weinstein and Weinstein (1962) were awarded US patents for high-expansion porcelains
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TABLE 1 COMPOSITION OF FELDSPATHIC PORCELAINS (wt%) ACCORDING TO WEINSTEIN et al (1962) SiO2
A12O3
CaO
MgO
Na 2 O
K2O
Li 2 O
57.8-73.0
11.1-17.1
0.1-2.6
0.1-1.8
1.9-9.6
6.4-19.3
0-5.0
and thermally compatible alloy formulations. These patents have contributed significantly to the clinical success of metalceramic restorations. Because of these patents, metal-ceramic restorations have become more successful than previously. For single-unit restorations, crowns made from porcelainfused-to-metal (PFM) systems have withstood the test of time during the past 30 years. These restorations are now used for over 60% of crown and bridge restorations. The properties of feldspathic porcelains depend on their composition, microstructure, residual stress state after being processed, and surface finish. Porcelains for metal-ceramic restorations are generally composed of a majority of silica (SiO2) and commonly contain oxides of Na, K, Ca, Al, B, and Zn. Colorant oxides of Fe, Ni, Cu, Ti, Mn, and Co may also be added in addition to Sn, Zr, and Ti, which are included as opacifiers. The compositional range of porcelains given in the patent of Weinstein et al. (1962) is shown in Table 1. Typical compositions of commercial feldspathic porcelains are given in Tables 2 and 3. As the concentration of alkali ions increases, disruption of -Si-O-Si- bonds occurs, with a concomitant increase in thermal expansion coefficient and a reduction in softening temperature and chemical durability. The thermal expansion coefficient is further increased by the formation of leucite crystals (K 2 0Al 2 0 3 -4Si0 2 ). The reduction in chemical durability is of importance, since an increased susceptibility to chemical attack may release ions of the above elements, which, in certain circumstances, could be considered as undesirable from a biocompatibility perspective. In addition, surface roughening could cause a significant weakening of the structure as well as an increased susceptibility to pellicle formation and plaque accumulation. It is generally believed that two dominant mechanisms are responsible for the aqueous corrosion of alkali-silicate glasses:
(1) the selective leaching of alkali ions and (2) dissolution of the glass network. At a pH of 9 or less, selective leaching of alkali ions will be the dominant mechanism (Charles, 1958; Newton, 1985). This mechanism is controlled by the diffusion of H + or H 3 O + ions from an aqueous solution into the glass and the loss of alkali ions from the glass surface. In general, alkali metal ions from glass are much less stable in the glass phase than in the crystalline phase and thus will be leached more rapidly. Further research is needed for the corrosion process in dental ceramics to be characterized. Little information is available on the corrosion of dental ceramics. Since most of these consist of glass-crystalline structures, the above mechanisms are assumed to be valid. Except in rare and usually contra-indicated uses of acidulated fluoride or HF on porcelain surfaces in the oral cavity, the chemical durability of dental porcelains and most other dental ceramics is generally considered to be superior to that of most restorative materials. DeRijk et al (1985) concluded that the dissolution of two dental ceramics and some experimental porcelains after one year occurred by slow dissolution of the glazed silica-rich outer surface rather than by the selective removal of a given phase. The solubility rates at 80°C in 4% glacial acetic acid were 0.92 mg/cm 2 /day for a nepheline syenite-based porcelain (Ney) and 0.57 x 10 2 mg/cm 2 /day for a conventional feldspathic porcelain (Ceramco). No explanation was given for the high solubility of the nepheline syenite-based porcelain. They concluded that immersion in artificial saliva at 22°C for 22 years would be required to produce the same degree of surface dissolution as an exposure to 4% acetic acid at 80°C for one week. The rank order of porcelain solubility for artificial saliva and acetic acid is the same. These values are low. For example, a 12-ounce serving of Coca Cola contains 778 times as much sodium as is released from 32 Ceramco
TABLE 2 MAIN CONSTITUENTS OF OPAQUE PORCELAIN (wt%) ACCORDING TO MEYER (1971) AND OKAMOTO AND HORIBE (1984)
TABLE 3 MAIN CONSTITUENTS OF DENTIN PORCELAINS (wt%) ACCORDING TO MEYER (1971) AND OKAMOTO AND HORIBE (1984)
Component
Biodent Cera 8 Ceramco Ivoclar
Vita
Component
Biodent Cera 8 Ceramco Ivoclar
Vita
SiO,
52.0
37.1
55.0
50.0
52.4
SiO 2
56.9
57.5
62.2
60.9
56.8
A1,O,
13.6
9.1
11.7
17.0
15.2
A12O3
11.8
14.2
13.4
14.4
16.3
Na,O
5.3
10.4
4.8
5.2
6.7
Na 2 O
5.4
14.4
5.4
4.0
8.6
K,0
11.1
5.3
9.6
11.1
9.9
K2O
10.0
8.8
11.3
14.0
10.3
TiO,
3.0
0.1
0.3
2.6
TiO 2
0.6
0.1
—
0.3
0.3
ZrO2
3.2
27.1
0.2
—
5.2
ZrO 2
1.5
0.2
0.3
0.1
1.2
SnO.
6.4
5.4
15.0
18.0
4.9
SnO.
0.5
0.5
—
—
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—
—
ADV DENT RES SEPTEMBER 1992
ANUSAVICE
TABLE 4 COMPOSITIONAL RANGE OF DICOR® GLASSCERAMIC (ADAIR, 1984) Weight Percent Component MgO
14-19
SiO2
55-65
ALO3
0-2
ZrO 2
0-7
F
4-9
porcelain crowns per day at 80°C in 4% HAc (0.09 mg). A12ounce can of tomato juice contains 13,333 times as much sodium (1200 mg) and 6666 times as much potassium (600 mg) as is released on a daily basis by the 32 crowns under the same conditions (0.09 mg). The chemical durability of glasses made with potash (K2O) is approximately half that of glasses made with soda (Na^), presumably because of the difference in ionic size (El-Shamy, 1973). A mixed alkali effect occurs when both Na and K are present (Hench and Clark, 1978), such as is often the case with feldspathic porcelains. This means that a glass which contains 3 mol% K2O and 12 mol% Na2O will be half as susceptible to chemical attack (twice as durable) as a glass which contains 15 mol% Na2O. The reduction in durability of glass by alkali ions can be offset by the addition of divalent network modifiers such as Ca, Mg, Sr, Zn, and Ba. Zirconia and alumina are also effective in improving the chemical durability of glass. The trivalent Al ion acts to immobilize alkali ions and therefore reduces ion exchange in the pH range of 4 to 9 (Paul and Zaman, 1978). FeO and MnO can be substituted for CaO in glasses containing Na2O and 70 mol% SiO2 to improve the durability in the pH range of 9 to 12 by immobilizing the alkali ions (Paul and Youssefi, 1978a). However, in the pH range of 0 to 2 the chemical durability decreases when these substitutions are made.
The use of acidulated fluoride is known to attack the surface of porcelain chemically (Gau and Krause, 1973; Copps et al., 1984; Rawson et a/., 1984; Sposetti et a/., 1986; Wunderlich and Yaman, 1986). Such chemical degradation can: (1) increase the abrasiveness of the ceramic against opposing teeth and other restorative materials, (2) change the appearance of the ceramic, (3) increase the retention of plaque, and (4) increase the susceptibility of the ceramic to future chemical attack. A neutral fluoride formulation should eliminate this chemical attack of dental porcelains. The use of HF in the mouth is sometimes advocated for repair of fractured porcelain surfaces. However, because of the serious danger to tissues, this chemical agent should be avoided for in vivo situations. Neither the fluoride nor the abrasive in toothpaste is expected to have a significant effect on either the integrity of surface stain or the unstained surface of dental porcelain. Bativala et al. (1987) used a simulated toothbrushing device to demonstrate that a 20- to 30-pm-thick extrinsic stain layer on porcelain was not affected significantly after 8.5 years (120,000 strokes) of being brushed with a fluoridated toothpaste. After 11.4 years (160,000 strokes) of being brushed, the glazed surface was removed, but no marked loss of stain was evident. Under normal conditions, the chemical durability of porcelain and other ceramic surfaces should be superior to that of dental amalgam. Normal conditions can be defined as: (1) a temperature range of 0°C to 100°C, (2) a wide variety of organic solutions, (3) inorganic solutions ranging in pH from 1 to 13, (4) a diet which is relatively free of highly abrasive substances, and (5) applied vertical forces of 1000 N (225 lbs) or less.
Chemical Attack by Organic Agents Certain organic acids, such as EDTA (sodium salt of ethylene diamine tetra-acetic acid) and citric acid, are relatively corrosive toward glasses because of their chelating effects, which lead to the formation of soluble complexes (Newton, 1985). Any agents which have a chelating effect, including sucrose and
TABLE 5 COMPOSITIONAL RANGE AND ACID DURABILITY OF MACHINABLE GLASS-CERAMICS IN 5% HCl at 95°C (GROSSMAN, 1973) Weight Percent A Component B E F D C MgO
13.8
16.2
12.9
12.0
11.2
11.6
MgF2
10.6
10.6
9.9
9.3
8.6
9.0
SiO2
61.8
64.2
57.5
53.4
50.1
51.8
K2O
13.8
9.0
7.5 11.4
10.6
BaO
12.2
SrO
7.4
Rb2O
13.9
Cs2O Acid Durability (mg/day)
5.1
3.1 /
12.0
13.0
19.5
20.2
12.0
9.2
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ethanol, can degrade glasses (Paul and Youssefi, 1978b). The influence of these agents on dental ceramics is not known.
TABLE 6 COMPOSITIONAL RANGE OF OPTEC HSP (KATZ, 1989)
GLASS CERAMICS
Glass ceramics are polycrystalline solids which are prepared by the controlled crystallization of glasses. The most widely used glass ceramic in the US today is sold under the trade name of DICOR® (Dentsply International, York, PA). A machineable glass ceramic which can be melted and cast into the form of dental inlays, onlays, and crowns was introduced by Adair (1984) and Grossman (1985). This glass-ceramic system is based on the growth of fluorine-containing, tetrasilicic mica crystals, first reported in a patent by Grossman (1973). Compositions enriched in K2O and SiO2 can be melted from 1350°C to 1400°C and then crystallized to yield a tetrasilicic fluormica glass ceramic (KMg2 5Si4O10F2). Addition of ZrO2 in amounts up to 7 wt% is believed to improve chemical durability and enhance translucency. Like other glass ceramics, this product must be stained on the external surface with shading porcelain for acceptable esthetics to be achieved. If too thin a layer is applied, the entire colorant layer may be lost during routine dental prophylaxis, because of the abrasiveness of the prophylaxis paste. The daily use of acidulated fluoride gels may cause the dissolution of the shading porcelain. Boudrias and Buithieu (1988) reported no alteration of the surface of shading porcelain after 64 min of immersion in neutral NaF (0.05% solution and 2.0% gel). Etching of DICOR® shading porcelain surfaces was observed after exposure to acidulated phosphate fluoride for only four min, and a loss of glaze occurred after an exposure time of 16 min. The application of two coats of Copalite varnish before exposure to the acidulated fluoride preserved the integrity of the porcelain surface. So that it can withstand the chemical environment and stresses which develop under intra-oral conditions, DICOR® glass ceramic is prepared within the compositional range shown in Table 4. For optimum durability, the formulation should contain at least 0.5 wt% A12O3 and 2 wt% ZrO2 (Adair, 1984). However, the original formulations identified by Grossman (1973) for DICOR® contained no ZrO2 or A12O3 and seem to exhibit acceptable durability in 5% HC1 at 95°C for 24 h. As shown in Table 5, the acid durability of the six possible formulations of tetrasilicic fluormica glass ceramics ranges from 3.1 to 13.0 mg/cm2. These conditions are quite severe and represent at least two years of exposure time at 37°C. Grossman and Walters (1984) reported that the durability of a tetrasilicic fluormica glass ceramic in 4% acetic acid at 80°C was only 4.2 x 10 3 mg/cm2/day. Corresponding durability values were 16.5 x 10 3mg/cm2/day for Vitadur porcelain, 20.0 x 103 mg/cm2/day for Vitadur aluminous core porcelain, and 9.5 x 103 mg/cm2/day for Ceramco feldspathic porcelain. These conditions are equivalent to a two-year exposure to the acid at a temperature of 37°C. It is unlikely that these low concentrations are of any clinical or toxicologic consequence. The maximum release of ions from 32 full-ceramic crowns exposed to HA at 80°C would amount to approximately 0.1 mg/day. The estimated depth of attack after one year was only 0.3 urn for the glass ceramic and 1.4 pm for Vitadur N opaque
General Composition (weight percent)
Preferred Composition (weight percent)
SiO2 A1
55-70
60-64
16-20
16-19
K2O
12.5-22.5
12.5-14.5
CaO
0.5-5.0
0.5-2.0
MgO
0.5-5.0
0.5-1.5
Li 2 O
1.0-5.0
1.0-3.0
Na 2 O
2.0-5.0
2.0-4.0
CeA
0.0-1.0
0.0-0.15
Component
A
porcelain.
Hydroxyapatite Glass Ceramics Hobo and Iwata (1985a) introduced a castable apatite ceramic in which crystallization of oxyapatite occurs after a heat treatment at 870°C for one h. Upon exposure to water, the crystals convert to hydroxylapatite. The components of this product consist of SiO2, CaO, P2O5, and MgO. The glass ceramic produced has a relatively low flexure strength (150 MPa) and appears to be susceptible to dissolution (Hobo and Iwata, 1985b). Lithium-containing Glass Ceramics
A glass-ceramic product, Olympus Castable Ceramic (OCC), which was recently introduced in Japan, contains SiO2, A12O3, Li2O, MgO, ZnO, TiO2, Na2SiF6, and small concentrations of oxide stains (Uryuef aL, 1989). The crystallization treatment produces mica crystals [NaMg3(Si3AlO10)F2] and beta spodumene crystals (Li2OAl2O3-4SiO2). No information on chemical durability is available on this material. Gelenberg etal. (1989) recommend that serum levels of Li+ not exceed 1.0 mmol/L. Such low doses are used in medicine to prevent or attenuate manic and depressive episodes in patients with bipolar disorder. This dose does not lead to serious nephrotoxicity (Gelenberg etal., 1987), although sideeffects such as tremor, diarrhea, urinary frequency, weight gain, and a metallic taste have been reported (Gelenberg et ai, 1989). Since 75 to 96% of a one-time lithium ingestion is excreted within 72 h, the risk of lithium ingestion from dental ceramics should be negligible. However, research is needed to determine the chemical durability of lithium-based glass ceramics and the release rates of lithium from dental prostheses. In the early stages of corrosion of a Li,O-2SiO2 glass ceramic in a neutral aqueous environment, Li+ is selectively leached from the glass surface by ion exchange with H+ or H3O+ from the solution. For a pH of 9 or higher, the corrosion mechanism consists of dissolution of the glass network by OHattack on the Si-0 bonds (Charles, 1958; McCracken et ai,
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1982). In general, acid exposure caused the least surface damage, and basic solutions caused the most extensive surface change in glass. A higher-strength feldspathic porcelain (Optec HSP, Jeneric Industries, Wallingford, CT) which has recently been introduced is believed to contain between 1.0 and 5.0 wt% of Li2O (Katz, 1989). Strengthening of conventional porcelains is attributed to an increase in crystalline content by the addition of up to 45 wt% crystalline leucite (K 2 OAl 2 0 3 4Si0 2 ). The composition range of this material is given in Table 6. Calcium oxide, which is converted from calcium carbonate when fired, strengthens the glass phase and, in the presence of a high potassium oxide content, reduces the solubility of the glass phase. Pigments— in the form of chromates, vanadates, and manganates—are also added in amounts ranging from 0.5 to 1.0 wt%. Lithium oxide is added to control the firing range and viscosity to favor the formation and growth of leucite crystals. Such a component would remain in the glass phase, which would be more susceptible to chemical attack. No data are available on the chemical durability of this material. Because of a higher crystal content, one would expect a decreased solubility rate compared with that of conventional feldspathic porcelain.
ABRASIVITY AND WEAR RESISTANCE OF DENTAL CERAMICS Chemical attack of ceramics can: (1) increase the abrasiveness of the ceramic against opposing teeth and other restorative materials, (2) increase the retention of plaque, (3) weaken the structure by creating critical surface flaws, (4) strengthen the structure by reducing the influence of surface flaws, and (5) increase the susceptibility of the ceramic to future chemical attack. One of the main disadvantages of inlays, onlays, crowns, and fixed partial dentures made of ceramic used as an alternative to small or large amalgam restorations is the high abrasive potential of the ceramic surface relative to opposing natural teeth or other dental materials. Mahalick et al. (1971) reported high volumetric wear rates of porcelain against porcelain (loss of 90 volume units/unit time), enamel against enamel (80), and enamel against porcelain (60) specimens compared with lower rates for porcelainacrylic (26), enamel-acrylic (25), acrylic-acrylic (8), porcelaingold (5.8), enamel-gold (3.6), acrylic-gold (2.3), and goldgold (0.88). These results suggest that the wear rate for porcelain occluding against another porcelain surface is similar to that for enamel against enamel. The in vitro results of Mahalick et al. (1971) suggest a 100-fold increase in wear of porcelain-porcelain compared with gold-gold specimens. Ekfeldt and Qilo (1990) reported a volume loss per unit time for porcelain-porcelain in an in vivo study to be only 5 and 10 times greater than that of Type III gold-Type III gold and Type IV gold-Type IV gold, respectively. The influence of abrasive food particles may explain the difference between the two studies. The ingestion of wear particles from porcelain should be considered as a minor biological concern. If ingestion of wear fragments represents any biological risk at all, the highest risk group would be individuals who show evidence of bruxism, a condition which would enhance the two-body wear situation which was
ADV DENT RES SEPTEMBER 1992
simulated in the study of Mahalick etal (1971). The presence of an abrasive substance such as sand in food is also an important factor in the wear of dental ceramics. Harrison (1978) reported that the presence of 600-grit carborundum abrasive caused the highest wear (1.2 mm) of a glazed porcelain pin which was abraded by a glazed porcelain disk compared with acrylic resin against acrylic resin (0.45 mm), acrylic resin against glazed porcelain (0.62 mm), and acrylic resin against an abrasive-blasted porcelain pin (0.62 mm). DeLong et al. (1986) reported a high coefficient of friction between enamel and dental porcelain and concluded that the wear of porcelain appears to be one order of magnitude greater than that of dental amalgam. Monasky and Taylor (1971) showed that, while the wear of tooth enamel opposed by porcelain increased nearly linearly with time, the porcelain surface wears rapidly initially until a "polish" of the porcelain surface is obtained. It is not known if this localized smoothing effect causes a reduction in subsequent wear rates.
FRACTURE RESISTANCE OF CERAMIC INLAYS The bulk fracture of ceramic prostheses may also be considered an effect of degradation. Christensen et al. (1991) reported the following failure levels after two years for the following ceramic and resin inlays: Cerapearl-hydroxyapatite glass ceramic (39%), Cerinate porcelain (26%), Mirage fiberreinforced porcelain (12%), Brilliant directly-processed resin (10%), Estilux CVS indirectly-processed resin (7%), and DICOR® glass ceramic (6%). The two resin systems caused virtually no wear of opposing teeth. Significant wear of opposing teeth was observed in 45% and 32% of the cases for DICOR® and Mirage, respectively. The implication of these results is that fragments of fractured ceramic inlays can be swallowed and exposed to hydrochloric acid in the stomach. Subsequent dissolution and absorption of ionic species under this condition has not been previously studied.
TOXICITY OF LEACHED COMPONENTS The toxicity potential of dental ceramics is believed to be negligible because of their excellent chemical durability and low wear rates of opposing materials during oral function. However, the potential toxicity of the components of ceramics should be considered to be aware of the potential consequences of unexpectedly higher release rates under unusual circumstances. Of greatest concern is the ingestion of ceramic fragments from fractured inlays, onlays, or crowns. The toxicity of the components of dental ceramics, although highly unlikely because of low release rates, is worth a brief review. The principal elements of dental ceramics and potential toxic effects are summarized below. Aluminum: Few cases of aluminum toxicity have been reported in humans except for those with impaired renal function. Absorption is relatively low in healthy individuals. Controversial evidence exists to suggest that aluminum may be involved in the pathogenesis of Alzheimer's disease, an insidious neurodegenerative disorder of unknown etiology (Bertholf^a/., 1988). Arsenic: The dominant source of arsenic in the human body is from seafood and other food substances. It does not normally
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accumulate to toxic levels. Arsenic is included as a component of some glasses to minimize the evolution of gases during melting and to refine the crystal size in glass ceramics. It is not believed that any current dental glass ceramics contain arsenic. Calcium: Although calcium compounds are relatively harmless, certain calcium compounds (such as the oxides) may be hazardous as dusts because of the mechanical effects on the integrity of lung tissue and the potential for induction of lung disease (Birch, 1988a). Cerium: Elements of the lanthanide series are of low toxicity on the Hodge-Sterner classification scale. Cesium: Salts of Cs are virtually non-toxic. Fluorine: The threshold limit value (time-weighted average) in the US is 2 mg/m3. However, the daily fluoride intake from food consumption varies from 0.3 mg for individuals in nonfluoridated areas to 2 mg for those in fluoridated areas. A level of 1 mg/kg in food and drinking water is not believed to produce adverse effects. The maximum amount of fluorine released from 32 DICOR® crowns exposed to 4% HAc at 80°C is estimated to be 0.1 mg/day. Lithium: Salts of Li have been used to treat recurrent affective disorders (manic-depressive psychoses). Some patients have been receiving lithium on a regular basis for over 20 years, and this is an indication of the safe use of Li over an extended period (Birch, 1988b). Toxicity can result from plasma concentrations of 2 mmol/L, and a concentration of 4 mmol/L may be fatal. For psychotherapy, a maintenance plasma level of 0.4-0.8 mmol/L is considered typical. Magnesium: There is virtually no danger of Mg toxicity except from certain compounds. Otherwise, only very high doses in the presence of kidney failure will produce symptoms of toxicity. Potassium: The K ion itself is not toxic, although the anions of some salts are toxic. The risk for toxicity from oxides is nil. Sodium: It is non-toxic except when its associated anion may be toxic, e.g., the cyanide ion. Forms included in ceramics are considered to be harmless. Tin: Metallic and inorganic tin compounds are usually nontoxic. There is only limited absorption of soluble tin salts after oral administration. Titanium: Because of its low absorption rate, titanium dioxide is considered physiologically inert when assimilated via ingestion and inhalation as well as via dermal and subcutaneous pathways. Titanium metal and salts are relatively non-toxic except for titanic acid. Zinc: Zinc toxicity is uncommon. Symptoms of hematologic, hepatic, or renal toxicity have not been observed even in individuals ingesting up to 12 g of elemental zinc over a twoday period. Zirconium: Zirconium and its salts are generally of low systemic toxicity, and their tolerance limits are fairly high. RADIOACTIVE COMPONENTS IN CERAMICS Another effect of the chemical and physical degradation of dental porcelains is the release of radioactive components to other organs of the body. Binns( 1983) reported that fluorescing agents such as uranium oxide (UO2) and cerium oxide (CeO2), when added to glass in equal amounts of 1000 ppm, were
87
capable of producing a reasonably good match for natural tooth fluorescence. When the potential hazard of radioactive uranium was recognized, attempts were made in 1978 and 1979 to investigate the fluorescing potential of rare-earth oxides. A mixture of cerium oxide and terbium oxide can produce a fluorescent effect ranging from bluish-white and neutral-white to greenish yellow. Warmer color tones can be developed by additions of dysprosium oxide and samarium oxide (Binns, 1983). Since radiation effects on biological tissues are dose- and time-dependent, the main effects of naturally occurring radioactive compounds in dental porcelains would be expected in the oral cavity. O'Riordan and Hunt (1974) calculated that the annual dose to epithelial tissue from a uranium-containing porcelain crown was 2.7 rem. This value exceeds the limit established by the International Commission on Radiological Protection (ICRP) for unspecified tissues (1.5 rem/year). However, to minimize the fear of ingestion of radioactive porcelain, O'Riordan and Hunt (1974) estimated that a patient could swallow 60 pulverized crowns every week without exceeding the limit. Likewise, a dental laboratory technician could swallow up to 300 g of porcelain powder per week without exceeding the ICRP limit. The international community has taken steps to minimize the use of naturally occurring radioactive materials. The International Standards Organization and the American Dental Association, in conjunction with the American National Standards Institute, have specified in their standards and guidelines for acceptance that radioactive substances shall not be added intentionally to the porcelain formulations. RECOMMENDATIONS FOR RESEARCH ON DENTAL CERAMICS The chemical durability of dental ceramics appears to be excellent overall. However, few studies have been performed to measure the effects of chemical attack over the entire pH range. Furthermore, no study has investigated the combined effect of chemical attack and abrasive degradation of dental ceramics and the possible cumulative uptake of ceramic components from the gastro-intestinal tract. The apparent high volume loss of ceramic in functional contact with opposing ceramic restorations has received little attention from a biocompatibility viewpoint. Research is needed to investigate the effects of chemical attack by HC1 and the risk from components leached from ceramic particles which have been released by abrasion or by fracture of a restoration. Such studies should analyze the effects of ceramic microstructure and surface preparation on the abrasion and abrasivity of dental ceramics. With the exception of the excessive exposure to acidulated fluoride or hydrofluoric acid, there appears to be little risk of being poisoned by the majority of current dental ceramics. Extensive exposure to acidulated fluoride is a possible problem for individuals who have received large doses of radiation, with associated damage to salivary glands. Such fluoride treatment is necessary for tooth demineralization to be minimized when saliva flow rates have been reduced because of radiation effects. Porcelain surface stains are also lost
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occasionally when abraded by prophylaxis pastes and/or acidulated fluoride. In these cases, the solutes are not usually ingested, but surface roughness may accelerate wear and further chemical breakdown. Research is needed on the development of more durable surface stains. The overall safety and effectiveness of dental ceramics have been well-accepted by the profession because of the freedom from adverse effects of feldspathic porcelains during this century. However, safety cannot be inferred or extrapolated from measurements of one ceramic formulation to other compositions or conditions. A greater emphasis is recommended for future research on safety aspects of glasses, porcelains, glass ceramics, coloring agents, and stains. These materials have received much attention recently from an esthetic viewpoint, but they have not been adequately characterized from a biocompatibility perspective. The research community should focus on the establishment of standardized test conditions for the measurement of chemical durability of dental ceramics. Acknowledgment
Preparation of this paper was supported by NIDR Grants DE09307 and DE06672.
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