The issue of cup slippage in the immediate postop
period is a controversial one.
While bone ingrowth takes around 6 wks. - the hydroxy apatite to bone chemical reaction can occur much more quickly.
If we surgeons feel that the
cup is not perfectly tight ( press fit) during the
surgery then we restrict activities for a 6 -8 wk
period .This is done in the hope that no
precipitating event would occur that would tilt the
balance adversely till some stability occurs as we
have not achieved primarily stability during
surgery. I must say that most of these times we are
able to 'escape' component loosening.
I have done this a few times in my very early cases
, many years ago. Of course these days we get such
spectacular fixation of the cup primarily that many
of my patients are visiting the gym in 5-6 days
following surgery.
Achieving primary stability in the resurfacing
surgery is more difficult as by definition there are
no screws in the acetabular cup of a resurfacing as
the entire cup is an articulating part ( monobloc )
cup. This is different from a cup in a THR where the
surgeon can easily get additional stability by
putting some screws if an adequate press fit is not
achieved. Since a liner is always used in a THR cup
, this is feasible.
Thus the early cup loosenings are certainly going to be more in resurfacings esp. when the surgeon is in the learning curve.
An
extension of this concept implies, that surgeons who
use screws routinely for the cups in the THR may
find the resurfacing cup without screws more
difficult to install.
Another issue is that if the cup is installed very
loose , a fibrous fixation occurs - very similar to
non-union in a fracture situation. If this occurs
this will prevent bony incorparation of the cup
permanently. This cup is at risk for many years
following surgery. One of the things that we look
for in the postop films is the bony incorporation (
osteointergration) of the cup.
with best regards
vijay bose
chennai
