Fixed prosthodontics has become a major part of American general dentistry. The reliability of crowns and fixed prostheses and the predictability of their service are well-known, and most practitioners can place them with relative ease. The longevity of a typical crown or simple short-span fixed prosthesis is sufficient enough that most patients feel the expenditure for the treatment is equal to the value of the service.
However, premature failure of crowns and fixed prostheses is not uncommon. The most frequent failures appear to be the fracture of the ceramic veneering material, the need for endodontic treatment after cementation, postoperative tooth sensitivity and release of crowns or prostheses from abutment teeth during service. This article discusses the subject of retention and what can be done to improve it in the long term with crowns and fixed prostheses.
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TOOTH PREPARATIONS
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I suggest that every dentist take a tour through a major dental laboratory in any part of the United States. Observation of the dies representing typical tooth preparations made by dentists highlights the need for some changes in many tooth preparations. The so-called "tepee-shaped tooth preparation" dominates the dies in many laboratories. It is commonplace to see dies for crown preparations with more than 40 degrees of divergence from parallelism. There is not a school in the country that teaches this much divergence for crown preparations. However, it is easy to understand why dentists make tooth preparations with proximal walls that are too divergent: the crowns or fixed prostheses can be seated more easily and without worry about inability to remove the restoration after trying it on before cementation. The result of tooth preparations that are too divergent is lack of retention during service.
I suggest that crown preparations should be cut with minimal divergence from parallelism. Approximately 10 degrees of divergence from parallelism is an appropriate goal, though it is not always achievable. If abutment teeth do not have at least one-half of the coronal tooth structure remaining, well-retained buildup material should be placed. Buildups, most commonly made with resin-based composite, provide adequate bulk of structure so that the near-parallel walls of the tooth preparation provide retention. The remaining tooth structure must provide sufficient retention for the buildups. I will discuss more about buildups later in this article.
Small pieces of filler material placed in holes left in tooth preparations on removal of previous restorations should not be considered to contribute to crown or prosthesis retention. However, such "fillers" are highly useful to fill the voids where previous restorations have been removed, thus providing unimpeded removal and lack of distortion of subsequent impression materials.
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BONDING AGENTS
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Over the past 10 years, use of bonding agents on dentin surfaces has become routine. Although these materials might enable remaining dentin to retain resin-based composite or other restorative materials, the bond strengths to dentin of various brands vary considerably,1 and the bond to dentin over long periods may be questionable.
In my opinion, the major reason to use dentin bonding agents before placing buildup materials is to reduce the potential for postoperative tooth sensitivity. Bonding the restorative material to the underlying dentin is a secondary reason for use of bonding agents. Undercuts in tooth structure, channels, "potholes," pins or posts should be used to ensure retention of the buildup material. Fixed prostheses have been observed by practitioners in which unsecured buildups have released from tooth structure, allowing release of the cemented prostheses.
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RETENTIVE FEATURES IN TOOTH PREPARATIONS
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It once was thought that the walls of tooth preparations should be extremely smooth and without scratches. However, irregularities made in tooth preparations by diamond cutting instruments provide mechanical retention for the subsequent crowns. The cementing medium flows into the scratches in the tooth structure, providing retention. On the other hand, smooth-margin areas of tooth preparations provide for better definition of the juncture between the tooth and the restoration, as well as better observation by the dental technician as the crowns or fixed prostheses are fabricated.
When buildups are placed, they should have interdigitation with underlying tooth structure to provide retention.2 Depending on bonding agents alone for retention is not adequate, as evidenced by previously stated examples of low bond strength and clinical observations of failed restorations when bonding agents alone have been used under buildups. Mechanical retention of buildups easily can be obtained by cutting retentive features such as undercuts, channels or potholes into remaining tooth structure. Placing retentive pins or posts where indicated by gross loss of tooth structure or endodontic therapy assists in retention of buildups when large amounts of buildup material are needed. Since the introduction of bonding agents, most dentists have used fewer retentive pins and posts. Although this trend is understandable, pins and posts still should be used when there is a significant loss of tooth structure.
One of the most important changes in materials that relates to retention of crowns and fixed prostheses has been new strong forms of dental cements.
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CEMENT TYPE
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One of the most important changes in materials that relates to retention of crowns and fixed prostheses has been the introduction and acceptance of new strong forms of dental cements. In the past 10 years, resin-based composite has grown in use as a cement.3–5 However, use of resin cement has led to an increase in postoperative tooth sensitivity, which can be avoided with proper use of bonding agents.6
Resin-reinforced glass ionomers, or RRGIs, are dominant in the American cementation market. They have shown phenomenal growth in use, thanks in part to their relative lack of undesirable characteristics compared with previous cements.7 In most cases, RRGIs such as GC Fuji Plus (GC America, Alsip, Ill.) or RelyX Luting Cement (previously Vitremer) (3M ESPE, St. Paul, Minn.) provide nearly ideal characteristics for cementation of crowns and fixed prostheses.
When abutment tooth preparations have questionable retentive potential, I strongly suggest use of resin cement with an acceptable bonding agent for prevention of postoperative tooth sensitivity. The most popular brand of resin cement for routine use has been Panavia F (Kuraray America, New York) used with ED Primer (Kuraray America). However, many other conventional brands of resin cement and bonding agents have similar properties.
The unique resin cement product, 3M ESPE RelyX Unicem Self-Adhesive Universal Resin Cement (3M ESPE), has made the use of strong resin cement very easy and predictable. This cement is essentially a filled, self-etching primer that provides the physical properties of resin cement without the threat of postoperative tooth sensitivity. Dentists have come to trust the ability of self-etching primers to reduce or nearly eliminate postoperative tooth sensitivity when placing resin-based composite restorations.8 Unicem is a cement variation of that concept that looks promising, but more long-term research is needed.
When inadequate retention is suspected for a crown or fixed prosthesis, resin cement is indicated to provide additional strength and retention over more conventional cements. Because of the threat of postoperative tooth sensitivity, practitioners are encouraged to consider carefully the brand of resin cement and the bonding-desensitizing technique they use.
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OCCLUSION
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When observing clinical practice in study clubs, I often am amazed to note that some practitioners seat relatively complex fixed restorations and provide little or no postcementation occlusal equilibration. This practice probably is one of the most frequent causes of postoperative failure of fixed restorations. To date, no articulator provides perfect clinical occlusion from the laboratory.
After delivering crowns or fixed prostheses, dentists should take care to adjust occlusion carefully and thoroughly. For complex fixed prostheses, I suggest scheduling an appointment about one week after the cementation procedure to adjust the occlusal prematurities that have developed after a week of chewing. I always am surprised to see that some objectionable occlusal disharmonies have developed during that time. These occlusal prematurities may be adequate to cause dislodgement of the restoration after a period of use.
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SUMMARY
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Crowns and fixed prostheses are well-proven, accepted and routinely used restorations. However, they occasionally come loose from tooth preparations. Many things can cause these failures. In this article, I have discussed the following reasons for lack of adequate retention of crowns and fixed prostheses: inadequate tooth preparation; too much trust in dentin bonding agents and lack of adequate tooth buildup; tooth preparations that lack irregularities; improper selection of cements; and lack of postoperative occlusal adjustment.
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TOOTH PREPARATIONS
BONDING AGENTS
