Scope and History of Restorative Materials

BASIC SCIENCES APPLIED TO RESTORATIVE MATERIALS

The sciences of primary interest to the dentist are derived from the three basic scientific fields: biology, chemistry, and physics. Fig. 1-1 indicates the relationship of the three basic fields to applied techniques and clinical dentistry. The practice of clinical dentistry depends not only on a complete understanding of the various applied techniques but also on an appreciation of the fundamental biological, chemical, and physical principles that underlie the applied techniques. A failure to know the scientific principle on which a technique is established often leads to its incorrect application.

It is evident that chemistry, physics, and related engineering sciences serve as the foundation for the science of restorative materials. Most persons working with restorative materials believe that physical, chemical, and mechanical studies cannot be separated from physiological, pathological, or other biological studies of the tissues that support and tolerate the restorative structures. Certainly from the clinical standpoint it is desirable to keep the studies and interpretations as practical as possible. If practical clinical dentistry and the theoretical scientific aspects of restorative materials are allowed to develop without suitable correlation, neither is likely to progress as it should or be useful to the other.

APPLICATION OF VARIOUS SCIENCES

The fundamental principles of the physical sciences find application in the comparison of the physical characteristics with the structural applications of restorative materials. In the chapters that follow, numerous practical examples of basic principles are presented, and throughout the discussions the fundamental characteristics are stressed, with a minimum of emphasis on test procedures and techniques of manipulation.

A more complete understanding of these and other fundamental principles is important to the dentist as an aid in understanding such typical phenomena as the melting and freezing of casting alloys, the volatilization of liquids with the accompanying cooling action, or the crystal structure produced in solidified metals as compared with the essentially noncrystalline structure of hydrocolloid impressions and denture base materials. The branch of physical chemistry that considers the colloidal state of matter has been applied for years successfully to the sciences of medicine, physiology, botany, and engineering. Numerous illustrations of the application of colloid and surface chemistry to dentistry, oral conditions, and dental materials are also known.

To understand the complex nature of metals and alloys used for cast inlays or removable partial dentures, one should know something of the physical and chemical reactions that influence the combination of metals and alloys in liquid and solid states. Alloys that demand heat treatment to produce the optimum properties offer an example of the application of such physicochemical principles. With knowledge of physicochemical principles and the numerous principles of good metallurgical casting practices and fabrication of structures through soldering and assembly, it is possible to design and construct remarkably effective dental restorations and appliances.

Knowledge of organic and polymer chemistry, the mechanics of restorative structures and mastication, and something of the biophysical principles involved in complete denture restoration is desirable for designing and constructing complete dentures. Stress analysis of the various types of restorations involves physical principles that are closely related to successful design as well as the biophysical analysis of the support structures.

The toxicity of and tissue reactions to dental materials are receiving more attention as a wider variety of materials are used and as federal agencies demonstrate more concern in this area. A further indication of the importance of the interaction of materials and tissues is the development of recommended standard practices and tests for the biological interaction of materials through the auspices of the American Dental Association (ADA).

After many centuries of dental practice, we continue to be confronted with the problem of replacing tooth tissue lost by either accident or disease. In an effort to constantly improve our restorative capabilities, the dental profession will continue to draw from contemporary arts and sciences to further develop an integrated science of dentistry.

EARLY HISTORY

Among the earliest recorded examples of dental prostheses are the gold structures of the Phoenicians, the Etruscans, and, a little later, the Greeks and Romans. Their restorations date back to a period several hundred years before the beginning of Christianity. Gold has been employed for prosthetic dental purposes for at least 2500 years. The ancient Babylonians, Assyrians, and Egyptians (4500 to 4000 BC) were familiar with gold, silver, copper, and lead. It remained for the Phoenicians (about 2700 BC) to spread the culture along the shores of the Mediterranean. They practically controlled the tin trade (which was important for the bronze industry) during the period 1000 to 300 BC, and were considered the most skillful metallurgists of the ancient world. Iron was known to them as early as 990 BC.

It is not certain exactly how or by whom the earliest appliances were constructed. Possibly they were made by skilled metalworkers and not by those who practiced the dental art. The role played by the goldsmiths and other artisans is comparable to that of modern laboratory technicians.

The practice of using gold crowns and bridgework apparently flourished in Etruria and Rome as early as 700 to 500 BC. These people must have understood the arts of soldering and riveting to have prepared a restoration from pure gold rings soldered in correct relations, with the artificial tooth held in place by a pin that passed through both the artificial tooth and the gold ring. The extensive use of solder to prepare the appliance implies some knowledge of the simple alloying of gold and the preparation and use of fluxes and perhaps antifluxes.

The teeth used in the ancient appliances were either human or carved from the teeth of an animal. The early Phoenician restorations represent an interesting example of the use of wire to hold the teeth in a more or less fixed position. It appears therefore that the art of wire fabrication was known to this ancient civilization. Hippocrates, who was born in 460 BC, apparently used gold wire and linen thread for ligatures in the repair of bone fractures. He was likewise reported to be the inventor of a type of crude dental forceps and other dental instruments. In modern dentistry the oral surgeon is interested in the properties and behavior not only of ligature materials, but also of hypodermic needles, cobalt-chromium and titanium alloy screws and appliances, tantalum or titanium plates, and various instruments.

Filling carious teeth for preservation apparently was not practiced extensively by ancient civilizations. Celsus (first century AD) recommended the filling of large cavities with lint, lead, and other substances before attempting extraction to prevent the tooth from breaking under the pressure of the instrument. This may have been the beginning of filling materials for carious teeth.

Restorative dental materials were relatively simple in character and few in number at the end of this ancient period. However, a beginning had been made, and mankind was conscious of the desirability of replacing lost tooth tissue.

MEDIEVAL AND EARLY MODERN PERIOD

BEGINNING OF DENTAL SCIENCE—1600 TO 1840

During the period from 1600 to 1840, the foundations for the science of dentistry were established. So little progress had been made up to this time that dentistry was merely an art practiced largely by the barber-surgeons or artisans. Few records of results were kept, and little thought was given to the improvement of methods before the beginning of the seventeenth century. Nevertheless, a special type of medical-dental practitioner was recognized by the medical profession.

By the end of the sixteenth century a limited knowledge of dentistry had spread to most of the countries in Europe. France, England, and other nations had been established after the age of the feudal system. Carved bone and ivory teeth held to neighboring teeth with gold and silver wire were used in France, Germany, and Italy.

Wax models used in prosthetic work are first mentioned by Matthaeus Gottfried Purmann about 1700. It is supposed that the wax was carved to the desired shape, after which it was reproduced in bone or ivory by a craftsman.

Much progress in dentistry was made during the eighteenth century. Pierre Fauchard described the materials and practices of his time in his book Le chirurgien dentiste, ov Traité des dents, published in 1728. He discussed many phases of dentistry, including operative and prosthetic procedures. He collected and catalogued much of the information that was good in dentistry at his time. There were dental texts before the time of Fauchard, but they were considerably more limited in extent and application.

Fauchard mentioned lead, tin, and gold as filling materials. He preferred tin because of the ease with which it could be adapted to cavity walls. Separate ivory or natural teeth with wood pivots were fastened in position with a cement compound of sealing wax, turpentine, and white copal, or were set into a low-melting-point alloy used to fill the canal. The use of the dental file had become common practice by the time of Fauchard, and emery wheels for grinding down teeth had been introduced by a Dutch physician, Kornelis van Soolingen, during the latter part of the seventeenth century. According to Vincenzo Guerini, it was Lorenz Heister (1683–1758) who first mentioned removable prosthetic appliances. The introduction of fused porcelain for teeth in 1789 is regarded as one of the most important events in the history of dentistry. It represents the beginning of scientific improvements in the restorative art of dental practice.

The first book to describe mechanical dentistry was that of Claude Mouton in 1746, Essay d’odontotechnie, ou dissertation sur les dents artificielles. He mentioned gold-shell crowns swaged from one piece of metal and the use of gold clasps instead of ligatures to retain artificial teeth. Clasps to retain partial dentures were in common use in 1796. Numerous other dental texts were written during the late eighteenth and early nineteenth centuries. Etienne Bourdet (1775) made the first reference to the use of a gold base to support artificial ivory teeth fixed with gold pins. Low-fusing metal alloy was introduced by Jean Darcet in 1770.

A baked-porcelain, complete denture made in a single block was first displayed by the French dentist Nicholas Dubois de Chemant in 1788. In 1797, he wrote a book in English, A Dissertation on Artificial Teeth, describing porcelain. The Italian dentist Guiseppangelo Fonzi, who lived in Paris, is credited with preparing the first baked-porcelain single tooth with attached platinum hooks about 1806 to 1808. He is also credited with preparing 26 shades of porcelain by using metallic oxides.

It is claimed that the first American book on dentistry was written by R.C. Skinner around 1801 (A Treatise on the Human Teeth). By this time dentistry was no longer entirely in the hands of barbers or artisans, but was practiced by professionally minded dentists or surgeons who warned the public against pretenders. Forty-four treatises on dentistry were published in the United States between 1800 and 1840.

The combination of silver and mercury to form amalgam “silver paste” was announced by O. Taveau of Paris in 1826. This was the beginning of dental amalgam, which is recognized as one of the outstanding developments in the field of restorative materials.

Although French dentists may well be considered the leaders of this period, dentists in other countries of Europe were quick to adopt French practices or their equivalent, and in a few instances made additional contributions. In Germany there was little progress until the sixteenth century. Mention of the use of gold foil in German writings during this century is common. Philip Pfaff (1756) is credited with being the first to use plaster models prepared from sectional wax impressions of the mouth.

In Great Britain, dentistry did not develop much until the eighteenth century. The work of Fauchard was not generally known in Great Britain, and the first comprehensive textbook in English appeared in 1768, although in 1686 Charles Allen had written a book on teeth in which he described a method of transplantation. By the early part of the nineteenth century, dental practice had apparently improved somewhat in Great Britain. Retentive cavities for gold fillings were prepared in Edinburgh in 1787.

James Snell (1832) wrote that he preferred forceps to the key for use in extractions. He chose gold for filling carious teeth, and he described two types of cement that might be used, although these cements offered little promise of success. Zinc oxychloride cement did not come into use until 20 years later.

Considerable progress was made toward the perfection of porcelain teeth in France, England, and the United States before 1840. These teeth had been introduced in the United States from France in 1817. By 1825, porcelain teeth were being produced and improved in America. The replacement of carved bone and ivory or natural teeth by the fused mineral product was another step forward for the profession and represents one of the first great improvements in dental materials. The Ash tube tooth introduced in 1838 was produced until recent times with only slight modification in form.

In the United States, Wooffendale is said to have introduced gold foil after he settled in New York in 1767 to practice dentistry. Tin and lead were also used as filling materials at this time. Carved ivory and bone dentures, ivory or natural teeth with metal pivots, silk and wire ligatures, and files for the removal of carious lesions were in common use. Paul Revere is credited with being a skilled ivory turner and goldsmith who applied his skill to the production of artificial teeth. Gold points were used to fill root canals in 1805 by Edward Hudson in Philadelphia. “Silver paste,” the amalgam of silver with mercury, was introduced in the United States as a filling material by the Crawcour brothers in 1833.

Dental materials were beginning to be produced in America during the early part of the nineteenth century. Before that time they were imported from Europe. Gold coins were rolled into a noncohesive gold filling material in 1800, and by 1812 gold foil was being produced by the beating method by Marcus Bull in Hartford, Connecticut. He founded a company that later became the J.M. Ney Company, a leading dental gold alloy manufacturer. Thus the first American-made dental products were gold foil and dental porcelain.

By 1840 the practice of dentistry in America had reached a definite turning point. The first dental journal in the world, The American Journal of Dental Science, was established in 1839. The first national dental society, the American Society of Dental Surgeons, was established in 1840. The first dental school, the Baltimore College of Dental Surgery, was established the same year. H.H. Hayden and C.A. Harris were both active in these three institutions. This same year Charles Goodyear discovered the process of dry-heat vulcanization of rubber, in which he heated together caoutchouc, sulfur, and white lead, which later made possible the introduction of a most useful dental material—vulcanite.

Progress still was not rapid in the early part of the nineteenth century, but dentistry was moving toward an improved and established science. It has been estimated that by 1830 the total number of dentists in the United States had increased to about 300, with an advancement in every department of dentistry as a science.

THE PERIOD OF MECHANICAL IMPROVEMENT—1840 TO 1900

Dentistry took full advantage of the mechanical developments of the last half of the nineteenth and the early twentieth centuries. During this period, applied mechanics was recognized as an essential supplement to the biological principles of dentistry. Apparently this was more quickly and completely recognized among American dentists than among those of other countries, thereby advancing American dentistry to the position it now holds.

Between 1839 and 1884, 44 dental journals were established in the United States, and between 1842 and 1884, 103 dental societies were organized. These assisted greatly in the dissemination of scientific dental information on practices and techniques throughout the profession. At the beginning of this period, dental materials were comparatively few in number, but this was the beginning of the application of physical principles to dental practices and processes, and the search had begun for better restorative materials.

By the end of the 60-year period from 1840 to 1900, many of the major present-day materials had been introduced to the profession, along with techniques for their manipulation and use. After 1840 America began to acquire leadership in creating and producing restorative dental materials, and a substantial industry developed in this field. From this industry dentistry derived many valuable contributions, relationships, and benefits in the form of research and scientific development.

Because of the great number of improvements and developments introduced by dentistry during this period, it is possible here to enumerate in chronological order only a few of the most important. In 1844, S.S. White became interested in the production of porcelain teeth and their improvements in color and form. White was later to become a leading manufacturer and distributor of dental materials, establishing the S.S. White Dental Manufacturing Company. The records indicate, therefore, that the Ney and White companies were among the oldest in the field.

One of the early actions of the American Society of Dental Surgeons was to forbid its members to use silver amalgam for restoring lost tooth structure. Like many other acts of prohibition, this action apparently served to stimulate thought on the use and study of the nature of amalgam. Years later, after much study, an improved amalgam was developed that eventually became one of the most popular and useful of all restorative materials.

About the time the society started the “war” against the use of silver amalgam, a companion material in the form of copper amalgam was introduced (1844). Mouth impressions were being taken in plaster about this same time. Gutta-percha was discovered in India in 1842, and by 1847 it was being used as a root canal filling material when mixed with chloroform. This material, chloropercha, remained in use until recent times as a cavity liner and varnish in deep cavities. Gutta-percha was mixed with zinc oxide for filling purposes by Asa Hill in 1848. In 1883, gutta-percha was dissolved in eucalyptol and used as a root canal filler. This was perhaps the beginning of the present-day gutta-percha points for root canals. Platinum-gold alloys, consisting of three-fourths gold and one-fourth platinum, were introduced in 1847.

In 1851, Nelson Goodyear announced the development of a method for producing vulcanite, or hard rubber, based on Charles Goodyear’s earlier discovery of a method of dry-heat vulcanization of rubber. The discovery of vulcanite and its subsequent use for “dental plates,” patented on March 5, 1855, was another outstanding advance in dental materials. Although the material was not the most ideal as a denture base, vulcanite served well as the first substitute for carved ivory dentures. Not long afterward (1869) celluloid was introduced by J. Smith Hyatt, who was searching for a suitable material for billiard balls, and soon it was used as a denture base material. Thus a substitute for vulcanite was sought soon after its introduction. It was not until about 80 years later (1937), however, that a satisfactory substitute for vulcanite was obtained in a material known as acrylic resin.

The restrictions on the use of amalgam had not been completely successful. A silver-tin-mercury alloy, or amalgam, was introduced in 1855 by Elisha Townsend, followed by another formula by J.F. Flagg in 1860. Gold foil was becoming increasingly popular at the same time, with the introduction of cohesive annealed foil by Robert Arthur of Baltimore in 1855. Zinc oxychloride cement was in common use by this time as a filling and cementing medium. Low-melting point alloy baseplates were developed by Alfred A. Blandy in 1856, and a flexible dental engine cable was introduced by Charles Merry of St. Louis in 1858, followed by the contra-angle handpiece in 1862. The rubber dam to isolate teeth from saliva was put into use by Phineas Taylor Barnum of Monticello in 1864, and 4 years later the profession was to benefit by the expiration of the patents controlling the use of vulcanite. About 1870 zinc phosphate cements were first used, and they were introduced to the profession in 1879. Silicate cements were developed a few years later.

The practice of malleting for condensation of gold foil had been common since 1838, when it was introduced by E. Merrit of Pittsburgh. Since that practice started, numerous automatic mallets have been introduced, beginning with one developed by J.C. Dean in 1867. The introduction of automatic mechanical condensing devices has continued with varying degrees of success to the present time.

In 1850, the pivot crown was a crude structure with its wooden pin set into the tooth. The Richmond crown was introduced in 1878, followed by the Davis crown in 1885, with a modified form of the Davis crown in the same year by H.D. Justi, all making use of metal pins to replace the wood. These were only three of the many forms of porcelain pivot crowns common at that time. Much experimentation was being done with fused porcelain for inlays, jacket crowns, porcelain teeth set into vulcanite bases, and other modified porcelain structures. Some years later these ambitions were realized with the introduction of gas and gasoline furnaces for baking porcelain, porcelain jacket crowns, and high-fusing-point inlays by Charles Land (1889); an electric furnace for porcelain by Levitt Ellsworth Custer (1894); high-fusing-point porcelain inlays by W.E. Christensen (1895); the gingival shoulder for the porcelain jacket crown by E.B. Spalding (1903); and the summary of porcelain inlay construction in the technical publication of J.Q. Byram (1905).

Circular inlays that were ground and fitted to position were in common use from 1858 to 1890. Aguilhon de Saran of Paris is credited with melting 24k gold in an investment mold to form inlays about 1884. J.R. Knapp in America invented a blowpipe in 1887, but not until 1907 did W.H. Taggart of Chicago succeed in introducing a practical casting method for the gold inlay. Solbrig in Paris, independent of Taggart, cast gold inlays by a similar method during the same year, and B.F. Philbrook had described in 1897 a method of casting metallic fillings. This indicates that much study was given to the problem throughout the profession and that its solution was a natural result of investigation. Why it was not done at an earlier date is now interesting speculation, because it is known that Cellini and Theophilus used the same principles 1000 years before in their arts.

Continued progress was made in the work on amalgam alloy throughout the last years of the nineteenth century. G.V. Black published the results of his studies in 1895, which marked the beginning of precision measurements on amalgam alloys. Black had previously published (1891) his theories on cavity design and preparation, which are only remotely related to dental materials but include certain principles of mechanics that involve properties of materials used for restorations.

These examples are only a few of the many techniques, practices, and principles that had their beginning in the late nineteenth century. The fundamental principles underlying such operations as shaping cavities for various restorations, impression-taking operations, wax patterns, models and indirect dies, the construction of complete and partial removable dentures, and many other types of structures were recognized before the beginning of the twentieth century.

ADVANCES SINCE 1900

With the beginning of the twentieth century came many refinements and improvements in the quality of various materials and processes used in restorative dentistry. Physical and mechanical tests and the fundamentals of engineering practice were applied to structural designs and restorative materials. From studies of physical and mechanical behavior, certain shortcomings of structures and materials were observed. When these shortcomings were detected, the process of improvement began with studies of methods of chemical combination or physical improvement in fabrication. Thus for the first time a concentrated effort was made to develop and improve products with specific properties designed for a definite purpose.

Before 1900, relatively few persons specialized in the improvement of dental materials or were able to verify the claims made for available materials. Today, many individuals who have a background of training and experience in physics, engineering, chemistry, and dentistry are engaged in research and development in this field, and many universities offer graduate training in biomaterials. From 1900 to about 1925, frequent references to modifications, tests, and improvements of materials and structures appeared in the literature. Unfortunately, a lack of uniformity of testing conditions prevailed, and it can now be seen that this often caused failure to duplicate results and led to some misunderstanding of the science and studies as a whole. Since the early 1950s, much has been done to establish uniform standards, thanks to a cooperative effort among some of the dental schools; leaders in the profession; the American Dental Association Council on Dental Materials, Instruments, and Equipment (now the Council on Scientific Affairs); the National Bureau of Standards (now the National Institute for Standards and Technology); and the research departments of many reputable manufacturers.

During the early part of the twentieth century, some of the persons engaged in improving the quality of restorative materials were associated with dental schools, and others were in practice or engaged in research with manufacturers. G.V. Black was still active in the profession and at Northwestern University Dental School. The various editions of his textbook Operative Dentistry contained references to various dental materials and in particular to the need for a balanced formula for an amalgam alloy. In addition, his rules on cavity design are still generally accepted, although there is a trend toward somewhat more conservative designs. In the related field of crown and bridge construction, F.A. Pesso was active in improving and modifying the technique and design of these restorations. At the University of Michigan School of Dentistry, M.L. Ward was active in improving methods to measure dimensional change, flow, and other properties of amalgam. The development of the optical lever micrometer for measuring dimensional change in amalgam was one of the first refinements in dimensional change–measuring equipment for this and related materials. Ward also studied cements, improved designs in instruments and cavities, and made numerous contributions to the literature. Many of these are described in several editions of the American Textbook of Operative Dentistry, which he edited.

The term dental metallurgy was commonly used during the early period of the twentieth century, and several books were written on the subject. The sixth edition of C.J. Essig and Augustus Koenig’s book Dental Metallurgy was published in 1909. This book describes the metallurgy of various elements and procedures for melting and alloying. After a complete treatment of the methods of extracting metals from ore and refining various metals, there is a chapter on amalgam alloys with emphasis on their use in dentistry. The sixth edition of another popular book, Practical Dental Metallurgy, written in 1924 by J.D. Hodgen and G.S. Millberry, follows the same pattern of subject treatment. By that time, considerable information had been accumulated on the various metals, and particularly on amalgam and certain alloys used in dentistry, so the book is somewhat more complete than previous editions. Dental metallurgy at that time, however, was not highly specialized, but perhaps that is to be expected because the subject was only beginning to take form and a limited amount of information was available on various materials and dental alloys. Books in this field written in later years, such as those of O.E. Harder (Modern Dental Metallurgy), K.W. Ray (Metallurgy for Dental Students), J.S. Shell (Hodgen-Shell Dental Materials), and E.W. Skinner (The Science of Dental Materials), adopted an entirely different style and included a broader subject matter.

Among the contributors to the periodical literature during the early part of the century were A.W. Gray, Paul Poetske, R.V. Williams, and W.S. Crowell. Gray reported numerous studies on amalgam alloy and its behavior when subjected to various practices of manipulation. He first offered a theory for the dimensional change resulting from the hardening of the amalgam mass. Poetske reported studies on amalgam alloy and dental cement. Williams described methods of testing and improving dental gold alloys, and Crowell contributed reports of investigations on cements and various other materials, as well as test practices.

At about this same time, James McBain and co-workers in England were studying the behavior of amalgam alloy subjected to different mixing procedures. At a later date M.L.V. Gaylor, working in the same laboratory, made significant observations on the method by which mercury and silver combine. Studies in Germany during this period dealt with the investigation of the theoretical behavior of metals and various alloys when combined under varying conditions. The theoretical behavior of structural designs was also being studied and reported from Germany. Significant studies on the method of combining gold and copper were reported from Russian laboratories in the early part of the twentieth century and were subsequently verified by studies in the United States and Great Britain. The discovery of copper-gold compound formation was most significant in the development and improvement of dental casting alloys.

In 1919, the National Bureau of Standards in Washington was requested by the United States government to formulate specifications for the selection of dental amalgam to be used in the federal services. Wilmer Souder directed this research and presented a well-received report in 1920 that led to subsequent study of other materials. Shortly thereafter the Weinstein Research Laboratories established a research associateship at the National Bureau of Standards, and studies were started on other materials. The first associates included R.L. Coleman, W.L. Swanger, and W.A. Poppe, who were under the direction of Dr. Souder. Their studies included investigations of the physical and mechanical properties of casting gold alloys, wrought gold alloys, and accessory casting materials. As a result of this investigation, research paper No. 32, which contained much fundamental information, was published in December 1928.

Since April 1928, the ADA has maintained a research fellowship at the National Bureau of Standards. Numerous reports presented on the progress of investigations made under this fellowship have stimulated the advancement of information on many dental restorative materials. This research body has formulated a number of specifications, based on qualified investigations into the characteristic properties of each type of material. These specifications have been of great value to the profession by ensuring greater uniformity and improved quality of restorative materials. The details of these specifications are described later in appropriate chapters.

Specifications have been developed in a number of countries, most notably in Australia, the United States, Europe, and Japan. Specifications for materials and devices are important in the practice of dentistry throughout the world, as evidenced by the establishment of international standards. Currently, all American standards (specifications) are developed and approved by the ADA Standards Committee on Dental Products and are reviewed for adoption by the ADA Council on Scientific Affairs. All adopted standards are forwarded for approval by the American National Standards Institute (ANSI) and, if accepted, become American National Standards. These standards may be submitted for acceptance by the International Organization for Standardization (ISO) and, if approved, become international standards. Of course, many countries contribute to the International Organization for Standardization, and specifications may be modified many times before they are finally accepted as international standards. The development of international standards has resulted in the improvement and reliability of materials and devices throughout the world and has eliminated the need for each country to develop its own standards and specifications.

For many years, standards have been designed for individual materials based on composition and design (vertical standards or specifications). Beginning in the early 2000s, standards began to be directed to application or use (horizontal standard). For example, ISO 1561 Dental casting wax and ISO 12163 Dental baseplate/modelling wax were replaced by ISO 15854 Dentistry—Casting and baseplate waxes. A current list of ANSI/ADA and ISO standards can be found in Chapter 4.

The Medical Devices Amendments, signed into law in 1976, were designed to protect the public from hazardous and ineffective devices. Responsibility for commercially available medical devices was divided among 19 panels, one of which was a dental panel. Each panel is to classify devices, identify known hazards, recommend characteristics for which standards should be developed, advise on the formulation of protocols and review premarket approval applications, recommend exemption for certain devices, and respond to requests from the FDA relating to the safety and effectiveness of devices. The Dental Device Classification Panel classified life-sustaining and life-supporting devices, implants, and priority items for standards development. The list of dental devices included 11 in the diagnostic area, 1 in monitoring, 51 in prosthetics, 82 in the surgical field, 2 in therapeutics, and 166 in the category of other devices.

If one were to list the major new materials, techniques, and processes that have been developed or introduced since 1900, he or she would immediately realize that a continual search has been in progress for new and improved items and practices. The search has focused on ways to make restorative dentistry more acceptable and serviceable to the patient and convenient for the operator.

In the field of restorative materials and practices since 1900, several major items have been introduced, such as the casting process, the use of acrylic resins to replace vulcanized rubber in dentures, base-metal casting alloys for partial dentures, stainless steel for orthodontic and other appliances, and a variety of flexible impression materials. Each has made modern dental practice more acceptable to both the patient and the dentist. The development of carbide burs and diamond cutting instruments and the successful introduction of increased speeds for rotary instruments have aided materially in the operation of cutting tooth tissue. The development of resin composite, glass ionomer, resin-modified glass ionomer and compomer restorative materials, new and modified polymers for restorations and impressions, new phenolic and resin cements, pit and fissure sealants, improved base-metal alloys and amalgams, low- or no-gold casting alloys such as palladium-based alloys, ceramic-fused-to-metal systems, and improved ceramics for single and multi-unit restorations have contributed to the service and function of restorative materials.

The extensively used acid etching of tooth structure, base metals, and ceramics to provide adhesion of resin composites has had a dramatic effect on restorative and orthodontic dental treatment. Recent improvements in bonding agents for composites and metals to enamel and dentin have provided the opportunity for major changes in cavity design. The improvement in composites has resulted in their extended application to the restoration of posterior teeth. Clinical and biological evidence of the success of titanium and titanium alloys for dental implants has made it possible to replace a tooth lost as a result of extraction.

Biophysical applications such as experimental stress analysis studies have resulted in better guidelines for materials used in the design of restorations. Materials used for maxillofacial applications or as dental implants have received increased attention; the urgent need for improvement has stimulated research in both of these areas.

The interaction of materials with oral tissues has become increasingly important in the evaluation of these materials, as indicated by the interim acceptance by the Council on Dental Materials and Devices in November 1971 of recommended standard practices for the biological evaluation of dental materials. A series of handbooks has also been published reviewing the current knowledge about the biocompatibility of dental materials.

Many of the advances in biomaterials during the twentieth century occurred after 1950. These advances include high-speed cutting, carbide burs, metal-ceramic systems, elastomeric impression materials, chemical- and light-cured composites, pit and fissure sealants, acid etching of enamel and other surfaces, bonding agents, auto mixing of various materials, vacuum forming of athletic mouth protectors, titanium and its alloys for implants, glass ionomers, high-copper amalgams, standardized endodontic instruments, strengthened ceramics, silicone maxillofacial materials, and compomers, to list only a few of the 100 or more major advances.

FUTURE DEVELOPMENTS IN BIOMATERIALS

Based on a 1996 report from the National Health and Nutrition Survey, it is expected that individuals younger than 20 years of age will need fewer removable appliances but will require single or partial tooth replacement. For persons older than 20, maintenance and fabrication of appliances will be needed for those at the high end of this group and fixed bridges and single tooth restoration will be required for those at the low end of this age group. Thus single tooth replacement will become more important than fixed bridges or removable partial dentures.

With more emphasis on preventive treatment, future restorative needs will move in the direction of restoring teeth with intracoronal and root caries. Thus, it is expected that future research would be directed toward improvement in esthetic restorative materials for both anterior and posterior applications and for bonding systems to attach these materials to tooth structures. Also, the need for single tooth replacement will result in continued research on dental implants and surface treatments to produce satisfactory osseointegration.

In addition, the interaction between the fields of biomaterials and biology will increase. Demands for ensuring the biocompatibility of restorative materials before they are marketed will be an incentive to develop in vitro short- and long-term tests. Studies on tissue regeneration will continue, as will studies controlling surface characteristics of materials. As the fields of cell and molecular biology continue to develop, applications of these technologies should have an impact on restorative dentistry and the development of biomaterials.

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