The history of Glass Ionomer Cement
Carel L. Davidson, PhD.
Throughout over 4000 years of history, those who worked on teeth in order to embellish teeth or to limit decay, were looking for a durable substance that could mimic or decorate tooth structure and that could be united to the substrate. The ideal material would be self-adhesive. It would last until halfway the twentieth century before materials became available with more or less proper adhesive properties. One was searching for suitable cements, that would be mechanically and chemically stable in the oral cavity for an satisfactory long time. The term ‘cement’ covers a variety of pastes that has the potential to adhere one way or the other to the substrate after hardening. Most known cements however do not adhere in the true physical-chemical sense but find retention through micro-mechanical interlock. Traditionally, the setting mechanism of cements was based on an inorganic acid-base reaction. Since halfway last century, also resin-based cements came available. Thereafter, also blends of inorganic and organic setting mechanisms are being employed.
The earliest successful dental cement was a reaction product of basic silica and phosphoric acid, leading to a more or less tooth structure mimicking hard substance. Notwithstanding the dimensional instability and thus poor sealing capacity of silicate dental fillings, secondary caries was seldomly observed. Many years later, this remarkable phenomenon could scientifically be explained when the cariostatic potential of fluorides was scientifically understood.
Parallel to the dental silicate cements, the fully opaque, but this time, highly dimensional stable, zinc-oxide phosphate cements were introduces in dentistry. Both cement families were not adhesive to tooth structure.
By replacing the inorganic phosphoric acid by aqueous solutions of various organic acids, particularly poly-acrylic acid, direct chemical bonding to mineralized tissues could be obtained. The cradle of exploring the possibilities of poly-acrylic acid for dental application stood in the UK. In 1968 Dennis Smith formulated zinc poly-carboxylate cement, the first direct bonding dental cement. After establishing a proper understanding of the chemistry of dental silicates, dental materials scientists at the Laboratory of the Government Chemist in UK were able to develop the logic, still missing dental cement with desirable properties such as adhesion to tooth structure and base metals, anticariogenic and remineralizing potential due to fluoride release, thermal compatibility with tooth structure and dependable biocompatibility. The material was even ascribed to be intelligent and bio-mimic. In short, this could be achieved by replacing also the basic component zinc-oxide (ZnO) in the carboxylate cement by translucent calcium, strontium aluminum-silicate oxides, thus generating the family of glass ionomer cements.
The more or less simple interrelation of the various cements is shown below:
|phosphoric acid||poly-acrylic acid|
|ZnO||phosphate cement||carboxylate cement|
|SiO2||silicate cement||glass-ionomer cement|
Because the composition of both the glass-ionomer cement constituents can chemically be modified in many ways, a wide range of glass-ionomer cements have been proposed, from which some appeared to hold a potential for becoming the ideal all-purpose dental restorative.
The earliest glass-ionomer cements were on paper very promising, but clinically almost unworkable and chemically unstable. Particularly shelf life, handling, working time and setting time were insufficient and also esthetics were clinically not acceptable. By increasing the glass activity through adding more alumina, the reactivity and thus the setting could be improved. In 1972, Alan Wilson
and Brian Kent formulated a high fluoride containing , fast reacting glass with which setting time could be shortened to a clinically acceptable time span. Amongst others, particularly John McLean in the UK and Graham Mount in Australia pioneered with this groundbreaking new dental restorative material and gathered following in the dental schools of these countries.
Yet for many years, amongst clinicians, glass-ionomers remained an only hesitantly accepted material. The first product was marketed by De Trey, Dentsply at the brand name ASPA, an abbreviation of Alumina- Silicate Poly-acrylic Acid. After ASPA I followed ASPA II, III and IV. Notwithstanding the direct-adhesive properties of ASPA, the material could not compete with the, then also still far from perfect, resin-based composites. Particularly when the light-curing resin-based composites were introduced, clinicians preferred these set-on-command and more esthetic materials above the disobediently hardening and limited esthetic glass-ionomers.
To get rid of the impractical viscosity of the high molecular poly-acid liquid also poly acrylic acid in solid form for glass-ionomer formulations was employed to be mixed with water or an aqueous solution of tartaric acid. Some commercial brands of such products were Chem- Fil and Ketac -Cem.
While resin-based composites were preferred in the USA and thus development and local marketing was mainly restricted to these restorative materials, glass-ionomer research got appropriate attention in Japan and Germany. An almost endless series of products, all with specific clinical applications, were since marked under various brand names. The Japanese GC company launched the Fuji-line products such as Fuji 1, Fuji Cem, Fuji IX and Fuji VII, while the German company ESPE developed and marketed their glass ionomer restoratives with names such as Ketac Fil, Ketac Molar or Ketac Silver. The latter, a so called silver glass ionomer was designed as an alternative to amalgam. Indeed, the resistance to premature abrasion could be enhanced by admixing or sintering metals to the glass. A number of metal powders were tried, including alloys of silver and tin, pure silver, gold, titanium and palladium. This glass-ionomer generation were called Cermet- ionomer cements. However, their strength showed still to be insufficient to replace amalgam alloys or resin-based composites in stress bearing restorations.
In the late 1980s attempts were made to overcome the moisture sensitivity and lack of command cure of the glass-ionomers by hybridizing the benefits of the glass-ionomer formula and those of resin-based materials into the so called resin modified glass ionomers cements (RM GICs). In these materials the acid-base curing reaction is accompanied by a succeeding light curing process. Due to the more or less independent curing mechanisms of the various components, these products are considered to be dual -cure or even tri -cure cements.
The first commercial RM GICs available were liner/bases, Vitrebond (3M
Dental, USA) being the first introduced. Later other companies followed this direction as well, like GC introducing the light-curing product Fuji LC.
Resin modified glass ionomer cements have properties in-between to conventional glass ionomer materials and resin-based composites. In general they have the advantages of both such as greater working time, command set on application of visible light, good adaptation and adhesion, acceptable fluoride release, aesthetics similar to those of composites, and superior flexural strength characteristics. However, resin modified glass-ionomer cements still suffer from certain drawbacks such as setting shrinkage, limited depth of .The subsequent development of compomers (poly-acid modified composites) was initially welcomed as the easy to manipulate restorative, everyone was waiting for, but soon recognized as a too limited and nondurable material.
Not amongst dental material scientists, nor amongst general dental practitioners, there exists a wide-ranging satisfaction and consensus about the quality of glass-ionomers and their derivates as an universal dental restorative, like there was for dental amalgam and still is for resin-based composites. However dental amalgam becomes gradually a material of the past with no future. Resin-based composites are still widely used, but its further development has little perspective, while concern about their bio-compatibility is increasing growing. In contrast, the very reputable biocompatible glass-ionomer concept still allows a variety of new formulations. Next to sophistication of the particle size and distribution as well as the chemistry, new application techniques have been proposed, such as moderate heating as a means to accelerate and intensify the chemical reaction, thus partly solving problems with command setting and strength.
In conclusion, the full history of glass-ionomer is all but settled. The prospective of glass-ionomers as a mineralizing agent for repair of the adjoining dental tissues is still not fully exploited and deserves intense consideration. It can be expected that further improvements will come to be and glass-ionomer cement formulations will gain more importance and recognition in preventive and restorative dentistry. In this perspective, it can be expected that glass-ionomer-like formulations will play the prominent role in the concept of biomimesis, the exploration of biomaterials that repair or reproduce the original tissues best.