A Blended Prosthesis Design to Manage Restricted Restorative Space

One method to create restorative space when needed is to perform an ostectomy (ridge height reduction), while another is to alter the vertical dimension of occlusion (VDO).^1,5 Clinical protocols for full-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.

Treatment Plan

A full-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 with 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) the implant positions were planned. Informed consent was obtained from the patient and the treatment was initiated.

Surgical Stage

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) 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, 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) and dental floss for splinting and 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.

Collaborative Restoration Planning

The co-author's dental laboratory approaches full-arch cases similar to complete dentures, starting with basic records: a wax bite rim (typically stabilized onto two implants), a verification jig, and then a denture setup. This allows the technician to achieve the proper mounting and tooth esthetics before planning the final restoration. It also allows them to provide the dentist and, importantly, the patient with a rough idea of what to expect in a final case. Of course, a setup includes wax and a flange that will not be present on the final case, but it helps to determine if the patient is happy with how much teeth show in their smile and whether they are comfortable with the proposed VDO, as well as providing a good idea about the ultimate tooth placement.

Once those steps are completed, a discussion can occur as to what to try to accomplish with the final restorative material and design. The restorative team has established the VDO as well as the ideal tooth placement. This information can help determine the best material choices and case design. For example, if the teeth need to be well off the ridge to provide lip support, an implant overdenture might be the best restorative option.

On this case, the laboratory worked closely with the prosthodontist as 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 Group), 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, due to the limited occlusal clearance, the framework was designed to receive layered feldspathic ceramic directly. 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) 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 protototype of the framework was fabricated based on the provisional, scanned, and copy-milled at a milling center (Panthera Dental) to produce a one-piece Co-Cr framework using 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 VDO was verified, the laboratory milled provisional crowns in the Nos. 8 and 9 positions in a PMMA temporary material for use in verifying 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.

Laboratory Fabrication

As the laboratory ensured the casts were verified and confirmed the bite and setups, the process to complete the rest of the case was relatively straightforward. With the tooth positions already known, the framework could be fabricated based on that outline with no need to grind or adjust the milled prosthesis. Tooth-shaded feldspathic ceramic was layered on the Co-Cr framework for Nos. 3 through 5 and 12 through 14, and pink ceramic was applied throughout for gingival contouring. Final baking, processing, and finishing of the ceramic was completed. The framework was then scanned with a laboratory scanner (D2000, 3Shape) in order to design the individual zirconia crowns for teeth Nos. 6 through 11. The crowns were designed with a minor facial cutback to allow for porcelain application and then milled (Figure 10). This design allowed the structural integrity of the restoration to be maintained with monolithic zirconia on the incisal edge and the lingual surface while maximizing esthetics via custom staining and translucency.

Finally, the zirconia crowns and porcelain application were completed based on the approved provisional and setup.

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

Try-in and 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) (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 authors thank Dylan Spendal, DMD, for the oral surgery treatment provided in this case, 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

Justin Hayes
Operations Director
Marotta Dental Studio
Farmingdale, NY