Biomedical Engineering Reference
In-Depth Information
causing an inflammatory reaction. In the case of hip prostheses, these wear
particles can induce an inflammatory response which results in local bone
loss around the implant (Jacobs
et al.
, 2001). Wear can be tested by several
methods, including the hip joint wear simulator (Lu and McKellop, 1997),
the multi-directional motion wear test (Wang, 2001) and the flat-on-ring
wear test (Chiesa
et al.
, 2000).
Coeficient of friction
if two materials are in contact, the best way to avoid unnecessary wear
is by having a low coefficient of friction between the two surfaces. The
coefficient of friction is determined by measuring the ratio of the parallel
force between the two surfaces in contact to the perpendicular force as the
surfaces pass over one another. Several techniques have been developed
to do this, the most simple of which are the dead load and inclined plane
test, described in detail in Ludema's textbook on tribology (1996). he does
recommend, however, seeking a published value for the coefficient for the
two materials in question, in the first instance, as it can be very difficult to
obtain consistent values of this property.
5.4.5 Surface properties
As the surface of a material is the first part of an implant the body's
environment encounters, it drives the subsequent biological reactions. the
body's response to surface characteristics and features is still not entirely
understood, although some general trends in the types of properties that
affect cell behaviour have emerged. Special methods must be employed
to characterise surface properties, as the surface has a different reactivity
compared with the bulk material. it also involves such a small amount of
material that particularly sensitive equipment is needed and, additionally,
the surface is easily contaminated by air or other substances. Ratner (2004)
gives a good description of the important surface properties and the current
technology involved in their characterisation. A review by Boyan
et al
. (1996)
summarises typical cell behaviour towards different surface properties.
Surface composition
Perhaps the most obvious surface characteristic that influences the body's
response to a material is the composition. the composition and chemical
structure affect whether cells attach to the surrounding tissue, resorb the
implant material or form a capsule around it. Bioceramics, particularly calcium
phosphates, are noted for their chemical similarity to the inorganic constituent
of bone. thus, coatings of hydroxyapatite (hA) and tri-calcium phosphate
