Zirconium: Is it everything it’s cracked up to be?  Since entering the dental scene, Zirconium has been accepted by the industry with open arms. It has become a popular alternative to metal ceramic restorations for several reasons. It’s easily milled in the green stage and sintered to clinically acceptable tolerances with the near strength of metal. It is white in color as well as its oxides, rather than grey brown to dark grey of metal ceramics. Zirconium oxide represents a large segment of ceramic restorations now performed. Strong enough for bridgework, zirconium can withstand the oral environment seemingly well.

Zirconium oxide has replaced metal ceramics in many instances; however, it cannot function in all indications required for dental restorative needs. It does not lend itself well for precision attachments nor longer span bridgework. It cannot replace metal completely but does work quite well in many situations. The highly reflective surface makes it undesirable in some aesthetic situations.

It has performed quite well clinically, especially with regard to strength of the frameworks. However, there have been reports of some problems with restorations fracturing whereby the veneering ceramic fractures from the framework. The low frequency of this problem does not warrant discontinuing the use of zirconium frameworks, but does occur often enough that the topic deserves attention.

There are several theories as to why this may be occurring. Most agree that it is not a problem with the coefficient of thermal expansion but may be related to the modulus of elasticity. Another theory is that the surface of the framework should be sandblasted. Others say sandblasting is contraindicated. Yet another possible cause may be the nanodust from the milling which cannot be washed completely away and survives sintering as a non- bonded layer. This layer then becomes a focus of crack propagation or separation of the veneering material. Some insist that the framework design is critical and the porcelain needs to be fully supported, without any sharp line angles. Another theory is that the initial bonding layer of veneering ceramic should be over-fired so it is “sweated” into the surface of the zirconium oxide. Still others insist that porcelains with weaker bond strengths actually perform better clinically. Another theory is that zirconium is an insulator and shields the veneering ceramic from the heat. It dissipates heat faster than the veneering ceramic thereby setting up crack propagation in the process.

Maybe any one of these factors is the problem. Maybe none of them are the reason and the real cause is not understood. The other scenario is a combination of some or all of these factors.

Well designed frameworks that support the marginal ridges and functional cusps should always be employed regardless of the framework materials. It seems it is more important when using zirconium as opposed to metal. Zirconium oxide is less forgiving.

Possible solutions to the problem suggest sandblasting the surface of the sintered framework or steam-blasting the unsintered framework to remove the entire dust residue from milling.
Deliberately over-firing the initial porcelain layer is mentioned by many technicians as another means to minimize veneering failures. Of course, one always needs to adhere to practical occlusal schemes since zirconium restorations are not as forgiving as metal ceramic systems.

The suggestion of using weaker strength porcelain systems has also been mentioned and certainly warrants more investigation. The last alterations to the techniques that I want to mention, which also is very important, are ways to overcome the insulating and cooling properties of zirconium oxide. By slow cooling the last firing cycle of the restoration one can more evenly distribute the heat dissipation of the system and hopefully minimize crack propagation.

I also would plead to the ceramic manufactures that zirconium frameworks actually exceed the clinical strengths required for most indications. Maybe a slight change in the engineering is possible such that we could gain translucency and bond strength at the expense of some framework strength. These problems and possible solutions are just some of the theories available. One needs to decide for themselves as to what works best for them.

Robert Zena, DDS, Prost.