JADA CLINICAL
PRACTICE
GLASS
Cermet ionom er cements are sintered m eta l/g la ss powders, which can be m ade to react w ith poly(acids). These new cements are significantly more resistant to abrasion than regular glass ionom er cements an d are w idely accepted as core build-up m aterials an d lin in g cements. They can strengthen teeth an d provide the clinician w ith an o p p o rtu n ity to treat early dental caries.
Cermet cements
John W. McLean, OBE, FDS, MDS, DSc, D O dont
he main criticisms leveled at the glass ionom er cem ents by c li n icia n s are ch iefly concerned w ith b rittlen ess, poor surface p o lish , porosity, and surface wear. Improvements in these areas are essential if the clinical use of glass ionomer cements is extended to high-stress bearing areas. T he incorporation of a disperse-phase such as corundum (A120 3) into the glass can double the flexural strength of the glass ionom er cem ent.1 However, little im p rovem en t in wear resistan ce was obtained. The incorporation of alumina fibers (Saffil, ICI Runcorn) also increased the flexural strength of the cement but resulted in decreased abrasion resistance.2 Seed and W ilson2 investigated the effect of incorporating metal fibers or powders into the glass ionomer powder. Flexural strengths were increased but abrasion resistance was poor, probably because of lack of strong bonding between the metal filler and the polyacrylate matrix. T h e p rob lem of o b ta in in g strong bonding of metal fillers to glass ionomer powders was solved by sintering the metal powder in to the glass p ow d er.s-4 Fine precious m etal powders such as silver or gold were intimately mixed in equal volume with a reactive glass powder and compressed in a pelletizer at pressures above 350 MPa (megapascal). The pelle tiz in g cham ber was evacu ated at a pressure of 100 MPa. T he com pressed m etal glass pow der p ellets were then fused at temperatures about 800 C. Strong
IONOMERS
CURRENT
T
Fig 1 ■ Electron m icrograph of silver-cerment powder (orig mag *400).
b ond ing of the m etal glass com posite resulted in the formation of a “cermet” (ceramic-metal) in which, unlike simple mixtures of metal and glass powder, the metal became part of the glass powder, and the strength of the bond was com parable to that of fu sin g porcelain to gold. Silver cermet cem ents have been developed for clinical use in which pure silver powder of about 3.5 /um average particle size is used. Im provem ents in color have been made by the incorpo ration of up to 5% by weight of titanium dioxide in the glass powder. Interestingly, after g rin d in g the fired cerm et p ellet to a powder, rou n d in g of the powder particles occurred, probably caused by the lubricating effect of the silver (Fig 1). T h is resu lted in better p a ck in g densities, less porosity, and easier m ix ing.3 Cermet powders can be made to
react w ith p oly(acid s) to form cerm et ionomer cements. Currently, two types of silver cermet ionom er cem ents are available, one for handm ixing (Chelon-Silver, ESPE) and the other for a u to m a tic m ix in g in capsules (Ketac-Silver, ESPE). The capsu lated m a teria l, a cco rd in g to sales demand, is the most popular despite some criticism that in su fficien t q u an tity of m aterial is availab le for core b u ild u p and som e variability in consistency of m ix can occur. T he capsulated material, K etac-Silver, can be injected in to the tooth from the capsule. However, it is still possible to inject the hand-m ixed m aterial, C h elo n -S ilv er, if the m ixed paste is loaded in a syringe (C entrix, Centrix Inc). Discussion T h e capsulated silver cermet ionom er K etac-Silver has better resistan ce to abrasion than regular glass ionom ers, and in a sim ulated occlusal wear test, it was close to that of am algam alloy and m icrofilled com posite resin (Table l ) .5 M cK inney and others6 also found Ketac-Silver more resistant to abrasion than regular glass ionomer cements, but the surface still showed some incidence of fa ilu re from b rittle fracture. T h ey postulated that the improved wear resis tance could be caused by the lubricating effect of the silver. McLean and Gasser3 suggested that the sintered silver powder JA DA, Vol. 120, Jan u ary 1990 ■ 43
J !0 )A \
GLASS
IONOMERS
Tooth surface preparation Table 1 ■ M a te ria l loss in s im u la te d occlusal w ear.* Amalgam alloy Microfilled composite resin Conventional composite resin Glass-ionomer Type II cement Ketac-Silver cermet ionomer
0.2 mm3 0.2 mm3 0.4 mm3 6.0 mm3 0.3 m m3
•M o o re BK, Sw artz ML, P h illip s RW. A brasion resistance of metal reinforced glass ionom er cements. J Dent Res [Abstract no. 1766] 1985;64:371.
was lowering the coefficient of friction. A lth ou gh the resistance to abrasion of the cermet ionomers has resulted in a considerable im provem ent in clinical perform ance,7 their flexu ral strengths and fracture toughness are insufficient to allow their use in stress-bearing cavities such as a large C lass II lesion . T h e flexural strength of the cermet ionomers is better than m ost com m ercial glass io n o m er cem en ts but, as yet, can n ot compare with the strength of composite resins and amalgam alloys (Table 2).
Cermet ionomer cements may fail adhe sively if the tooth surface is contaminated by c u ttin g debris, sa liv a , b lo o d , or p rotein aceous debris. Partial rem oval of the smear layer before the cement is a p p lied is e s se n tia l.7 R ecom m ended procedures are: (1) for devitalized teeth or sm all areas of cut d en tin , a 25% solution of polyacrylic acid is used for 10 seconds, washed off, and lightly dried; (2) for extensive areas of vital cut dentin, a 25% so lu tio n of tann ic acid is used for 30 seconds, lightly washed, and dried. Surface conditioning of the tooth before in jectin g Ketac-Silver sh ould be m an datory, as the cermet ionom ers do not w et the surface of the tooth as easily as the th in n er m ixed bonded liners. Overdried surfaces and mixes of cermet ionomer that are drying out w ill reduce wettability and decrease the chances of long-term bonding to dentin. Core buildup
Table 2 ■ F lexu ral s tre n g th of re s to rative m aterials. Flexural strength (MPa) Amalgam alloy Composite posterior resin Composite conventional resin Composite Microfil Glass ionom er Type II Glass ionom er Ketac-Silver
140 120-150 110-135 60-80 20 40 Yield strength (MPa)
Gold alloy
Ideally, cermet ionom ers are best used for core b u ild u p in posterior teeth in which at least 2 mm of coronal dentin is still intact. In a recent in vitro study by T a leg h a n i and L ein felder,8 s ig n if ica n tly few er cases o f root fracture occurred w hen K etac-Silver w as used in con ju n ctio n w ith en d oposts (ParaPosts, W haledent Int) com pared w ith cast posts and cores. In the latter case, all the specimens tested underwent root failure, whereas only 60% of the Ketac-
Silver specimens failed in this way. T he clinician is strongly advised not to rely solely on molecular bonding to tooth structure, as the strength of the bond is inadequate where high stresses occur. In the case of devitalized molars, at least two endoposts should be inserted at different angles and extended through the core b u ild u p (F ig 2). W here vital teeth are involved, accessory pin anchor age may be necessary. After the surface of the dentin is conditioned w ith 25% p o ly a cr y lic acid, the p osts sh o u ld be cemented with a standard glass ionomer Type I lu ting cement. Ketac-Silver may then be injected over the posts and tooth; and if necessary, a second m ix can be in jected in to a cop p er band lig h tly lubricated with a silicone separator (NonS tick , H ager and W erken) w h ich is pressed over the tooth (Fig 3). Before the tooth is prepared for a full veneer crown, the cement must be allow ed to set for at least 6 m inutes; ideally, the preparation sh ould be done at a su b seq u en t v isit. F in e d iam on d stones running under copious water spray are recommended for trimming the set cermet cement. Light pressure is used and the tooth is prepared under m agnification and fiberoptic lighting. If all stages of the foregoing operation are carried out carefully, then the cermet ionomer and rem aining tooth should form a m ono lith ic structure (Fig 4). M icroleakage w ill be reduced and the cement has the added advantage of releasing fluoride, w h ich p rovid es som e a n tic a rio g en ic action. Ketac-Silver may be used as an interim
350-500
Clinical use T h e m ain c lin ic a l use of the cerm et ionomer cements is as a dentin substitute. T h e clin ica l ap p lication s follow : core buildu p; lin in g s for in la y s, am algam restoration s, and p osterior com p osite resins; fissu re fillin g s; restoration of prim ary teeth; restoration o f Class II lesions using an internal occlusal fossa (tunnel) preparation; treatment of root caries; repair of defective metal margins in crow ns and inlays; retrograde root fillings; and sealing of bifurcation areas in periodontally involved molars. Before a cermet ionom er is applied, the tooth surface should be conditioned to improve wettability and adhesion. 44 ■ JADA, Vol. 120, Ja n u a ry 1990
Fig 2 ■ Diagram of correct use of Ketac-Silver in a core buildup. At least two Para-Posts should be inserted at different angles to retain cermet ionomer. Conservation of all rem aining dentin is essential to support the future restoration.
Fig 3 ■ A copper band filled with Ketac-Silver is pressed over the tooth and will assist in closely a d ap tin g the cerm et ionom er to the prepared dentin.
jm A direct com posite bonding or laboratory processed com posite inlays are contem plated. If the cermet ionom er is used in bulk as a dentin substitute, the volume of o v er la p p in g co m p o site is less and shrinkage is reduced. A cid -etch in g of K etac-Silver has also produced strong b o n d in g to co m p o site re sin s.10 If the cement is carried up to the margin, then it acts as a cariostatic seal and w ill protect the pulp against any accidental exposure to phosphoric acid during the etching procedure.
IONOMERS
restoration w hen fu ll-m o u th re h a b il itation is being carried out. The cermet ionomer will form a good seal with the tooth and allow secondary dentin for mation to take place before final tooth preparation for crowning.
Fig 4 ■ K etac-Silver core prepared on a de vitalized first premolar. The endopost has been extended to the surface of the cermet ionom er and m ore th a n 2 mm of rem ain in g d e n tin supports the cervical base.
Linings Cermet ionom ers are useful for lin in g amalgam alloy restorations. If the patient has a h ig h -caries in cid en ce, then the proximal box may be filled at the cervical to a depth of 2 mm and the am algam packed on top (Fig 5, 6). A thin appli cation of 25% polyacrylic acid may assist b o n d in g of the am algam to the glass ionomer.9 The good cervical seal of the cermet ionomer and its ability to leach fluorid e may prevent a further attack by caries. Where the esthetic appearance is not compromised, cermet ionomers can also be used to line posterior teeth in which
In Europe, using glass ionomer cements as fissure fillings has shown that these materials can have a major role in the co n tro l o f early ca r ie s.11’12 A fissure w idening procedure is recommended in which fine diamond points are used to w iden the fissure before any dentinal caries is removed with small round burs (F ig 7). After c o n d itio n in g the cavity w ith 25% polyacrylic acid, Ketac-Silver may be injected into the opened fissures and covered with foil or green wax during the 5 -m in u te se ttin g p eriod. Excess material may then be trimmed with small round burs before sealin g w ith a coat of b on d in g agent. Fissures filled with K etac-Silver are illu stra ted in Figure 8. A lthough the material is not toothcolored , the lig h t tita n iu m co lo r is unobtrusive and has the added advantage of form ing a long-term cariostatic seal not subject to corrosion . T h e wear resistance of Ketac-Silver is good and, w hen enclosed in a fissure, the cermet ionomer is not placed under high-tensile or shear stress.
F ig 7 ■ F ine d iam ond p o in ts used to w iden the occlusal fissure and any dentinal caries may be removed subsequently with sm all round burs.
Restoration of primary teeth T h e use of g la ss-io n o m er cem en ts in primary teeth is valuable in preventing recurrent caries and in protecting adjoin ing enamel surface in permanent teeth. F lu o rid e release from these cem en ts d u rin g 2 years is often su ffic ie n t to prevent early approxim al lesions from d ev elo p in g at a later date. C roll and P h illip s 13 have made extensive use of Ketac-Silver, for restoring primary teeth; they advocated its use in bulk to prevent fracture in narrow isthmuses. They also advised that restored m argin al ridges should be left slightly out of occlusion, and the basic p rin cip les o f “retention form” must be observed for best results. M echanical retention w ill enhance the success of an adhesively bonded glass ionomer cement restoration. Restoration of Class II lesions
Fig 5 ■ Disto-occlusal cavity in second premolar filled w ith Ketac-Silver. T he cermet ionom er has been carried u p the proximal box to a depth of 2 mm and the occlusal isthm us lined only th inly to allow space for the am algam alloy. T he m atrix band has been tightly wedged to ensure a good cervical seal.
Fig 6 ■ Amalgam alloy packed over set KetacS ilver after c o n d itio n in g the surface w ith polyacrylic acid.
T he destruction of marginal ridges during treatment of the Class II lesion is being challenged and considerable interest has been aroused in trea tin g a p p ro x im a l caries via an internal occlusal fossa or “ tu n n e l” approach.71416’17 T h e in itia l approach to the lesion should be made th ro u g h the o cc lu sa l fossa, a v o id in g where possible occlusal centric-holding stops. Entry is made to the soft lesion by a d ia g o n a l approach w ith a sm all round diam ond stone, leaving at least 2-mm w idth of m arginal ridge intact. T h e access ch a n n el m ay be w id en ed buccolingually to gain entry to the caries (F ig 9). M ag n ifica tio n and fib erop tic lighting are essential tools for this type of microcavity preparation. The proceMcLean : CERMET CEMENTS ■ 45
GLASS
Fissure fillings
the flu o rid e release from the cerm et ionomer can help control the spread of root caries.
Miscellaneous uses
*
s"
4 ,*
f e «
'
A
Fig 8 ■ F issure in m a n d ib u la r m olars filled with Ketac-Silver. After 5-7 years, no corrosion is seen. T he color of the cermet ionom er has also been maintained.
Fig 9 ■ Internal occlusal fossa preparation in m a n d ib u la r second m o lar. Access ch an n el widened buccolingually b ut a ll the m arginal ridge has been retained. O cclusal preparation in m andibular first molar (lower half of picture) has also been extended to restore approxim al lesion, which is clearly visible.
Fig 10 ■ D en tin of b o th m a n d ib u la r m olars restored with Ketac-Silver. A m atrix band should be applied before restoration to prevent excess m a te ria l e x tru d in g th ro u g h the a p p ro x im al enamel aperture.
Fig 1 1 b K etac-Silver lin in g s etched for 30 seconds w ith a 37% ph o sp h o ric acid solution an d a p o ste rio r co m p o site resin in se rted by sta n d ard procedure. T h e m a rg in a l ridge is stren g th en ed by th is p ro ced u re and avoids excessive wear of the composite resin.
GLASS
IONOMERS
JAD)A
dures for restoring the Class II lesion via a tunnel approach have been fully described.7 E ssentially the preparation should be regarded as an occlusal one in w h ich the a p p ro x im a l caries is removed via an internal approach and the d en tin restored w ith K etac-Silver (Fig 10). One investigation of tunnel restorations restored w ith 9 standard T yp e II glass io n o m er cem ent (K etac-F il, ESPE) revealed that the strength o f prepared teeth was increased to 92% of their sound valu e com pared w ith am algam resto ra tio n s, w h ich produced a sm aller, nonsignificant increase.18 However, Corey and Schulein,19 in another study found little difference between tunnel resto ration s restored w ith com p osite resin or am algam alloy, and both materials caused significant strengthening of the teeth compared with the marginal ridge stren gth of prepared teeth that were unrestored. Therefore, a composite resin may therefore be inserted over the Ketac 46 ■ JA DA, Vol. 120, Ja n u a ry 1990
-Silver lin in g using the acid-etch tech nique to strengthen the marginal ridge (Fig 11). Treatment of root caries Cermet ionom ers are useful m aterials for restoring areas of root caries in the p osterior reg io n . O ften , in the older patient, su bgin gival caries is d ifficult to treat, and problems of moisture control make placem ent of alloy or com posite restorations very difficult. Ketac-Silver is fa st-settin g and may be in jected subgingivally even where some moisture is present. If a metal or plastic matrix is used, the pressure exerted w ill adapt the cermet ionomer to the root face and avoid excessive salivary contam ination. R etention of the cermet ionom er w ill be im proved w ith slig h t m ech an ical reten tion but w here th is ca n n o t be achieved, bonding in a nonstress-bearing area is generally successful. In the older patient, when oral hygiene is often poor,
T h e cermet ionom ers are particularly u sefu l for rep a irin g d efective m etal margins in crowns and inlays. Providing the margins are thoroughly cleaned, if necessary with small round burs, mate rials such as Ketac-Silver w ill provide a long-term m arginal seal. In all cases in v o lv in g repair or recem en tation of crowns, surface conditioning of the tooth w ith polyacrylic acid is advised before injecting Ketac-Silver. Where crowns have loosened, as the result of further decay at the margins, it is possible to remove the carious dentin m echanically and recement the crown w ith K etac-Silver. T h e crow n sh ould be sandblasted internally and Ketac-Silver injected. Further m aterial sh o u ld be injected over the tooth and the crown seated by asking the patient to bite on a co tto n -w o o l pad. P ro tectio n of the m argins can be achieved by covering w ith a sheet o f soft green wax. After setting, any excess cement covering the exposed dentin may be trimmed w ith fine diamond stones running under water spray. A procedure that may be regarded as a last resort in v o lv es reattachm ent of a loose abutment crown on one end of a fixed p rosth esis w here en d o d o n tic treatment has p reviously been carried out. Often, rem aking of the prosthesis can be very exp en sive. As an interim procedure, p a rticu la rly in the older p a tien t, it is p o ssib le to reattach the crown using the follow ing procedure: —T h e en d odon tic access channel is w idened and the in tern al area of the crown is cleaned with round burs. —The root canal is reamed and a ParaPost is fitted to a su itable d im ension of 1- to 1- to 2-mm diameter. The ParaPost should be approximately two-thirds the length of the root canal and extent to the crown surface. At this stage, cement is not applied. —T h e canal is clea n ed w ith a 25% polyacrylic acid solution for 10 seconds and w ashed and dried. Paper p o in ts should be used to dry the apical areas. —A g la ss-io n o m er lu tin g cem ent (Ketac-Cem, ESPE) is m ixed as KetacCem and placed in to the canal with a spiral filler. The Para-Post is gently seated to position and a m ix of Ketac-Silver
Cermet ionom ers are now b ein g used for repair of perforated root canals or as retrograde root fillings.7 Bone healing has been sh ow n to take place after 7 months, and Jonck and others20 consider the glass ionomers to be bioactive and able to promote bone growth. Further work is necessary to confirm the use of these materials as a root canal sealant. Another interesting use for the cermet ionomers as sealants for molar bifurcation areas exposed in periodontal disease has been in vestigated (J. Zamet, personal com m unication, 1989). T he root bifur cation area is exposed su rgically and the roots planed. Any osseous surgery required should then be carried out, and the gin gival tissue apically p ositioned and allow ed to heal for 6 to 8 weeks. At this stage the root bifurcation area should be conditioned with polyacrylic acid for 10 seconds before injecting the cermet ionomer cement into the bifur cation area to seal the open channel. After setting, the cement may be smoothed level w ith the root face and protected w ith a coat of varnish or preferably a light-cured bonding agent. Summary Cermet ionomer cements are sintered
m etal/glass powders, which can be made to react w ith p oly (a cid s). T h ese new cements are significantly more resistant to abrasion than regular glass ionomer cements and have found wide acceptance as core b uild-up m aterials and lin in g cem en ts. T h eir use in C lass I and II restorations is dependent on the adoption by the dental profession of new concepts in cavity preparation in v olvin g micro, not macro, cutting of teeth. These new materials can strengthen teeth and prevent cusp splitting later as the marginal ridges of teeth are preserved. The glass ionomer and composite resin laminate technique, when used in posterior teeth, harnesses the best properties in both materials and provides the clin icia n with an op por tunity to treat early dental caries, without resorting to extensive cavity preparation or the use of am algam a llo y s. T h e an tica rio g en ic properties of the glass ionomer cements, from fluoride leaching, can maintain the integrity of approximal enam el and give long-term protection to the enamel cap.
------------------- J W A ------------------Dr. McLean is developer of the cermet ionomer cements and is currently clinical consultant at the Laboratory of the G overnm ent Chemist E ngland and senior research fello.v at Eastman Dental Hospital in London. Address reprint requests to Dr. McLean at 38 Devonshire St., London W IN 1LD, England. 1. P rosser H J, Pow is DR, W ilson AD. G lass ionom er cem ents of im proved flexural strength. J Dent Res 1986;65:146-8. 2. Seed IR, W ilson AD. P oly(carboxylic acid) hardenable compositions. Br Pat Appln 2, 1980;028855A. 3. McLean JW , Gasser O. Glass cermet cements. Quintessence 1985;16:333-43. 4. M cLean JW , G asser O. P ow dered denial m aterials and process for the preparation thereof. US Pat No 4,527,979. 1985. 5. Moore BK, Swartz ML, Phillips RW. Abrasion resistance of metal reinforced glass ionomer cements.
J Dent Res [Abstract no. 1766] 1985;64:371. 6. McKinney JE, Antonucci JM, R upp NW. Wear and microhardness of a metal filled ionomer cement. J Dent Res [Abstract no. 1577] 1986;65:344. 7. Wilson AD, McLean JW. Glass ionomer cement. Berlin: Quintessence: 1988. 8. T aleg h an i M, L einfelder KF. E valu atio n of a new glass-ionom er cement w ith silver as a core b u ild -u p under a cast resto ratio n . Q uintessence 1988,19:19-24. 9. Warren JA, Soderhold KJM. Bonding amalgam to glass ionomer with PAA. Dent Mater 1988;4:191 -
6.
10. Joynt RB, Williams D, Davis EL, et al. Effect of etching time on surface morphology and adhesion of a p o ste rio r resin to glass io n o m er cem ent. J Prosthet Dent 1989;61:310-4. 11. McLean JW, Wilson AD. Fissure sealing and filling with an adhesive glass ionom er cement. Br Dent J 1974;136:269-76. 12. M cLean JW . B ritish D ental A sso ciatio n Workshop on Alternatives to Amalgam Alloys. Br Dent J 1984;157:432-63. 13. Croll T P , Phillips RW. Glass ionomer-silver cermet restorations for primary teeth. Quintessence 1986;17:607-15. 14. Jink GM. Fluoride-impregnated cements and their effect on the activity of interproxim al caries. J Dent Child 1963;30:87-92. 15. K night GM. T he use of adhesive m aterials in the conservative restoration of selected posterior teeth. Aus Dent J 1984;29:324-31. 16. H u n t PR. A m odified Class II cavity p rep a ra tio n for glass io n o m er resto rativ e m aterials. Quintessence 1984;15:1011-8. 17. M cLean JW . New concepts in cosm etic dentistry using glass ionom er cem ents and com posites. CDA J 1986;21:20-7. 18. H ill FJ, Halaseh FJ. A laboratory investigation of tunnel restorations in prem olar teeth. Br Dent J 1988;165:364-7. 19. Corey D, Schulein TM, Kohout FJ. Marginal ridge strength of restored teeth with modified Class II cavity preparations. JADA 1989;118:199-202. 20. Jonck JM , G robbelaar C J, Strating H. T he biocom patibility of glass-ionom er cement in jo in t replacem ent: bulk testing. C lin M ater 1989;4:85107.
McLean : CERM ET CEMENTS ■ 47
IONOMERS
im m ediately injected under pressure, through the canal orifice. If the crown is lifted, excess m aterial w ill extrude from the margins. The crown is seated firm ly and the Ketac-Silver sets for 5 minutes before excess is removed. T he material at the access channel may then be trimmed and smoothed with a rubber wheel.
a
GLASS
jm
PRACTICE
GLASS
Cermet ionom er cements are sintered m eta l/g la ss powders, which can be m ade to react w ith poly(acids). These new cements are significantly more resistant to abrasion than regular glass ionom er cements an d are w idely accepted as core build-up m aterials an d lin in g cements. They can strengthen teeth an d provide the clinician w ith an o p p o rtu n ity to treat early dental caries.
Cermet cements
John W. McLean, OBE, FDS, MDS, DSc, D O dont
he main criticisms leveled at the glass ionom er cem ents by c li n icia n s are ch iefly concerned w ith b rittlen ess, poor surface p o lish , porosity, and surface wear. Improvements in these areas are essential if the clinical use of glass ionomer cements is extended to high-stress bearing areas. T he incorporation of a disperse-phase such as corundum (A120 3) into the glass can double the flexural strength of the glass ionom er cem ent.1 However, little im p rovem en t in wear resistan ce was obtained. The incorporation of alumina fibers (Saffil, ICI Runcorn) also increased the flexural strength of the cement but resulted in decreased abrasion resistance.2 Seed and W ilson2 investigated the effect of incorporating metal fibers or powders into the glass ionomer powder. Flexural strengths were increased but abrasion resistance was poor, probably because of lack of strong bonding between the metal filler and the polyacrylate matrix. T h e p rob lem of o b ta in in g strong bonding of metal fillers to glass ionomer powders was solved by sintering the metal powder in to the glass p ow d er.s-4 Fine precious m etal powders such as silver or gold were intimately mixed in equal volume with a reactive glass powder and compressed in a pelletizer at pressures above 350 MPa (megapascal). The pelle tiz in g cham ber was evacu ated at a pressure of 100 MPa. T he com pressed m etal glass pow der p ellets were then fused at temperatures about 800 C. Strong
IONOMERS
CURRENT
T
Fig 1 ■ Electron m icrograph of silver-cerment powder (orig mag *400).
b ond ing of the m etal glass com posite resulted in the formation of a “cermet” (ceramic-metal) in which, unlike simple mixtures of metal and glass powder, the metal became part of the glass powder, and the strength of the bond was com parable to that of fu sin g porcelain to gold. Silver cermet cem ents have been developed for clinical use in which pure silver powder of about 3.5 /um average particle size is used. Im provem ents in color have been made by the incorpo ration of up to 5% by weight of titanium dioxide in the glass powder. Interestingly, after g rin d in g the fired cerm et p ellet to a powder, rou n d in g of the powder particles occurred, probably caused by the lubricating effect of the silver (Fig 1). T h is resu lted in better p a ck in g densities, less porosity, and easier m ix ing.3 Cermet powders can be made to
react w ith p oly(acid s) to form cerm et ionomer cements. Currently, two types of silver cermet ionom er cem ents are available, one for handm ixing (Chelon-Silver, ESPE) and the other for a u to m a tic m ix in g in capsules (Ketac-Silver, ESPE). The capsu lated m a teria l, a cco rd in g to sales demand, is the most popular despite some criticism that in su fficien t q u an tity of m aterial is availab le for core b u ild u p and som e variability in consistency of m ix can occur. T he capsulated material, K etac-Silver, can be injected in to the tooth from the capsule. However, it is still possible to inject the hand-m ixed m aterial, C h elo n -S ilv er, if the m ixed paste is loaded in a syringe (C entrix, Centrix Inc). Discussion T h e capsulated silver cermet ionom er K etac-Silver has better resistan ce to abrasion than regular glass ionom ers, and in a sim ulated occlusal wear test, it was close to that of am algam alloy and m icrofilled com posite resin (Table l ) .5 M cK inney and others6 also found Ketac-Silver more resistant to abrasion than regular glass ionomer cements, but the surface still showed some incidence of fa ilu re from b rittle fracture. T h ey postulated that the improved wear resis tance could be caused by the lubricating effect of the silver. McLean and Gasser3 suggested that the sintered silver powder JA DA, Vol. 120, Jan u ary 1990 ■ 43
J !0 )A \
GLASS
IONOMERS
Tooth surface preparation Table 1 ■ M a te ria l loss in s im u la te d occlusal w ear.* Amalgam alloy Microfilled composite resin Conventional composite resin Glass-ionomer Type II cement Ketac-Silver cermet ionomer
0.2 mm3 0.2 mm3 0.4 mm3 6.0 mm3 0.3 m m3
•M o o re BK, Sw artz ML, P h illip s RW. A brasion resistance of metal reinforced glass ionom er cements. J Dent Res [Abstract no. 1766] 1985;64:371.
was lowering the coefficient of friction. A lth ou gh the resistance to abrasion of the cermet ionomers has resulted in a considerable im provem ent in clinical perform ance,7 their flexu ral strengths and fracture toughness are insufficient to allow their use in stress-bearing cavities such as a large C lass II lesion . T h e flexural strength of the cermet ionomers is better than m ost com m ercial glass io n o m er cem en ts but, as yet, can n ot compare with the strength of composite resins and amalgam alloys (Table 2).
Cermet ionomer cements may fail adhe sively if the tooth surface is contaminated by c u ttin g debris, sa liv a , b lo o d , or p rotein aceous debris. Partial rem oval of the smear layer before the cement is a p p lied is e s se n tia l.7 R ecom m ended procedures are: (1) for devitalized teeth or sm all areas of cut d en tin , a 25% solution of polyacrylic acid is used for 10 seconds, washed off, and lightly dried; (2) for extensive areas of vital cut dentin, a 25% so lu tio n of tann ic acid is used for 30 seconds, lightly washed, and dried. Surface conditioning of the tooth before in jectin g Ketac-Silver sh ould be m an datory, as the cermet ionom ers do not w et the surface of the tooth as easily as the th in n er m ixed bonded liners. Overdried surfaces and mixes of cermet ionomer that are drying out w ill reduce wettability and decrease the chances of long-term bonding to dentin. Core buildup
Table 2 ■ F lexu ral s tre n g th of re s to rative m aterials. Flexural strength (MPa) Amalgam alloy Composite posterior resin Composite conventional resin Composite Microfil Glass ionom er Type II Glass ionom er Ketac-Silver
140 120-150 110-135 60-80 20 40 Yield strength (MPa)
Gold alloy
Ideally, cermet ionom ers are best used for core b u ild u p in posterior teeth in which at least 2 mm of coronal dentin is still intact. In a recent in vitro study by T a leg h a n i and L ein felder,8 s ig n if ica n tly few er cases o f root fracture occurred w hen K etac-Silver w as used in con ju n ctio n w ith en d oposts (ParaPosts, W haledent Int) com pared w ith cast posts and cores. In the latter case, all the specimens tested underwent root failure, whereas only 60% of the Ketac-
Silver specimens failed in this way. T he clinician is strongly advised not to rely solely on molecular bonding to tooth structure, as the strength of the bond is inadequate where high stresses occur. In the case of devitalized molars, at least two endoposts should be inserted at different angles and extended through the core b u ild u p (F ig 2). W here vital teeth are involved, accessory pin anchor age may be necessary. After the surface of the dentin is conditioned w ith 25% p o ly a cr y lic acid, the p osts sh o u ld be cemented with a standard glass ionomer Type I lu ting cement. Ketac-Silver may then be injected over the posts and tooth; and if necessary, a second m ix can be in jected in to a cop p er band lig h tly lubricated with a silicone separator (NonS tick , H ager and W erken) w h ich is pressed over the tooth (Fig 3). Before the tooth is prepared for a full veneer crown, the cement must be allow ed to set for at least 6 m inutes; ideally, the preparation sh ould be done at a su b seq u en t v isit. F in e d iam on d stones running under copious water spray are recommended for trimming the set cermet cement. Light pressure is used and the tooth is prepared under m agnification and fiberoptic lighting. If all stages of the foregoing operation are carried out carefully, then the cermet ionomer and rem aining tooth should form a m ono lith ic structure (Fig 4). M icroleakage w ill be reduced and the cement has the added advantage of releasing fluoride, w h ich p rovid es som e a n tic a rio g en ic action. Ketac-Silver may be used as an interim
350-500
Clinical use T h e m ain c lin ic a l use of the cerm et ionomer cements is as a dentin substitute. T h e clin ica l ap p lication s follow : core buildu p; lin in g s for in la y s, am algam restoration s, and p osterior com p osite resins; fissu re fillin g s; restoration of prim ary teeth; restoration o f Class II lesions using an internal occlusal fossa (tunnel) preparation; treatment of root caries; repair of defective metal margins in crow ns and inlays; retrograde root fillings; and sealing of bifurcation areas in periodontally involved molars. Before a cermet ionom er is applied, the tooth surface should be conditioned to improve wettability and adhesion. 44 ■ JADA, Vol. 120, Ja n u a ry 1990
Fig 2 ■ Diagram of correct use of Ketac-Silver in a core buildup. At least two Para-Posts should be inserted at different angles to retain cermet ionomer. Conservation of all rem aining dentin is essential to support the future restoration.
Fig 3 ■ A copper band filled with Ketac-Silver is pressed over the tooth and will assist in closely a d ap tin g the cerm et ionom er to the prepared dentin.
jm A direct com posite bonding or laboratory processed com posite inlays are contem plated. If the cermet ionom er is used in bulk as a dentin substitute, the volume of o v er la p p in g co m p o site is less and shrinkage is reduced. A cid -etch in g of K etac-Silver has also produced strong b o n d in g to co m p o site re sin s.10 If the cement is carried up to the margin, then it acts as a cariostatic seal and w ill protect the pulp against any accidental exposure to phosphoric acid during the etching procedure.
IONOMERS
restoration w hen fu ll-m o u th re h a b il itation is being carried out. The cermet ionomer will form a good seal with the tooth and allow secondary dentin for mation to take place before final tooth preparation for crowning.
Fig 4 ■ K etac-Silver core prepared on a de vitalized first premolar. The endopost has been extended to the surface of the cermet ionom er and m ore th a n 2 mm of rem ain in g d e n tin supports the cervical base.
Linings Cermet ionom ers are useful for lin in g amalgam alloy restorations. If the patient has a h ig h -caries in cid en ce, then the proximal box may be filled at the cervical to a depth of 2 mm and the am algam packed on top (Fig 5, 6). A thin appli cation of 25% polyacrylic acid may assist b o n d in g of the am algam to the glass ionomer.9 The good cervical seal of the cermet ionomer and its ability to leach fluorid e may prevent a further attack by caries. Where the esthetic appearance is not compromised, cermet ionomers can also be used to line posterior teeth in which
In Europe, using glass ionomer cements as fissure fillings has shown that these materials can have a major role in the co n tro l o f early ca r ie s.11’12 A fissure w idening procedure is recommended in which fine diamond points are used to w iden the fissure before any dentinal caries is removed with small round burs (F ig 7). After c o n d itio n in g the cavity w ith 25% polyacrylic acid, Ketac-Silver may be injected into the opened fissures and covered with foil or green wax during the 5 -m in u te se ttin g p eriod. Excess material may then be trimmed with small round burs before sealin g w ith a coat of b on d in g agent. Fissures filled with K etac-Silver are illu stra ted in Figure 8. A lthough the material is not toothcolored , the lig h t tita n iu m co lo r is unobtrusive and has the added advantage of form ing a long-term cariostatic seal not subject to corrosion . T h e wear resistance of Ketac-Silver is good and, w hen enclosed in a fissure, the cermet ionomer is not placed under high-tensile or shear stress.
F ig 7 ■ F ine d iam ond p o in ts used to w iden the occlusal fissure and any dentinal caries may be removed subsequently with sm all round burs.
Restoration of primary teeth T h e use of g la ss-io n o m er cem en ts in primary teeth is valuable in preventing recurrent caries and in protecting adjoin ing enamel surface in permanent teeth. F lu o rid e release from these cem en ts d u rin g 2 years is often su ffic ie n t to prevent early approxim al lesions from d ev elo p in g at a later date. C roll and P h illip s 13 have made extensive use of Ketac-Silver, for restoring primary teeth; they advocated its use in bulk to prevent fracture in narrow isthmuses. They also advised that restored m argin al ridges should be left slightly out of occlusion, and the basic p rin cip les o f “retention form” must be observed for best results. M echanical retention w ill enhance the success of an adhesively bonded glass ionomer cement restoration. Restoration of Class II lesions
Fig 5 ■ Disto-occlusal cavity in second premolar filled w ith Ketac-Silver. T he cermet ionom er has been carried u p the proximal box to a depth of 2 mm and the occlusal isthm us lined only th inly to allow space for the am algam alloy. T he m atrix band has been tightly wedged to ensure a good cervical seal.
Fig 6 ■ Amalgam alloy packed over set KetacS ilver after c o n d itio n in g the surface w ith polyacrylic acid.
T he destruction of marginal ridges during treatment of the Class II lesion is being challenged and considerable interest has been aroused in trea tin g a p p ro x im a l caries via an internal occlusal fossa or “ tu n n e l” approach.71416’17 T h e in itia l approach to the lesion should be made th ro u g h the o cc lu sa l fossa, a v o id in g where possible occlusal centric-holding stops. Entry is made to the soft lesion by a d ia g o n a l approach w ith a sm all round diam ond stone, leaving at least 2-mm w idth of m arginal ridge intact. T h e access ch a n n el m ay be w id en ed buccolingually to gain entry to the caries (F ig 9). M ag n ifica tio n and fib erop tic lighting are essential tools for this type of microcavity preparation. The proceMcLean : CERMET CEMENTS ■ 45
GLASS
Fissure fillings
the flu o rid e release from the cerm et ionomer can help control the spread of root caries.
Miscellaneous uses
*
s"
4 ,*
f e «
'
A
Fig 8 ■ F issure in m a n d ib u la r m olars filled with Ketac-Silver. After 5-7 years, no corrosion is seen. T he color of the cermet ionom er has also been maintained.
Fig 9 ■ Internal occlusal fossa preparation in m a n d ib u la r second m o lar. Access ch an n el widened buccolingually b ut a ll the m arginal ridge has been retained. O cclusal preparation in m andibular first molar (lower half of picture) has also been extended to restore approxim al lesion, which is clearly visible.
Fig 10 ■ D en tin of b o th m a n d ib u la r m olars restored with Ketac-Silver. A m atrix band should be applied before restoration to prevent excess m a te ria l e x tru d in g th ro u g h the a p p ro x im al enamel aperture.
Fig 1 1 b K etac-Silver lin in g s etched for 30 seconds w ith a 37% ph o sp h o ric acid solution an d a p o ste rio r co m p o site resin in se rted by sta n d ard procedure. T h e m a rg in a l ridge is stren g th en ed by th is p ro ced u re and avoids excessive wear of the composite resin.
GLASS
IONOMERS
JAD)A
dures for restoring the Class II lesion via a tunnel approach have been fully described.7 E ssentially the preparation should be regarded as an occlusal one in w h ich the a p p ro x im a l caries is removed via an internal approach and the d en tin restored w ith K etac-Silver (Fig 10). One investigation of tunnel restorations restored w ith 9 standard T yp e II glass io n o m er cem ent (K etac-F il, ESPE) revealed that the strength o f prepared teeth was increased to 92% of their sound valu e com pared w ith am algam resto ra tio n s, w h ich produced a sm aller, nonsignificant increase.18 However, Corey and Schulein,19 in another study found little difference between tunnel resto ration s restored w ith com p osite resin or am algam alloy, and both materials caused significant strengthening of the teeth compared with the marginal ridge stren gth of prepared teeth that were unrestored. Therefore, a composite resin may therefore be inserted over the Ketac 46 ■ JA DA, Vol. 120, Ja n u a ry 1990
-Silver lin in g using the acid-etch tech nique to strengthen the marginal ridge (Fig 11). Treatment of root caries Cermet ionom ers are useful m aterials for restoring areas of root caries in the p osterior reg io n . O ften , in the older patient, su bgin gival caries is d ifficult to treat, and problems of moisture control make placem ent of alloy or com posite restorations very difficult. Ketac-Silver is fa st-settin g and may be in jected subgingivally even where some moisture is present. If a metal or plastic matrix is used, the pressure exerted w ill adapt the cermet ionomer to the root face and avoid excessive salivary contam ination. R etention of the cermet ionom er w ill be im proved w ith slig h t m ech an ical reten tion but w here th is ca n n o t be achieved, bonding in a nonstress-bearing area is generally successful. In the older patient, when oral hygiene is often poor,
T h e cermet ionom ers are particularly u sefu l for rep a irin g d efective m etal margins in crowns and inlays. Providing the margins are thoroughly cleaned, if necessary with small round burs, mate rials such as Ketac-Silver w ill provide a long-term m arginal seal. In all cases in v o lv in g repair or recem en tation of crowns, surface conditioning of the tooth w ith polyacrylic acid is advised before injecting Ketac-Silver. Where crowns have loosened, as the result of further decay at the margins, it is possible to remove the carious dentin m echanically and recement the crown w ith K etac-Silver. T h e crow n sh ould be sandblasted internally and Ketac-Silver injected. Further m aterial sh o u ld be injected over the tooth and the crown seated by asking the patient to bite on a co tto n -w o o l pad. P ro tectio n of the m argins can be achieved by covering w ith a sheet o f soft green wax. After setting, any excess cement covering the exposed dentin may be trimmed w ith fine diamond stones running under water spray. A procedure that may be regarded as a last resort in v o lv es reattachm ent of a loose abutment crown on one end of a fixed p rosth esis w here en d o d o n tic treatment has p reviously been carried out. Often, rem aking of the prosthesis can be very exp en sive. As an interim procedure, p a rticu la rly in the older p a tien t, it is p o ssib le to reattach the crown using the follow ing procedure: —T h e en d odon tic access channel is w idened and the in tern al area of the crown is cleaned with round burs. —The root canal is reamed and a ParaPost is fitted to a su itable d im ension of 1- to 1- to 2-mm diameter. The ParaPost should be approximately two-thirds the length of the root canal and extent to the crown surface. At this stage, cement is not applied. —T h e canal is clea n ed w ith a 25% polyacrylic acid solution for 10 seconds and w ashed and dried. Paper p o in ts should be used to dry the apical areas. —A g la ss-io n o m er lu tin g cem ent (Ketac-Cem, ESPE) is m ixed as KetacCem and placed in to the canal with a spiral filler. The Para-Post is gently seated to position and a m ix of Ketac-Silver
Cermet ionom ers are now b ein g used for repair of perforated root canals or as retrograde root fillings.7 Bone healing has been sh ow n to take place after 7 months, and Jonck and others20 consider the glass ionomers to be bioactive and able to promote bone growth. Further work is necessary to confirm the use of these materials as a root canal sealant. Another interesting use for the cermet ionomers as sealants for molar bifurcation areas exposed in periodontal disease has been in vestigated (J. Zamet, personal com m unication, 1989). T he root bifur cation area is exposed su rgically and the roots planed. Any osseous surgery required should then be carried out, and the gin gival tissue apically p ositioned and allow ed to heal for 6 to 8 weeks. At this stage the root bifurcation area should be conditioned with polyacrylic acid for 10 seconds before injecting the cermet ionomer cement into the bifur cation area to seal the open channel. After setting, the cement may be smoothed level w ith the root face and protected w ith a coat of varnish or preferably a light-cured bonding agent. Summary Cermet ionomer cements are sintered
m etal/glass powders, which can be made to react w ith p oly (a cid s). T h ese new cements are significantly more resistant to abrasion than regular glass ionomer cements and have found wide acceptance as core b uild-up m aterials and lin in g cem en ts. T h eir use in C lass I and II restorations is dependent on the adoption by the dental profession of new concepts in cavity preparation in v olvin g micro, not macro, cutting of teeth. These new materials can strengthen teeth and prevent cusp splitting later as the marginal ridges of teeth are preserved. The glass ionomer and composite resin laminate technique, when used in posterior teeth, harnesses the best properties in both materials and provides the clin icia n with an op por tunity to treat early dental caries, without resorting to extensive cavity preparation or the use of am algam a llo y s. T h e an tica rio g en ic properties of the glass ionomer cements, from fluoride leaching, can maintain the integrity of approximal enam el and give long-term protection to the enamel cap.
------------------- J W A ------------------Dr. McLean is developer of the cermet ionomer cements and is currently clinical consultant at the Laboratory of the G overnm ent Chemist E ngland and senior research fello.v at Eastman Dental Hospital in London. Address reprint requests to Dr. McLean at 38 Devonshire St., London W IN 1LD, England. 1. P rosser H J, Pow is DR, W ilson AD. G lass ionom er cem ents of im proved flexural strength. J Dent Res 1986;65:146-8. 2. Seed IR, W ilson AD. P oly(carboxylic acid) hardenable compositions. Br Pat Appln 2, 1980;028855A. 3. McLean JW , Gasser O. Glass cermet cements. Quintessence 1985;16:333-43. 4. M cLean JW , G asser O. P ow dered denial m aterials and process for the preparation thereof. US Pat No 4,527,979. 1985. 5. Moore BK, Swartz ML, Phillips RW. Abrasion resistance of metal reinforced glass ionomer cements.
J Dent Res [Abstract no. 1766] 1985;64:371. 6. McKinney JE, Antonucci JM, R upp NW. Wear and microhardness of a metal filled ionomer cement. J Dent Res [Abstract no. 1577] 1986;65:344. 7. Wilson AD, McLean JW. Glass ionomer cement. Berlin: Quintessence: 1988. 8. T aleg h an i M, L einfelder KF. E valu atio n of a new glass-ionom er cement w ith silver as a core b u ild -u p under a cast resto ratio n . Q uintessence 1988,19:19-24. 9. Warren JA, Soderhold KJM. Bonding amalgam to glass ionomer with PAA. Dent Mater 1988;4:191 -
6.
10. Joynt RB, Williams D, Davis EL, et al. Effect of etching time on surface morphology and adhesion of a p o ste rio r resin to glass io n o m er cem ent. J Prosthet Dent 1989;61:310-4. 11. McLean JW, Wilson AD. Fissure sealing and filling with an adhesive glass ionom er cement. Br Dent J 1974;136:269-76. 12. M cLean JW . B ritish D ental A sso ciatio n Workshop on Alternatives to Amalgam Alloys. Br Dent J 1984;157:432-63. 13. Croll T P , Phillips RW. Glass ionomer-silver cermet restorations for primary teeth. Quintessence 1986;17:607-15. 14. Jink GM. Fluoride-impregnated cements and their effect on the activity of interproxim al caries. J Dent Child 1963;30:87-92. 15. K night GM. T he use of adhesive m aterials in the conservative restoration of selected posterior teeth. Aus Dent J 1984;29:324-31. 16. H u n t PR. A m odified Class II cavity p rep a ra tio n for glass io n o m er resto rativ e m aterials. Quintessence 1984;15:1011-8. 17. M cLean JW . New concepts in cosm etic dentistry using glass ionom er cem ents and com posites. CDA J 1986;21:20-7. 18. H ill FJ, Halaseh FJ. A laboratory investigation of tunnel restorations in prem olar teeth. Br Dent J 1988;165:364-7. 19. Corey D, Schulein TM, Kohout FJ. Marginal ridge strength of restored teeth with modified Class II cavity preparations. JADA 1989;118:199-202. 20. Jonck JM , G robbelaar C J, Strating H. T he biocom patibility of glass-ionom er cement in jo in t replacem ent: bulk testing. C lin M ater 1989;4:85107.
McLean : CERM ET CEMENTS ■ 47
IONOMERS
im m ediately injected under pressure, through the canal orifice. If the crown is lifted, excess m aterial w ill extrude from the margins. The crown is seated firm ly and the Ketac-Silver sets for 5 minutes before excess is removed. T he material at the access channel may then be trimmed and smoothed with a rubber wheel.
a
GLASS
jm
