Written by Randall G. Cohen, DDS Wednesday, 01 February 2006 00:00
Designing a partial-coverage casting to address an incisal edge fracture becomes complicated when a large cervical lesion is present. These kinds of defects create path of insertion problems, since a casting that wraps through the proximal surfaces needs to be inserted in an axial direction. Also, any attempt to make an impression in the presence of a large facial-cervical undercut will result in distortion of the material, an inaccurate model, and a poorly fitting casting. Further, the narrowed facial-lingual portion of the cervical third of the tooth creates a weakened tooth structure, making the case more susceptible to fracture. Clearly, large cervical defects must be restored in a way that will strengthen the underlying tooth in a manner that is compatible with bonded porcelain.
In this case, the author uses a new core build-up material with a new self-etching primer that accomplishes this objective.
An 85-year-old male presented to the office complaining of a fractured front tooth. He had been a patient of record in this practice for many years, having had some teeth restored as they became badly decayed or fractured. Tooth No. 8 had been restored 6 years prior with a porcelain crown, but he had declined to have tooth No. 9 done at the same time. Now that No. 9 had fractured, he was ready to have it restored.
Pretreatment examination revealed a large incisal edge fracture and a large deeply notched cervical lesion. There was an adequate band of attached gingiva, although generalized recession of the gingival tissues was also present. No mobility was noted for tooth No. 9 (Figure 1).
|Figure 1. Pretreatment view.||Figure 2. Initial preparation.|
As with many older patients, local anesthesia was not required for tooth prep-aration due to the heavy de-posit of sclerotic dentin. Using a high-speed handpiece, all broken and de-cayed tooth structure was removed. As the bond strength to sclerotic dentin is less than would be found in deeper dentin,1 the facial cervical portion of the tooth was roughened with a No. 4 round bur at low speed to improve the dentin permeability and to create a better bond (Figure 2).
The glaring problem of the large cervical lesion was easily managed using a light-cured composite core build-up material together with a self-etching primer and light-cured bonding agent.
Following the removal of old filling material and caries, the facial and proximal surfaces of the tooth were reduced appropriately for placement of a cast-ceramic three-quarter porcelain veneer. This prep design affords better strength, aesthetics, fit, and easier cleanup after bonding than a design that does not include proximal reduction. This prep design also preserves the cingulum and avoids reducing the lingual surface by the 2 mm necessary to avoid porcelain fracture.2
The preparation was then scrubbed with a wet cotton pellet and rinsed for 15 seconds. The substrate was left wet; however excess water was removed with a vacuum tip.
Using a self-etching primer avoids use of the usual 32% phosphoric acid that had been part of the dentin bonding regimen for years. Accordingly, much postoperative sensitivity is avoided, since this process eliminates the dentin demineralization from the phosphoric acid that occurs prior to penetration of the adhesive primer.3 Self-etching systems do not require phosphoric acid, so tubules are not opened during the bonding process, therefore less postoperative sensitivity is experienced.4 Other difficulties arise from the phosphoric acid denaturing dentinal proteins if it is left on the tooth too long, thus compromising bond strength. Thus, avoiding the phosphoric acid gel represents a real improvement in the bonding process.
|Figure 3. Self-etching primer application.||Figure 4. Bonding agent application.|
The self-etching primer (Clearfil Protect Bond [Ku-raray Dental]) is water-based rather than alcohol- or acetone-based, therefore it is applied to the preparation surface in a single coat (Figure 3). Besides avoiding overwetting the dentin prior to bonding, the clinician should vigorously dry the preparation to evaporate the water solvent fully.4 Unlike other primers, this material does not need to be applied continuously as it evaporates. Rather, a single layer left in place for 20 seconds and air-dried is the required technique. Following the application of the primer, the bonding agent is applied (Figure 4), lightly air-dried, and light-cured for 10 seconds. Clearfil Protect Bond further reduces sensitivity by its antibacterial property and by its ability to release fluoride ions into the dentin.
The core build-up material (Clearfil DC Core Automix [Kuraray Dental]) is easily mixed by depressing on the plunger and extruding the material through a very small mixing tip, which enables the clinician to control precise amounts of the material with minimal waste.
|Figure 5. Creating the preparation.|
Once the material is applied to the tooth, a 20-second cure time will set the composite to a depth of 2 mm. In a slight departure from the chamfer finish line advocated by Strupp,5 the author used a rounded and polished porcelain butt joint. This margin design works well in the lab and at the chair, since it allows for more bulk of porcelain at the margin and makes it easier to verify the fit and positive seating during insertion. Accordingly, the handpiece is held perpendicular to the long axis when preparing the proximal and lingual surfaces. This technique offers better control in developing the butt joint circumferentially (Figure 5).
The author utilizes the F-82 diamond (Pollard Dental) to do the axial reduction as well as to create the proximal, buccal, and lingual finish lines. Sharp line angles are smoothed with flexible sandpaper disks (3M ESPE) and rubber polishing points under irrigation (Dedeco).
|Figure 6. Cord placed, ready for impression.|
The surface of the preparation is cleaned for 15 seconds with a scrub of Tubulicid Red (Global Dental) in a syringe with a brush tip, then rinsed. Then retraction cord (No. 7 Siltrax) soaked in astringent (Hemodent [Premier]) is placed and rinsed immediately (Figure 6). After 10 minutes by the clock, the cord is rinsed, dried, and removed, and an impression is made (Flexitime [Heraeus Kulzer]).
A provisional crown (Figure 7, shown prior to cementing) was custom made by block carving methylmethacrylate powder and liquid (Pollard Dental). The provisional was contoured and polished, then inserted with polycarboxylate cement (Durelon [Premier Dental Products]).
|Figure 7. Provisional crown, prior to cementing.||Figure 8. Completed case.|
|Figure 9. Occlusal contact on enamel.||Figure 10. Full-face view.|
The lab used the impression to fabricate a feldspathic porcelain three-quarter crown. The casting was tried in, the fit was verified clinically, and it was bonded in place using a dual-cured resin (Panavia F2.0 [Kuraray Dental]). Occlusal contact was designed to occur on enamel, so protrusive guidance was the only aspect of occlusal adjustment that was needed (Figures 8 to 10).
Broken down teeth that are to be restored with castings will benefit from a composite core buildup in many ways: pulpal protection; preservation and strengthening of tooth structure; easier fabrication of provisional restorations; fewer path of insertion problems; easier impression making; fewer laboratory complications.
This preparation is designed to allow as much bulk of porcelain at the margins as possible, so the flat end of the diamond was held at a right angle to the long axis of the tooth when preparing the proximal surfaces. The preparation was also designed to avoid the centric occlusal contact, a precaution that greatly reduces the likelihood of fractured porcelain during normal function.
The procedure went very smoothly because the existing cervical lesion was restored prior to final preparation and impression making. The core material (Clearfil DC Core Automix), with the associated bonding agent (Clearfil Protect Bond), is ideally suited for this application: it is easy to manage, quick curing, reduces or eliminates sensitivity, and has excellent bond strength. As an added benefit, this material saves time by eliminating the application of the 32% phosphoric acid and the subsequent rinsing step. Further, the primer requires only 20 seconds on the tooth, without the need for bottle shaking or repetitive applications.
The bonding system contains an antibacterial agent and a fluoride-releasing property that greatly reduce postoperative pain while affording excellent bond strength.
When presented with a large cervical lesion, the clinician must restore it prior to placing a casting, a process for which this bonding agent and core material are well suited. A feldspathic porcelain three-quarter crown is the restoration of choice; it affords better strength, durability, fit, aesthetics, tooth preservation, and cleanup than other restorations.
1. Yoshiyama M, Sano H, Ebisu S, et al. Regional strengths of bonding agents to cervical sclerotic root dentin. J Dent Res. 1996;75:1404-1413.
2. Strupp W. The three-quarter porcelain veneers. Dent Today. Apr 1996;15:44-49.
3. Haller B. Recent developments in dentin bonding. Am J Dent. 2000;13:44-50.
4. O'Keefe KL, Pinzon LM. Management of the prepared tooth for placement of direct posterior composite restorations. Dent Today. 2005;24:70-77.
5. Strupp W. Creating laser-like margins. Crown and Bridge Update. 1996;1:26, 32.
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