Dentaltown February 2022 - 25

ztech
May I join the fun, to add some facts and a couple of opinions about zirconia milling and
strength?
Zirconia is a great material, and as with most materials it has its strengths and weaknesses.
As a material, its greatest weakness is when it is placed in shear stresses. Conversely, its greatest
strength is when it is under compression. This is true of all ceramic materials that we use. The
majority of fractures you will see in ceramics occur because of this condition. Occlusal fractures
are most often caused by axial walls being placed in stress that accumulates at the thinnest
portion of axial walls and sometimes across the occlusal surface.
This fracture will travel through the thinnest axial walls to relieve the shear stress. This
shear stress is inherent to fi tting a material on a tapered object and can't be avoided, only minimized.
The ways to overcome this accumulation of stress is to allow more space between the
axial walls and the prep. Add more material to resist the stress. Or eliminate the taper, which
is only possible in machine-to-machine fi ts (ti-base). On the 0-degree fi ts, it's also important to
understand the cement's properties to optimize retention and to minimize the hydraulic effect.
My cement's optimal thickness is 30 microns (Ivoclar Multilink Hybrid Abutment).
The area that is hugely variable in our industry is margin design. The ideal design for
strength would be a 90-degree shoulder prep, but that is not possible with the need to preserve
tooth structure. The least favorable for strength is the knife edge or feather, with those being
the most favorable for preservation of tooth structure. The one I advocate is a heavy chamfer
that is no less than 0.5 mm at the terminus. This is so I can dial in 40-plus microns of spacer for
all areas except that 0.5 mm, which if designed correctly
will be under compressive forces. Even though I advocate
this as a good compromise, I also understand that you as
a dentist have different considerations than I do. This is
why understanding what causes shear failure and what
prevents this is important to understand when you do a
preparation.
The other property of zirconia is that it should not be
bulk-fi nished after sintering. The adjustments should be
done in green state when at all possible (and most can be
if extreme care is taken).
Any grinding postsintering has the propensity to
create areas of microfractures, and a phase change in that
area that will negate the healing properties of zirconia. If
this is needed, the only viable options are to either perform
a regeneration bake of the zirconia or remake the
restoration closer to the correct dimensions. ■
12/11/2018
Member Since: 03/28/17
Post: 171 of 222
WE WANT
YOUR FILES!
Let dental professionals design
and fi nish your full Zirconia
* We work with all digital systems
* All work done in our lab in the U.S.
serving dentists since 1976
* Restorations designed and fabricated
by experienced dental techs
* Townie Doctor on staff
* We use only the highest grade Zirconia
and Lithium disilicate on the market
Catch up on crown tips
This thread has more than 200 responses from
Townies spanning three years-including new
responses and more photos. View the entire
conversation and jump into the discussion.
Head to dentaltown.com/messageboard and
search " Best Margin Style. " This board will be
among the top results.
* We know we're not the cheapest
option, but we make up for that by
providing superior results and service
Find us on your digital OS
Call 801-942-1874 or Email: sculptec@yahoo.com
FREE FACTS, circle 17 on card
dentaltown.com \\ FEBRUARY 2022 25
Sculptec Dental Design

http://www.dentaltown.com/messageboard http://www.dentaltown.com

Dentaltown February 2022

Table of Contents for the Digital Edition of Dentaltown February 2022