The best possible fit of prosthetic components should be the aim of the clinician. The technical report presented here describes the use of a modified plastic shell technique for modeling a cement-retained single-implant restoration. This approach will help to prevent potential errors caused by the wax modeling technique.
Passive and well-fitting restorations are an important factor in clinical success for any kind of dental prostheses. In addition, a passive and well-fitting framework is the primary objective necessary for accomplishing the balanced distribution of masticatory forces in the implant-supported prostheses.1–4 An ill-fitting implant framework can transmit unfavorable stresses to the bone-implant interface, which may lead to biological and especially mechanical complications. The degree of misfit is directly related to the occurrence of loosening or fracture of screws and prosthetic components.1,4–6 Framework fit discrepancies may also be associated with a higher rate of porcelain fracture in metal-ceramic implant restorations.2,7 An evidence-based value for the optimal level of implant prosthesis fit has yet to be defined. Thus, the best possible fit of restorative components should be achieved.
The precision of an implant superstructure is determined by the entire clinical and laboratory fabrication process. Elastic deformation of impression materials (dimensional shrinkage), stone or investment expansion, analog variance, wax distortion, metal, acrylic, or porcelain shrinkage, soldering inaccuracies, and manufacturer variances among several implant components all are interrelated in the fabrication of completely passive superstructures; however, all of these cannot be fully controlled directly by the clinician.4 The goal of the restorative dentist should be to prevent or minimize such factors, which may occur most often in laboratory procedures.
The lost wax technique is the preferred technique for cast restorations. Once the wax pattern is fabricated by the technician, several conditions may lead to wax distortion. As the wax is heated and cools, strain may occur within the material. When the wax pattern is removed from the abutment and is sprued before casting, the strain may be released, which may distort the pattern. Handling the wax pattern may also distort its accuracy.4
Thinning the wax in the areas to be veneered with porcelain also can create problems during fabrication of metal ceramic restorations, particularly in the implant-supported fixed dental prosthesis, for which passive fit is of particular importance. The wax becomes very fragile and breaks easily, force generated during the cutback stage may distort the adaptation of the wax, and it is difficult to judge the thickness of the coping wax pattern.8
Recently, technology has made it possible to design virtual abutments and frameworks with nearly unlimited design alternatives. By scanning the dental cast of interim abutment, some manufacturers can fabricate final ceramic or titanium abutments of any shape or angle with computer-assisted design/computer-assisted machining (CAD/CAM) technology. Interimplant titanium frameworks fabricated with this technology have been reported to fit more accurately and passively than those fabricated with standard casting technology.9–11 However, standard casting technology could be improved with alternative techniques. Use of a plastic shell coping8 may overcome some of the problems encountered in fabricating wax patterns for fixed partial dentures for implant-supported restorations.
The aim of this technical report is to describe a method for modeling implant abutments. This method relies on the use of a plastic shell technique and pattern resin in the finish line of the prepared abutments. In the authors' opinion, this method may prevent potential errors occurring in the marginal finish lines when the wax modeling technique is used. This procedure can be used when modeling is performed on intraorally prepared abutments or abutments prepared under laboratory conditions, as well as with prefabricated abutments prepared for cement-retained implant-supported fixed prostheses.
Using routine laboratory procedures, obtain the definitive cast from type IV dental stone (Galaxy, Ultima, Lafarge, Seiches Sur Le Loir, France) (Figure 1).8
Remove the prepared abutment (OPA/5, SwissPlus, Zimmer Dental, Carlsbad, Calif) from the definitive cast and attach it with another implant analogue (OPR, SwissPlus, Zimmer Dental).
Place a 4.0 cm diameter/0.1 mm thick spacer disk over a 4.0 cm diameter/0.6 mm disk of coping material (Adapta System, Bego, Bremen, Germany) (Figure 2).
Place the coping material and spacer disk onto a wire-holding frame (Adapta System Bego) (Figure 3).
Heat the disks slowly and evenly by holding them approximately 10 cm above a Bunsen burner flame. Soften the material uniformly and carefully until it becomes transparent.
Place the heated coping disk and spacer over the mouth of the molding apparatus (Adapta System, Bego), a plastic jar filled (Adapta System, Bego) with silicone putty. The spacer should be facing upward (Figure 4).
Press the prepared abutment (OPA/5, SwissPlus, Zimmer Dental) attached with an implant analogue (OPR, SwissPlus, Zimmer Dental) forcefully against the softened spacer and coping disk until the finish line of the preparation is completely submerged in the plastic jar filled with silicone putty (Adapta System, Bego).
Continue to exert pressure against the disk until the sheets become cloudy (approximately 10 seconds).
With a sharp movement, remove the prepared implant abutment and analogue assembly with the adapted spacer and coping disk from the silicone putty in the molding apparatus (Figure 5).
After the foil becomes stiff, cut off excess border material 1.0 mm above the preparation finish line with a sharp knife and separate the spacer from the inside of the coping. Inspect and evaluate the fit of the plastic coping disk on the prepared implant abutment.
Add pattern resin (GC America Inc, Alsip, Ill) to the gap between the edge of the coping and the preparation finish line.
Remove the plastic shell from the prepared implant abutment, and adjust the pattern resin finish line with a diamond rotary cutting instrument (Figure 6).
Complete the full-contour wax pattern (Classic modeling wax–blue, Renfert GmbH, Hilzingen, Germany) and the cutback stage for metal-ceramic copings with usual procedures (Figures 7 and 8).8
Sprue and invest the pattern using routine laboratory procedures.
A passive and well-fitting restoration is of paramount importance for clinical success, particularly for implant-supported prostheses, and is the primary objective for achieving the balanced distribution of masticatory forces. As described in this report, the plastic shell technique with the use of pattern resin for modeling cement-retained implant restorations may enable optimum fit of the framework while preventing errors that may occur when the conventional wax modeling technique is used.