In this chapter:
■ Traditional implant restorations supported by stock/customized abutments
■ Monolithic implant restorations supported by titanium-base abutments
■ Factors for predictable outcomes: adhesive cementation of monolithic ceramics to titanium-base abutments
Implant-supported fixed dental prostheses (FDPs) are well established and considered a safe and reliable treatment option for partial edentulous spaces (see also Part III).
Traditionally, the fixation of these restorations was achieved by either screw retaining them directly onto the implant or through an intermediate abutment, or by cementing them onto a standardized or customized abutment (see also Part I, Chapter 10)1. Current evidence demonstrates that screw-retained restorations have a lower risk of developing biologic complications (eg, marginal bone loss or peri-implant disease) and, therefore, should be preferred over cement-retained restorations1,2. Furthermore, the retrievability of the restoration makes the handling of complications easier if they do occur1. To allow for screw retention and achieve a reasonable clinical outcome, a comprehensive prosthetic planning is mandatory, followed by a correct three-dimensional (3D) implant position and axis. Concerning the material selection, both metallic and all-ceramic abutments have been suggested3. While ceramic abutments may be better in the esthetic area, titanium abutments have proven to be mechanically more stable4–7 and should, therefore, be considered as first option in posterior regions8.
Until the last decade, metal-based, handmade restorations fully or partially veneered with feldspathic ceramics were considered to be the golden standard. Currently, the developments in computer-assisted design/computer-assisted manufacturing (CAD/CAM), coupled with the high demand for esthetics, have opened up a range of possibilities for all-ceramic-based restorations, allowing for less costly and more time-effective production workflows9,10. Designing and subsequent milling of high-strength ceramics is now possible, with an adequate fit and favorable clinical outcomes11–13. For these reasons, zirconia has emerged as a very popular material, also due to its excellent biocompatibility and improved mechanical properties14. The use of zirconia as an alternative to conventional metallic frameworks in multiple-unit FDPs is currently accepted15. As with metal-based structures, zirconia frameworks are usually manually veneered for esthetic reasons. However, it is reported in the literature that the incidence of chipping of this veneering ceramic is still high15.
To overcome the aforementioned problems, different experts have suggested a monolithic restoration design, ie, the fabrication of the entire restoration out of one material without the need for veneering ceramic. In addition, when the optimal polishing is performed, monolithic zirconia has been shown to cause less wear to antagonist teeth than veneering ceramic16,17. One possibility is to limit the use of veneering ceramic to the esthetic area of the restoration, avoiding its use in the more functional areas, especially in posterior regions where stress forces are higher and esthetics is less important. In any event, abutments are required in order to connect these monolithic restorations to the implant (Fig 1-11-1).
The development of titanium-base abutments allowed for the use of implant-supported monolithic restorations in a digital workflow. Different base configurations are available according to each clinical indication, from single-unit restorations to multiple-unit FDPs supported by the respective titanium-base abutments (Fig 1-11-2).
These prefabricated titanium-base abutments appear to be a very promising solution as they combine a metallic implant connection with a ceramic outer part, thereby offering good esthetics and mechanical stability at the same time18–20. Following a digital workflow, the monolithic restoration can be designed, milled, and adhesively cemented to the titanium-base abutment, and then directly screw-retained to the implant as a conventional one-piece, screw-retained restoration21–23. This type of restoration has shown positive results in laboratory and short-term clinical investigations6,18,24.
Different configurations of titanium-base abutments are available from different manufacturers, according to the indication, ie, a single-unit or multiple-unit restoration. To allow for the compensation of implant axis divergences, and to achieve a passive fit, titanium-base abutments for FDPs usually have more convergent walls, a smaller bonding surface, and a shorter internal connection to the implant. One might assume that these factors could be possible causes for technical complications such as bonding failures or screw loosening.
In an attempt to reduce the risk of these complications, the use of titanium-base abutments for crowns has been suggested even for multiple-unit restorations, as the mechanical retention is expected to be higher. In a compromised situation, eg, if higher implant divergences need to be compensated for, a combination of titanium-base abutments for FDPs and crowns can theoretically also be applied. A recent, yet unpublished study showed that the retentive strength of the conical titanium-base abutments for multiple-unit implant FDPs was less reliable than the cylindrical titanium-base abutment25. Significantly more debonding of the monolithic zirconia multiple-unit FDPs was found in the conical group than in the cylindrical group25. Furthermore, the combination of one conical and one cylindrical abutment only led to slightly improved outcomes25. In contrast, the FDPs cemented to cylindrical titanium-base FDP abutments performed significantly better than all other groups25 (Fig 1-11-3).
Hence, in the current concept only the (new) cylindrical types of titanium-base abutments can be recommended for multiple-unit implant FDPs. Until now, the concept of titanium-base abutments supporting fixed implant restorations has demonstrated very promising results when applied to single-unit restorations6,18,24. When the ideal 3D implant position is respected, the use of titanium bases made for crowns can also be applied to multiple-unit restorations. What remains unclear is whether the difference in the design between these abutments for crowns and multiple-unit implant FDPs can lead to problems.
Notwithstanding all advantages of the titanium-base abutment concept, its success is highly dependent on the bonding stability between the titanium base and the ceramic components. In order to achieve a high and durable adhesive retention, several surface pretreatments have been proposed. Besides that, some implant manufacturers explicitly do not recommend airborne-particle abrasion of the titanium bases. From the conventional airborne-particle abrasion with aluminum oxide particles (Al2O3) to the tribochemical silica-coating systems, numerous airborne-particle abrasion methods with different particles sizes have been suggested26–