1997-3-Guest Editorial - Saudi Dental Journal

ISSN (Print) 1013-9052
EISSN 1658-3558

The Saudi Dental Journal,
P.O. Box 52500,
Riyadh 11563,
Kingdom of Saudi Arabia

Tel.	966-1-467-7328
Fax.	933-1-467-7308 / 966-1-467-7534
Email	saudidj@ksu.edu.sa

Editorial

Restorative Materials for the 21st Century

The oral environment makes heavy demands on prosthetic materials. Stress corrosion, abrasion, mechanical failure and hydrolytic breakdown are some of the major problems encountered in clinical service.

The increasing needs of industry for high strength metals, plastics and ceramics has added a considerable impetus to new materials research during the last decade. It is from such research that dentistry may find new and improved restorative materials. Three main groups of materials that are of interest to the dentist are metal alloys, ceramics and cements, and plastics.

Metal Alloys

If we ignore the aesthetic factor, metals are the obvious choice in areas of widespread tooth destruction and heavy occlusal loading. One cannot envisage the replacement of the current dental alloys which meet most of our mechanical requirements and gold alloy castings may be used in thin section to restore occlusion, maintain proximal and occlusal contact areas for the lifetime of the patient. Gold occlusions will continue to set the standard for restorative materials well into the next century and the question remains as to whether they can be replaced with plastics or ceramics.

Adhesion and Osseointegration

The most important property required from prosthetic materials, besides mechanical strength and hydrolytic stability, is longterm physical and chemical adhesion to tooth and bone structure. The impact of the titanium implant and its establishment as a biologically acceptable material capable of integration with bone is receiving increasing attention. The osseointegrated implant developed by Branemark will be recognized as the outstanding dental discovery of the 20th century.

Adhesion of materials to tooth structure and the capability of materials to integrate with bone will be a main area of research in the next century. We now have a better understanding of the properties required from dental materials for long-term bonding to both hard and soft tissues.

Osseointegration and titanium dioxide

An explanation for the success of the titanium implant may lie in its oxide layer. Titanium is covered by a dense layer of titanium dioxide which is self healing. In bulk titanium oxide, each titanium atom is surrounded by six oxygen atoms, that is Ti sits at the center of an oxygen polyhedron. The formula for titanium oxide is normally given as Ti0₂ and this is the case when the titanium oxygen polyhedra are normally linked via oxygen atoms at the corners. However, these polyhedra sometimes link at edges when the formulae depart from Ti0₂; the general formulae for titanium oxide is: Ti_n0_2n.i and n can have the values 4,5,6,7, 8,9,10. The last gives Ti0₂. At the surface Ti is 4 or 5-, rather than 6coordinate as in the bulk oxide. At the surface there is a layer of oxygen and titanium atoms arrayed in the same plane. Proud, above there is a layer with few oxygen atoms, these superficial oxygen atoms do not form a continuous layer. Water is absorbed on this layer and is dissociated; this process is known as dissociated chemisorption of water.

H₂0 -H⁺ + OH
The proton H⁺ react with the superficial oxygen atoms to form OH groups and the OHbecomes attached, as hydroxy 1 groups, to oxygen atoms in 5fold coordination. There are at least two kinds of hydroxy Is, basic and acidic. It seems likely that the hydroxyls formed from protons and superficial oxygens are acidic and those formed by the direct attachment of hydroxyls to 5coordinated oxygens are basic. The acidic hydroxyls react to form surface bicarbonates, and basic hydroxyls will react with bases provided they are not very weak; trimethylamine does react but not pyridine.

It seems likely that the oxygen layer on titanium implants will react with moisture to form both acidic and basic hydroxyl groups. It is almost certain that these will react with acidic and basic substances in the body providing the implant is inserted under sterile conditions.

Glass-ionomer Cements

The principal barrier to effective adhesion to dental tissue is water. Dentine is permeated by aqueous fluids transported from the pulp, and there is both loosely and tightly bound water in the surface of enamel. In the oral situation, organic adhesives are either insufficiently polar to compete with water for the surface of dental tissue, or, if highly polar, the bond they form is hydrolytically unstable. By contrast the hydrophilic, highly ionic glass-ionomer cement competes successfully with water because of its multiplicity of carboxyl groups that form hydrogen bonds with the substrate. Wa ter is displaced or even incorporated into the cement. Adhesion is permanent, because of the multiplicity of hydrogen and ionic bonds that attach the glass-ionomer cement to the substrate in an octopus like fashion, making temporary scission of one bond inconsequential and probably accounts for the cement's unique property of being permanently adhesive under oral conditions.

The clinical role of the glass-ionomer cements is now well established and they are of particular value as dentine substitutes under restorations and for the treatment of early carious lesions using microcavity preparation techniques. If used correctly, these cements can avoid the use of more expensive metal or porcelain restorations. However, their mechanical strength is poor and, at the present time, restricts them to use in low stressbearing areas.

What of the future? Resin-modified cements of higher mechanical strengths have now been produced in which the polyacid is modified by grafting methacrylate groups onto the poly (acrylic acid) chain. These resin modified cements can then have photo-iniators incorporated to permit photocuring. Unfortunately these cements have a tendency to swell in water due to the incorporation of hydroxyethyl methacrylate which is needed as a cosolvent and is a potential source of discoloration and may reduce strength. Work is now continuing to try and eliminate these problems and, there is little doubt, that in the next century we shall see cements with mechanical properties that can compete with the carbon plastics.

Polymers and bonding agents

The original invention of the epoxy resins by Castan at De Trey's Zurich in 1938 really set the stage for all future developments of the composite resins as restorative materials. More recently, the urethane dimethacrylates were developed at ICI for dental use and the first visible light-curing systems were marketed. These new resins have the advan tage of higher molecular weight, lower viscosity, and a certain degree of toughness in the urethane moiety, together with less staining than the bisGMA resins.

The acid-etch technique for bonding these resins to enamel, suggested by Buonocore in the 50's, has now become firmly established. Dentinal bonding agents for bonding anterior resin composites to dentine are also used for bonding porcelain veneers, inlays and some metal restorations to tooth structure. They possess higher bond strengths than do glass-ionomer cements but require greater attention to the preparation of surfaces for bonding. Modern techniques are employing weaker acids, such as maleic acid, to prevent damage to the pulp in the deeper cavity, and these etchants facilitate the formation of a hybrid layer. The success of dentinal bonding is still dependent on the morphology of the dentine and, in areas lacking a high percentage of intertubular dentine, problems can arise.

Future research on dentinal bonding agents employing glass-ionomer technology could provide some chemical bonding as well as materials with increased fracture toughness, negligible setting shrinkage, and thermal expansion similar to that of tooth structure. Long-term stability at the enameldentine interface can only be achieved with restorative materials that have these properties.

Polymer research appears to have reached a plateau, and progress in the resin composite field has not moved very much in the last two decades. Unless there is a major breakthrough by one of the major chemical companies, it is unlikely, in the foreseeable future, that we shall see any new dental composites that have significantly better properties than our current materials. Improved filler systems and better bonding between the filler and the resin matrix may allow the composites to compete with amalgam alloys but, at present, it is hardly surprising that the profession is turning to bonded ceramic inlays and porcelain veneers where an aesthetic restoration is required.

Ceramics

Ceramics are brittle materials with low fracture toughness and may cause excessive wear to opposing dentitions, properties that are less than ideal for replacing hard tissues in the body. However, despite these disadvantages, their use is increasing due to the need for a durable aesthetic material which can be used for crowns, bridges, veneers and inlays. Providing they are not used in clinical situations where very high tensile stresses occur and the design of the restoration conforms to good engineering practices where cross-sectional areas are thickened in the higher stress-bearing areas, then longterm resistance to fracture may be achieved.

The first high strength ceramics using alumina crystal reinforcement were marketed in the 1960's and termed aluminous porcelain. Since then much work has been done on the development of high strength ceramic for dentistry. Castable glassceramics have been produced for the construction of crowns and inlays but it was not until acid-etch resin bonding techniques were used that the full potential of the bonded ceramic inlay or veneer was realized. Resin bonding appears to redistribute stresses on the brittle porcelain and reduce the risk of crack propagation from the fit surface of the restoration. More recently, machinable ceramics have been produced for making inlays and it is likely that this work will be carried further and enable simpler and more commercially viable techniques to be used. At the present time, the accuracy of fit of these restorations leaves much to be desired but it is not inconceivable that improved luting cements will be produced with significantly higher wear resistance that can take care of this problem.

Sintered high-alumina of 98% purity is being used to replace metal copings in the ceramic crown and has also found a place as an implant superstructure in order to improve aesthetics in the gingival area. Slip cast alumina ceramics are being used to make copings for aluminous porcelain crowns and their greatly improved strength has allowed the construction of all-ceramic crowns on anterior teeth even in high stress-bearing areas.

Until an abrasion resistant and hydrolytically stable resin polymer is produced, it is likely that the profession will continue to expand the use of high strength ceramics. However, it should be recognized that improvements in the metal-ceramic systems, particularly with regard to color and the range of porcelains marketed, has reached a stage where anterior aesthetics is no longer a problem for the skilled technician. Metal-ceramic systems will continue to be used well into the 21st century, and if we look back at the last century most of the progress made in new materials research has been in the latter half. However, even these improvements were built on fundamental knowledge gathered before World War II and major breakthroughs in materials science are rare events.

Metal-ceramics, glass-ceramics, high alumina ceramics, cements based on poly (acrylic acid), epoxy resins and the acid-etch attachment of visible light-cured resins to tooth structure are some of the newer materials and technologies that have had a major impact on clinical dentistry in the last century.

John W. McLean, OBE, FDSRCS (Eng), MDS, DSc (London), D Odont (Lund)
Consulting Professor in Fixed Prosthodontics and Biomaterials,
Louisiana State University, New Orleans, LA, USA
and Senior Research Fellow, Eastman Dental Hospital, London, UK