MY APPROACH to Using Monolithic Zirconia for Full-Coverage Restorations

 

Monolithic zirconia is a restorative material with increasing popularity due to its excellent mechanical, optical, and biological properties and compatibility with CAD/CAM technology. However, a common concern remains regarding zirconia’s abrasiveness against antagonist teeth.

In my opinion, a well-polished zirconia crown delivered in harmonious occlusion does not result in significantly more tooth wear than does enamel. When using monolithic zirconia for my patients, I manage its wear properties by evaluating the following factors:

• Patient-specific risk factors
• Impression and mounting methods
• Instructions to dental laboratory
• Crown delivery methods
• Follow-up and maintenance

Patient-specific factors

It is important to recognize types of patients who will demonstrate more tooth wear in general. They include those with a low Frankfort-mandibular plane angle, lack of mutually protected occlusion, parafunctional habits, and a highly acidic oral environment from extrinsic acid, such as diet, or intrinsic acid, such as gastroesophageal reflux disease. These patients are more likely to experience more severe abrasive wear from a zirconia crown that is delivered in hyperocclusion. It is important to educate these patients on management of controllable factors first and foremost.

Impression and mounting methods

To minimize the risk of fabricating a zirconia crown with occlusal interferences, an accurately mounted cast is necessary. For patients with a stable, mutually protected occlusion, no facebow transfer is required. In these patients, for a single posterior tooth bounded by a mesial and distal tooth (eg, tooth #4, with #3 and #5 present), my preference is a triple tray impression, utilizing a rigid tray with two-bodied polyvinyl sulfide impression material. If there is no distally bound tooth, or it is an anterior tooth, my preference is to make a full arch impression and obtain an occlusal registration record with a rigid material, such as a bisacryl bite registration material.

For a patient with an unstable occlusion and/or lack mutually protected occlusion, I will obtain a facebow transfer to mount the casts. A facebow transfer will allow more accurate occlusal adjustment of the crown in excursive movements on the articulator.

Instructions to dental laboratory

For posterior teeth, my preference is to use multilayered monolithic 4Y zirconia. It has satisfactory fracture toughness and translucency, even for premolars in the esthetic region. I avoid glazing or external characterization on any occluding surfaces because the glaze and staining will wear away in 6 months and may contribute to more wearing of the antagonist teeth. I will prescribe external characterization on the buccal surface if necessary. For single anterior teeth, I will prescribe porcelain-fused-to-zirconia (PFZ) crowns.

Crown delivery method

Prior to delivery of the zirconia crown, I evaluate its fit and occlusion on stone dies and mounted cast in the maximal intercuspal position (MIP) and excursive movements. When trying the crown in the mouth, I first evaluate the interproximal contact and adjust using fine-grit diamond burs and an electric handpiece with an RPM of 6000–20,000, with copious water and minimal pressure. Following adjustment, these areas should be polished with zirconia polishing kits to minimize interproximal tooth wear from rough zirconia. Next, I evaluate the margin seal via visual and tactile sensations, and, if necessary, use a small amount of Fit Checker to visualize possible interferences on the intaglio aspect of the restoration.

Following, I evaluate the crown occlusion in MIP and eliminate all premature occlusal contact while the patient is in the supine position and in the upright position. I ask the patient to help me by pointing to the teeth that contact first when occluding. The goal is to achieve bilateral even simultaneous occlusal contacts. The articulating mark produced by the functional cusp on the zirconia crown should be slightly lighter than those of the adjacent teeth. Special attention should be given to buccal inclines of the palatal cusp on maxillary crowns and to lingual inclines of the buccal cusps on mandibular crowns. Excessive occlusal contact in these regions will also result in more wear of an antagonist tooth.

Any adjustment made on the crown surface should be polished with a zirconia polishing kit. I prefer using extraoral polishing kits over intraoral kits since they produce a more polished surface. Always follow the manufacturer’s instructions, including recommended RPM and sequence of burs. After cementation of the crown, I verify occlusion again. If adjustments are needed, the intraoral polishing kit can be used for polishing.

Follow-up and maintenance

The most amount of wear of antagonist teeth occurs in the first 6 months after delivery, and most wear is self-limiting as the contact area of the opposing surfaces increases over time. During follow-up visits, I will reevaluate occlusion of my zirconia crowns by examining the central cusp and the inclines as mentioned above, as well as any presence of fatigue wear on the opposing tooth surface. Especially for patients at higher risk as indicated above, I will examine more frequently and thoroughly for any localized occlusal factors that will contribute to more tooth wear.