Implants and components: entering the new millennium.

T elusive dream of replacing missing teeth with artificial analogs has been part of dentistry for a thousand years. The coincidental discovery by Dr P-I Branemark and his coworkers of the tenacious affinity between living bone and titanium oxides, termed osseointegration, propelled dentistry into a new age of reconstructive dentistry. Initially, the essential tenets for obtaining osseointegration dictated the atraumatic placement of a titanium screw into viable bone and a prolonged undisturbed, submerged healing period. By definition, this required a 2-stage surgical procedure. To comply, a coupling mechanism for implant placement and the eventual attachment of a transmucosal extension for restoration was explored. The initial coronal design selected was a 0.7-mm-tall external hexagon. At its inception, the design made perfect sense, because it permitted engagement of a torque transfer coupling device (fixture mount) during the surgical placement of the implant into threaded bone and the subsequent second-stage connection of the transmucosal extension that, when used in series, could effectively restore an edentulous arch. As 20 years of osseointegration in clinical practice in North America have transpired, much has changed. The efficacy and predictability of osseointegrated implants are no longer issues.1–7 During the initial years, research focused on refinements in surgical techniques and grafting procedures. Eventually, the emphasis shifted to a variety of mechanical and esthetic challenges that remained problematic and unresolved.8–10 During this period, the envelope of implant utilization dramatically expanded from the original complete edentulous application to fixed partial dentures, single-tooth replacement, maxillofacial and a myriad of other applications, limited only by the ingenuity and skill of the clinician.11–13 The external hexagonal design, ad modum Branemark, originally intended as a coupling and rotational torque transfer mechanism, consequently evolved by necessity into a prosthetic indexing and antirotational mechanism.14,15 The expanded utilization of the hexagonal resulted in a number of significant clinical complications.8–11,16–22 To mitigate these problems, the external hexagonal, its transmucosal connections, and their retaining screws have undergone a number of modifications.23 In 1992, English published an overview of the thenavailable external hexagonal implants, numbering 25 different implants, all having the standard Branemark hex configuration.14 The external hex has since been modified and is now available in heights of 0.7, 0.9, 1.0, and 1.2 mm and with flat-to-flat widths of 2.0, 2.4, 2.7, 3.0, 3.3, and 3.4 mm, depending on the implant platform. The available number of hexagonal implants has more than doubled. The abutment-retaining screw has also been modified with respect to material, shank length, number of threads, diameter, length, thread design, and torque application (unpublished data, 1998).23 Entirely new secondand third-generation interface coupling geometries have also been introduced into the implant milieu to overcome intrinsic hexagonal deficiencies.24–28 Concurrent with the evolution of the coupling geometry was the introduction of a variety of new implant body shapes, diameters, thread patterns, and surface topography.26,27,29–36 Today, the clinician is overwhelmed with more than 90 root-form implants to select from in a variety of diameters, lengths, surfaces, platforms, interfaces, and body designs. Virtually every implant company manufactures a hex top, a proprietary interface, or both; “narrow,” “standard,” and “wide” diameter implant bodies; machined, textured, and hydroxyapatite (HA) and titanium plasma-spray (TPS) surface implants; and a variety of lengths and body shapes (Table 1). In the wide-diameter arena alone, there are 25 different offerings, 15 external hexagonal, and 10 other interfaces available in a number of configurations. 1Adjunct Professor of Prosthodontics, Graduate Prosthodontics, Indiana University, Indianapolis, Indiana; Assistant Research Scientist, Department of Restorative Dentistry, University of California at San Francisco; and Private Practice, Roseville, California.