PFM Preparation for Premolars: The “T-Prep” Technique
Introduction
This document outlines a specialized method for Porcelain-Fused-to-Metal (PFM) crown preparation on a maxillary first premolar (Tooth #4). Developed by Dr. Richard Stevenson of Stevenson Dental Solutions, the T-Prep Technique borrows from carpentry principles. It utilizes a “tenon”—a small, central strip of unprepared tooth structure—to serve as a built-in reference point for measuring reduction depth without relying solely on external guides.
The T-Prep Concept
The “T” in T-Prep stands for tenon
{{FRAME:00:00:40}} . By leaving a vertical strip of tooth structure untouched in the center of the facial and occlusal surfaces during the initial reduction, the clinician can:
- Accurately measure the depth of tooth structure removed.
- Visualize the angulation of the bur relative to the original tooth anatomy.
- Identify areas of under-reduction (A, B, and C planes)
{{FRAME:00:00:53}} more effectively than with traditional depth cuts.
Step-by-Step Preparation Procedure
1. Occlusal Reduction (C-Plane)
- Instrument: 847KR-016 (Coarse, Tapered Flat-End Cylinder)
{{FRAME:00:01:10}} .
- Technique: Hold the bur parallel to the triangular ridge. Perform plane reduction while leaving a central “tenon” strip
{{CLIP:00:01:20-00:01:32}} .
- Measurement: Aim for a reduction range of 1.5 mm to 2.0 mm. Use the RGS4 (1.5 mm) gauge to verify depth against the tenon
{{FRAME:00:01:32}} .
2. Functional Cusp Bevel (A-Plane)
- Technique: Maintain the A-plane parallel to the C-plane
{{CLIP:00:02:42-00:02:55}} .
- Benefit: The tenon provides a sagittal reference similar to a preoperative putty guide, making it easy to see exactly how much structure has been removed
{{FRAME:00:03:02}} .
3. Facial Reduction (B-Plane)
- Technique: Angle the bur to align with the cusp tips of adjacent unprepared teeth
{{CLIP:00:03:54-00:04:07}} .
- Gingival Extension: Extend the T-prep technique down to the gingival margin
{{CLIP:00:05:05-00:05:18}} .
- Bur Angulation: Hold the bur more vertically near the gingival margin and angle it more toward the occlusal as you move upward
{{FRAME:00:05:54}} .
4. Lingual Axial Reduction
- Instrument: 878K-012 (Tapered Chamfer)
{{FRAME:00:06:10}} .
- Rationale: A smaller diamond is used here to accommodate a metal collar (typically 1–3 mm). This bur is excellent for creating the necessary taper
{{CLIP:00:06:41-00:07:07}} .
5. Proximal Breakthrough and Blending
- Instrument: 878K-012 or 859-010 (Needle bur).
- Technique: Break through the proximal contacts and blend the axial walls
{{CLIP:00:07:13-00:07:22}} . Ensure the handpiece follows the long axis of the preparation to avoid undercuts
{{CLIP:00:07:53-00:08:03}} .
Finishing and Refinement
Evaluation of the Tenon
Before removing the tenon, use it to visualize the intersection of the prepared surfaces. This is the ideal time to identify under-reduced areas, particularly at the transition between the A, B, and C planes
{{FRAME:00:09:18}} .
Final Reduction Measurements
| Tool | Dimension | Target Area |
|---|---|---|
| RGS4 | 1.5 mm | Occlusal/Facial clearance |
| RGS3 | 1.0 mm | Facial shoulder width |
{{FRAME:00:10:48}}
{{FRAME:00:11:04}}
Tenon Removal and Smoothing
- Removal: Use a KS0F or similar fine diamond to remove the remaining tenon
{{CLIP:00:11:19-00:11:32}} once reduction is verified.
- Lingual Refinement: Use an 887-7009 (Fine Chamfer) to create a smooth, finesse gold finish line
{{CLIP:00:11:47-00:12:05}} .
- Facial Refinement: Use an 8847KR-016 (30-micron fine grit) to smooth the facial shoulder
{{CLIP:00:12:27-00:12:44}} .
- Hand Instruments: A sharp hatchet or bin-angle chisel can be used to remove minor irregularities from the facial shoulder
{{CLIP:00:13:26-00:13:37}} .
Clinical Tips
- Avoid Coarse Burs at the End: Once the tenon is removed, switch exclusively to fine diamonds (30-micron) to avoid over-reduction or scarring the margins.
- Shoulder Extension: The facial shoulder should extend from the mesial line angle to the distal line angle. While “wings” may remain, it is often preferable to smooth them for a continuous outline.
- Versatility: The T-prep technique is applicable to both anterior and posterior teeth and is particularly useful in examination settings where closing the typodont to check occlusion may be restricted.