136 Theories and techniques of oral implantology

and lymph can freely circulate around the base of the implant and speed healing. In addition to permitting faster healing, the convolutions provide an-other advantage. A band of collagenous connective tissue forms between the implant and its surrounding bone. If the implant is relatively immobile upon insertion, this tissue will firmly wrap around and bind itself to the implant. On the side of the tissue facing the bone, strands of the connective tissue extend into the bone, providing so-called false periodontal ligaments. As pressure is put on the implant it is compressed against the bone in an apical direction. Because the connective tissue is tightly bound around the irregularities, it too is pushed down. This

pulls the fibers extending into the bone and this is vitally important   recreates the tension on bone
necessary for its healthy and continued growth. Thus the relationship between an irregularly shaped implant post and its tightly bound connective tissue substitutes the normal functions of Sharpey's fibers.

In an implant where the lowermost part of the post is relatively smooth or simple in design, the connective tissue grows around the implant in the shape of a capsule. When pressure is exerted on the implant, it merely slips within its capsule. Not only is this irritating to the soft tissues, but it creates no pull on the bone and the danger of resorption is as apparent as with any loose tooth.

As bone grows, it follows the convolutions of the post. If spaces are provided for it to grow around and through certain parts of the post, greater stability and insurance against exfoliation are provided.

The shape of that part of the post nearest the alveolar crest is also a vital design consideration. This part of the implant, being uppermost, will directly receive the impact from an opposing tooth. Unless some provision is made for directing the force of the impact away from the alveolar crest, the great pressure exerted each time the prosthesis moves against its opposing tooth will result in the bone's not healing around the uppermost part of the implant. Then oral epithelium will invaginate down toward the base of the implant. This means that the implant will become loose soon after its insertion.

Generally, the larger the diameter of the upper-most part, the greater the impact area. The ideal is to localize the force of the impact by reducing the diameter and to thus direct the impact's force down-ward.

In directing the impact force toward the base of the implant, a cast or fused implant is superior to an implant composed of distinct parts. In the case of separate parts, the force has to "leap the gap" between the parts. Also, because certain minimal size requirements are involved in making an implant of several parts just so they can be fitted together without microinstrumentation, these implants tend to have their largest diameters near the surface of the alveolar crest. This automatically creates a large impact area, inviting undue stress and subsequent problems.

In addition to downward pressure, there is also pressure applied on the sides of the protruding post or prosthesis during lateral movements of the jaws and by the tongue and lips. Even the most carefully stabilized implant is affected. A loose-fitting post type implant, like any other loose post, can act as a lever. The fulcrum in this case is the widest part of the implant. If the fulcrum is too deep, the post may be moved a considerable distance by lever action. Again, and this cannot be stressed too often, the more undue pressure on bone, the less its chance of healing. Designing the widest part of the implant away from its base lessens the danger from lever action.

Having established some basic criteria for post type implants, the following endosseous implants will be evaluated in these terms. It would be ungenerous not to acknowledge that what are now established as vital design considerations in post type implants were not recognized when most of these implants were proposed. The flaws of a particular design will seem immediately obvious to the modern, experienced practitioner. However, it is only because of the pioneer research of early implantologists that some of today's principles have evolved. It is interesting to note, in surveying early designs, that some operators were well aware of certain considerations and oblivious to others. Even in the earliest designs one recognizes excellent features that might have succeeded if only certain other provisions had been incorporated.

The following survey presents the wide range of ideas and designs that produced today's most successful types of implants. We do not assume that the evolution of implant design is complete with the presentation of some workable implant models. Improved operative procedures and instrumentation and a better understanding of and control over the biologic features typical of implant sites will doubt-less lead to more efficient implant designs and methods of insertion. Although it is doubtful that a "universal" implant will be designed—the variations in even one individual patient's mouth are too great—a few designs have proved successful and applicable