What wear issues exist when full-arch implant cases use two opposing materials such as zirconia and denture teeth?

The wear process is more closely related to the roughness of the two opposing materials. The roughness or abrasiveness of a material is determined by how smooth it is. Abrasion is defined as the property of one material to wear away another material by means of frictional contact.³ Where friction exists by mechanical interlocking between two wear bodies, abrasion will occur; the greater the roughness the more wear will take place. A smooth-surfaced material will not lead to excessive antagonist abrasion. Like using coarse and fine sandpaper, both remove the surface of wood but at different rates.

Volumetric loss of material characterizes surface wear and can also contribute to abrasiveness. For example, porcelain is composed of a feldspathic glass and leucite crystal grains. As wear occurs, the exposure of large leucite crystal grains on the surface increases the abrasiveness of the porcelain. Subsurface cracks from repeated load-cycles fatigue form and propagate, surrounding ceramic material, which is eventually lost. Wear will act by fracture and not by plastic deformation, as with metals, when ceramic slides against ceramic or enamel.

Zirconia has a fine and homogeneous microstructure,⁴ which accounts for its high wear resistance. Different zirconia formations have different grain size, which is an important consideration in surface topography. The wear resistance of zirconia is reported to increase by a factor of 8 when the grain size decreases from 1.5 µm to 0.18 µm.⁵

Polished zirconia has shown to compare to gold alloy in enamel loss and be less abrasive than feldspathic and leucite-reinforced porcelains.⁶ However, glazed zirconia has a feldspathic surface that exhibits similar wear as a porcelain restoration.

Occlusal adjustments to any restorations will damage the surface and increase the surface roughness. These grinding procedures can produce a surface roughness that exceeds 1 µm. It has been reported that a patient can distinguish a roughness value of 0.5 μm. Polishing procedures can reduce the average surface roughness to as low as 0.06 µm to 0.2 µm.⁷ Clinically, the wear of zirconia and its antagonist is proportional to the amount of roughness of the occluding zirconia surface.

Therefore, a highly polished zirconia surface will have a minimal wear effect on opposing denture teeth.

The first determination to make is what kind of denture teeth are involved. Clinicians typically use composite denture teeth because, obviously, they can be re-polished and they self-adjust if the occlusion is off slightly. Composite makes sense both structurally and biologically because the denture is going to move somewhat, which is actually desirable if there is a slight amount of hyperocclusion.

Extensive research has thus far shown that good-quality, well-polished zirconia wears favorably.^8,9 This includes the original material, which is the tetragonal crystal form, all the way to the newer, more translucent material that contains a cubic phase. Imagine a large diamond (cubic phase); it is the more lucid material that is well-polished. Zirconia wears very similar to enamel, but as long as it is well polished it does not wear enamel more than enamel wears enamel. Actually, zirconia hardly wears at all.

Having said this, if there are unbalanced occlusal areas either in centric occlusion or in excursive, zirconia will not “self-adjust,” unlike composite, which does tend to self-adjust somewhat. Opposed to natural teeth, for example, there is a potential for zirconia to cause cracking, but, similar to the smooth side of a diamond, it does not seem to cause wear. Today, a much-discussed issue is “smooth” versus “rough.” When adjusting occlusion on a full-contour, full-occlusal zirconia restoration, if clinicians do not perform the adjustment and re-polishing as well as they should, which may often be the case, the result will likely be increased wear.

From both a clinical and biologic perspective, to avoid, for example, an increase of loss of value of bone from resorption from a loose denture moving around, it would probably be advisable to have a high-quality composite opposite zirconia because it would not show all the flaws. I would question the treatment planning if a fixed bone-anchored bridge or implant-supported full-zirconia restoration was used in the upper without using a semi-fixed restoration in the lower, and I would probably recommend zirconia against zirconia. This approach would seem to make the most sense both materially and biologically.

Understanding that the science is clear that zirconia wears the least of any restorative dental material, if a clinician is going to place zirconia against any material, including zirconia itself, he or she will need to be an outstanding “occlusionist.” Clinicians cannot perform an average occlusal adjustment and expect it to wear itself in, as occurred in the past with other materials. We would exhibit composite denture teeth or perhaps gold or something similar, which, though not an ideal choice, had the potential to wear itself in. This simply does not happen with zirconia. The good news, though, is that zirconia is not abrasive to enamel. This seems counterintuitive, as one would think that if the material is not going to wear then it should be abrasive; however, this does not necessarily have to be so. Thus, zirconia has an interesting physical property—again, think of the smooth side of a diamond. You can rub it against something all day long and it won’t cause wear as long as it is smooth.

The question of wear issues when a full-arch implant case features opposing materials such as zirconia and denture teeth is an interesting one, and one that I deal with on a regular basis. In short, yes, there are issues. Although zirconia is less abrasive than other ceramic materials, it is still more abrasive than acrylic or composite. Denture teeth will eventually wear against zirconia. The premise behind using a softer material against an opposing layered fixed case on implants was based on the supposition that layered ceramic has the potential to fracture over time if it opposes a strong or layered material. Incorporating a softer material, such as acrylic or composite, opposite this layered system allowed the softer material to become the weak link so that it could act as a buffer or “shock absorber” for the system.

Today, with the availability of various monolithic zirconias, there is no issue of fracture of the overlay ceramic because all the functioning surfaces are monolithic. Even so, many clinicians still use acrylic or composite denture teeth opposing zirconia because of cost or dogma. Wear will eventually manifest on denture teeth, although this is not the major issue. Of greater concern is the fact that denture teeth tend to loosen and de-bond over time, necessitating a lifetime of recall. Once they have worn to the point of needing to be changed, there is no reference to replace them and they need to be set on the base by hand, one by one. The clinician needs to be able to restore back to the original treatment position.

One way to combat de-bonding of denture teeth from the base is to mill a full arch out of layered polymethyl methacrylate (PMMA) material. The advantages are that the block of teeth is milled in one piece and can even be designed with mechanical extensions that can be used to hold the block in the denture base, negating the denture teeth from popping out. Additionally, a digital file can be used to mill an exact duplicate of the original teeth whenever needed. If a matrix is made of the final prosthesis before delivery, replacing the teeth at some future time is almost effortless. Lastly, the virtual library contains nicer tooth forms than the available denture and, hence, are even more esthetic.

Monolithic zirconia against monolithic zirconia is another option; however, patients often complain of noise when chewing, though many—but not all—adapt to it. If this is a problem, PMMA can be used. The newer zirconia blocks offer much greater esthetics than the original materials, because they are layered with opacity and translucency and can be milled, fired, and stained. They do not need to have layering ceramic, which is much more abrasive than polished monolithic zirconia.

Gregg A. Helvey, DDS, MAGD, CDT
Associate Professor
General Dentistry
Virginia Commonwealth University School of Dentistry
Richmond, Virginia
Private Practice
Middelburg, Virginia

Ed McLaren, DDS, MDC Professor
University of Alabama at Birmingham School of Dentistry
Birmingham, Alabama
Program Director
Advanced Dental Esthetics, Restorative & Biomaterials Program; Founder and Director
Master Ceramist and Digital Dentistry Continuing Education Program
Founder and Director, Esthetic and Restorative Continuing Education Fellowship

Tal Morr, DMD, MSD
Private Practice
Miami, Florida