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New Materials on the Horizon
24
JADA, Vol. 122, July 1991
MATERIALS N EW
HORIZON ABSTRACT
The status, advantages, disadvantages and potential future of amalgam, cast alloys, cohesive gold, resins, glass ionomer materials and porcelain are reported and a bright future for light-cured glass ionomer and reinforced direct composite resin technologies is predicted.
his article reviews the status of preventive and restorative dental materials, and predicts which areas will develop significantly in the future. Although the status quo of m aterials perform ance is empirical, the future, limited by speculative ability, is difficult to predict. The status, advantages, disadvantages and potential future of amalgam, cast alloys, cohesive gold, resins, glass ionomer m aterials and porcelain are discussed. AMALGAM
R IC H A R D J. S IM O N S E N , D.D .S., M .S .
As a restorative material, dental amalgam has served the profession well for approximately 150 years. Repeated controversies concern ing the safety of the m ercury content of amalgam continue and have affected material usage, but such controversies have not sunk the amalgam ship—yet. Amalgam has some clear advantages, as well as some distinct disadvantages, in clinical
use. Clinical data on the longevity of amalgam restorations show a relatively short half-life.1It seems clear, however, th at if clinicians allow im perfect m argins to rem ain an acceptable part of an older amalgam restoration, and then only replace amalgam restorations based on the diagnosis of secondary caries or catastrophic restoration failure, the clinical serviceability of amalgam restora tions could increase substantially. Clinicians do not agree about the criteria for amalgam restoration replacem ent, making studies of restoration longevity subjective.2 The long history of clinical use, and the relative ease of placem ent as a result of this long experience, is an advantage to the use of dental amalgam. In addition, the clinical technique has been developed over many years and the m aterial is convenient and efficient to use. Compared w ith competitive materials, amalgam is an inexpensive restorative m aterial JADA, Vol. 122, July 1991
25
for the patient because of place m ent speed. Perhaps the major disadvantage of amalgam at this tim e is w hat can be called the “ ‘60 M inutes’ syn drom e.” The television program, aired in December 1990, was m erciless in attacking dental amalgam. The presentation pam pered advertisers and ratings while attacking the credibility of the profession. This angle was probably used to capture viewer interest. This, and other negative m edia events, along with environm ental concerns could eventually spell the death of amalgam as it is used today—the leading restorative m aterial for dental caries. A recent publication by Consum er Reports provided a m ore balanced and rational approach to the issue than did the “60 M inutes” program .3Negative m edia reports can, and probably will, have a considerable negative im pact on amalgam restoration use, no m atter w hat the scientific evidence to the contrary may be. Additionally, amalgam is more unattractive than other restorative m aterials. Its dark color and propensity to stain teeth dark gray can be a major problem in areas of the m outh w here appearance is im portant. Tooth preparation for any restorative procedure weakens the structural integrity of the rem aining tooth, particularly with preparations for amalgam in which the preparation m ust incorporate mechanical undercuts. Removing such tooth structure can predispose the tooth to subsequent fracture because the amalgam restorative m aterial cannot strengthen the rem aining tooth structure. Additionally, the m aterial properties of amalgam determ ine cavity preparation size. For 26
JADA, Vol. 122, July 1991
example, all dental students are taught to prepare Class I cavities for amalgam into dentin, even if the caries is only in enamel. Since amalgam is not strong in a thin layer, it m ust be placed in a layer thick enough to w ithstand fracture. To have space for this sufficiently thick layer of amalgam, healthy tooth structure m ust be removed in addition to carious tissue removal. Similarly “extension for prevention,” removing healthy tooth structure to prevent caries from attacking adjacent cariessusceptible pits and fissures, is an integral part of amalgam prepara tions. Again, this is perform ed to accommodate another amalgam deficiency—its lack of cariostatic properties. If adjacent cariessusceptible pits and fissures are not incorporated into the cavity preparation for amalgam restorations in the extension-forprevention technique, further caries is likely to develop adjacent to the amalgam restoration, resulting in additional need for restorative intervention. The paradox is clear; healthy tooth structure is removed and replaced with a restorative m aterial. While it makes little
sense to further weaken rem aining tooth structure to make the restor ative m aterial successful, this is essential for the clinical success of amalgam. A restorative m aterial that allows the rem aining healthy tooth structure to be left intact, while removing only diseased tissue, would be preferable. If an amalgam restoration is replaced, the restoration increases to the “next size,” initiating the “molar life cycle.” The life cycle is a facetious look at w hat is routinely perform ed in operative dentistry. As small restorations are replaced with increasingly larger ones, the rem aining tooth becomes weaker and fatigue causes cusp fractures. Perhaps the pulp is exposed and a crown is needed after endodontic treatm ent. Subsequent problem s of perio dontal disease or endodontic failure may result in the eventual loss of the tooth—a loss th at can stem directly from the initial restoration and the radical restorative procedures th at were the only alternative to short-term tooth loss. Then, as a recently docum ented last-resort treatm ent, an im plant may stave off remov able dentures. If the first restoration on a tooth
can be avoided by the use of preventive m aterials such as fluoride and p it and fissure sealant, or m inimized in size and restored with a bonded m aterial such as composite resin in a preventive resin restoration, the long-term potential for successful tooth m aintenance is increased. The m olar life cycle is a chain of events th at clinicians should prevent. Finally, amalgam placem ent requires two visits to polish and finish the restoration. Few practitioners, however, polish amalgam restorations, despite the fact th at m ost educational program s teach the procedure" as a part of the complete restoration. Scientific studies support the prevailing clinical approach of not routinely polishing amalgam restorations because the longevity of the restoration is not affected.4,5 Amalgam corrodes, stains and has lim ited bonding potential. The clinical efficacy and longevity of some recently m arketed amalgam adhesives are undeterm ined. Conservative cavity preparation or preventive resin restoration concepts are not attainable with amalgam and the future improve m ent or developm ent of amalgam is, therefore, limited in restorative dentistry. CAST ALLOYS
Cast alloys w ere the foundation of prosthetic dentistry for many years. With the advent of the stronger nonprecious alloys, cast gold is used less frequently, bu t is still one of the best restorative materials. The higher strength of the nonprecious alloys, and the developm ent of composite resin luting cements, has led to heretofore impossible conserv ative applications of alloys such as for the etched-cast restoration, commonly referred to as the
“Maryland bridge.”6Concerns about biocompatibility and the inevitable problems with appearance rem ain disadvantages with the alloys. The major advantage of the cast alloys is the strength of the m aterials, w hen com pared with m ore esthetic options. The long term , high-gloss finish and polishing ease after occlusal or m arginal adjustm ent are significant benefits. The alloys w ear well and are especially compatible with opposing tooth enamel, unlike the well-known ravages of porcelain. Disadvantages of alloys, in addition to biocompatibility of the nonprecious alloys and the ever present esthetic problem, center around the extensive laboratory support required and procedural complexity. These problems result in higher consum er costs. While m icrom echanical retention with m ost resin systems is easily attainable, it is still difficult to routinely couple the alloys with adhesive systems. Additionally, some base metals may stain the oral tissues, and more im portantly, a considerable am ount of hard tissue is usually removed when preparing a tooth for a cast alloy
restoration. Tooth structure loss is a significant disadvantage of the cast alloys—a disadvantage that will rem ain no m atter how m uch the m etals are improved. The “Maryland bridge” was a major advance in using alloys conservatively, but the future of restorative m aterials lies in metal replacem ent, rather than in alloy improvement. For the foreseeable future, however, cast alloys are here to stay. For the long term , the im pact of cast alloys will decrease to the point w hen one day the profession may practice metal-free dentistry. COHESIVE GOLD
Few valid reasons support the routine use of cohesive gold in dentistry today, although the m aterial has a fervent following among a small m inority in the profession. It is a fine restoration, but the alternative m aterials provide major advantages, m aking cohesive gold obsolete. The competitive m aterials described later provide more conservative, m ore esthetic, less traum atic and much less costly alternatives. The gold foil restora tion may have a longer life than, for example, a bonded composite JADA, Vol. 122, July 1991
27
resin restoration, but this assum p tion m ust be confirm ed by long term studies of conservative composite resin restorations that consider the fact th at less tooth structure is removed in the preparation. The two restoration types cannot, in reality, be com pared because it is not possible to pu t a value on saving tooth structure. University classes in cohesive gold restorations are usually elective courses. As the
of operators skilled in this techni que decreases, it is likely th at the restoration will rem ain the dom ain of a few skilled clinicians. RESIN SYSTEMS
The resin systems constitute an increasingly im portant role in preventive and 28
JADA, Vol. 122, July 1991
restorative dentistry. Preventive dentistry affords a look at the present status of pit and fissure sealant. PIT AND FISSURE SEALANT
Pit and fissure sealant was first m arketed in 1971, and many studies have docum ented its reten tion and caries prevention effica cy.78 Sealants prevent dental caries in pits and fissures where the sealant is applied and re tained. F urther more, the seal ant, if properly applied under norm al clinical conditions, should be retained in m ost cases for approxim ately 10 fore reappli cation becomes necessary. Addi tionally, sealant on the occlu sal pits and fis sures prevents the restora tion of such teeth. The belief that the perm anent molars would even tually become carious and need restoration was so pervasive that some clinicians routinely prescribed restoration of such teeth in the belief that it was better to restore a tooth w ith a small restoration immediately, than wait for caries to develop and the
necessity for carious tooth structure to be removed. This technique was made famous by Thaddeus P. Hyatt in his article on the prophylactic odontotomy.9 Unfortunately, the routine Class I prophylactic odontotomy was no more conservative, in m any cases, than later caries restoration techniques, and it was invasive and injurious to individuals who may never have developed caries. Pit and fissure sealant application requires m eticulous technique for success. Ten-year results have been published in JADA10and 15-year results from the sam e study are in press. Prelim inary exam ination of the 15-year data is prom ising with continued great differences betw een the sealed and the control groups th at were presented in the 10-year report. Two other studies have reported retention data over (not after) 10 years, in private practice years be settings, with or public health positive results.1112 It is hard to find disadvantages for the pit and fissure sealant prevention technique. The m eticulous technique necessary is perhaps a disadvantage, b u t the skill level required is not unusually high and all m em bers of the dental 'team, including dentists, hygienists or assistants, can be trained to apply pit and fissure sealant.13 Regrettably, the latest national data available indicate th at only 7.6 percent of five- to 15-year-old children in the United States have had sealants applied.“1It would be satisfying to see the molar life cycle changed to a preventive life cycle—let’s say for generations born after 1970. In this scenario, the tooth gets sealed, the sealant is reapplied 10 to 15 years after initial application, and the restorative life cycle is avoided. The future im pact of pit and
fissure sealant will be great. The need for prevention is still high in the United States. Only 16 percent of 17-year olds are caries-free and 84 percent of the caries experience of five- to 17-year-olds is on pit and fissure (sealant protectable) surfaces.'4 The developing nations of the world th at have yet to experience the ravages of dental caries associated with the W estern diet, will be challenged to adopt prim ary prevention program s to avoid the massive cost to society and to individuals of restoring the teeth th at could rem ain caries-free w ith judicious pit and fissure sealant application and fluoride use.15 BONDING AGENTS
Enamel and dentinal bonding agents are confusing and require more space than is available here to discuss thoroughly. The advantages of bonding to enam el are clear. Of m ajor im portance is th at bonding to enamel allows conservative cavity preparation which m eans th at the inherent structural integrity of the tooth will be minimally compromised. Additionally, microleakage prevention is a major benefit of bonding to enamel. Dentinal bonding agents are still in their infancy, relying on m icrom echanical bonding rath er than any true chemical bond. Yet, an ideal system is one that provides routine reliable bonding to dentin and can be perform ed by the average practitioner under normal, or even difficult, clinical conditions. Too often, the operator is faced with m oisture control problems th at are anathem a to reliable dentinal bonding. The use of the rubber dam for m oisture control is, regrettably, low for routine operative dentistry. Two dentinal bonding agents
have received ADA Provisional Acceptance (Scotchbond 2, 3M Dental Products, and Tenure, DenMat Corporation). Recently Scotchbond 2 was aw arded full acceptance by the American Dental Association Council on Dental Materials, Instrum ents and Equipment. The future of bonding agents is bright, at least until m aterial im provem ents based on simplif ication of the clinical techniques elim inate the bonding agent step. Improvements in the reliability of the bond and the user-friendliness of the m aterials should secure the short-term future for these materials.
Conserving tooth structure through the acid-etch tech nique remains the major benefit of the composite resin systems over some of the stronger, but more radical alternative materials requiring tooth structure removal. ANTERIOR AND POSTERIOR COMPOSITE RESINS
Light-curing technology has been an im portant advance th at has affected both anterior and posterior resin systems. Light curing has simplified the proce dure for clinicians and increased the color stability of the restoration for the patient. Appearance is a major advantage of the composite resin systems over the previously discussed materials. Composite resins, with the aid of bonding resins, can be bonded to enamel thus allowing conservative cavity preparation, such as the prevent
ive resin restoration,16and saving tooth structure. Conserving tooth structure through the acid-etch technique rem ains the major benefit of the composite resin systems over some of the stronger, but m ore radical alternative m aterials requiring tooth structure removal. For inter proxim al restor ations, conservative preparations such as the tunnel preparation have been advocated.17 Perhaps the major disadvantage of composite resin systems is the polym erization shrinkage of the resin. This shrinkage can lead to marginal leakage as the force of the resin shrinkage may, under certain circum stances, exceed the bond strength of resin to tooth structure and cause the m arginal seal to fracture. Additionally, the technique-sensitivity of the acidetch technique can be regarded as a disadvantage in the use of composite resin systems inasm uch as the procedure is clinically dem anding and more timeconsum ing than an amalgam restoration. Another disadvantage is the cost to the patient which will be higher than th at for a simpler, faster procedure. Some simplification of clinical procedures, m aking resin bonding to tooth structure a m ore userfriendly procedure, is likely to occur soon. F uture developments, particularly in the area of filler particles and in filler particle size distribution, will improve clinical performance. Hopefully, the polymerization shrinkage problem will be solved or minimized. GLASS IONOMER MATERIALS
This group of m aterials has m et with greater clinical success than clinical acceptance by the profession. The limited acceptance can be traced to the dem anding clinical technique and the JADA, Vol. 122, July 1991
29
relatively long setting tim e for glass ionom er m aterials in the mouth. Recent developments that have modified the glass ionomer m aterial with resin, thus allowing light curing of the material, indicate a bright future for this group of m aterials. Restorative m aterials and bases based on the light-cured glass ionom er technology will be particularly beneficial in the restorative treatm ent of young children and geriatric adults, w here a simple-touse, fast-setting, strong, fluoridereleasing m aterial is ideal. Although glass ionomer m aterials have been used as sealants, the retention rates have been low.18-19The glass ionomer sealants cannot m atch the docum ented retention rates of the resin sealants. Additionally, the clinical technique for glass ionom er m aterials use as initially developed is cumbersome com pared w ith the comparable technique for resins. The major advantage of using a glass ionom er m aterial as a restorative m aterial is that fluoride is released from the material and the m argins of the restoration are, therefore, less caries-susceptible. Glass ionom er restorations are microleakage resistant inasm uch as they bond to enamel and dentin and are kind to the pulp. Without the benefit of fluoride release, however, glass ionomer m aterials should not be used, except in the restoration of root caries, for they are not as strong or as esthetically pleasing as competitive resin materials. The developm ent by Mitra of the resin-modified glass ionomer m aterials has been one of the most im portant recent developments in dental m aterials research and will have a great im pact on this group of m aterials.20'22Mitra’s developm ent resulted in the 30
JADA, Vol. 122, July 1991
production of a light-cured glass ionom er liner base. When used as a lining m aterial, such resinmodified glass ionom er materials bond to dentin with considerably higher bond strength (approxi m ately 11 megapascals shear bond strength to dentin), than conventional glass ionomer m aterials (approximately 4 MPa shear bond strength). Much of this difference is caused by the greater cohesive strength of the resinmodified glass ionomer material. Applying this technology to light-cured glass ionomer restor ative m aterials would radically alter the caries treatm ent of the prim ary dentition and treatm ent of root caries in elderly patients. Conventional glass ionomer restorative m aterials are ham pered by the sometimes inconvenient technique—the m aterials m ust neither become dehydrated, nor become too wet during curing, and conventional setting can take four to five m inutes. Incorporating lightcuring technology into glass ionomer restorative materials would elim inate these disadvant ages, while m aintaining the advantages of fluoride release, and of a strong bond to dentin. When this is done, the im portance of glass ionom er m aterials in restora tive dentistry will increase greatly. PORCELAIN
For crown and bridge, porcelain has the advantages of appearance, strength (single units), and a long history of clinical success. Recent development in fine-particle ceramics, where fine particles of alum inum oxide are infiltrated with glass for use as a core under conventional porcelain, has greatly improved strength. If clinical testing is favorable, this developm ent may expand the general use of porcelain, after
appropriate clinical testing, into short-span anterior, and perhaps even posterior, Dr. Simonsen is bridges. professional The services manager, Dental Products development of Division, 3M Health etching Care, 3M Center Bldg. 225-4S-11, St. porcelain has Paul, Minn. 55144increased 10OO and is currently applications for on a leave of ab sence as professor, this material. It Department of was stated as a General Dentistry, College of Dentistry, conclusion of University of Tenne the original 1983 ssee, Memphis. presentation of Address requests for reprints to the the etched author. porcelain research that “bond strengths of the m agnitude reported here are expected to be clinically significant for the retention of porcelain veneers and for other intraoral uses of porcelain.”23 While porcelain has been increasingly used in recent years in bonded inlays and onlays, it is only as a labial veneer that the material has met with incontro vertible success as a bonded restoration. The fragile nature of a thin veneer is not a problem during clinical bonding and the adoption of the porcelain veneer procedure prom oted esthetic dentistry consciousness by both dentists and the public. However, the fragility of an inlay or onlay is a significant disadvantage because the restoration m ust be bonded before occlusal adjustm ent. Necessary occlusal adjustm ent removes the glaze, exposing the particulate nature of the material, thus causing potentially disastrous consequences in wear on the opposing enamel, depending on the porcelain used. The disadvantages, therefore, of porcelain are related mainly to the
wear on opposing enamel and to the material’s brittleness. In the future, some of the current wear problems will be eliminated by a reinforced resin system that will not have the brittleness of current porcelains. INDIRECT RESTORATIVE MATERIALS
Burke and others evaluated five indirect composite resin inlay systems.24The authors had generally positive conclusions about the indirect systems. Based on the current poor acceptance, however, by the profession for the commercially available indirect composite inlay systems, it is doubtful that such systems will become well accepted at this time. The techniques increase cost with little time savings or performance improvement of the restorations. SUMMARY
Two areas of research will prosper and change operative dentistry in the future: developing light-cured glass ionomer technology further, and reinforcing direct composite resin technology. Successful development of restorative materials in these areas will have an immediate and long-term effect on the practice of operative dentistry in both affluent and poor countries of the world. ■ Publication of names of products does not imply endorsement by the American Dental Association. This paper was presented April 25,1991 at the Scientific Frontiers in Clinical Dentistry Symposium, National Institute for Dental Research, Bethesda, MD. The author thanks Dr. William H. Douglas for his contributions during the planning of the oral presentation of this paper. 1. Smales RJ, Webster DA, Leppard PI, Dawson AS. Prediction of amalgam restoration longevity. J Dent 1991;19:18-23 2. Mjor, IA. Amalgam and composite resin restorations: longevity and reasons for replacement. In: Quality evaluations of dental restorations: criteria for placement and replacement. Anusavice K J, ed. Chicago: Quintessence; 1989:61-8. 3. "Hie mercury in your mouth. Consumer Reports May 1991:316-9. 4. Letzel H, Vrijhoef MM. The influence of polishing on the marginal integrity of amalgam restorations. J Oral Rehabil 1984;11:89-94.
5. Jeffrey 1WM, Pitts NB. Finishing of amalgam restorations: to what degree is it necessary? J Dent 1989;17:55-60. 6. Simonsen RJ, Thompson VP, Barrack G. Etched cast restorations: clinical and laboratory techniques. Chicago:Quintessence;1983. 7. Horowitz HS, Heifetz SB, Poulsen S. Retention and effectiveness of a single application of an adhesive sealant in preventing occlusal caries: final report after five years of study in Kalispell, Montana. JADA1977; 95:1133-9. 8. Rock WP. The effectiveness of fissure sealant resins. National Institutes of Health Consensus Development Conference. Dental sealants in the prevention of tooth decay J Dent Educ (Supplement) 1984;48:27-31. 9. Hyatt TP. Prophylactic odontotomy: the cutting into the tooth for the prevention of disease. Dent Cosmos 1923; 65:23441. 10. Simonsen RJ. Retention and effectiveness of a single application of white sealant after ten years. JADA 1987;115:31-6. 11. Wendt LK, Koch G. Fissure sealant in permanent molars after 10 years. Swed Dent J 1988;12:181-5. 12. Romke RG, Lewis DW, Maze BD, Vickerson RA. Retention and maintenance of fissure sealants over 10 years. J Can Dent Assoc 1990; 56:235-7. 13. Stiles HM, Ward GT, Woolridge ED, Meyers R. Adhesive sealant clinical trial: comparative results of application by a dentist or dental auxiliaries. J Prev Dent 1976;3:8-11. 14. US Public Health Service. Oral health of United States children. The national survey of dental caries in United States school children, 1986-1987. Hyattsville: National Center for Health Statistics, 1989; NIH publication No. 89-2247. 15. Ripa LW. A critique of topical fluoride methods (dentifrices, mouthrinses, operator- and self-applied gels) in an era of decreased caries and increased fluorosis prevalence. J Public Health Dent 1991;1:23-41. 16. Simonsen RJ. Clinical applications of the acid etch technique. Chicago: Quintessence;1978:89-101. 17. Croll TP. Glass ionomer-silver cermet bonded composite resin Class II tunnel restorations. Quintessence Int 1988; 19:533-9. 18. Boksman L, Gratton DR, McCutcheon E, Plotzke OB. Clinical evaluation of a glass ionomer cement as a fissure sealant. Quintessence Int 1987; 18:707-9. 19. Hickel R, Voss A. Comparative studies on fissure sealing: composite versus Cermet cement. Dtsch Zahnarztl Z 1989; 44:4724. 20. Mitra SB. European patent application 323,120. 21. Mitra SB. Adhesion to dentin and physical properties of a light-cured glass-ionomer liner/base. J Dent Res 1991; 70:72-4. 22. Mitra SB. In vitro fluoride release from a light-cured glass-ionomer liner/base. J Dent Res 1991:70:75-8. 23. Simonsen RJ, Calamia JR. Tensile bond strength of etched porcelain. J Dent Res (abstract no. 79) 1983; 62:297. 24. Burke FJT, Watts DC, Wilson NHF, Wilson MA. Current status and rationale for composite inlays and onlays. Br Dent J 1991;170:269-73.
JADA, Vol. 122, July 1991
MATERIALS N EW
HORIZON ABSTRACT
The status, advantages, disadvantages and potential future of amalgam, cast alloys, cohesive gold, resins, glass ionomer materials and porcelain are reported and a bright future for light-cured glass ionomer and reinforced direct composite resin technologies is predicted.
his article reviews the status of preventive and restorative dental materials, and predicts which areas will develop significantly in the future. Although the status quo of m aterials perform ance is empirical, the future, limited by speculative ability, is difficult to predict. The status, advantages, disadvantages and potential future of amalgam, cast alloys, cohesive gold, resins, glass ionomer m aterials and porcelain are discussed. AMALGAM
R IC H A R D J. S IM O N S E N , D.D .S., M .S .
As a restorative material, dental amalgam has served the profession well for approximately 150 years. Repeated controversies concern ing the safety of the m ercury content of amalgam continue and have affected material usage, but such controversies have not sunk the amalgam ship—yet. Amalgam has some clear advantages, as well as some distinct disadvantages, in clinical
use. Clinical data on the longevity of amalgam restorations show a relatively short half-life.1It seems clear, however, th at if clinicians allow im perfect m argins to rem ain an acceptable part of an older amalgam restoration, and then only replace amalgam restorations based on the diagnosis of secondary caries or catastrophic restoration failure, the clinical serviceability of amalgam restora tions could increase substantially. Clinicians do not agree about the criteria for amalgam restoration replacem ent, making studies of restoration longevity subjective.2 The long history of clinical use, and the relative ease of placem ent as a result of this long experience, is an advantage to the use of dental amalgam. In addition, the clinical technique has been developed over many years and the m aterial is convenient and efficient to use. Compared w ith competitive materials, amalgam is an inexpensive restorative m aterial JADA, Vol. 122, July 1991
25
for the patient because of place m ent speed. Perhaps the major disadvantage of amalgam at this tim e is w hat can be called the “ ‘60 M inutes’ syn drom e.” The television program, aired in December 1990, was m erciless in attacking dental amalgam. The presentation pam pered advertisers and ratings while attacking the credibility of the profession. This angle was probably used to capture viewer interest. This, and other negative m edia events, along with environm ental concerns could eventually spell the death of amalgam as it is used today—the leading restorative m aterial for dental caries. A recent publication by Consum er Reports provided a m ore balanced and rational approach to the issue than did the “60 M inutes” program .3Negative m edia reports can, and probably will, have a considerable negative im pact on amalgam restoration use, no m atter w hat the scientific evidence to the contrary may be. Additionally, amalgam is more unattractive than other restorative m aterials. Its dark color and propensity to stain teeth dark gray can be a major problem in areas of the m outh w here appearance is im portant. Tooth preparation for any restorative procedure weakens the structural integrity of the rem aining tooth, particularly with preparations for amalgam in which the preparation m ust incorporate mechanical undercuts. Removing such tooth structure can predispose the tooth to subsequent fracture because the amalgam restorative m aterial cannot strengthen the rem aining tooth structure. Additionally, the m aterial properties of amalgam determ ine cavity preparation size. For 26
JADA, Vol. 122, July 1991
example, all dental students are taught to prepare Class I cavities for amalgam into dentin, even if the caries is only in enamel. Since amalgam is not strong in a thin layer, it m ust be placed in a layer thick enough to w ithstand fracture. To have space for this sufficiently thick layer of amalgam, healthy tooth structure m ust be removed in addition to carious tissue removal. Similarly “extension for prevention,” removing healthy tooth structure to prevent caries from attacking adjacent cariessusceptible pits and fissures, is an integral part of amalgam prepara tions. Again, this is perform ed to accommodate another amalgam deficiency—its lack of cariostatic properties. If adjacent cariessusceptible pits and fissures are not incorporated into the cavity preparation for amalgam restorations in the extension-forprevention technique, further caries is likely to develop adjacent to the amalgam restoration, resulting in additional need for restorative intervention. The paradox is clear; healthy tooth structure is removed and replaced with a restorative m aterial. While it makes little
sense to further weaken rem aining tooth structure to make the restor ative m aterial successful, this is essential for the clinical success of amalgam. A restorative m aterial that allows the rem aining healthy tooth structure to be left intact, while removing only diseased tissue, would be preferable. If an amalgam restoration is replaced, the restoration increases to the “next size,” initiating the “molar life cycle.” The life cycle is a facetious look at w hat is routinely perform ed in operative dentistry. As small restorations are replaced with increasingly larger ones, the rem aining tooth becomes weaker and fatigue causes cusp fractures. Perhaps the pulp is exposed and a crown is needed after endodontic treatm ent. Subsequent problem s of perio dontal disease or endodontic failure may result in the eventual loss of the tooth—a loss th at can stem directly from the initial restoration and the radical restorative procedures th at were the only alternative to short-term tooth loss. Then, as a recently docum ented last-resort treatm ent, an im plant may stave off remov able dentures. If the first restoration on a tooth
can be avoided by the use of preventive m aterials such as fluoride and p it and fissure sealant, or m inimized in size and restored with a bonded m aterial such as composite resin in a preventive resin restoration, the long-term potential for successful tooth m aintenance is increased. The m olar life cycle is a chain of events th at clinicians should prevent. Finally, amalgam placem ent requires two visits to polish and finish the restoration. Few practitioners, however, polish amalgam restorations, despite the fact th at m ost educational program s teach the procedure" as a part of the complete restoration. Scientific studies support the prevailing clinical approach of not routinely polishing amalgam restorations because the longevity of the restoration is not affected.4,5 Amalgam corrodes, stains and has lim ited bonding potential. The clinical efficacy and longevity of some recently m arketed amalgam adhesives are undeterm ined. Conservative cavity preparation or preventive resin restoration concepts are not attainable with amalgam and the future improve m ent or developm ent of amalgam is, therefore, limited in restorative dentistry. CAST ALLOYS
Cast alloys w ere the foundation of prosthetic dentistry for many years. With the advent of the stronger nonprecious alloys, cast gold is used less frequently, bu t is still one of the best restorative materials. The higher strength of the nonprecious alloys, and the developm ent of composite resin luting cements, has led to heretofore impossible conserv ative applications of alloys such as for the etched-cast restoration, commonly referred to as the
“Maryland bridge.”6Concerns about biocompatibility and the inevitable problems with appearance rem ain disadvantages with the alloys. The major advantage of the cast alloys is the strength of the m aterials, w hen com pared with m ore esthetic options. The long term , high-gloss finish and polishing ease after occlusal or m arginal adjustm ent are significant benefits. The alloys w ear well and are especially compatible with opposing tooth enamel, unlike the well-known ravages of porcelain. Disadvantages of alloys, in addition to biocompatibility of the nonprecious alloys and the ever present esthetic problem, center around the extensive laboratory support required and procedural complexity. These problems result in higher consum er costs. While m icrom echanical retention with m ost resin systems is easily attainable, it is still difficult to routinely couple the alloys with adhesive systems. Additionally, some base metals may stain the oral tissues, and more im portantly, a considerable am ount of hard tissue is usually removed when preparing a tooth for a cast alloy
restoration. Tooth structure loss is a significant disadvantage of the cast alloys—a disadvantage that will rem ain no m atter how m uch the m etals are improved. The “Maryland bridge” was a major advance in using alloys conservatively, but the future of restorative m aterials lies in metal replacem ent, rather than in alloy improvement. For the foreseeable future, however, cast alloys are here to stay. For the long term , the im pact of cast alloys will decrease to the point w hen one day the profession may practice metal-free dentistry. COHESIVE GOLD
Few valid reasons support the routine use of cohesive gold in dentistry today, although the m aterial has a fervent following among a small m inority in the profession. It is a fine restoration, but the alternative m aterials provide major advantages, m aking cohesive gold obsolete. The competitive m aterials described later provide more conservative, m ore esthetic, less traum atic and much less costly alternatives. The gold foil restora tion may have a longer life than, for example, a bonded composite JADA, Vol. 122, July 1991
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resin restoration, but this assum p tion m ust be confirm ed by long term studies of conservative composite resin restorations that consider the fact th at less tooth structure is removed in the preparation. The two restoration types cannot, in reality, be com pared because it is not possible to pu t a value on saving tooth structure. University classes in cohesive gold restorations are usually elective courses. As the
of operators skilled in this techni que decreases, it is likely th at the restoration will rem ain the dom ain of a few skilled clinicians. RESIN SYSTEMS
The resin systems constitute an increasingly im portant role in preventive and 28
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restorative dentistry. Preventive dentistry affords a look at the present status of pit and fissure sealant. PIT AND FISSURE SEALANT
Pit and fissure sealant was first m arketed in 1971, and many studies have docum ented its reten tion and caries prevention effica cy.78 Sealants prevent dental caries in pits and fissures where the sealant is applied and re tained. F urther more, the seal ant, if properly applied under norm al clinical conditions, should be retained in m ost cases for approxim ately 10 fore reappli cation becomes necessary. Addi tionally, sealant on the occlu sal pits and fis sures prevents the restora tion of such teeth. The belief that the perm anent molars would even tually become carious and need restoration was so pervasive that some clinicians routinely prescribed restoration of such teeth in the belief that it was better to restore a tooth w ith a small restoration immediately, than wait for caries to develop and the
necessity for carious tooth structure to be removed. This technique was made famous by Thaddeus P. Hyatt in his article on the prophylactic odontotomy.9 Unfortunately, the routine Class I prophylactic odontotomy was no more conservative, in m any cases, than later caries restoration techniques, and it was invasive and injurious to individuals who may never have developed caries. Pit and fissure sealant application requires m eticulous technique for success. Ten-year results have been published in JADA10and 15-year results from the sam e study are in press. Prelim inary exam ination of the 15-year data is prom ising with continued great differences betw een the sealed and the control groups th at were presented in the 10-year report. Two other studies have reported retention data over (not after) 10 years, in private practice years be settings, with or public health positive results.1112 It is hard to find disadvantages for the pit and fissure sealant prevention technique. The m eticulous technique necessary is perhaps a disadvantage, b u t the skill level required is not unusually high and all m em bers of the dental 'team, including dentists, hygienists or assistants, can be trained to apply pit and fissure sealant.13 Regrettably, the latest national data available indicate th at only 7.6 percent of five- to 15-year-old children in the United States have had sealants applied.“1It would be satisfying to see the molar life cycle changed to a preventive life cycle—let’s say for generations born after 1970. In this scenario, the tooth gets sealed, the sealant is reapplied 10 to 15 years after initial application, and the restorative life cycle is avoided. The future im pact of pit and
fissure sealant will be great. The need for prevention is still high in the United States. Only 16 percent of 17-year olds are caries-free and 84 percent of the caries experience of five- to 17-year-olds is on pit and fissure (sealant protectable) surfaces.'4 The developing nations of the world th at have yet to experience the ravages of dental caries associated with the W estern diet, will be challenged to adopt prim ary prevention program s to avoid the massive cost to society and to individuals of restoring the teeth th at could rem ain caries-free w ith judicious pit and fissure sealant application and fluoride use.15 BONDING AGENTS
Enamel and dentinal bonding agents are confusing and require more space than is available here to discuss thoroughly. The advantages of bonding to enam el are clear. Of m ajor im portance is th at bonding to enamel allows conservative cavity preparation which m eans th at the inherent structural integrity of the tooth will be minimally compromised. Additionally, microleakage prevention is a major benefit of bonding to enamel. Dentinal bonding agents are still in their infancy, relying on m icrom echanical bonding rath er than any true chemical bond. Yet, an ideal system is one that provides routine reliable bonding to dentin and can be perform ed by the average practitioner under normal, or even difficult, clinical conditions. Too often, the operator is faced with m oisture control problems th at are anathem a to reliable dentinal bonding. The use of the rubber dam for m oisture control is, regrettably, low for routine operative dentistry. Two dentinal bonding agents
have received ADA Provisional Acceptance (Scotchbond 2, 3M Dental Products, and Tenure, DenMat Corporation). Recently Scotchbond 2 was aw arded full acceptance by the American Dental Association Council on Dental Materials, Instrum ents and Equipment. The future of bonding agents is bright, at least until m aterial im provem ents based on simplif ication of the clinical techniques elim inate the bonding agent step. Improvements in the reliability of the bond and the user-friendliness of the m aterials should secure the short-term future for these materials.
Conserving tooth structure through the acid-etch tech nique remains the major benefit of the composite resin systems over some of the stronger, but more radical alternative materials requiring tooth structure removal. ANTERIOR AND POSTERIOR COMPOSITE RESINS
Light-curing technology has been an im portant advance th at has affected both anterior and posterior resin systems. Light curing has simplified the proce dure for clinicians and increased the color stability of the restoration for the patient. Appearance is a major advantage of the composite resin systems over the previously discussed materials. Composite resins, with the aid of bonding resins, can be bonded to enamel thus allowing conservative cavity preparation, such as the prevent
ive resin restoration,16and saving tooth structure. Conserving tooth structure through the acid-etch technique rem ains the major benefit of the composite resin systems over some of the stronger, but m ore radical alternative m aterials requiring tooth structure removal. For inter proxim al restor ations, conservative preparations such as the tunnel preparation have been advocated.17 Perhaps the major disadvantage of composite resin systems is the polym erization shrinkage of the resin. This shrinkage can lead to marginal leakage as the force of the resin shrinkage may, under certain circum stances, exceed the bond strength of resin to tooth structure and cause the m arginal seal to fracture. Additionally, the technique-sensitivity of the acidetch technique can be regarded as a disadvantage in the use of composite resin systems inasm uch as the procedure is clinically dem anding and more timeconsum ing than an amalgam restoration. Another disadvantage is the cost to the patient which will be higher than th at for a simpler, faster procedure. Some simplification of clinical procedures, m aking resin bonding to tooth structure a m ore userfriendly procedure, is likely to occur soon. F uture developments, particularly in the area of filler particles and in filler particle size distribution, will improve clinical performance. Hopefully, the polymerization shrinkage problem will be solved or minimized. GLASS IONOMER MATERIALS
This group of m aterials has m et with greater clinical success than clinical acceptance by the profession. The limited acceptance can be traced to the dem anding clinical technique and the JADA, Vol. 122, July 1991
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relatively long setting tim e for glass ionom er m aterials in the mouth. Recent developments that have modified the glass ionomer m aterial with resin, thus allowing light curing of the material, indicate a bright future for this group of m aterials. Restorative m aterials and bases based on the light-cured glass ionom er technology will be particularly beneficial in the restorative treatm ent of young children and geriatric adults, w here a simple-touse, fast-setting, strong, fluoridereleasing m aterial is ideal. Although glass ionomer m aterials have been used as sealants, the retention rates have been low.18-19The glass ionomer sealants cannot m atch the docum ented retention rates of the resin sealants. Additionally, the clinical technique for glass ionom er m aterials use as initially developed is cumbersome com pared w ith the comparable technique for resins. The major advantage of using a glass ionom er m aterial as a restorative m aterial is that fluoride is released from the material and the m argins of the restoration are, therefore, less caries-susceptible. Glass ionom er restorations are microleakage resistant inasm uch as they bond to enamel and dentin and are kind to the pulp. Without the benefit of fluoride release, however, glass ionomer m aterials should not be used, except in the restoration of root caries, for they are not as strong or as esthetically pleasing as competitive resin materials. The developm ent by Mitra of the resin-modified glass ionomer m aterials has been one of the most im portant recent developments in dental m aterials research and will have a great im pact on this group of m aterials.20'22Mitra’s developm ent resulted in the 30
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production of a light-cured glass ionom er liner base. When used as a lining m aterial, such resinmodified glass ionom er materials bond to dentin with considerably higher bond strength (approxi m ately 11 megapascals shear bond strength to dentin), than conventional glass ionomer m aterials (approximately 4 MPa shear bond strength). Much of this difference is caused by the greater cohesive strength of the resinmodified glass ionomer material. Applying this technology to light-cured glass ionomer restor ative m aterials would radically alter the caries treatm ent of the prim ary dentition and treatm ent of root caries in elderly patients. Conventional glass ionomer restorative m aterials are ham pered by the sometimes inconvenient technique—the m aterials m ust neither become dehydrated, nor become too wet during curing, and conventional setting can take four to five m inutes. Incorporating lightcuring technology into glass ionomer restorative materials would elim inate these disadvant ages, while m aintaining the advantages of fluoride release, and of a strong bond to dentin. When this is done, the im portance of glass ionom er m aterials in restora tive dentistry will increase greatly. PORCELAIN
For crown and bridge, porcelain has the advantages of appearance, strength (single units), and a long history of clinical success. Recent development in fine-particle ceramics, where fine particles of alum inum oxide are infiltrated with glass for use as a core under conventional porcelain, has greatly improved strength. If clinical testing is favorable, this developm ent may expand the general use of porcelain, after
appropriate clinical testing, into short-span anterior, and perhaps even posterior, Dr. Simonsen is bridges. professional The services manager, Dental Products development of Division, 3M Health etching Care, 3M Center Bldg. 225-4S-11, St. porcelain has Paul, Minn. 55144increased 10OO and is currently applications for on a leave of ab sence as professor, this material. It Department of was stated as a General Dentistry, College of Dentistry, conclusion of University of Tenne the original 1983 ssee, Memphis. presentation of Address requests for reprints to the the etched author. porcelain research that “bond strengths of the m agnitude reported here are expected to be clinically significant for the retention of porcelain veneers and for other intraoral uses of porcelain.”23 While porcelain has been increasingly used in recent years in bonded inlays and onlays, it is only as a labial veneer that the material has met with incontro vertible success as a bonded restoration. The fragile nature of a thin veneer is not a problem during clinical bonding and the adoption of the porcelain veneer procedure prom oted esthetic dentistry consciousness by both dentists and the public. However, the fragility of an inlay or onlay is a significant disadvantage because the restoration m ust be bonded before occlusal adjustm ent. Necessary occlusal adjustm ent removes the glaze, exposing the particulate nature of the material, thus causing potentially disastrous consequences in wear on the opposing enamel, depending on the porcelain used. The disadvantages, therefore, of porcelain are related mainly to the
wear on opposing enamel and to the material’s brittleness. In the future, some of the current wear problems will be eliminated by a reinforced resin system that will not have the brittleness of current porcelains. INDIRECT RESTORATIVE MATERIALS
Burke and others evaluated five indirect composite resin inlay systems.24The authors had generally positive conclusions about the indirect systems. Based on the current poor acceptance, however, by the profession for the commercially available indirect composite inlay systems, it is doubtful that such systems will become well accepted at this time. The techniques increase cost with little time savings or performance improvement of the restorations. SUMMARY
Two areas of research will prosper and change operative dentistry in the future: developing light-cured glass ionomer technology further, and reinforcing direct composite resin technology. Successful development of restorative materials in these areas will have an immediate and long-term effect on the practice of operative dentistry in both affluent and poor countries of the world. ■ Publication of names of products does not imply endorsement by the American Dental Association. This paper was presented April 25,1991 at the Scientific Frontiers in Clinical Dentistry Symposium, National Institute for Dental Research, Bethesda, MD. The author thanks Dr. William H. Douglas for his contributions during the planning of the oral presentation of this paper. 1. Smales RJ, Webster DA, Leppard PI, Dawson AS. Prediction of amalgam restoration longevity. J Dent 1991;19:18-23 2. Mjor, IA. Amalgam and composite resin restorations: longevity and reasons for replacement. In: Quality evaluations of dental restorations: criteria for placement and replacement. Anusavice K J, ed. Chicago: Quintessence; 1989:61-8. 3. "Hie mercury in your mouth. Consumer Reports May 1991:316-9. 4. Letzel H, Vrijhoef MM. The influence of polishing on the marginal integrity of amalgam restorations. J Oral Rehabil 1984;11:89-94.
5. Jeffrey 1WM, Pitts NB. Finishing of amalgam restorations: to what degree is it necessary? J Dent 1989;17:55-60. 6. Simonsen RJ, Thompson VP, Barrack G. Etched cast restorations: clinical and laboratory techniques. Chicago:Quintessence;1983. 7. Horowitz HS, Heifetz SB, Poulsen S. Retention and effectiveness of a single application of an adhesive sealant in preventing occlusal caries: final report after five years of study in Kalispell, Montana. JADA1977; 95:1133-9. 8. Rock WP. The effectiveness of fissure sealant resins. National Institutes of Health Consensus Development Conference. Dental sealants in the prevention of tooth decay J Dent Educ (Supplement) 1984;48:27-31. 9. Hyatt TP. Prophylactic odontotomy: the cutting into the tooth for the prevention of disease. Dent Cosmos 1923; 65:23441. 10. Simonsen RJ. Retention and effectiveness of a single application of white sealant after ten years. JADA 1987;115:31-6. 11. Wendt LK, Koch G. Fissure sealant in permanent molars after 10 years. Swed Dent J 1988;12:181-5. 12. Romke RG, Lewis DW, Maze BD, Vickerson RA. Retention and maintenance of fissure sealants over 10 years. J Can Dent Assoc 1990; 56:235-7. 13. Stiles HM, Ward GT, Woolridge ED, Meyers R. Adhesive sealant clinical trial: comparative results of application by a dentist or dental auxiliaries. J Prev Dent 1976;3:8-11. 14. US Public Health Service. Oral health of United States children. The national survey of dental caries in United States school children, 1986-1987. Hyattsville: National Center for Health Statistics, 1989; NIH publication No. 89-2247. 15. Ripa LW. A critique of topical fluoride methods (dentifrices, mouthrinses, operator- and self-applied gels) in an era of decreased caries and increased fluorosis prevalence. J Public Health Dent 1991;1:23-41. 16. Simonsen RJ. Clinical applications of the acid etch technique. Chicago: Quintessence;1978:89-101. 17. Croll TP. Glass ionomer-silver cermet bonded composite resin Class II tunnel restorations. Quintessence Int 1988; 19:533-9. 18. Boksman L, Gratton DR, McCutcheon E, Plotzke OB. Clinical evaluation of a glass ionomer cement as a fissure sealant. Quintessence Int 1987; 18:707-9. 19. Hickel R, Voss A. Comparative studies on fissure sealing: composite versus Cermet cement. Dtsch Zahnarztl Z 1989; 44:4724. 20. Mitra SB. European patent application 323,120. 21. Mitra SB. Adhesion to dentin and physical properties of a light-cured glass-ionomer liner/base. J Dent Res 1991; 70:72-4. 22. Mitra SB. In vitro fluoride release from a light-cured glass-ionomer liner/base. J Dent Res 1991:70:75-8. 23. Simonsen RJ, Calamia JR. Tensile bond strength of etched porcelain. J Dent Res (abstract no. 79) 1983; 62:297. 24. Burke FJT, Watts DC, Wilson NHF, Wilson MA. Current status and rationale for composite inlays and onlays. Br Dent J 1991;170:269-73.