Since its emergence into the dental arena, Zirconia has increasingly become the material of choice for clinicians who wish to provide their patients with the most technologically advanced metal-free restorations. Zirconia has improved significantly with the introduction of a wider array of available milling pucks which allow for greater shade variation and translucencies that closely resemble natural dentition. In addition to improved esthetics, the Zirconia physical properties allow for durability, strength, and precision-fitting restorations. To maximize on the success of seating Zirconia restorations, and minimizing chair time, it is essential to ensure that proper preparation guidelines are being followed.
Why Tooth Preparation Design Is Important
Smooth edges result in lower stress on the crown. This lower stress decreases the percentage of fractures occurring.
Ceramic restorations require a passive fit.
Uniform reduction results in ideal ceramic strength.
Sufficient reduction leads to the best esthetic results.
Digital scanners read smoother preparations with more accuracy.
Preparation Guidelines for an Anterior Zirconia Crown
When prepping a tooth for an anterior Zirconia crown, you will need to ensure that there is sufficient room for the wall thickness to have a minimum of 0.3 mm and ideally between 1.0 mm and 1.5 mm, or 1.8 to 2.0 mm incisal reduction. There should be a clearly visible and continuous circumferential chamfer with a reduction of at least 0.5 mm at the gingival margin. The horizontal and vertical preparation of the tooth should have an angle of approximately 5° and a bevel is not advisable. All the incisal edges should be rounded and you want to reduce the linguals of anteriors with football diamond to create a concave lingual.
Preparation Guidelines for a Posterior Zirconia Crown
When prepping a tooth for a posterior Zirconia crown, you will need to ensure that there is sufficient room for the wall thickness to have a minimum of 0.5 mm and ideally between 1 mm and 1.5 mm or 1.5 to 2 mm occlusal reduction. The prep should be tapered between 4°and 8°. It will also need to have a clearly visible and continuous circumferential chamfer and a reduction of at least 0.5 mm is required at the gingival margin. Just as with the preparation for an anterior crown, a bevel is not recommended. Ensure that all occlusal edges should be rounded.
Although the Shoulder and Chamfer preparations are the most ideal, Feather edge preparations are typically not recommended, but can be acceptable for full-Zirconia crowns. Check with your dental laboratory to see if their fabrication process will allow for this form of prep, as different types of Zirconia require different guidelines.
Factors That May Make a Crown Preparation Unacceptable for a Zirconia Restoration
To be acceptable for a Zirconia crown restoration, the preparation should not have any undercuts and it should not have a gutter preparation. Both 90° shoulder and parallel wall preparation are unacceptable. Sharp incisal or occlusal edges are not suitable for a zirconia restoration.
Layered or Monolithic Restorations
For restorations within the esthetic zone, a Zirconia crown layered on the facial, or full layering, will provide optimal results. Over the past few years layering porcelain techniques have improved significantly so a Zirconia substructure that has been layered with porcelain is unlikely to chip or fracture on the occlusal or incisal. However, if maximum strength is required because a patient has bruxism, a heavy bite or where there is only limited occlusal clearance, a monolithic crown may be a better posterior solution. Thanks to newer monolithic zirconia (single, solid blocks of zirconia), this type of crown is nearly unbreakable and built to withstand the harsh environment the back of the mouth endures better than all-porcelain crowns. Monolithic restorations are providing an increasingly aesthetic result with the introduction of High Translucency Zirconia or the DDS Lab product reference, Zirconia HT.
For a full-contour monolithic Zirconia crown, there should be a 1.0 to 1.5 mm occlusal depth cut to achieve appropriate occlusal anatomy. You will need to ensure a 1.0 to 1.5 mm functional cusp tip reduction, a 0.5 mm gingival chamfer reduction, a 6 to 8 degree taper to the axial walls, and a 1.0 mm occlusal 1/3 reduction of the functional cusp. For the marginal design, prepare a 0.3 to 0.5 mm chamger to allow for a more accurate mill of the pre-sintered zirconia.
Cementing and Finishing Zirconia Restorations
Quite often marginal finishing is required when fitting and cementing Zirconia restorations. Excess cement must be removed to avoid plaque formation which can lead to tooth sensitivity and periodontal disease. Gingival margins can be finished using non-cutting, safe-end finishing burs that will protect the soft tissues. Although these adjustments may slightly roughen the surface of a Zirconia restoration, it should be easy to polish, creating an exceptionally smooth surface.
If adjustments are required it is important to use a fine diamond bur suitable for a Zirconia restoration. Too much heat or sparking from an aggressive reduction will lead to micro-fractures in the Zirconia. It is important to use as little pressure as possible to reduce the amount of heat that is produced. Using water while adjusting is also a helpful way to keep the restoration cool.