ZIRCONIA … making it work in dentistry

To get the best from our zirconia based restorations we first need to know about the material and its manufacture, then how to understand how coping design is vital to support the ceramic layers, and finally we must prepare the zirconia surface to maintain its strength and achieve a satisfactory bond with the porcelain.

Zirconia, the material

Zirconia makes an ideal support for porcelain and has a number of advantages over the traditional metal bonded crown, but what is it …

Zirconium is an element (Zr) in its own right and is obtained from the mineral zircon. Zirconium is highly resistant to corrosion and heat and is often used for surgical appliances, crucibles and furnaces but mainly as a refractory material. It can be cut into gemstones for jewellery, but most of the world’s zircon is used in nuclear reactors.

Zirconia can be found in space vehicle parts for their resistance to heat and also in some abrasives and sandpaper, and of course now in dentistry because of its superb biocompatibility, which is superior to titanium. The mineral form, zircon, can be found in Brazil, India, Russia and the USA, but 80% comes from Australia and South Africa.

Zirconia is the transition metal form and has exceptional toughness and chemical resistance making it ideal for dental restorations. In industry, zirconia is known as ‘ceramic steel’ as it has a similar resistance to fracture.

Strength and biocompatibility but also translucency make zirconia a welcome addition to the range of materials we have at our disposal.

The manufacture of zirconia

Zirconia can be made into many shapes for many uses and depending on that use various grades of the raw powder are selected. For the dental industry it should of course be of a medical grade. Purity, particle size and homogeneity all have a part to play in realizing the best translucent zirconia substructures. If purity is compromised, then inclusions may show up in areas that could affect longevity.

Translucent glass-ceramic crown (left) compared with a zirconium oxide-based restoration (right) with a higher opacity of the framework.

It is vital that the actual laboratory/factory where the powder is pressed is a controlled and clean environment, seems obvious…but not all are the same

Particle size should be below one micron to achieve the translucency we need. Added to the zirconia will be Yttrium for stability and other trace elements, so an even mix of the powder is essential for the best results.

Types of pressing of the zirconia to form the blocks or discs for milling is another important process to be considered. Uniaxial pressing, i.e. pressure is applied in one direction only, produces a variation in density across the block. This is not desirable for consistent results and can cause variations in strength and shrinkage which is particularly unwelcome for bridgework, as the fit will be compromised.

Fortunately, isostatically pressed zirconia is available which guarantees the consistency we need for dental restorations. ‘Isostatic’ means that the powder is formed into shape with an even pressure from all directions. This produces an even density across the block or disc and gives us the qualities we need for successful dentalwork: consistent strength and excellent, reproducible marginal fits.

All zirconia is not the same and compromises made by using lower grade powders, cheaper manufacturing operations and simpler pressing techniques are maybe risks we would not wish to take, as we construct more and more zirconia based work in our laboratories.

So (just to emphasise the point), if you want a more predictable zirconia with a homogenous density and a superior fired density, with even shrinkage then you will be looking for isostatically pressed material. It will save you time, money and heartache in the long run.

Coping Design

We all design our metal frameworks to support the porcelain buildup, or we risk tensile fractures on cooling, or worse still, after fitting. The same risks apply when zirconia is used. lt is generally accepted that we should design our substructures so that no more than 2mm of porcelain is needed to complete the veneering.

Some reports of chipping and fracture are coming in which are being blamed on the zirconia. Research quite clearly shows that a restoration, properly designed and prepared, is at least as resistant to fracture as traditional metal bonded is. So, invest a little time making sure your zirconia crownwork is not subject to stress by designing out excess porcelain build up and remember… 2mm maximum of porcelain veneer!

Surface Preparation

This is vital to avoid stresses and cracks and to achieve a good Zr / Ceramic bond. You may need to adjust the zirconia frame first, so this should be done as follows:

1. Use diamond impregnated wheels at approx 4000 revs/m, or heatless silicon wheels at approx 6000 revs/m, or,diamond impregnated burs at approximately 8,000 revs/m. Do not use a lot of force. Keep the part cool by dipping in water as necessary and be sure not to leave any sharp edges or corners.

It has been shown that there is not a lot to be gained by using water cooled handpieces as only the surface is cooled and the point of contact is also dry, but letting us believe we can grind with abandon, and creating microcracks below the surface.

2. Smooth over the surface to remove debris using stones or ‘specially for zirconia’ mounted points dipped in water and at approximately 8,000 revs/m only

3. Sandblast with new (ie.clean) AlO2, 50 micron grit, not too close to the surface (around 2 cm) and at a pressure of just 2-4 bar to produce a smooth satin finish.

4. Heat treat at 920°C for 2 minutes under vacuum with a temperature rise of 40°c/m. Slow cool to 300°C over 6- 8 minutes.

Do not:

1. Use ultrasonic to clean zirconia. This can cause microcracks and lead to fractures. Cleaning can be done after sandblasting by placing in Chloroform for 15 minutes which completely degreases the frame and also increases electronic activity at the surface, which in turn solidifies certain particles and eliminates others during heat treatment.This increase in the vitreous phase at the surface improves the Zr/porcelain bond.

2. Steam cleaning is also not recommended as it can cause destabilisation at the surface.

Finally, don’t forget to slow cool your porcelain buildup at every furnace cycle, 6-8 minutes down to 300 deg C is ideal. The zirconia cools far slower than the ceramic veneer, particularly when pontics are involved, so rushing work out of the furnace will bring disaster.