Preparation guidelines.
Commonly cited figures and requirements by material and restoration type. Treat them as guidance rather than absolutes — the specific product and whether the restoration is bonded both shift them.
Reduction by material
Metal-ceramic needs the most total reduction of the tooth-coloured options, because coping, opaque layer and veneering porcelain all consume space before any anatomy is developed.
Lithium disilicate needs meaningful occlusal thickness — around a millimetre and a half for full coverage, reducing somewhat for bonded partial coverage where the adhesive interface carries part of the load.
Monolithic zirconia performs at appreciably less, with posterior units workable around a millimetre and lower-yttria formulations tolerating less again. Cast metal performs in the thinnest section of anything on the form.
Margin type by material
Chamfer of roughly half a millimetre for metal-ceramic copings and for zirconia, which retains strength in thin section and does not need bulk at the finish line.
Shoulder or rounded shoulder of around a millimetre for lithium disilicate and all-ceramic generally, providing the thickness those materials need at their most vulnerable point.
Knife edge or feather margin only for cast metal, which finishes to a fine edge and burnishes. Under ceramic it produces either an unacceptably thin margin or an overcontoured one.
Geometry that applies everywhere
Total convergence of ten to twenty degrees is realistic and works. Beyond that, retention falls away quickly; an undercut prevents seating altogether and cannot be compensated for at the bench.
Reduce anatomically rather than flattening the occlusal table — it achieves uniform clearance while removing considerably less structure and preserves resistance form.
Round the internal line angles. Sharp angles concentrate stress and are where ceramic fracture initiates, and they are also where impression material tears and scanner reconstruction is least reliable.
Verification before the record
Check clearance with a silicone index, a bite registration record measured with a gauge, or depth-limiting reduction burs — not by eye, which consistently overestimates how much has been removed.
Check in excursion as well as intercuspation. A restoration with adequate space at maximum intercuspation can still be struck on a cusp incline in lateral movement.
Review the preparation along the intended path of insertion for undercuts, run an explorer around the finish line to confirm continuity, and inspect the adjacent teeth for damage.
Special situations
Short clinical crowns need grooves or boxes added at preparation to restore resistance form. They cannot be introduced later without re-preparing and re-recording.
Post-restored teeth need a ferrule of sound structure coronal to the finish line — commonly one and a half to two millimetres — and the margin should sit on tooth rather than on core material wherever possible.
Multi-unit work needs a common path of insertion across all abutments, assessed while preparing rather than discovered at the laboratory.
Impression and scan requirements
Whatever the material, the record has to show a continuous, unambiguous finish line around the full circumference, with a short band of unprepared tooth or tissue visible beyond it so the margin can be identified rather than inferred.
It also needs the adjacent teeth in full — contacts are established against them — the opposing arch to a comparable standard, and an accurate interocclusal record that seats stably on the casts.
Inspect before the patient is dismissed. Almost every record problem that reaches a laboratory was detectable at that moment. See impression quality.
If something is not ideal, say so
Most preparation compromises are workable if the laboratory knows about them. A short preparation, clearance deliberately accepted as tight, a margin clear on three surfaces and uncertain on the fourth — each changes design decisions.
Note them in Special Instructions rather than leaving them to be inferred from a die. Common crown preparation errors covers what we see most often and how each is caught at the chair.
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