An Interview with Rella Christensen, PhD
Rella Christensen, PhD, team leader of the nonprofit Technologies in Restoratives and Caries Research (TRAC Research), says new indirect restorative materials are exciting but need clinical studies to confirm performance.
Inside Dental Technology (IDT): What made you decide to make long-term clinical studies of restorative materials such an important part of your work?
Rella Christensen (RC): Approximately 40 years ago, when we first started CRA (now called Clinicians Report), my husband, Gordon, asked if I would look around the world and try to find something that performed like cast gold (good fit, durable, kind to opposing dentition and the gingival tissue) but was white. We have now conducted controlled clinical trials on more than 131 white materials produced all over the world, including polymers, ceramics, and some very unique materials. After 5 years of clinical evaluation, I believe we can say that BruxZir zirconia and IPS e.max lithium disilicate are beginning to look like they may fulfill Gordon’s request. This month, we start a controlled clinical evaluation of the new translucent zirconias, which have new formulations, new molecular structures, and physical properties different from each other and different from the original zirconias used for substructures since 2000 and later in BruxZir in 2009. There is now an aggressive race building among competing companies selling the new translucent zirconias. Brand names in our new controlled clinical study we just started include 5 zirconia formulations: BruxZir Anterior (Glidewell), cubeX2 (DAL DT Technologies), Katana STML (Kuraray Noritake), Pavati Z40.1 (CAD/CAM Research Institute), and Zenostar LT (Ivoclar Vivadent).
IDT: How would you evaluate PFM versus zirconia at this point in time?
RC: There is still a place for PFM for very long spans, heavy bruxing/clenching patients, and when precision attachments are planned. However, a 10-year study we completed this year that compared PFM to zirconia and included more than 900 units of 3-unit posterior prostheses shows that zirconia is up to the challenge of replacing metals in most other situations. For single-unit monolithic restorations, we will continue to recall our cases of BruxZir and e.max CAD posterior full-contour crowns indefinitely into the future because these materials have now become study controls for monolithic crowns. Initially, the new translucent zirconias were all laboratory-fabricated materials. However, this will change this month with the introduction of Pavati Z40.1 from CAD/CAM Research Institute (CCRI), which will sell blocks for in-office milling. Sirona will sell a new furnace called the inFire HTC superspeed high-temperature furnace, which will be capable of post-mill sintering Pavati in 10 minutes. Sirona will sell a similar ceramic branded as inLab to dental laboratories in puck form. The fast-moving developments with zirconia today are tending to eclipse the PFM market, whether or not it is warranted.
IDT: Under what circumstances should technicians and dentists perhaps be more careful regarding the use of new materials?
RC: It is crucial to simply realize that the materials are new, and not get carried away in their use before objective, comparative clinical data become available. Generally that takes several years. We begin reporting on progress of our studies after 1 year of clinical service. Dentists and laboratories can encounter problems with new materials if they later prove not to work as expected. When studying new ceramic materials, I really cannot overemphasize the importance of fracture-toughness data. Advertisements generally report flexural strength numbers, but fracture toughness is a physical property we need to pay more attention to because it is particularly relevant to ceramic clinical longevity.
IDT: With the sheer amount of products on the market, do you see dental professionals making the correct materials choices?
RC: It is very difficult for both clinicians and laboratory technicians to know what is fact and what is marketing hype. Private labeling presents a critical challenge. Many different brands are actually the same materials sold under different names. It becomes almost impossible to know which products are the same and which are different. Generally, private labeling is highly protected information because different people often charge differently for the same material. If dentists and laboratories were aware that materials were identical, they would tend to purchase the product with the least cost. The best interest of the patient would be served if all concerned were able to identify which brands are the same formulation and which are different. Both clinicians and laboratories want to use the best materials in terms of durability, esthetics, handling, etc. We have advised dentists to communicate with their laboratories to identify the name of the company that formulates the zirconia used in their restorations since different zirconia formulations differ in particle size, particle size distribution, purity, porosity, additives, production methods, etc. These differences can influence clinical performance, patient hypersensitivity, material durability, etc.