A Blended Prosthesis Design for an Implant-Supported Fixed Complete Denture to Manage Restricted Restorative Space

Many implant planning guidelines are based on factors that provide optimal physical properties for restorative materials and respect anatomical structures in the patient.¹ Historically, implant-supported fixed complete dentures were fabricated by creating a metal framework from cast alloy and wrapping it with acrylic resin and denture teeth. The minimum recommended restorative space requirements for these metal-acrylic resin hybrid restorations is 15 mm.^1,2 Newer materials such as zirconia can be used either as a monolithic material or layered with a glassy ceramic, and high-performance polymers derived from polyacryletherketones, such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), have recently become popular as frameworks.³ Zirconia has shown highly promising resultsin the medium term; PEEK- and PEKK-based restorations, however, do not have enough scientific evidence to suggest routine clinical use, at least not for the long term.^3,4 Although these new materials have good physical properties, certain dimensional limitations still preclude them from being used where restorative space is limited.

One method to create restorative space when needed is to perform an ostectomy (ridge height reduction), while another is to alter the occlusal vertical dimension (OVD).^1,5 Clinical protocols for complete-arch implant-supported solutions, particularly for tilted implants with immediate loading protocols, recommend an ostectomy to allow room for prosthetic materials even in patients with low smile lines.⁵ This procedure has advantages, but if implant complications or implant loss arise, prosthetic rehabilitations with removable prostheses become extremely challenging because of minimal to no alveolar bone being available and/or a lack of vestibular depth to provide stability for a removable prosthesis.

When restorative space is inadequate there is a high risk for prosthetic complications.⁶ Currently, metal alloys are still the preferred material for restorations when there is limited restorative space.^7,8 With CAD/CAM technology, casting distortions for ceramic alloys can be overcome by milling from a block of finished cobalt-chromium (Co-Cr) alloy, which also helps keep cost low compared to the higher price of noble and high noble alloys that have traditionally been used for metal-ceramic restorations.^8,9 Newer compositions of ceramics have their coefficient of thermal expansion compatible with these alloys, and, therefore, stacking ceramic has become more predictable.^9-11 The following case report illustrates the benefits of using CAD/CAM  technology and various new materials when restorative space is limited.

A 57-year-old woman presented with a chief concern of having an impaired ability to eat, and she was dissatisfied with the appearance of her front teeth. Her medical history was unremarkable. Her dental history revealed high anxiety over having had multiple dental visits throughout the years with failed attempts to restore her teeth.

An extraoral examination revealed no significant findings. An intraoral examination showed multiple failing restorations in the maxillary arch with secondary caries, mismatched shades of maxillary anterior crowns, a partially edentulous mandibular arch with failing crown No. 30 with secondary caries extending subgingival, and a fractured crown No. 21 (Figure 1 through Figure 3). The periodontal condition was fair with only mild gingivitis. The patient also noted that she clenched her teeth occasionally.

A complete mouth series of radiographs was taken along with a panoramic radiograph (Figure 4). Diagnostic casts were obtained. Multiple treatment options were discussed with the patient, including retention of her existing teeth with fixed dental prostheses, extraction and removable prostheses, and implant-supported restorations with a combination of fixed and removable prostheses. Based on her smile line and buccal corridor display, coupled with the fact that she clenched, the author preferred not to use tilted implants with cantilevers. After thorough discussion of the various options, the patient decided to have all of her maxillary teeth extracted and replaced with an implant-supported fixed complete prosthesis and retain her natural teeth on the mandibular arch and receive an implant-assisted removable partial denture (RPD).

To avoid cantilevers, bilateral sinus lifts with six implants were planned in the maxillary arch. In the mandibular arch, due to the resorption pattern in the edentulous site and the proximity of the inferior alveolar nerve, an implant could not be placed distal to No. 21, as adjunctive grafting procedures and possibly nerve lateralization would be required. The treatment plan was communicated with the oral surgeon, and with the use of an implant planning software (coDiagnostiX^®, Dental Wings, dentalwings.com) the implant positions were planned. Informed consent was obtained from the patient and the treatment was initiated.

During the surgical visit all maxillary teeth were extracted, bilateral vertical sinus lift procedures were performed, and implants (Straumann^® Roxolid SLActive Bone Level Tapered [BLT], Straumann, straumann.com) were placed in the maxillary arch in sites Nos. 3, 5, 7, 10, 12, and 14. The implant sizes were as follows: Nos. 3 and 14: 4.8 mm x 10 mm; Nos. 5 and 12: 4.1 mm x 12 mm; Nos. 7 and 10: 3.3 mm x 12 mm. In the mandibular arch, teeth Nos. 21 and 30 were extracted, and two implants (Straumann Roxolid SLActive BLT) were placed in these sites. The implant in site No. 21 was 4.1 mm x 10 mm, and the implant in site No. 30 was 4.8 mm x 12 mm (Figure 5). Cover screws were placed on all implants, primary closure was obtained, the flaps were sutured, and a maxillary immediate complete denture and a mandibular acrylic resin RPD were provided as provisional restorations.

After 6 months of uneventful healing, second-stage surgery was planned, and multiunit abutments were placed on the maxillary implants and attachment abutments were placed on the two mandibular implants. After 2 weeks of soft-tissue healing an abutment-level splinted pick-up impression was made using acrylic pattern resin (GC Pattern Resin^™, GC America, gcamerica.com) and dental floss for splinting and polyvinyl siloxane (PVS) impression material in a custom tray for the maxillary arch. In the mandibular arch, an abutment-level transfer impression was made using PVS in a custom tray.

Maxillomandibular relation records were made using record bases and wax rims. Following this, a diagnostic denture wax-up was performed (Figure 6). Once the patient approved the esthetics and the occlusion was verified, the artificial tooth arrangement was indexed on the master cast with condensation silicone (Sil-Tech^®, Ivoclar Vivadent, ivoclar.com), and a space analysis was performed (Figure 7). It was determined that there was approximately 9 mm of restorative space in the maxillary posterior region and more than 12 mm of space in the maxillary anterior region.

With this knowledge and an understanding of material properties, individual ceramic crowns that could be cemented onto the framework were planned in the anterior region. This would enhance esthetics and allow for easier replacement if a fracture or damage occurred to the crowns. In the posterior region, the framework was designed to receive layered feldspathic ceramic. Because the opposing mandibular arch would have denture teeth on an implant-assisted RPD the chances of veneering ceramic chipping on the maxillary prosthesis was low. A resin pattern (Pi-Ku-Plast HP 36, Bredent Medical, bredent-group.com)reflecting this design was fabricated using the provisional matrix as a guide. This framework allowed room for 1 mm of layered ceramic in the posterior dentition and abutments with enough retention and resistance form to receive zirconia crowns of 1.5 mm in thickness in the anterior region (Figure 8).

This resin framework was scanned and milled using a five-axis milling machine (Panthera Dental, pantheradental.com) to produce a one-piece Co-Cr framework using computer numeric controlled (CNC) technology. The mandibular prosthesis was an implant-assisted cast-frame RPD retained by locator attachments in sites Nos. 30 and 21. The maxillary and mandibular frameworks were returned to the clinician for try-in and fit verification. Once the OVD was verified acrylic resin provisional crowns in the Nos. 8 and 9 positions were used to verify incisal edge position, midline, and shade (Figure 9). Passivity of the maxillary framework was confirmed with the one screw test and radiographic images.¹² The RPD framework fit was also confirmed. At this stage a protrusive record was obtained to program a semi-adjustable articulator. A mutually protected occlusal scheme was planned for this treatment.

In the dental laboratory, with the use of the silicone matrix as a guide, tooth-shaded feldspathic ceramic was stacked on the Co-Cr framework for Nos. 3 through 5 and 12 through 14, and pink ceramic was stacked on the entire framework in the region of the artificial gingiva. Final baking, processing, and finishing of the ceramic was completed. The framework was then scanned with a laboratory scanner (D2000, 3Shape, 3shape.com), and zirconia crowns were designed with a minimal facial cutback and milled (Figure 10). This design allowed the incisal edge and the lingual surface to be in monolithic zirconia to maximize esthetics without compromising structural integrity of the restoration.

After staining and glazing, the cubic zirconia crowns were cemented onto the framework in the region of Nos. 6 through 11 with a resin luting system (Multilink^® Automix, Ivoclar Vivadent). The interface between the crown and the pink ceramic was sealed with pink composite (Pala^® cre-active^®, Kulzer, kulzer.com).¹³ The mandibular implant-assisted RPD was processed using denture teeth and heat-polymerized pink acrylic resin (Lucitone 199 Denture Base Resin, Dentsply Sirona, dentsplysirona.com). The completed maxillary and mandibular prostheses were returned to the clinician for delivery.

Once received, the maxillary framework was tried in, and the esthetics were accepted by the patient (Figure 11). The delivery was completed in a routine manner with the prosthetic screws torqued to 15 Ncm per the manufacturer's recommendations and access holes closed with polytetrafluoroethylene tape and composite resin (Filtek^™ Universal, 3M Oral Care, 3m.com) (Figure 12). The mandibular prosthesis was inserted and fit evaluated (Figure 13). Minor adjustments were made to the occlusion.

Post-delivery periapical radiographs were taken (Figure 14), and an occlusal device was fabricated and delivered. The patient followed-up for routine periodic evaluations at 1 week, 1 month, 6 months, and 12 months and was very satisfied with the result.

When restorative space is restricted in some regions of a prosthesis but acceptable in other areas, a blended framework may be designed and fabricated that is capable of accepting layered ceramic as well as serving as abutments for individual crowns. This strategy can keep costs down without compromising the esthetics and function of the prosthesis. A thorough knowledge of modern material properties and technology is imperative to provide optimal patient care.

The author thanks Dylan Spendal, DMD, for the oral surgery treatment provided in this case, Marotta Dental Studio for the laboratory work, and Avinash Bidra, DDS, MS, for reviewing the manuscript.

Amit Punj, BDS, DMD, MCR, MFDS RCPS(Glasg)
Chief of Prosthodontics and Director of Prosthodontic Residency Program, Montefiore Medical Center, Bronx, New York; Diplomate, American Board of Prosthodontics; Fellow, American College of Prosthodontists

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