Numbness can happen with both THR's as well as hip resurfacing since the surgery is so close to the sciatic nerve. If the doc accidentally puts too much pressure on the sciatica or nicks it by accident it can cause some serious problems. Most problems are what they call drop foot and usually disappear within a year, it can be accompanied with a burning sensation and if it does not go away after two years, then unfortunately your nerve damage is permanent. This is another reason to pick an extremely skilled resurfacing surgeon. I know of one surgeon in the Boston area that has has quite a few patients with drop foot and several that ended up with permanent nerve damage, even though he will tell each one that they are the first.
Some numbness by the incision site is common and I believe most people will experience that and there is no worries or concerns about that, it eventually goes away, but the numbness that goes all the way down the leg is the kind where a nerve was somehow involved and is a risk you take having any type of hip surgery. Just again, make sure you pick a skilled surgeon and your risks of it go down.
Vicky