For a successful restoration, teeth must be properly prepared, often requiring substantial tooth reduction. The crown must be thick enough to hide the metal matrix and the opaque porcelain used to hide the alloy. When preparing teeth, a systematic and organized method helps to ensure that the prepared shape is correct. Tooth preparation includes several different steps, including creating guide grooves to reduce incisors or bites, reducing lip or buccal surfaces, and reducing lingual and proximal surfaces axially. After these selected steps are completed, all prepared surfaces can be completed.
1. Preparing Front Teeth for PFM Crowns
A minimum of 1.5 mm should be reduced from the lip surface of the front teeth for aesthetic purposes. The tongue surface is reduced by 1 mm with a rotating instrument, and the tangential direction is reduced by 2 mm. In order to successfully prepare the labial surface, the placement path along the long axis of the tooth should be parallel to the central groove of the neck. Two more secondary grooves were made on each side. The cutting edges reduce slot placement, these must be about 1.8 mm deep. The depth of these grooves can be verified with a periodontal probe, and then they should extend to half of the lip surface. Second, reduce the edge of the incisor, cut off the proximal contact, while maintaining the enamel of the lips, protecting the adjacent teeth from damage.
Ideally, the incisal margin of the anterior teeth should be reduced by 2 mm, as this will allow sufficient material thickness to enable the potter to create a crown with good incisal transparency. Reduce proximal contact and form a 0.5 mm lingual chamfer. Remove the tooth structure between the deep grooves to form a neck and shoulders about 1 mm wide. It should extend to the proximal hole. The easiest way to shape the tongue of the front teeth is to use diamonds in the shape of a football. Generally speaking, the front teeth have a single guide groove in the centre of the tongue. The preparation is done with a fine sandstone drill. When possible, be sure to consider using a metal collar, as this will help reduce the scope of cervical tooth preparation. This is especially beneficial for teeth with root dentin or receding gums.
2. Preparation of posterior teeth with a porcelain crown
Create a deep occlusal incision on the posterior teeth when preparing them for porcelain crowns. Once these deep cuts are completed, the bite can be reduced and tongue chamfers and cheek shoulders created. The cheek and shoulder preparation should extend at least 1 mm toward the proximal contact tongue. Because it is easier to see, the surgical preparation should be extended slightly proximally rather than distally. The occlusal surface of the posterior teeth generally requires a clearance of 1.5-2mm. If a metal occlusal surface or metal bite stopper is used to make the crown, the bite reduction may be less.
All margins should be obvious and continuous. A rounded edge must be present on all other angles, and the finished preparation must have no obvious drill marks. Areas often missed in the trimming process include the pre-prepared incisal edge and the post-prepared shaft wall to the transition from occlusion. The edges should be finished with diamonds or hand tools.
The design of the shoulder depends on the selected margin. For example, porcelain edges need proper support, preferably at a 90-degree angle. This kind of shoulder strap is also suitable for crowns with traditional metal collars, keeping the collar narrow and unobtrusive. However, the slanted edge or slanted it allows for better support of the porcelain.
3. Errors to avoid in PFM tooth preparation:
- Chisel for lips and shoulders. Be sure to consider your choice of the sword to avoid this mistake.
- Uneven lips on the shoulders. When preparing your teeth, make sure your depth is consistent.
- Some places are over-prepared and some are under-prepared.
- Insufficient proximal clearance between the prepared tooth margin and adjacent teeth. Remember, the technician must be able to cut the mold, so there must be enough space between the prepared tooth and the other teeth.
- If you prefer porcelain edges, be sure to use shoulder preparation
- If the patient has bruxism, use metal occlusion
- If the situation is different or the patient has special conditions, be sure to contact your laboratory. The procedure for preparing the PFM crown may differ, particularly when it is a precision accessory.