Audio Recording - Working Safely With Zirconia
Speaker: Dave Dalgleish, RDT - Director - The ADAM Centre
Venue: A.D.T (Academy of Dental Technology)
Date: Winter 2010
Format: Table clinic
Time: 10 minute Recording

Dave Dalgleish has over 25 years experience as a top dental ceramist and is the Technical Director of The ADAM Centre.

Dave has provided hundreds of presentations on dental ceramics over the years and is a recognized expert in the profession.
In this recording, Dave answers a few of the most important questions that laboratories have relating to the safely working with zirconia.

Listen in on the time-limited table-clinic format as Dave answers the following questions and issues dealing with the safe handling of zirconia.
There is a little background noise, but the information available here is valuable.

• Safe trimming and adjustments - what to do and what to avoid.
• Sandblasting - Why it's important. How and when to do it safely.
• Porcelain application - how to guarantee a complete and solid bond.
• Furnace - Special heating and cooling requirements.
• Clinical Adjustments - can dentists make internal adjustments?
• Clinical Adjustments - can dentists make occlusal zirconia adjustments?
• Reglazing or polishing - after adjustments?

How to safely work with Zirconia - 
Zeno-Zr02 / Zirox - 2010

1. Contra-indications
2. Firing Recommendations
3. Design of the Substructure and the Veneering Porcelain
4. Technical Adjustments and Sandblasting before veneering
5. Dentist – Adjustments and Cementation - recommendations
6. Analysis of Chipped Restorations – Detected Causes

Contra-indications

• Veneering of substructures not within the CTE range.
• Improper preparations (Dentist: always follow prep-guidelines!)
• Lack of veneering space
• Patients with parafunctions (e.g. bruxism)
• Temporary seating of restoration
• Heavy adjustment of fit with rotary instruments at high pressure and high speed without water cooling by dentist or dental technician.
• Glaze-bake is always necessary after adjustment of occlusion.

Firing recommendation I

Note! Never underfire the veneering-ceramic (overfiring does not harm the veneering-ceramic).
Note! Zirconium oxide is a poor heat conductor. Never use a heating rate of more than 45°C/min for veneering.
Note! Avoid placing too many objects on the firing tray, since they absorb too much energy and might cause underfiring.
Note! With large bridge substructures, massive pontics or many small units raise the first and second dentin bake firing temperatures.

Firing Recommendation II

• Rule of Thumb: The final temperature for each additional unit (crown, pontic etc.) should be raised by 2°C each, however the maximum temperature increase should not exceed 15°C.
• example 1st dentin bake 6-unit bridge: 5 additional units -> 900 °C + (5 x 2°C) -* firing temperature = 910 °C
• Pre-drying and oven closing time may also be extended with large bridge substructures, massive pontics or many small units.

Firing Recommendation III

• Note! Using massive firing trays will cause excessive heat absorption and will influence the firing results negatively. Therefore honeycomb firing trays are strongly recommended. If a massive firing tray is used, the firing temperature must be raised approximately 10°C.
• The same is true when using massive firing pins. We recommend thin wire pins.
• Note! Never place objects directly in the middle of the firing tray.
  Use area indicated in the illustration below.

placement of ziconia on firing tray

Firing Recommendation IV

• Note! In order to reduce thermal tension, a slow opening phase of the oven is advised.
• Note! After opening the oven leave veneered zirconium oxide frameworks on the firing tray to allow slow cooling-down to room temperature. This procedure will reduce unwanted thermal tensions.

Firing Recommendation V

• Use of Calibration-Kit is recommended! It contains sample former plus a master-pellet (Zirox) and instructions.
  Call Dave if you would like additional information.

Design of the Substructure and the Veneering Porcelain

• The substructure must be designed in anatomical form and support the veneering porcelain in the incisal and occlusal area.
• The minimum dimension of the substructure must be at least 0.6 mm.
• Excessive loss of substance must be compensated by the framework and should never be buffered by using veneering porcelain.

Technician: how to handle the Zr02-substructure (before veneering)

• Frameworks of zirconium oxide should never be worked on with rotary instruments by using high pressures or without using water cooling. This is also true for adjustments made by the dentist.
• Always ensure that after grinding the dimensions of the substructure will stay within the limits.
• To avoid microcracks, in general all ceramic substructures should be ground with proper burs and with low pressure.
• Never treat the sintered bridge framework with separating disks. Otherwise the strength of the restoration will be decreased.

Technician: How to handle the Zr02-substructure (before veneering)

• After adjusting (before application of veneering-ceramic), the framework is recommended to be sandblasted with 110 µm aluminium oxide at a pressure of approx. 1.0 bar and then cleaned under rinsing water (also compare: Yang et al., J Dent Res 2007; 86 (8):749-753 (cleaning by sandblasting))
• Check for cracks or defects before veneering (intensive light source can be helpful - e.g. Zeno Light). Faulty bridgeworks must never be veneered ! (will always cause cracks + chipping of veneering ceramic).

Dentist: How to handle veneered Zr02-restorations

• Never adjust the fit of zirconium oxide frameworks with rotary instruments by using high pressures or without using water cooling. If the Zr02-framework is damaged by cracks it can not be repaired and provokes chipping of the veneering ceramic.
• After massive adjustment of the occlusion, the restoration must be sent back to the dental technician for a final glaze-bake (damaged parts of the veneering-ceramic can thus be healed)
• No temporary seating recommended!

WDC: Analysis of Chipped Restorations - Detected causes:

• Underfiring of veneering-ceramic
• Grinding on veneering-ceramic by dentist without final glaze-bake
• Veneering-dimension of Zirox too thin (min. requirement see working-instruction)
• Sandblasting with 110 µm Al2O increases fracture strength of the framework
• Bond strength of Zeno-Zirox is not affected by sandblasting. (values are high in both cases)
• After simulated grinding of Zirox (by dentist), the fracture strength drops by approx. 50 % and can not be healed by polishing. Only a final glaze-bake can restore the original strength and is therefore highly recommended !

Conclusion: Clinical situation based on limited data

• All current veneering-systems for Zr02 on the market deal with similar chipping-problems, more or less independent of physical properties (CTE, leucite-containing or non-leucite-containing microstructures, fracture strength etc..)
• Under identical circumstances (correct firing-level, identical framework design etc..) Zirox displays the highest possible safety when compared to competitors (highest fracture strength, fracture toughness, fracture forces etc..)
• Disadvantage of Leucite-containing ceramics: Lower fracture strength and CTE-drift during multiple firing and upon slow opening of the oven.