Management of Technical Complications During Full-Mouth Implant Rehabilitation With Hybrid Prostheses Over a 7-Year Period

The longitudinal effectiveness of dental implants for the restoration of function and esthetics in partially and completely edentulous patients has been scientifically validated.¹ Rough implant surfaces and immediate or early loading protocols have led to faster healing times and immediate restoration of function in carefully selected cases.¹ Prosthodontic protocols and materials have also evolved significantly since the earlier use of mandibular hybrid prostheses with acrylic denture teeth on cast gold alloy frameworks, largely because of the introduction of computer-assisted design/computer-assisted manufacturing (CAD/CAM) technology.^2,3 Selection of the most appropriate prosthetic material for the rehabilitation of the edentulous jaw may, however, still represent a challenge.

Papaspyridakos et al reported that technical complications after placement of a definitive prosthesis may not affect the implants negatively but will result in an increased number of mechanical problems, repairs, and maintenance events.⁴ The 10-year cumulative rate of "prosthesis free of complications" was 8.6% (95% CI: 7.1-10.3) with metal-resin and porcelain-fused-to-metal (PFM) hybrid prostheses.⁴ Compared to traditional PFM prostheses, porcelain-fused-to-zirconia (PFZ) prostheses have been associated with increased porcelain chipping rates.^5,6 Monolithic zirconia was employed in an effort to overcome the problem of veneered porcelain chipping by milling the prostheses from monolithic blocks using CAD/CAM technology.^7-10 Another approach includes the use of titanium/zirconia frameworks with multiple abutment preparations design and individual single crowns cemented on the framework.^11-13 A retrospective case series study with 108 edentulous jaws restored with titanium frameworks with individual cemented single crowns reported favorable 10-year outcomes.¹⁴

This clinical report describes the 7-year follow-up of a full-mouth implant rehabilitation that used different prosthetic materials. The report highlights the maintenance and management of technical complications that occurred during the course of the restoration. Benefits and limitations of various materials are also discussed.

A 35-year-old man presented for prosthodontic consultation in 2014 with concerns regarding his dental esthetics and ability to eat. He had mandibular and maxillary metal-resin hybrid prostheses that were 5 years old and were failing. Generalized denture tooth wear, along with loss of occlusal vertical dimension (OVD), was evident. The mandibular prosthesis was supported by six regular implants that had been placed in 2008 and immediately loaded, while the maxillary prosthesis was supported by three zygoma, one pterygoid, and one regular implant (Brånemark System, Nobel Biocare, nobelbiocare.com), which had been placed in early 2009.

Review of the patient's dental history revealed that his full-mouth rehabilitation was completed in early 2010 with metal-resin hybrid prostheses. After the set of hybrid prostheses was delivered, multiple technical complications were encountered over the next 4 years. Multiple repairs had been attempted to restore chipped or fractured acrylic denture teeth, while the gold framework was exposed in several areas (Figure 1 and Figure 2). Food and plaque accumulation was also noticed in the premaxilla due to ridge lap design and inappropriate contours (Figure 3). Repeated incidents of peri-mucositis were also recorded and had been treated. Radiographic examination revealed stable bone levels. Finally, by 2014 the maxillary hybrid prosthesis fractured and the mandibular counterpart showed signs of excessive wear, and, thus, it was decided to replace them.

After a comprehensive diagnostic work-up and subsequent presentation of various treatment options, the decision to fabricate new hybrid prostheses was made with the patient. For the new hybrid prostheses, it was decided that acrylic material would be eliminated and newer prosthetic materials and design would be used instead. A six-visit prosthodontic protocol would be utilized that incorporated a digital workflow.

During the first visit, abutment-level impressions were taken for both maxillary and mandibular arches using an open-tray, splinted technique, and the working casts were fabricated.¹⁵ Verification jigs also were made to verify the accuracy of the working casts. At the next two clinical visits, new denture teeth set-ups were tried-in to restore appropriate phonetics, esthetics, OVD, and tooth spatial position. Each denture teeth set-up engaged three implants in each arch to achieve greater accuracy in occlusal registration.¹⁶ The occlusion between the two screw-retained denture set-ups 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

At the laboratory, dual scanning of the denture teeth set-ups and respective working casts was performed using a laboratory scanner (Activity 880, Smart Optics, smartoptics.de) (Figure 4). The generated standard tessellation language (STL) files were overlapped and imported into a CAD software program (exocad^® DentalCAD, exocad GmbH, exocad.com) coupled with a CAM milling unit (Tizian Cut eco plus; Schütz Dental Group, schuetz-dental.de). Prosthesis prototypes were milled from prefabricated polymethyl methacrylate (PMMA) blocks (ZCAD^™ Temp-Fix 98, Harvest Dental Products, harvestdental.com). The PMMA prototype frameworks were cutback to allow for design of individual abutment preparations and subsequently re-scanned. The STL files from the re-scanning were sent to a CAM facility for milling of the titanium frameworks (NobelProcera, nobelbiocare.com) (Figure 5).

At the fourth clinical visit, the milled titanium frameworks were tried in and accuracy of fit was confirmed clinically (Figure 6) and radiographically.¹⁷ After clinical fit was confirmed, the individual single crowns were designed with CAD software (exocad^® DentalCAD) and milled in wax at the laboratory (Figure 7 and Figure 8). Mutually protected occlusion with anterior guidance was the prescribed occlusal scheme, and the wax single crowns were refined by a dental technician as needed before they were pressed in lithium disilicate (e.max; Ivoclar Vivadent, ivoclarvivadent.com). At the fifth clinical visit, bisque try-in was done and minor esthetic adjustments were made. Occlusal assessment was performed using red articulating paper(AccuFilm^® II, Parkell, parkell.com) and shimstockfoil (GHM-Hanel).

At the laboratory, the frameworks were sandblasted for 20 seconds and covered with opaque and gingiva-colored composite resin was applied at the cervical area of the framework (Gradia pink, GC America Inc, gcamerica.com) to replicate the missing soft and hard tissues (Figure 9). The internal surfaces of the lithium disilicate single crowns (e.max; Ivoclar Vivadent, ivoclarvivadent.com) were sandblasted-etched for 20 seconds with 4.9% hydrofluoric acid, rinsed with water for 1 minute, and air-dried with oil-free air.¹⁰ Then, a 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-containing bonding/silane coupling agent mixture (Clearfil^™ Ceramic Primer, Kuraray, kuraraydental.com) was applied on both the single crowns and the titanium frameworks for 60 seconds before cementation to the titanium frameworks with a self-adhesive resin cement (Clearfil^™ SA Cement, Kuraray).

At the sixth clinical visit, the screw-retained titanium frameworks 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 on how to clean the hybrid prostheses.¹⁸ One week later the patient expressed his complete satisfaction with the esthetics and function (Figure 10 and Figure 11). A nightguard was delivered to protect the prostheses from porcelain chipping and parafunctional activity. The patient was instructed to wear the nightguard every night. At the 2-year clinical follow-up a stable and pleasant outcome was evident (Figure 12 and Figure 13).

This clinical report describes the digital workflow for full-mouth implant rehabilitation with titanium frameworks with multiple abutment preparations design and individual lithium-disilicate single crowns cemented on the screw-retained framework. The main benefit from the use of this technique was that the possibility of porcelain fracture was reduced. Additionally, when using this approach any major failure of porcelain crowns can be managed by CAD/CAM milling of new crowns. This can lower the cost of maintenance and repair in the long term.

The utilization of a titanium framework and ceramic crowns reduces the bacterial accumulation reported with PMMA restorations.¹⁴ However, the use of titanium framework limits this approach to patients with adequate restorative space, because the space required is more than what is needed for other types of ceramic hybrid prostheses.

All prosthetic materials are subject to time-dependent wear and a variety of factors that affect them. Technical complications after placement of the definitive prosthesis may not affect the implants negatively but will result in an increased number of complications, repairs, and maintenance events.^19,20 The treatment modality of using metal-resin hybrid prostheses has been well-documented longitudinally, and increased technical complication rates in terms of denture tooth chipping and wear have been reported.According to Papaspyridakos et al, the 10-year cumulative rate of "prosthesis free of complications" was 8.6% (95% CI: 7.1-10.3) with metal-resin hybrid prostheses.⁴ A retrospective study by Balshi et al assessing 205 edentulous arches restored with metal-resin hybrids reported that the replacement of denture teeth (retread) occurred after an average of 7.8 years.¹⁸ Alternative options with gold or ceramic onlays on the posterior quadrants of the maxillary denture or even amalgam occlusal stops to mitigate the rate of wear on the maxillary denture teeth have been proposed.

In terms of the gingiva-colored portion of the prosthesis, composite resin (Gradia pink) was used in the present treatment instead of acrylic resin. All polymers exhibit time-dependent degradation and microleakage over time, which is an inherent limitation of acrylic resin. However, the design of the prosthesis coupled with the use of screw retention makes the repair or replacement of the gingiva-colored resin cost-efficient and easy by stripping and re-application, if necessary in the future.

As described and illustrated in this case report, digital and CAD/CAM technologies offer significant benefits for the prosthodontic treatment of the edentulous patient.

Bahaa Alshawaf, BDS
Resident
Division of Postgraduate Prosthodontics
Tufts University School of Dental Medicine
Boston, Massachusetts

Yukio Kudara, CDT, MDT
Technical Instructor and Laboratory Chief Technician
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

Panos Papaspyridakos, DDS, MS, PhD
Assistant Professor
Division of Postgraduate Prosthodontics
Tufts University School of Dental Medicine
Boston, Massachusetts