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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 ?
Adler 1
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 Esop -
Primary
stability -
Insert
stability -
Insert
thickness -
Secondary
stability -
Explant
analysis -
Low-friction
Surgical
procedure -
Clinical
data -
Quality
requirement -
The
inventor -
Contact
- International
distribution
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