Today's patients are very much aware of dentistry and its benefits to oral health. In general, it has been my experience that in this COVID pandemic, patients have taken greater interest in their overall health. I have observed that patients are much more engaged in their treatment planning and are choosing to accept elective treatments that will help them maintain health and stability. This greater engagement has led to our patients asking more questions about proposed treatments and taking a more active part in the decisions around the suggested restorative materials and about long-term outcomes.
Dental marketing and social media have provided our patients with awareness and insight into the alternative types of restorations and materials utilized for treatment. Patients have a keen awareness of terms like “bonding,” “implants,” ”veneers” and “crowns” and that they are services that can improve health, replace missing teeth, and alter esthetic appearance. This knowledge creates the desire in patients to have input into the suggested treatments.
Most patients expect that dental crowns will last a long time. They understand that this restorative therapy is a significant investment in their health and welfare. With this perspective, many will desire explanation and insight into which material may be best for their particular situation.
I recently received a question from a patient who desired to know the difference between and benefits of zirconia crowns and PFM crowns, relative to their longevity, repairability, and usage as an implant restoration. As this is a valid, common, and relevant question, I thought it might be important to share my comments to Spear Digest. My hope is that this response provides insight and content that can be applied to future and subsequent patient requests.
When a patient asks: PFM or Zirconia crown?
A patient asked:
“Dr. Bonk, I recently retired and moved from Chicago to San Diego, California. My newly acquired dentist needs to restore a newly placed molar implant for me. He recommended a PFM crown rather than a zirconia crown as the restoration of choice. He said it is a 'better marriage' for the titanium custom abutment and it is more easily repairable should a chip or fracture occur. I have heard, anecdotally, that most dentists restore posterior implant teeth with zirconia. Is this true and what would your recommendation be for restoration of my implant crown? I understand that you teach dentists for continuing education so I thought I would ask you these questions. Thanks in advance for your help.”
Yes, I agree that a PFM (porcelain fused to metal) crown would be a good choice for the implant tooth. As your dentist has indicated, the metal within the PFM crown would block the dark color/shadow (most titanium posts are dark gray in color) that would likely result from using a zirconia crown.
There are a few different types of zirconia used in dentistry. Assuming that 5-Y (translucent zirconia) is used, it is likely that a shadowing coming from within (the metal implant abutment post) would show through no matter how thick the zirconia crown would be. 3-Y zirconia is much denser in color, but would still likely not totally block out the metal post from showing show through.
One alternative to a gray custom titanium post is a “gold-hue” titanium post from a company called Atlantis. This company custom makes titanium implant posts in gray and gold color. The gold surface coating blocks the dark post color and would allow the use of an all-ceramic crown without shadowing. This is an alternative.
PFM vs. Zirconia: Strength and repairability
The truth is that the ceramic on PFM crowns have lower compressive strength to fracture than ceramic crowns (zirconia or Emax Lithium Disilicate). Depending upon the type of zirconia, the compressive strength varies from 600 to 1200 mega pascals (MPs). The compressive strength of PFM ceramic is about 80 MPs. That being said, the PFM ceramic has a greater likelihood of fracture than ceramic under load. All crowns withstand compressive forces well, even with varying compressive strengths. But no ceramic does well with shearing type forces. This is when we see fractures of porcelain in need of repair.
Both PFM crowns and zirconia crowns can be repaired should a minimal fracture occur. In the event of a catastrophic fracture, the crown would need to be remade. If the fracture is significant and not catastrophic, once the crown (PFM or ceramic) has been placed into the oral environment, it becomes predictably impossible to repair the fractured porcelain and re-fire the restoration without it disintegrating with the heat. However, even though ceramic is repairable with composite resin, the bonding strength of resin to ceramic is weaker than bonding resin to natural tooth structure. Therefore, ceramic repairs of any sort have a limited life span. Thus, I would not be picking PFM over zirconia from a comparison of repairability.
I believe most dentists use zirconia as the go-to restoration of posterior crowns whether they are implant crowns or crowns on natural teeth. Their belief is that zirconia is stronger than other ceramics. However, as outlined above, different zirconias have differing strengths.
It turns out that 3-Y zirconia is the strongest, but it is also the most opaque in color or appearance (e.g. Bruxir). It does not look like tooth structure. Sometimes it is even referred to as “white gold” because it stands out as a bright white, reflective restoration. This is much like a gold crown-reflective: opaque and non-tooth-like.
Since this 3-Y zirconia is so poor in color, most dentists defer to 5-Y zirconia (translucent zirconia – e.g. Katana). This is much more tooth-like in appearance. Emax (lithium disilicate) ceramic is the same strength as 5-Y zirconia, has natural tooth color and esthetics, but is not used as often for posterior crowns. The reality is that lithium disilicate actually blocks out underlying colors better than zirconia at 0.8mm thickness.
Zirconia crowns, even at thicker extents, will result in lower value (less brightness) rather than “block-out” of the shadow. The recommended material thickness for posterior ceramic crowns (Lithium disilicate and 5-Y zirconia) is 0.8-1.2mm. Thus, for control of underlying shadows, lithium disilicate would be a better choice than zirconia (5-y) and PFM due to weaker overlay ceramic.
The bottom line is that we, as dentists, must be attuned to the fact that our patients are hearing about dental materials and esthetics in the media. They are interested in health and longevity. From this information, they make decisions on proceeding with dental care based upon how they have interpreted the information. It is up to us to help them understand the options they have available and to aid them in making good dental decisions moving forward. “A crown is a crown is a crown” is no longer true. New materials require improved knowledge and skills around creating predictable outcomes. Our job is to help patients achieve those outcomes through consultation and implementation.
Jeffrey Bonk, D.D.S., is a member of Spear Resident Faculty.