Evidence Behind Today's Millable Materials
Putting the patient first when selecting restorative products
Rella Christensen, PhD
Efforts have been ongoing for decades to develop a white material that performs and acts like cast gold. TRAC Research has performed long-term clinical trials with more than 200 materials, including polymers, PFM, glass-ceramics, and the many different formulations of zirconia. For the past 13 years, full-contour zirconia restorations have been a primary focus, as they have soared in popularity. There are important differences in clinical performance between different types of zirconia, especially tetragonal vs cubic containing, of which the dental laboratory, dentist, and patient should all be aware.
What are the differences between tetragonal and cubic containing zirconia?
Zirconia was introduced to dentistry as a full-contour restoration in 2009 in the US with a tetragonal formulation. As the clinical evidence of its durability mounted, the competition identified its weak spots: brightness and lack of translucency. A number of cubic containing formulations were developed to overcome these deficiencies, but improved translucence meant the tetragonal strength was reduced. I think the patient needs to know that today they have two choices: (1) maximum durability and longevity, or (2) maximum esthetics. Tetragonal zirconia has fracture toughness and flexural strength that give it maximum longevity and a clinical margin of safety, in clinical unknowns or compromises. However, there is an important indication for cubic containing zirconia in the anterior portions of the oral cavity and for patients who place esthetics above all else, but they need to realize they may have a crown fracture occasionally.
What makes the cubic containing zirconia so beautiful?
The improved translucence relates to alteration of the refractive index of the zirconia. It is important to remember, however, that these various degrees of translucence are in the hands of the laboratory technologist. Properly nesting a design within a transition disc to take advantage of its esthetic characteristics and characterization requires more time and calls for artistic and technical skill. The sintering process also can change the translucence and color, so that must be performed very precisely in terms of both temperature and time. Manufacturers can lose control of what they have formulated if the laboratory is not careful.
What other millable materials are catching your eye right now?
We have continued to study polymers, which have been around since the beginning of CAD/CAM. There are benefits to working with polymers. For example, in some parts of the world, this is the type of restoration that the governments support, rather than ceramics. We have found that polymers are very kind to the opposing dentition, they blend well with surrounding dentition, and sometimes they can even be repaired in the event of, for example, a small chip in a place that is not stress-bearing. However, they do undergo surface wear over time. Like any material, we need to be mindful of occlusion. But polymers have a place, just like glass-ceramics have a place. Glass-ceramics have served very well in the anterior portions of the oral cavity and are definitely still a very viable treatment option.
Key Takeaway
I think we need to put the patient first at all times. If we do that, I do not see how we can miss, but it runs all the way up and down the chain—industry, laboratory technologist, and dentist. We all need to put the patient first to help them make informed decisions about which material to choose for their treatment.
About the Author
Rella Christensen, PhD, leads TRAC Research, which is the human studies section of Clinicians Report, a nonprofit foundation. TRAC Research is dedicated to in-depth and long-term clinical studies of restorative materials, preventive dentistry, and dental caries.
