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The ATLAS® and «low
friction» : A few facts
result from Atlas®
observations since 1987
- The advantage of the cup in
providing impaction stability,
- The excellent stability of the
Atlas® insert
proved by explant clinic and studies 1 ,
- The usefulness of hydroxyapatite
in obtaining rapid, lasting and total bony
adhesion,
- The crucial importance of a
polyethylene thickness of at least 10 mm.
Our series after ten years have a survival rate
close to 100 %, and osteolysis was rare on the femur and
absent on the acetabulum . This experience conflicts with
some publications describing rapid and frequent osteolysis
with metal back acetabular prosthesis. 3
In our opinion, even if favourable factors
contributed to the excellent long term results, these were
also achieved thanks to low friction.
  ADVANTAGES OF THE
22.2 HEAD
The better long term results of cemented
Charnley prostheses compared with other prostheses has been
widely proved and discussed. Indeed, small size heads
provide unquestionable advantages : smaller wear debris
volume and polyethylene thickness.
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1) The (V) volume of polyethylene debris
released is directly dependant on the head volume : V = U ½
R2, where U is the linear wear and R the head radius. Thus,
with the same linear wear, the volume of a 32 mm is twice
that of a 22.2 mm (low friction). The 28 mm head, according
to the unchanging physical law, released an intermediate
volume.
2) The increased polyethylene thickness
associated with the use of 22.2 mm heads is especially
noticeable with cup diameters under 54 mm. A polyethylene
thickness of at least 10 mm helps prevent any creeping from
reaching the outside surface of the insert 2 .
This prevents insert deterioration due to micro-fretting
between the insert and the metal back.
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That is why, in our opinion, it is vital to use
22.2 mm heads with cups under 54 mm with polyethylene
inserts (new alumina alumina friction are not subjected to
that principle, and 28 mm heads can then be
used).
DISADVANTAGES OF 22.2. HEADS
Some surgeons thought they could improve the
Charnley prosthesis by increasing the femoral head diameter,
in order to reduce the dislocation risk. This had led to
early osteolysis without really solving the stability
problem. Huten, in his Conférence d'Enseignement
4 showed that even though large diameter heads
are theoretically less prone to dislocation, the lowest
dislocation rates have been experienced with 22.2 mm head
series. The Mayo Clinic experience on 10 500 THR, published
by Woo 5 showed that dislocation rates were not
dependent on the head diameter (2.9 % with the 22.2 mm, 4.7
% with the 28 and 3.3 % with the 32). No scientific study
has never found a decrease in the dislocation rate
associated with the use of large size heads. The issue of
diameter head is not crucial, and there are many other more
important factors associated with the approach, the
operating techniques, the gluteus leverage arm. Wanting to
solve approach and technical failures by increasing the head
size is not only illusive, it is also a dangerous process
leading to the major disadvantages of debris volume increase
and insert deterioration risk due to insufficient
polyethylene thickness.
In our opinion, that is the reason for the
differences in the long term results of our experience and
that of pejorative metal back acetabulum series. With
Atlas® like with any other prosthesis, failing to
comply with those principles will lead to imperfect long
term results..
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