Gotta jump in here, having had both a total knee replacement (October 2012) AND subsequent sciatica:

The TKR has been successful; the orthopedic surgeon surmises that the sciatica may have been due to his having corrected about 4 degrees of valgus (knock knee) and as a result, my hip, back & soft tissue needed to adjust and adapt, ergo, pressure on the sciatic nerve. It was extremely painful--more so than the knee pain--at about 6 weeks, but with a lot of PT and some gabapentin, it was completely gone by week 11.

I am slim and fit and was biking up until a few days before surgery; someone who would say, while the sciatic pain was most severe, 'get outta the chair and start walking a bit,' has obviously never had either a TKR or sciatic pain.