CHAPTER 26 Implant Prosthodontics: Overdentures and Their Mesostructure Bars
Overdentures are the recommended prostheses for many types of implant systems and are relatively economical and easy to use. Proper positioning of a mesostructure bar is essential to a successful result with overdentures, both esthetically and functionally, and for complete subperiosteal, ramus frame, transosteal, or root form implants. If the bar is in an inappropriate location (e.g., impinging on the tongue space or placed too far labially, creating an unesthetic or dysfunctional condition), a surgically acceptable implant may not be restorable. In addition, if inadequate vertical dimension has been left for construction of the overdenture, successful use of the implant may be impossible.
The preoperative procedure of making a surgical template (see Chapters 4 and 20) should serve as a guide for the placement of a bar or implants designed to hold a bar in the optimum position. The template can be used at the time of surgery to guide the placement of transosteal, blade, and root form or ramus frame implants, at least within the limits of the available bone. It also can be used after implant integration to guide placement of the bar. Furthermore, after making a template, the dental surgeon may realize that the procedure must be abandoned entirely because prosthetic restoration is not possible.
If a complete subperiosteal casting is planned, the clear surgical template should be placed over the bone model (although its fit is only approximate) to guide bar placement by the laboratory. If the computer-aided design/computer-aided manufacture (CAD/CAM) or stereolithographic method was not chosen, then at the time of first-stage surgery, an impression is made of the opposing arch and a wax or Optosil putty interocclusal recording with the bone exposed at the correct vertical dimension and centric relationship (Fig. 26-1). This allows accurate mounting of the bone model and proper placement of the abutments (Fig. 26-2). CAD/CAM technology creates a more significant problem, because even the most splendidly executed cast is returned without its relationship to the opposing jaw (although the CAD/CAM section in Chapter 14 provides a solution to this problem). The tube and stylus technique ensures accurate placement of the bar or abutment.
Chapter 5 explains bar attachment designs. If the bar is to be made separately from the implants, it should be the fixed-detachable or cemented type and should be fabricated in accordance with the techniques outlined in Chapters 24 and 25. Of course, a bar may also be a part of the implant infrastructure casting, as with ramus frame and subperiosteal designs.
The prosthetic technique of choice for the complete subperiosteal implant is an overdenture. Full arch fixed splints have been used, but they are not the recommended treatment primarily because of hygiene considerations.
If centric relationship and vertical dimension records were taken at the first stage of bone surgery, the laboratory can articulate the bone impression model reliably. Once the impression model has been articulated, the information needed to fabricate the implant is available, and an interim or temporary denture can be made for the patient (Fig. 26-3). The surgeon may insert this at the time of surgery and use it during the healing period (Fig. 26-4).
FIGURE 26-4. A temporary overdenture may be worn over a newly placed subperiosteal implant throughout the postoperative period and until the patient is supplied with the final prosthesis. It must be totally implant borne.
The temporary denture usually consists of anterior teeth and posterior bite blocks. The laboratory also can make a temporary denture with a CAD/CAM model if the soft tissue duplication or tube and stylus method is followed. However, if a 1-day, two-stage procedure is being followed, the laboratory will not have time to fabricate a temporary overdenture. In such cases, the patient’s existing denture can be modified to fit over the implant. After aggressive reaming, a soft lining material (e.g., Coe-Comfort, Viscogel, or Softone) is used. It is important that there be no soft tissue contact anywhere beneath the denture base; the denture should be completely implant borne. This is also true for the final superstructure restoration. Tissue contact or pressure could lead to dehiscence of the implant infrastructure.
During the impression procedure, care must be taken not to lock material under the bar or force material beneath the tissues surrounding the implant cervices. The area under the bar should be blocked out with a soft wax (e.g., periphery wax) or block-out material and the impression is then made with an alginate material in a stock tray (Figs. 26-5 and 26-6). The impression is poured with die stone, and the cast is used to fabricate a custom tray (Fig. 26-7). This tray is used to record the final impression, which is done with an elastomer (e.g., Impregum), and for which the areas under the bar again are blocked out (Fig. 26-8). The final cast is made of an epoxy material using the centrifuge technique to ensure accuracy, density, and strength (Fig. 26-9). This method ensures a detailed duplication of the bar and its retention devices, such as Locator attachments, O-rings, trailer hitches, or Zest anchors.
Next, an acrylic, heat-cured, final denture base is made to establish the maxillomandibular records (Fig. 26-10). If retentive devices are to be used as the means of attachment, the laboratory should incorporate them into the base so that its final position allows accurate records to be made (Figs. 26-11 and 26-12). If the trial setup is satisfactory, this base is used as the final one onto which the teeth are processed (Figs. 26-13 to 26-15).
FIGURE 26-11. The retentive devices planned for the superstructure are placed in the base before prosthetic registrations are recorded. This ensures the stability required for recording accurate relationships.
FIGURE 26-12. A, The Lew attachment (Park Dental Research) is a successful retentive device. A pair of them are shown here in opened (released) and closed (fixed) positions. The protruding pin fits into a groove in the mesostructure bar, providing retention. B, If the mesostructure bar has not been designed with a retentive groove or notch, one can be cut into it with a diamond or fissure bur. C, The relationship of the Lew attachment to the fenestration cut for it in the Brookdale bar. D, The overdenture buccal flange is opened to reveal the attachment site. The Lew attachment is processed into this site. E, Light-cured composite is used to tack the attachment to the denture. F, A profile view of the flange, with the attachment in open position fully processed into it. When the patient presses the button after complete seating of the denture, the pin locks it in place, and the head becomes flush with the flange.
The universal (or bilateral) subperiosteal implant is a full arch casting that is placed with natural teeth, which the prosthesis’ peripheral struts circumvent. The usual reconstructive technique is an overdenture with cast gold copings placed on the natural teeth (Figs. 26-18 to 26-20).
Telescopic copings are made on the natural teeth after the tissues have healed. If these copings are placed before healing is complete, gingival recession occurs at their margins. If a single-cast gold coping splint is cemented for the natural teeth, standard impression techniques are used to fabricate the overdenture. The bases for anchorage of natural teeth and subperiosteal implant abutments are dissimilar; therefore a one-piece coping bar splint is not advisable. Individual gold copings protect the natural teeth from caries and facilitate hygiene procedures.
FIGURE 26-21. A, A mesostructure planned for reception of Hader clips. B, Precious metal Hader clips, each of which is supplied with retention wings. C, Clips are placed in selected strategic numbers and locations on the bar. Retention wings should be readily accessible. D, Windows are cut through the lingual flange of the overdenture at each clip site. The denture is seated, offering clear access to each device. E, The area beneath each clip is blocked with wax. F, A strong, quick-setting, autopolymerizing resin (e.g., GC Pattern) is used to process the clips to the denture. G, After processing, the stark contrast between the repair material and the denture acrylic makes correction, retrieval, and clip changes a simple task. H, Final finish/>