Biomedical Engineering Reference
In-Depth Information
phases of fixed appliance treatment. At the beginning, during the initial alignment stage, a light,
low-module Ni
Ti alloy into
malaligned teeth/brackets activates the wire and creates internal forces. Upon deactivation, an inter-
nal shear force drives wire straightening and force is applied on the teeth. As a result, the wire
slides through the neighboring brackets and at this point, the important role of friction forces. As
treatment progresses the Ni
Ti archwire is used. Engaging the flexible shape memory Ni
Ti wires are replaced by rigid SS archwires. At this stage, tooth move-
ment occurs by sliding motion of the tooth over the wire and the orthodontic force needs to over-
come the friction resistance to their sliding motion. In some cases, a group of teeth are being
translated (i.e., retracting incisors distally) and the orthodontic force makes the SS wire slide
through the slots of the posterior teeth. The concept of friction force in orthodontics was re-
emphasized in 1997 by Kusy and Whitley
[5]
. Mathematically, friction resistance to sliding (RS) is
described as an additive effect of classical friction (FR), binding (BI), and physical notching (NO).
RS
FR
BI
NO
5
1
1
The FR is the result of the contact between the wire and the slot walls during the sliding
motion. This force directly reduces the effective force delivered to the tooth. When initial tooth
movement occurs, an angle (
) forms between the slot and the wire (
Figure 13.1
). At the point
where the wire first contacts the edges of the slot walls, the BI component starts contributing to the
RS and this point is called the critical contact angle for binding (
θ
θ
c
). From this point on, as
θ
increases the BI component grows and the term FR becomes negligible. At a higher angle (
θ
z
),
notching begins and the NO component takes a major role in RS (
Figure 13.2
), causing notches in
the wire due to plastic deformation that stop the sliding completely.
IBD/2
θ
=
θ
c
Width
Size
Slot
FIGURE 13.1
Photograph of an archwire engaged in a bracket showing the geometric parameters that are important to
adequately describe orthodontic sliding mechanics: the archwire size (Size), the bracket slot (Slot), the
bracket width (Width), the interbracket distance (IBD), and the angulation (
) that corresponds to the critical
θ
contact angle of second-order angulation
[6]
.
