BUILDING UP TOOTH PREPARATIONS FOR FULL CROWNS–2000
GORDON J. CHRISTENSEN, D.D.S., M.S.D., PH.D.
Fixed prosthodontic procedures have continued to increase in dentistry over the past several decades, but tooth preparation concepts for full crowns have not changed significantly. There has been a marked reduction in the use of retentive pins over the past 10 years and many changes in the materials used for buildup. In 2000, is it necessary to build up full-crown tooth preparations to the degree that was accomplished 20 or 30 years ago? If not, what are the current guidelines for building up tooth preparations? This article provides the author’s observations on the current state of the art in building up teeth.
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CEMENTS
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Materials for cementing full crowns have changed significantly over the past few years. Although so-called conventional cements such as zinc-phosphate, polycarboxylate and glass ionomer are still used by some dentists, they are rapidly being replaced by hybrid ionomers (examples include Fuji Plus, GC America Inc.; ProTec Cem, Ivoclar-Vivadent; and RelyX Luting Cement [Vitremer], 3M Dental Products). Resins have also become more popular for routine fixed prosthodontic cementations (examples include Panavia 21 and Panavia F, J. Morita; RelyX Arc, 3M Dental Products). Both hybrid ionomer and resin cements have been shown to be much stronger than conventional cements when cemented test crowns are subjected to tensile stress to remove them from tooth preparations.1 The greater strength and retentive properties of these cements over conventional cements makes building up tooth preparations less necessary than it was in the past.
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SINGLE UNITS VS. FIXED PROSTHESES
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Less preparation strength and retention are required for crowns placed on single teeth than when a crown is used as a fixed partial denture retainer. Clinical Research Associates’ in vitro research2 has shown that natural teeth, built up with various types of posts and cores, have strength characteristics equal to those of prepared natural teeth that are similar in size. Buildups on nonendodontically treated teeth seldom have strength equal to that of unrestored natural teeth prepared to the same size and shape. These findings should persuade clinicians to seriously consider the amount of remaining coronal tooth structure on potential fixed prosthesis abutments. If only a small amount of coronal tooth structure is present, en-dodontic therapy and adequate post-and-core procedures may be more appropriate than building up the remaining vital tooth structure. If a tooth with minimal remaining coronal tooth structure is to be restored as a single crown, buildup of the broken-down vital tooth may provide adequate strength and retention for the single crown.
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OCCLUSAL FORCES
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Occlusal forces expected on crowns and fixed prostheses should be considered in relation to buildup needs. Patients with bruxism or clenching habits place far more force on crowns and fixed prostheses than patients without aggressive occlusion. Tooth preparations for crowns for patients with bruxism or clenching should be stronger and more retentive than tooth preparations for nonaggressive chewers. Teeth that support crowns or fixed prostheses with heavy canine rise or incisal guidance should have more strength than those without heavy expected loads. Posterior teeth "protected" by canine guidance do not have significant lateral stresses on them. The canines take the majority of the load in lateral function. Fortunately, patients with canine rise make up the majority of patients.
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RETENTION FOR BUILDUPS
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Forty years ago, retentive pins were introduced and became popular. At that time, pins were cemented, screwed or held by friction lock because of their small size in relation to pin holes in dentin. I routinely saw teeth with eight to 10 or more retentive pins holding amalgam to the tooth structure. Over the years, dentists have reduced their use of retentive pins because of the increased strength of dentin and enamel bonding agents and the revived use of retentive slots, "potholes," grooves and channels in tooth structure. Is this reduction in use of pins acceptable? Fixed prosthodontics is being accomplished more than ever before, and reports of lack of crown retention are not increasing. From this empirical perspective, reduced use of retentive pins appears to be clinically acceptable.
There is a growing problem reported by practitioners. Many popular bonding agents do not bond to composite resin buildup materials. This situation may be easily evaluated in your own practice by cutting off the coronal part of an extracted tooth, putting the bonding agent you currently use on the dentin surface, and placing your regular buildup material over the bonding agent. Place the test specimen in a cup of water, and after 24 hours attempt to separate the buildup from the tooth. Many current bonding agents are not compatible with popular buildup materials.
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MATERIALS USED FOR BUILDUPS
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The most popular material for buildup use is bonded composite resin, followed by bonded amalgam. Since amalgam use is decreasing in dentistry, it is expected that use of composite resin as a buildup material will continue to increase. Composite resin buildup materials have about the same strength as tooth structure, while amalgam is stronger than tooth structure. However, composite resin can be chemically bonded to the tooth structure better than amalgam. Resin-reinforced glass ionomer and compomer are both significantly weaker than tooth structure. Their use should be limited to situations in which only minimal tooth structure is missing, and increased tooth strength and abutment retention are not required.
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FILLERS VS. BUILDUPS
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When teeth are prepared for full crowns, there is often amalgam remaining in box forms, Class V areas and other locations. In my opinion, all previously placed materials should be removed, allowing teeth to be rebuilt with the assurance that there are no surprises left under old restorations. If more than one-half of the coronal tooth structure is remaining in the tooth preparations, and increasing tooth preparation strength is not mandatory, I prefer replacing the remaining old restorative material with bonded compomer (Dyract AP, DENTS-PLY/L.D. Caulk; F-2000, 3M Dental Products; Compoglass F, Ivoclar-Vivadent; Elan, SDS Kerr). Hybrid ionomer could also be used as a filler in these situations (Vitremer Restorative Material, 3M Dental Products or Fuji II LC, GC America Inc.).
If more than one-half of the coronal tooth structure is gone, and there is not a 2- to 3-millimeter collar of sound tooth structure remaining around the gingival portion of the tooth preparation, I prefer a buildup to a filler. The buildup is used to increase tooth strength and crown retention. The material of choice for the buildups, as reported by practitioners, is composite resin bonded to potholes, grooves, channels or undercuts. Occasionally, when minimal tooth structure remains for mechanical retentive features, pins may be used.
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FEES FOR TOOTH BUILDUP
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Only a few minutes are required to place a bonded compomer filler in a crown preparation. I consider fillers to be included in the crown fee. However, in my opinion, placement of a bonded composite buildup, accompanied by the hazards of placing retentive features and the need for an adequate matrix, should require an appropriate fee, separate from the crown fee.
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SUMMARY AND CONCLUSIONS
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Many factors influence whether teeth planned for crowns or fixed prosthesis abutments need to be built up. Teeth may now be treated more conservatively with minimal fillers because of the greater strength of current cements, including hybrid ionomers and resins. Teeth planned as fixed prosthesis abutments require more remaining tooth structure and greater buildup strength than those designed for single full crowns. It appears that crown buildup is still necessary, but this procedure is needed less frequently than it was in previous years. Fillers can be used to fill holes left when previous restorations were removed, and are more commonly placed than buildups.
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FOOTNOTES
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The views expressed are those of the author and do not necessarily reflect the opinions or official policies of the American Dental Association.
Educational information on topics discussed by Dr. Christensen in this article is available through Practical Clinical Courses and can be obtained by calling 1-800-223-6569.
Dr. Christensen is co-founder and senior consultant of Clinical Research Associates, 3707 N. Canyon Road, Suite No. 7A, Provo, Utah 84604, and is a member of JADA’s editorial board. He has a master’s degree in restorative dentistry and a doctorate in education and psychology. He is board certified in prosthodontics. Address reprint requests to Dr. Christensen.
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REFERENCES
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- Hunsaker KJ, Christensen GJ, Christensen RP, Cao DS, Lewis RG. Retentive characteristics of dental luting agents (abstract 140). AADR 1992;71:123.
- Clinical Research Associates. CRA in-vitro post & core study. Clin Res Assoc Newsletter 1998;22(11):2–3.
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CEMENTS
SINGLE UNITS VS. FIXED...
