Chapter 21 Porcelain-Fused-to-Metal and Zirconium Crowns and Bridges
To put things in perspective, this chapter looks first at the development of porcelain-fused-to-metal (PFM) restorations and then introduces zirconium as it was developed. PFM crowns in dentistry have been the mainstay of esthetic restorations since facings were first placed on the facial surfaces of gold crowns. In the 1960s and 1970s dentists did acrylic or ceramic facings in gold restorations as the esthetic alternative to a full-coverage crown for anterior teeth. They were also the mainstay for posterior teeth when crown restorations were needed. When it was discovered that porcelain could be fused to metal through an oxide layer, this offered a significant improvement in the esthetic quality of the restoration that could be delivered. PFM has been actively used as an anterior crown, posterior crown, and bridge restoration material for esthetic dentistry for several decades. About 70% of the laboratory work done for indirect restorative procedures is PFM, so this technique is probably the most widely used esthetic restoration offered. Since computer-generated materials and processes have come into vogue, specifically termed computer-aided design and manufacturing (CAD-CAM), laboratories have been able to mill both ceramic restorations and understructures out of metal and out of zirconium. Zirconium offers a benefit over conventional PFM in that it has a very light tooth like color and is dimensionally stabile during the firing process when the veneering porcelain is added. One of the challenges of PFM is hiding the color of the metal by layering ceramic on top of it. The goal is to produce a naturally colored, translucent tooth. Zirconium offers the dentist more leeway in the ability to create a beautiful, translucent, tooth-like restoration.
Dr Harold M. Shavell has stated that “if it looks good, it functions well; and if it functions well, it looks good.” If this is the case, the requirements of dental restorative materials have been met. The materials must be strong enough to withstand masticatory forces yet still look tooth like and esthetic. The aim of every dentist using indirect restorations is to create a facsimile of natural tooth form while recreating or replacing lost function resulting from dental disease, caries, or extraction.
The advent of dentin bonding and high-strength microcrystalline porcelain has moved dentistry toward the ultimate goal of a long-lasting tooth-colored restorative material that can be micro-mechanically or chemically affixed to tooth structure. This helps create a restoration that resembles natural tooth structure and at the same time has strength and longevity.
The indications for PFM and zirconium overlap, and the requirements for tooth reduction or preparation are quite similar. A full-coverage PFM restoration is indicated when the tooth structure is so badly broken down that it requires a complete crown. These teeth could be endodontically treated teeth with posts and cores or heavily restored teeth that were filled with amalgam or composite restorations but suffered recurrent decay or damage that could not be addressed by direct filling material. Whether the indicated indirect restoration will be an inlay or onlay, which conserves some surrounding healthy tooth structure, or whether it will be a full-coverage restoration, such as a crown, depends on the amount of healthy tooth structure that remains. The clinical indications for these restorations are to restore badly decimated posterior and anterior teeth with full-coverage restoration in an esthetic fashion.
The contraindications deal with the delivery and cementation when there is inadequate tooth structure for resistance and retention form. With conservative clinical preparation, dentin bonding and total etching increase the retention of the restorative materials. It is not possible to etch zirconium or PFM to gain the same micro-mechanical retentive quality seen with an all-ceramic restoration, although there is great promise with the addition of zirconium primers, which are helping to increase the bond strength of zirconium and metal to tooth structure. The author generally draws the line at a preparation height of 3 mm or higher. If the height from the exit of the gingiva to the top of the preparation (axial height) is shorter than 3 mm, it is preferable to use total etch and dentin bonding to gain additional retention and restore the tooth. (The limit is 2.5 mm of axial height remaining.)
A range of materials is available, beginning on the laboratory end at the material from which the metal substructure is made. That metal substructure could be a non-precious metal, semi-precious metal, noble metal, or high noble metal. Higher noble metals with a higher gold content are more expensive, and non-precious metals are less costly. The cost of the fabrication for the PFM restoration is a factor of the metal used and the time ceramists need to build or stack porcelain on top of that metal. Zirconium restorations are more expensive than traditional non-precious or semi-precious metal or PFM restorations because of the CAD-CAM technology and the labor needed for milling. Once the zirconium or PFM understructure has been fabricated, the process of stacking the porcelain is virtually the same. Although many material options are available when ordering the standard PFM restoration, price and the specific laboratory used dictate many of the material choices.
Another PFM substructure with which dentists have considerable clinical experience is Captek (Precious Chemicals Company, Altamonte Springs, Florida). Captek composite metal is a combination of platinum, palladium, and gold. No oxide layer is needed for bonding to block out the metal. Captek offers several advantages over conventional PFM substrates such as a thinner, warmer substructure that has been shown to be more biologically compatible in the gingival sulcus.
In general, with PFM the metal substructure is dark and will not allow light transposition through it. Often it is difficult for the laboratory to create a cervical third of the restoration at a high enough value. When a brighter shade or higher value than A2 is needed, the technician faces the challenge of masking the low value of the metal without having the crown look too opaque or masking the gingival margins when light does not penetrate the metal substructure and illuminate the root. The marginal areas around PFM restorations create dark lines at the gingival area. For crowns in anterior areas and with patients who have high smile lines, this could be an esthetic problem.
Alumina and zirconium both have the advantage of being a substructure that is more tooth colored or lighter than the metal counterparts. Therefore the porcelain layered on top of it has less trouble blocking out the underneath color. These restorations tend to be more translucent and more lifelike and have less of a problem with dark lines at the gingival areas because of the lack of metal in the restoration.
Alumina or zirconium can definitely be a more esthetic restoration than PFM, particularly in the gingival third and marginal area. The margin placement for esthetics is crucial for PFM, and that placement is usually subgingival or intracrevicular because these placements will mask or hide the dark line around the marginal area. This is much less critical with zirconium because there is no dark line. Clinically, the author uses an intracrevicular margin on the facial surface in two situations only:
In other cases an attempt is usually made to end the margins of the restoration either at the level of the gingiva or above. Both of those areas are much more esthetically pleasing with a zirconium than a PFM restoration.
From the scientific standpoint, PFM technology has changed little from the fabrication standpoint. There have been, however, advances in the types of porcelains that are used to cover the substructures. Today’s porcelains shrink less, polish better, are more esthetic, and wear opposing dentition at a lesser rate than the older porcelain materials. Basically a PFM crown is constructed via the lost-wax technique to create the understructure. The dies are coated with wax and the understructure is designed in wax. That wax is invested on a refractory die and burnt out, then the metal is melted and spun via centrifuge into the investment to create the metal understructures or copings. Next the ceramic is stacked onto the coping, then that is placed in the ceramic oven. The heating of the metal creates an oxide layer, which allows the specific ceramic material to adhere to the outer surface. With PFM it is important to remember that the esthetic quality is highly dependent on the proper preparation design. Fit and longevity depend on an excellent impression, as noted in the clinical cases presented later in this chapter.
The creation of Captek involves two materials—Captek P and Captek G. Captek P is a blend of platinum, palladium, and gold suspended in a waxy medium that is press-fit over the die. This wax coping is put in a burn-out oven and the wax is burned away, leaving a honeycomb structure of platinum, palladium, and gold. The second layer, Captek G, is 97% gold suspended in a wax medium; it is press-fit on top of Captek P then placed in a burn-out oven. The 97% gold actually liquefies and fills all the capillary networks created in the original Captek P, forming a composite coping that is about 0.2 mm thick. The average conventional PFM coping is 0.3 to 0.5 mm thick. The newer, thinner Captek castings do not usually require opaquing because of the high value of the surface. Overall these require less tooth reduction, about 0.5 mm, so for younger patients and those who require high-value restorations the Captek PFM may be a good alternative to the conventional PFM.
Other innovative elements in the laboratory involve the use of CAD-CAM technology to mill understructures, frameworks, or complete restorations With chairside CAD-CAM units such as CEREC or E4D, dentists can create inlays, onlays, and single-unit ceramic crowns chairside for the patient. Typically the zirconium CAD-CAM approach from the laboratory is purely to mill the understructures for zirconium crowns and bridges. The laboratory CAD-CAM machine scans the dies; from that scanning the computer designs and mills a coping out of a solid block of zirconium. The technician then places that on the die and builds the ceramic on top of it. These innovative elements in the laboratory in both materials and techniques give the dentist several options in the types of restorations available.
The artistic result of the ceramic restoration, in part, depends on providing enough space via tooth preparation, for the dental technician to create a beautifully esthetic restoration. For a PFM crown, zirconium crown, or bridgework created on either substrate, the technician needs a minimum of 1.0 to 1.5 mm of space for porcelain on top of the coping. This means the tooth in the axial dimension must be reduced at least 1.5 mm. For posterior crowns and bridges the opposing occlusion is also a consideration. There must be enough space or enough of a reduction of the occlusal portion of the preparation to allow for the proper thickness of metal and ceramic or zirconium-ceramic on the occlusal stress-bearing surface. For posterior teeth, long-lasting results require the amount of space to be 1.5 to 2 mm, which will then be taken up by the metal or zirconium coping and porcelain that stacks on top of that. This allows the technician to create all of the elevations and depressions inherent in posterior anatomic form without compromising the thickness of the ceramic underneath that form, which would reduce its strength. Often in failures of PFM crowns in posterior teeth, the ceramic breaks or separates from the metal substructure. Usually this is attributable to occlusal forces and lack of space for the proper thickness of porcelain. The dentist demonstrates artistry in the preparation design, which allows the technician proper space and dimension to create a natural-looking tooth in proper anatomic form.
The other artistic element goes beyond the scope of the dentist. This is the artistic technique needed for a technician to recreate a human-looking tooth. This requires much more than picking two or three shades of porcelain for some patients. It always requires the ceramist’s eye for and knowledge of color—hue, value, and chroma—and his or her ability to mix the powders in the correct proportions to create the desired result. The ceramist must know and picture the end result while building this restoration to contour the various layers one at a time.
When planning treatment for full-coverage restorations, a good portion of the tooth structure is missing, previously filled, or decayed and the decay has been removed. Usually this area has been restored with a foundation restoration, possibly a resin-based buildup material, or, in older cases, amalgam, which uses a buildup material. It may also be an endodontically treated tooth or a post and core structure that was used to reestablish lost tooth form. For full-coverage options in PFM restorations, it is possible to use a wide variety of cements in a conventional cementation technique. In today’s clinical practice of dentistry/>