It is not possible to avoid runaway bur completely, but it is
possible to minimize the chances of accident.
The first step is solving any problem is to understand the problem.
It is a bit cumbersome, without using diagrams, but I’ll give it a
A bur, while cutting, is a rotating disk. When bur just spins (no
cutting), the only forces on the scene is centrifugal and
centripetal, and they are in balance.
If it has been a long time you open a book on physics - centripetal
is towards the center of rotation, and centrifugal is in opposite
direction ( the 3-rd law of Newton ).
When you start cutting, you will apply some force in the direction
of the cut. Since you cutting is away from the center of the bur, the
force will add to centrifugal, and subtract from centripetal,
creating imbalance. However, as long as the direction of your force,
coincides exactly with bur radius at the point of the cut, there is
no problem. That imbalance is actually makes cutting more
The problem arises when applied force does not coincides with
centrifugal force, which is almost all the time. What we are talking
about is to be able to cut exactly towards the center of a prong,
which given the size of most prongs is practically impossible. The
imbalance of forces now is your enemy and causes runaway bur effect.
It does not matter how you hold your handpiece. John says
the smaller the bur relative to the size of the prong, the more
likely it is to catch on that edge, just because the bur itself is
so small. Bigger is better, but too big is bad, too, naturally...
It is somewhat correct. The stated reason is wrong, and the last
sentence is incorrect as well. To re-state it - there is an optimal
combination of bur size, rotational speed, bur coarseness, amount of
force used, bur profile, and probably many others, which will
minimize that imbalance. Most setters come to know this combination
through years of experience.
How one can minimize the chance of an accident ? by minimizing
forces involved and try to learn from every occurrence. When it
happens, try changing bur size, or speed, or profile, and etc…