There are many reasons why the cement-retained implant restoration gained popularity over the last few years, which can be attributed to esthetics, ease of use and familiarity with cementation techniques. However, Pauletto, Gapski (see references 1) and others reported that cement excess was problematic; then Wilson’s study established a positive relationship between excess residual cement and peri-implant disease.
Surveys on cements used for implant restorations indicated a diversity in material selection, application technique and volume. This suggested a lack of conformity and understanding of cement usage within the dental profession. To overcome the cement problem, it became evident that improved understanding was required for cement material selection, abutment design and the determination of cement margin depths. Even with the very best intentions, however, the residual excess cement can lead to disease affecting the health of the implant/tissue interface and remains a dominant risk factor.
The association of residual excess cement and peri-implant disease has resulted in the need to re-examine alternatives such as the screw-retained implant crown. For many implant systems, the ability to use a screw-retained implant restoration is limited to regions where the screw access channel emerges in an esthetically “safe” site.
Usually the anterior maxilla and mandible present the greatest challenges, as the long axis of the implant often projects through the proposed incisal edge or even facial to the final restoration. (Fig. 2A.) Occasionally, when the surgeon places the implant in a compromised site—or the implant is inappropriately placed—the traditional screw-retained implant restoration may seem to provide more of a challenge than a solution (Fig. 2B).
ASC saves the day
An innovative solution to the offaxial implant is the Angulated Screw Channel (ASC) abutment system developed by Nobel Biocare (Fig. 3). With the ability to alter the screw channel up to 25 degrees it eliminates the need for cementation in the vast majority of cases like these.
The ASC provides for an active synthesis of health, esthetics, and excellent structural and mechanical abutment joint stability.
With use of the ASC abutment system, cement extrusion into the fragile peri-implant soft tissues is eliminated. The ASC puts an end to the insult of cement fluid pressure and unset chemicals from the cement material.
It also gets rid of the potential for foreign bodies being pushed around the implant site, which can jeopardize implant health (Fig. 4). In addition, the use of zirconia abutmentsuperstructures in combination with titanium bases provides optimized materials for biocompatibility and health.
With the ASC, the screw access channel can be projected away from high-esthetic-risk areas and placed appropriately at a variety of different angulations. CAD/CAM design enables the restorations to be efficiently designed and quickly manufactured at Nobel Biocare’s production facilities (Fig. 5). Milled zirconia is highly esthetic, thus especially useful at the soft tissue emergence site.
Fig. 6. (A) The screw access from Fig. 2A has been redirected using the ASC abutment and crown (B), producing a pleasing natural appearance thanks to a screw– retained implant restoration (C).
CAD/CAM utilization (Fig. 6 A–C) allows for optimized screw access site planning, and the machining of components provides a precise, dedicated connection, optimized for the implant-abutment joint. As with all implant-to-abutment connections, the optimized passive fitresults when these surfaces are in intimate contact and forces distributed universally. Casting abutments cannot always provide even connection joint contact, as they are often inadvertently damaged through cleaning and polishing which alters the consequent fit (Fig. 7). When this occurs, the joint connection may fail, with screw loosening or even failure of the implants as a result.
Titanium alloy abutment bases provide the most accurate fit with machining tolerances readily controlled. Abrasive wear, i.e., the release of titanium metal into the peri-implant tissues from the inside of the implant, is not an issue.
The zirconia abutment, with its well-designed circumferential wall strength, is held through the abutment screw, optimizing the ceramic’s ability to withstand forces that have been seen to fracture non-titanium base abutments.
The benefits of the ASC abutment system are numerous, reflecting a multiple symbiosis of engineering ingenuity and biocompatible materials, and allowing for the combination of good esthetics and excellent health.
The author wishes to acknowledge and personally thank the following dental laboratories for their assistance with this material: Avots Dental laboratory, Nakanishi dental laboratory and Myron Choi.
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