TheraCal LC Resin-modified Calcium Silicate Liner/Pulp Protectant

September 2017
Volume 13, Issue 9

When the Patient Has a Full-Arch Zirconia Restoration, What Is Your Material of Choice When an Opposing Full-Arch Restoration Is Needed?

Brandon James, DDS | M. Nader Sharifi, DDS, MS | Wael Garine, DDS

Brandon James, DDS: A great question, and one that is certainly pertinent in my practice today. My answer has evolved over the years of restoring these types of cases. When compared with resin-based conventional full-arch restorations, the monolithic restorations are associated with a significant reduction in future maintenance. When we place these restorations in clinical situations that are opposing another full-arch restoration, we have started opposing the zirconia restoration in the maxilla with a resin-based/titanium framework (conventional hybrid) in the mandible. The primary reason for this is that I believe it allows me to better control the anticipated maintenance of the restorations. I also believe, based on empirical clinical findings, that in this way, the mandibular, resin-based restoration acts like a biomechanic “shock absorber.” It will dissipate forces at the occlusal level, which will show up clinically as wear of the mandibular teeth, thus reducing the biomechanical force at the implant/bone interface. During treatment planning, it is important to explain this phenomenon to patients up front. They need to understand that we are planning their cases with these types of materials because we believe that we can better control the outcome from a long-term maintenance standpoint, and that we anticipate that it will keep them out of the dental chair in the future. Part of this understanding is that the mandibular teeth are expected to wear at a much higher rate than those of the maxilla and will require tooth replacement (ie, placing new denture teeth on the existing titanium framework) approximately every 3 to 5 years with virtually no or very limited maintenance required on the zirconia restoration.

M. NADER Sharifi, DDS, MS: The short answer is to use a similar material in the opposing arch. It has long been accepted in dentistry that opposing materials should exhibit similar characteristics of wear. However, zirconia is such a strong material that opposing surfaces of it potentially eliminates a restorative weak link by placing more stress on the biologic system. Combining that with the lack of proprioception and increased bite force raises concerns that bone loss and implant failure would manifest in lieu of the wear and tear that’s been reported with other materials.

The alternative solution is to use an opposing material that is not as strong as zirconia, essentially inserting a planned failure point. For example, an acrylic reconstruction could be used for the opposing arch. Of course, this may raise concerns that the differences between zirconia and acrylic are so great that the weak link may “fail” too quickly to be considered reasonable.

In a study published in The International Journal of Prosthodontics in 2003, Parker and colleagues found that problems and complications with opposing implant-supported, fixed bridges made of acrylic increased three fold when compared with an acrylic implant-supported, fixed bridge that opposed the natural dentition. The loss of proprioception is often cited, but never fully understood in this process of protection. In addition, in a study published in the Journal of Prosthodontics in 2012, Malo reported that opposing another implant-supported, fixed prosthesis was a key risk factor for denture tooth fracture. These findings lend support to the concept that the use of an acrylic restoration opposing one of zirconia at least results in a known failure point that can be managed proactively over time.

Conversely, a recent report by Rojas in the Journal of Prosthodontics points to a lack of bone loss or implant failure with opposing zirconia implant-supported, fixed bridges.

Therefore, comprehensive and personalized solutions are the answer to the question of what material should be used in a full-arch fixed implant restoration that opposes a full-arch, fixed implant zirconia restoration. There are indications for the use of an acrylic opposing restoration and the creation of a “weak link.” When the proper protocols of polishing zirconia before glazing are followed, this combination will produce less functional noise, provide excellent function, and possibly limit the wear to no greater than if the acrylic was opposing natural teeth. However, there are some patients who will desire the functional stability and longevity of the opposing zirconia option despite the contact noise. Patient selection will be key for each of these viable treatment solutions.

Wael Garine, DDS: Material choice for a full-arch, implant-supported restoration opposing a full-arch, implant-supported zirconia restoration is controversial. Typically, a similar material is chosen for the opposing restoration. However, zirconia presents a challenge due to the uncertainty of the long-term biomechanical effect on the osseointegrated implant and the surrounding environment, the difficulty of adjusting occlusion and polishing it intraorally without creating a rough abrasive surface, and the objectionable clicking sound that the restorations present while in function.

One of the most common choices is to use a screw-retained acrylic hybrid restoration in the opposing arch, usually the mandible. The rationale is that acrylic hybrid restorations are better tested for long-term use on implants, are easier to adjust for occlusion and polish, and don’t create loud sounds while in function.

Some of the drawbacks of using acrylic include the well-reported high rate of delamination and/or acrylic fracture, the low wear resistance, and the difficulty of matching the shade between the denture teeth and zirconia, which may be objectionable to some patients.

Currently, my first choice to restore a full-arch, implant-supported restoration opposing zirconia is the use of lithium disilicate restorations supported by a high performance polymer framework (Pekkton by Cendres Metaux). The lithium disilicate crowns are milled or pressed, then bonded to a milled or pressed Pekkton framework to make a screw-retained prosthesis.

The Pekkton framework offers multiple benefits over other materials. It provides an excellent fit, is biocompatible, has been used in orthopedics for years, and provides a shock-absorbing effect due to the similarity of its properties with surrounding biologic materials. Lithium disilicate restorations provide excellent esthetics, high strength, and good wear resistance. The lithium disilicate/Pekkton prosthesis can be fabricated using a digital workflow or by using the burnout denture tooth technique. I believe that, when opposing zirconia, this restoration provides an excellent biomechanical and cost effective choice for a full-arch, implant-supported restoration. However, more studies are needed to determine the long-term effects of these different materials on the surrounding environment and their prosthetic longevity.

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