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Anti-Rotational Features for Osseointegrated Implants

Osseointegrated implants have broadened the scope of prosthetic treatment to include both edentulous and partially dentate patients. The standard externally hexed implant was originally used on a large scale to treat edentulous patients. After much success in long-term clinical trials in such patients, the implant was then utilized in partially dentate subjects to include single- and multiple-tooth replacement. The design of single tooth prostheses was predicated on the use of the platform hexagon as an anti-rotational feature for either screw-retained or cemented restorations.

The original purpose of the external hex was to allow the surgeons to drive the implant into the correct surgical position, not as an anti-rotational device. Early attempts to modify existing components to achieve this anti-rotation clearly demonstrate that this application was an afterthought.1 The long-term literature from single and multiple-tooth studies cite abutment screw loosening as a common prosthetic complication.2 In fact, the literature is abundant with citations concerned with abutment screw joint loosening after only one year of function.

The machining tolerances of the external hex and its abutment components have also been directly associated by some with the predictability of screw loosening. In fact, rotational misfit in excess of 2 degrees may result in screw joint instability, and the degree of misfit can be as high as 6 degrees of rotation for certain types of implants.3 Some of the screw loosening phenomena can be addressed by the use of standardized torque wrenches for maximum torque force delivery. The standardized force enables the delivery of correct magnitude that is appropriate for the specification of indicated abutment screws. Each system has a specific torque wrench to ensure that the correct magnitude of force is applied. Sequential tightening strategies can also be applied to the screw to provide maximum torque delivery for preload. An initial torque delivery is accomplished prior to a 5-minute waiting period, followed by a subsequent torque for maximum torque delivery.

Solid lubrication has also been introduced to allow the maximum torque applied to the screw to be effective in establishing a sufficient preload. The placement of new abutment screws at the time of delivery also prevents premature screw loosening.4 Since manipulation of abutment screws in the laboratory may wear the abutment screw thread's ability to acquire a maximum clamping force, the use of try-in screws in the laboratory and new abutment screws at delivery will facilitate effective and maximum preload delivery.

The spline connection was introduced as a result of innovative strategies for using anti-rotational features. This device uses a series of six interlocking projections to reduce the degree of anti-rotation and maintain abutment/implant interface stability. The internal hexagon, internal abutment anti-rotational connections, and morse tapers were introduced to aid with loosening of the abutment connection and to create a more "hermetic seal" from implant to abutment connection. Internal connections have been considered effective for abutment stability and do not encroach upon interarch restorative distances as implants with external anti-rotational features.5

Since lateral loads increase shear force on the abutment rather than the abutment screws of internal connections, improvements in abutment connection have evolved to provide more predictable results for osseointegrated implants. The predictability of implant dentistry relies upon the stability of the implant/abutment connection. It will be a clinical milestone to achieve a stable implant/abutment connection so that implant dentistry becomes a more predictable treatment modality.

*Assistant Professor, Department of Otolaryngology, University of Nebraska Medical Center, Omaha, Nebraska.

 

References

  1. Lewis SG, Beumer J 3rd, Perri GR, Hornburg WP. Single-tooth implant supported restorations. Int J Oral Maxillofac Impl 1988;3(1):25-30.
  2. Laney WR, Jemt T, Harris D, et al. Osseointegrated implants for single-tooth replacement: Progress report from a multicenter prospective study after 3 years. Int J Oral Maxillofac Impl 1994;9(1):49-54.
  3. Binon PP. The effect of implant/abutment hexagonal misfit on screw joint stability. Int J Prosthodont 1996;9(2):149-160.
  4. Binon PP. The external hexagonal interface and screw joint stability: A primer on threaded fasteners in implant dentistry. Quint Dent Technol 2000;23:91-105.
  5. Merz BR, Hunenbart S, Belser UC. Mechanics of the implant-abutment connection: An 8-degree taper compared to a butt joint connection. Int J Oral Maxillofac Impl 2000;15(4):519-526.
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