Many mechanical methods have been suggested to increase retention for restorations and crowns. Also, there have been numerous suggestions about the use of bonding agents for retention. Failure of bonding agents often is noted in practice when intracoronal restorations come out or when veneers fall off during service. Similarly, buildups for crown preparations or posts and cores occasionally come out during service. Questions often are asked about when bonding agents should be used, when mechanical retention should be used or when both are desirable. In this article, I discuss the various types of mechanical retentive methods available and make suggestions about the use of bonding agents.
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TECHNIQUES COMMONLY USED TO INCREASE RETENTION
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Acid-etched enamel.
This procedure has been very successful for more than 50 years. When cut or uncut enamel is etched properly for 15 seconds with phosphoric acid, the microscopic undercuts produced provide reliable retention for subsequently placed liquid resin bonding agents and restorations. Bonding of liquid resin bonding agents to acid-etched enamel has been shown, both in clinical observation and in research, to be reliable and predictable. The many bonding procedures accomplished during the past several decades attest to the acceptability of this type of retention. However, sometimes there is not much enamel present on the tooth surface, in which case bonding to dentin is one of the alternatives.
Dentin bonding.
When dentin bonding agents were introduced 50 years ago, the bonds were weak and unpredictable. Fortunately, bonding materials have been improved significantly since then. The "total etch of dentin" concept has been practiced for many years, showing moderate success with retention to dentin, but occasionally resulting in significant postoperative tooth sensitivity. In the past 10 years, the "self-etching of dentin" concept has gained popularity, accompanied by moderate success in bonding to dentin and significantly less postoperative tooth sensitivity than in the total-etch concept.1
However, clinical observations show that when either type of dentin bonding is used alone, without mechanical retention, some restorations fall off during service. Examples include bonding failures of ceramic veneers placed primarily on tooth preparations cut deeply into dentin, or failure of Class V restorations that depend primarily on dentin bonding for retention. These failures present a confusing phenomenon, since dentin bond strengths in vitro show values as strong as, or stronger than, bonds to enamel. Speculation on reasons for failure of dentin bonds usually indicts expansion and contraction of the restorative resin or cement during service, as well as forces placed on the restoration during service.
Clinical observation of enamel bonding versus dentin bonding generally would conclude that enamel bonding is very effective, and that dentin bonding provides less predictable clinical service.
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WHAT CAN BE DONE TO CREATE AND INCREASE RETENTION?
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Whenever most of a tooth preparation has been cut into dentin, and the majority of the retention of the restoration is expected to be supplied by bonding agents, additional mechanical retention is advisable. Examples are Class V preparations in which most of the preparation is in dentin, or veneer preparations in which more than 50 percent of the preparation is in dentin. There are several methods of creating retention in intracoronal preparations or veneers, including pins, "potholes" or channels, and undercuts, which I describe below. After that, I will summarize various methods of increasing retention for crown, inlay and onlay preparations: parallelism, box forms, grooves, cements, buildups and roughening of tooth preparations.
Creating retention.
When attempting to create retention in intracoronal preparations or veneers, the clinician has a few options.
Pins.
Twenty years ago, pins were used to excess.2 I have seen as many as 15 pins placed in one tooth. However, use of pins has been reduced significantly, and some dentists have eliminated their use altogether, because of alleged cracking of tooth structure and confidence in the retention of dentin bonding agents. In some situations, there is not enough remaining tooth structure for potholes or channels to be placed in the tooth preparation. In these situations, additional retention is needed, and pins are an excellent retentive choice.
Pins are available in three types of metal content: stainless steel (nickel-chrome), titanium alloy (titanium, vanadium and aluminum) and pure titanium. Current thought supports use of titanium alloy or pure titanium pins instead of stainless steel. This opinion relates to the desire to reduce the use of nickel in the body. Titanium alloy pins and especially pure titanium pins can be bent easily, and when used with care, they do not crack tooth structure significantly. They should be placed into dentin at least two millimeters deep, and they should be placed at least 15 degrees from parallelism with the long axis of the tooth, to prevent their release after dentin has relaxed. In my opinion, pins should not be avoided when they are indicated.
Potholes or channels may be used to assist retention for restorations and buildups or in any other situation in which additional retention is needed.
Potholes or channels.
Potholes or channels may be used to assist retention for restorations and buildups or in any other situation in which additional retention is needed. These retentive features may be placed easily with small burs that are pear-shaped (no. 329 or no. 330) or round (no. 
, no. 
or no. 1). The retentive features should be undercut for optimum retention, with an adequate opening to allow placement instruments to put restorative material into the retentive features. Research and clinical observation through the years have shown that retention created by these methods is successful.
Undercuts.
Some dental schools have suggested that mechanical retention is not needed in the proximal box forms of Class II restorations for amalgam or composite. This may be true in some situations if the preparation is deeply cut and there is a large bulk of restorative material in the area of the box form. However, placement of slight undercuts in the proximal box form does not remove much tooth structure, and it certainly provides assurance that the restoration will stay in place. I suggest that undercuts be placed with a small bur in the facial and lingual internal line angles of Class II box forms, or in any other location where additional retention is needed in a direct restoration.
Increasing retention.
Several methods exist for increasing retention for crown, inlay and onlay preparations.
Parallelism.
In the past, parallelism of the axial walls of crown, inlay or onlay preparations was mandatory for optimum retention of restorations. Stronger cements have reduced the need for extreme parallelism, and most crown preparations today are far from parallel. The more parallel the axial walls of crown preparations are, the more retention there will be for the restoration. When short crown, inlay or onlay preparations are encountered, the axial walls should be made as parallel as possible to augment retention.
Box forms.
Box forms can be made in any crown preparation if additional retention is needed. The box form should be made with parallel walls and should be as long as possible in an occlusal-gingival direction.
Grooves.
Short crown preparations often need more retention. One of the easiest and most effective methods to make additional retention is to place several parallel occlusal-gingival grooves on the axial surfaces. Suggested burs for these grooves are long burs nos. 1170, 1171 and 1171. The grooves should be as long and as parallel as possible for optimum retention.3
Cements.
Some cements provide significant resistance to restoration removal, while others are not as retentive. When optimum retention of indirect restorations is desired, resin cement is suggested. Resin cement has the greatest strength and the most retention of all cements. Because of the tendency of some resin cements to produce postoperative tooth sensitivity, an adequate self-etching primer should be used before the restoration is cemented. Examples of cements with self-etching primers are Panavia F2.0 cement (Kuraray America, New York), Linkmax cement (GC America, Alsip, Ill.) and Multilink (Ivoclar-Vivadent, Amherst, N.Y.). Another resin cement introduced recently, RelyX Unicem (3M ESPE, St. Paul, Minn.), has been observed to reduce or eliminate postoperative sensitivity because of a self-etching primer incorporated into the cement.4,5
Buildups.
When more than one-half of the coronal tooth structure on a full-crown tooth preparation is absent, building up the tooth structure with well-retained buildup material is desirable. Resin-based composite is the most popular material. It should be retained to the remaining tooth structure with the same retentive features as described above (pins, potholes, channels and undercuts) for final restorations.2
Roughening of tooth preparations.
When lack of retention is observed in an indirect restoration, the walls of the tooth preparation and the internal walls of the restoration should be roughened with a coarse diamond rotary instrument or metal bur at the time of cementation, and the restoration should be cemented with a strong resin cement.
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CONCLUSIONS
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Release of direct or indirect restorations from supporting tooth structure during service is frequently observed, causing frustration to both patients and dentists. I have discussed in this article many ways to increase retention, and I have cautioned against overdependence on the somewhat unpredictable retention of dentin bonding agents. Increased use of titanium alloy or pure titanium pins and judicious use of potholes, channels, undercuts, grooves and box forms can yield good results. There are many ways to increase retention for both direct and indirect restorations, and thoughtful planning for the most appropriate method to obtain optimum retention for each specific case is necessary.
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TECHNIQUES COMMONLY USED TO...
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