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Question: Where and when is it appropriate to place monolithic vs. layered restorations?
Clearly, you should layer with higher-than-average esthetics in the anterior part of the mouth as required. Mono lithic zirconia, while it is getting better, is still lacking as a dedicated monolithic material primarily because of the material’s lack of translucency. 3M EPSE has refined their zirconia material to make the grains significantly smaller. I have seen this material and it is very, very good in terms of improved translucency. It’s not the same as enamel in terms of translucency, but its certainly much closer than we’ve ever come before. I think it will be a highly acceptable material for posterior restorations. One thing we know about teeth is the color that is expressed is because of the various levels of translucencies in a tooth. That’s impossible to create with non-layered restorations.
Monolithic lithium disilicate (e.max®) is also a very good product. It has much higher level of translucency than the current versions of zirconia because the refractive index of the material is less than zirconia and there is a glass matrix along with the high crystalline complex of lithium disilicate involved with this system. Lithium disilicate can look very good with a little paint on the surface in the posterior region. I think that it is the alternative of choice for monolithic in the posterior as long as you can adhesively bond the final restoration. If you’re focusing on the anterior, you can get a decent to good esthetic result with a monolithic restoration that is surface stained, but it still lacks that three-dimensional effect that the layered restoration offers.
Now why would we do monolithic? The obvious answer is that it’s a stronger material. When we layer anything, whether it’s a PFM or zirconia or aluminum, we layer on a much weaker porcelain. The thicker the layering becomes the weaker the system becomes regardless of the core system. So the clear simple answer is layer in the anterior for esthetics and monolithic in the posterior when strength is of primary concern.
Monolithic restorations have been in the literature quite a bit lately and are being promoted by dental laboratories heavily for their strength and reasonable cost, with the most popular being lithium disilicate and zirconia. They are both good restorations, but they should be used in different areas of the mouth for maximum strength and esthetics. In my practice, I use full zirconia restorations sparingly in the posterior region of the mouth for full-coverage molar crowns when the patient does not want a gold restoration and I do not have room for a porcelain-fused-to-metal crown. The esthetics can range from bad to good depending on the laboratory that fabricates the restorations. These restorations can be conventionally cemented, and they are very cost effective. However, they are surface stained so if you need to adjust the occlusion, you may need to reglaze them. The other drawback is if you need to remove the crown or have endodontics done, they are very hard to drill through.
Lithium-disilicate monolithic restorations are more esthetic, but the material is not as strong. They have about 400 MPa of strength and are used in the molar region and some second bicuspids. The material is more translucent than zirconium, but the final esthetics still depends on the laboratory that fabricates them. They also are cost effective and very popular with many laboratories. They are easily fabricated, which makes them affordable. For maximum esthetics the lithium-disilicate material should be cut back and a layer of porcelain fired over the core. This would be necessary in the anterior region
I prefer only layered restorations in the anterior region where strength is not as important and esthetics is. These types of restorations should be bonded in for maximum strength.
Obviously, the primary goal of either monolithic or layered restorations is to reintegrate form, function, and esthetics with minimal damage and maximum longevity to the remaining natural dentition. State-of-the-art technology is currently available in both realms to provide above average to excellent esthetic results. The clinical choice between one or the other may depend on several factors that include how crucial strength and esthetics are when restoring the anterior or the posterior segments. The layering porcelain stacked over the core of all restorations is the weak link that gives “under shear” or flexural loads between 90 MPa and 140 MPa. Because of the high flexural strength (380 MPa to 1,000 MPa), monolithic restorations are quite strong and are ideally indicated for stress-bearing areas; they can be used as a single bulk material without the need for the weaker outer layer of stacked porcelain, especially in the posterior zone or in the form of short-span anterior or posterior bridges.
Optimizing the esthetic outcome with monolithic restorations has been the greatest challenge to date. More recently, however, blocks and ingots that present improved color and optical properties have been introduced, which minimizes the use of surface stains. CAD/CAM technology now allows the milling of blocks that have a dentin-like color bulk (eg, CEREC® Block PC), with more pronounced hue and chroma, topped with an enamel-like, more translucent layer. The milling can be adjusted to achieve the desired color result while final staining is still an option to further customize the restoration through extrinsic characterization. This option comes in handy in the posterior segment where the esthetic challenge is not as high as with anterior restorations. Other CAM/CAM systems are available (eg, Lava™ DVS) that allow the characterization to be applied internally, rendering the restoration more polychromatic and natural looking. Other monolithic systems now present a high-translucency coping material that preclude the use of a veneering layer due to improved optical characteristics (eg, e.max® HT). This restorative option is particularly important in cases of anterior veneers in bruxers or where occlusion poses a challenge. Feldspathic veneers are, however, my primary choice in most cases because I can achieve better and consistent esthetics, with minimum tooth reduction. Following biomimetic restorative principles of tooth preparation and adhesion enhances the strength of these otherwise frail restorations.
About the Authors
Edward A. McLaren, DDS | Dr. McLaren is a professor and the director of the UCLA Center for Esthetic Dentistry in Los Angeles, California, and has a private practice limited to prosthodontics and esthetic dentistry in Los Angeles, California.
Robert Margeas, DDS | Dr. Margeas is an adjunct professor in the department of operative dentistry at the University of Iowa College of Dentistry in Iowa City, Iowa, and has a private practice in Des Moines, Iowa.
Newton Fahl, Jr., DDS, MS | Dr. Fahl holds a private practice dedicated to restorative dentistry and is the director of the Fahl Center, located in Curitiba, Brazil.