Author: Bernard T. Carr/David A. Dersh/Wayne R. Harrison/Richard P. Kinsel
This article was published in The International Journal of Oral & Maxillofacial Implants, Vol 16, No 1 (January/February 2001).
Bernard T. Carr, DDS, maintains a private practice in Alexandria, Virginia, emphasizing gnathological, aesthetic, and implant prosthetics. He is a fellow of the American College of Dentists, an Active Member of the Academy of Osseointegration, and a board member of the Northern Virginia Implant Society.
When I draw up a treatment plan using endosseous implants, the following are the most important clinical and laboratory factors that influence my choice of an implant system. The initial need for different implant systems to be developed in implant dentistry arose from the loose screw syndrome with the original external hex design. The loosening of small gold cylinder screws, as well as the larger titanium screws used to secure gold cylinders and machined abutments to the implants, was a constant problem when the restoration lacked a passive-fitting prosthetic framework. This lack of a passive fit would cause microvibrations, which would loosen the screws. These vibrations were the result of built-in stress from the forced fitted framework returning to its original dimension. Obviously, there were far fewer problems with screws loosening in a passive-fitting prosthetic framework.
All of the original-design frameworks were screw-down implant prostheses. The need and desire to restore intact and stronger occlusal morphology with porcelain-fused-to-gold crowns led to the development of the custom-milled cast UCLA abutment for a cemented prosthesis. The advent of the 32 N/cm square drive gold screw, used to secure the custom abutment directly to the implant, as well as improved manufacturing tolerances, has practically eliminated the loose screw problem.
Therefore, designs of the other implant systems, although they may have higher and stronger engineering values, limit prosthetic versatility, both from a laboratory and clinical point of view, in restoring implants that are surgically or positionally compromised. The external hex system provides more versatility for the technician to solve problems with emergence profile and esthetics, since the technician is able to bring the porcelain of a porcelain-fused-to-gold crown closer to the implant interface.
If the use of screws was intended to prevent implant fracture in cases of excessive occlusal load, then the internal hex implant may be more prone to fracture because of its stronger design at the abutment/implant interface.
The MicroMiniplant, used to replace congenitally missing maxillary lateral incisors, has an internal hex interface. It is easier to engage the impression and/or final custom abutment in the internal hex design. However, if the implant platform level is not significantly subgingival, esthetic problems can arise at the abutment/implant interface. These are easier to correct with a custom UCLA abutment and the external hex design system.
Strong consideration should be given to a tapered root-form implant where the apical root forms of the natural roots adjacent to the implant site are too close to permit placement of a cylinder implant.
Personally, there seem to be no problems with the esthetics and longevity of the external hex design implant system, especially if the necessary density and volume of bone is available at the implant site. Clinically, I have had few problems with the external hex implant design and do not see the need for using another implant system.