Zirconia in Dentistry was first used in 2000’s due to its similarities to titanium in terms of biocompatibility and strength. It is used in Orthopedics for making prosthetic joints and first used in Dentistry for making Crowns and bridges. Many think of Zirconia as a non metal but in reality it is a metal oxide. FCZir or Full contour zirconia crowns are highly aesthetic similar to lithium disilicate or Emax crowns. Zirconia is now being used in all cases – Bridges, Single unit crowns, Implant crowns or bridges and even Full mouth prosthesis.
The one major drawback found in Zirconia is the dislodgement of Crowns in just a couple of weeks due to difficulty in getting proper Bonding. For Bonding Ceramic or Metal Ceramic Crowns there is no need to prepare the tooth or Crown surface but in Zirconia to get a proper Bond we need to prepare both the inner surface of the Zirconia Crown and the Tooth surface.
According to some studies it have been found that the use of Primer containing 10-MDP has exponentially increased the bond strength of Zirconia Crown. Certain cements such as SpeedCem plus by Ivoclar has been seen to contain MDP molecules in the self etching cement.
Steps to Bond Zirconia Crowns to Teeth:
We are going to divide the protocol into 2 parts – preparing the Crown and preparing the tooth.
Preparing the Tooth for bonding Zirconia Crown:
Isolate the tooth and prepare it with 50 micron aluminum oxide or pumice using Air Abrasion.
In case Enamel is present, you have to etch it using phosphoric acid for 15-30 seconds and dry it.
Now apply Adhesive on the Dentin surface and spread it evenly using Air leaving it for 15-20 seconds (do not light cure)
Preparing the Zirconia Crown for bonding:
As you will trying the Zirconia crown on the prepared tooth before going in for insertion to check for the fit and finish, it is important to clean the Crown to remove any saliva or contaminants which might interfere in the bonding between the crown and the tooth.
Step 1: Clean the inner surface of the tooth using a solution such as Ivoclean from Ivoclar which removes any saliva, blood etc. To clean with Ivoclean, place the Ivoclean liquid onto an applicator tip and place it on the inner surface of the crown and leave it for 20 seconds. Now rinse and dry it with Air.
You can also clean the Crown using 5% Sodium Hypochlorite which has been found very effective almost similar to using Ivoclean and Ultrasonic Cleaning bath with Alcohol for 5 minutes.
Note: Do not use Phosphoric acid or Hydrofluoric acid to clean Zirconia Crowns.
Step 2: Now a Primer has to be placed on the intaligo or inner surface of the crown. Make sure that you apply the primer only to the inner surface and not cross the borders as cleaning it will be difficult.
Step 3: Now using the three way syringe, using Air evenly distribute the primer onto the Crown.
Step 4: After preparing the Crown Load the Cements – Most commonly used Panavia V by Kurrary or SpeedCem plus by Ivoclar. into the Crown and place it on the tooth.
Note: SpeedCem by Ivoclar or Unicem does not need any adhesive as it is a Self Etching cement.
Step 5: Now you have to Light cure the crown from all angles, all four sides on the Molars or just Facial and Lingual in the Cuspids.
Step 6: Remove any excess Cement from the tooth surface and also from the interdental spaces using a Floss. I usually place Glycerine around the margins which prevents any Oxygen inhibition layer from forming as done with Composite.
Step 7: Now Cure the Crown for 20 seconds at the Line Angles.
When compared to Self Etching cements, Resin bonded cements are more reliable and have the strongest bond strength. So in clinical practice I use Resin Bonded cements to etch Zirconia Crown in teeth with short clinical height while using Self Etching cements in Anterior or teeth with good Crown length.
Now that we have seen how to properly bond Zirconia Crown to the tooth, let us look at what exactly happens during Bonding. Zirconia crown is made up of Zirconium Oxide and the oxide part of the crown is what bonds with the phosphate of the Cements. This is why proper cleaning of the crown before inserting Adhseive is important as, Saliva contains high amounts of Phosphates which if not cleaned properly will bond the Oxide layer and interfere with the proper bonding of Zirconia crown using the Resin or Self Etching Cements.