Complete Crown Tooth Preparation Protocol
Understanding Complete Dental Crowns
Dental crowns represent a crucial intervention, often considered a last resort to preserve a compromised tooth. They are particularly useful when a tooth requires significant recontouring to restore its normal anatomy and function. Crowns offer superior retention and resistance compared to other restorative options. A complete crown, also known as a full crown or full coverage casting, encases the entire tooth structure.
Indications for Crown Placement
- When significant tooth reduction is necessary.
Contraindications for Crown Placement
- Crowns are contraindicated in cases where caries have not been adequately controlled.
- Furthermore, this procedure should not be used to prolong the longevity of a crown in an oral environment that has not been thoroughly assessed and managed.
Crown Preparation Steps to Follow
Occlusal Reduction in Layers
Begin occlusal reduction using a round, conical diamond tip (e.g., bur 171). Achieve an occlusal reduction of 1-1.5 mm. This reduction creates an inclined plane, crucial for maintaining the tooth’s natural occlusal morphology.
Orientation Grooves
Create orientation grooves using a conical diamond tip to guide the depth of reduction for triangular ridges and developmental grooves.
- Working cusps: 1.5 mm deep
- Non-working (balancing) cusps: 1.0 mm deep
The depth can be calibrated by the diameter of the bur used for reduction, or by using an enamel chisel (1.0-1.5 mm) for measurement.
Functional Cusp Bevel
Perform this step with the same tapered, round-ended diamond bur used previously. The bevel must be parallel to the opposing tooth’s apex, 1.5 mm deep, and at a 45° angle to the axial wall.
Occlusion Space Check
Check free space occlusion (with the patient’s mouth closed) using a pink ‘utility’ wax.
Buccal and Lingual Axial Reduction
Perform axial reduction using a torpedo-shaped diamond bur. This reduction creates a chamfered finish line. A chamfer is ideal for the gingival finish line as it is distinct and easily captured in an impression. It also ensures adequate metal thickness at the margin.
- Buccal axial reduction should extend as much as possible into the interproximal areas without cutting adjacent teeth.
- Lingual axial reduction should have a less pronounced chamfer if there is a significant lingual inclination of the teeth. It should also be extended as much as possible without affecting adjacent teeth.
From an occlusal view, the interproximal areas should appear intact.
Finishing Axial Reduction
Finish the axial reduction using a short, fine tapered diamond and a fine torpedo-shaped diamond. The short, fine tapered diamond is placed on the buccal surface of the remaining interproximal area, held upright, and moved up and down, aiming towards the lingual aspect with gentle tip pressure. It can be used in particularly narrow areas or horizontally on the marginal ridge.
When sufficient space is available, move the bur from side to side to smooth the surface evenly, avoiding inclination towards the center to prevent an overly tapered preparation. This is performed in both mesial and distal interproximal areas.
The bevel on the proximal surface is created with the torpedo-shaped diamond.
The edges of the preparation, where the proximal and buccal/lingual axial reductions meet, are refined with long, sweeping movements to create a smooth festooned margin. Incorrect contouring can lead to a poorly fitting crown. Ensure sufficient axial reduction in each transition area around an edge, aiming for a smooth and continuous finish line, while avoiding overly conical angles.
Chamfer Finish Refinement
Refine the axial walls with the torpedo bur, also adjusting the corners for a precise and clearly defined finish line.
Retention Groove Placement
Create a retention groove on the axial surface using a bur 171 (e.g., a large, conical, serrated fissure bur). The groove should not exceed the bur’s diameter and should extend over the corner by up to 0.5 mm.
This groove aids in seating the crown during cementation. A second groove can be added in any part of the preparation to enhance crown seating and stability.