Written by Juergen Manhart, DDS, PhD Sunday, 31 August 2003 19:00
Ceramic inlays and partial crowns are considered some of the most aesthetic posterior restorations available to dentistry, but excellent aesthetics is only one of the important components that has led to this restorative technique becoming increasingly popular. The outstanding biocompatibility of ceramic also promotes acceptance by the patient.
When they are adhesively luted, ceramic restorations permit dentists to use new methods for replacing lost tooth structure. Bonded high-strength ceramic restorations frequently avoid the necessity for a full crown since the severely weakened cusps are stabilized by the adhesive technique. Adhesive restorations do not require cavities to be prepared with mechanical retention, which allows minimally invasive preparations.
In comparison to composite inlays, where the material properties (eg, low modulus of elasticity, inferior wear resistance, lower shade stability) restrict the indications to small- to medium-sized cavities, ceramic inlays can be used for restoring large cavities in occlusally loaded posterior regions. Partial crowns replacing cusps or even full crowns are now feasible and have good long-term prognoses, owing to the excellent properties of ceramics. The low coefficient of thermal expansion and minimal mechanical distortion, which ceramics undergo when loaded, stress the bond to the tooth structure considerably less than composite restorations. This lowers the risk of marginal gap formation, which could result in secondary caries long-term. If the higher costs are ignored, ceramic inlays exhibit considerable advantages when restoring extensive defects.
A few rules must be strictly adhered to while preparing the cavity and while adhesively luting the restoration if this technique is to be successful long-term. If these guidelines are not heeded, the restorations may fail prematurely. Generally speaking, all patients provided with adhesive inlays should be highly motivated as far as oral hygiene is concerned and be very conscious of their teeth.
The following clinical case shows step-by-step how an insufficient amalgam filling is replaced with a ceramic inlay.
|Figure 1. Preoperative view. Inadequate amalgam filling (viewed in a mirror).|
|Figure 2. As the amalgam severely shines through the enamel, the shade of the adjacent teeth was used for color analysis.|
|Figure 3. The amalgam filling has been removed.|
The preoperative status shows the tooth (viewed in a mirror) with an inadequate MOD amalgam filling with marginal gaps and very limited aesthetics due to the grey metal and discolored areas shining through the enamel (Figure 1). Prior to commencing the treatment, the tooth was cleaned with a rotary prophylaxis brush and nonfluoride paste. The basic shade of the ceramic restoration was taken in daylight with a Vita shade guide and matched to the adjacent teeth. The shade could not have been taken with the affected tooth because the metal shining through the enamel made it impossible to analyze the shade accurately (Figure 2). The amalgam filling was removed using a high-speed, water-cooled handpiece and suction (Figure 3). Remains of the cement cavity lining and carious tooth structure were drilled out with a round-head bur. Any dentin that was simply discolored but felt hard when probed was left in place.
The basic shape of the cavity was prepared using a slightly tapered diamond with rounded tip. A minimum thickness of the inlay of 1.5 mm was provided for. All interior point and line angles were finished with round-tip finishing diamonds. Avoiding sharp line and point angles simplifies the fabrication of ceramic restorations without marginal imperfections and prevents stress concentrations that may cause fractures in the cavity. The side walls and floors of the approximal extensions were then finished using the SONICflex (KaVo) sonoabrasive preparation system with SONICflex prep ceram (KaVo) tips for adhesive inlays. The tips oscillate to remove the tooth structure, and because they are only diamond-coated on one side, they can be supported on the adjacent tooth to prepare the cavity precisely but without harming the adjacent tooth.
Once the cavity had been prepared with the SONICflex system, no chipped areas were observed along the margins. This is essential if the ceramic restoration is to fit precisely. At this point, several additional important guidelines for preparing cavities for ceramic inlays must be followed. The occlusal opening angle of the opposing walls should be approximately 10° to 12°. This is best achieved with slightly tapered finishing diamonds. The restoration must be at least 1.5 mm thick overall, since ceramic exhibits limited flexural and tensile strength values. (Pay particular attention to the central fissure.) If the restoration is not adequately thick, it may fracture, leading to premature failure. It is crucial to avoid overhanging and knife-edge areas around the cavity margin. Sharp, tapering inlay margins must be avoided, since they are prone to fracturing. The approximal cavosurface angle must not be less than 60°. Also, restoration margins should not coincide with static occlusal contact areas.
|Figure 4. Taking an impression of the cavity.||Figure 5. Placing and contouring a direct, light-curing temporary material.|
|Figure 6. Light-curing.||Figure 7. The temporary restoration has been adjusted to the correct level.|
The mesial and distal preparation margins were exposed with retraction cords, and the impression taken with polyether (Impregum, 3M ESPE, Figure 4). The inlay cavity was filled temporarily with direct, light-curing temporary filling material (Clip, VOCO) and roughly contoured with hand instruments (Figure 5). The temporary restoration was light-cured with a lamp (Figure 6), and the height of the restoration was adjusted with a finishing diamond, keeping the preparation margins intact (Figure 7).
|Figure 8. Laboratory-fabricated Empress porcelain inlay.||Figure 9. Isolating the teeth from the mouth with a rubber dam.|
Figure 8 is an occlusal view of the ceramic inlay (Empress, Ivoclar Vivadent), fabricated by a dental technician on the master model. Once the temporary restoration had been removed and the cavity was cleaned thoroughly, the cavity was isolated with a rubber dam. A section of the cavity lining in the distal box was dislodged when the impression was removed (Figure 9). The rubber dam isolates the operating field from the oral cavity to enable the operator to work effectively and neatly, and it keeps the bonding area clean from contaminants such as saliva, blood, and crevicular fluid. Contamination of the enamel and dentin would significantly decrease the adhesive bond strength of the luting composite to the tooth structure and reduce the chances of producing a long-lasting restoration with optimum marginal integrity. In addition, the rubber dam protects the patient against irritating substances such as phosphoric acid and dentin bonding agent. The dam is an important aid that simplifies working procedures and enhances the quality of adhesive dentistry.
The restoration was tried in to check the precision of fit, ensuring that no sections of the rubber dam interfered with the fit of the inlay once it has been fully seated. The occlusion must never be checked during the try-in since this would risk fracturing the restoration. Also, inlays that jam on the cavity walls or proximal surfaces must not be pressed in with force, since this may cause microcracks that cannot be seen and may propagate under masticatory loading, thus causing the restoration to fail. If necessary, in such cases, premature areas must be relieved slightly.
|Figure 10. Etching the inner surface of the restoration with 3% to 5% hydrofluoric acid for 60 seconds.||Figure 11. Applying the Bifix Ceramic Bond.|
Once the ceramic inlay had been tried in properly, a trained assistant prepared it for luting. To ensure that the luting composite would bond well to the leucite-reinforced glass ceramic, its inner surface was etched for 60 seconds with 3% to 5% hydrofluoric acid (Figure 10). For this purpose, the inlay was fixed firmly to an application instrument with a sticky tip. It is advisable for the staff and patient to wear safety glasses when dealing with hydrofluoric acid. Etching creates retentive structures on the ceramic surface, which provides for micromechanical retention. The acid is rinsed off thoroughly with a jet of pure water before using compressed air/water spray to clean off the remains of acid and deposits released during etching. The restoration was then dried, and silane (Bifix Ceramic Bond, VOCO) was applied to further enhance the bond strength (Figure 11). The silane was allowed to react for 60 seconds before carefully blasting off the residual solvent with compressed air.
|Figure 12. Etching the enamel section with 37% phosphoric acid.|
|Figure 13. Etching the dentin areas of the cavity (total etching).|
|Figure 14. Conditioned tooth structure|
While the inlay was being prepared for adhesive cementation, the operator prepared the tooth for bonding the restoration. Thirty-seven percent phosphoric acid gel (Vococid, VOCO) was applied selectively to the enamel margins (Figure 12) and allowed to react for approximately 20 seconds before etching the total enamel and dentin surface of the cavity for an additional 15 seconds using etching gel (Figure 13). Enamel tolerates longer etching times better than dentin; the latter should not be etched for more than 15 seconds. Once the acid and remains of deposits had been sprayed off thoroughly with compressed air/water for 30 seconds, the cavity was dried carefully with oil-free compressed air. If the enamel has been etched properly, it will have a frosted appearance (Figure 14), which is an indication that the microretentive demineralization pattern required for bonding the restoration to the enamel has been established successfully.
Before applying the adhesive materials, the dentin in the cavity was rehydrated with a clean, moist (via distilled water) foam pellet (Pele Tim, VOCO), because if the etched dentin is desiccated, the bond strength will be reduced considerably. Acetone-based adhesive systems in particular require a certain amount of residual moisture in the etched dentin to prevent the 3-dimensional network of exposed collagenous fiber from collapsing and to ensure that the monomers in the bonding agent penetrate the demineralized zone adequately. Once the cavity appears damp, it has been moistened adequately. Avoid creating a wet surface with pools of water.
Figure 15 shows copious amounts of acetone-based Solobond Plus primer (VOCO) being applied to the enamel and dentin with a brush. As described in the manufacturer’s instructions, the primer was worked in for 30 seconds before the solvent was evaporated carefully with oil-free compressed air. Following this, Solobond Plus Adhesive (VOCO) was applied to the entire surface of the cavity with a small brush and worked in for 15 seconds, blasted with compressed air to create a thin coating, and then light-cured for 20 seconds. The clinician should make absolutely certain that no pools of adhesive are overlooked in the cavity and cured, since this would compromise the fit of very congruent restorations. The result is a shiny cavity surface uniformly coated with adhesive. This should be checked carefully, since any areas that appear dull have probably not been adequately coated with bonding agent. In the worst case, this could reduce the bond strength of the restoration in these areas and compromise dentin sealing, possibly leading to postoperative hypersensitivity. Should such areas be seen, apply a further coat of bonding agent specifically to them.
|Figure 15. Applying the adhesive.||Figure 16. Placing the porcelain inlay in the cavity.|
“Universal” shade Bifix QM (VOCO) dual-curing luting composite was mixed homogeneously in a QuickMix double-barrel syringe and applied directly from the mixing tip onto the underside of the ceramic inlays. The luting composite was also applied to the adhesively pre-treated cavity and adapted carefully to the floor and walls with a spatula. The restoration was then placed in the cavity, the application instrument twisted off, and the inlay seated fully without applying excessive force, allowing the excess composite to flow out. A plastic-coated applier (a USI applier without ultrasonics) was used to avoid metal being rubbed off (Figure 16). The restoration can also be placed with other suitable instruments of the clinician’s choosing.
|Figure 17. Prior to curing, excess luting composite must be removed from the proximal surfaces with a foam pellet.||Figure 18. The luting composite beneath the contact areas is removed with dental floss.|
Before curing the luting composite, foam pellets (Pele Tim, VOCO) were used to remove excess material from the approximal areas (Figure 17) and along the occlusal inlay margins. The proximal contact areas were cleaned with Superfloss (Oral-B, Figure 18). Glycerine gel was applied around the luting joint to prevent an oxygen-inhibited layer forming on the surface of the luting composite during polymerization. Following this, the luting composite was cured using a curing lamp with an adequately high light intensity. Each accessible aspect of the restoration (occlusal, distolingual, mesiolingual, mesiobuccal, distobuccal, buccal, and lingual) was cured for 40 seconds with the curing lamp. The additional chemical curing mechanism of the dual-curing luting composite ensures that even those areas that cannot be accessed properly for light-curing actually polymerize sufficiently.
|Figure 19. Checking and adjusting the static and dynamic occlusion.||Figure 20. The restoration after finishing and polishing.|
Once the rubber dam had been removed, the minimal remains of bonding resin and luting composite were carefully removed from the proximal areas with a sharp scaler and finishing strips. The gingival sulcus was probed carefully for remains of dentin bonding resin, which were removed where necessary. Since the restoration had been cleaned and the excess resin carefully removed prior to light-curing, finishing only involved minimal correction of the static and dynamic occlusion. This could only be carried out after adhesive cementation of the restoration to stabilize it and the use of articulating paper to analyze the occlusal premature contacts (Figure 19). The luting gap was high-lustre polished using flexible aluminium oxide disks (Sof-Lex, 3M ESPE), and the ceramic surface polished to a high lustre with diamond particle containing polishers, followed by diamond polishing paste on mounted foam polishers.
Figure 20 shows the finished restoration. The occlusal anatomy has been restored, and due to the chameleon effect, the shade of the ceramic inlay cannot be differentiated from that of the adjacent tooth structure. Finally, fluoride varnish (Bifluorid, VOCO) was applied to the tooth to protect the enamel adjacent to the restoration, which was unavoidably touched during conditioning and finishing.
Laboratory-fabricated ceramic inlays are sophisticated restorations that create top-quality aesthetics. Due to the excellent properties of ceramic materials and luting composites, even extensive occlusal load-bearing posterior cavities can now be provided with adhesive restorations with favorable long-term prognoses.
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