|
Mr. McMinn at the the
McMinn Centre
Malposition of the cup in relation to the
head in such a way that the wear patch
approaches the edge of bearing surface leads to
excessive wear.
Cup abduction (or Inclination) is a two
dimensional assessment. Malposition in the third
dimension is represented by anteversion or
retroversion. You are right in saying that
excess anteversion leads to edge loading towards
the front of the cup. Retroversion leads to
psoas tendinitis and also anterior impingement
(where the femoral neck hits against the edge of
the cup in flexion). This leads to posterior
subluxation of the head from the cup and to edge
loading in the back portion of the cup. All of
these are detrimental to long-term survival of
the bearing. As a rule of thumb, provided there
is no femoral abnormality, the surgeon should
try to achieve around 40 degrees of cup
abduction and 20 degrees of anteversion.
Mr. McMinn has written in his book and teaches
his surgeon colleagues regularly how this
problem is particularly complex in young women
who have hip dysplasia, because the cup will
have to be dialed in to adjust for the femoral
abnormalities which abound in dysplasia. In
these women the femoral neck is often more
vertical which means the cup will have to be
implanted in an even lower abduction.
Furthermore in dysplasia the femur is anteverted
more than the average femur. This will need to
be taken into account and the cup placed in
reduced anteversion. If the femoral anteversion
is too high (i.e. greater than 45 degrees) then
resurfacing is not advisable.
Implantation angles are as critical in hip
replacement as in hip resurfacing. In view of
the narrower hip replacement stem, there is
marginally greater elbow room before impingement
occurs but edge loading is as critical to hip
replacement as it is with hip resurfacing.
|
