Modern Metal-Ceramic Restorations
Edward A. McLaren, DDS, MDC
Metal-ceramic restorations are still the most widely used type of indirect restorative system, and they have been used with great success for nearly 40 years. Porcelain remains the material of choice for esthetic veneering of teeth, metal, or high-strength ceramic copings.1 It has proven to be highly biocompatible, which favors a healthy gingival response. Generally, only adequate esthetics have been achievable with conventional metal-ceramic restorations, primarily because of the opaque dark metal oxide layers created on conventional alloys. The dark oxide can create shadowing in the adjacent soft tissue because of lack of light transmission, and it can corrode and invest the surrounding tissue, creating a tattoo. The abrasiveness of conventional feldspathic metal-ceramic porcelains against the opposing dentition has also been problematic.
The development of new high-gold metal systems provide a more esthetic outcome than conventional castable metals.2 Newer generation porcelains have been developed with improved physical properties and decreased abrasion potential. With these newer metal and ceramic combinations, proper preparation, and metal framework design, it is possible to rival all-ceramic restorations in esthetic appearance.
New Generation of Metal-Ceramic Porcelains
Research has led to the evolution of metal-ceramic materials that can optimize the leucite crystalline phase. Unlike conventional metal-ceramic porcelains with larger multi-sized and irregularly dispersed leucite crystals (Figure 1), the leucite crystals are more evenly dispersed and much smaller. This more homogeneous nature of the crystalline phase not only raises the coefficient of thermal expansion of the material to match that of the alloys used for copings, the absolute tensile stresses between the crystals and the glass matrix are so negligible that no tensile cracks occur (Figure 2). This homogeneous nature to the crystalline phase greatly improves the physical properties of this ceramic class, and flexural strengths twice that of conventional metal-ceramic porcelains have been reported.3 A longtime concern of conventional metal-ceramic porcelains is their abrasion potential, a result of the rather large (30 µm) average particle size of the leucite crystals.4 One of the main benefits of the fine crystalline structure is the decreased potential for abrasion, attributed to a significantly smaller particle size of about 1 µm to 2 µm.4 The size and shape of crystalline particles of leucite on the surface of ceramics appear to be the critical factor for abrasion of the opposing dentition, not the hardness of the material.
Along with the development of an optimized crystalline phase, there has been the concomitant development of lower firing materials, which allows sintering on alloys that maintain a warm gold color. High-gold alloys have the added benefit of having greatly reduced amounts of nonprecious elements that easily oxidize, creating the increased likelihood of corrosion. Corrosion products can invest the surrounding tissues causing local toxic reactions and unsightly discolorations.
Alternate Alloys for Metal-Ceramic Restorations
Because of the demand for esthetics, it is desirable to use metal-ceramic restorations with a high gold content to create a warmer appearance to the final restoration. One recently introduced high-gold alloy system, Captek™ (Precious Chemicals Company, Altamonte Springs, FL), has no nonprecious metals, which eliminates the possibility of toxicity.2 This system uses advanced metallurgic principles of capillary casting to create metal frameworks for the veneering of porcelain. The final framework is a high-gold, oxide-free alloy unique to dentistry for its composition of 2 distinct alloy phases; as a result, they can be fabricated to have better physical properties than single-phase materials.
Clinical Technique
The techniques for using this class of metal-ceramic materials are the same as conventional metal-ceramic systems, which can be a benefit over many of the all-ceramic systems. Teeth can be prepared with any traditional margin design, but for truly esthetic metal-ceramic restorations, a shoulder preparation that allows for the creation of a 1-mm porcelain margin is preferred. Ideally, a minimum of a 270° or 360° shoulder preparation on teeth in the anterior region facilitates optimal esthetics (Figure 3). Facial reduction can be slightly less than conventional metal-ceramics as the granular gold surface gives a light scattering effect that improves the perception of depth. Generally, an overall facial thickness of 1.2 mm to 1.3 mm gives a highly esthetic result. Accepted tissue management and impression making procedures should be followed.
Laboratory Technique
Framework design should allow for maximum thickness of porcelain, within the accepted limits to minimize susceptibility to fracture. Frameworks can be safely thinned after casting to 0.15 mm in esthetic areas for single teeth without an increased potential for ceramic fracture.5 Margin design can be a conventional metal margin (collar), or the metal framework can be cut back to create a porcelain butt margin. Metal-ceramic restorations can rival all-ceramic restorations in esthetics by using a vertically reduced metal framework as developed by Willi Geller. The framework is reduced up the axial wall a minimum of 2 mm (Figure 4); this allows for more translucent porcelains to be used in the marginal area, improving optics in this region. As long as the margin design is a shoulder with a 90° exit angle, this amount of cutback does not affect the strength of the cemented system.6 After opaquing, fluorescent porcelains (VM 13 Effect Liner 2 shade for brighter shades, Vident™, Brea, CA) are built up as a porcelain margin and a thin layer is also placed over the whole Captek framework and subsequently fired over (Figure 5; Figure 6; Figure 7). The authors then use base dentins and opal translucents called Effect Opals with the VM13 using the skeleton build-up technique to complete the restoration (Figure 8 and Figure 9).7 Contouring, staining, and glazing are accomplished by the same techniques as conventional metal-ceramic materials.
Try-in and Cementation
The crowns and/or fixed partial dentures are tried on the teeth to verify complete seating. Fit-checking medium (Fit Checker, GC America, Chicago, IL) can be usedto highlight binding areas, which are then adjusted. Once complete seating is obtained, the teeth are thoroughly cleaned with a slurry of pumice, or an intraoral air-abrasion unit. It is recommended to etch the porcelain margin with a gel hydrofluoric acid-etching material. This will increase the surface area, creating micromechanical retentive areas for better adhesion. Conventional or chemical-cure resin cements can be used depending on clinical requirements. Experience has shown that more opaque cements like zinc phosphate limit the esthetic result with a vertically reduced metal framework. In the critical anterior areas where esthetics is paramount, more translucent cements are preferred. Rely X™ luting cement (3M™ ESPE™, St. Paul, MN), which is in the so-called compomer class of cements, works well in posterior regions because of its ease of use. For maximum translucency the author prefers Panavia 21 TC (Kuraray America, Inc, New York, NY) or Rely X™ Unicem (3M ESPE). Panavia 21 TC, as with all resin cements, is significantly more technique sensitive and requires the use of dentin bonding agents. The patient case in Figure 10; Figure 11; Figure 12 demonstrates that even with a single central it is possible to achieve a highly esthetic restoration with modern metal-ceramic materials.
Summary
Because of the short clinical history and technique difficulty of all-ceramic restorations, metal-ceramic restorations should still be considered the restoration of choice for full-coverage esthetic posterior applications (Figure 13), especially in high stress areas or for bridge applications. The ultimate esthetic dentistry takes into account biologic and long-term functional requirements. Materials and techniques that address these inseparable issues are necessary for true excellence. New metal-ceramic systems with improved optical and physical properties, coupled with esthetic designs for metal framework fabrication, can rival all-ceramic restorations in esthetic appearance. The decreased potential for abrasion and the ability to use long-established clinical and laboratory techniques make these materials an ideal choice for esthetic metal-ceramic indications. One such material has been presented for the esthetic reconstruction of teeth.
References
1. Kelly JR. Dental ceramics: current thinking and trends. Dent Clin North Am. 2004;48(2):513-530.
2. Shoher I, Whiteman A. Captek: a new capillary casting technology for ceramometal restorations. Quintessence Dental Technol. 1995;18: 9-20.
3. Kappert HF. Modern metal ceramic systems. Zahnnarztl Mitt.1996;18:1-8.
4. McLaren EA, Giordano RA, Pober R, Abozenada B. Material testing and layering techniques of a new two phase all glass veneering porcelain for bonded porcelain and high alumina frameworks. Quintessence Dental Technol. 2003;26:69-81.
5. McLean JW, Sced IR. Reinforcement of aluminous dental porcelain crowns using a platinum alloy preformed coping technique. Br Dent J. 1987;163(11): 347-352.
6. Lehner CR, Mannchen R, Scharer P. Variable reduced metal support for collarless metal ceramic crowns: a new model for strength evaluation. Int J Prosthodont. 1995;8(4):337-345.
7. McLaren EA. The skeleton buildup technique: a systematic approach to the three-dimensional control of shade and shape. Pract Periodont Aesthet Dent. 1998;10(5): 587-587.
About the Author
Edward A. McLaren, DDS, MDC
Director Center for Esthetic Dentistry
Founder and Director
Master Dental Ceramist Residency Program
Adjunct Associate Professor
The University of California, Los Angeles
School of Dentistry
Adjunct Assistant Professor
Oregon Health Sciences University
School of Dentistry
Portland, Oregon
Private Practice limited to Prosthodontics and Esthetic Dentistry
Los Angeles, California
