Fabrication of a Retrievable Cement- and Screw-Retained Implant-Supported Zirconium Fixed Partial Denture: A Case Report

The success of all-ceramic crowns and patient demands for metal-free, tooth-colored restorations have led to the development and introduction of restorative systems for all-ceramic fixed partial dentures (FPDs). Sintered, glass-infiltrated ceramic core material with high alumina content,1 zirconia-reinforced glass-infiltrated alumina core material,2 and lithium disilicate glass-ceramic have been recommended for anterior FPDs,1 but none of these materials fulfill the strength requirements for posterior FPDs. These systems continue to be evaluated in clinical studies for their predictability and long-term success.3 

This problem can be solved by use of zirconium-oxide ceramics,1 which are now available for posterior 3-unit FPDs.2 The biocompatibility, pleasing color, and increased strength of these materials make them an attractive alternative to conventional ceramometal FPDs.4 

A number of zirconium-oxide ceramic systems have been recently introduced, such as Cercon (Dentsply, Amherst, NY), DCS System (DCS Dental AG, Allschwil, Switzerland), LAVA (3M ESPE, St Paul, Minn), and Procera AllZircon (NobelBiocare, Loma Linda, Calif). Zirconium-oxide ceramic is indicated for conventional and resin-bonded FPDs, full-coverage crowns, implant abutments, and endodontic posts.5 

To achieve optimal esthetics, it has been suggested that single-tooth implants should be restored with all-ceramic abutment-crown combinations.6,^–9 However, ceramics are sensitive to tensile stresses because of their inherent brittleness, and concerns remain regarding the capability of all-ceramic implant restorations to withstand functional forces in the oral cavity. Moreover, all-ceramic abutments cannot be machined to the same degree of precision as metal abutments, and an imprecise fit between the abutment and implant can lead to clinical problems.10 

With zirconium-based materials, adhesive cementation is not mandatory. Conventional acid etching has no positive effect on the resin bond to zirconium-oxide ceramics, and traditional cementation procedures can be used predictably.5^,11 

Rajan and Gunaseelan12 described a technique for fabricating a retrievable cement- and screw-retained implant prosthesis that combines the advantages13,^–16 of both cement retention and screw retention for implant-supported restorations, for example, retrievability of the abutment and the prosthesis, avoidance of the use of provisional cements, and ease of intentional removal of the prosthesis to clean excess cement. This article presents a case that illustrates this technique with a retrievable cement- and screw-retained implant-supported zirconium FPD luted with conventional cementation.

A 53-year-old man previously treated with implants in a private dental office presented at the Department of Prosthodontics at Ankara University, School of Dentistry, for an implant-supported prosthesis. Intraoral and radiographic examinations of the patient revealed that implants (Tapered Screw-Vent, Zimmer Dental, Carlsbad, Calif) had been placed in the locations of the maxillary right second premolar and the maxillary right second molar area (Figures 1 and 2). The patient's right first molar was missing, and he had a worn porcelain-fused-to-metal restoration in the maxillary right first premolar. An implant originally placed in the first molar area had failed to osseointegrate. The implant in the second molar location was placed in conjuction with a sinus lift as as a replacement for the failed implant.

After waiting for nearly 1 year for healing, prosthodontic procedures were started. The patient was given the choice of a conventional ceramometal or an all-ceramic FPD as treatment options. To satisfy the patient's esthetic demands, a zirconium FPD was selected to restore both the implants and the maxillary right premolar.

A complete-arch impression of the dentition, implants, and prepared tooth was made using a closed-tray technique with silicone impression material (Speedex, Coltene/Whaledent, Cuyahoga Falls, Ohio). An opposing arch impression and interocclusal records were made. The impressions were poured in Type IV stone (Whip-mix prima rock, Louisville, Ky), and the resulting casts were transferred to a semiadjustable articulator (Denar, Denar Corp., Anaheim, Calif) using the occlusal records.

A surveyor (Bego, Paraskop, Germany) was used to mill a rounded shoulder finish line on the abutment surface. Zirconia (3 mol% yttrium stabilized) frameworks were machined from blanks using the Cercon Smart Ceramics System (DeguDent, Hanau, Germany). Wax FPD framework models were fabricated with the screwdriver positioned to maintain the screw access channel, as described by Rajan and Gunaseelan (Figure 3). The shape of the wax model was digitized by laser scanning (Cercon), enlarged, and milled out of a homogeneous porous prefabricated Cercon blank and oven-sintered to full density. During firing, the framework shrinks to the desired dimensions. After the framework was fabricated (Figure 4), the first try-in was done to assess the complete seating of the prosthesis. The shade of the restoration was determined and recorded (Figure 5), then the zirconium framework was veneered using the dental porcelain (Noritake, Kizai Co, Nagoya, Japan) specially designed for zirconia. During the veneering process, ceramic was added incrementally to keep the occlusal screw access channel open. A second try-in was performed to evaluate occlusion.

After glazing, the FPDs were cemented with zinc-phosphate cement (Heraeus Kulzer, GmBH, Hanau, Germany) (Figure 6). Excess cement from the access channel was cleaned, and the excess cement from the margins was removed by loosening the abutment screw through the screw access channel and removing the abutment with the prosthesis (Figure 7). The FPD was repositioned and secured in place with screws, and the screw access holes were sealed with chemical-cure composite (AlphaDent, Dental Technologies, Lincolnwood, Ill) (Figure 8). After delivery of the definitive FPDs, occlusion was checked and the patient was dismissed until the first annual recall appointment.

New high-strength core/framework materials have been developed for all-ceramic FPDs; however, strength limitations in most of these systems only indicate their use for replacement of anterior and premolar teeth. Systems using Y-TZP (yttrium tetragonal zirconia polycristals) as a core material may be an alternative treatment modality for replacing missing teeth in both anterior and posterior segments.3 The increased strength of zirconia and its possible resistance to crack propagation make it the material of choice in cases with high occusal load.

Cement-retained, implant-supported restorations have advantages over screw-retained restorations.13^,14 The passive fit of castings, reduced complexity of clinical and laboratory procedures, enhanced esthetics, and reduced cost factors are advantages of cemented restorations.15^,16 They have the potential to compensate for any minor dimensional discrepancies in the fit of restorations to abutments, which can contribute to lack of passivity.13 Minor dimensional discrepancies may be compensated for by using cement and cement space. However, a disadvantage of cementing implant-supported restorations is the potential difficulty in retrieving the restoration. The combination cement- and screw-retained implant-supported zirconium FPD combines the advantages of both designs. The described method enables removal of the prosthesis and abutment without a crown remover in case of repair or for other reasons.

Parallel alignment of the abutments will simplify this technique on applying 2 implant-supported restorations; otherwise, unfavorable stresses can be induced while repositioning the FPD on the misaligned implants after removing the excess cement. In this case report, difficulty in repositioning of the prosthesis on the implant in the second molar site was attributed to the poor alignment and the anatomic limitations of the soft tissue.

High-strength all-ceramic systems have been developed for FPDs. Zirconium ceramics may be an alternative treatment modality both in the anterior and the posterior regions. This clinical report decribes the fabrication of a retrievable cement- and screw-retained implant-supported zirconium FPD.