Since their introduction more than four decades ago, endosseous implants have transformed prosthetic dentistry. Removable prosthodontics ceased to be the only option for completely edentulous patients and the functional and esthetic outcomes of restorations were greatly improved to achieve unprecedented patient satisfaction.
In spite of the clear benefits of implant dentistry, long‐term, large‐scale data on restoration of implants are scarce. This scarcity applies to specifically implant abutments and crowns, and as a result, there is no strict guideline or consensus on designing and fabricating implant restorations. Studies do indicate implant restorations produced using digital technology can be as good as, if not better than, those produced using conventional methods (Long et al. 2017). Moreover, digital technology has led to many different types of materials and permutations of methods in implant restorations as it has allowed flexibility due to innovative thinking by clinicians and laboratory technicians. Thanks to the high accuracy and flexibility of digital technology in dentistry, restoring implants has become streamlined and predictable.
Restoring dental implants follows similar principles to those described for the restoration of natural teeth. An accurate rendition of implant position in the dental arch (impression) is necessary to design and fabricate a foundation for the restoration – an implant abutment. An implant‐retained restoration, whether cement or screw‐retained, can then be fabricated on top of the implant abutment.
Screw‐retained restorations, designed to be attached directly to the implant or to an intermediate abutment, are easy to fabricate, retrievable, and cannot cause foreign body reactions like those observed with cement‐retained restorations due to remnant cement. Therefore, whenever possible, screw‐retained restorations should be the first choice of treatment (Long et al. 2017). Cement‐retained restorations should be reserved for instances when a screw‐retained restoration is not possible. An example of such instance is shown in Figure 10.1. The right lateral incisor is missing, and an implant was placed to restore the missing tooth (Figure 10.1a). The implant appears to be labially positioned and gingival zenith is apical to adjacent teeth and contralateral lateral incisor. The severe angulation of the implant is illustrated in Figure 10.1b,c. Due to implant angulation, corono‐apical position of the implant, and esthetic needs, a cement‐retained restoration is preferable. A milled titanium custom abutment was fabricated and coated with opaque material to mask gray color of the abutment and a milled zirconia crown was fabricated to match the shade of adjacent teeth (Figure 10.1b–d).
Intraoral data in implant restoration can be acquired in ways similar to those in tooth‐borne restoration:
- Make a conventional impression using elastomeric impression material and impression coping. Scanning the resulting master cast allows digitization of the cast.
- If an intraoral scanner and scan body are available, the intraoral situation can be digitized by using the scanner.
The clinician and/or laboratory technician can now enter the design phase in the digital workflow using a design software, which will help design both coronal restoration and abutment.
10.2 Implant Abutments
Implant abutments can be generally classified into two types: (i) prefabricated implant abutments and (ii) custom‐made implant abutments.
10.2.1 Prefabricated Abutments
Prefabricated abutments are manufactured using subtractive manufacturing technology described in Chapter 3. The seating surface of these abutments is precision milled to passively fit the head of the implant with very low machining tolerance (Ma et al. 1997, Malaguti et al. 2011). The height of the abutment, location, and width of the finish line, and the axial walls can be modified manually by the technician or the dentist to accommodate a full coverage restoration. Like any item used in dentistry, these abutments are manufactured from biocompatible materials, typically a titanium alloy or ceramic, that do not promote plaque accumulation and can withstand masticatory forces. An example of a prefabricated abutment is shown in Figure 10.2a.
Prefabricated abutments are readily available, economical, and can be easily modified, thus, they are widely used. An example of commonly used prefabricated abutments is titanium base (“Ti base,” or titanium insert; Figure 10.2b). Ti bases in different dimensions of height and angulations are available in the digital design software for most implant brands, allowing the clinician and/or laboratory technician to design the coronal restoration after acquiring an intraoral scan of the intraoral presentation. An example of a Ti base and a coronal restoration designed to be fabricated in lithium disilicate is shown in Figure 10.2c,d. In comparison, an implant restoration requiring a custom abutment would first need a design of the custom abutment, which is described later in this chapter.
Once the lithium disilicate restoration has been fabricated and crystallized, it is cemented to the Ti base extraorally. To start this process, the screw access channels of both Ti base and ceramic restoration are obturated with a material that can be easily removed after cementation (Figure 10.2e). The internal surfaces of the restoration are etched (5% hydrofluoric acid) for 20 seconds, rinsed, and allowed to dry while the axial surface of the Ti base is air‐abraded, cleaned with steam, and dried (Figure 10.2f). A primer is then applied evenly to both Ti base and restoration (Figure 10.2g). After 60 seconds, both components are dried. An even coat of a self‐curing resin cement is applied to the internal axial walls of the coronal restoration and the external axial walls of the Ti base (Figure 10.2h). The coronal restoration and Ti base are firmly connected, and an excess amount of cement can be observed (Figure 10.2i). After setting, the excess can be removed (Figure 10.2j), and the joint at the restoration and Ti base can be closely examined under a magnification for any residual cement, which can be removed carefully with a rubber point.
The clinical performance of implant‐retained crowns restored using Ti bases has not been thoroughly investigated; however, laboratory studies suggest that these restorations may exhibit promising outcomes (Conejo et al. 2017). One of the major drawbacks of Ti base and other prefabricated abutments is that their contours are rarely anatomic, especially at the junction between the implant and the abutment, and do not support the surrounding soft tissues making managing the emergence profile of an implant restoration difficult. They are also difficult to use in patients where there is excessive implant angulation. To circumvent these caveats, prefabricated abutments that allow for the correction of angles were introduced. These abutments use angled screw channel technology to allow for the correction of implant angulation of up to 20° (Friberg and Ahmadzai 2019, Gjelvold and Sohrabi 2016).
In addition to the disadvantages of this workflow based on the limitations of prefabricated abutments mentioned above, should the crown (zirconia or lithium disilicate) cemented to the Ti base need addition of interproximal and/or occlusal contact, it must be de‐cemented before adding a contact using a ceramic material. Alternatively, the clinician may choose not to have the Ti base and coronal restoration to be cemented prior to try‐in. The soft tissue may interfere with a proper seating of the coronal restoration if the Ti base does not mimic the final gingival anatomy at the implant and abutment junction. Unsupported ceramic may also be a concern for a large restorative area again due to the limitation of the Ti base dimension.
10.2.2 Custom Abutments
Custom abutments were first described in 1988 (Lewis et al. 1988, Lewis et al. 1989). Traditionally, these abutments consisted of a plastic sleeve or a gold cylinder that could be waxed to specification and cast in metal alloy to fabricate an abutment that sits on dental implants. These abutments are prepared, finished, and polished like any traditional casting and can be designed for cement‐ or screw‐retained restorations. In Figure 10.3, a custom abutment is shown before fabrication.
With the continued evolution of dental technology, abutments can now be designed digitally and fabricated using milling technology for each individual patient. Thus, by definition, these computer‐aided design and computer‐aided manufacturing (CAD/CAM) abutments are classified custom abutments.
With CAD/CAM custom abutments being economical to fabricate and predictable to provide optimum contours for the restoration resulting in proper esthetics and function, there is no longer a need to wax and cast abutments. The CAD/CAM custom abutments provide convenience as well, since dentists and technicians can design and fabricate custom abutments from the comfort of their seats.
However, should an implant restoration with a cast custom abutment be needed because a conventional manufacturing method is desired, digital technology can still be utilized. An example of this combination of conventional and digital workflows would be lithium disilicate pressed on metal (Figure 10.4). In the conventional workflow, a custom abutment is made out of wax to be cast or alternatively, the abutment could be scanned and the “wax‐up” could be produced via CAD/CAM in the form of a castable resin. Either the wax‐up or castable resin can be put through a burnout process, after which lithium disilicate would be pressed onto the abutment.
Whether digital technology is used entirely or it is combined with conventional methods, the goal remains constant: the design and manufacturing process must be predictable, helping choose the most appropriate material to produce accurate restorative components that satisfy biology, mechanics, and esthetics. Choosing the most appropriate design and manufacturing method and materials would require consideration of various aspects of dental laboratory technology. As some areas of dental laboratory technology still rely on conventional materials and methods, the workflow will still include certain conventional processes, such as wax‐up for casting and layering feldspathic ceramic material manually for coronal restoration (Figure 10.5).
Another reason for considering both conventional and digital processes is the availability of materials and devices used in digital workflow. For example, a clinician who does not have access to a digital intraoral scanner will choose to make a conventional master impression using an elastomeric impression material. If the receiving dental laboratory has the digital capability (e.g. desktop scanner) to digitize the cast, the laboratory can scan the impression (or the cast from the impression), and the workflow would change to digital, with the goal of designing and sending the restoration design to a manufacturing machinery for fabrication of a crown (3D printer or milling machine; see Figure 10.6).