Radiopaque acrylic resins containing miscible heavy-metal compounds H.R. Rawls1. J . Starr 1 F.H. Kasten1 M. Murray 2 J. Smid~ I. Cabasso3
*To whom correspondence should be addressed-Present address: University of Texas Health Science Center 7703 Floyd Curl Dr. San Antonio, FX 78284-7890 1Louisiana State University Medical Center New Orleans, LA 70119-2799 2Louisiana State University Medical Center New Orleans, LA 70112-1393 3State University of New York Polymer Institute Syracuse, NY 13210 Received October 5, 1989 Accepted February 5, 1990 This investigation was supported in part by USPHS Research Grants DE06170 and DE06179 from the National Institute of Dental Research.
Dent Mater 6:250-255, October, 1990
Abstract-Radiopacity is needed in order to facilitate diagnosis of polymeric appliances, which may be dislodged and become impacted in the upper respiratory or digestive tracts. In order for a stable, optically transparent, radiopaque material to be provided, heavy-metal compounds were investigated which we had previously shown to form homogeneous structures with methyl methacrylatebased systems. It was found that, when present in PMMA at 11 to 14%, several compounds of either bismuth or uranium or 35% of an organo-zirconium compound impart radiopacity equivalent to that of aluminum. A low level of cytotoxicity and lack of mutagenicity indicated that a high level of biocompatibility can be expected. Processing characteristics are somewhat altered, but formulations satisfactory for use in various dental devices were found.
he use of dental biomaterials presents few life-threatening situations. However, removable appliances such as full and partial dentures, bite splints, night guards and the like have the potential to be accidentally dislodged and find their way into the body, requiring surgical removal (Adelman, 1988). The American Dental Association and several dental journals regularly receive reports of such incidents (Jacobs, 1980; Littner et al., 1982), and pleas are often heard for the use of radiopaque materials to facilitate the retrieval of impacted dental objects (Stewart, 1981; Lohman, 1979). Restorative filling materials represent no significant hazard of ingestion, but do require a contrasting radiopacity for the accurate diagnosis of leaking margins, overhangs, and other problems-especially in posterior filling resin composites (Leinfelder, 1988). The requirement for radiopacity in dental resins was fully documented by a recent report prepared for the ADA Council on Dental Materials and Devices (Brauer, 1981). In the report, various approaches to the problem were reviewed and found to be unsatisfactory, and a call for "novel techniques to incorporate radiopacity into the cured resins" was made (Brauer, 1981). The more important requirements for radiopaque denture-type resins having optimum utility are listed in Table 1. Current and previous methods have used either dispersions of metals, heavy-metal-containing glasses or metallic compounds; or solutions of low-molecular-weight halogenated organic compounds (Brauer, 1981). Mixtures and other inhomogeneous structures scatter light, are inherently susceptible to failure at the interface between phases, and are therefore susceptible to liquid penetration and leaching of the additives. Low-molecular-weight organohalide
T
250 RAWLS et aL/RADIOPAQUE DENTAL RESINS
compounds plasticize the resin and are also susceptible to leaching. A recently r e p o r t e d method overcomes these problems with acrylic copolymers containing pendent dib r o m o p r o p y l g r o u p s (Davy and Causton, 1982). Our approach is to form heavy-metal compounds which are chelated or otherwise solubilized to form a single phase with the macromolecular structure of the resin. Such compounds effectively bind a metal within the resin, forming a homogeneous stl~cture that is optically transparent and inherently resistant to degradation (Obligin et al., 1986; Staid et al., 1987). Certain metal salts were found to be adequately bound by the methacrylate carboxylic groups of poly(methyl methacrylate). For others, chelating comonomers were required. Previously we reported on the preparation and properties of these latter compounds (Obligin et al., 1986; Smid et aL, 1987). In the present work, formulations containing acrylic-monomer-soluble heavy-metal compounds, without chelating co-monomers, were prepared and used to investigate their potential utility as practical biomedical materials. MATERIALS AND METHODS
Additives, Monomers, and Polymers.
-
Analytical grades of bismuth trichloride, bismuth tribromide (Alpha), uranyl nitrate hexahydrate (SPIChem), zirconyl dimethacrylate ("ZrDM", Polysciences), and methyl m e t h a c r y l a t e ("MMA", Polysciences) were used without further purification. Non-colored, unpigmented versions of p r o p r i e t a r y methacrylic monomer and polymer materials formulated for the manufacture of heat-cured dentures (Hygenic Co.) and self-cured removable appliances (ORTHOJET, Lang Dental Mfg. Co.) were used as the starting compositions. The manufactm'ers
state that these two-part, liquid/ powder p r o d u c t s contain predominantly methyl methacrylate in the liquid and poly(methyl methacrylate) in the powder. HYGENIC (H) requires heat for curing and also contains a small portion of an unspecified dimethacrylate cross-linker. ORTHO-JET (OJ) has an amine accelerator in the liquid component to provide "self-curing" (polymerization at ambient temperature). Amine accelerators are strong electron donors and rapidly combine with and precipitate metal ions, which prevents the use of heavy-metal salts with self-cure resin systems (Staid et aL, 1987). Thus, the liquid portion of the OJ system was not used with the bismuth and uranyl salts. ZrDM does not affect the amine accelerators and was thus used for preparation of both self-cure (O J) and h e a t - c u r e (H) specimens (see Table 2). The OJ powder component also contains an unspecified methacrylate comonomer ( p r o b a b l y ethyl- or butylmethacrylate) which enhances monomer u p t a k e and formation of a doughy consistency during mixing and results in greater flexibility of the cured material. It was in order to take advantage of these properties that the powder from the OJ selfcure resin s y s t e m was used, together with accelerator-free MMA, to form certain of the heat-cured specimens.
Specimen Preparation. - M e t a l compounds were added to either MMA or the HYGENIC monomer formulation. The resulting liquid composition was then combined with one of the proprietary polymer powder formulations shown in Table 2. The metal additives dissolve in the monomer liquid and alter its ability to solvate and diffuse into the powder in such a way that manipulation and processing characteristics are worsened. Thus it was not possible for normal mixing ratios and temperatures to be used in preparation of satisfactory specimens. Therefore, a series of preliminary experiments was conducted to establish compositions and processing conditions that would minimize alterations in handling characteristics and visual appearance. These experiments employed radiopaque additives at or
somewhat above the proportions needed to match the radiopacity of aluminum. From this work, conditions were selected, as shown in Table 2, and used to prepare specimens for further testing. Curing was accomplished with gypsum molds and o t h e r c o n d i t i o n s as o u t l i n e d in American Dental Association/ANSI Specification 12 (ADA, 1976). Radiopacity. -Specimens, 2 mm thick, were placed on Kodak Ultraspeed DF49 occlusal film along with aluminum and copper step-wedges and exposed for one min at 70 kVp and 15 mA at 24.5 inches source-tospecimen distance, with a General Electric dental x-ray machine. The conditions conformed to ASTM stand a r d s for p l a s t i c s i n t e n d e d for biomedical applications (ASTM, 1979). Films were developed in an automatic processor, and the film densities of step-wedge and specimen images were measured. Radiopacity was calculated as the relative
thickness of A1 or Cu required to produce a film density equal to that of t h e 2-mm s p e c i m e n , and expressed in mm specimen p e r mm of reference metal (A1 or Cu)
Physical and Mechanical Properties. After being cured, specimens were measured for hardness by use of a Durometer, D-Scale indentor. Transverse deflection and water absorption (per unit of exposed surface area) were determined according to ADA/ANSI Specification 12 (ADA, 1976). BiocompaUbility. - T w o types of i n vitro tests were used: the Ames mu-
tagenicity assay (Maron and Ames, 1983) and a cytotoxicity test based on human epithelial cells (Kasten et al., 1989). In the cytotoxicity test, a time-distance cytotoxicity index (TDCI) is determined by measurement of the average number of surviving cells at two radial distances from three specimens after two ex-
TABLE 1 REQUIREMENTS FOR RADIOPAQUEDENTAL RESINS FOR REMOVABLEAPPLIANCES Radiopacity
Mechanical Properties Storage Stability Biocompatibility Utility Esthetics
Equal to or greater than that of aluminum (ASTM, 1979) or at least 1/lOth that of copper (McArthur and Taylor, 1975). Equivalentto or better than non-radiopaque resins (Brauer, 1981). Equivalentto or better than non-radiopaque resins. Equivalentto or better than non-radiopaque resins. Easily cured, shaped, adjusted, and repaired. Transparent, colorless, and capable of being dyed and pigmented.
TABLE 2 CONDITIONS USED TO PREPARE RADIOPAQUERESIN SPECIMENS Dev. of Dough Metal Compound Added None BiCI3 BiBr3 UO2(NO3)2-6H20 ZrDM (heat-cured) ZrDM (self-cured)
Final Concentration (Wt%) 0 11 15 10 15 20 10 15 35 35
Powder-toStage Monomer Temp. Time (Wt/Wt) (°C) (Min.) 2.27 2.27 1.59 2.27 2.18 2.27 2.27 2.27 1.75 + 1.75 +
23 23 47 23 47 23 23 23 37 23
10 55 18 85 5 180 40 60 25 13
Monomer Liquid Used*
Polymer Powder Used*
H H H H OJ H H H H OJ
H H H H MMA H H H H OJ
*The compositions of HYGENIC (H) and ORTHO-JET (OJ) are proprietary compositions, as explained in the text. MMA = Methyl methacrylate. +Based on total amount of powder in the system, which contains 45% polymer powder and 55% zirconyl dimethacrylate (ZrDM) powder. Powder/Liquid = 0.96 based on polymer powder alone. An additional 0.5% benzoyl peroxide was also added to compensate for the added monomer content due to the ZrDM.
Dental Materials]October 1990 251
TABLE 3 RADIOPAClTY OF METAL-CONTAININGPOLY(METHYLMETHACRYLATE)RESINS
Metal Compound*
Target Values None BiCI3 BIBr3
UO2(NO3)2"6H20 ZrDM (H, heat-cured) ZrDM (OJ, self-cured)
Concentration ONt%) 11 15 10 13 15# 20 10 11 15 35 35§ 40§
Radiopacity (mm reference per mm resin) (AI) (Cu)
_>1.0 0.0 1.0 1.5 0.7 1.0 1.7 1.9 0.7 1.0 1.2 1.1 1.3 1.9
_>0.10 0.00 0.03 024 0.007 0.05 0.06 0.00 0.00 0.02 0.04 0.05 -
*Additives were dissolved in HYGENIC (H) monomer, combined with H powder and heat-cured, unless otherwise stated. #In these specimens, additives were dissolved in MMA, combined with ORTHOJET(OJ) powder, and heat-cured. §Additives were dissolved in OJ monomer, combined with OJ powder and "self"-cured at ambient temperature. -Not applicable or not measured.
These concentrations were found to produce radiopacities equivalent to or greater than the target values obtained when the aluminum standard was used, but only 20% to 60% of the target radiopacity obtained when copper was used as the standard. These results are shown in Table 3. The Cu standard is based on the detectability needed to locate objects in the chest cavity (McArthur and Taylor, 1975), while the A1 standard is for medical implants. The task of locating a swallowed or impacted object whose location is unknown would require a greater level of detectability than would non-invasive examination of an implanted material. Thus, the more demanding Cu standard may be the more appropriate one to use for removable appliances such as dentures. This would imply that either higher additive levels or thicker resin sections are needed for objects that offer a high risk of being accidentally dislodged. Physical and Mechanical Properties. -
posure times, normalized to a control (Kasten et al., 1982). Four replicates (12 specimens) were used for each test. The control was the adhesive used to bond the specimen to the plastic culture dish. RESULTS
As shown in Tables 3 and 4, it was found that sevei'al salts of either bismuth and uranium or an organo-zirconium compound are capable of imparting diagnostic levels of radiopacity. Processing properties were found to be somewhat altered, and it was more difficult to attain a doughy stage having the desired putty-like consistency. Although moldable, the compound exhibited a slightly rubbery nature, always present, which heating and raising the polymer-powder/monomer-liquid ratio of the precured resin could only partially eliminate (see Table 2). The results of the biological response studies are shown in Tables 5-7. For the bismuth bromide resins, Table 5 shows a very low level of cytotoxicity, and Table 6 shows an absence of mutagenicity. Table 7 demonstrates a very low level of cytotoxicity for the bismuth chloride
and zirconyl dimethacrylate-containing resins. In a leaching experiment, only 0.049% Br (w/v) and less than 0.0002% Bi could be detected after exhaustive Soxlet extraction of 20% BiBrs in PMMA with refluxing hot water (microanalyses by Galbraith Labs., Knoxville, TN). DISCUSSION
Radiopacily.-The bismuth, uranyl, and zirconium compounds were found to be soluble in MMA and poly(methylmethacrylate) in concentrations that impart radiopacity. Combe (1972) suggested bismuth halides as radiopacifying agents for PMMA. Our results confirm that bismuth halides are indeed good radiopacifying agents and that they readily dissolve in the acrylic matrix. The phenomenon appears to be general and can be utilized to incorporate a number of x-ray-absorbing species in polymeric resins. For the bismuth compounds, 1:1 Bi-carbonyl interaction has been shown to occur (Smid et al., 1987). Thus, carbonylcation adduct foz~mation can account for the solubility of these metal compounds, which ranges from 11 to 35%.
252 R AWL S et aL/RADIOPAQUE DENTAL RESINS
Tables 3 and 4 demonstrate that at levels that produce adequate radiopacity compared with the A1 standard, resins having an entirely satisfactory balance of physical properties are difficult to attain. Resins containing 10% BiBrs and 1015% UOe(NO3)2"6H20 pass the transverse strength specification test, while those with 15% BiC13 and 20% BiBr3 are only slightly outside the ADA specification limits. The formulations that used ORTHOJET powder (15% BiBr3 and 35% ZrDM) were substantially more flexible (deflection > 5 ram, Table 4) and reached a doughy consistency significantly faster (Table 2) than those using HYGENIC powder. Since OJ is formulated for use in self-curing systems, it can be expected to be more readily penetrated and swollen by monomer than polymer powders intended for heat-curing systems such as H (Craig, 1989). The greater flexibility and the decreased time to develop a doughy consistency using OJ powder in both heat- and self-curing formulations (Table 2) demonstrate the influence of the powder component. With these additives, then, it should be possible for satisfactory manipulation and mechanical prop-
TABLE 4 PHYSICAL AND MECHANICAL PROPERTIES OF RADIOPAQUE RESINS
Metal Compound* TARGET or ADA Spec. 12 None (H, heat-cured) None (OJ, self-cured) BiCI3 BiBr3
UO2(NO3)2-6H20 ZrDM (heat-cured) ZrDM (self-cured)
Concentration (Wt%) 0 0 0 11 15 10 15§ 20 10 15 35 35§ 40§
Hardness Durometer D 80 80 81 80 76 78 75 76 80 77 83 83 76
- 0.5 ± 0.5 -+ 1 _ 2 _ 2 __+_1 -+ 2 ± 3 ± 1 - 2 ± 3 ± 2
Transverse Deflection (ram) 1.5-3.5 Kg 1.5-5 Kg ÷ 0 to 2.5 1.6 _ 0.3# 1.5 ± 0.4# 2.7 ± 0.4 1.0 _ 0.7 >5(broke)** 2.6 _ 0.05 2.1 ± 0.4# 1.7 ± 0.2# -
2 to 5.5 3.1 _ 0.5# 2.6 _ 0.7# >5(broke)** 2.3 ± 0.7 4.6(broke)** 2.8 ± 0.1# 3.1 ± 0.5# -
Absorption at 4 weeks (mg/cm 2) - 0.05)
Salmonella Salm. + S-9
+
Mutagenlcity# Reversions > 0 (p < 0.05) m
+
Salmonella Salm. + S-9 Salmonella Salm. + S-9 Salmonella Salm. + S-9 Salmonella
+
m
+
m
+
+
R
+
+ +
m
*Salmonella strains 97A, 98, 100, & 102, with or without metabolizing with S-9 extracts.
*If the dose-response curve is linear (+), the data fit the assay's model and are thus reliable. Linearity is demonstrated by a lack of a significant difference (p > 0.05) between the data points and a linear least-squares regression line, using analysis of variance. #If the number of reversions is significantly different from zero and increaseswith dose (+), the test demonstrates mutagenicity; if not ( - ) , then the substance is not mutagenic at p < 0.05. TABLE 7 BIOCOMPATIBILITYOF BiCl3 AND ZIRCONYL DIMETHACRYLATERESINS AS DETERMINEDBY GINGIVAL EPITHELIALCELL CYTOTOXlCITY
Material*
Cytotoxicity (TDCI) (% of adhesive)
Dish + adhesive Heat-cured PMMA Heat-cured PMMA(,) + 11% BiCIs + 15% BiCI3 Self-cured PMMA + 10% ZrDMro) + 40% ZrDM(o)
0.0 37.1 ÷ 35.5* 36.9* 40.7* 41.1 *
*In addition to PMMA, the pre-cured formulations included (Wt%): (a) 1% benzoyl peroxide (BP)initiator, (b) 18% triethylene glycol dimethacrylate (TEGDMA) cross-linker, 3% BP, and 1.5% dihydroxyethyl-p-toluldlne (DHEPT) accelerator, and (c) 11% TEGDMA, 2% BP, and 1% DHEPT. *Significantly different from the dish + adhesivecontrol at p < 0.05, but not different from each other, according to the Student Newman-Keulstest for least-significant differences of the means for 12 specimens each.
5% BiBrs have no additional effect. The TDCI values shown in Table 5 are very low, amounting to only 1/4th to 1/6th of that found for several currently used cements and dental filling materials, with both gingival fibroblasts (Kasten et al., 1982) and gingival epithelial cells used (Kasten et al., 1989). For example, TDCI was shown to be 10% for a glass-ionomer cement, 23% for a silicate cement, 82% for a glass-ionomer filing material, and 91% for a light-cured filing resin composite (Kasten et aL, 1989). Poly(methyl
methacrylate) resins containing BiCls or zirconyl d i m e t h a c r y l a t e w e r e tested for cytotoxicity in a separate experiment. As shown in Table 7, neither additive significantly affects cytotoxicity compared with either the specimen-bonding adhesive or with PMMA alone. These results show that the Bi- and Zr-containing compounds generally induce a small increase in cytotoxicity that is at the low end of the cytotoxicity range of materials in current clinical use, and that BiBrs has no mutagenic potential. We have observed a similar small
254 RAWLS et ak/RADIOPAQUE DENTAL RESINS
increase in cytotoxicity for acrylic resins which contained fluoride additives (Kasten et al., 1989). The elevated cytotoxicity was attributed to a reduction in monomer-polymer conversion due to the presence of the additive, t h u s p r o v i d i n g an increased amount of monomer available for extraction into the cell culture medium. The radiopaque compounds are all sparingly water-soluble. Thus, it is likely that reduced monomer conversion also accounts for the small elevation in cytotoxicity observed here. Unreacted monomer would also account for the increased transverse deflection and water absorption (Table 4) discussed above. Overall, the results indicate that a high level of biocompatibility and no significant hazard for oral use can be expected for acrylic resin materials containing these compounds. Based only on the physical properties shown in Tables 3 and 4, uranyl nitrate would appear to be the more promising radiopaque additive of those investigated. However, the biocompatibility was not evaluated for this compound because of the known toxic effects of trace levels of ~85U that are present with the nonradioactive isomer, zssU. Depleted uranium is available with as low as 2.4 ppm 2ssU and a very low specific activity (3.3 x 10 -7 curies/g, Oak Ridge Natl. Lab., private communication). In principle, uranyl nitrate made from this source may be suitable for biomaterials applications. However, it is quite expensive and would undoubtedly be suspect. Thus, in practical terms it is more suitable for non-biomedical applications. Nevertheless, for completeness, the results for it are presented here. CONCLUSIONS
The approach of incorporating soluble, heavy-metal compounds into acrylic compositions is capable of producing optically transparent, radiopaque resins suitable for dental prostheses and other biomedical devices. However, further formulation refinements are required to gain the relatively small improvements in manipulation properties, transverse deflection, and resistance to water
that would provide the balance of properties demanded by the dental profession.
ACKNOWLEDGMENT The help of Dr. K. H. Thunthy, Department of Oral Diagnosis, .Oral Medicine and Oral Radiography, LSU School of Dentistry, in developing procedures for the radiopacity measurements is gratefully acknowledged.
REFERENCES ADELMAN, H.C. (1988): Asphyxial Deaths as Result of Aspiration of Dental Appliances: A Report of Three Cases, J Forensic Sci 33: 389-395. AMERICAN DENTAL ASSOCIATION COUNCIL ON DENTAL MATERIALSAND DEVICES (1976): Revised A.D.A. Specification No. 12 for Denture Base Polymers. Fourth revision, Chicago: ADA. AMERICAN SOCIETY FOR TESTING AND MATERIALS (1979): Standard Test Methods for Radiopacity of Plastics for Medical Use. Method B. ANSI/ASTM F 640-79. Philadelphia, PA: ASTM, pp. 1137-1141.
BRAUER, G.M. (1981): The Desirability of Using Radiopaque Plastics in Dentistry: A Status Report, J A m Dent Assoc 101: 347--349. COMBE, E.C. (1972): Further Studies on Radio-Opaque Denture-base Materials, J Dent 1: 93-97. CRAIG, R.G., Ed. (1989): Restorative Dental Materials, 8th ed. St. Louis: Mosby, pp. 509-547. DAVY, K.M. and CAUSTON,B.E. (1982): Radio-Opaque Denture Base: A New Acrylic Co-polymer, J Dent 10: 254265. JACOBS, L.I. (1980): Ingestion of Partial Denture, J A m Dent Assoc 101: 802. KASTEN, F.H.; FELDER, S.M.; GETTLEMAN, L.; and ALCHEDIAK, W. (1982): A Model Culture System with Human Gingival Fibroblasts for Evaluating the Cytotoxicity of Dental Materials, In Vitro 18: 650--660. KASTEN, F.H.; PINEDA, L.F.; SCHNEIDER, P.E.; RAWLS, H.R.; and FOSTER, T.A. (1989): Biocompatibility Testing of an Experimental Fluoride Releasing Resin Using Human Gingival Epithelial Cells in vitro, In Vitro Cellular & Dev Biol 23: 57-62. LEINFELDER, K.L. (1988): Posterior Composite Resins, J A m Dent Assoc 117: 21E-26E.
LI2"rNER, M.M.; gAFFE, I.; and TAMSE, & (1982): Foreign Bodies in the Orofacial Region, Quint Int 6: 701-706. LOHMAN,J.W. (1979): Letter to the ADA From the Executive Council of the Montana Dental Assoc., May 17. MARON, D. and AMES, B.N. (1983): Revised Methods for the Salmonella Mutagenicity Test, Murat Res 113: 173215. MCARTHUR, D.R. and TAYLOR, D.F. (1975): A Determination of the Minimum Radiopacification Necessary for Radiographic Detection of an Aspirated or Swallowed Object, Oral Surg 39:329-338. OBLIGIN, A.; XIA~ D.W.; SILBERMAN,R.; RAWLS, H.R.; CABASSO,I.; and SMID, J. (1986): Novel Radiopaque BariumPolymer Complexes. US Patent application 852,990, Washington, DC: US Patent Office. SMID, J.; CABASSO, I.; RAWLS, H.R.; 0BLIGIN, A.; DELAVIZ, Y.; SAHNI, S.K.; and ZHANG, Z. (1987): Novel Homogeneous Polymer-Heavy Metal Salt Complexes for X-ray Imaging, Makromol Chem Rapid Co~r~mun 8: 543-547. STEWART, J.A. (1981): Radiopaque Plastics (letter to the editor), J A m Dent Assoc 103: 604.
Dental Materials~October 1990 255
*To whom correspondence should be addressed-Present address: University of Texas Health Science Center 7703 Floyd Curl Dr. San Antonio, FX 78284-7890 1Louisiana State University Medical Center New Orleans, LA 70119-2799 2Louisiana State University Medical Center New Orleans, LA 70112-1393 3State University of New York Polymer Institute Syracuse, NY 13210 Received October 5, 1989 Accepted February 5, 1990 This investigation was supported in part by USPHS Research Grants DE06170 and DE06179 from the National Institute of Dental Research.
Dent Mater 6:250-255, October, 1990
Abstract-Radiopacity is needed in order to facilitate diagnosis of polymeric appliances, which may be dislodged and become impacted in the upper respiratory or digestive tracts. In order for a stable, optically transparent, radiopaque material to be provided, heavy-metal compounds were investigated which we had previously shown to form homogeneous structures with methyl methacrylatebased systems. It was found that, when present in PMMA at 11 to 14%, several compounds of either bismuth or uranium or 35% of an organo-zirconium compound impart radiopacity equivalent to that of aluminum. A low level of cytotoxicity and lack of mutagenicity indicated that a high level of biocompatibility can be expected. Processing characteristics are somewhat altered, but formulations satisfactory for use in various dental devices were found.
he use of dental biomaterials presents few life-threatening situations. However, removable appliances such as full and partial dentures, bite splints, night guards and the like have the potential to be accidentally dislodged and find their way into the body, requiring surgical removal (Adelman, 1988). The American Dental Association and several dental journals regularly receive reports of such incidents (Jacobs, 1980; Littner et al., 1982), and pleas are often heard for the use of radiopaque materials to facilitate the retrieval of impacted dental objects (Stewart, 1981; Lohman, 1979). Restorative filling materials represent no significant hazard of ingestion, but do require a contrasting radiopacity for the accurate diagnosis of leaking margins, overhangs, and other problems-especially in posterior filling resin composites (Leinfelder, 1988). The requirement for radiopacity in dental resins was fully documented by a recent report prepared for the ADA Council on Dental Materials and Devices (Brauer, 1981). In the report, various approaches to the problem were reviewed and found to be unsatisfactory, and a call for "novel techniques to incorporate radiopacity into the cured resins" was made (Brauer, 1981). The more important requirements for radiopaque denture-type resins having optimum utility are listed in Table 1. Current and previous methods have used either dispersions of metals, heavy-metal-containing glasses or metallic compounds; or solutions of low-molecular-weight halogenated organic compounds (Brauer, 1981). Mixtures and other inhomogeneous structures scatter light, are inherently susceptible to failure at the interface between phases, and are therefore susceptible to liquid penetration and leaching of the additives. Low-molecular-weight organohalide
T
250 RAWLS et aL/RADIOPAQUE DENTAL RESINS
compounds plasticize the resin and are also susceptible to leaching. A recently r e p o r t e d method overcomes these problems with acrylic copolymers containing pendent dib r o m o p r o p y l g r o u p s (Davy and Causton, 1982). Our approach is to form heavy-metal compounds which are chelated or otherwise solubilized to form a single phase with the macromolecular structure of the resin. Such compounds effectively bind a metal within the resin, forming a homogeneous stl~cture that is optically transparent and inherently resistant to degradation (Obligin et al., 1986; Staid et al., 1987). Certain metal salts were found to be adequately bound by the methacrylate carboxylic groups of poly(methyl methacrylate). For others, chelating comonomers were required. Previously we reported on the preparation and properties of these latter compounds (Obligin et al., 1986; Smid et aL, 1987). In the present work, formulations containing acrylic-monomer-soluble heavy-metal compounds, without chelating co-monomers, were prepared and used to investigate their potential utility as practical biomedical materials. MATERIALS AND METHODS
Additives, Monomers, and Polymers.
-
Analytical grades of bismuth trichloride, bismuth tribromide (Alpha), uranyl nitrate hexahydrate (SPIChem), zirconyl dimethacrylate ("ZrDM", Polysciences), and methyl m e t h a c r y l a t e ("MMA", Polysciences) were used without further purification. Non-colored, unpigmented versions of p r o p r i e t a r y methacrylic monomer and polymer materials formulated for the manufacture of heat-cured dentures (Hygenic Co.) and self-cured removable appliances (ORTHOJET, Lang Dental Mfg. Co.) were used as the starting compositions. The manufactm'ers
state that these two-part, liquid/ powder p r o d u c t s contain predominantly methyl methacrylate in the liquid and poly(methyl methacrylate) in the powder. HYGENIC (H) requires heat for curing and also contains a small portion of an unspecified dimethacrylate cross-linker. ORTHO-JET (OJ) has an amine accelerator in the liquid component to provide "self-curing" (polymerization at ambient temperature). Amine accelerators are strong electron donors and rapidly combine with and precipitate metal ions, which prevents the use of heavy-metal salts with self-cure resin systems (Staid et aL, 1987). Thus, the liquid portion of the OJ system was not used with the bismuth and uranyl salts. ZrDM does not affect the amine accelerators and was thus used for preparation of both self-cure (O J) and h e a t - c u r e (H) specimens (see Table 2). The OJ powder component also contains an unspecified methacrylate comonomer ( p r o b a b l y ethyl- or butylmethacrylate) which enhances monomer u p t a k e and formation of a doughy consistency during mixing and results in greater flexibility of the cured material. It was in order to take advantage of these properties that the powder from the OJ selfcure resin s y s t e m was used, together with accelerator-free MMA, to form certain of the heat-cured specimens.
Specimen Preparation. - M e t a l compounds were added to either MMA or the HYGENIC monomer formulation. The resulting liquid composition was then combined with one of the proprietary polymer powder formulations shown in Table 2. The metal additives dissolve in the monomer liquid and alter its ability to solvate and diffuse into the powder in such a way that manipulation and processing characteristics are worsened. Thus it was not possible for normal mixing ratios and temperatures to be used in preparation of satisfactory specimens. Therefore, a series of preliminary experiments was conducted to establish compositions and processing conditions that would minimize alterations in handling characteristics and visual appearance. These experiments employed radiopaque additives at or
somewhat above the proportions needed to match the radiopacity of aluminum. From this work, conditions were selected, as shown in Table 2, and used to prepare specimens for further testing. Curing was accomplished with gypsum molds and o t h e r c o n d i t i o n s as o u t l i n e d in American Dental Association/ANSI Specification 12 (ADA, 1976). Radiopacity. -Specimens, 2 mm thick, were placed on Kodak Ultraspeed DF49 occlusal film along with aluminum and copper step-wedges and exposed for one min at 70 kVp and 15 mA at 24.5 inches source-tospecimen distance, with a General Electric dental x-ray machine. The conditions conformed to ASTM stand a r d s for p l a s t i c s i n t e n d e d for biomedical applications (ASTM, 1979). Films were developed in an automatic processor, and the film densities of step-wedge and specimen images were measured. Radiopacity was calculated as the relative
thickness of A1 or Cu required to produce a film density equal to that of t h e 2-mm s p e c i m e n , and expressed in mm specimen p e r mm of reference metal (A1 or Cu)
Physical and Mechanical Properties. After being cured, specimens were measured for hardness by use of a Durometer, D-Scale indentor. Transverse deflection and water absorption (per unit of exposed surface area) were determined according to ADA/ANSI Specification 12 (ADA, 1976). BiocompaUbility. - T w o types of i n vitro tests were used: the Ames mu-
tagenicity assay (Maron and Ames, 1983) and a cytotoxicity test based on human epithelial cells (Kasten et al., 1989). In the cytotoxicity test, a time-distance cytotoxicity index (TDCI) is determined by measurement of the average number of surviving cells at two radial distances from three specimens after two ex-
TABLE 1 REQUIREMENTS FOR RADIOPAQUEDENTAL RESINS FOR REMOVABLEAPPLIANCES Radiopacity
Mechanical Properties Storage Stability Biocompatibility Utility Esthetics
Equal to or greater than that of aluminum (ASTM, 1979) or at least 1/lOth that of copper (McArthur and Taylor, 1975). Equivalentto or better than non-radiopaque resins (Brauer, 1981). Equivalentto or better than non-radiopaque resins. Equivalentto or better than non-radiopaque resins. Easily cured, shaped, adjusted, and repaired. Transparent, colorless, and capable of being dyed and pigmented.
TABLE 2 CONDITIONS USED TO PREPARE RADIOPAQUERESIN SPECIMENS Dev. of Dough Metal Compound Added None BiCI3 BiBr3 UO2(NO3)2-6H20 ZrDM (heat-cured) ZrDM (self-cured)
Final Concentration (Wt%) 0 11 15 10 15 20 10 15 35 35
Powder-toStage Monomer Temp. Time (Wt/Wt) (°C) (Min.) 2.27 2.27 1.59 2.27 2.18 2.27 2.27 2.27 1.75 + 1.75 +
23 23 47 23 47 23 23 23 37 23
10 55 18 85 5 180 40 60 25 13
Monomer Liquid Used*
Polymer Powder Used*
H H H H OJ H H H H OJ
H H H H MMA H H H H OJ
*The compositions of HYGENIC (H) and ORTHO-JET (OJ) are proprietary compositions, as explained in the text. MMA = Methyl methacrylate. +Based on total amount of powder in the system, which contains 45% polymer powder and 55% zirconyl dimethacrylate (ZrDM) powder. Powder/Liquid = 0.96 based on polymer powder alone. An additional 0.5% benzoyl peroxide was also added to compensate for the added monomer content due to the ZrDM.
Dental Materials]October 1990 251
TABLE 3 RADIOPAClTY OF METAL-CONTAININGPOLY(METHYLMETHACRYLATE)RESINS
Metal Compound*
Target Values None BiCI3 BIBr3
UO2(NO3)2"6H20 ZrDM (H, heat-cured) ZrDM (OJ, self-cured)
Concentration ONt%) 11 15 10 13 15# 20 10 11 15 35 35§ 40§
Radiopacity (mm reference per mm resin) (AI) (Cu)
_>1.0 0.0 1.0 1.5 0.7 1.0 1.7 1.9 0.7 1.0 1.2 1.1 1.3 1.9
_>0.10 0.00 0.03 024 0.007 0.05 0.06 0.00 0.00 0.02 0.04 0.05 -
*Additives were dissolved in HYGENIC (H) monomer, combined with H powder and heat-cured, unless otherwise stated. #In these specimens, additives were dissolved in MMA, combined with ORTHOJET(OJ) powder, and heat-cured. §Additives were dissolved in OJ monomer, combined with OJ powder and "self"-cured at ambient temperature. -Not applicable or not measured.
These concentrations were found to produce radiopacities equivalent to or greater than the target values obtained when the aluminum standard was used, but only 20% to 60% of the target radiopacity obtained when copper was used as the standard. These results are shown in Table 3. The Cu standard is based on the detectability needed to locate objects in the chest cavity (McArthur and Taylor, 1975), while the A1 standard is for medical implants. The task of locating a swallowed or impacted object whose location is unknown would require a greater level of detectability than would non-invasive examination of an implanted material. Thus, the more demanding Cu standard may be the more appropriate one to use for removable appliances such as dentures. This would imply that either higher additive levels or thicker resin sections are needed for objects that offer a high risk of being accidentally dislodged. Physical and Mechanical Properties. -
posure times, normalized to a control (Kasten et al., 1982). Four replicates (12 specimens) were used for each test. The control was the adhesive used to bond the specimen to the plastic culture dish. RESULTS
As shown in Tables 3 and 4, it was found that sevei'al salts of either bismuth and uranium or an organo-zirconium compound are capable of imparting diagnostic levels of radiopacity. Processing properties were found to be somewhat altered, and it was more difficult to attain a doughy stage having the desired putty-like consistency. Although moldable, the compound exhibited a slightly rubbery nature, always present, which heating and raising the polymer-powder/monomer-liquid ratio of the precured resin could only partially eliminate (see Table 2). The results of the biological response studies are shown in Tables 5-7. For the bismuth bromide resins, Table 5 shows a very low level of cytotoxicity, and Table 6 shows an absence of mutagenicity. Table 7 demonstrates a very low level of cytotoxicity for the bismuth chloride
and zirconyl dimethacrylate-containing resins. In a leaching experiment, only 0.049% Br (w/v) and less than 0.0002% Bi could be detected after exhaustive Soxlet extraction of 20% BiBrs in PMMA with refluxing hot water (microanalyses by Galbraith Labs., Knoxville, TN). DISCUSSION
Radiopacily.-The bismuth, uranyl, and zirconium compounds were found to be soluble in MMA and poly(methylmethacrylate) in concentrations that impart radiopacity. Combe (1972) suggested bismuth halides as radiopacifying agents for PMMA. Our results confirm that bismuth halides are indeed good radiopacifying agents and that they readily dissolve in the acrylic matrix. The phenomenon appears to be general and can be utilized to incorporate a number of x-ray-absorbing species in polymeric resins. For the bismuth compounds, 1:1 Bi-carbonyl interaction has been shown to occur (Smid et al., 1987). Thus, carbonylcation adduct foz~mation can account for the solubility of these metal compounds, which ranges from 11 to 35%.
252 R AWL S et aL/RADIOPAQUE DENTAL RESINS
Tables 3 and 4 demonstrate that at levels that produce adequate radiopacity compared with the A1 standard, resins having an entirely satisfactory balance of physical properties are difficult to attain. Resins containing 10% BiBrs and 1015% UOe(NO3)2"6H20 pass the transverse strength specification test, while those with 15% BiC13 and 20% BiBr3 are only slightly outside the ADA specification limits. The formulations that used ORTHOJET powder (15% BiBr3 and 35% ZrDM) were substantially more flexible (deflection > 5 ram, Table 4) and reached a doughy consistency significantly faster (Table 2) than those using HYGENIC powder. Since OJ is formulated for use in self-curing systems, it can be expected to be more readily penetrated and swollen by monomer than polymer powders intended for heat-curing systems such as H (Craig, 1989). The greater flexibility and the decreased time to develop a doughy consistency using OJ powder in both heat- and self-curing formulations (Table 2) demonstrate the influence of the powder component. With these additives, then, it should be possible for satisfactory manipulation and mechanical prop-
TABLE 4 PHYSICAL AND MECHANICAL PROPERTIES OF RADIOPAQUE RESINS
Metal Compound* TARGET or ADA Spec. 12 None (H, heat-cured) None (OJ, self-cured) BiCI3 BiBr3
UO2(NO3)2-6H20 ZrDM (heat-cured) ZrDM (self-cured)
Concentration (Wt%) 0 0 0 11 15 10 15§ 20 10 15 35 35§ 40§
Hardness Durometer D 80 80 81 80 76 78 75 76 80 77 83 83 76
- 0.5 ± 0.5 -+ 1 _ 2 _ 2 __+_1 -+ 2 ± 3 ± 1 - 2 ± 3 ± 2
Transverse Deflection (ram) 1.5-3.5 Kg 1.5-5 Kg ÷ 0 to 2.5 1.6 _ 0.3# 1.5 ± 0.4# 2.7 ± 0.4 1.0 _ 0.7 >5(broke)** 2.6 _ 0.05 2.1 ± 0.4# 1.7 ± 0.2# -
2 to 5.5 3.1 _ 0.5# 2.6 _ 0.7# >5(broke)** 2.3 ± 0.7 4.6(broke)** 2.8 ± 0.1# 3.1 ± 0.5# -
Absorption at 4 weeks (mg/cm 2) - 0.05)
Salmonella Salm. + S-9
+
Mutagenlcity# Reversions > 0 (p < 0.05) m
+
Salmonella Salm. + S-9 Salmonella Salm. + S-9 Salmonella Salm. + S-9 Salmonella
+
m
+
m
+
+
R
+
+ +
m
*Salmonella strains 97A, 98, 100, & 102, with or without metabolizing with S-9 extracts.
*If the dose-response curve is linear (+), the data fit the assay's model and are thus reliable. Linearity is demonstrated by a lack of a significant difference (p > 0.05) between the data points and a linear least-squares regression line, using analysis of variance. #If the number of reversions is significantly different from zero and increaseswith dose (+), the test demonstrates mutagenicity; if not ( - ) , then the substance is not mutagenic at p < 0.05. TABLE 7 BIOCOMPATIBILITYOF BiCl3 AND ZIRCONYL DIMETHACRYLATERESINS AS DETERMINEDBY GINGIVAL EPITHELIALCELL CYTOTOXlCITY
Material*
Cytotoxicity (TDCI) (% of adhesive)
Dish + adhesive Heat-cured PMMA Heat-cured PMMA(,) + 11% BiCIs + 15% BiCI3 Self-cured PMMA + 10% ZrDMro) + 40% ZrDM(o)
0.0 37.1 ÷ 35.5* 36.9* 40.7* 41.1 *
*In addition to PMMA, the pre-cured formulations included (Wt%): (a) 1% benzoyl peroxide (BP)initiator, (b) 18% triethylene glycol dimethacrylate (TEGDMA) cross-linker, 3% BP, and 1.5% dihydroxyethyl-p-toluldlne (DHEPT) accelerator, and (c) 11% TEGDMA, 2% BP, and 1% DHEPT. *Significantly different from the dish + adhesivecontrol at p < 0.05, but not different from each other, according to the Student Newman-Keulstest for least-significant differences of the means for 12 specimens each.
5% BiBrs have no additional effect. The TDCI values shown in Table 5 are very low, amounting to only 1/4th to 1/6th of that found for several currently used cements and dental filling materials, with both gingival fibroblasts (Kasten et al., 1982) and gingival epithelial cells used (Kasten et al., 1989). For example, TDCI was shown to be 10% for a glass-ionomer cement, 23% for a silicate cement, 82% for a glass-ionomer filing material, and 91% for a light-cured filing resin composite (Kasten et aL, 1989). Poly(methyl
methacrylate) resins containing BiCls or zirconyl d i m e t h a c r y l a t e w e r e tested for cytotoxicity in a separate experiment. As shown in Table 7, neither additive significantly affects cytotoxicity compared with either the specimen-bonding adhesive or with PMMA alone. These results show that the Bi- and Zr-containing compounds generally induce a small increase in cytotoxicity that is at the low end of the cytotoxicity range of materials in current clinical use, and that BiBrs has no mutagenic potential. We have observed a similar small
254 RAWLS et ak/RADIOPAQUE DENTAL RESINS
increase in cytotoxicity for acrylic resins which contained fluoride additives (Kasten et al., 1989). The elevated cytotoxicity was attributed to a reduction in monomer-polymer conversion due to the presence of the additive, t h u s p r o v i d i n g an increased amount of monomer available for extraction into the cell culture medium. The radiopaque compounds are all sparingly water-soluble. Thus, it is likely that reduced monomer conversion also accounts for the small elevation in cytotoxicity observed here. Unreacted monomer would also account for the increased transverse deflection and water absorption (Table 4) discussed above. Overall, the results indicate that a high level of biocompatibility and no significant hazard for oral use can be expected for acrylic resin materials containing these compounds. Based only on the physical properties shown in Tables 3 and 4, uranyl nitrate would appear to be the more promising radiopaque additive of those investigated. However, the biocompatibility was not evaluated for this compound because of the known toxic effects of trace levels of ~85U that are present with the nonradioactive isomer, zssU. Depleted uranium is available with as low as 2.4 ppm 2ssU and a very low specific activity (3.3 x 10 -7 curies/g, Oak Ridge Natl. Lab., private communication). In principle, uranyl nitrate made from this source may be suitable for biomaterials applications. However, it is quite expensive and would undoubtedly be suspect. Thus, in practical terms it is more suitable for non-biomedical applications. Nevertheless, for completeness, the results for it are presented here. CONCLUSIONS
The approach of incorporating soluble, heavy-metal compounds into acrylic compositions is capable of producing optically transparent, radiopaque resins suitable for dental prostheses and other biomedical devices. However, further formulation refinements are required to gain the relatively small improvements in manipulation properties, transverse deflection, and resistance to water
that would provide the balance of properties demanded by the dental profession.
ACKNOWLEDGMENT The help of Dr. K. H. Thunthy, Department of Oral Diagnosis, .Oral Medicine and Oral Radiography, LSU School of Dentistry, in developing procedures for the radiopacity measurements is gratefully acknowledged.
REFERENCES ADELMAN, H.C. (1988): Asphyxial Deaths as Result of Aspiration of Dental Appliances: A Report of Three Cases, J Forensic Sci 33: 389-395. AMERICAN DENTAL ASSOCIATION COUNCIL ON DENTAL MATERIALSAND DEVICES (1976): Revised A.D.A. Specification No. 12 for Denture Base Polymers. Fourth revision, Chicago: ADA. AMERICAN SOCIETY FOR TESTING AND MATERIALS (1979): Standard Test Methods for Radiopacity of Plastics for Medical Use. Method B. ANSI/ASTM F 640-79. Philadelphia, PA: ASTM, pp. 1137-1141.
BRAUER, G.M. (1981): The Desirability of Using Radiopaque Plastics in Dentistry: A Status Report, J A m Dent Assoc 101: 347--349. COMBE, E.C. (1972): Further Studies on Radio-Opaque Denture-base Materials, J Dent 1: 93-97. CRAIG, R.G., Ed. (1989): Restorative Dental Materials, 8th ed. St. Louis: Mosby, pp. 509-547. DAVY, K.M. and CAUSTON,B.E. (1982): Radio-Opaque Denture Base: A New Acrylic Co-polymer, J Dent 10: 254265. JACOBS, L.I. (1980): Ingestion of Partial Denture, J A m Dent Assoc 101: 802. KASTEN, F.H.; FELDER, S.M.; GETTLEMAN, L.; and ALCHEDIAK, W. (1982): A Model Culture System with Human Gingival Fibroblasts for Evaluating the Cytotoxicity of Dental Materials, In Vitro 18: 650--660. KASTEN, F.H.; PINEDA, L.F.; SCHNEIDER, P.E.; RAWLS, H.R.; and FOSTER, T.A. (1989): Biocompatibility Testing of an Experimental Fluoride Releasing Resin Using Human Gingival Epithelial Cells in vitro, In Vitro Cellular & Dev Biol 23: 57-62. LEINFELDER, K.L. (1988): Posterior Composite Resins, J A m Dent Assoc 117: 21E-26E.
LI2"rNER, M.M.; gAFFE, I.; and TAMSE, & (1982): Foreign Bodies in the Orofacial Region, Quint Int 6: 701-706. LOHMAN,J.W. (1979): Letter to the ADA From the Executive Council of the Montana Dental Assoc., May 17. MARON, D. and AMES, B.N. (1983): Revised Methods for the Salmonella Mutagenicity Test, Murat Res 113: 173215. MCARTHUR, D.R. and TAYLOR, D.F. (1975): A Determination of the Minimum Radiopacification Necessary for Radiographic Detection of an Aspirated or Swallowed Object, Oral Surg 39:329-338. OBLIGIN, A.; XIA~ D.W.; SILBERMAN,R.; RAWLS, H.R.; CABASSO,I.; and SMID, J. (1986): Novel Radiopaque BariumPolymer Complexes. US Patent application 852,990, Washington, DC: US Patent Office. SMID, J.; CABASSO, I.; RAWLS, H.R.; 0BLIGIN, A.; DELAVIZ, Y.; SAHNI, S.K.; and ZHANG, Z. (1987): Novel Homogeneous Polymer-Heavy Metal Salt Complexes for X-ray Imaging, Makromol Chem Rapid Co~r~mun 8: 543-547. STEWART, J.A. (1981): Radiopaque Plastics (letter to the editor), J A m Dent Assoc 103: 604.
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