For nearly 50 years, the procedure of etching tooth enamel and dentin with phosphoric acid to gain retention of resin by the tooth surface has been researched, recommended and practiced by the dental profession. In recent years, numerous techniques have evolved in support of "self-etching" of tooth structure, which uses weaker acids and does not require washing of the tooth surface after acid application. Self-etching systems leave the smear layer on the dentin and, in most cases, produce a significantly less aggressive etching of the enamel.
In my opinion, the dental literature holds conflicting suggestions relative to phosphoric acid etching or etching with weaker acids. It is apparent to me that practitioners are confused about when to etch with phosphoric acid, when to use self-etching agents and when to not etch at all.
The observations in this article are based on my interpretations of some of the reported projects and surveys in the literature, as well as on surveys of practicing clinicians I have conducted in continuing education courses involving many thousands of dentists around the world in the last few months.
This article describes the situations in which various types of etchants and techniques for etching teeth appear to be appropriate as related to the current research literature and the behavior of clinicians around the world as I have observed it.
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ETCHING SURFACES THAT ARE ALL ENAMEL
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There are only a few restorative situations in which the entire surface of the tooth to be restored is enamel. Usually, some of the tooth structure being restored or sealed is dentin or cementum, and etching with phosphoric acid may or may not be indicated for those surfaces. Although some of the self-etch primers have pH characteristics similar to those of phosphoric acid, most of them are significantly less aggressive in etching enamel than is phosphoric acid.
Sealant usually is placed on tooth surfaces that are all enamel, at least on the external surfaces of the teeth. There may be dentin inside the teeth, but that is not visible when a typical sealant is placed. In my opinion, etching enamel before sealant placement is done improperly far too often. Phosphoric acid will not penetrate dental plaque. Usually, typical sealant placement is accomplished after only minimal surface treatment of the enamel grooves with prophylaxis paste on a rubber cup or brush, or with an explorer run along the tooth grooves. It is not possible for these techniques to remove dental plaque in the depths of the grooves, which often are only a few micrometers wide. Experienced dentists have observed that when plaque remains on an enamel surface being etched with phosphoric acid, the surface does not etch and remains shiny in appearance after the plaque is removed by a rotating rubber cup and pumice.
Relatively flat, smooth enamel surfaces to be etched that are accessible to conventional rubber-cup polishing should be cleaned of plaque with a rubber cup and prophylaxis paste or pumice before they are etched with phosphoric acid. Grooves and depressions in enamel that are inaccessible to conventional rubber-cup polishing should be cleaned of plaque by an air slurry polisher, which blasts the surface of the tooth with water-soluble sodium bicarbonate. Neither rotary instruments, explorers nor other instruments can clean plaque from inaccessible surfaces, and these surfaces remain unetched.
The placement of veneers on enamel surfaces is another instance in which phosphoric acid etching should be accomplished rather than self-etching. Currently, it is popular to place thin ceramic or polymer veneers directly onto unprepared or minimally prepared enamel surfaces. The external surfaces of teeth are highly impregnated with acid-resistant fluoride in the form of fluorapatite, which is about 50 µm deep. It is logical to remove about 50 µm of external enamel (about the breadth of a thick human hair) to allow optimal acid etching of the underlying enamel.
Attachment of orthodontic brackets to enamel surfaces requires total etching of the enamel with phosphoric acid.
There are situations in which the retention of a restoration is highly dependent on the potential mechanical retention known to be achieved with enamel etching. Examples of these situations are Class IV restorations, restoration of the incisal or occlusal cavosurface margins of abfractions, restoration of the incisal or occlusal margins of Class V tooth preparations for carious lesions and preparation of the enamel portions of tooth preparations for onlay restorations in which the tooth preparations are short in an occlusalapical dimension. In these situations, it is logical to use a controlled-placement phosphoric acid gel etchant on the specific areas of enamel in which optimum retention is needed.
Some enamel surfaces near cavosurface margins of tooth preparations have microcracks or macrocracks with some staining. This type of situation typically is observed on occlusal surfaces that have been subjected to heavy occlusal forces such as bruxing or clenching or in mature patients who have chewed hard foods for many years. These conditions often are accentuated visually when Class I or Class II tooth preparations are being made. The visually observable cracks commonly are present in mature patients’ anterior teeth in which large Class III tooth preparations are being made or in teeth that have been traumatized by athletic activity or aggressive chewing. Whenever teeth to be restored have visually apparent cracks that extend into planned margin areas, localized etching of the enamel with phosphoric acid gel is recommended before the dentin and enamel are treated with self-etching primers. In teeth without cracks, self-etching primers appear, from my clinical experience and observation, to be adequate for the tooth preparations before bonding and restorative resin placement occur.
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ETCHING BOTH ENAMEL AND DENTIN
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For many years, dentists used—most of the time successfully—phosphoric acid to etch both enamel and dentin to be restored, but postoperative tooth sensitivity frequently resulted. Dentists used various desensitizing solutions, flowable resins and resin-modified glass ionomer tooth liners to prevent or reduce the unpredictable postoperative tooth sensitivity and the occasional need for endodontic therapy. Research and surveys of practitioners have shown that dentists who use total-etch systems produce up to twice the postoperative tooth sensitivity, as reported by patients, when compared with dentists who use self-etching products.1–4
Self-etching bonding agents can be used directly on cut enamel or dentin surfaces. The wetting agents in these liquids penetrate into the dentinal canals, taking with them acid and resin. The constituents of the self-etching primer polymerize in the canals, combining with the debris in the canals, obturating the canals, and reducing or preventing postoperative sensitivity. Usually, these self-etching materials prevent postoperative tooth sensitivity, but it still occurs infrequently.1 As a further precaution, many dentists are using resin-modified glass ionomer liners (such as Vitrebond Light Cure Glass Ionomer Liner/Base [3M ESPE, St. Paul, Minn.] or GC Fuji Lining LC [GC America, Alsip, Ill.]) and/or glutaraldehyde solutions (such as Gluma Desensitizer [Heraeus Kulzer, Armonk, N.Y.] or Micro-Prime [Danville Materials, San Ramon, Calif.]) before applying self-etching primers to completely prevent the occurrence of postoperative tooth sensitivity related to resin-based composite restorations.
When the enamel of a tooth has been etched selectively with phosphoric acid gel and the dentin has not been etched, the dentist may use the following technique, which has been successful in my experience in producing an effective bond and preventing postoperative tooth sensitivity. An example of a situation in which this technique is needed is a veneer tooth preparation that involves both exposed enamel and dentin. A simple procedure follows:
- – Selectively etch the enamel with a well-controlled, viscous phosphoric acid gel.
- – Wash the phosphoric acid gel rapidly from the tooth with a significant amount of water spray. If it is washed slowly, the gel acid is spread all over the tooth preparation, thus etching it and reducing the sensitivity-prevention advantages obtained with self-etching primer application.
- – Place the self-etching primer and bonding agent on the entire preparation, including the enamel that has been etched with the phosphoric acid gel. If the restoration is a direct-placement restoration, cure the self-etching primer material and place the resin-based composite material. If the restoration is an indirectly placed, tooth-colored, translucent restoration—such as a veneer, inlay, onlay or crown—and the self-etching primer and/or bonding agent has a thick viscosity, do not cure the primer and/or bonding agent before seating the restoration. Cure the self-etching primer/bonding agent through the veneer or tooth-colored restoration.
If this sequence is not followed, the indirect restoration will not seat totally owing to the film thickness of the self-etching bonding agent. Some of the newer-generation self-etching primers have an extremely thin film (for example, Brush&Bond [Parkell, Edgewood, N.Y.], iBond [Heraeus Kulzer] and GC G-Bond [GC America]). Because of this, these materials may be cured before seating the restoration.
The typical, routinely occurring restorative situations are discussed below, including suggestions for etching with total-etching or self-etching materials. This is an extremely volatile and confusing area, and readers are encouraged to monitor the research and clinical articles in the literature to stay up to date with constantly changing techniques.
Class I, II and III restorations.
Most tooth preparations need self-etching only. Clinicians report that total-etching is unnecessary and even undesirable because of postoperative sensitivity in some situations. Selective total-etching gel can be used on margins where enamel looks suspicious. To provide more assurance of eliminating the possibility of postoperative tooth sensitivity, self-etching primers and bonding agents may be used after elective or routine use of preventive desensitizers, such as resin-modified glass ionomer or glutaraldehyde.
Class IV and V restorations.
Clinicians know well that the majority of the retention for these restorations comes from the etched enamel surfaces and not from mechanical retention or dentin bonding. The enamel portions should receive selective total etching using phosphoric acid gel. Elective or routine use of desensitizers on more deeply prepared portions of dentin followed by self-etching on the entire preparation has produced observable, predictable, effective restorations.
Veneers on all-enamel surfaces.
All of the retention comes from the etched enamel surfaces. All enamel surfaces of the tooth preparation should be etched with phosphoric acid. I prefer liquid phosphoric acid for this technique to ensure that the etchant goes into the interproximal areas.
Veneers on enamel and dentin.
Selective total etching of enamel should be accomplished with phosphoric acid gel, followed by elective use of desensitizing solutions, self-etching materials on the entire preparation, seating of the veneer and curing the resin cement and bonding agent together. An exception to this technique is when the thin self-etching primer brands discussed earlier in this article are used.
Crown or fixed prostheses seated with resin cement.
Total etching is not needed. Usually, the restorations are retentive because of parallel walls on the crown or abutment preparations. The dentist may use one of two self-etching methods:
- – application of self-etching liquid, followed by application of resin cement (ED Primer II followed by Panavia F2.0 [both manufactured by Kuraray, New York]) or Multilink (used with a separate liquid primer, both manufactured by Ivoclar Vivadent [Amherst, N.Y.]);
- – application of self-etching/resin cement combination products (RelyX Unicem [3M ESPE]) or Maxcem [Kerr, Orange, Calif.]).
Inlays or onlays on enamel or dentin seated with resin cement.
Many clinicians placing these restorations routinely are using selective total-etching agents on enamel surfaces, followed by desensitizing solutions and self-etching materials on the entire preparation. Another option is to use the self-etching/resin cement combination products described in the preceding item.
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SUMMARY
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Etching teeth with phosphoric acid has been a major technique in the profession for many years. Total etching of both enamel and dentin, popular for several decades, is slowly being replaced by selective enamel acid etching of tooth preparation margins and/or use of self-etching products only. I have described the various possible acid-etching techniques and suggested appropriate techniques for each specific clinical situation, as observed in successful clinical practice.
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ETCHING SURFACES THAT ARE...
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