July 2012, Volume 8, Issue 7
Published by AEGIS Communications
Resin-Bonded Bridge Pontic Repair Using CAD/CAM
Patients are becoming more aware of the technology and are pleased with the results, especially the time saved.
Rochette first described the resin-bonded splint or fixed partial prosthesis technique in 1973.1 The prosthesis consisted of a perforated, anatomically designed metal framework bonded to periodontally compromised anterior teeth in order to splint them. Macro-retention was gained by mechanically locking resin material into the perforations. There was a high failure rate because of the weakening effect of the perforations in the metal wings.2 Howe and Denehy3 used the perforated metal framework design and modified the technique by adding a pontic to replace a missing anterior tooth. This technique provided a fixed partial denture (FPD) tooth replacement with minimal tooth preparation.
Over time, a number of problems surfaced that were corrected through alternations to the framework materials and bonding agents. Low-film-thickness composite cements were designed.4 A new configuration was introduced that eliminated the perforations and conditioning of the metal through electrolysis, providing micro-retention and a stronger framework.4,5 Different modifications to the abutment teeth evolved to create a distinct path of insertion and to maximize the surface area for bonding. Lateral resistance form was also created through the specific preparation of the interproximal enamel. Resistance to vertical displacement was provided through small, well-defined occlusal rest seats. A #5 or #6 round bur was used to create a countersink preparation into the enamel 1 mm deep and 1 mm to 1.5 mm wide in a buccal-lingual direction, and the same dimensions in a mesio-distal direction. The location could be anywhere on the marginal ridge provided it did not interfere with the opposing cusp in centric occlusion. Because of the strength provided by the metal, feather-edged margins could be used so that the framework design mimicked the amount of enamel that would be removed during the preparation.4
The resin-bonded—or "Maryland" bridge—has been used in the anterior and posterior regions for many years. Practitioners have used this bridge either as an interim prosthesis or an alternative to implant replacement therapy or conventional fixed prosthodontics with a success rate of 87.7%.6 While other studies7 have found the success rates to be as high as 93.8%, the most common failures have been debonding of the framework followed by ceramic veneer debonding. Esthetically, the metal lingual wings, in some cases, have been responsible for altering the value or causing a "graying" of the abutment teeth. To remedy this, all-ceramic zirconia frameworks have recently been introduced.8
Given the success rate and conservative approach of some resin-bonded bridges, when problems arise (excluding repetitive prosthetic debondings), treatment can be offered to remedy the problem rather than changing the entire treatment design. This case report focuses on an alternative method of correcting a failure of the veneering porcelain.
A 29-year-old man presented with a fractured ceramic facing in hand from a 3-unit, resin-bonded bridge that had replaced tooth No. 7, the right lateral incisor. He reported that this had happened several times over the last few years and a dentist would just "glue" it back on (Figure 1 and Figure 2). He also reported that the framework had never debonded since it was inserted 10 years prior. Although he was pleased with the stability and retention of the appliance, he was becoming concerned with the increased frequency of the ceramic facing debonding.
Upon examination, the margins of the framework proved to be intact. The patient was instructed to move his mandible in all excursive movements, which showed sufficient incisal clearance. The integrity of the bond between the tooth and the framework appeared to be acceptable. Several different treatment options were presented to the patient, which included repair or replacement with different scenarios. After lengthy discussions, the patient’s request was to, if at all possible, use the intact framework and create a new ceramic facing that could be cemented to the existing framework. If that was possible, the patient also asked that when making the new facing, could the dimensions be enlarged because the original facing was smaller than the contralateral tooth. He was concerned about the difference in the gingival height between the pontic and the left lateral incisor (Figure 3 and Figure 4).
An underlying factor to the patient’s request was that he was getting married in a week. With the understanding that the long-term retention of a new ceramic facing fixed to the existing framework may be guarded, the patient fully accepted the proposed treatment plan which was to fabricate a new ceramic pontic using computer-aided design/computer-aided manufacturing (CAD/CAM) technology and bond it to the existing framework. The procedure would be completed in one appointment.
The ceramic facing was temporarily placed back on the framework. Composite resin was added to the gingival margin to duplicate the contralateral tooth contour (Figure 5). Once the patient approved the new shape and size, then the composite resin was light-cured. The pontic and the adjacent teeth were powdered with titanium dioxide (Figure 6) and digitally captured using a CAD/CAM acquisition unit (CEREC®, Sirona Dental Systems, www.sirona.com). Images were taken from an incisal direction as well from the labial to capture the new gingival margin. The ceramic facing was then removed. Some alteration to the framework was necessary to provide sufficient space for the lingual aspect of the ceramic facing and to create a distinct path of insertion.
To prevent a "graying" effect of the metal framework bleeding through the new ceramic facing, the exposed metal had to be blocked out. The exposed metal in the area of the pontic was treated with the CoJet™ system (3M ESPE, www.3mespe.com), which applies a layer of silica to the surface (Figure 7). Embedding silica on the metal surface significantly improves the bond strength between metal and resin material.9 The technique involves the use of a small sandblaster filled with the CoJet sand and air-abrading the metal surface at 50 psi. The 30-µm particle strikes the metal surface, generating a high temperature that is produced by the energy of impact. The manufacturer refers to this process as "tribochemical coating." The silica particles are incorporated into the metal surface up to a depth of 15 µm. This leaves a silica surface on the metal framework that increases the effectiveness of the adhesion of resin to metal, leading to greater bond strength between the two materials.10
The silica-coated metal surface was then hydrogenized by applying phosphoric acid for 5 seconds (Figure 8), rinsing with water, then drying with an air-water syringe. A ceramic primer (CLEARFIL™ CERAMIC PRIMER, Kuraray America Dental, www.kuraraydental.com) was applied and dried in the same manner (Figure 9). This type of ceramic primer, according to the manufacturer, has an increase in bond strength when the ceramic surface has been treated with phosphoric acid. The acid hydrolyzes the Si=O (silanon group) of the ceramic surface, creating a Si-OH (silanol group). Several thin layers of an opaque resin material (Kolor + Plus®, Kerr Corporation, www.kerrdental.com) were applied. Each layer was light-cured before the next, until the metal was completely blocked out (Figure 10 and Figure 11).
The acquisition unit then digitally scanned the prepared metal framework and the adjacent teeth. The pontic margin was placed on the digital model that represented the borders of the pontic. The software program was advanced to the design phase. The pontic was then designed in the correlation mode, which made a copy of the modified pontic. Further modifications were made using the design tools in the system. Once the final design was completed, the computer program was advanced to the milling stage. Lithium disilicate with a low translucency was selected based on its strength and ability to mask underlying shades while still maintaining a vital appearance. The shade was selected based on the adjacent teeth. A collection of leftover segments of lithium-disilicate material from previous millings had been shaped into tabs and used to select the shade. Many commercial shade guides use material other than the actual restorative material. Using the actual ceramic material and placing it between the adjacent teeth is a tremendous aid during the shade-selection process.
The blue-colored lithium-metasilicate ceramic block, which is in a softer pre-sintered phase, was placed into the milling machine. After the milling process was completed, the pontic was tried in to verify fit and adjust the contours. A fine-tapered diamond was used with water to finalize the contours (Figure 12). The labial and lingual surfaces were polished using a series of diamond-impregnated wheels (Dialite®, Brasseler USA, www.brasselerusa.com). Surface stains were then applied to the cervical (Khaki) and incisal areas (Blue) creating a more natural appearance (Figure 13). The pontic was carefully removed from the framework and secured to the firing tray with object fix putty to protect the margins during the crystallization process. A final layer of glaze was then sprayed over the labial and lingual surfaces.
The final step was the crystallization process where the restoration was fired in a two-stage ceramic furnace. During this phase, the restoration experienced 0.2% shrinkage, which the computer software program accounted for during the milling. The flexural strength of the lithium metasilicate before crystallization was around 125 MPa, similar to pressed leucite-reinforced ceramic.11 During the crystallization cycle, the lithium-metasilicate restoration reached a temperature of 840ºC to 850ºC (1,544°F to 1,562°F). During the temperature rise, a controlled growth of lithium-disilicate crystals occurred, producing a transformation of the microstructure that resulted in an increase of the final flexural strength to 360 MPa, approximately three to four times stronger (170%) than leucite-reinforced glass ceramics.12 After complete crystallization, the glass ceramic is made up of 70% of prismatic lithium-disilicate crystals (0.5 µm to 5 µm long) dispersed in a glassy matrix.13 The lithium-disilicate microstructure is made up of numerous small, randomly oriented, interlocking plate-like crystals. This crystal size and orientation cause cracks to deflect, branch, or blunt, which can account for the increase in flexural strength and fracture toughness over leucite-reinforced ceramics.14
After the cycle and the restoration cooled, the restoration was steam-cleaned to remove the hardened object fix putty.
Preparing the Ceramic Bonding Surface
There are two basic ways to prepare a ceramic surface for resin bonding. One way is to use a strong etchant to create micro-porosities on the bonding surface. Hydrofluoric acid can be used for the amount of time specified by the ceramic manufacturer. Not all ceramics are conditioned for the same amount of time. Over-etching can lead to micro-flaws that can have a negative impact on the integrity of the ceramic.15 After the etching process with hydrofluoric acid, a white chalky ceramic residue comprised of water-insoluble crystalline residue/salts mix remains after the etchant is rinsed away with water.15 This residue can be removed by either steam-cleaning or using an ultrasonic bath in distilled water for 1 minute. The presence of residue can interfere with obtaining an optimal bonding surface.16,17 Once the residue is removed, then the dry ceramic surface can be primed with a conventional silane.
The use of hydrofluoric acid is not permitted by law in some countries and so alternative methods for conditioning the ceramic surface for bonding have been developed. A ceramic primer that contains the functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) can be used with conditioning with hydrofluoric acid. According to the manufacturer, the 10-MDP phosphate monomer has the ability to bond to metal oxides and ceramics that include leucite-reinforced alumina, zirconia, and lithium disilicate.11,12 To maximize the effectiveness of the ceramic primer, the ceramic surface must first be hydrogenized by applying phosphoric acid, then rinsed with water and dried. The manufacturer states that the acid changes the surface configuration of the ceramic surface that promotes the reaction of the 10-MDP monomer.
To prepare the framework for insertion, a microbrush was used to cleanse the labial and lingual surfaces with phosphoric acid to remove salivary protein contaminates. A drop of CLEARFIL ceramic primer was applied to increase the adhesion between the block-out resin and the resin cement. The framework was then thoroughly dried.
A dual-cure resin cement containing the 10-MDP monomer18 was applied to the bonding surface of the restoration and then inserted. While holding the restoration in place, the labial surface was light-cured, then the lingual surface was light-cured. Excess cement was removed. The occlusion was verified and any eccentric contacts were removed to minimize shear forces to the bonding interface of the pontic (Figure 14 through Figure 16).
The techniques used in conjunction with CAD/CAM technology are constantly developing. Patients are becoming more aware of the technology and are pleased with the results and especially the time saved. The technique used in this article could have be completed in a conventional way—taking an impression followed with by placing a provisional restoration, with the time lapse from sending it to an off-site laboratory. In this case, as the patient had a time constraint with his upcoming wedding and needed the restoration as soon as possible, the CAD/CAM technology was able to fulfill his needs.
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About the Author
Gregg A. Helvey, DDS
Adjunct Associate Professor
University School of Dentistry