Notas / Notes - Active Congress.......

MARTES / TUESDAY
26
interlock between implant and bone. The European designs
had instead a roughened surface produced on a titanium
alloy stem by “corundum blasting” the surface to a specific
roughness but with out a specific porous coating. This
treatment was expected to provide a surface onto which
bone could grow to provide for long-term, secondary stability.
There have been reports in the literature of successes with
the Zweymüller and Spotorno (CLS) prostheses. With now
more than 1,000,000 of these types of stems in place world
wide, the clinical success of these tapered, wedge shaped
prostheses suggests that this European philosophy is one
that will continue into the decades to come. Survivorships in
excess of 95% at as long as 20 years have been reported with
no indication on radiograph of impending failure on the horizon.
The results of these prostheses, coupled with the good results
obtained with the AML type prostheses, suggest that
we look at the similarities of the implants to discern the requirements
for obtaining fixation of a femoral stem. The stages
of fixation can be summarized as primary fixation, intermediate
fixation and secondary fixation.
- Primary fixation is that obtained in the operating room
by driving a slightly oversized or wedge shaped implant
into the bone. The interference of the stem in the bone
leads to high pressure at the implant-bone interface and
a “press-fit”.
- Intermediate fixation is the means by which the implant
remains stable while bone attaches itself to the implant
for long-term stability.
- Secondary fixation is that obtained by apposition of bone
to a rough surface or apposition of bone to the surface
and ingrowth into the depth of a porous coating. Interference
probably disappears with bone remodeling and
all long-term stability is from bone apposition to the surface
of the implant.
After secondary fixation is obtained the effects of bone loss
through osteoporosis, fracture of the interface or debris or
infection induced dissolution of the interface may cause an
implant to become unstable in the bone. Continued postimplantation
surveillance of implanted prostheses is imperative
to determine the rate and potential solutions to late problems.
Since different femoral components have similar results with
respect to fixation, it is of more importance today to consider
the functional restoration of a hip rather than the specifics of
the stem. The surgeon should choose a stem with which he
or she is comfortable and can reliably attain good primary
fixation. Post operative care should take into account the
shape and history of the stem so that the intermediate period
can lead to osseointegration.
In my practice, both porous coated and surface roughened
implants have been successful in the long term. Having had
to revise porous coated implants, particularly those that have
achieved extensive ingrowth into porous coating in the diaphysis
(such as the AML), I prefer the surface roughened alternative.
REFERENCES
Blaha JD: Pressfit Femoral Components in The Adult Hip Callahan
JJ, Rosenberg AG and Rubash HE eds. 2nd ed. in press.
BASIC SCIENCE OF HA-COATINGS
Rudolph Geesink, MD PhD
Professor of Orthopaedic Surgery, Maastricht University,
Netherlands
Cementless fixation of hip implants has evolved through
several stages of evolution since the last decades. Results
of mechanical presfit implantation resulted in lack of adequate
long term fixation. Unavoidable increase in micro-motion between
the stiff implant and the more flexible bone during
loading through difference in elastic modulus of both components
resulted in unacceptably high loosening rates. More
rigid fixation would be required between implant and bone
to withstand all forces during daily activities. Second generation
fixation using porous-coatings using beads or meshes
represented a significant step forward although the interface
biology was far from ideal. Both experimental and autopsy
retrieval studies confirmed that average ingrowth surface of
porous-coated implants rarely exceeded 10 - 15 % of available
ingrowth surface. In clinical practice this may be adequate
to provide long term stability, however the reliability of the
system to provide durable fixation can easily be impaired by
adverse actions of the surgeon such as accidental undersizing,
varus positioning or adverse patient parameters such as
impaired bone quality as in osteoporosis, rheumatoid arthritis
or otherwise. Substantial increase in ingrowth fixation potential
of implants can be achieved by providing HA-coatings in
implant surfaces. HA-coatings of good quality should have
both adequate mechanical and biological properties.
Main advantages of HA-coatings in clinical practice:
- reduce the time for implant fixation & rehabilitation
- minimize relative motion between bone and implant
- allow more normal physiological stress transfer
- encourage gap filling between implant and bone
- seal periprosthetic access channels for fluid & particles
Results of experimental studies by many authors confirm the
superior behavior of HA-coatings to achieve these goals
while autopsy retrieval studies confirm improved results in
clinical practice for a great variety of patient populations, including
rheumatoid arthritis patients, patients suffering from
osteoporotic bone or other adverse phenomena.