January 2017
Volume 38, Issue 1

Peer-Reviewed

Full-Mouth Implant Rehabilitation With Monolithic Zirconia: Benefits and Limitations

Sarah Amin, BDS, MSc; Hans-Peter Weber, DMD, Dr. Med. Dent.; Yukio Kudara, CDT, MDT; and Panos Papaspyridakos, DDS, MS, PhD

ABSTRACT

As increased chipping rates have been reported with porcelain-fused-to-zirconia fixed dental prostheses, monolithic zirconia has been introduced in an effort to reduce the technical complications associated with bilayered ceramics. This clinical report illustrates the steps for achieving full-mouth implant rehabilitation with monolithic zirconia prostheses and minimal facial porcelain veneering. The benefits and limitations of this technique are also discussed. The incisal edges and occluding surface areas comprised monolithic zirconia to reduce the possibility of breakage and improve the esthetic outcome. Up to 1 year in function, no porcelain fracture was found.

With the aid of computer-assisted design/computer-assisted manufacturing (CAD/CAM) technology or copy milling, zirconia is currently used in implant prosthodontics for the fabrication of implant abutments, single crowns, fixed dental prostheses (FDPs), and complete arch frameworks.1-4 Compared to traditional porcelain-fused-to-metal (PFM) prostheses, higher failure rates have been reported in porcelain-fused-to-zirconia (PFZ) prostheses.5 To date, three studies have been published regarding full-arch PFZ implant FDPs; all have reported good survival rates though this is coupled with increased porcelain chipping rates.6-8

Several methods have been employed in an effort to overcome the problem of veneering porcelain chipping. One approach is to modify the zirconia framework design as it relates to the anatomical contour, such that an even, consistent thickness of the veneering ceramics can be obtained.9 Another way of manufacturing zirconia FDPs is to mill the prostheses from monolithic blocks with CAD/CAM technology.10 However, reduced low-light transmission may compromise the esthetic outcome of FDPs manufactured from monolithic zirconia.

Regarding full-mouth implant rehabilitation with monolithic zirconia, only three case studies have reported on 6-, 12-, and 24-month follow-ups, all with good outcomes.10-12 A recent retrospective case series with 26 edentulous jaws restored with monolithic zirconia FDPs reported favorable 12- to 20-month outcomes.13 Major complications have not yet been reported, except for a catastrophic failure of a mandibular monolithic prosthesis observed at the 18-month follow-up.14

The purpose of the present clinical report is to describe a full-mouth implant rehabilitation with minimally veneered monolithic zirconia FDPs and present the results and complications after 1 year in function. Benefits and limitations of this technique are also discussed.

Case Report

A 63-year-old edentulous man with complete dentures presented for implant consultation (Figure 1 and Figure 2). The patient’s chief complaint was dissatisfaction with the stability of his dentures while chewing, and he wanted to receive a fixed prosthesis. Following a comprehensive diagnostic work-up and subsequent presentation of various treatment options, the patient decided on full-mouth implant rehabilitation. Additional options consisting of implant-supported overdentures, mandibular fixed prosthesis opposing a maxillary complete denture, and mandibular overdenture opposing a maxillary complete denture had been also discussed previously with the patient.

Clinical and radiographic examination revealed that the quality of the dentures was acceptable, in terms of phonetics, esthetics, occlusal vertical dimension, and tooth spatial position. Radiographic templates (duplicates of the dentures) were used for cone-beam computed tomography (CBCT) scanning, which was followed by virtual implant planning with software (coDiagnostix™, Dental Wings, codiagnostix.com). The radiographic templates were converted into surgical templates, and on the day of the implant surgery eight implants were placed in the maxilla and six implants (Bone Level®, Straumann, straumann.com) were placed in the mandible (Figure 3).

After a 2-month uneventful healing period, second-stage surgery was done, at which time the dentures were converted into fixed interim prostheses using the conversion technique (Figure 4).4,7,10,11 One month later, definitive implant impressions were taken at the implant level using an open-tray, splinted technique, and the working casts were fabricated (Figure 5 and Figure 6).15 At the next visit, the occlusion between the two interim prostheses was registered with an interocclusal centric relation record, and the prostheses were screwed to the working casts and then mounted on a semiadjustable articulator using a facebow transfer technique.16 Verification jigs were also made intraorally to verify the accuracy of the working casts. Subsequently, dual scanning of the interim prostheses and the respective working casts was done using a laboratory scanner (Activity 880, Smart Optics, smartoptics.de).

The generated standard tessellation language (STL) files were overlapped and imported into a CAD software (exocad® DentalCAD, exocad GmbH, exocad.com) coupled with a CAM milling unit (Tizian Cut eco plus; Schütz Dental Group, schuetz-dental.de) (Figure 7). Prosthesis prototypes were milled from prefabricated polymethyl methacrylate (PMMA) blocks (ZCAD™ Temp-Fix 98, Harvest Dental Products, harvestdental.com) (Figure 8).

At the next clinical visit, try-in of the PMMA prostheses was performed, followed by confirmation of esthetics and phonetics after minor modifications. The adjusted PMMA prototypes were scanned again and the CAD files were used to mill three maxillary FDPs (Figure 9) and three mandibular FDPs. Minor facial cutback was incorporated on the maxillary FDPs and the anterior mandibular FDP for porcelain veneering to enhance the esthetics. The definitive prostheses were milled (Tizian Cut eco plus) from zirconia monoblocks (Katana™ Zirconia HT, Kuraray Noritake Dental Inc, kuraraynoritake.com) and tried in (Figure 10). Feldspathic porcelain was used to veneer the facial aspect of the FDPs. Mutually protected occlusion with anterior guidance was the prescribed occlusal scheme.

Titanium inserts (Variobase™, Straumann, straumann.com) had previously been cemented to the zirconia frameworks at the laboratory. The internal surfaces of the zirconia FDPs were sandblasted-etched for 20 seconds with 4.9% hydrofluoric acid, rinsed with water for 1 minute, and air-dried with oil-free air. Then, a 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-containing bonding/silane coupling agent mixture (Clearfil™ Ceramic Primer, Kuraray, kuraraydental.com) was applied on both the zirconia FDPs and the titanium inserts for 60 seconds before adhesive cementation. Furthermore, self-adhesive resin cement (Clearfil™ SA Cement, Kuraray) was used for the bonding procedure.

At the following clinical visit, the screw-retained monolithic zirconia prostheses were tried in. The accuracy of fit was confirmed both clinically and with periapical radiographs.17 Occlusal assessment was performed using red articulating paper (AccuFilm® II, Parkell Inc, parkell.com) and shimstock foil (GMH, Hanel Medizinal). Then, the zirconia FDPs were torqued at 35 Ncm and the screw-access holes were filled with Teflon tape and composite resin. The patient was given oral hygiene instructions regarding cleaning around the implant FDPs.

One week later the patient expressed his complete satisfaction with the esthetics and function. A nightguard was delivered to protect the prostheses from porcelain chipping and parafunctional activity. At the 1-year clinical follow-up, a stable and pleasant outcome was evident (Figure 11 and Figure 12).

Discussion

This clinical report describes a full-mouth implant rehabilitation with modified monolithic zirconia FDPs. The main benefit from the use of monolithic ceramics was the reduced possibility of porcelain fracture due to the frameworks being designed with a monolithic zirconia occlusal surface. Veneering porcelain was layered on the buccal surfaces of the zirconia-based FDPs to improve the esthetic outcome. The incisal edges and occluding surfaces remained in monolithic zirconia.

While zirconia does not seem to cause further antagonistic tooth wear,18 the effects of grinding zirconia remain unknown. Occlusal grinding could trigger the tetragonal to monoclinic phase transformation, which may compromise the mechanical properties of zirconia.10-12 This could be a potential limitation of monolithic ceramics, and, hence, every attempt should be made to carefully polish the external surfaces of monolithic zirconia to reduce abrasive effects.14 In the present case, efforts were made to minimize occlusal adjustments. First, a screw-retained interim prosthesis was used instead of occlusal wax rims to take interocclusal registration. This method allowed an accurate interarch relationship to be obtained. Second, full-contoured PMMA prototype FDPs were tried in the mouth and adjusted to obtain a harmonious occlusion. Titanium inserts were used so excessive wear at the titanium–zirconia interface could be avoided.19 Bonding or cementing titanium inserts to zirconia superstructures can compensate for machining errors and provide a passive fit while maintaining the titanium–zirconia interface at the implant level.

For minimally veneered monolithic zirconia prostheses, the cooling temperatures have to be extended, because crack propagation and subsequent zirconia fracture can be encountered if the unit is removed rapidly from the porcelain furnace.13 Another limitation of this technique is the interface between the apical cylinder portion of the zirconia superstructure and the titanium insert. Zirconia in this area may be very thin and could potentially fracture.14 The weak link is the interface between the zirconia apical cylinder (chimney) and the underlying titanium insert; either the zirconia chimney may fracture due to inadequate thickness14 or de-cementation between the titanium and zirconia surfaces may occur. The first scenario may be catastrophic while the second can be easily addressed by re-cementation. It is recommended that when the implant is placed deep, then a titanium insert with higher transmusocal profile is used to bring the interface higher and equigingivally so that the zirconia chimney is thicker and stronger, in order to reduce the possibility of fracture.

A segmented prosthodontic design was used in the present clinical case. Where applicable, a segmented design with multiple FDPs could be used for full-arch implant rehabilitation because of ease of fabrication and simplified prosthetic maintenance.20 Monolithic zirconia prostheses represent an alternative option to PFZ or PFM restorations.21 Implant-supported monolithic zirconia prostheses are gaining popularity because they address porcelain chipping problems, though further clinical trials should be conducted to investigate the longitudinal effectiveness of this novel prosthetic design.

Conclusion

The advent of CAD/CAM technology and monolithic zirconia has heralded a new era in the prosthodontic treatment of edentulous patients. While this treatment concept does have certain limitations as presented, the primary benefit of using monolithic ceramics is the reduced potential for porcelain fracture. This is due to the frameworks being designed with a monolithic zirconia occlusal surface and veneering porcelain being layered on the buccal surfaces of the zirconia-based FDPs for enhanced esthetics.

ABOUT THE AUTHORS

Sarah Amin, BDS, MSc

Implant Fellow, Division of Postgraduate Prosthodontics, Tufts University School of Dental Medicine, Boston, Massachusetts

Hans-Peter Weber, DMD, Dr. Med. Dent.

Professor and Chairman, Division of Postgraduate Prosthodontics, Tufts University School of Dental Medicine, Boston, Massachusetts

Yukio Kudara, CDT, MDT

Laboratory Instructor and Chief Technician, Division of Postgraduate Prosthodontics, Tufts University School of Dental Medicine, Boston, Massachusetts

Panos Papaspyridakos, DDS, MS, PhD

Assistant Professor, Division of Postgraduate Prosthodontics, Tufts University School of Dental Medicine, Boston, Massachusetts

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