D2 · 22 min
Preparation design
D2
Reduction, taper, rounded internals, ferrule — the prep is the design.
Mandibular left first molar
#19
Path of insertion
0% of the wall above the margin is undercut
Teal faces the path. Gold fades. Red will not seat.
Look at this #19 prep. Reduction, taper, rounded internals. CAD cannot invent a ferrule that is not on the tooth.
Read this
The mill can only cut what the prep allows
CAD does not rescue an over-tapered, under-reduced, sharp-edged prep. Reduction for a posterior monolithic zirconia crown is typically 1.0–1.5 mm occlusal (some systems want 1.5–2.0). Axial walls need a path of insertion.
Taper
Total occlusal convergence of about 10–20° is realistic in clinic (the textbook 6° is a unicorn). Parallel walls lock the crown. A knife-edged internal line angle will not mill and will not seat.
Ferrule
A 1.5–2 mm band of sound axial tooth structure around a core. Without it, a post-and-core crown is a lever on a root. CAD cannot print a ferrule that is not there.