CHAPTER 22 Root Form Implant Prosthodontics: Abutments
After second-stage surgery has been completed, and the healing collars have been removed, the final restoration is started. To affix a final prosthesis to the implants, abutments must be used as intermediate devices. In most implant systems (two surgical stages), abutments are the components that extend through the gingival tissues overlying the implants. For implants placed in a single-stage procedure (e.g., implants by Nobel Biocare, BioHorizons, and Park Dental Research), the abutments, when attached, are positioned above the exposed gingival cuffs (see chapters 10 and 11).
Abutments may consist of a single unit (Fig. 22-1); two pieces (Fig. 22-2); or three pieces. The parts of the three-piece abutment, which may be separate or unified, are the base, which fits into the antirotational component of the implant; the head, which protrudes permucosally and serves as the prosthetic retainer; and the retaining screw, which affixes the other two parts to the implant.
Abutments can be obtained from the manufacturer in machined form, or they can be custom cast by a laboratory using manufactured gold or plastic components. Several recently introduced variations include ceramic, zirconium and other wide esthetic abutments. Tables 22-1 and 22-2 list the specific characteristics of each company’s spectrum of available abutments for screw-retained and cementable crowns and copings. In this atlas, implant abutments are referred to as transepithelial abutments (TEAs).
|Astra Tech||+||+||+||a, b, c, d|
|MIS||+||+||+||a, b, c, d|
+, A feature of this implant.
a, Wide emergence profile.
b, Tooth-shaped bioesthetic abutments.
c, Prepable ceramic inverse cone, allows for up to 20-degree angulation.
d, Prepable titanium inverse cone, allows for up to 20-degree angulation.
|Name||Flat Top||Tapered||Tapered Angled||Direct Gold|
+, A feature of this implant.
Wherever esthetics allows, the margins of final restorations should be placed 2 mm above the gingival tissues. This provides easier access for oral hygiene and minimizes the risk of gingival inflammation, which can lead to implant pericervical saucerization. In such areas, a TEA is selected that will put the planned crown margin at an optimum location. This location is determined by placing a plastic periodontal probe along the gingival margin adjacent to the healing collar and down to the cervix, or by using the collar itself as a guide. The proper collar height is established by adding 2 mm to that measurement (Fig. 22-3). This can be done by the laboratory technician or the surgeon. (Chapter 24 presents the steps required to transfer an implant to a cast.)
FIGURE 22-3. A, After removal of the gingival healing cuff, a plastic periodontal probe is used to measure the tissue height, which determines the selection of the transepithelial abutment (TEA). TEAs can accommodate a range of gingival thicknesses. B, After the soft tissue thickness has been measured, abutments with 2 mm of additional height are screwed into the implants.
If the implant’s location requires that the margins be placed subgingivally, such as for esthetic reasons or because the patient wants the implant below the gum margin, the collar height is shortened commensurately.
In some cases, the cervix of an implant is already level with the gingival tissues or possibly slightly below them (Fig. 22-4), and no space has been left for a collar. In these cases, an esthetic restoration is made by fabricating a crown directly to the head of the implant. This commonly is done by using a manufactured gold or polymeric coping (described later in the chapter).
Another alternative, which allows cementation of the prosthesis, is to insert the abutment with a torque wrench and prepare the implant-abutment combination intraorally with a tapered diamond bur and copious irrigation, creating a subgingival margin. Some systems do not have an abutment or attachment designed to satisfy the surgeon’s every requirement. A wide variety of abutments are made with internal thread patterns (Fig. 22-5) and matching antirotational bases that are compatible with implants made by several manufacturers. The abutments of different companies (e.g., Biomet-3i, BioHorizons), therefore, can be substituted if they provide a required angulation, dimension, or emergence profile.
Proponents of screw retention cite retrievability as this method’s major advantage. Screw-retained prostheses also offer other benefits that deserve serious consideration. The primary advantage is greater prosthetic flexibility. Although cementation is used primarily for conventionally designed crown and bridge prostheses (whether single tooth or full arch), they lose efficacy if height is insufficient. In such cases, screw retention allows the use of short or low-profile abutments. In addition, screws provide reliable security when mesostructure bars (see Chapter 26) of limited vertical dimension are made quite close to the mucosa, and screws offer reliable support to hybrid prostheses, which must be removed for hygiene (see Chapter 24) (Fig. 22-6).
FIGURE 22-6. A, Nobel Biocare’s Curvy system supplies its own abutment. It is similar in shape to other well-proven, esthetic abutments, except that it has a concave waist. The abutment is machined from titanium and works with NobelReplace and Brånemark System NP, RP, and WP implants and corresponding healing abutments. The Curvy abutment consists of a threaded central screw and a cone component, which is available in three heights to meet the esthetic requirements of a variety of clinical situations. B, Curvy components when coupled. C, The proper collar height brings the cervical margin of the abutment level with or just below the free gingival margin.
Abutment-implant interfaces vary, depending on whether antirotational devices are included. Implants that do not have antirotational elements are flat surfaced and usually require attachment of one-piece abutments (Fig. 22-7). Traditionally, these implants are used only when multiple units are to be splinted by connecting their overlying crowns or bars, which generally are cement retained, preventing abutment malrotation. However, flat-surfaced implants should not be used for single-tooth restorations, because the lack of an antirotational feature results in persistent loosening of the abutment and prosthesis.
Antirotational features on implants prevent unwanted movement of the overlying abutment. Antirotational features currently used include the external hex, the internal hex, the spline-type interface, the Park star, and the Morse taper. Table 22-3 shows the distribution of these features in the various implant systems.
|Name||External Hex||Internal Hex||Spline||Morse Taper|
|Park Dental||+||+ (internal)|
+, A feature of this implant.
The external hex (Fig. 22-8) is the second most widely available antirotational feature in implants. This design offers a wide variety of restorative options because of the interchangeability of abutments among manufacturers. For example, the hexagonal geometry atop the Lifecore implant is compatible with the abutments available for Biomet-3i implants.