Biomedical Engineering Reference
In-Depth Information
Hydroxyapatite
High mechanical stability is required for hip and knee prostheses. To impart stability
to stem and cup, Prof. Charnley cemented femoral and acetabular parts by a PMMA
cement (1960s!). Although this cement is immensely improved since Charnley's
days (lower concentration of monomer), cementing is not without risk for the patient
and remains a point of discussion as well as the cementless alternative. To impart
'biological' fixation, a common practice is coating the stem with a bioactive coat-
ing, almost exclusively HA. The coating is applied by the well-established technique
of plasma-spraying. The plasma is energized in a stream of ionized argon (or mix-
tures with by passing through a high frequency coil 200-2,000 kHz. The core of the
plasma is extremely hot:
10,000
ı
C but with steep gradients to the cooler periph-
ery. Most compounds will be decomposed at these temperatures were it not that the
powder, fed to the Ar stream, is accelerated to nearly sonic velocities (
>
200 m s
1
).
It reduces the residence time of the particles in the plasma to milliseconds, so that
only partial melting takes place with relative small changes in composition. The
substrate is moving in front of the torch to expose the area to be coated which at the
same time reduces excessive heating of the substrate. Nevertheless, some changes
take place. The coating becomes with respect to the original powder, partly amor-
phous and may contain other crystallographic phases (analysis by X-ray diffraction).
HA coating by plasma spraying allows to keep composition, texture and structure
under control and to obtain adhesion strengths of
80 MPa.
Many manufacturers coat only the proximal one third of the stem and the whole
backside of the acetabular cup, while others coat the whole stem. The CORAIL total
hip prosthesis (DePuy) is a particular successful example of the latter. The coating
has a thickness of 150
>
˙
40
m, a porosity
<
10%, a crystallinity around 60% and an
adhesion strength
35 MPa. Excellent overall survival rates were reported: 99.5%
after 4.5 year, 95% after 10 year for the total hip replacement. For the stem only,
however, a 0.977% survival rate after 10 year is reported. In general, it is accepted
that a primary mechanical fixation is needed which is followed by a secondary,
biological, one. The interface seems to act as an efficient barrier to wear debris,
which is carried toward the pelvis and not into the interface. After all those years,
HA is definitely proven not to be toxic and does not produce inflammatory reac-
tions. Resorption happens, an unescapable consequence of (bio)activity. It has been
observed that, where HA coating has disappeared, new bone in direct contact with
the metal surface is formed without interposition of fibrous tissue [
309
,
310
]. Tight
mechanical fit remains a major issue when pursuing osseointegration.
8
>
8
For another support of the statement that a tight fit is maintaining the integrity of bone tissue, see
Chap. 11
for a similar effect in the elastomer-coated prosthesis.
