The effects of third-order torque and self - Saint Louis University

sizes of forces delivered to the crowns by the wire during
deactivation, especially with a light leveling wire. The
authors concluded that careful ligation to minimize
friction was important for delivery of expected force
magnitudes. It is as early as placement of this initial
wire that friction becomes a concern to the orthodontist.
The classical laws of friction offer only a partial
explanation of friction in orthodontic systems. The
original theories by Amontons and Coulomb were derived from
dry friction mechanics with distinct static and kinetic
frictional phases, defined by zero velocity and constant
velocity, respectively. Tooth movement in orthodontics,
however, occurs at such low velocities, that these two
phases are clinically interrelated. 3 Although the first two
laws of friction would be unaffected by this application,
the third law of friction may not accurately describe
orthodontic friction in vivo. As the velocity of tooth
movement approaches zero, instability in steady sliding is
likely, due to interlocking and shearing of asperities, the
peaks of microscopic surface irregularities. This effect
is evident in the phenomenon known as “stick-slip motion”;
a cycle involves a “stick state” where elastic loading
occurs, and a “slip” where sudden sliding and stress-
relaxation occur. 3,8 Nonetheless, Rossouw’s group 3 concluded
8