September 2009, Volume 5, Issue 8
Published by AEGIS Communications
In-office Provisional Restorative Materials for Fixed Prosthodontics: Part 2—Preformed Crown Forms
By Howard E. Strassler, DMD
Interim restorations are a critical component of fixed prosthodontic treatment, biologically and biomechanically.1 These restorations are also referred to as provisional restorations or temporary restorations. Part 1 of this series described the important functions of provisional restorations when used in fixed prosthodontics, and focused on polymeric resin provisional materials. This article will provide insight for the clinician in the evaluation and choice of preformed single-unit crown forms.
For the single crown, a clinician has many choices when deciding what type of provisional crown they want to fabricate. In Part 1, custom-made single crowns were described using acrylic and composite resins. Also, the use of a preformed composite crown form, Protemp Crown (3M ESPE, St. Paul, MN) was described. Independent of the restorative material, the final crown is fabricated from all-ceramic, porcelain-metal, or all-metal, and there are requirements for the provisional restoration to ensure that the final crown, when returned from the laboratory, has physiologic proximal contact and occlusal contacts when necessary. It is critical that the provisional crown used be well-made, smooth, and well-adapted so that it will be retained between visits while the definitive crown is being fabricated. When in place, the provisional single crown must provide the tooth preparation with pulpal protection between visits as well as ensure gingival health.2-5
Provisional restorations must be easy to remove without damaging the existing tooth preparation. Other important purposes for the provisional restoration include the maintenance of the tooth preparation position both occlusally and proximally.7-10 The provisional restoration also provides for positional stability of the tooth preparation while the definitive restoration is being fabricated.
The fabrication of temporary restorations for a single crown requires proficiency with a variety of materials and techniques that can be used to make well-adapted and functional provisionals. There are many choices to temporize a single crown with a preformed crown (Table 1). These choices include prefabricated metal crowns, polycarbonate crowns, celluloid crowns, and composite resin crowns. In Part 1 of this series, the uses of acrylic resin for custom provisionals, bis-acryl or bis-GMA automix composite resin materials, and composite resin for custom fabrication were described.2,8-12
Preformed Metal, Polycarbonate, and Acrylic Crowns
Preformed stock crowns refer to interim restorationsthat are commercially available through avariety of manufacturers as kits. These kitstypically contain either anterior-premolarshells, posterior crown shells, or both. Thesizes and shapes have been determined by themanufacturers to fulfill many clinicalsituations in regard to tooth dimensions including buccal-lingual, mesial-distal, andocclusal- (or incisal-) gingival. Thesestock crowns are typically ill-fitting at the gingival margins withoutrelining with a resin restorative material andwill be responsible for adverse periodontalreactions. Preformed crowns are generally usedfor single-toothrestorations, many times in an emergency situation where there has been no planning for use of acustom-fabricated, resin-based restoration.
Metal Preformed Crowns
Metal preformed crowns can be made from aluminum, stainless steel,nickel-chromium, and tin-silver/tin-bismuth. These are available in a varietyof sizes and shapes for premolars and molars. Theyusually require trimming of the height of thecrown with scissors and further gingivalshaping by bending, and the occlusion willusually need to be adjusted by bending the crown when the patient occludes on it with thesofter metals (aluminum and Iso-Form withtin-silver/tin-bismuth) and by grinding with the harder metals (stainlesssteel and nickel-chromium). This author recommends that whenever a preformed metal crown form is used, it be trimmed short of the gingival margin, air-abraded internally, and relined with polymethyl methacrylate (PMMA) acrylic resin to improveadaptation to the crown preparation and gingival margin.
Aluminum shell crowns are available in two forms: a flat-topped, cylindrical shell and an anatomic, contoured shell. The anatomic aluminum shell is available in sizes with widths for most premolars and molars. If the crown is adjusted, the patient may complain of an aluminum “taste” to the crown.
Stainless-steel crowns were originally introduced to restore extensively decayed deciduous teeth and permanent molars for the pediatric and adolescent patient.13 Stainless-steel alloys for these preformed crowns is sufficiently ductile to allow for contouring with pliers and burnishing so that a reasonable adaptation to the gingival margin that will maintain its shape is achieved. Stainless-steel crowns are the most abrasion-resistant of all preformed metal crowns, and because of their durability as well as resistance to tarnishing and corrosion, they can be used as long-term interim restorations.11
Nickel-chromium crowns (Ni-Chro crown, 3M ESPE) can be differentiated from stainless-steel crowns because of their low iron content. The crowns can be shaped intraorally similar to that procedure used for a stainless-steel crown. The Ni-Chro crown has better surface hardness and smoothness than a stainless-steel crown.
Although this article is focused on the use of preformed crowns as an interim restoration, both stainless-steel and Ni-Chro crowns have a significant use as a long-term provisional crown in molars for patients who cannot afford laboratory-fabricated restorations. In this author’s experience, the use of a well-fitted stainless-steel crown cemented with an adhesive resin technique with a dual- or self-cure composite core material provides a patient with more than 5 to 10 years of service.
Another preformed metal crown that is available in a variety of shapes and sizes for posterior teeth is the Iso-Form crown (3M ESPE) made from tin-silver and tin-bismuth. These crowns are structurally different from stainless steel and nickel-chromium crowns in that they are fabricated for reinforcement by thickening at the occlusal surfaces and rounding at the cervical-most extent of the crowns.11
Plastic, Celluloid Preformed Crowns
Plastic and clear crown forms offer the advantage over metal preformed shells because of the ability to provide the patient with a tooth-colored provisional crown. They can more easily be adapted at the gingival margin of atooth preparation and when relined withacrylic resins,they can be contoured and shaped to achieve an improved fit.
Plastic Preformed Crowns
Plastic crowns can be made from either polycarbonate or polymethylmethacrylate. They are provided in kits for both anterior and posterior interim restorations (Figure 1A and Figure 1B). These crowns are preformed plastic reinforced with glass fibers. The size of the crown is selected by tooth type; posterior crown forms use a guide to be placed in themesial-distal dimension of the tooth that theprovisional crown will be fabricated for (Figure 2), or anterior crowns canbe sized using a crown form guide where the crowns are connected with aplastic connector and sized intraorally (Figure 3). Once sized and adjusted with an acrylic bur or abrasive disk, the crown can be relined with an acrylic resin. Both polycarbonate and PMMA crowns are chemicallycompatible with both PMMA and polyethyl methacrylate (PEMA) when theseresins are used to reline the crown forms. Thesecrown types are easy to reline with acrylicresin and shape in a timely fashion (Figure 4A and Figure 4B).9,10,13,14
Celloid Crown Forms
Clear celluloid crown forms are made from a thin shell of cellulose acetate. They are generally available foranterior teeth (permanent and deciduous) and premolars and molars depending on the manufacturer. The crown form is meant tobe used as a carrier for provisional resins when fabricating an interim restoration. When using these clear crown forms (eg, Odus Pella Crown Forms, E.C. Moore, Dearborn, MI; Strip CrownForms, 3M ESPE; Crown Forms, DENTSPLY Caulk, Milford, DE) they should be trimmed with a scissor to fitthe tooth preparation. It is advisable to place a hole using an explorer tip to avoid trapping air bubbles when placing the resin into the crown form. One disadvantage of celluloidcrown forms is that many times after removing the crown form, proximal contacts must be addedback using the acrylic resin.
While custom-fabricated provisional crowns and fixed partial dentures provide an esthetic and functional preview of the definitive restoration being fabricated, there are times when a preformed crown will more than fulfill the needs while waiting for a crown to return from the laboratory. These single-unit, preformed crowns provide an important diagnostic function while in place, as well as being critical in maintaining positional stability of the crown preparation. There are many choices of preformed crowns. It is important that the clinician make a selection based on the clinical needs of each patient. As part of these considerations, the clinician must understand the physical properties, handling characteristics, patient response to the appearance of the interim restoration, durability of the restoration, and the material cost. There is no doubt that no one material meets all of the requirements for provisional restorations. Selection of single-unit preformed crowns should be made on a case-by-case basis.
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About the Author
Howard E. Strassler, DMD
Professor and Director of Operative Dentistry
Department of Endodontics, Prosthodontics, and Operative Dentistry
University of Maryland Dental School