CHAPTER 5 The evolution

of

dental implants

For years attempts have been made to replace lost teeth with some kind of esthetically pleasing, secure prosthesis that approximates natural dentition in both appearance and function. One of the most fertile areas of exploration has been the dental implant. Several workable designs of proved value are in use today. These usually fall into either of two categories: the endosseous implant and the subperiosteal implant. There are also some implants that are difficult to classify as either type because of their design, location in the mouth, or material.

It has become quite evident that a good implant design, no matter what its type, must fulfill certain requirements. First, the design itself must either complement or supplement the natural biomechanical forces of the site. Second, the operative procedure used to insert the implant must be so precise as to cause as little trauma and destruction to the site as possible. The best designed implant will not succeed in a site where extensive damage has occurred. For this reason, an implant design cannot be separated from the kind of operative procedure necessary for its insertion.

Although this chapter will cover the evolution of many kinds of implants, the emphasis will be on the endosseous implant. Here some of the most exciting and exacting work in modern dentistry has been concentrated. What has been revealed through concentrated effort in evolving a workable endosseous implant helps explain observations in other areas of implantology.

The endosseous implant, as well as other types of dental implants, is usually made of metal, a ma-

terial foreign to the body. In planning the insertion of any foreign body into living tissues, the reaction of these tissues to the foreign material must be considered. Numerous researchers have spent many years establishing what kinds of metals can be tolerated by the body and why. An intensive review of this vast body of research lies outside the scope of this book, but certain findings pertinent to dental implants should be included.

Some of the earliest metals used in implantation were gold, silver, and platinum. Their choice was probably predicated upon the false assumption that because these metals were "precious" they would be well tolerated. However, implantations with these metals usually failed since they were not inert; this eventually caused the rejection of the metallic inclusion.

In 1937, Venable, Stuck, and Beach published the results of their extensive research on metallic inclusions.* Their studies showed that most metals, when in contact with body fluids, underwent an electrolytic reaction. The apparent subsequent production of metallic salts in the local tissues caused the abnormally excessive proliferation of some tissues on the one hand and the inhibition of bone formation on the other. Concomitantly, an immune reaction set in. In other words, those metals that ionized on contact with body fluids were bound to fail as implant materials. However, relatively inert metals

*Venable, C. S., Stuck, W. G., and Beach, A.: The effects on bone of the presence of metals; based upon electrolysis, experimental study, Ann. Surg. 105:917-938, 1937

134.