Maximizing Stability and Prosthetic Durability

INTRODUCTION
When treatment planning a fixed full-arch implant restoration, selecting a prosthesis that addresses the long-term needs of the patient is paramount. It is also imperative to utilize dental implants that can be optimally positioned to support the restoration while establishing a maximal amount of primary stability. For clinicians seeking a durable prosthesis, full-arch implant restorations can now be milled from 100% monolithic zirconia. To establish the foundation needed for such a restoration, tapered implants with a pronounced thread design afford flexibility in implant positioning as well as high initial stability, smoothing the path toward an aesthetic, predictable outcome.

Although screw-retained hybrid dentures with titanium frameworks have offered patients maximal function and prosthetic stability, their vulnerability to acrylic chipping, stains, and fractures has created a demand for a more durable fixed prosthesis. This need has been addressed by the emergence of the solid zirconia full-arch implant prosthesis, which offers clinicians unrivaled strength and lifelike aesthetics. Indeed, these restorations have been shown to withstand functional forces remarkably well.^1,2 The only reported vulnerability has been cracks in the layering porcelain, but this concern has been addressed by monolithic restorations, the entire body of which, including the gingival areas and teeth, is milled from a single block of solid zirconia, eliminating the need for veneering ceramic.

Dental implant designs have advanced to the point where a strong majority of systems offer an excellent long-term prognosis.^3,4 Nonetheless, practitioners stand to benefit from utilizing implants with a tapered design, which can be better situated in areas constrained by limited bone volume or adjacent anatomical structures. The flexibility in positioning afforded by a tapered implant design is particularly advantageous for screw-retained full-arch restorations, in which the anterior-posterior (A-P) spread must be maximized to provide the support needed for the fixed prosthesis.

Further, choosing a design with deep, prominent threads can help maximize primary stability.^5,6 In addition to establishing the support needed for a predictable restorative outcome, high initial stability is required in order to immediately load the implants with a fixed provisional restoration, which appeals strongly to many fully edentulous patients. A pronounced thread design also helps the clinician maintain control as the implant is threaded into place, engaging the lingual or palatal wall and thus helping keep the proper distance from the facial plate, which is essential in screw-retained full-arch restorations.

The following case demonstrates how implants designed with a tapered shape and prominent threads can be used in combination with monolithic zirconia to deliver fully edentulous patients an aesthetic, highly stable restoration with a superb long-term prognosis.

CASE REPORT
Diagnosis and Treatment Planning
A female patient in her early 60s with terminal dentition presented for consultation (Figure 1). The patient had received extensive treatment in the past, including conventional crowns and root canals, most of which either had failed or were breaking down as a result of severe decay. Due to the collapse and fracture of many of her posterior teeth, the patient suffered from a diminished vertical dimension. In addition to the discomfort and functional difficulties her dental condition was causing, the patient was self-conscious about her unaesthetic smile and halitosis. The patient desired nonremovable prostheses resembling the form and function of natural teeth as closely as possible, as the prospect of losing her dentition had prevented her from seeking out treatment for some time. It was also critical to the patient that she leave the surgical appointment with fixed teeth of some kind in place, as she did not want to wear a removable appliance at any point during treatment.

Figure 1. Preoperative condition of the patient, whose dentition had a generally poor prognosis resulting from severe caries, recurrent decay, and periodontal abscesses.	Figure 2. Wax try-in over implants.
Figure 3. Polymethyl methacrylate (PMMA) temps over implants.	Figure 4. Postoperative photo showing final zirconia bridge.

She accepted a treatment plan in which 6 implants would be placed in each arch to support a BruxZir Full-Arch Implant Prosthesis (Glidewell Laboratories) (Figures 2 to 4 show various stages of treatment). Although a few of her teeth could have been salvaged through further endodontic therapy, the added time and cost of such treatment was not accepted by the patient. Radiographic examination and CBCT scanning indicated sufficient bone volume to place the implants following a freehand surgical protocol (Figure 5).

The implants would be placed immediately following the extraction of her terminal dentition. In comparison to protocols in which the extraction sites are left to heal prior to implant placement, this approach would shorten the duration of treatment. Provided satisfactory initial stability, the implants would be provisionalized with temporary dentures. In addition to providing the same-day temporary restoration that the patient desired, immediate implantation and provisionalization would help preserve the patient’s alveolar bone.^7-9 Hahn Tapered Implants were selected for the case in order to establish a high level of initial stability, maximizing the potential for provisionalization following an immediate loading protocol. The implants would also ease placement toward the palatal and lingual aspects, while simplifying the positioning needed to avoid vital anatomical structures.

Following implant placement, a restorative protocol would be followed that is remarkably similar to that of the screw-retained hybrid denture. One key difference is the use of a provisional appliance that ensures the proposed final restoration is 100% accurate prior to milling the monolithic zirconia implant prosthesis. This extra step, along with the accuracy of the CAD design process, results in an extremely well-fitting final prosthesis that addresses the precise aesthetic and functional needs of the patient.

Surgical Implant Placement
To begin the surgical procedure, the patient’s teeth were extracted in an atraumatic manner that preserved the facial plate adjacent to the socket sites (Figure 6). Then, the implant osteotomies were created following the manufacturer’s streamlined drilling protocol. Because there was adequate bone height and width throughout the ridges of the patient’s maxilla and mandible, osteotomies were created that would allow for 6 axially placed implants in each arch, maximizing the A-P spread. The osteotomies were situated in the key implant positions of the molar, canine, and incisor regions of each arch.

Figure 8. The deep, sharp threads of the implants helped achieve the desired positioning toward the palatal aspect of the maxillary arch.

Next, the Hahn Tapered Implants were placed, including diameters of 3.5 mm and 4.3 mm (Figure 7). The pronounced threads of the implants firmly engaged the palatal and lingual bone during placement (Figure 8). In addition to maintaining the proper distance from the facial aspect, this would help situate the screw access holes of the eventual restorations away from the buccal and labial surfaces of the prostheses. Because the Hahn Tapered Implant features a machined collar, designed to be friendly to the crestal bone as well as the soft tissue, the prosthetic platform was positioned subcrest­ally with confidence.

Inclusive Multi-Unit Abutments (Glidewell Direct) were then attached to the implants, creating a level prosthetic platform for the restoration, and the surgical flaps were sutured (Figure 9). The conical prosthetic connection of the implants provided a secure and stable interface with the multiunit abutments. Because one of the posterior mandibular implants was situated in the position of the left first molar, where the bone quality was not conducive to favorable initial stability, the site was sutured closed for the duration of healing. The other 5 Hahn Tapered Implants in the patient’s mandible established high primary stability, so a screw-down temporary denture was indicated nonetheless.

Immediate Loading of Temporary Dentures
Next, temporary cylinders were added to the upper and lower immediate dentures, which were fabricated in advance of surgery based on preliminary impressions (Figure 10). The dentures were then trimmed, polished, and backfilled with acrylic to establish a convex intaglio surface (Figure 11). The dentures were attached to the implants through the temporary cylinders and would serve as fixed temporaries that the patient could wear for the duration of healing (Figure 12). Note that the posterior regions of the lower denture were shortened to remove a molar bilaterally, which would help to limit forces in the area of tooth No. 19 during healing (Figure 13).

Figure 13. Postoperative radiograph illustrates the multiunit abutments connecting the implants to the temporary cylinders of the fixed provisional dentures, with the exception of the implant near the bony defect in the area of tooth No. 19.

Impressions, Jaw Relation Records, Wax Setups
The patient returned 4 months after surgery so that the integration of the implants and healing of the peri-implant soft tissue could be evaluated. The patient’s temporary dentures were unscrewed, revealing pink, healthy soft tissue (Figure 14). After verifying the stability of the implants, open-tray transfer copings were attached so preliminary impressions could be taken (Figure 15). From the open-tray impressions, the laboratory fabricated master casts, which were first used to produce wax rims. At the next appointment, the wax rims were attached to the multiunit abutments then shaped and contoured. The jaw relationship was recorded utilizing standard denture technique (Figure 16). Next, wax setups were provided by the laboratory and tried in to evaluate the teeth positioning, centric relationship, incisal edges, bite, function, and aesthetics (Figure 17).

The dental laboratory team provided custom final impression trays and implant verification jigs for each arch. The individual sections of the verification jigs were connected to the implants and luted together (Figure 18). Then, the custom trays were filled with a vinyl polysiloxane (VPS) impression material (Capture VPS [Glidewell Laboratories]) and seated in the patient’s mouth. Because the individual sections of the implant verification jig were splinted together, the relative positions of the implants remained fixed as the impression material set (Figure 19). This method leads to a more accurate final impression, which is crucial in achieving a passive fit for the final prosthesis.¹⁰

Figure 16. Conventional wax rims were used to record the vertical dimension and jaw relationship and returned to the lab so wax setups could be created.

Figure 18. The verification jig, which captures the precise positions of the implants in the final impression, was attached to the multiunit abutments.

Provisional PMMA Implant Prosthesis
New master casts were poured from the final impressions. These were scanned along with the wax setups so that dental CAD software could be utilized to design the full-arch restorations. The prostheses were digitally designed to replicate the form, contours, and teeth positioning of the final-approved wax setups (Figure 20). CAM software readied the prosthetic designs for fabrication, and BioTemps provisional implant prostheses (Glidewell Laboratories) were milled from blocks of polymethyl methacrylate (PMMA) (Figure 21). The provisional implant prosthesis affords the doctor and patient an opportunity to make certain that they are satisfied with the proposed restoration before the final prosthesis is milled from BruxZir Solid Zirconia (Glidewell Laboratories).

At the next appointment, the PMMA provisionals were attached to the multiunit abutments. The fit, function, bite, and aesthetics of the restorative designs were verified (Figure 22). The implant in the area of tooth No. 19 was uncovered, and an additional screw access hole and temporary cylinder were incorporated into the mandibular provisional, resulting in a molar-to-molar A-P spread. Radiography was used to confirm complete seating of the provisional implant prostheses (Figure 23).

Figure 21. Provisional full-arch implant prostheses were digitally milled from PMMA, an aesthetic and versatile material that can accommodate chairside adjustments.	Figure 22. The PMMA provisionals were tried in, and only minor occlusal modifications were needed. The patient would wear the provisionals for an interim to confirm the function and aesthetics of the restorative designs.

Figure 23. Radiograph illustrates complete seating of the provisional implant prostheses and crestal bone preservation 4 months after surgery. Note the temporary cylinder in the area of tooth No. 19, which was added to the prosthesis and later incorporated by the lab into the definitive design.

The patient wore the provisional restorations for a trial period in order to “test-drive” the prostheses during day-to-day function. After 4 weeks, final approval was obtained from the patient, who was now extremely confident in the restorative designs. The provisionals were returned to the lab team so that the CAD design could be updated for the fabrication of the final prostheses. The patient wore her original converted dentures while the monolithic zirconia implant restorations were produced, fulfilling her desire to have fixed appliances of some kind in place throughout treatment.

Delivery of the Implant Prostheses
The final-approved PMMA provisionals were scanned (Figure 24) to essentially serve as blueprints for the monolithic zirconia full-arch implant restorations. Each final prosthesis was milled from a block of BruxZir Solid Zirconia, stain and glaze were applied, and titanium connections were bonded into the screw access holes (Figure 25).

Figure 25. The final full-arch implant prostheses were milled from BruxZir Solid Zirconia (Glidewell Laboratories).	Figure 26. The BruxZir Full-Arch Implant Prostheses (Glidewell Laboratories) were delivered and, as a result of the verification steps built into the restorative protocol, met the exact functional and aesthetic expectations of the patient.

Figure 27. Final radiograph exhibits full-mouth monolithic zirconia reconstruction and fully integrated Hahn Tapered Implants that were essential in establishing the positioning and primary stability needed to execute the treatment plan.

The BruxZir Full-Arch Implant Prostheses were delivered at the final appointment with ease (Figure 26). Because they were exact copies of the test-driven prostheses, the full-arch implant restorations fit perfectly, establishing the form, function, and bite the patient had grown accustomed to during the trial period. Final radiography illustrated complete integration of the Hahn Tapered Implants, establishing a stable foundation for the monolithic zirconia restorations (Figure 27). In combination, the well-integrated implants and durable prostheses provided the patient with a superb long-term prognosis.

CLOSING COMMENTS
Tapered implants with pronounced thread designs can be placed with a great deal of flexibility and can predictably provide the initial stability needed to provide the patient with fixed temporary prostheses on the day of surgery. Once the implants have integrated, monolithic zirconia restorations can be delivered following a protocol, and at a cost similar to those of the acrylic hybrid denture. The result is a fixed prosthesis that affords the same prosthetic stability as its acrylic predecessor, with the added benefit of better long-term durability.

References

1. Larsson C, Vult Von Steyern P. Implant-supported full-arch zirconia-based mandibular fixed dental prostheses. Eight-year results from a clinical pilot study. Acta Odontol Scand. 2013;71:1118-1122.
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Dr. Patel is a graduate of the University of North Carolina at Chapel Hill School of Dentistry and the Medical College of Georgia/American Academy Implant Dentistry MaxiCourse. He is co-founder of the American Academy of Small Diameter Implants and a clinical instructor at the Reconstructive Dentistry Institute. He has placed more than 2,500 small-diameter implants and has worked as a lecturer and clinical consultant on mini implants for various companies. He belongs to numerous dental organizations, including the ADA, North Carolina Dental Society, and American Academy of Cosmetic Dentistry. He can be reached via email at pareshpateldds2@gmail.com or via the website dentalminiimplant.com.

Disclosure: Dr. Patel reports no disclosures.