|
The
Esop® and «low
friction» : A few facts
result from Esop® observations since 1987
- The advantage of the cup in
providing impaction stability,
- The excellent stability of the
Esop® 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.
|
|
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.
|
|
|
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 Esop® like
with any other prosthesis, failing to comply with those
principles will lead to imperfect long term results..
|
