Enhanced Verifiable CPD from the
University of Birmingham

Indirect Restorations
Metal-Ceramic (PFM) Crown Prep. Upper 1st Premolar

Metal-ceramic crowns are also known as "Porcelain Fused to Metal" (PFM) crowns. Where porcelain is needed, extra reduction must be undertaken to make room for both metal and ceramic.

In places where porcelain is not needed, for example the palatal margin, less reduction is done.

Slide show: Metal-Ceramic Crown Prep. Upper 4.

  • Plan where you will place your finishing lines, and what type of margin will be where.

  • *The 501 bur acts as your depth gauge, its dimensions are: Tip 1.1mm; Maximum diameter 1.6mm; length 7.0mm ; Taper 4.5° (9° convergence)

  • Cut 3 slots in the palatal cusp, at depth 1.5mm if it is to be covered in alloy only or 2.0mm if porcelain coverage is required.

  • Note that a thin band of marginal ridge can be left unprepared to prevent bur contact with the adjacent tooth and aid in depth orientation.

  • Repeat for palatal cusp.

  • Reduction is 1.5mm for alloy; 2.0mm for gold, and can be begun with depth grooves.

  • Use 501 bur to cut 1.5mm depth slots in the occlusal half of the buccal surface, fading out at the maximum bulbosity.

  • Using 501 bur cut 1.0mm deep marginal depth slots parallel to the cervical half of the buccal surface. Finish depth slots supragingivally.

  • Alternative view

  • - Leave occlusal 1/3 of axial wall under-prepared, resulting in a finished crown of poor contour.

  • Join the buccal depth slots in the occlusal half of the preparation.

  • Join the buccal depth slots in the occlusal half of the preparation.

  • Join the buccal depth slots in the occlusal half of the preparation.

  • Keep the long axis of the bur parallel to the buccal surface, in the same way as the depth slots.

  • NB: These are not slots/grooves.

  • The aim of this part of the preparation is simply to separate the teeth.

  • Clinical note: your technician is going to have to saw the model between your prep and the adjacent tooth. If you don’t leave enough room, your margin and/or the proximal surface of the adjacent tooth may be lost.

  • Using your 877 Torpedo diamond prepare the palatal axial surface to create the correct convergence relative to the cervical half of the buccal axial preparation.

  • - Monitor the bur angulation to create the correct taper between proximal axial walls.

  • Round any line angle between palatal and proximal axial preparations to ensure alloy will have the correct contour and a natural emergence profile. Do not over-round your preparation.

  • The 501 will also create a rounded lip towards the outside of the shoulder, this must now be carefully flattened with a flat-tipped bur eg: 847 tapered diamond (Tip diameter 1.01mm) and/or a sharp margin trimmer.

  • ... using the chisel

  • Smooth any sharp line angle where the buccal axial planes meet, to reveal any part of the shoulder hidden when viewed from above.

  • Using composite finishing burs, fine discs or rubber cups + pumice, gently smooth line and point angles.

  • FINISH.

  • FINISH.

Show all slides


Why should you avoid sharp angles when preparing a crown?


What is "convergence" in this context?


What is the ideal convergence angle. And what is clinically achievable and acceptable?


What are the problems of over-tapered and under-tapered preparations?


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Wow. I actually understand occlusion now, thank you so much. I saw those BDJ articles years ago when newly qualified and I didn't understand any of it. It is all so much more understandable now, thank you.
TD

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