Types of Crown preparation for Bench Exams
- Full Cast /Metal/Gold Crown Preparation
- Porcelain Fused to Metal
- Full Porcelain
Technique of crown preparation is same for all except for the type of margins and amount of reductions needed.
Various types of cervical margin preparations exist but for bench exams most commonly tested are Chamfer and shoulder margins.
Chamfer margins have an internal rounded angles and shoulder has a 90 degree internal angle.
Full Cast preparation – Chamfer margins all around
PFM preparation – Shoulder on buccal surface and proximal surface 1mm lingual to proximal contact and chamfer on the lingual side.
Full porcelain – Shoulder margins are placed
During crown preparation maintain the morphology of the tooth structure. Producing flat surfaces will not provide retention for the crowns.
Functional cusp bevels are placed on the functional cusps of the posteriors, Buccal cusps of the mandibular and lingual/Palatal cusps of maxillary teeth. The bevel reduction is 0.5mm after the initial preparation.
The measurements for Full cast crown preparation are
1mm Occulsal reduction
1mm Buccal and Lingual reduction
6 degrees Total occlusal convergence – ie. 3 degrees on each proximal surface.
If the convergence is more then retention for the crown is questionable.
Functional cusp bevel of 0.5mm
0.5mm wide Chamfer margin all around the tooth
The measurements for Porcelain Fused to Metal crown are
1.5mm occlusal reduction, if porcelain will be veneered to the occlusal surface then 2mm is reqd.
1.3mm Buccal reduction – extra material is removed to give room for the porcelain.
3 degree taper and 2 plane reduction – occlusal half and gingival half.
1mm Lingual reduction with 3 degree taper.
0.5mm Functional cusp bevel.
1.3mm Shoulder margin on the buccal surface
0.5mm Chamfer margin on the lingual portion
Full porcelain crown
2mm occlusal reduction
1.5mm buccal reduction, 2 plane reduction and 3 degree taper.
1.3mm lingual fossa reduction
1.5mm Cingulum reduction. Try to maintain the lingual wall height as it is one of the retention feature for the crown.
1.5mm wide Shoulder margin all around the tooth.
All crown margins should not have any ledges or undercuts.
Ways to keep track of the width of the margin preparations
- Tip of bur is used. If 856-016 bur (tapered round end) is used , the diameter of the bur head is 1.6mm , hence less than half of the bur should be placed comfortably to create the chamfer margin.
- Use a Williams probe to measure the width of the bur.
Maintain a uniform reduction all around the tooth to produce a uniform preparation.
Some Universities follow the above guidelines. IF you are given the details of what preparation and what measurements they are looking for in the bench exam, you HAVE to practice and follow those instructions.