Сraig. Dental Materials

xiv CONTENTS

Base Metals, 453

Binary Combinations of Metals, 456

CASTING ALLOYS, 459

Types and Composition, 459

Grain Size, 462

Properties, 462

Gold-Based Alloys for Porcelain-Metal

Restorations, 466

WROUGHT ALLOYS, 466

Microstructure, 466

Composition, 467

Properties, 467

SOLDERS AND SOLDERING

OPERATIONS, 468

Types of Solders, 469

Microstructure of Soldered Joints, 472

16 1 Cast and Wrought Base Metal

Alloys, 479

George R. Baran

GENERAL REQUIREMENTS OF A DENTAL

ALLOY, 480

COBALT-CHROMIUM AND NICKEL-

CHROMIUM CASTING ALLOYS, 481

ANSI/ADA Specification No. 14, 481

Composition, 481

Microstructure of Cast Base Metal Alloys, 482

Heat Treatment of Base Metal Alloys, 483

Physical Properties, 485

Mechanical Properties, 485

Corrosion, 487

Crown and Bridge Casting Alloys, 487

Other Applications of Cast Base Metal

Alloys, 488

TITANIUM AND TITANIUM ALLOYS, 488

Commercially Pure Titanium, 488

Titanium Alloys: General, 488

Ti-6Al-4V, 489

Cast Titanium, 489

Dental Implants, 491

Other Applications of Wrought Titanium, 494

WROUGHT STAINLESS STEEL ALLOYS, 495

Composition, 495

Function of Alloying Elements and Chemical

Resistance, 495

Stress-RelievingTreatments, 496

Stainless Steel Orthodontic Wires, 496

Stainless Steel Endodontic Instruments, 499

Nickel-Titanium Endodontic Instruments, 502

Base Metal Prefabricated Crowns, 503

WROUGHT COBALT-CHROMIUM-NICKEL

ALLOY, 503

Composition, 503

Processing and Manipulation, 503

Properties, 504

WROUGHT NICKEL-TITANIUM ALLOY, 504

Composition and Shape-Memory Effect, 504

Properties and Manipulation, 505

WROUGHT BETA-TITANIUM ALLOY, 505

Composition and Microstructure, 505

Manipulation, 506

Properties, 506

OTHER ORTHODONTIC WIRES, 506

SUMMARY OF ORTHODONTIC WIRES, 506

SOME OTHER ALLOYS, 507

17 1 Casting and Soldering

Procedures, 515

John C. Wataha

CASTING, 516

Lost-Wax Technique, 516 Formation of the Wax Pattern, 519 Spruing the Wax Pattern, 521 Investing the Wax Pattern, 523 Burnout of the Wax Pattern, 526 Casting, 529

Special Casting Situations, 531 Casting Problems, 539

Cleaning and Pickling Alloys, 542

SOLDERING TECHNIQUES, 543

General Suggestions for Soldering, 543 Infrared Soldering, 544

CASTING AND SOLDERING FLUXES, 545

Noble and High-Noble Alloys, 545 Chromium-Containing Alloys, 546

18 I Ceramics, 551

Isabelle L. Denry

HISTORICAL BACKGROUND, 552

CLASSIFICATION OF DENTAL

CERAMICS, 552

Fusion Temperature, 552

Applications, 553

Fabrication Technique, 553

Crystalline Phase, 554

METAL-CERAMIC RESTORATIONS, 554

COMPOSITION AND MANUFACTURE, 555

Composition, 555

Manufacture, 555

Processing, 556

ALL-CERAMIC RESTORATIONS, 561

Sintered All-Ceramic Materials, 561

Slip-Cast All-Ceramic Materials, 564

Machinable All-Ceramic Materials, 564

GENERAL APPLICATIONS OF CERAMICS IN

RESTORATIVE DENTISTRY, 565

Ceramic-Metal Crowns and Fixed Partial

Dentures, 566

All-Ceramic Crowns, Inlays, Onlays and

Veneers, 566

MECHANICAL AND THERMAL PROPERTIES

OF DENTAL CERAMICS, 566

Test Methods, 567

Comparative Data, 567

OPTICAL PROPERTIES OF DENTAL

CERAMICS, 568

PROPERTIES OF PORCELAIN DENTURE

TEETH, 570

19 I Ceramic-Metal Systems, 575

Robert G. Craig

Requirements for a Ceramic-Metal System, 576

CERAMIC-METAL BONDING, 578

Evaluation of Ceramic-Metal Bonding, 579

CERAMICS FOR CERAMIC-METAL RESTORATIONS, 580

CONTENTS XV

ALLOYS FOR CERAMIC-METAL RESTORATIONS, 581

Composition and Properties of Noble Metal Alloys, 581

Composition and Properties of Base Metal Alloys, 584

PREPARATION OF CERAMIC-METAL RESTORATIONS, 587

EFFECT OF DESIGN ON CERAMIC-METAL RESTORATIONS, 588

20 1 Cements, 593

John M. Powers

ZINC PHOSPHATE CEMENT, 595

Composition, 595

Setting Reaction, 596

Manipulation, 596

Characteristic Properties, 597

Applications, 604

ZINC OXIDE-EUGENOL (ZOE) AND NON-

EUGENOL CEMENTS, 604

Composition, 604

Setting Reaction, 605

Manipulation, 606

Characteristic Properties, 606

Applications, 608

ZINC POLYACRYLATE CEMENT, 611

Composition, 611

Setting Reaction, 612

Manipulation, 612

Properties, 612

Applications, 614

GLASS IONOMER CEMENT, 614

Composition, 614

Setting Reaction, 614

Manipulation, 615

Properties, 615

Applications, 616

HYBRID IONOMER CEMENT, 616

Composition, 616

Setting Reaction, 617

Manipulation, 617

xvi CONTENTS

Properties, 617

Applications, 618

COMPOMERS, 6 1 8

Composition, 618

Setting Reaction, 618

Manipulation, 6 18

Properties, 618

COMPOSITES AND ADHESIVE RESINS, 6 1 8

Cementation of Alloy Crowns and Bridges,

Resin-Bonded Bridges, and Provisional

Restorations, 619

Bonding of Esthetic Restorations, 620

Resin-Metal Bonding, 620

Bonding of Orthodontic Brackets, 620

CAMIT VARNISHES, 622

Composition, 622

Manipulation, 622

Properties, 623

Applications, 623

C A W LINERS, 6 2 3

Composition, 623

Manipulation, 624

Properties, 624

LOW-STRENGTH BASES, 6 2 4

Composition and Chemistry of Setting, 624

Manipulation, 624

Properties, 624

Zinc Oxide-EugenolBases, 626

HIGH-STRENGTH BASES, 6 2 6

Properties, 626

21 I Prosthetic Applications

of Polymers, 635

Andrew Koran III

PROPERTIES OF DENTURE BASE

MATERIALS, 636

Physical Form and Composition, 636

Other Denture Materials, 639

ANSI/ADA Specification No. 12 for Denture Base Resins, 639

PROPERTIES OF DENTAL PLASTICS, 6 4 0

Strength Properties, 640 Thermal Characteristics, 645

Other Properties of Denture Plastics, 647

MANIPULATION AND PROCESSING OF DENTURE BASE PLASTICS, 651

Heat-Accelerated Acrylic Denture Plastics, 651 Chemically Accelerated Acrylic Denture

Plastics-Compression Molding, 659 Fluid Resin Acrylic Denture Plastics, 660 Light-Cured Denture Plastic, 662

Factors Involved in Denture Retention, 662 Effect of Auxiliary Materials on Denture

Plastics, 662 Repair Materials, 665

Relining and Rebasing Dentures, 666

DENlVRFi TEETH, 6 7 2

ANSI/ADA Specification No. 15 for Synthetic Polymer Teeth, 675

MAXILLOFACIAL MATERIAIS, 675

Poly(Methy1 Methacrylate), 675 Plasticized Polyvinylchloride, 675 Polyurethane, 676 Heat-Vulcanized Silicone, 676

Room Temperature-Vulcanized Silicones, 677 Other Elastomers, 677

Fabrication of the Prostheses, 677 Physical Properties, 678

PLASTIC FACINGS FOR CROWN AND BRIDGE APPLICATIONS, 6 7 9

TEMPORARY CROWN A.ND BRIDGE

RESTORATIONS, 6 7 9

OCCLUSAL SPLINTS, 6 8 0 INLAY P A ~ R N S 6, 8 1

IMPRESSION TRAYS AND RECORD BASES, 6 8 1

APPENDIX, 691

Humankind has always been plagued by the problem of restoring parts of the body lost as a result of accident or disease. Practitioners of

dentistry have been confronted with this problem since the beginning of dental practice, and the means of replacing missing tooth structure by artificial materials continues to account for a large part of dental science.

The replacement of lost teeth is desired for two primary reasons: esthetics and restoration of function (partial or complete). The ability of the dentist to accomplish the desired results has been limited by certain basic factors. One is the availability of suitable materials for the construction of the restorative appliance; another is the development and control of a suitable technical procedure for using the materials that are available. This search for the correct materials, and for a method of manipulation or applied techniques, has continued from the beginning of dental art to the present. Throughout the ages, dentistry has depended to a great degree on advances of the contemporary arts and sciences for improvements in materials and procedures, and this relationship continues. The field of restorative materials is extensive with regard to not only the wide variety of materials and techniques of manipulation, but also the related sciences that are employed.

SCOPE OF MATERIALS COVUUED . , .

IN RESTORATIVE DENTISTRY

Restorative dental materials include such items as noble and base metals, amalgam alloys, cements, composites, glass ionomers, ceramics, gypsum compounds, casting investments, dental waxes, impression-taking compounds, denture base resins, and other materials used in restorative dental operations. In describing these materials, comparisons are usually made on the basis of physical and chemical characteristics. The line dividing restorative dental materials from therapeutic agents often is not clear. For example, the distinction is not pronounced in such cases as medicated cements, cavity liners, or root canal-filling materials. Usually in such borderline instances

the materials are included in both fields of study, with emphasis placed on the properties related to the application involved.

This subject should be approached from the point of view of determining what the material is chemically, why it functions as it does physically and mechanically, and how it is manipulated technically to develop the most satisfactory properties.

The application of dental materials is not limited to any one branch of dentistry. There is scarcely a dental procedure that does not make use of dental materials in one or more forms. The very existence of some phases of restorative dentistry depends largely on various materials and their favorable properties. Other branches of dentistry, such as minor oral surgery and periodontics, require less use of materials, but even in these fields the physical characteristics of the equipment and the chemical characteristics of materials used are important. However, because most materials are used in restorations, either directly or indirectly, the subject is described as one dealing with restorative dental materials.

Most restorative materials are measured by a set of physical, chemical, or mechanical tests that lend themselves to duplication, and as a result of these tests, efforts are being made to control the quality of and claims for the materials. This approach has led to a number of gradual improvements in the materials available to the profession. As improvements in properties have occurred, refinements in technique of application have become necessary.

BASIC SCIENCES APPLIED T O RESTORATIVE MATERIALS

The sciences of primary interest to the dentist are derived from the three basic scientific fields: biology, chemistry, and physics. No clear distinction can be made regarding the relative importance of these three sciences to dentistry, for two reasons: (1) the overlapping of these sciences tends to produce a continuous field of knowledge, and (2) investigations within each science are constantly encroaching on the others.

Clinical Dentistry

Applied Techniques

Physical, Chemical, and

Biological Sciences

Fig. 1-1Steps to understanding relations of materials science in dentistry.

Fig. 1-1indicates 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.In no way are the physiological and biological aspects of dentistry subordinated by this physical science treatment of the materials, but rather the whole structure of restorative, corrective, and preventive dentistry is strengthened by its inclusion. Whereas the subject depends on one or more of these exact sciences, the chief problem in this field is to make the correct practical application and interpretation of information available from scientific studies. 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 tissue 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. Often this situation is not fully appreciated because the application of the various scientific principles to dentistry has not been emphasized in undergraduate textbooks. 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.

Not all the theoretical aspects and applications of physical, chemical, and engineering principles can be described in detail in the limited space available. Such treatment should not be necessary because predental training includes an understanding of many of the basic principles, even though the practical applications may have been neglected. Therefore emphasis is placed on applying significant fundamentals to dental operations.

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 impression compounds 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 or 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 structures 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.

HISTORY

An examination of the history of dentistry shows that the various materials available in any given period have always been important to contemporary restorative dental operations. Improvements came slowly and steadily over the

centuries at about the same rate as related developments in other fields of science. It becomes evident therefore that many accepted techniques, materials, and practices have resulted from systematic evaluation and development, so that now the branch of restorative materials has become an accepted part of the science of dentistry.

During the past generation the quality of restorative dental materials has seen more improvement than during any other period of dental history. An understanding of the factors that contributed to this progress enables us to better appreciate the limitations, developments, and future possibilities of this phase of dentistry.

Although no complete history of restorative materials has been written, it is possible to follow the general development of the subject by the progress that was made in the art and science of restorative dentistry throughout the centuries. Until recently this subject was not a distinct science, but only an aspect of the art and science of dentistry.In the early development of dentistry the subject was sufficiently elementary that no separate study was devoted to materials. As dentistry developed and became more complex, so did the development of restorative materials. Therefore the accumulated mass of information regarding materials inevitably became so great as to be established as a separate science. As a science the subject is new, but as part of restorative practices, it is as old as dentistry itself.

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, and for practical purposes it makes little difference which of the civilizations produced the first prostheses. It is much more interesting to observe that many of the materials and practices now in common use were first used more than thousands of years ago.

Gold is one of the oldest materials used. It 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 seems reasonable to assume that the practice of using gold for dental appliances was common for centuries before the known examples. Evidence in favor of the antiquity of the art is found in the examination of the appliances. Many were prepared by soldering after reasonably careful assembly, and certainly this art was not developed for the convenience of only the few surviving examples.

It is not certain exactly how or by whom the appliances were constructed. Possibly they were made by skilled metalworkers and not by those who practiced the dental art. As pointed out by historians, the physicians and barber-surgeons probably performed the treatment and extractions, whereas goldsmiths and other artisans constructed the artificial restorations. The role played by the goldsmiths and other artisans is comparable to that of modern laboratory technicians. Persons qualified by practice and experience can often prepare a more artistic structure in less time than the one who visualized it, even though the functional design is the responsibility of the dentist.

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. Inasmuch as tools and supplies were simple, persons who practiced the dental art depended on nature to provide materials and on artisans to fashion restorations.

MEDIEVAL AND EARLY MODERN PERIOD

Dental historians describe little progress in the dental art from the beginning of the Christian era to about AD 1500. Historians doubt, however, that it was a period of retrogression or a "dark age" of inactivity. There was likely much activity, creative thought, and invention, but records either were not kept or were later destroyed through acts of superstition or religious fanaticism. The chief contribution to dentistry of this period appears to have been some shift in practice from

prosthetic restorations to the restoration of carious teeth.

Some historians consider the sixteenth century to be the end of the Middle Ages. The invention of the printing press (1436), which aided in the dissemination of knowledge, and the emigration of Greek men of letters and science to Italy, were important events toward the close of this period. During the latter part of this period (between AD 1116 and 12891, universities with medical faculties were established at Bologna, Oxford, Paris, and Montpellier.

The development of books and writing on dental subjects was important to the progress of restorative dental materials. One of the first books to treat dentistry independently of medicine was written in German in 1548 by Walter Herman Ryff. This book, written in the language of the people, is significant because all previous works describing the teeth were in Latin.

The use of gold leaf to fill cavities was perhaps the most significant development of the period from the standpoint of restorative materials. The first authentic record of the use of gold fillings to preserve human teeth appears to have been about 1480 by an Italian, Johannes Arculanus, who was at the University of Bologna and later at Padua. A description of the removal of carious matter from the teeth before filling them with gold leaf was given by Giovanni de Vigo (14601520). The practice of using gold leaf for fillings probably was not original with either of these writers because there is some indication that the custom may have dated back to the Middle East, several centuries before. It is certain, however, that gold leaf has been used for the past 500 years.

Gold leaf was used for gilding and other commercial purposes in antiquity. The ancient Egyptians, Hebrews, and Greeks were familiar with the art, although the origin of gold leaf production was perhaps in the Far East. The early Greeks produced leaf of approximately 1/100,000-inch thickness, approaching the thickness of modern leaf or foil, which is approximately 1/300,000 inch. The methods for producing gold leaf have changed little through the ages.

Carious teeth were filled with ground mastic, alum, and honey or other substances during the period from about AD 1050 to 1122, according to the Arabian author Rhazes (al-Rgzi). Oil of cloves (eugenol) is mentioned by Riviere (1589) as being applicable to dental operations, but may have been used earlier (1562) by Ambroise Pare to alleviate toothache. Pare is also credited with having prepared artificial teeth from bone and ivory. Jacques Guillemeau, who was a pupil of Pare, prepared a substance by fusing together certain waxes, gums, ground mastic, powdered pearl, and white coral. This may have been the forerunner of esthetic fused porcelain.

Some Contemporary Arts of the Middle Ages

Contemporary arts were also being developed during this period. The writings of Pliny (AD 23-79), Theophilus (eleventh century), and Cellini (1558) describe how painters, goldsmiths, ceramists, metalworkers, and others applied their art. None of the authors claim credit for complete originality of all their practices, but rather indicate that the methods were routine.

In his Natural Histoy, Pliny described bronze statuettes and other cast bronze or silver household articles such as candelabra and cups common to the period before An 100. Pliny listed many dental practices that appear to be based on popular belief rather than on the practices of specialists.

In "An Essay Upon Various Arts," the priest and monk Theophilus shows certain improve- ,ments over the earlier writers and has omitted certain erroneous practices of the previous period. In Book One he deals primarily with painting and the allied arts. Of considerable interest is his description of the method for forming gold leaf from gold of high purity by hammering, which is similar to the recent practices of beating gold to form foil. Book Two deals predominantly with the ceramic art and gives a good description

of early practices in glassworking.

In Book Three, Theophilus describes metalworking and devotes considerable space to the work of the goldsmiths. There is a description of

the casting of handles for a silver cup in which the "lost wax" method is given in detail. The

casting practice was so clearly described by Theophilus that it is possible to follow him in practice as well as in principle. After fashioning the handle in wax, a wax "sprue," which was described as being round like a slender candle and half a finger in length, was attached. It was somewhat thicker at the top. This wax, called the funnel, was made fast with a hot iron. Wellbeaten clay was used to cover the wax carefully so that all details of the wax sculpture were filled. Afterward these molds were placed near warm coals, so when the molds became warm it was possible to pour out the wax. After the clay mold was well baked and still hot, the molten metal was poured in through the funnel. When the mold and casting became cold, the clay mold was removed, and a metal replica of the wax model remained.

If a suitable dental casting investment, or mold material, had been used with an appropriate inlay wax, a balanced casting alloy, and a modern casting machine, the description might well apply to the dental casting process of today.

A metal casting process very much like that of Theophilus is described by the Florentine artist Benvenuto Cellini in Chapter 41 of his Menzoilpes, written in 1558. Cellini prepared a wax model and surrounded it with a plastic clay, which he allowed to dry and harden before attempting to melt the wax and pour the metal. He implied that he had frequently used wax patterns in previous castings. He claimed no originality for the casting method, though he indicated that the melting furnace he used was original.

Cellini, like Pliny and Theophilus, also soldered gold by using copper acetate, nitre, and borax, a method that was considered very effective. Thus it is seen that certain metals and materials were available to the artisans for use in their trades. The secrets of these practices were no doubt often guarded so they were not recognized outside the trade. Such dentistry was more an art than a science, so that full use possibly was not made of existing skills and techniques. Certainly the practice of casting restorations by the

"lost wax" method was to wait several centuries before it was adopted by dentistry.

BEGINNING OF DENTAL SCIENCE1600 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.

The contemporary sciences of chemistry and physics were being developed at the beginning of the seventeenth century. Galileo had stated his law of falling bodies and invented the telescope. The compound microscope and the printing press were in use. By the end of the century chemical elements had been defined by Robert Boyle, and Sir Isaac Newton had demonstrated the law of gravitation. Similar developments were taking place in the biological sciences of bacteriology, anatomy, and physiology.

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, ou Trait6 des dents, published in 1728. He discussed many phases of dentistry, including operative and prosthetic procedures. He collected and cata-