Biomedical Engineering Reference
In-Depth Information
crack formation. Second, a highly porous metal matrix with increased surface
area may exhibit higher corrosion rates than its solid counterpart (Reclaru et al.,
2005). To address this concern, surface treatment of porous metals is necessary
prior to implantation.
Although porous metals are promising for titanium and cobalt±chromium
alloy hip implants, it is only now that researchers are starting to understand the
multifactorial design criteria that include mechanical properties under loading
conditions, corrosion resistance, bone attachment and ingrowth, and, finally,
parameters such as pore size, shape, and distribution that will optimize
mechanical/corrosion properties and bone ingrowth.
2.4.3 Bioactive coatings
Hydroxyapatite (HA) has been used as a surface coating on metallic hip implants
(mostly on femoral stems and limited use on acetabular components) since the
mid-1980s (Geesink, 1989; Furlong and Osborn, 1991). HA is the inorganic
mineral component of bone and it exhibits excellent biocompatibility and
bioactivity, such as quick bone bonding capability and osteoconductivity
(Stephenson et al., 1991; Cook et al., 1992). Clinical practice with HA-coated
hip prostheses indicates that HA coatings generally achieved earlier fixation and
stability with more bone in- and on-growth (Capello et al., 1997; Donnelly et al.,
1997; Nelissen et al., 1998; Roynesdal et al., 1998). However, the long-term
performance of such HA-coated prostheses depends on the quality of the HA
coating, that is, the purity, crystallinity, Ca/P ratio, microstructure, porosity,
thickness, and implant surface properties.
Specifically, two major concerns exist in achieving continued bone fixation
via HA coatings. First, degradation or resorption of HA coatings may result in
the loss of interfacial bonding strength. Second, delamination or disintegration
of the HA coating with the formation of granular debris may increase wear, thus
leading to osteolysis and finally implant loosening. These should be addressed
by improving the HA coating quality via optimizing the deposition techniques
and implant surface properties.
HA coatings are usually applied to both titanium and cobalt±chromium alloys
using a plasma spray method. This process involves high velocity spraying of
HA powders and high temperatures (up to 30 000 K) so conventional plasma-
sprayed HA coatings are composed of micron-size HA particles. Before plasma
spraying, the surface of metallic implants can be grit blasted to create
microroughness, macrostructured to form patterns, or made porous (as discussed
in the previous section) to enhance coating adhesion strength and implant
fixation. Studies have shown that compared with CoCr alloys, titanium alloys
had a 33% increase in bonding strength to the HA coatings in vitro, which might
be a result of chemical bond between Ti and HA. Moreover, titanium showed a
closer coefficient of thermal expansion (9±1010
ÿ6
/ëC) to HA (1210
ÿ6
/ëC)
