Biomedical Engineering Reference
In-Depth Information
have Cr, Go, and Ti. Depending on different needs and proportions, they can make
cobalt-chromium alloy, nickel-chromium alloy, cobalt-nickel-chromium alloy,
titanium alloy, nickel-titanium alloy, etc. These elements have a common character-
istic, which is an excellent resistance to corrosion. In addition, some elements, such
as Mo, Be, Cu, W, Al, Si, Mn, etc., are used to improve the functions of dental alloys
[
31
]. Since titanium and its alloys were used for dental implants in the 1950s [
32
],
they have been increasingly applied to dental restorations. Due to its excellent
biocompatibility, titanium and its alloys have gradually replaced traditional dental
metals and alloys such as gold alloys, nickel, copper, and silver amalgam. Today
titanium alloy is the fi rst choice among dental alloys in dentistry [
33
].
Corrosion has been considered the most important factor in the selection of
metallic materials in dentistry because poor biocompatibility and the cytotoxicity of
their corrosion or wear products may make the materials worse for either restoration
or implantation purposes in the mouth [
1
]. A vast amount of literature is available
concerning the corrosion of dental alloys, and from the available information, it
emerges that leaching of metallic ions and food habits are the main causes of corro-
sion of metallic restorations and implants [
2
]. The purity, casting, and melting tech-
niques also affect the corrosion behavior of metal alloys. In general, the nature of
metal alloys plays a major role in the initiation and propagation of corrosion.
Recently, Liu et al. reported that the wear and corrosion resistance of nickel-based
and chromium-based dental alloys could be improved with the presence of titanium
aluminum nitride fi lm [
34
].
Generally, most metals are strong enough to withstand the maximum possible
oral forces. However, with quality of life increasing greatly, the appearance of den-
tal materials may be the most important factor for many patients. As a result, metals
and alloys have increasingly been applied today to orthodontic appliances and den-
tal implants.
Friction in fi xed orthodontic appliance systems is recognized by most clinicians
to be very harmful to tooth movement [
35
,
36
]. Various factors affect the friction
resistance process of orthodontic metallic bracket-wire combinations, such as the
archwire and bracket materials, their size and shape, width and slot dimensions, and
the surface composition, roughness, and cleanliness. Other important parameters
are the bracket-to-wire positioning in a three-dimensional space, the ligature force
and the type of ligation, the interbracket distances, and lubrication. Fretting tests of
stainless bracket-wire combinations during sliding processes, carried out by Willems
et al. [
35
], indicated the signifi cant role of the centered positioning method on the
friction value. In addition, the slot-fi lling, bracket-wire combination resulted in an
increased coeffi cient of friction and therefore is not recommended as a sliding sys-
tem. It is also notable that pain and discomfort of the oral mucosa often can be
experienced as a result of trauma from the metallic appliance caused by the increased
friction between the mucosa tissue and the surface of the brackets [
37
]; however,
only limited research has been conducted in this area.
Fretting wear has been frequently mentioned by clinicians as a possible cause for
the failure of dental implants [
38
,
39
]. Yu et al. investigated the tangential fretting
behaviors of pure titanium (TA2) and its alloy (TC4) against human thighbone
