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Atlas
Alumina-Alumina
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  Why
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A In
the long run, i.e. after twenty years, polyethylene, despite
tribology progresses, will unavoidably result in osteolysis,
metal metal friction cause potential local or general
toxicity risks. Any incident, dislocation, impingement,
three bodies wear may release large quantities of chrome and
cobalt. Although very small quantities of chrome and cobalt
are tolerated by the body, large quantities are highly
toxic. Cobalt is eliminated through the urine, but chrome is
not or little eliminated, and builds up in the body. The
long term consequences of such a build up cannot be
forecasted.
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Alumina head surface ( 1 micron )
Acetabular
implant design
In
light of the lessons drawn from the use of solid alumina
ceramic implants, with or without metal back, it seems
advisable to insert the ceramic cup, the female element of
the unit, in a PE core..
Combining
both materials helps take advantage of the undisputed
advantages of some of PE's properties.
- PE helps soften and spread
stress
- With PE, the ceramic cup edge can
be covered and protected, preventing a possible conflict
with the neck of the femoral prosthesis
- In the event of an installation
with a metal back ring, the PE core and its ceramic
insert is installed precisely, without any misplacing
risk
- The PE inserted between the
ceramic and the metal back is subjected to little stress:
the stress is spread over larger surfaces.
Polyethylene
stress transmission is different with an all polyethylene
cup and with a sandwich cup. The head sends loads to spots,
which concentrates the stress and leads to creeping. In the
event of a sandwich, the alumina cup transmits broad loads
to the polyethylene, and the better stress distribution
reduces the polyethylene deterioration risks. The alumina
cup is inserted in the polyethylene core by hot crimping.
The spreading of the stress helps
reduce creeping risks significantly.
The PE is not in contact with the
head, and therefore is not subjected to any
friction.
In
such conditions, the use of PE helps take advantage of PE's
properties while preventing its deterioration and serious
resulting problems.
The
minimum polyethylene thickness is 5 mm. The polyethylene is
UHMW and reticulated in order to provide increased break
resistance, as in this instance, there is no need to
increase the friction resistance, since none of the
polyethylene will be subjected to friction.
The quality and resistance of the
crimping has been tested. The alumina cup resistance and
that of the polyethylene have also been tested..
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Laboratory
tests
The
Burst test consists in applying a force to a head placed in
the cup, which is held in a support with a 25 % angle. No
fracture of the alumina or polyethylene cup or metal back
was observed under a 100 kn load equivalent to a 10 ton
load.
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The
Lever out test was used to check the resistance to a
rotation torque exerted on the edge of the alumina cup. The
lever out test resistance was 3 295 N
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The
push out test exerts a force in the head axis on the alumina
cup embedded in a polyethylene core placed on a flat basis,
without any metal back and any peripheral support. This test
showed a 2300 N resistance.
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Lastly,
the torsion test, which exerts a rotation on the axis of the
alumina cup, showed resistance to a force of 2200 N per
centimetre.
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The
burst tests is rather similar to the forces exerted in a
hip, and the resistance to a 10 ton + load is much higher
than any usual stress.
The
lever out test could simulate a conflict between the cup
neck and edge (impingement), prevented by the polyethylene
edge and the end-of-operation testing we describe below.
The Push out test corresponds to the
barely imaginable absence of metal back or bony support.
The Torsion test is satisfactory,
since the particularly low alumina alumina friction torque
is much less than the required 220 kg per cm for mobility.
All the values above are much higher
than human clinic conditions, and therefore are highly
reliable.
Technical
requirements
* The
special impactor for alumina alumina FH acetabulum provides
support all around the core in the polyethylene section, not
at the centre on the alumina. The use of this impactor
is mandatory as any impaction on the alumina
cup using a conventional impacter would weaken the cup.
* As
a remainder, for the metal back, the cup surroundings need
to be cleared before impacting the core, in order to prevent
any conflict between the insert and soft tissues that could
interfere. *
Impingement, a conflict between the femoral neck and the
edge of the cup, fearsome with all types of prosthesis,
regardless of the friction combination. It stresses the
acetabulum anchoring and results in debris. Impingement in
metal metal and ceramic ceramic friction is all the more
fearsome when the neck wears out in contact with the cup
edge, weakens, may break and release metal debris. That
is why we have added a polyethylene edge on the acetabulum
insert, which comes between the neck and the cup
automatically in the event of impingement.
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As a
reminder, like for any prosthesis, great care should be
taken in placing the femoral and acetabulum prosthesis parts
and in checking, at the end of the operation, the absence of
neck-cup contact in maximum positions, especially in forced
external extension-rotation.
It
is essential to use a perfectly compatible alumina alumina
combination and therefore strongly inadvisable to mix both
parts : it is safe to use the alumina alumina combination
delivered by FH-Orthopedics or by distributors, which
guarantees compatibility.
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Welcome
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various Atlas -
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stability -
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thickness -
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