Passive fit of a long-span screw-retained implant prosthesis is an important criteria for the success of the restoration. This article describes a technique for fabricating a ceramometal implant fixed dental prosthesis (FDP) for a long-span partially edentulous situation by altering the conventional screw-retained design. The possibility of a passive fit is maximized by intraoral luting of the cast frame to milled abutments, and the potential framework distortion during fabrication is compensated to a major extent. Retrievability is ensured by screw retention of the prosthesis to the implants. Compared with conventional porcelain fused to metal screw-retained FDP, this prosthesis is relatively inexpensive to fabricate.
Introduction
The inability to consistently achieve a passive fit with a multi-implant screw-retained fixed ceramometal prosthesis is well documented in literature.1–3 Screw-retained implant fixed prostheses have been traditionally advocated because of the ease of retrievability, benefit of splinting, and low profile retention. However, lack of passive fit and distortion of the framework that may result upon firing porcelain at high temperatures remain problem areas.4–6
Cement-retained fixed implant prostheses have the advantages of passively fitting frameworks and better esthetics.7,8 However, retrievability, repair and maintenance, choice of cement, and excess cement in the sulcus remain areas of concern.9 A combination of screw and cement retention would combine the advantages of both approaches.10 Some earlier reports have discussed the advantages of cement and screw retention for retrievable single-tooth implant restorations11 and 3- to 4-unit FDPs.12,13 Other authors14–17 have presented multiple variations of this technique basically aimed at creating passively fitting frameworks for implant-supported metal-resin fixed dentures.18 This article describes an alternative technique for fabricating a long-span, ceramometal implant FDP with cement and screw retention.
Clinical Report
Four dental implants (Nobel Biocare Replace Select Tapered TiU, NobelBiocare AB, Göteborg, Sweden) were placed in the partially edentulous grafted anterior maxillary region (Figure 1) of a patient for implant-supported replacement of 8 missing teeth (6 anterior and the maxillary right and left first premolars), using a surgical guide stent. After 7 months of postoperative healing, prosthodontic procedures were initiated. A modified-design, screw-retained, implant-supported FDP was planned for the restoration of the maxillary arch.
A maxillary complete arch polyether (Impregum, 3M ESPE, Seefeld, Germany) open-tray implant-level impression was made after coating the impression copings with polyether adhesive (3M ESPE) and splinting with polyether interocclusal recording medium (Ramite, 3M ESPE).19 Interocclusal recording material was essentially used to hold the impression copings rigidly during impression making and transfer, to maintain the interimplant relationships, in an attempt to improve the accuracy of the cast. Definitive cast was poured (Ultrarock Type IV dental stone, Kalabhai, Mumbai, India) with implant analogues (Nobel Biocare AB) and transferred to a semiadjustable articulator (Artex, Amann Girrbach AG, Koblach, Austria) using face-bow transfer and interocclusal records. Prefabricated titanium abutment cylinders (Temporary Abutment Non-Engaging, Nobel Biocare AB) were then secured to the implant analogues on the master cast and milled to a 2° taper with a milling rotary instrument (F200 2H 23, Bredent, Senden, Germany) attached to a milling machine (BF 1,Bredent) to produce a uniform path of insertion for all the abutments (Figure 2). The wax pattern (Biotec, Bredent) of the FDP framework was fabricated on the milled abutments with a spacer thickness of 100 µm (Figure 3) and the screwdriver positioned to maintain the screw access channels, as described by Rajan and Gunaseelan.11 The wax pattern was cast with ceramic cobalt-chromium alloy (d.SIGN 30, Ivoclar Vivadent AG, Schann, Leichtenstein; Figure 4). The cast framework's fit on the milled abutments was checked (Figure 5), and once found satisfactory, the framework was veneered with dental porcelain (IPS d.SIGN, Ivoclar Vivadent AG) in a conventional manner. During the veneering process, ceramic was added incrementally to keep the occlusal screw-access channels open.
Figure 2.</emph> Master cast with milled titanium abutments. Figure 3. Wax pattern of the fixed dental prosthesis framework. Figure 4. Cast framework with milled abutments. Figure 5. Cast framework fitted on the abutments. Figure 6. Resin index for intraoral transfer of the abutments. Figure 7. Anterior view of the implant fixed prosthesis 14 months after issue.
Figure 2.</emph> Master cast with milled titanium abutments. Figure 3. Wax pattern of the fixed dental prosthesis framework. Figure 4. Cast framework with milled abutments. Figure 5. Cast framework fitted on the abutments. Figure 6. Resin index for intraoral transfer of the abutments. Figure 7. Anterior view of the implant fixed prosthesis 14 months after issue.
After final finishing of the ceramometal framework, the abutments were transferred intraorally from the working cast using an acrylic resin index to maintain the spatial orientation of the abutments to the implants (Figure 6). The abutments were fastened securely to the implants using abutment screws. The polished ceramometal framework was then luted intraorally to the milled abutments with a dual-cure composite adhesive (DTK adhesive; Bredent). After the composite adhesive was completely set, the abutment screws were loosened, and the ceramometal framework, along with the titanium abutments, was removed from the implants in one piece. The implant-engaging part of the abutments was protected, and the excess adhesive was removed from the assembled framework.
Aesthetics and function were verified, the prosthesis was repositioned on the implants, and the abutment screws were tightened to 35 Ncm torque (Figure 7). The teeth contours in the cervical region of the FDP had been specifically altered and designed to avoid any saddle area under the pontics and ensure easy access for oral hygiene maintenance. The screw-access holes were filled with gutta percha and sealed with light-cure composite. The patient was given oral hygiene instructions and placed on a periodontal maintenance protocol (Figure 8).
Discussion
The prosthesis described in this article provides an esthetic and relatively inexpensive alternative to conventional porcelain fused to metal screw-retained implant-supported FDP. Though ceramometal implant restorations have superior aesthetics and strength, the cost is a limitation for many patients. The use of a base metal alloy, with its higher modulus of elasticity, enables the framework to be designed with smaller dimensions and ensures the feasibility of this approach.20 Prefabricated titanium abutments milled to fabricate the prosthesis serve as an alternative to cast gold cylinders.
Long-span ceramometal frameworks that are cast as one piece may have clinically unacceptable inaccuracies.3,21 Cumulative factors that cause distortion may inhibit passive fit of the metal implant framework.22–24 Improper fit is considered to be a cause of screw loosening, and possibly loss of integration.25–27 The technique described in the article attempts to minimize the effect of potential factors causing distortion during the fabrication stage of the prosthesis. The adhesive layer between the abutments and the metal framework compensates for the fit discrepancies, to an extent.
Though the concept of intraoral luting mentioned in this article has similarities with earlier techniques discussed in literature,11–17 some variations were incorporated into this current technique. In this article, the ceramometal prosthesis (FDP) was luted to the abutments all at once, rather than in stages of 1 or 2 abutments at a time, as advocated by the earlier studies.14–17 The earlier studies had most often applied this technique to provide long-span metal-resin implant FDPs.14–17 The studies using this technique with ceramometal or zirconia ceramic prostheses have been restrictive to single-tooth11 or short-span FDPs,12,13 and all or at least 1 of the abutments used were the engaging or indexing type.
A limitation of this technique is the challenge of milling, that is, aligning abutments to insert a ceramometal framework in cases of severe implant angulations. In most cases, implant angulations beyond 15° may require an angled abutment or a castable abutment to achieve an acceptable path of insertion. With this technique, numbering or other methods of matching the correct abutment and orientation with the correct implant fixture is imperative during the fabrication stage. In addition, an accurate intraoral transfer of the abutments using an index is important to ensure proper fit of the framework. Inadequate abutment height in patients with limited interarch space may compromise retention and resistance form of the framework/abutment interface.
Summary
A technique for fabricating a screw-retained ceramometal implant FDP is described. The ceramometal framework is intraorally cemented to the titanium milled abutments, and the FDP is connected to the implants using screws. The technique may be useful in situations where screw retention is intended and a cost-effective option is desired. The porcelain fused to metal provides the desired aesthetics and strength.
