Conservative Esthetic Replacement of a Missing Anterior Tooth Using Monolithic Zirconia One-Wing Fixed Dental Prosthesis

The replacement of single missing anterior teeth presents the clinician with challenges. Several treatment options exist, each having specific advantages and limitations. Options include a single-tooth implant, fiber-reinforced composite bridge, metal-ceramic (fixed dental prosthesis or resin bonded) or ceramic (fixed dental prosthesis or resin bonded) restoration, or a removable partial denture. Alternatively, the space may be closed by orthodontics.^1,2 The cause of the missing or lost tooth may be either congenital, trauma, or decay. The placement of implants in a growing child may be contraindicated, because the implant acts like it is ankylosed and does not follow the continuous eruption of natural teeth. Implants may even disturb the normal development of the jaws.^3,4 The use of a resin-bonded fixed prosthesis in the anterior region created by electrolytic etching of the metal wing may be problematic, as the teeth often appear grayish from the metal on the lingual aspect.⁵

Conventional crown preparation on a young patient is not recommended due to the size of the pulp chamber and the amount of tooth structure requiring removal to achieve an esthetic result.⁶ Orthodontic treatment to close spaces may involve complicated multidisciplinary treatment and may be limited to specific clinical circumstances. Proper tooth size and proportion may be difficult to achieve. Autotransplantation is an option, as is a removable prosthesis. However, recent improvements and innovations in adhesive techniques and ceramic materials have made the choice of an all-ceramic bonded dental prosthesis a viable and desirable option.^7,8

Zirconia has taken on an increasingly valuable role in restorative dentistry due to its excellent mechanical^9-11 and biocompatible characteristics.¹² Traditionally, two wings were used for resin-bonded anterior bridges, but it was found that the facial-lingual movement of teeth created a shearing action and stress in the adhesive layer, which eventually may lead to one of the wings debonding^13-17

This article presents two case reports that demonstrate innovative, minimally invasive treatment for replacement of a missing maxillary anterior tooth.

A 17-year-old female patient presented to the Advanced Program for International Dentists in Esthetic Dentistry at New Yok University with the desire to improve her smile by replacing the missing maxillary right central incisor. Her medical history was noncontributory. Her dental history included previous endodontic treatment on tooth No. 8, orthognathic surgery, orthodontic treatment, and extraction of tooth No. 8, the coronal portion of which was attached by a bracket and orthodontic wire to maintain proper spacing and for esthetic purposes (Figure 1 and Figure 2).

The treatment objective was to provide an esthetic replacement for the missing tooth No. 8 that was similar in size, shape, and color to the contralateral tooth.

An esthetic evaluation of the patient was performed, and diagnostic data was collected. This included a facebow record, study models, and the completion of an esthetic analysis. After assessing the patient's needs, a treatment plan was formulated and presented to the patient and her mother. They elected to proceed with the resin-bonded zirconia cantilevered bridge. The evaluation and indication for use of a resin-bonded fixed dental prosthesis (RBFDP) requires that there be adequate bonding surface of approximately 30 mm², in enamel, and the tooth should not present with periodontal mobility.^18,19 To assess the area, a piece of tin foil may be adapted to the lingual surface of the abutment intraorally or on the model. The piece of foil can then be measured to ensure adequate bondable area. Sufficient clearance to create a retainer wing 0.5 mm to 0.7 mm thick is also needed. The degree of horizontal and vertical overlap will determine if any reduction of the lingual surface is required. Minor orthodontic movement may also be an option by itself or in conjunction with the occlusal adjustment. Excessive reduction must be avoided, as it will weaken the tooth and possibly expose dentin. Adequate space for a proximal connector at least 3 mm high and 2 mm thick is also necessary. Occasionally, a tooth in the opposing arch may require reduction to obtain enough space if it is not detrimental to occlusal function and esthetics.^20,21

The lingual aspect of tooth No. 9 was reduced slightly and a lingual dimple was formed. An impression was taken with polyvinyl siloxane (Reprosil^®, Dentsply Sirona, dentsplysirona.com), and the laboratory fabricated a stone model from the impression. The preparations on the model and wax-up were scanned to create an STL file for milling (Figure 3 through Figure 5). The laboratory milled the bridge from a commercial zirconia puck (DLMS-Crystal^® Zirconia, Digital Dental, digitaldental.com). Adequate cement space was created.

The bridge was presintered at 1350°C (ie, 100°C below the zirconia sintering temperature) for 1 hour in air (Figure 6), and then the intaglio surface of the bridge was coated with an in-house aqueous-based slurry of powdered glass composition using a conventional enameling method. The glass composition was similar to conventional dental feldspathic ceramics, but its coefficient of thermal expansion was adjusted substantially so that it was similar to that of zirconia, and its melting temperature was significantly higher than that of feldspathic ceramics, while it exhibited an excellent resistance to crystallization during cooling from the sintering temperature.^22,23 The powdered glass slurry was allowed to dry, and the glass-coated zirconia was subjected to infiltration and sintering at 1450°C for 2 hours in air (Figure 7).²⁴

The bridge was sent back to the laboratory for porcelain and staining to match the adjacent central incisor (Figure 8). It was tried in on the model (Figure 9) and then intraorally. The fit and occlusion of the bridge were verified, and the patient and parent gave their approval. The bridge was cleaned with a universal cleaning paste (Ivoclean, Ivoclar Vivadent, ivoclarvivadent.com) to remove possible salivary contamination and then re-etched (Figure 10). The abutment tooth was cleaned with chlorhexidine 2% with 70% ethanol (Consepsis^™ Scrub, Ultradent, ultradent.com), rinsed, and dried, and then 37% phosphoric acid was applied for 20 seconds (Figure 11) and then rinsed and dried. An adhesive primer (Clearfil Ceramic Primer Plus, Kuraray, kuraraydental.com) containing the original 10-MDP (10-methacryloyloxydecyl dihydrogen phosphate) monomer was applied to the intaglio of the wing (Figure 12), and then a tooth primer (Panavia^™ V5 Tooth Primer, Kuraray) was applied to the isolated tooth and left alone for 20 seconds and then air-dried for 20 seconds (Figure 13). The Panavia V5 paste was placed on the bridge wing, which was then placed in position and light-cured at the margins after initial self-cure. The insertion of the resin-bonded bridge with one wing and a pontic provided the patient with a conservative, esthetic result (Figure 14 through Figure 16).

The patient was a 41-year-old man who presented to the Advanced Program for International Dentists in Esthetic Dentistry at New Yok University wanting to improve his smile by replacing the missing left canine, tooth No. 11. His medical history was noncontributory. His dental history included extraction of tooth No. H, which had occurred a year prior to treatment and an implant and bone graft had failed, and he now wanted an alternative solution. The treatment objective was to provide an esthetic replacement for the missing tooth No. 11 that was similar in size, shape, and color to the contralateral tooth.

Using the same protocol as in Case 1, this case illustrates the replacement of missing tooth No. 11 with a cantilevered zirconia bridge (Figure 17 and Figure 18).

The replacement of a single missing tooth is an esthetic challenge, especially in young patients. Restorative options include a dental implant, fixed partial denture, removable partial denture, and RBFDP. The latter is a reliable and minimally invasive choice that is especially well-suited for a young patient. A RBFDP can be constructed either with a two-retainer configuration or as a single-retainer structure.²⁵ Clinical studies have demonstrated advanced reliability and fewer debonding problems with single-retainer RBFDPs compared with the two-retainer design in the anterior region.^26,27

RBFDPs are minimally invasive and less expensive than other options and require less time to fabricate. The two-retainer design that was used for decades presented a major drawback involving one wing debonding or partial debonding. The debonding has been described as the result of the differential mobility of the two abutment teeth during protrusion and movement from the retainer to the abutment teeth.²⁸Metal-ceramic resin-bonded bridges have been utilized for many years for the replacement of an anterior tooth. With children, a metal-ceramic resin-bonded bridge presents several disadvantages. The unworn incisal edges of anterior teeth in children often have increased translucency because of the lack of dentin in the region. The metal behind this area creates a grayish appearance.²⁹

Recent advances in adhesive dentistry in conjunction with new ceramic materials with improved mechanical features have led to the development of RBFDPs being fabricated with zirconia. The advantages of yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) as a dental ceramic have been recognized.³⁰ Kern and Sasse suggested the preparation of a palatal veneer, a cingulum dimple, and possibly a box on the proximal.³¹ Van Dalen et al reviewed the literature examining the debonding rates of RBFDPs with two-retainer designs and single-retainer designs,³² and their conclusions agreed with those of Kern and Sasse, which found that anterior zirconia ceramic cantilever RBFDPs provided excellent clinical longevity and that the reasons for missing incisors did not influence the longevity of the cantilever RBFDPs.³¹

From an esthetic standpoint, 3 mol% yttria-stabilized zirconia polycrystalline (3Y-TZP) is inadequate, but it is an excellent core material. Research has confirmed that it satisfies the demand as a core material for ceramic restorations, with a survival rate compared to that of metal-ceramic^33,34 The high flexural strength of 3Y-TZP allows it to be either cemented or bonded adhesively with a resin cement. A resin cementation is advisable when there is poor retention, a short preparation, or minimally invasive treatment such as a RBFDP, such as those used in the present cases.^35.36 Although composite cements are recommended for cementing zirconia restorations, zirconia's high crystallinity makes them micromechanically inert with respect to bonding. While these restorations may be chemically bonded, additional retention is advisable for a single-wing cantilever fixed dental prosthesis of glass-ceramics.^37-39Various mechanical and chemical surface treatments have been suggested to improve composite cement-zirconia bonding, such as tribochemical silica sandblasting with 30-μm or 110-μm silica-coated aluminum-oxide (Al2O3) particles. This treatment has demonstrated the ability to not only roughen but also chemically activate zirconia surfaces.^40,41 Surface pretreatments with airborne-particle abrasion using Al2O3 particles are required to achieve a durable bond to zirconia followed by the application of an agent containing special (phosphate) monomers such as 10-MDP.⁴²

Research has shown that 10-MDP promotes adhesion to zirconia. Tanis and Akcaboy described that the use of 10-MDP monomer, including in resin cement, increased the bond strength of sandblasted zirconia.⁴³ Byeon et al proved that the application of 10-MDP-based primers after Al2O3 sandblasting augmented the resin bond strength to Y-TZP.⁴⁴ Kern et al recommended that reducing the pressure during sandblasting can be valuable in reducing the negative surface effects caused by sandblasting.⁴⁵ In several studies on Y-TZP zirconia ceramic bonding, sandblasting with 50-μm Al2O3 at 0.25 MPa at 10-mm operation distance was used to condition the ceramic surface in order to increase the surface roughness and stimulate the surface to increase bond strength.^46,47 Although chemical bonding of zirconia to tooth structure has been successfully reported in numerous studies, the present authors aimed to improve on this technique through the use of a highly adherent glass-infiltrated surface on the intaglio. This surface is made of glass and is, thus, etchable, which affords the clinician the opportunity to take advantage of micromechanical bonding. This interface has produced overwhelmingly positive clinical success. A previous study showed that the long-term bond strength of glass-infiltrated zirconia was more than three times stronger than zirconia with a sandblasted surface and was similar to that of feldspathic ceramics. In that study, the bond strength was challenged with 50,000 thermal cycles between 55°C and 4°C water baths and an additional 2-month aging in a 37°C humidor.⁴⁸

In addition, glass infiltration can also enhance the strength of zirconia through a surface elastic gradient. Previous studies have shown the flexural fracture resistance of zirconia was increased by almost 30% following surface glass infiltration.^49-51 This is significant as the flexural resistance of a ceramic restoration is proportional to its thickness squared. For minimally invasive treatments, increasing the flexural resistance of zirconia allows thinner restorations to be used, preserving tooth structure. This also provides an effective method to enhance the fracture resistance of the connectors of multi-unit bridges.^52,53 Finally, by infiltrating the cameo surfaces of the monolithic zirconia restorations, the shades of the restoration can be tailored. If necessary, the infiltrated zirconia can be further stained to enhance its esthetics.^54-56 In vitro studies also have shown that the glass-infiltrated zirconia surface also exhibited better wear characteristic relative to glazed and even polished zirconia.^57,58

This work was supported by the US National Institutes of Health/National Institute of Dental and Craniofacial Research (grant numbers R01 DE026279, R01 DE026772, and R01 DE017925). The authors thank Phanita Rojanasakul, DDS, for the photographs of infiltration (Figures 6 and 7) and Larry Passaro, MDT, of New-Dent Dental Laboratory for the excellent laboratory work.

Richard D. Trushkowsky, DDS
Adjunct Clinical Professor, Department Cariology and Comprehensive Care, Associate Director, International Program in Advanced Esthetic Dentistry, New York University College of Dentistry (NYUCD), New York, New York

Dhanny R. Medianti, DDS
Fellow, NYUCD International Program in Advanced Esthetic Dentistry, New York, New York

Paul L. Giotopoulos, DDS
Adjunct Clinical Instructor, Department Cariology and Comprehensive Care, NYUCD, New York, New York

Siriwadee Prathompat, DDS
Resident, NYUCD International Program in Advanced Esthetic Dentistry, New York, New York

Yu Zhang, PhD
Professor, Department of Preventive and Restorative Sciences, Director for Restorative Research, University of Pennsylvania, School of Dental Medicine, Philadelphia, Pennsylvania