Preparation Design: Beginning with the End in Mind

I wanted to share with you some traditional principles of crown preparation as well as some esthetic and functional techniques that I have found to be useful in my practice.

For all types of crown preparations the key principles still apply including retention form or parallel walls to prevent displacement of the crown along its path of insertion and resistance form to prevent dislodgment of a restoration by oblique forces. An appropriate marginal finish line is important to minimize microleakage and allow accessibility for optimal oral hygiene.  In addition, the marginal shoulder should have sufficient reduction to allow the cervical contour of the crown to follow the emergence profile of the tooth and preserve a healthy periodontium. 

Adequate Reduction

Adequate reduction is one of the most critical aspects of tooth preparation.  Inadequate reduction can result in unesthetic or over contoured crowns that have periodontal and occlusal consequences.  In addition, without appropriate thickness of restorative material, the structural integrity of the crown may be compromised resulting in fracture or perforation of the crown. (Table 1.)  Belle de St Claire occlusal reduction guides are extremely helpful in checking the occlusal/ lingual reduction for single unit crowns, and are available in 1.0mm (pink), 1.5mm (green), and 2.0mm (blue).

By contrast, over preparation can result in pulpal pathology or reduced tooth strength.  Determining the ideal amount of reduction can be difficult, especially in a mouth that has broken down or worn teeth.  Also clinical situations that require multiple crowns or veneers present special challenges in determining proper tooth preparation.  This is where beginning with ‘the end in mind’ is particularly helpful.

Functional Esthetic Wax-up

For all of the anterior or multiple unit crown and bridge cases in our practice, we have a functional esthetic wax-up created as part of our 3-D treatment planning for the case. For multiple unit anterior cases, we invite the patient into the office to have an intraoral mock-up done.

At the mock-up appointment, we use a PVS impression of the wax-up to make an intraoral matrix which is then filled with a bisacryl temporary material.  Without etching or preparing the teeth, we insert the bisacryl filled matrix into the patient’s mouth to re-create the wax up intraorally. By making a preliminary cut in the labial surface of the matrix between the canine and the first premolar, it can be removed easily from the mouth without dislodging the temporary material. (See Pre-op & Mock-up photos 1 & 2) This not only allows the patient to visualize the proposed results, but it also allows the dentist to evaluate them with the 11- step checklist to determine the anterior tooth position and contour from Dr Dawson’s The Concept of Complete Dentistry. 

This valuable opportunity to evaluate issues like phonetics and lip closure path on the patient can help determine if any adjustments need to be made to the wax-up prior to the preparation day.  This practice has become an important step in our communication with the patient about their esthetic desires and more. Afterward the bisacryl is easily removed from the teeth using a scaler and a high-speed section.

Preparation and Provisionalization

On the day of preparation and provisionalization, we often etch the teeth and place a thin layer of an inexpensive unfilled resin on the enamel and re-insert the bisacryl-filled matrix on the teeth.

Then we actually prep right through the provisional material to obtain the ideal reduction depth according to the material of choice (see Table 1).  This is invaluable when it is necessary to open the patient’s vertical dimension to make room for restorative material as in patients with bulimia or severe acid erosion as often times these patients will require little if any occlusal/ lingual reduction.  Prepping through the mock-up material with a bur that permits the proper amount of tooth preparation makes the procedure more accurate and predictable.

The mock-up technique works well for cases in which we are lengthening the teeth or closing spaces but is not practical for a case in which the teeth have been previously restored with over contoured crowns or veneers since it is an additive procedure.  For those cases, a thin plastic essix-type matrix can be made over a solid model duplication of the wax-up.  Holes are then made in the mid-labial and lingual portion of the matrix that can accommodate a periodontal probe to measure the amount of reduction according to the restorative material of choice.  Since the plastic material is also transparent, it can be used to visually inspect the reduction of the incisal edge and other critical areas.

Hopefully these methods will aid in the planning and preparation of future cases and provide some helpful communication tools between the doctor and the patient.