Biomedical Engineering Reference
In-Depth Information
13.1
Introduction
Orthodontics deals with prevention and correction of malaligned teeth in the jaws as well as the
proper positioning of jaws in the face (dentofacial orthopedics). Esthetics and function are the main
reasons to seek orthodontic treatment. Since the beginning of the twentieth century, orthodontic
tooth movement has been performed by the metal appliances available. These consisted of attach-
ments bonded onto the tooth (brackets) and wires engaged within a specific slot incorporated within
the brackets. This clinical setup is also referred to as fixed orthodontic appliances. Until the
emergence of etching-bonding procedures on the tooth enamel
[1,2]
, metal bands were cemented
onto the teeth. This setup allows the orthodontist to control tooth movement during the entire
course of treatment. In the early 1900s, orthodontists used gold, platinum, silver, steel, gum rubber,
vulcanite, and occasionally zinc, copper, and brass. During the 1950s, stainless steel (SS) was
introduced in orthodontics and became the popular material for making brackets and archwires. In
the early 1970s, nickel
titanium (Ni
Ti) archwire was used for the first time in orthodontics
[3]
which originated from the Ni
Ti alloys developed at the Naval Ordnance Laboratory by Buehler
[4]
.
Currently, orthodontists are mostly using SS brackets and archwires made of SS and Ni
Ti metal
alloys with different size and shape (round and rectangular) for archwires. There are also wires made
of cobalt
-titanium, available for use in orthodontics.
In order to initiate orthodontic tooth movement, force has to be applied on the tooth. This force is
exerted either by the archwires engaged into the bracket's slots or by springs and elastics which are
attached to the brackets or by extra-oral devices such as headgear. Apart from the metallurgical
aspect of orthodontics, teeth movement is entirely dependent on the biological changes occurring
within the tissues encircling the teeth, mainly the periodontal ligament and the alveolar bone.
Consequently, biomechanics is considered as the basis for orthodontic treatment. There are additional
principles which will assure successful orthodontic therapy such as proper diagnosis and treatment
plan, successful bonding, prevention of caries formation and root resorption (shortening the length of
the tooth), patient's compliance, and retention protocol. However, the level of the orthodontic force
is the core of the treatment. Force magnitude of 40
chromium
nickel and
β
60 g exerted directly on the tooth for an adequate
continuous period, without the metal fixed appliance setup, is sufficient to move the tooth in the jaw.
However, application of orthodontic force using fixed appliances requires a significant increase in
the force level in order to overcome the friction created at the bracket-wire interface
[5]
.
13.2
Friction in orthodontics
Friction is defined as the force resisting the relative motion of solid surfaces, fluid layers, and
material elements sliding against each other. There are two types of friction: static friction which
occurs between two objects that are not moving relative to each other and it prevents an object
from sliding down, and kinetic friction which occurs when two objects are moving relative to each
other and its value is usually less than the static friction for the same material. In orthodontics, the
kinetic friction is irrelevant because tooth movement is not a continuous motion of the tooth along
the wire. It is a series of tipping and root uprighting movements resulting from the biological
response of the bone, a pseudostatic condition. The orthodontist encounters friction during two
