Adhesive Placement of a Zirconia FPD to Replace a Maxillary Central Incisor

The advancement of adhesive techniques and materials has created new restorative possibilities. Current options for replacement are the conventional three-unit fixed partial denture (FPD), a single-tooth implant, or a resin-bonded FPD. The three-unit FPD can be a traditional ceramometal or an all-ceramic using a high-strength core material, such as lithium disilicate or zirconia. The use of traditional FPDs has a high success rate but requires a substantial amount of tooth reduction to achieve the best esthetics. Long-term preservation of the pulp is always uncertain. Light transmission may also be reduced despite the use of a porcelain butt joint. All-ceramic FPDs require a substantial amount of tooth structure removal. If the adjacent abutments have no restorations, an implant or resin-bonded FPD may be a better alternative.

In cases in which space is limited or hard and soft tissues are inadequate, implant placement is not always feasible. Additional surgical procedures may be required to improve biomechanical and esthetic needs. However, the patient may not want the additional time and expense for these surgical procedures. Another viable alternative is the resin-bonded FPD metal, ceramic, or fiber-reinforced composite for caries-free FPDs, designed as single abutments.¹ It has been demonstrated that FPDs created as single-retainer restorations have better survival rates than traditional two-retainer resin-bonded FPDs. The single-retainer resin-bonded FPD may allow less tensile stress at the connector and adhesive interface.^2-4 Recent advances in adhesive technology and ceramic materials allow all-ceramic, single-retainer resin-bonded FPDs to be used in addition to metal retainers. Ceramics possess desirable characteristics: chemical stability, biocompatibility, high compressive strength, and a coefficient of thermal expansion similar to tooth structure.⁵ Clinicians can choose from a wide range of ceramic products, and zirconia has been used in dentistry for a broad array of indications.

Zirconia has favorable esthetic, mechanical, and biomechanical properties. Densely sintered zirconia possesses the highest flexural strength of approximately 1,000 MPa. Zirconia can be used for conventional FPDs or resin-bonded FPDs due to its superior mechanical properties and high fracture toughness. Hydrofluoric acid is used as a predictable basis for roughening the surface of feldspathic ceramic for bonding composite resin. High alumina content or zirconia content cannot be treated in this manner because they do not contain the silicon dioxide (silica) phase.⁶ Air abrasion with silica-containing particles (silica coating), followed by silanation, has been used on zirconia ceramics with various results. Silica coating has produced durable resin bonding for some zirconia ceramics.⁷ Several bonding agents have previously been investigated, and only those containing an organophosphate ester monomer have been proven to be effective. According to recent studies, the combination of airborne-particle abrasion and a 10-methacryloyloxydecyl dihydrogen phosphate (MDP) monomer is the recommended method for bonding resin composites to zirconia.⁸

A 29-year-old woman was seen at the Advanced Program in Aesthetic Dentistry at New York University College of Dentistry wanting to replace her missing left central incisor and improve the color of the right central incisor (Figure 1 and Figure 2). A comprehensive examination was conducted, including a full-mouth radiographic series, caries detection, periodontal probing, and temporomandibular examination. Her medical history was noncontributory, and she had no contraindication to dental treatment. Intraoral and extraoral photographs were taken in order to aid in the esthetic evaluation. Study models were obtained with Reprosil^® (DENTSPLY International, www.denstply.com). A bite registration was taken in centric occlusion. Facial analysis revealed the interpupillary line was parallel to the occlusal plane, the midline relationship (central incisor to philtrum) was symmetric, and upper and lower lips were full and prominent. At rest, the patient displayed 4 mm to 5 mm of the maxillary central incisors and the mandibular incisors were not visible. She had a normal nasolabial angle of 90° and the Ricketts’ E plane of 4 mm. She had a high smile line, an incisal edge to lower lip parallel but not touching, exposure of 12 teeth when smiling, a straight midline, and an increased bilateral negative space (Figure 3).

The option of a porcelain veneer on the upper right central incisor and implant and a crown to replace the upper left central incisor was presented first. Other alternatives presented were a traditional three-unit FPD or a resin-based FPD. An intraoral mock-up using a putty stent obtained from the wax-up and Luxatemp^® (DMG America, www.dmg-dental.com) was used to demonstrate to the patient the potential esthetic benefits of the recommended treatment. The patient elected to have the tooth replaced with an all-ceramic FPD combining full coverage on the maxillary right central incisor (to mask the darkness) and a bonded wing on the left lateral incisor (to conserve the tooth structure). The shade of the unprepared tooth was noted (Figure 4).

In the following visit, the sites were anesthetized with 2% lidocaine with 1:100 000 and the maxillary right central incisor was prepared for full coverage. The left lateral incisor was prepared to provide 0. 5-mm clearance with a groove at the position of the connector to allow adequate bulk in this area. A full-mouth impression was obtained with Impregum™ Penta™ Soft polyether impression material (3M ESPE, www.3mespe.com) and Blu-Bite (Henry Schein, www.henryschein.com) bite registration was used to record the relationship between the maxilla and mandible in centric occlusion. The shade of the prepared central incisor was obtained (Figure 5). The laboratory-fabricated die models (Figure 6) were scanned, and the zirconia framework was designed and milled (Figure 7, Figure 8, Figure 9 and Figure 10) to support the overlying porcelain (Figure 11 and Figure 12). The dimensions of the connectors are critical to the restoration’s longevity (Figure 13). Feldspathic porcelain compatible with the framework was placed (Figure 14).

In the following visit, the restoration was tried in and the margins were verified. The FPD was then bonded with Panavia™ F2.0 (Kuraray America, www.kuraraydental.com). The provisional bridge was removed and the adherent surfaces were cleaned and decontaminated with K-Etchant Gel (Kuraray) to prepare for cementation. Clearfil^® Ceramic primer (Kuraray) was placed on the intaglio surface of the FPD. ED Primer A and B was mixed one drop each in a well and applied to the abutment teeth. Both enamel and dentin were coated using a disposable brush tip, and left in place for 60 seconds. The primer was then dried with a gentle air flow. Pooling has to be avoided as this would prevent seating of the restoration. Panavia F paste was then mixed and applied to the restoration. The restoration was placed in position, and excess cement was removed from the margins. (When the adhesive cement contacts the ED Primer, the polymerization of the adhesive cement accelerates.) The margins were light-cured for 20 seconds in several areas, and Oxyguard II (Kuraray) was placed with a disposable tip to ensure curing. After 3 minutes, Oxyguard II was removed with a cotton roll and water spray. Scalers were used to remove any remaining excess cement. Occlusion was confirmed in centric, protrusive, and lateral excursions. The patient was pleased with the esthetic result (Figure 15, Figure 16 and Figure 17).

A missing central incisor is usually replaced by a conventional three-unit FPD, simple implant, resin-based FPD, or removable partial denture.⁹ The selection of the best option requires the consideration of both function and esthetics. The goals should include the longevity of the restoration, minimal invasion, and cost effectiveness. These considerations may be modified by interdental spacing, ridge contour, orientation of the roots or crowns of adjacent teeth, anterior guidance, potential shade-matching problems, parafunctional habits, and the patient’s esthetic concerns.

The most conservative technique of restoring the missing central incisor for this patient would have been implant placement with bleaching and a porcelain veneer to mask the darker right central incisor. However, the patient refused implant placement, which would have required ridge augmentation on the labial to achieve a contour similar to the adjacent central incisor. This would have been beneficial for any FPD replacement. However, the patient did not want surgery and opted for tooth replacement as expeditiously as possible. The laboratory technician thought masking the darkness of tooth No. 8 with only a porcelain veneer would be difficult and a ceramic restoration with an opaque core would provide better results. When it was decided to use full coverage with a high-strength ceramic (zirconia) on the maxillary right central incisor with the left central incisor as a pontic, the decision had to be made whether to use full coverage on the maxillary left lateral incisor, cantilever the upper left central incisor off the upper right central incisor, or place a bonded wing on the upper left lateral incisor.

One of the main causes of failure of ceramic resin-based FPDs is where the pontic connects to the abutment. This failure may be due to the differential movement of the abutment teeth during protrusive and lateral excursions with the teeth in contact. The strength of this connector depends on its dimensions and also the physical properties of the framework. The recommendations for connector dimensions for yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) FPDs vary from 2 mm to 4 mm in occlusal-gingival height and 2 mm to 4 mm in buccolingual width. To reduce the fracture probability when designing all-ceramic FPDs, the shape of the connector is an important factor that also has to be considered. The radius of curvature at the gingival embrasure has a major role in the load-bearing capacity. Y-TZP FPDs with small gingival embrasure radii are subjected to high stress concentrations in the connector area during loading, compared with FPDs with large embrasure radii.¹⁰ Another potential problem is the debonding of the wing to either the luting cement or the luting cement to tooth structure. Also, the veneering porcelain to the zirconium framework may chip.^11,12

Several options for high-strength core materials can be used in the anterior region. The core material can be either glass-infiltrated alumina ceramic (Vita In-Ceram^® Alumina, Vident, www.vident.com), pure alumina ceramic (Procera^®, Nobel Biocare, www.nobelbiocare.com), lithium disilicate (IPS e.max^® Press, Ivoclar Vivadent, www.ivoclarvicadent.com), or Y-TZP (Lava™, 3M ESPE). The bond strength of luting cements to various ceramic surfaces is critical for resin-based FPDs to function. Various techniques have been introduced to provide a mechanical attachment between resin and ceramic.¹³ Hydrofluoric acid and subsequent placement of a silane coupling agent will increase the bonding strength to feldspathic ceramic. However, this technique is not applicable to the high-alumina or zirconia ceramic materials because these do not contain silicon dioxide.

For most restorations, high-strength ceramics do not require adhesive bonding to tooth structure and conventional cementation can be used.¹⁴ Adhesion can reduce microleakage, increase retention, and increase fracture and fatigue resistance.¹⁵ Usually, silanes are used to coat the inorganic filler particles to allow the adhesive bonding of resin luting materials to porcelain. A silane coupling agent has two types of reactivity in the same molecule. A silane coupling agent will act as a link between an inorganic substrate and an organic material to bond or couple the two dissimilar materials. The metal hydroxyl groups on the surface of minerals are usually hydrophobic and incompatible with organic polymers. Silanes are added to treat the surface of the mineral to make the mineral more compatible with the polymer and allow the particle to be dispersed in the polymer (eg, composites). High-strength ceramics (alumina and zirconia) are more chemically stable than silica glasses and ceramics that contain silica. This stability and glass-free aspect does not allow the zirconia to be treated with hydrofluoric acid etching and silanization to create a stable bond.¹⁶

The increased use of zirconia has resulted in greater interest in adhesion between the zirconia restoration and tooth structure. Some clinicians have suggested the use of surface abrasion with alumina and the subsequent placement of a tribochemical silica coating (Rocatec™, 3M ESPE) to allow the creation of chemical bonds between silane and a resin cement.¹³ The bonded silica particle reacts with the organosilane monomer; however, the bond is still weaker than the bond to conventional porcelain.¹⁷ It is thought that air-particle abrasion may cause microfractures that would ultimately result in failure of the restoration.¹⁸

Another technique is the application of molecular vapor deposition of gas-phase chlorosilane (SiCl4) pretreatment to place an ultra-thin silica seed layer. This is achieved by combining chlorosilane with water vapor to form a more reactive surface. This 2-nm to 3-nm layer allows the creation of bond strength statistically equivalent to traditional bonded porcelain materials.¹⁹

A modified rough surface zirconia, tentatively named the Maryland Surface (Nobel Biocare), has become available. It is completely different from the company’s ZiUnite™ surface. "The modified surface is produced by coating a presintered or a fully sintered and milled zirconia framework with slurry containing zirconia ceramic powder and a pore former. Then, the slurry-coated ceramic is sintered while the pore former burns off, leaving a porous surface. The porosities of the surface can be modified by using different sizes of pore formers or repeating the coating process."20 The coating is about 20 µm to 40 µm. However, its effect on strength remains unknown.^21,22

Aboushelib et al22 found that structural changes can occur on the grain level to Y-TZP. Grain growth and cubic grain formation can occur when zirconia is heated to 1,450°C for 2 hours. Heating at lower temperatures (700°C to 900°C) causes thermal aging of the surface of the zirconia. The thermal aging Results in the formation of surface elevations, grain pullout, and detachment with increased grain boundary thickness. Thermal etching of zirconia at 1,350°C for 12 minutes resulted in surface elevations, rippled grain surfaces, and vertical grooves at grain margins. These changes correspond to the tetragonal monoclinic transformation of the zirconia crystals. Heat-induced maturation creates stresses in the grain boundary region using two short thermal cycles that do not create enough energy for grain growth or cubic grain formation. Initially, the zirconia is heated to 750°C for 2 minutes, cooled to 650°C for 1 minute, and reheated to 750°C for 1 minute, causing the grain boundaries to become pre-stressed. This allows other materials to infiltrate these areas.

Developed by Aboushelib et al16 and others, a surface treatment called selective infiltration etching makes use of the heat-induced maturation concept and grain boundary diffusion to form a highly retentive surface on Y-TZP. Initially, heat-induced maturation creates pre-stresses in the boundary region, and then these areas are widened by the application of a thin layer of an infiltration glass on a treated-zirconia surface. This Results in a 3-dimensional network of intergrain porosity only at the site where the surface grains contact the infiltration glass. The infiltration agent has a thermal expansion coefficient that is similar to that of zirconia. This is important as no detrimental pre-stressors occur during room temperature cooling. This infiltration glass can then be dissolved with hydrofluoric acid to create areas of nano-mechanical retention to a luting resin. This would be preferred to creating chemical bonds (MDP monomer) or creating surface roughness with airborne-particle abrasives because this was the only way that bond strength values were maintained when stored in water.²³ Other zirconia materials with different stabilizing agents may need different heat-induced maturation protocols and infiltration agents. In addition, because selective infiltration etching is only a surface treatment, the mechanical properties of Y-TZP are not detrimentally affected. This is because on a nano-scale, zirconia crystals can transform from tetragonal to monoclinic phases when suitably stimulated. However, on a microlevel, the grains of zirconia can rearrange by moving, splitting, and sliding in the presence of the dopant phases and the appropriate amount of temperature increase.²⁴ Laser treatment has also been shown to create to create acid-etch type effects.

Several phosphate/phosphonate monomer-based primers can also be used with zirconia that have not been treated or sandblasted. These include Monobond Plus (Ivoclar Vivadent), Clearfil^® Ceramic primer (Kuraray), AZ Primer (Shofu Dental, www.shofu.com), and Z-Prime™ Plus (Bisco, www.bisco.com). Most of these materials have a phosphate monomer with solvents only (eg, ethanol, acetone). The new Bisco product contains phosphoric, carboxylic acid, and other monomers that can be used with different surface treatments (ie, sandblasted or polished) and is compatible with self-cured or light-cured cements.

The use of zirconia for FPDs has increased the potential for providing long-lasting esthetic restorations. Sailer et al concluded that zirconia will provide sufficient stability as a framework for three- and four-unit posterior FPDs but the veneering ceramic needs improvement. This study was for only 5 years and ideally durability compared with porcelain-fused-to-metal should, in the authors’ opinion, be the standard.⁸ The use of ceramic resin-based FPDs has resulted in restorations that provide esthetics and conservation of tooth structure. Replacing a single missing tooth with an implant is usually one of the best treatment options. Sometimes an implant placement is not feasible because of medical reasons, habits, inadequate bone not amenable to grafting, and patient desires. The use of adhesively placed ceramic restorations provides a viable alternative that needs to be considered in the restorative practice. The adhesion to zirconia needs further research and clinical studies to ascertain long-term benefits of the various techniques currently promoted.

The laboratory photographs and laboratory work were provided by Adrian Jurim, MDT, Jurim Dental Studio, Great Neck, New York.

Marietta Kalogirou, DDS;
Resident, International Program in Advanced Esthetic Dentistry,
New York University College of Dentistry, New York, New York

Richard Trushkowsky, DDS
Clinical Associate Professor, New York University Division College of Dentistry Department: Cariology and Comprehensive Care; and Associate Director, International Program in Advanced Esthetic Dentistry
New York University College of Dentistry, New York, New York

Jorje Andrade, DDS
Instructor, General Dentistry and Management Sciences
New York University College of Dentistry, New York, New York

Steven David, DMD
Clinical Professor
New York University College of Dentistry, New York, New York; and Director, International Program in Advanced Esthetic Dentistry, New York University College of Dentistry, New York, New York