1 - Heavy archwires do not necessarily mean
heavy force. This patient will show how a heavy archwire can protect the teeth
from root resorption. We removed upper first bicuspids to make room to get the
upper incisors lined up in about the position they are in right now.
2 - First, we align the crowns of the teeth so
they can place heavy archwires later in the treatment. We will go through a
series of very small 12 nitinol archwire is to a very large 19 x 25 stainless
steel archwire in the upper arch.
3 - Now we are four months into the treatment and the
cuspid is moved down into the extraction space. In the next six months the
lower arch will be bonded and both arches will be leveled and aligned so larger
archwires can be placed.
4 - This is month nine and the crowns of the
teeth are now aligned, but we have a large extraction space on the upper left
side. We are still in a round 18 stainless steel archwire and closing the space
now would tip the incisor teeth and make them subject to root resorption.
5 - This is month 10 and a 19 x 25 stainless steel upper archwire has been placed. We call this heavy archwire a "Passive Archwire" because it is placed passively. This means that the archwire does not act on any of the teeth until a linear force is applied to close spaces in the buccal segments. As the closing force is applied, all the teeth move in translation as the remaining extraction spaces are closed.
6 - A month later we have now doubled the closing force in the buccal segments from upper arch and all the teeth are now moving in translation. When teeth are translating the resistance per square millimeter at the apex of the root reduces to 20% of the resisting force at the apex of a tipping tooth. For this reason we can up the linear force in the buccal segments by five times and still stay below the root resorption threshold of 1 g/mm² at the apex of the root.
7 -
This patient finished treatment in 20 months. There was a minimal amount of
root resorption of one millimeter on the upper incisor roots.
Closing
the extraction space on the upper left side would have taken much longer if we
were not able to increase the linear force with the straight-backs on a very
heavy 19 X 25 stainless steel archwire. The heavy upper archwire did not let
either the incisors or the teeth in the buccal segments to tip. Meaning all the
teeth moved into position by translation and the roots of the vulnerable
incisor teeth were protected from root resorption.
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