Biomedical Engineering Reference
In-Depth Information
Table 13.1
select properties for common materials used as porous
coatings for orthopaedic applications
Cancellous
bone
Porous
tantalum
Porous
titanium
Porous
magnesium
Porosity (%)
30-95
75-85
60-72
35-50
Pore size (μm)
20-1000
400-600
150-600
100-400
Modulus of
elasticity (GPa)
2.3-20
2.5-3.9
1.6-4.2
0.9-1.8
Compressive
strength (MPa)
20-193
50-70
100-200
12-17
Coefficient of
friction
—
0.88
0.54 to 1.2
Marketed titanium-based foams include Biofoam (Wright Medical
Technology), Stiktite (Smith and Nephew), Gription (Depuy), Tritanium
(Stryker), and Regenerex (Biomet). Zimmer's Trabecular Metal is the
only tantalum foam currently available. Porous magnesium coatings are
not yet available, but this approach is considered a candidate for future
innovations for implant fixation. Further details on the properties and
character of these metals are given in Chapter 7.
Hydroxyapatite coatings
As an alternative to porous coatings, hydroxyapatite (HA) was intro-
duced for biological fixation during the 1980s, with the intent of pro-
viding strong bone bonding. HA is a nontoxic, biocompatible, and
osteoconductive material. In some coatings, the chemical purity of HA
can be above 99.99% and have a maximum porosity of 2%. Clinical
studies have shown good outcomes following the use of HA coatings,
though some have raised concerns about continuous absorption of HA.
HA coating can be plasma sprayed onto a porous coating to further
enhance the fixation capabilities of the porous coating.
Fixation for bone fractures
When a bone absorbs energy above its ultimate strength, a fracture occurs
along line(s) of least resistance. The treatment of bone fractures may
include casting, external fixation, or internal fixation, which is currently
the preferred method in a majority of cases. Internal fixation makes use
of rigid bone plates secured to the endosteal surface with screws or pins,
nails implanted into the intramedullary canal, or lag screws and wires
that directly secure bone fragments. The aim of treatment is to provide a
safe environment for fracture healing, while minimizing the intrusion of
the prosthesis on the host region.
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