In this monthly feature, Dr. Gordon Christensen addresses the most frequently asked questions from Dental Economics® readers. If you would like to submit a question to Dr. Christensen, please send an e-mail to [email protected].
I have placed several of the new zirconia-supported, all-ceramic crowns (both Lava and Cercon), and have been unsure about how to provide additional retention on the internal surfaces of the crowns before cementing them. Should I sandblast the internal surfaces, roughen them mechanically, or use hydrofluoric acid on them?
Answer from Dr. Christensen
The all-ceramic zirconia-framework-supported crowns and fixed prostheses are being accepted well by dentists around the world. Currently, Lava™ (3M ESPE) and Cercon® (DENTSPLY) are the most popular brands; however, there are still many aspects of their characteristics that need further investigation. Our research group, Clinical Research Associates, under the direction of Dr. Rella Christensen, is accomplishing a major, long-term clinical study on most of the brand names of zirconia-supported crowns and fixed prostheses. We are learning more about them each day.
The clinical technique for these restorations is quite demanding and specific. These types of restorations require meticulous care when preparing the tooth, making the impression, and seating the restoration. The tooth must be prepared deeply enough to provide about 1.0 to 1.2 mm of reduction at the margin locations and 1.5 to 2.0 mm of reduction on the occlusal surface. This amount of reduction allows about 0.3 mm of zirconia understructure on the facial, lingual, and proximal surfaces and about 0.5 mm of zirconia understructure on the occlusal surface. The preparation walls must be nearly parallel and long enough to retain the restoration without gaining additional retention by roughening the internal areas of the restoration. The margins must be clearly identifiable in the impression and on the die to allow adequate interpretation by the computer designing and milling devices.
Now I will provide you with information concerning your question. If typical sandblasting with an aluminum oxide blaster, such as the MicroEtcher™ (Danville Materials) is accomplished, available research shows that micro cracks may be produced in the zirconium oxide ceramic. This weakens the restoration and potentially compromises its longevity. More research is needed on this subject.
Typical hydrofluoric acid-etching to increase retention is not successful on zirconium oxide.
If the tooth preparation is not as retentive as desired, try the following. Tell the laboratory technician you desire more internal retention on the restoration. Before the restoration goes through its final sintering (firing), the technician can place mechanical scratches on the internal surfaces of the restoration. When the final sintering is accomplished, any micro cracks produced by the technician will be fused (healed) by the melting ceramic.The internal roughness will still be present to enhance retention.
Blood, mucus, and other liquid debris are always present on the internal surfaces of restorations after the trial seating to evaluate fit, color, contacts, and occlusion. Place liquid phosphoric acid on the internal surfaces of restorations to remove the organic debris before seating and wash the internal surfaces well with water to remove the acid. Roughening the internal surfaces of zirconium oxide restorations before seating them is not a good idea because of the potential of producing micro cracks in the zirconium oxide. These restorations may be cemented with any currently popular cement of your choice.
One of our recently completed videos, V19-15 “3M ESPE Lava - The Beautiful PFM Replacement,” shows the clinical placement of Lava restorations from start to finish in high-magnification, close-up detail. For more information, contact Practical Clinical Courses at (800) 223-6569, or visit our Web site at www.pccdental.com.
I am confused about whether I should attempt to polish ceramic or PFM crown surfaces after adjusting them, or should I send them back to the laboratory for another glaze?
Answer from Dr. Christensen ...
There are various opinions on this topic. I will give you my opinions and observations based on the adjustment of many ceramic crowns and watching them serve over several decades. The structure of ceramic restorative materials is somewhat similar to resin-based composites. The high-fusing ceramic particles of which they are composed have varied particle sizes before they are fired to become a crown. Typical dental fired ceramic particles are fused together at a lower temperature than some of the higher fusing particles of ceramic contained in the powder. A flux that usually fuses (melts) at a temperature lower than the higher fusing ceramic particles melts and fuses all of the particles together. Although the ceramic appears to be a homogeneous surface to you and me, it actually varies internally in hardness and texture. The lower fusing flux that holds all of these particles of the ceramic restoration together usually comes to the surface on the final firing of the restoration, creating the shiny, smooth surface that we clinicians see.
When the glaze is removed, the underlying surface can be polished to a high shine. But some types of ceramic surfaces will roughen again after service in the mouth and degeneration or wear of the flux that originally made the surface smooth. When possible, maintaining the glaze is the most desirable technique. When this is not possible due to the need to change contour or occlusal contacts, you have only the following choices:
☛ Polish the surfaces with instruments such as Brasseler Dialite™ wheels followed by ceramic polish such as VH Technologies (888) 628-8300 polish on a brush wheel, or
☛ Send the restoration back to the laboratory for a reglaze, or
☛ Before trying on the restoration, tell the patient that you may want to “smooth” the surfaces of the opposing arch teeth to provide a more acceptable bite. With his or her permission, slightly adjust the tooth occlusal surfaces opposing the new ceramic restoration, thus avoiding cutting the glaze away on the new restorations.
Having done these three options many times, I prefer to have the interocclusal record as accurate as possible to avoid much need for adjustment. I then slightly adjust the opposing teeth, if necessary. Sending the restoration back to the laboratory is time consuming and expensive; however, some dentists have porcelain ovens in their offices that allow them to reglaze the restorations in minimal time. Seating many units of crowns and fixed prostheses usually requires some occlusal adjustment after cementation. This forces you to polish the ceramic surfaces after they have been cemented. In other words, there are many answers to your question depending on the ceramic used, the complexity of the clinical situation, and the availability of ceramic ovens in your facility.
Dr. Christensen is a practicing prosthodontist in Provo, Utah. He is the founder and director of Practical Clinical Courses, an international continuing-education organization for dental professionals initiated in 1981. Dr. Christensen is a co-founder (with his wife, Rella) and senior consultant of Clinical Research Associates which, since 1976, has conducted research in all areas of dentistry and publishes its findings to the dental profession in the well-known “CRA Newsletter.” He is an adjunct professor at Brigham Young University and the University of Utah. Dr. Christensen has educational videos and hands-on courses on the above topics available through Practical Clinical Courses. Call (800) 223-6569 or (801) 226-6569.