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Primary
stability
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The resting forces are conveyed to the
acetabulum roof through two bony row systems, one
upper-external stretching to the foot of the sacroiliac and
the other lower-internal one towards its top
Both rows join in the upper section of the
ilium, achieving a system very similar to a gothic arch.
Looking at normal and pathologic pelvis x rays shows that in
most cases, a clear cover of the little mineralised
acetabulum , marked out by darker internal and external
rows, joining in the thick section of the ilium and forming
a rib.
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acetabulum rows
(according to Kapandji)
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For an acetabular prosthesis to convey the
stress to the acetabulum in the closest manner possible to
physiology, it should not only be hemispherical, to fit the
acetabulum anatomy, it should also achieve close
osteo-adherence throughout its whole surface. Thus it will
be able to convey the stress to the whole bone-contacting
surface, especially in surrounding tangential areas where
the rows leave from.
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Stress conveyed by a
hemispheric cup
(press-fit)
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The use of screws transfixing the acetabulum
roof tends to concentrate the stress in the cover and oo
relieve tangential areas. That is probably why lucencies are
observed in zone 3 with screw acetabulum.
In the long run, screws can help free metal
micro-particles due to friction with the cup . Last and
overall, they help polyethylene micro-debris penetrate the
cover.
That is why it seems advisable to avoid screws,
and to use the "press-fit" technique, consisting in
impacting a cup slightly larger than the reaming.
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The screws keep the
stress in the cover
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Why an elastic cup ?
Adler1 Mac Enzie15 showed that impacting an oversized rigid
acetabulum prosthesis results in stress peaks around the
bony acetabulum , making it difficult to introduce the metal
cup into the acetabular cavity. Kim 13 in a corpse study, showed that the impaction
of a oversized cup larger than the reaming resulting in
micro-fractures in 65 % cases.
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The impaction of a
oversized cup acetabulum results in over-stress around the
bony acetabulum (according to Adler)
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The impaction of a
oversized cup acetabulum results in micro-fractures
(according to Kim).
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Conversely, an impacted elastic cup with a slot
goes to the bottom of the bony acetabulum and fits in the
hole, without any surrounding hyper-pressure. After the
impaction, the cup exerts an expansion force that increases
the press-fit effect. Lab calculations show that with an
elastic cup with a 2.5 mm titanium thickness and 50
diameter, the expansion force was 400 newtons. Such an
expansion force constitutes an easy cohesion force to check
in per-operation, since, when introducing a hook through a
hole of the cup , a strong traction will not dislodge
it.
Using an elastic cup larger in size than the
reaming, 2 mm on average, prevents from using screws and
helps avoid their disadvantages.
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An oversized elastic
acetabulum will close on impaction and go to the bottom of
the bony acetabulum.
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An impacted elastic
acetabulum exerts a strong expanding force, increasing the
"press-fit" effect.
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Welcome - The
various Atlas -
Primary
stability -
Insert
stability -
Insert
thickness -
Secondary
stability -
Alumina/Alumina - Explant
analysis
Low-friction - Surgical
procedure -
Clinical
data - Quality
requirement -
The
inventor -
Contact - International
distribution
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