Delivering Truly Screw-Retained Hybrid Prostheses

The utilization of screw-retained restorations eliminates these as well as other potential problems that can occur with cemented restorations over time. Should the screw loosen on a screw-retained restoration, access to the screw channel is simplified and damage to the restoration can be avoided while accessing the screw. Moreover, the process of removing screw-retained prostheses is simplified should it be required for other reasons, such as for periodontal maintenance.

Screw-retained zirconia restorations are traditionally fabricated to be luted to a titanium base that sits between the zirconia and a multi-unit abutment at the implant's platform. The restoration seats over the titanium base and is luted to it using resin cement. Unfortunately, problems are often found with implant-supported restorations with titanium bases that cause issues chairside. For example, the restoration may separate from its titanium base under function.^1,2 One cause for this is that the height of many of the titanium bases that are available from the implant manufacturers is insufficient, so less surface area is present to lute them to the overlying ceramic restoration.

When attempts to eliminate the use of a titanium base and connect a zirconia crown directly to a multi-unit abutment are made using traditional fixation screws, which have flat bottoms that are designed to seat in titanium bases, the thickness of the surrounding zirconia is decreased (Figure 1). This may lead to fracture of the zirconia under function.^3,4 Alternatively, elimination of the titanium base using a crown fabricated to accommodate a fixation screw with a tapered head permits the zirconia to be thicker around the multi-unit abutment, which greatly decreases the fracture potential of the zirconia (Figure 2). In addition, because traditional prosthetic screws have flat-seating heads and parallel walls, they do not create a constant load to prevent screw losing. Utilization of a screw with a tapered head and a conical shape creates loading pressure that retains the prosthesis and helps to prevent screw loosening. These screws are compatible with multiple implant systems on the market and may be utilized in-office or with an outside laboratory for the fabrication of milled or 3D printed provisional and final single or multi-unit fixed prostheses.⁵

A 65-year-old male patient presented for a consultation related to a failing maxillary anterior bridge. As a part of the evaluation, a panoramic radiograph was acquired (Figure 3). Radiographically, it was noted that the only three remaining teeth in the anterior region of the maxillary arch were associated with a bridge that spanned from the right canine to the left lateral incisor (teeth Nos. 6 through 10). These three abutment teeth (Nos. 6, 8, and 10) had all undergone prior endodontic treatment and exhibited significant bone loss. Moreover, the bridge demonstrated grade 2 or greater mobility. Regarding the maxillary posterior region, the patient's maxillary left second molar (tooth No. 15) was also present and in occlusion with his mandibular left second molar (tooth No. 18). This was the only stable occlusal stop intraorally.

The patient expressed that he only desired to treat his maxillary arch at this time. Therefore, a discussion was held with him about initiating an implant-based treatment plan to replace the maxillary dentition. This would include extraction of the three abutment teeth supporting the anterior bridge, followed by socket grafting and a bilateral sinus lift. Tooth No. 15 would be maintained as an occlusal stop. After extraction, a provisional traditional denture would be fabricated and inserted for the patient to wear during a healing period prior to implant placement. Planning would be performed for the placement of implant fixtures between site Nos. 3 and 13 for an "all-on-6" approach. Following implant placement, should adequate insertion torque be achieved, the implants would be immediately loaded with a provisional hybrid restoration. Then, once the integration period was complete, the provisional hybrid restoration would be used to inform the design of a final zirconia hybrid restoration. The patient agreed to the treatment plan as presented. Both of his arches were scanned with an intraoral scanner (CS 3700, Carestream Dental), his vertical dimension of occlusion was captured for fabrication of the provisional denture, and he was dismissed with an appointment to return for the extraction phase of the planned treatment.

In order to design a full denture that took maintaining tooth No. 15 into consideration, the intraoral scan data were imported into dental planning software (Blue Sky Plan^®, Blue Sky Bio), and teeth were virtually added to the maxillary arch in the plan. Following completion of the provisional denture design, a desktop 3D printer (Form 2, Formlabs) was used to print the base (Denture Base Resin, Formlabs) and the teeth (Denture Teeth Resin, Formlabs) separately. The teeth were then luted to the base using the base resin.

Extraction, Grafting, and Sinus Lift

When the patient returned to the practice, he signed a consent form approving the extraction of teeth Nos. 6, 8, and 10, subsequent socket grafting at all three sites, and a bilateral sinus lift. A local anesthetic was administered, and the teeth were extracted uneventfully. Upon extraction, the sockets were curetted to remove any residual tissue, and socket grafting was accomplished with a corticocancellous allograft (MinerOss^® Blend, BioHorizons) and collagen plugs (OraPLUG^®, Salvin Dental Specialties).

Next, a crestal incision was made distal to the extraction sockets bilaterally, and the tissue was elevated to expose the crestal bone inferior to the maxillary sinuses. A lateral window approach was then used to elevate the sinus floor bilaterally. To complete the sinus augmentation and provide sufficient bone height for implant placement, the areas were filled with a 50/50 mix of the corticocancellous allograft material and a cancellous bovine xenograft material (MinerOss^® X, BioHorizons), and the soft tissue was sutured closed. The provisional denture was inserted intraorally, and its occlusion was checked and adjusted as needed (Figure 4).

The patient returned after 1 week for a postsurgical check and removal of the sutures. After 4 weeks, the patient returned again so that the arch could be examined to check on the progress of healing. The provisional denture was removed, and it was noted that the soft tissue presented without inflammation and that complete healing had been achieved (Figure 5).

Fabrication of the Surgical Guide and Provisional Prosthesis

Within the planning software, a virtual design was created for the delivery of a 12-unit prosthesis that would extend from the site of the maxillary right first molar (tooth No. 3) to the site of the maxillary left first molar (tooth No. 14). A space would be present between the terminal left tooth of the prosthesis and the remaining natural second molar due to its position in the arch. Based on the availability of sufficient bone for implant placement, implants were virtually placed into the design at the sites of teeth Nos. 3, 5, 8, 9, 11, and 13 (Figure 6). A surgical guide was then designed based on these planned positions, 3D printed from resin (Surgical Guide Resin, Formlabs), and finished by adding the metal guide sleeves.

After fabrication of the surgical guide, the provisional hybrid prosthesis was designed in the dental planning software for chairside pickup with multi-unit abutment copings that would be placed on the implants (Figure 7). The pontic areas of the provisional hybrid prosthesis were designed to have a convex adaptation to the ridge with slight pressure on the soft tissue to help eliminate potential food traps and aid in patient home care (Figure 8). Once the design for the provisional hybrid prosthesis was finalized (Figure 9), it was exported and 3D printed (Permanent Crown Resin [shade A2], Formlabs). Pink composite (Anaxgum, Anaxdent North America) was then applied to the gingival aspects of the provisional hybrid prosthesis to provide it with an esthetic, natural-looking appearance (Figure 10).

Implant Placement and Immediate Provisionalization

At the surgical appointment, the consent form for implant placement was reviewed and signed by the patient, and then a local anesthetic was administered. The surgical guide was then tried in, and its adaptation to the arch was confirmed. Utilizing a guided surgical approach, three 4.6 × 10.5 mm implants (Tapered Plus, BioHorizons) were placed at the sites of teeth Nos. 3, 5, and 13; two 3.8 × 12 mm implants (Tapered Pro, BioHorizons) were placed at the sites of teeth Nos. 8 and 9; and one 4.2 × 12 mm implant (Tapered Pro, BioHorizons) was placed at the site of tooth No. 11. Following placement of the implants, a panoramic radiograph was acquired to document their position and angulation in relation to the anatomy and crest (Figure 11). Multi-unit abutments were then placed on each implant (Figure 12). Next, the 3D printed provisional hybrid prosthesis was tried in and fixated to the implants using a chairside pickup technique (Luxatemp Plus, DMG America) with multi-unit abutment copings at each fixture. A panoramic radiograph was taken to confirm seating (Figure 13), and the occlusion was checked to verify uniform contacts. Upon delivery, the patient was given a mirror to visualize the provisional hybrid prosthesis. He expressed that he was satisfied with its esthetics (Figure 14). The patient was scheduled to return for 1-week and 4-week postoperative checkups.

Fabrication and Delivery of the Final Prosthesis

After the implants were allowed to integrate for a period of 6 months, the patient was recalled to the practice to discuss any esthetic changes that he might desire in the final hybrid prosthesis. He indicated that he was happy with the esthetics and did not have any changes. The design of the final hybrid prosthesis would follow that of the provisional hybrid prosthesis, eliminating the need for titanium bases with screw access channels that accommodate the use of prosthetic screws with tapered heads. The data file was transferred to the in-office milling unit (Roland 52D, Roland DGA), and the final hybrid prosthesis was milled from a multilayer zirconia disc (CopraSupreme Symphony, Whitepeaks Dental Solutions). Following milling, the zirconia structure was sintered, and a pink stain (MiYO^® Pink, Jensen Dental) was placed on the gingival aspects to replicate the patient's natural esthetics (Figure 15 and Figure 16).

The patient presented for delivery of the final restoration. After the provisional hybrid prosthesis was removed from the mouth (Figure 17), the monolithic zirconia hybrid prosthesis was inserted and fixated directly to the multi-unit abutments using prosthetic screws with tapered heads (MUA Rosen Screw^®, Rosen Implant Solutions) that were tightened to 35 Ncm per the implant manufacturer's recommendations. A final check to verify the occlusion was performed, completing the restorative phase of the treatment (Figure 18). The patient expressed that he was satisfied with the esthetics of the clinical results (Figure 19).

The delivery of monolithic zirconia hybrid prostheses has become routine when restoring implants in a partial or fully edentulous arch. Although titanium bases have been utilized to fixate these prostheses to their multi-unit implant abutments, two common problems have been reported with this approach. First, the use of titanium bases results in a decrease in the thickness of the surrounding zirconia in order for the prosthesis to demonstrate natural dimensions that meet esthetic demands. This may lead to fracture of the zirconia adjacent to the titanium base and subsequent failure of the prosthesis. And second, because titanium bases are luted into zirconia hybrid prostheses, those bonds have the potential to fail, which may result in separation of the bases from the zirconia. This can affect a single unit or the entire prosthesis, requiring the prosthesis to be removed from the mouth for rebonding. The elimination of titanium bases from zirconia hybrid prostheses avoids these two common problems. Furthermore, when fixation is accomplished with the zirconia in direct contact with the multi-unit abutment, the use of a prosthetic screw with a tapered head enables the zirconia to be thicker in this area and more resistant to fracture under function. The weak link between the zirconia and the titanium base is also eliminated, so there is no potential for debonding to result in separation. And finally, the geometry of a tapered prosthetic screw head puts pressure on the prosthesis, maintaining screw torque and decreasing the potential for loosening.

Rick Ferguson, DMD
Diplomate
American Board of Oral Implantology/Implant Dentistry
Director
Implant Educators Academy
Private Practice
Davie, Florida

Gregori M. Kurtzman, DDS
Master
Academy of General Dentistry
Diplomate
International Congress of Oral Implantologists
Private Practice
Silver Spring, Maryland