A Method for Fabrication of Implant-Supported Fixed Partial Dentures

During the restoration of partially edentulous arches with implant-retained fixed partial dentures (FPDs), several procedural steps may influence the fit and function of the suprastructure. These include: (1) the correct transfer of the implant position, (2) the correct transfer of vertical height and maintenance of the maxillomandibular relationship, (3) the determination of optimal occlusion, and (4) the correct shaping and angulation of the implant abutments.^1–4 

This case report is presented to introduce a method of restoration with FPDs that allows the transfer of the correct implant position, interocclusal relationship, and occlusal recording, as well as the provisional restoration of the implants.

A 62-year-old man with a partially edentulous left posterior mandible presented in the office of one of the authors (G.G.Z.) in 2008 for implant placement and prosthetic restoration. Teeth number 19–21 had been extracted due to root caries 5 years previously. Two screw cylinder implants (straight line, 3.75-mm diameter, 11.5-mm length, Dentegris, Duisburg, Germany) were placed without augmentation in the areas of teeth number 19 and number 21. After the elevation of a full-thickness flap, the implant site was prepared at 875 rpm, and the implants were placed manually at a torque of 35 Ncm, following a 2-step surgical protocol. Prosthetic restoration with an implant-retained FPD was planned.

The implants were uncovered 8 weeks after placement, healing abutments were placed, and a closed-tray impression was taken using a transfer system consisting of a titanium impression post and a plastic impression coping (Dentegris; Figure 1a and b). A polyether material (Impregum, 3M ESPE, St Paul, Minn; Figure 2a and b) was used to take the impression. The titanium impression posts were left on the implants until the interocclusal relationship was registered (1–2 days), to ensure that they remained in the exact same position.

A master cast was then fabricated using system-specific implant analogs and a new set of titanium impression posts (Figure 3a and b). The cast was used to fabricate a transfer key. For this purpose, resin copings were made on top of the titanium impression posts (pattern resin, GC America Inc, Alsip, Ill) and connected to each other using a light-curing resin (tray pink transparent, Omnident, Rodgau, Germany; Figure 4a). The transfer key was placed on the patient's titanium impression posts, and a bite registration was made in centric occlusion using pattern resin (Figure 4b and c). The titanium impression posts were then removed from the implants and the healing abutments were placed. The transfer key and bite record were then placed onto the master cast, and the casts were placed into the articulator in this position (Figure 5a).

After waxing up the FPD, a key was fabricated using C-silicone (Zetalabor, Zhermack SpA, Badia Polesine, Italy; Figure 5b). The ideal angulation, position, and shape of the implant abutments (prefabricated or customizable; see Discussion section) were determined using this key. In the case presented here, customizable abutments (platinum-iridium abutments [PTIR]; Dentegris) were used for implant abutment fabrication. After casting with a chromium cobalt (CrCo) alloy (Ankatit, Anka Guss, Waldaschaff, Germany), the customized implant abutments were ground and polished (Figure 6a).

Over the implant abutments, were fabricated: (1) a resin key (pattern resin, GC America) for the transfer of the abutments on the implants (Figure 6b) and (2) electroformed gold copings (AGC Galvanogold, 0.25-mm thickness, Wieland, Pforzheim, Germany; Figure 7a and b).^5–7 

The master cast with the mounted implant abutments and electroformed gold copings in place was scanned (D700, 3-Shape, Copenhagen, Denmark), and a mock-up from clear poly(methyl methacrylate) (PMMA) (Zenotec, Wieland, Pforzheim, Germany) was milled (Triton, Kern, Eschenlohe, Germany). The master cast with the mounted implant abutments but lacking the electroformed gold copings was then scanned again, and a temporary FPD was milled from colored PMMA (Figure 8).

At the next clinical session, the implant abutments were mounted on the implants using the transfer key and torqued to 35 Nm (Figure 9a and b). The screw openings of the abutments were then filled with a single-component light-cured resin (Fermit, Ivoclar Vivadent, Schaan, Liechtenstein; Figure 9b).

The electroformed copings were placed on the abutments (Figure 9c). The fit of the abutments was assessed using radiographic images taken with and without the gold copings in place (Figure 10a and b). The mock-up from clear PMMA was placed over the electroformed copings, and the occlusion was checked (Figures 11 and 12). A bite registration was made and a final impression was taken over the electroformed copings and the mock-up using a polyether material (Impregum, 3M ESPE; Figure 13a).

After the impression was taken, the abutments were left in the patient's mouth, and the temporary FPD from colored PMMA was placed on them using temporary cement (TempBond, Kerr, Orange, Calif; Figures 13b and 14). The FPD was left in place until the delivery of the final restoration 10 days later.

In the dental laboratory, a final master cast was fabricated using the mock-up and electroformed copings to transfer the position of the gold implant abutments (Figure 15). The metal framework was milled from a CrCo alloy (Zenotec) and veneered with porcelain (Vintage MP, Shofu, Ratingen, Germany; Figure 15b).

After veneering, the gold copings were placed into the framework and fixed with self-curing compomer cement (AGC Cem, Wieland, Pforzheim, Germany).

Finally, the FPD was fixed over the implant abutments using an acrylic/urethane-based temporary cement (Implant Provisional, Alvelogro Inc, Snoqualmie, Wash; Figures 15c and 16).

The reconstruction of partially edentulous arches with implant-supported FPDs must fulfill biological, mechanical, and esthetic requirements. Several clinical steps significantly influence the success of the restoration, including the accurate recording of the interocclusal relationship (ie, maxillary-mandibular relationship, vertical height) and the transfer of the correct implant position.^8,9  Occlusal forces and the passive fit of the suprastructure may influence the long-term success of the implants.¹⁰  A multitude of methods have been suggested to facilitate these procedures, such as the intraoral fabrication of acrylic resin copings,^11,12  the use of customized impression elements,¹³  or prefabricated acrylic resin bars,¹⁴  the splinting of transfer copings,^15–17  and the fabrication of transfer keys.¹⁸ 

Although the method described in this report used customized implant abutments, prefabricated titanium can also be used (Figure 17). However, customized abutments could be casted either with a gold alloy (UCLA abutments) or with a CrCo alloy (PTIR) and allowed the achievement of more ideal angulation, height, diameter, and shape. Such optimization improved the ability to address problems related to interocclusal and interproximal distances, implant angulation, and related soft-tissue responses.¹⁹  It is the sole responsibility and independent decision of the dentist, whether customized or prefabricated abutments should be used.

Further, it should be clarified that although this report has described the fabrication of a 3-unit FPD supported by 2 dental implants, the method used is not restricted to the rehabilitation of small edentulous areas with 3-unit FPDs. This technique can also be used for the rehabilitation of larger partially edentulous areas with multiple-unit FPDs retained on more than 2 implants (Figure 18a and b). In the present report, the closed-tray impression technique was used. However, this should not be considered a limitation of the method presented. The choice between closed-tray or open-tray impression technique should always be the responsibility and free decision of the dentist.

In the method presented here, the customized abutments were not removed after mounting and torquing until the final restoration was fitted and placed. Thus, the position of the abutments remained unchanged, eliminating or minimizing errors that might occur during repeated attachment of the abutments (for various test fittings of the restoration) to the implants and master cast (Figures 16 and 17).

Ideal tooth position is esthetically and functionally important. The use of a silicon key, as described in the present report, allows the definition of the optimal abutment position, angulation, and shape.

The passive fit of the restoration is necessary to achieve optimal long-term success. A proper fit requires the accurate transfer of the intraoral implant position to the master cast and an exact fit of the impression copings (Figure 2).^{10,16,20–23}  A precise fit to the abutment can be achieved with electroformed gold copings, which are normally used as the external part of telescoping systems for removable dentures (Figures 10, 19, and 20). Due to their properties, electroformed copings were used in the method presented here to achieve the precise transfer of implant abutment form, angulation, and position in the fabrication of FPDs.^24–26 

The fabrication of a mock-up yields several advantages. It allows the evaluation of FPD fit, occlusion, and shape (Figure 12). A mock-up also allows the fabrication of an exact final master cast because the electroformed copings remain in a fixed position while impressions are taken (Figure 11). Any necessary change in shape or occlusion can also be made on the mock-up and transferred to the final denture.

The method described here facilitates the precise fabrication of implant-retained FPDs due to the high accuracy of the transfer of implant position and occlusal and bite relationships. Although this method requires 1 or 2 more clinical treatment sessions than other traditional techniques, this does not represent a real disadvantage given the superiority of the final result (Table). In brief, the implants were uncovered and impressions were taken in the first session; the interocclusal relationship was registered in the second session on the following day; and the gold implant abutments were mounted, the occlusion was checked using a mock-up, and the patient received a temporary FPD in the third session (1 week later). The treated area could be restored with the final metal-ceramic FPD 1 week later (fourth session); however, the time of loading with the final FPD depends on the dentist's treatment protocol and philosophy (Table).^27,28  The disadvantages of this method include the higher cost and the need for a very skilled laboratory technician.

Abbreviations

FPD

fixed partial denture

PMMA

poly(methyl methacrylate)

PTIR

platinum-iridium abutments

The authors would like to thank Mr Marcus Brückner, Laboratory for Material Testing/Preparation and Microscopy, Mannheim, Germany, for his valued collaboration and outstanding technical work.