Full Gold Crown Preparation: Mandibular Molar
Instructor: Dr. Richard Stevenson, Director of Stevenson Dental Solutions
Location: San Dimas, California
This document outlines the systematic approach to a full gold crown (FGC) preparation on a mandibular molar, emphasizing the “depth plane” reduction technique over traditional depth grooves to achieve a smoother, more refined result.
1. Armamentarium: The Bur Block
A successful preparation begins with a specialized bur block.
While these burs can be purchased individually, using a pre-assembled block ensures efficiency for various procedures ranging from gold onlays to all-ceramic crowns.
Key Burs Used in this Procedure:
| Bur Reference | Purpose |
|---|---|
| 57 010 | Occlusal reduction (1mm diameter) |
| 56 | Functional cusp beveling |
| 878K 012 | Primary axial reduction |
| 859 010 | Interproximal contact breaking (Thin needle diamond) |
| 8877 009 | Chamfer refinement and axial smoothing |
| 7102 | Occlusal finishing and rounding line angles |
| 44s / 45s Chisel | Hand refinement of margins and sharp edges |
2. Occlusal Reduction
The Depth Plane Technique
Instead of traditional depth grooves—which often leave “bumps” that require extensive smoothing—this technique utilizes depth planes.
- Method: Reduce the tooth by following the natural anatomical planes.
- Measurement: Use the 1mm diameter of the 57 bur as a gauge.
By visualizing the bur against the remaining unprepared tooth structure, you can ensure a consistent 1.5mm of total clearance.
- Angulation: Keep the bur parallel to the natural inclines. Avoid a horizontal orientation, which leads to over-reduction of the lingual cusps.
Sequence of Reduction
- Lingual Cusps: Work from distal-lingual to mesial-lingual, maintaining anatomical morphology.
- B-Plane: Reduce the area between the functional cusp bevel and the non-functional cusp.
- Distal Cusp: Respect the small distal cusp anatomy, even though much of it will be removed during axial reduction.
- Functional Cusp Bevel: Use a 56 bur held parallel to the lingual cusps.
Create a crescent-shaped bevel on the facial side.
Ensure the new cusp tips line up with the adjacent teeth to restore original anatomy.
3. Axial Reduction
Axial reduction is divided into two distinct functional components:
A. Primary Plane (Retention Zone)
- Location: The gingival third (2–3mm band above the margin).
- Execution: Align the bur with the long axis of the tooth.
This area provides the primary retention for the crown.
- Lingual Lean: On mandibular molars, the bur must tip slightly lingually to follow the Curve of Wilson.
B. Secondary Plane (Contour/Aesthetics)
- Location: The occlusal third of the facial wall.
- Purpose: To follow the natural facial contour of the arch.
Without this secondary plane, the laboratory will struggle to create an aesthetic and structurally sound restoration.
4. Interproximal Reduction
Breaking the contact is often the most difficult step.
- Tool: The 859 010 (thin needle diamond).
- Technique: Work “uphill” from the facial or lingual toward the center of the contact.
Do not attempt to pass the entire length of the bur through the contact at once, as this risks nicking adjacent teeth.
- Goal: Simply break the contact and establish a rough “box” or shell
. Refinement of the finish line comes later.
5. Final Refinement and Finishing
Axial Reshaping
Once contacts are broken, return to the 878K 012 bur to unify the axial walls.
This step rounds off the “corners” between the proximal and facial/lingual surfaces, ensuring a uniform taper and a consistent zone of retention.
The True Chamfer
- Bur: 8877 009.
- Definition: A true chamfer is a bevel declining at approximately 60 degrees from the axial wall.
- Application: Use the side of the bur, not the tip, to skid along the finish line.
This creates a smooth, 60-degree declination that provides superior marginal closure.
Occlusal Smoothing and Hand Finishing
- Rounding: Use the 7102 bur to round the occlusal-lingual line angles.
Sharp edges must be eliminated to ensure the crown can be scanned, cast, and fitted properly.
- Hand Instruments: Use the 44s off-angle chisel to remove undermined enamel and refine the chamfer.
This instrument is essential for high-end restorative dentistry.
Conclusion
A professional full gold crown preparation should exhibit:
- 1.5mm uniform occlusal clearance.
- A distinct secondary plane on the facial surface.
- A smooth, 60-degree chamfer finish line located just above the gingival tissue.
- Rounded internal and external line angles to facilitate laboratory fabrication.