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