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Composition and Characteristics of Glass Ionomer Cements
vW
GLASS
IONOMERS
CURRENT
A V
CLINICAL
PRACTICE
Glass ionom er cem ent m aterials are currently a va ila b le fo r restoration, caxrity lining an d basing, luting, an d preventive applications. T he in vitro perform ance is a function o f com position , m a n ipu lation , an d placem ent. In general, the cem ents m ay be characterized as strong, stiff, hard m aterials th at are adhesive to calcified tissue, have low toxicity, an d are p oten tially anticariogenic through fluoride leaching.
Composition and characteristics of glass ionomer cements Dennis C. Smith, MSc, DSc, PhD , FRSC
lass io n o m er cem en ts were developed in the early 1970s by com bining the strength, rigid ity, and fluorid e release properties of a silicate glass powder with the biocom p a tib ility and adhesive characteristics o f a p o ly a cry lic acid liq u id .1 D u rin g the past decade, changes have been made in both the glass pow der co m p o n en t and the polycarboxylic acid liquid. Most of the research and development on cem en ts has con cern ed m aterials comprised of a calcium fluoro-aluminosilica te glass pow der and an aq u eou s so lu tion of a polyacrylic acid-itaconic acid/copolym er containing tartaric acid. M odifications in both com ponents have been made in various brands for both patent and practical reasons. As a result, there are sign ifican t differences in the com position and properties of commer cial materials for the various applications.
G
Material composition Glass composition The original ion leachable glasses were based on an S i0 2-A103-CaF2-A lP 04N a 3A lF 6 c o m p o sitio n .2 A ccord in g to W ilson and McLean,1 the Al203/S i0 2 ratio is required to be 1:2 or more, and the fluoride content can be up to 23%. These g la sses m ay be o p a q u e, c o n ta in in g fluorite (CaF2) or corundum (A1203). Little work has been p ublished on the com p o sitio n and structure of the glasses, but recent materials contain more sodium and less fluoride (Table l) .1 Materials 20 ■ JADA, Vol. 120, Ja n u a ry 1990
for tooth restoration may use clear rather than opal glasses to provide improved translucency, although this may result in in trin sically low er strength.3 Radiopacity may be achieved by incorporation of stron tiu m (Sr), barium (Ba), or lanthanum (La), by fusing silver to the
Table 1 . Com position of glass ionomer. Component Calcium Sodium Aluminum Fluoride Phosphorus Silicon Oxygen
A 8
Percent B 10
5
2
16 13 1 20 37
14 13 3 16 35
C 7 7 16 10 5 13 42
(Adapted from Wilson, AD and McLean, JW. Glassionomer cement. 1988. Courtesy of Quintessence).
glass, or m ix in g in zinc o x id e or zir conium oxide. T w o m o d ific a tio n s to these b asic glasses are of com m ercial im portance. A longer w orking tim e in the cem ent and decreased water sen sitiv ity o f the set cement can be obtained by depleting the outer 10-100 fim of the pow der p articles o f ca lciu m by treatin g w ith acids such as hydrochloric a cid .4 T h e powdered glass can be blended or fused with metal powders such as silver, silver alloy, gold, platinum, or palladium. The resulting silver-fused and ground product, for example, is claimed to yield cement fillings with greater wear resistance than the calcium-depleted glasses.5
Liquid composition T he liquid com ponent of the original glass io n o m er cem en ts w as a q u eo u s polyacrylic acid. A low molecular weight was required to achieve a high concen tration w ith o u t g e la tio n . T h e latter problem was overcome by using an acrylic a cid -ita co n ic acid cop o ly m er. O ther materials in practical use are acrylic acidmaleic acid67 and acrylic acid-3-butene1,2,3 tricarboxylic acid.8 T he introduction of d ica rb o x y lic or tricarb oxylic acids (Fig 1) into the polymer chain not only prevents gelation of the liquid but also provides greater reactivity because of the increased number of carboxyl groups per chain unit and the higher acidity. Increased chain crosslin k in g probably also occurs in the final set cement, leading to better physical properties. Higher m olecular w eight copolymers improve physical properties of cements,'911 and the lim itation of higher viscosity may be partly overcome by incorporating the dried polyacrylic acid or copolymer p ow der w ith the g la ss to p rod uce a material which is mixed with water or a tartaric acid solution. Setting and polymer modifiers T he viscosity, rate of setting, and initial and final properties of the cem ent are d eterm ined as fo llo w s: a d ju stm ent of the glass com position and powder particle size; and the p o ly a cid co m p o sitio n , m o lecu la r w eig h t, d istrib u tio n , and concentration. The water content of the
ch
2
1
1 CH-COOH
Acrylic acid unit
CH ,C O O H
K c
Itaconic acid unit x
ch
c h 2c o o h
2
1 CH-COOH
|
Maleic acid unit
1 CH-COOH F ig 1 ■ S tru c tu re of m onom er u n its in poly carboxylic acid polym ers in glass ionom er cement liquids.
reaction is in itia te d on m ix in g but proceeds slow ly and is overlaid by the ligh t-activated p olym erization m echa nism . In systems in w hich part of the water in the acid-base system is replaced by monomers or polymers adequate water must be available for hydration for early setting. Setting reaction considerations T h e settin g reaction o f the cem ent is com plex and may vary with composition. In general, it is represented (Fig 2) as an acid-base reaction between the p oly acid liquid and the glass in which calcium and alum inum ions are released by attack on the surface of the glass particles and ultimately crosslink the polyacid chains in to a n etw o rk .1 T h is io n release is facilitated by tartaric acid which readily forms complexes with these ions, and
Fig 2 ■ Setting reac tion of glass ionom er cement (Adapted from W ilson, AD and M cLean, JW . Glass Ionomer Cement, 1988. C ourtesy of Q u in tessence).
in turn forms a molecular structure with the polyacrylate chains.19 Initial setting (gelation) is regarded as result of chain en ta n g le m e n t as w ell as w eak io n ic cr o sslin k in g w h ich corresponds w ith the viscoelastic behavior of the freshly set m aterial. As the cem ent m atures d uring the first 24 hours and beyond, progressive crosslinking occurs (possibly w ith hydrated alu m in u m ions) as the sensitivity to moisture of the set cement decreases and the percentage of bound water and glass transition temperature increases.1 T h e final set structure is a com plex com posite of the original glass particles sheathed by a siliceous hydrogel and bonded by a matrix phase consisting o f hydrated flu o rid a ted ca lciu m and alum inum polyacrylates (Fig 3). T h e stru cture o f the lig h t-cu red polymer-reinforced cement is a sim ilar com posite of glass particles and hydrogel matrix. T he latter is assumed to be an interpenetrating polymer network con sistin g of the io n ic metal polyacrylate h ydrogel en ta n g led w ith polyhyd roxyethyl m ethacrylate hydrogel. H yd ro p h ilic and hydrophobic d om ains may be present. T h e rate o f s e ttin g depends on the glass powder com position and particle size, the liq u id com p osition in clu d in g the tartaric acid co n cen tra tio n , the p ow d er liq u id ra tio and the se ttin g mechanisms involved. Commercial materials O rigin ally, glass ionom er cem ent was used as a C lass V lesio n restorative material. As a result of the developments in fo r m u la tio n p rev io u sly d iscu ssed , cements have becom e available for res toration, cavity lining, cementation, and preventive applications. The early com
Powder M a trix A lum in u m , c a lc iu m so diu m s a lts
Smith : COMPOSITION OF GLASS IONOM ER CEMENTS ■ 21
GLASS
cem ent is im portant to hydration and com pletion of the setting reaction.1112 V arious salts affect the se ttin g rate of p o ly ca rb o x y la te cem en ts, but the positive isomer of tartaric acid is unique in both p ro lon gin g w orking tim e and increasing the setting rate.11* About 10% of this form of tartaric acid is incorporated in most if not all ionomer compositions. Sm ith14 suggested the need for tougher materials with reduced water sensitivity. A pp arently, less b rittle m aterials can be produced by the in situ polymerization of w a ter-so lu b le p olym ers or by the addition of water-soluble or compatible polym ers to the glass ionom er cement com position.16’16 In this work, monomers such as h yd roxyeth yl m ethacrylate (HEMA) were incorporated with chemical initiator systems or visible light-curing systems, or both, based on camphorquin o n e /te rt-a m in o m ethacrylate. T h u s, such hybrid materials have a dual-setting mechanism involving the acid-base reac tion of the polyacid w ith the glass as well as the polymerization reaction. In co rp o ration o f w ater-solu b le or compatible polymers into glass ionomer cem ents a lo n e or w ith p olym erizab le system s in v olves the form ation of an interpenetrating polymer network com b in in g acid-base crosslink in g reaction of metal ion-polyacid with crosslinking polym erization of m onom er system or additive action of polymers. Antonucci and Stansbury17 found that w ith both p olym erizab le system s and added polym ers products w ith higher strengths cou ld be obtained. T h e test specimens still showed brittle-type frac ture but some plastic deform ation was observed. T w o light-cured hybrid glass ionomer systems are com m ercially available. In Vitrabond (3M Co), the pow der co m p o n en t is co m p osed p rim arily o f a radiopaque fluoroalum ino-silicate glass pow der c o n ta in in g a p h otosen sitizer. T h e liq u id is an aq ueous solu tio n of a polyacrylic acid copolymer containing pendent m ethacrylate groups together with approximately 10% of HEMA and a p h o to in itia to r . T h e HEM A aids in the solubilization of the copolymer. In the second system called X R -Ionom er (Kerr M anufacturing Co), the powder com p onent is a calciu m alu m in oflu oro silica te g la ss, and the liq u id is a polymerizable polyacid copolym er with a p h o to in itia to r . T h is is p robably a polyacrylic acid h avin g grafted m eth acrylate grou p s on the c h a in .18 B oth materials are dual-cured: the acid-base
IONOMERS
JAD)A
GLASS
IONOMERS
jm > A mercial materials used sim ilar com p o sition s for glass and polyacid, respec tively, w ith m o d ific a tio n s in particle size and concentration, respectively, for the different applications. In recent years, specifically designed m aterials for the various clinical applications have become available. Thus glasses used for cement in g a p p lic a tio n s m ay be op al for im proved properties su ch as strength and radiopacity, sin ce translucence is not as critical com pared to restorative m aterials.1 Therefore, the com position of commercial materials may vary even w ith in a g iv e n cla ss, and in d iv id u a l evaluation is necessary as there may be a wide variation in properties. Because of the differences in dispensing systems, predispensed materials that are m ixed m ec h a n ica lly m ay have m ore favorable and more consistent component ratios, resulting in better and uniform performance.20 O n ly lim ited in fo rm ation has been published on the com position of com mercial materials. Analytical data have been given by Brune and Smith,21 Ban and others22 and W ilson and M cLean.1 This information suggests that the glass com positions are similar to the calcium fluoroalum inosilicate described by W il son and McLean1 (Table 1) with fluoride contents in the range 10%-16%-weight. As m en tion ed earlier, however, recent materials may contain strontium in place of calcium (Base-Line, Caulk), barium, or lanthanum and silver, (Ketac-Silver, ESPE). T an n ic acid is also present in one brand by Shofu. T he liquids may be 55%-60% solutions of copolymers of polyacrylic acid with itaconic or maleic acid c o n ta in in g p o sitiv e tartaric acid and other setting modifiers as described p rev io u sly . In the case of w ater-m ix m aterials, freeze-dried, or precipitated polyacrylic acid (Base-Line and ChemFil II, L. D. Caulk) or poly (acrylic acidmaleic acid) copolym er powder (KetacCem, ESPE) is incorporated in the powder com ponent (Ketac-Cem, ESPE). In KetacFil (ESPE) polym aleic acid is present.23 T h e silver-m od ified m aterial KetacSilver developed by McLean and Gasser5 may be used for several applications such as a restorative material and as a lining and core b u ild -u p m aterial. However, the different applications require different physical properties. Specific comments on the relations between physical and biological properties and clinical appli-
22 ■ JA DA, Vol. 120, Ja n u a ry 1990
;[m
m
M
Glass core
¡¡¡ III p ll
Siliceous hydrogel
□
Hydrogel matrix
■
Fig 3 ■ Structure of set glass ionom er cement (Adapted from W ilson, AD and McLean, JW . G lass io n o m er cem ent, 1988. C ourtesy of Quintessence).
cation have been made by Knibbs24 (1988), Mount23 and McLean and W ilson.1 These ob servations in d icate that a m on g the p rop erties that may be im p o rta n t to clin ica l practice are b iocom patib ility; w o rk in g and se ttin g characteristics; mechanical properties; hydration, leach in g , and d isso lu tio n characteristics; a d h esion to en am el and d entin; and esthetics including opacity. Summary T here are few w ell-esta b lish ed corre lations between in vitro properties and clinical performance of cements.25 Supe rior p erform an ce in the a p p lica b le properties listed may indicate a superior material. Adequate controlled clin ical trial data for glass ionomer cements as op p osed to an ecd o ta l ex p erien ce is b egin n in g to becom e available, and it is from this experience that proper criteria for assessment must be developed. -----------------------J1SOA ----------------------Publication of names of products in this article or in any o th er article pub lish ed in this special section devoted to glass ionomer cements does not imply endorsement of the product by the American Dental Association. Dr. Smith is a professor and head of the department of biomaterials at the University of Toronto, Canada. Address re p rin t requests to Dr. Sm ith, professor and head, departm ent of biom aterials, University of Toronto, 124 Edward St, Toronto, Ontario M5G 1G6, Canada. 1. Wilson AD, McLean JW. Glass-ionomer cement. Chicago: Quintessence 1988. 2. Wilson AD, Kent BE. T he glass-ionomer cement.
A new translucent dental filling material. J Appl Chem Biotech 1971,21:313. 3. Prosser H J, Powis DR, Brant P, W ilson AD. T he characterisation of glass ionom er cem ents 7. The physical properties of current materials. J Dent 1984;12:231-40. 4. Schm itt W, P urrm ann R, Jochum P, Zahler W-D, G rim m -L enz R. M ixing liq u id for silicate cements. U.S. Patent no. 3,906,998, 1976. 5. McLean J W, Gasser O. Powdered dental material and process for the preparation thereof. US Patent no. 4,527,979, 1985. 6. Tezuka C, Karasawa. Setting solution for dental glass ionom er cements. US P atent no. 4,089,830, 1978. 7. Schm itt W, P urrm ann R, Jochum P, Gasser O. M ix in g co m p o n en t for dental glass io n o m er cements. US Patent no. 4,360,605, 1982. 8. S uzuki N. D ental cem ent c o m p o sitio n , US Patent no. 3,962,267, 1976. 9. Smith DC. A review of the zinc polycarboxylate cements. J Can Dent Assoc 1971;37:22-30. 10. Wilson AD, Crisp S, Abel G. Characterization of glass ionom er cem ents 4. Effect of m o lecu lar w eight on physical properties. J Dent 1977;5:11720 11. Prosser H J, Powis DR, W ilson AD. Glassionom er cements of im proved flexural stren g th . J Dent Res 1986;65:146-8. 12. Wilson AD, Prosser HJ. Aluminosilicate dental cem ents. In: S m ith DC, W illiam s DF, eds. Biocompatibility of dental materials. Boca Raton, Fla.: CRC Press 1981;41-77. 13. H ill R G , W ilson AD. A rheo lo g ical study of the role of additives on the setting of glass ionomer cements. J Dent Res 1988;67:1446-50. 14. Sm ith DC. Glass ionomer cements. In: Kawahare H, ed. Proceedings of the international congress of im plantology and biom aterials in stomatology. Tokyo: Ishiyaku;1980;26-54. 15. M cK inney JE , A ntonucci JM . W ear and m icrohardness of two experim ental d en tal co m posites. J Dent Res 1986;66:1134-9. 16. A ntonucci JM. Form ulation and evaluation of resin modified glass ionomer cements. T ransactions 13th A n n u al M eeting. New York: Society of Biomaterials; 1987;225. 17. Antonucci JM, Stansbury JW. Polymer m od ified glass ionom er cements. J Dent Res [Abstract no. 68] 1989;68:555. 18. Jandourek H. C om position and m ethod for im p ro v in g adherence of po ly m eric m aterials to substrates. US Patent no. 3,872,047, 1975. 19. N ich o lso n JW , B rookm an P J, Lacy OM, Wilson AD. Fourier transform infrared spectroscopic study of the role of tartaric acid in glass ionomer cements. J Dent Res 1988;67:1451-4. 20. W ong T C C , B ryant RW. G lass io n o m er cem ents: d isp e n sin g and stren g th . A ust J D ent 1985;30:336-40. 21. B rune D, S m ith DC. M ic ro stru ctu re and strength of properties of silicate and glass ionomer cements. Acta Odontol Scand 1982;40:389-96. 22. Ban S, Hasegawa J. Heat of polym erization of dimethacrylate monomers studied by isothermal DSC measurement. Dent Mater J 1984;3:85-92. 23. M ount GS. Glass ionom er cements in gerodontics. A status report for the American Journal of Dentistry. Am J Dent 1988;1:123-8. 24. Knibbs P J. Glass ionom er cement: 10 years of clinical use. J Oral Rehabil 1988;15:103-15. 25. S m ith DC. D ental cem ents. Adv D ent Res 1988;2:134-41.
.
GLASS
IONOMERS
CURRENT
A V
CLINICAL
PRACTICE
Glass ionom er cem ent m aterials are currently a va ila b le fo r restoration, caxrity lining an d basing, luting, an d preventive applications. T he in vitro perform ance is a function o f com position , m a n ipu lation , an d placem ent. In general, the cem ents m ay be characterized as strong, stiff, hard m aterials th at are adhesive to calcified tissue, have low toxicity, an d are p oten tially anticariogenic through fluoride leaching.
Composition and characteristics of glass ionomer cements Dennis C. Smith, MSc, DSc, PhD , FRSC
lass io n o m er cem en ts were developed in the early 1970s by com bining the strength, rigid ity, and fluorid e release properties of a silicate glass powder with the biocom p a tib ility and adhesive characteristics o f a p o ly a cry lic acid liq u id .1 D u rin g the past decade, changes have been made in both the glass pow der co m p o n en t and the polycarboxylic acid liquid. Most of the research and development on cem en ts has con cern ed m aterials comprised of a calcium fluoro-aluminosilica te glass pow der and an aq u eou s so lu tion of a polyacrylic acid-itaconic acid/copolym er containing tartaric acid. M odifications in both com ponents have been made in various brands for both patent and practical reasons. As a result, there are sign ifican t differences in the com position and properties of commer cial materials for the various applications.
G
Material composition Glass composition The original ion leachable glasses were based on an S i0 2-A103-CaF2-A lP 04N a 3A lF 6 c o m p o sitio n .2 A ccord in g to W ilson and McLean,1 the Al203/S i0 2 ratio is required to be 1:2 or more, and the fluoride content can be up to 23%. These g la sses m ay be o p a q u e, c o n ta in in g fluorite (CaF2) or corundum (A1203). Little work has been p ublished on the com p o sitio n and structure of the glasses, but recent materials contain more sodium and less fluoride (Table l) .1 Materials 20 ■ JADA, Vol. 120, Ja n u a ry 1990
for tooth restoration may use clear rather than opal glasses to provide improved translucency, although this may result in in trin sically low er strength.3 Radiopacity may be achieved by incorporation of stron tiu m (Sr), barium (Ba), or lanthanum (La), by fusing silver to the
Table 1 . Com position of glass ionomer. Component Calcium Sodium Aluminum Fluoride Phosphorus Silicon Oxygen
A 8
Percent B 10
5
2
16 13 1 20 37
14 13 3 16 35
C 7 7 16 10 5 13 42
(Adapted from Wilson, AD and McLean, JW. Glassionomer cement. 1988. Courtesy of Quintessence).
glass, or m ix in g in zinc o x id e or zir conium oxide. T w o m o d ific a tio n s to these b asic glasses are of com m ercial im portance. A longer w orking tim e in the cem ent and decreased water sen sitiv ity o f the set cement can be obtained by depleting the outer 10-100 fim of the pow der p articles o f ca lciu m by treatin g w ith acids such as hydrochloric a cid .4 T h e powdered glass can be blended or fused with metal powders such as silver, silver alloy, gold, platinum, or palladium. The resulting silver-fused and ground product, for example, is claimed to yield cement fillings with greater wear resistance than the calcium-depleted glasses.5
Liquid composition T he liquid com ponent of the original glass io n o m er cem en ts w as a q u eo u s polyacrylic acid. A low molecular weight was required to achieve a high concen tration w ith o u t g e la tio n . T h e latter problem was overcome by using an acrylic a cid -ita co n ic acid cop o ly m er. O ther materials in practical use are acrylic acidmaleic acid67 and acrylic acid-3-butene1,2,3 tricarboxylic acid.8 T he introduction of d ica rb o x y lic or tricarb oxylic acids (Fig 1) into the polymer chain not only prevents gelation of the liquid but also provides greater reactivity because of the increased number of carboxyl groups per chain unit and the higher acidity. Increased chain crosslin k in g probably also occurs in the final set cement, leading to better physical properties. Higher m olecular w eight copolymers improve physical properties of cements,'911 and the lim itation of higher viscosity may be partly overcome by incorporating the dried polyacrylic acid or copolymer p ow der w ith the g la ss to p rod uce a material which is mixed with water or a tartaric acid solution. Setting and polymer modifiers T he viscosity, rate of setting, and initial and final properties of the cem ent are d eterm ined as fo llo w s: a d ju stm ent of the glass com position and powder particle size; and the p o ly a cid co m p o sitio n , m o lecu la r w eig h t, d istrib u tio n , and concentration. The water content of the
ch
2
1
1 CH-COOH
Acrylic acid unit
CH ,C O O H
K c
Itaconic acid unit x
ch
c h 2c o o h
2
1 CH-COOH
|
Maleic acid unit
1 CH-COOH F ig 1 ■ S tru c tu re of m onom er u n its in poly carboxylic acid polym ers in glass ionom er cement liquids.
reaction is in itia te d on m ix in g but proceeds slow ly and is overlaid by the ligh t-activated p olym erization m echa nism . In systems in w hich part of the water in the acid-base system is replaced by monomers or polymers adequate water must be available for hydration for early setting. Setting reaction considerations T h e settin g reaction o f the cem ent is com plex and may vary with composition. In general, it is represented (Fig 2) as an acid-base reaction between the p oly acid liquid and the glass in which calcium and alum inum ions are released by attack on the surface of the glass particles and ultimately crosslink the polyacid chains in to a n etw o rk .1 T h is io n release is facilitated by tartaric acid which readily forms complexes with these ions, and
Fig 2 ■ Setting reac tion of glass ionom er cement (Adapted from W ilson, AD and M cLean, JW . Glass Ionomer Cement, 1988. C ourtesy of Q u in tessence).
in turn forms a molecular structure with the polyacrylate chains.19 Initial setting (gelation) is regarded as result of chain en ta n g le m e n t as w ell as w eak io n ic cr o sslin k in g w h ich corresponds w ith the viscoelastic behavior of the freshly set m aterial. As the cem ent m atures d uring the first 24 hours and beyond, progressive crosslinking occurs (possibly w ith hydrated alu m in u m ions) as the sensitivity to moisture of the set cement decreases and the percentage of bound water and glass transition temperature increases.1 T h e final set structure is a com plex com posite of the original glass particles sheathed by a siliceous hydrogel and bonded by a matrix phase consisting o f hydrated flu o rid a ted ca lciu m and alum inum polyacrylates (Fig 3). T h e stru cture o f the lig h t-cu red polymer-reinforced cement is a sim ilar com posite of glass particles and hydrogel matrix. T he latter is assumed to be an interpenetrating polymer network con sistin g of the io n ic metal polyacrylate h ydrogel en ta n g led w ith polyhyd roxyethyl m ethacrylate hydrogel. H yd ro p h ilic and hydrophobic d om ains may be present. T h e rate o f s e ttin g depends on the glass powder com position and particle size, the liq u id com p osition in clu d in g the tartaric acid co n cen tra tio n , the p ow d er liq u id ra tio and the se ttin g mechanisms involved. Commercial materials O rigin ally, glass ionom er cem ent was used as a C lass V lesio n restorative material. As a result of the developments in fo r m u la tio n p rev io u sly d iscu ssed , cements have becom e available for res toration, cavity lining, cementation, and preventive applications. The early com
Powder M a trix A lum in u m , c a lc iu m so diu m s a lts
Smith : COMPOSITION OF GLASS IONOM ER CEMENTS ■ 21
GLASS
cem ent is im portant to hydration and com pletion of the setting reaction.1112 V arious salts affect the se ttin g rate of p o ly ca rb o x y la te cem en ts, but the positive isomer of tartaric acid is unique in both p ro lon gin g w orking tim e and increasing the setting rate.11* About 10% of this form of tartaric acid is incorporated in most if not all ionomer compositions. Sm ith14 suggested the need for tougher materials with reduced water sensitivity. A pp arently, less b rittle m aterials can be produced by the in situ polymerization of w a ter-so lu b le p olym ers or by the addition of water-soluble or compatible polym ers to the glass ionom er cement com position.16’16 In this work, monomers such as h yd roxyeth yl m ethacrylate (HEMA) were incorporated with chemical initiator systems or visible light-curing systems, or both, based on camphorquin o n e /te rt-a m in o m ethacrylate. T h u s, such hybrid materials have a dual-setting mechanism involving the acid-base reac tion of the polyacid w ith the glass as well as the polymerization reaction. In co rp o ration o f w ater-solu b le or compatible polymers into glass ionomer cem ents a lo n e or w ith p olym erizab le system s in v olves the form ation of an interpenetrating polymer network com b in in g acid-base crosslink in g reaction of metal ion-polyacid with crosslinking polym erization of m onom er system or additive action of polymers. Antonucci and Stansbury17 found that w ith both p olym erizab le system s and added polym ers products w ith higher strengths cou ld be obtained. T h e test specimens still showed brittle-type frac ture but some plastic deform ation was observed. T w o light-cured hybrid glass ionomer systems are com m ercially available. In Vitrabond (3M Co), the pow der co m p o n en t is co m p osed p rim arily o f a radiopaque fluoroalum ino-silicate glass pow der c o n ta in in g a p h otosen sitizer. T h e liq u id is an aq ueous solu tio n of a polyacrylic acid copolymer containing pendent m ethacrylate groups together with approximately 10% of HEMA and a p h o to in itia to r . T h e HEM A aids in the solubilization of the copolymer. In the second system called X R -Ionom er (Kerr M anufacturing Co), the powder com p onent is a calciu m alu m in oflu oro silica te g la ss, and the liq u id is a polymerizable polyacid copolym er with a p h o to in itia to r . T h is is p robably a polyacrylic acid h avin g grafted m eth acrylate grou p s on the c h a in .18 B oth materials are dual-cured: the acid-base
IONOMERS
JAD)A
GLASS
IONOMERS
jm > A mercial materials used sim ilar com p o sition s for glass and polyacid, respec tively, w ith m o d ific a tio n s in particle size and concentration, respectively, for the different applications. In recent years, specifically designed m aterials for the various clinical applications have become available. Thus glasses used for cement in g a p p lic a tio n s m ay be op al for im proved properties su ch as strength and radiopacity, sin ce translucence is not as critical com pared to restorative m aterials.1 Therefore, the com position of commercial materials may vary even w ith in a g iv e n cla ss, and in d iv id u a l evaluation is necessary as there may be a wide variation in properties. Because of the differences in dispensing systems, predispensed materials that are m ixed m ec h a n ica lly m ay have m ore favorable and more consistent component ratios, resulting in better and uniform performance.20 O n ly lim ited in fo rm ation has been published on the com position of com mercial materials. Analytical data have been given by Brune and Smith,21 Ban and others22 and W ilson and M cLean.1 This information suggests that the glass com positions are similar to the calcium fluoroalum inosilicate described by W il son and McLean1 (Table 1) with fluoride contents in the range 10%-16%-weight. As m en tion ed earlier, however, recent materials may contain strontium in place of calcium (Base-Line, Caulk), barium, or lanthanum and silver, (Ketac-Silver, ESPE). T an n ic acid is also present in one brand by Shofu. T he liquids may be 55%-60% solutions of copolymers of polyacrylic acid with itaconic or maleic acid c o n ta in in g p o sitiv e tartaric acid and other setting modifiers as described p rev io u sly . In the case of w ater-m ix m aterials, freeze-dried, or precipitated polyacrylic acid (Base-Line and ChemFil II, L. D. Caulk) or poly (acrylic acidmaleic acid) copolym er powder (KetacCem, ESPE) is incorporated in the powder com ponent (Ketac-Cem, ESPE). In KetacFil (ESPE) polym aleic acid is present.23 T h e silver-m od ified m aterial KetacSilver developed by McLean and Gasser5 may be used for several applications such as a restorative material and as a lining and core b u ild -u p m aterial. However, the different applications require different physical properties. Specific comments on the relations between physical and biological properties and clinical appli-
22 ■ JA DA, Vol. 120, Ja n u a ry 1990
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Glass core
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Siliceous hydrogel
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Hydrogel matrix
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Fig 3 ■ Structure of set glass ionom er cement (Adapted from W ilson, AD and McLean, JW . G lass io n o m er cem ent, 1988. C ourtesy of Quintessence).
cation have been made by Knibbs24 (1988), Mount23 and McLean and W ilson.1 These ob servations in d icate that a m on g the p rop erties that may be im p o rta n t to clin ica l practice are b iocom patib ility; w o rk in g and se ttin g characteristics; mechanical properties; hydration, leach in g , and d isso lu tio n characteristics; a d h esion to en am el and d entin; and esthetics including opacity. Summary T here are few w ell-esta b lish ed corre lations between in vitro properties and clinical performance of cements.25 Supe rior p erform an ce in the a p p lica b le properties listed may indicate a superior material. Adequate controlled clin ical trial data for glass ionomer cements as op p osed to an ecd o ta l ex p erien ce is b egin n in g to becom e available, and it is from this experience that proper criteria for assessment must be developed. -----------------------J1SOA ----------------------Publication of names of products in this article or in any o th er article pub lish ed in this special section devoted to glass ionomer cements does not imply endorsement of the product by the American Dental Association. Dr. Smith is a professor and head of the department of biomaterials at the University of Toronto, Canada. Address re p rin t requests to Dr. Sm ith, professor and head, departm ent of biom aterials, University of Toronto, 124 Edward St, Toronto, Ontario M5G 1G6, Canada. 1. Wilson AD, McLean JW. Glass-ionomer cement. Chicago: Quintessence 1988. 2. Wilson AD, Kent BE. T he glass-ionomer cement.
A new translucent dental filling material. J Appl Chem Biotech 1971,21:313. 3. Prosser H J, Powis DR, Brant P, W ilson AD. T he characterisation of glass ionom er cem ents 7. The physical properties of current materials. J Dent 1984;12:231-40. 4. Schm itt W, P urrm ann R, Jochum P, Zahler W-D, G rim m -L enz R. M ixing liq u id for silicate cements. U.S. Patent no. 3,906,998, 1976. 5. McLean J W, Gasser O. Powdered dental material and process for the preparation thereof. US Patent no. 4,527,979, 1985. 6. Tezuka C, Karasawa. Setting solution for dental glass ionom er cements. US P atent no. 4,089,830, 1978. 7. Schm itt W, P urrm ann R, Jochum P, Gasser O. M ix in g co m p o n en t for dental glass io n o m er cements. US Patent no. 4,360,605, 1982. 8. S uzuki N. D ental cem ent c o m p o sitio n , US Patent no. 3,962,267, 1976. 9. Smith DC. A review of the zinc polycarboxylate cements. J Can Dent Assoc 1971;37:22-30. 10. Wilson AD, Crisp S, Abel G. Characterization of glass ionom er cem ents 4. Effect of m o lecu lar w eight on physical properties. J Dent 1977;5:11720 11. Prosser H J, Powis DR, W ilson AD. Glassionom er cements of im proved flexural stren g th . J Dent Res 1986;65:146-8. 12. Wilson AD, Prosser HJ. Aluminosilicate dental cem ents. In: S m ith DC, W illiam s DF, eds. Biocompatibility of dental materials. Boca Raton, Fla.: CRC Press 1981;41-77. 13. H ill R G , W ilson AD. A rheo lo g ical study of the role of additives on the setting of glass ionomer cements. J Dent Res 1988;67:1446-50. 14. Sm ith DC. Glass ionomer cements. In: Kawahare H, ed. Proceedings of the international congress of im plantology and biom aterials in stomatology. Tokyo: Ishiyaku;1980;26-54. 15. M cK inney JE , A ntonucci JM . W ear and m icrohardness of two experim ental d en tal co m posites. J Dent Res 1986;66:1134-9. 16. A ntonucci JM. Form ulation and evaluation of resin modified glass ionomer cements. T ransactions 13th A n n u al M eeting. New York: Society of Biomaterials; 1987;225. 17. Antonucci JM, Stansbury JW. Polymer m od ified glass ionom er cements. J Dent Res [Abstract no. 68] 1989;68:555. 18. Jandourek H. C om position and m ethod for im p ro v in g adherence of po ly m eric m aterials to substrates. US Patent no. 3,872,047, 1975. 19. N ich o lso n JW , B rookm an P J, Lacy OM, Wilson AD. Fourier transform infrared spectroscopic study of the role of tartaric acid in glass ionomer cements. J Dent Res 1988;67:1451-4. 20. W ong T C C , B ryant RW. G lass io n o m er cem ents: d isp e n sin g and stren g th . A ust J D ent 1985;30:336-40. 21. B rune D, S m ith DC. M ic ro stru ctu re and strength of properties of silicate and glass ionomer cements. Acta Odontol Scand 1982;40:389-96. 22. Ban S, Hasegawa J. Heat of polym erization of dimethacrylate monomers studied by isothermal DSC measurement. Dent Mater J 1984;3:85-92. 23. M ount GS. Glass ionom er cements in gerodontics. A status report for the American Journal of Dentistry. Am J Dent 1988;1:123-8. 24. Knibbs P J. Glass ionom er cement: 10 years of clinical use. J Oral Rehabil 1988;15:103-15. 25. S m ith DC. D ental cem ents. Adv D ent Res 1988;2:134-41.
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