A Novel, Minimally Invasive Approach to Replacing Missing Teeth

Advances in zirconia and bonding protocols have greatly changed the nature and application of RBFPDs since Rochette first described them decades ago.^6-11 This is particularly true in the area of esthetics, where the most common complication of significance, aside from debonding, has been discoloration of the teeth due to the presence of metal wings.^3,12 In the past few decades, considerable improvement has been made in the long-term prognosis for all complications with RBFPDs.¹³ However, this success is dependent on preparation design and case selection. This includes mechanical retention in the preparation as well as a single-retainer design. When the recommended preparation is followed in an ideal case, the literature reports success rates of 95% or higher.¹⁴ When considering all cases, a 2013 meta-analysis found success rates as low as 72.5% for all-ceramic restorations at 3 years.¹⁵

Contraindications for RBFPDs include limited interocclusal space, parafunctional habits, and short clinical crowns. Additionally, esthetic contraindications include highly translucent teeth and interdental spaces that may allow the wings to be visible. There are times, however, when a patient may fall into one or more of these categories that offer a decreased prognosis, yet is not a candidate for dental implants. The purpose of this clinical report is to describe a novel way to decrease the likelihood of debonding and esthetic complications of an RBFPD in less-than-ideal cases through the use of a unilateral approach with a "c-wing" design.

Case 1

A 25-year-old female patient presented to a private practice with congenitally missing maxillary lateral incisors and mandibular central incisors (Figure 1). She had not previously sought tooth replacement, because she had only been aware of fixed replacement options that involved surgery or significant tooth preparation. Although physically healthy, the patient was adamant about avoiding surgery, removable prosthetics, and what she considered to be invasive treatment options. She had extensive interdental edentulous spacing on both arches, as well as limited bonding surface on the lingual of the maxillary canines, making her a less-than-ideal candidate for RBFPDs.

Because the edentulous spaces were disproportionately large and the adjacent teeth were relatively small, redistribution of the space with restorative material was proposed. To confirm this plan, a digital smile design and subsequent trial smile were completed. An intraoral scan was taken, and the smile design was performed using a tabletop design module (3Shape Dental System, 3Shape) (Figure 2). The design was then milled in PMMA, allowing the trial smile to fit over the patient's unprepared teeth (Figure 3). This let the patient see an attainable result with minimal preparation while enabling the clinician and laboratory technician to realistically evaluate the space distribution.

Once it was determined that restorative space was adequate and the proposed tooth proportions were acceptable, the design was finalized. In order to meet the design parameters requested by the dentist, the laboratory used the onlay module in the software to allow the needed flexibility for the margins. In addition to the standard single lingual retainer wing, a buccal extension was added to the zirconia frameworks (Figure 4), in a "c-wing" design. The frameworks were milled in zirconia (IPS e.max ZirCAD, Ivoclar Vivadent), sintered, and returned to the dentist, who tried them in to confirm fit and retention (Figure 5). Following the approved smile design, individual lithium disilicate veneers (IPS e.max^® CAD, Ivoclar Vivadent) were then pressed to fit retainer teeth, adjacent teeth, and prepared pontics on the framework (Figure 6). The laboratory fabricated the veneers by attaching the frames to the model with a small amount of sticky wax to allow for stability during the waxup of the prescribed restorations. The veneers were hand waxed and then pressed to allow for the maximum amount of strength. They were fit and finished with appropriate stones and diamond burs. The veneers were left in a monolithic state to maintain the full strength of the material. A stain and glaze system (IPS Ivocolor, Ivoclar Vivadent) was utilized to obtain the desired esthetics. The final veneers were etched for 20 seconds with hydrofluoric acid and rinsed with water.

After try-in and patient approval, the zirconia frameworks (IPS e.max^® ZirCAD, Ivoclar Vivadent) were treated and bonded per manufacturer's instructions and excess cement was cleaned. The veneers were then treated and bonded with a dual-cure resin cement (Variolink^® Esthetic, Ivoclar Vivadent) per manufacturer's instructions (Figure 7). At follow-up, the occlusion was verified, and the patient's home care was evaluated.

At 24 months post-delivery, the patient was experiencing no discomfort and the prostheses were stable. She has remained very satisfied with the outcome.

Case 2

A 77-year-old female patient presented to a private practice as an emergency with her endodontically treated lower right central incisor fractured at the gumline (Figure 8). Her primary objective was to find a long-term fixed prosthetic solution. She was at a relatively high risk for implant failure, as her medical history included a diagnosis of osteopenia and bisphosphonate use. Considering that and her narrow ridge width, a treatment option that minimized surgery was chosen.

After discussing treatment options, root submergence and a minimal-preparation cantilever FPD was chosen as the most conservative approach. The surgical treatment included reducing the fractured tooth structure to a slightly subcrestal position using a coarse diamond bur (Figure 9) and subsequently obtaining primary soft-tissue closure.

Upon review of the risks, benefits, and costs of treatment, the patient was concerned about the possibility of debonding, because she often traveled for several months at a time. There were also esthetic concerns with the proposed retainer tooth (lower left central incisor) and apprehension regarding the adequacy of restorative space. Additionally, the proposed retainer tooth was short with mottled enamel, making long-term bonding unpredictable. As such, facial coverage of the retainer tooth with restorative material was proposed to improve esthetics and decrease the potential for debonding. To confirm clearance and esthetics, and following the same protocol as the previous case, a digital smile design and subsequent trial smile were completed. After clinician and patient approval, the design was finalized.

The esthetic goal for this case was not as extensive as in the previous case; therefore, a design modification was proposed. The entire case was designed in monolithic zirconia (IPS e.max ZirCAD, Ivoclar Vivadent), and the buccal wing was extended to function as a veneer (Figure 10 and Figure 11). This satisfied the patient's esthetic concerns while improving retention. The laboratory designed the framework using the onlay feature in the software to allow for flexibility of the margin definition. The final restoration was textured with diamond burs running at low speeds, and a stain and glaze system (IPS Ivocolor, Ivoclar Vivadent) was utilized. The intaglio surface was cleaned using glass beads and subsequent steam cleaning.

After try-in and patient approval, the zirconia restorations were treated with silane and bonded with a dual-cure resin cement (Panavia^™ V5, Kuraray America Inc.) per manufacturer's instructions, and excess cement was cleaned (Figure 12). At 12 months post-delivery, the prosthesis and tissue levels were stable. New impressions for a retainer were made and the composite resin restoration on the adjacent tooth was refinished (Figure 13). Through 2.5 years, clinical evaluation revealed no functional or biologic complications.

Discussion

Although the designs in both cases deviated from the standard, the RBFPDs were fabricated with a single retainer. This was in accordance with literature that indicates a more favorable prognosis for a single-retainer (cantilevered) RBFPD versus a fixed-fixed two-wing RBFPD.^2,7,8,16-19 Zirconia was chosen as the framework material due to its esthetic properties, as well as its physical properties. Literature indicates that it may be a suitable alternative to metal for RBFPDs^2,20,21 and that a relatively predictable bond can be attained.^7,10,11

This design was proposed as a modification of the Encore^® bridge (trademark of Jeneric/Pentron). The Encore bridge is an RBFPD with a resin framework that achieves additional mechanical retention and improved esthetics with porcelain veneers on the pontic and adjacent teeth, because there is a dual path of insertion between the framework and the overlaying veneers.²²

A recent report by Mourshed et al reviewed the literature regarding anterior cantilever RBFPDs and suggested that cantilever RBFPDs had better survival than two-retainer RBFPDs.²³ Their report agrees with a 2016 systematic review and meta-analysis study by Wei et al.²⁴ The use of unilateral RBFPDs shows promising results and high survival rates. The use of zirconia ceramic and phosphate monomers containing resin cements was also suggested, as this helped to minimize the debonding rate and/or fracture of anterior all-ceramic cantilever RBFPDs.

The primary advantage of the treatments presented in this report is that while neither patient was an ideal candidate for a conventional RBFPD, an alternative minimally invasive option was completed. The veneers and RBFPD both required minimal preparation, and the patients' goals were achieved. These goals included short treatment time, minimal preparation, and no administration of local anesthesia. In addition, the "c-wing" design and the material choices should address the two most common complications with RBFPDs: debonding and discoloration. In a unilateral approach, the pontic will move with the abutment tooth during function, which can prevent shear and torque forces that result when splinting to the adjacent teeth bilaterally.

There are a few notable disadvantages of this technique. The long-term success depends primarily on the bond of zirconia to enamel, which, albeit promising, still requires further evidence. Additionally, execution of this prosthesis demands excellent communication between the clinician and laboratory technician. Fortunately, this is now easier to accomplish with CAD/CAM technology, which was utilized in both of these cases. Lastly, the "c-wing" design is an untested RBFPD design, with short follow-ups to this point. Additional cases, larger in vitro studies, and a longer follow-up are indicated to consider this a proof-of-concept rather than a clinical report.

Summary

Two clinical reports were presented to describe restorative solutions for patients who desire minimal interventions with short treatment times. To manage edentulous spaces, minimal-preparation RBFPDs and veneers were designed and fabricated using CAD/CAM technology for improved communication and predictability. A novel "c-wing" design was utilized to improve mechanical retention and esthetic results for patients who are not ideal candidates for RBFPDs but still desire a nonsurgical approach.

About the Authors

Wendy Auclair Clark, DDS, MS
Assistant Professor,
Department of Prosthodontics,
University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina

Alexandra B. Yarborough, DDS
Assistant Professor,
Department of Prosthodontics,
University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina

David A. Garber, DMD
Private Practice,
Atlanta, Georgia

Craig J. Holland
Owner,
3 Point Dental Studio,
Fremont, Ohio