Issues Easily Overlooked in the Preparation of All-Ceramic Crowns

I. Standards for All-Ceramic Crown Preparation:

For anterior teeth: The incisal edge should be prepared to create an angled bevel of 45°
relative to the long axis of the tooth, removing 1.5mm to 2.0mm of tooth structure. The labial surface should have a thickness reduction of 1.2mm to 1.5mm, prepared separately in accordance with the cervical and incisal edges. The lingual surface requires a uniform reduction of 0.5mm (if the lingual surface needs veneering, the preparation amount should be increased). The shoulder width should be 0.6mm.

II. Considerations for All-Ceramic Crown Preparation:

Contraindications for all-ceramic crowns include the absence of a shoulder preparation and a shoulder preparation with an angled surface. If the zirconia restoration is prepared without a shoulder or with an angled shoulder, it may lead to excessively thin margins, increasing the risk of fracture. The complexity of the occlusal surface contour should be simplified: An open angle of 120° to 140° on the occlusal surface ensures precise replication of the inner crown surface during milling, achieving an optimal fit.

Determining the insertion direction: When establishing the insertion direction, all abutment teeth should have a taper of at least 2° to 4° along their respective axes, which is crucial, especially for fixed bridges. During the scanning process, overly steep angled surfaces may be interpreted as vertical, leading to inaccurate representation in the software. The shoulder position should not be prepared too deeply; it is recommended to be 0.3mm subgingival. If too deep, it can negatively impact periodontal health and complicate the removal of excess bonding agent during cementation.

III. Considerations During Impressions:

It is advisable to use stainless steel trays and silicone for impressions. Aluminum or plastic trays are prone to deformation, and alginate impression materials can undergo micro-changes, both of which are unstable factors that can affect the final model quality. Ensure that trays do not have undercuts to allow smooth flow of the impression material. Maintain a uniform thickness of impression material on the buccal and lingual sides to effectively prevent distortion.

IV. Common Issues in Tooth Preparation:

1. **Shoulder with Flash:**

When preparing the shoulder, the diameter of the bur must be larger than the width of the shoulder; otherwise, flash may appear at the edge of the abutment tooth, significantly affecting the marginal integrity of the restoration. Sharp flashes can easily be damaged during the separation of the gypsum model from the impression tray, and technicians may also inadvertently damage the flash while finishing the shoulder, resulting in:

   – A compromised marginal fit of the restoration.

   – The restoration being unable to seat completely, leading to a high occlusion.

2. **Insufficient Tooth Reduction:**

For anterior teeth, the labial surface must be adequately reduced; the incisal edge should form a 45° angle related to the long axis of the tooth. Failure to prepare with this bevel may result in inadequate porcelain space, preventing the restoration from achieving the desired color and leading to an unsupported labial inclination, thus being misaligned with adjacent teeth. Adequate preparation on the labial side is essential for optimal restorative outcomes.

For posterior teeth, the occlusal surface must not be flat after preparation. The angle between the buccal cusp’s lingual slope and the lingual cusp’s buccal slope should be 120° to 140°.

3. **Sharp Margins After Tooth Preparation:**

If the intersection of the incisal edge and the axial angles (point angle and line angle) creates sharp edges, this can lead to:

   – Incomplete seating of the restoration, as these sharp angles become obstruction points. The internal surface of the base crown should be smooth and continuous; sharp angles cannot exist due to material and equipment constraints, with the finest diameter of a zirconia bur being 1.0mm and manual zirconia burring being 0.6mm.

   – If patients receive such a restoration, there may be an increased incidence of porcelain chipping during use due to stress concentration from sharp angles.

4. **Undercuts Present After Tooth Preparation:**

Undercuts may occur on single abutment teeth, between abutment teeth, or between an abutment tooth and an adjacent tooth. If a single abutment tooth exhibits an undercut, the undercut area should be filled with gypsum to create a flush surface for crown fabrication. Although the edges may be fitting on the model, gaps may exist between the crown margin and the abutment in the patient’s mouth.