This is the result of relying on the milled
post of a bicuspid endocrown! Small teeth like this require
a conventional post.
The following text is copied from a post
to the cerec.net discussion group by Mike Birkhoff:
Basically the difference in regular and endo mode is the order
that you give to the machine:
- in regular mode, you tell the machine to take
away any material that could prevent seating. Because the
1.6 or even the 1.2 burr are larger in diameter than many
of the prep details, it´s just logic that this will
lead to voids and reduced posts (if there are any). Still,
this is no problem with regular preps.
- in Endo mode, you tell the machine to keep
the ceramic as strong as possible and to NOT reduce a post
that is due to be milled. The problem with milling a post
is that the calculation of milling data must be opposite to
a regular crown:
To mill a crown, there is usually a hole in
the crown that must be milled; i.e. the bur´s cutting
edge is calculated on INSIDE milling. It changes to the other
side once half of the hole is cut out, because if you retain
the cutting edge the burr would cut away the crown´s
walls with it´s back side. So the former cutting side
is now the inactive side. This is also the reason why endo
posts are reduced in regular milling
mode: while the "face" of the burr is cutting the
inside of the crown, the bur´s "butt" accidentally
cuts away the post... unless the distance from crown inside
walls to post is larger than the bur diameter (sometimes on
molars).
To mill an endo crown, at a certain point the
cutting edge (or "face") of the bur must change
from inside milling to outside milling to prevent reducing
the post. This is done in that "secret area" (about
half a mm from the drawn BL, about 1mm wide). Here the bur
"cutting edge" turns around. This is the reason
why in this area there CANNOT be any detailed milling. This
is no problem in flat preps or V-style preps since here you
have descending z-values from margin towards the center of
the tooth. If, however, in this area there are rising z-values
(i.e. prep walls) these will NOT be milled out of the porcelain...
resulting in binding.
(There have been discussions on this forum that
the "sacred area" has disappeared or been moved
to a different place with 3D, but my tests have shown that
this is not the case. It´s still there and still gives
us the same "trouble" like in all earlier s/w versions)
However, if you know about this, you can easily
use this rule to adjust your prep and still have great fit.
For example, flat and V-preps are best milled in endo mode
because their z-values are always descending towards the toot´s
center and the endo mode gives you additional strength. Regular
crown preps, however, will give you a 99% chance of binding
if you mill them in endo mode.
Contrast the way endo crown works with the
way the software governs the cutting tool in non-endo mode
in making a crown.
Notice the sharp projection in the prep
above.
Allowances must be made in the internal
surface of the crown to accept the sharp projection of the
prep.
The diamond cutting instrument can go to
the outside......
or the inside of the relieve area without
damaging the crown.
The exception to this rule is when a projection
is near the margin as shown above.
To avoid damaging the margin, a .5 protected
zone is established at the edge of the bottom line.
The diamond is now directed to the internal
of the crown, protecting the margin.
