Foot drop, as I hear is the drop of the foot during walking due to weakness or injury so that the muscles which would normally raise the foot appropriately are not able to.
This causes the toes to drag where they normally would not, which is then compensated for by an exagerated walk to raise the toes high enough to clear. There are other compensatory movements also.
In our case, it might be tied to damage of the sciatic nerve during surgery, since the nerve that controls foot movement extends from the sciatic nerve at the knee (please correct if I'm wrong on any of this). There are other ways for this to happen (spinal injury, more rarely muscle disease, etc.). Apparently it is possible to recover some or all mobility, but I'm not sure what the percentages are.
Here's a reference, I'm sure Pat also has other references and if you do a search of the site, you'll probably find it discussed.
http://en.wikipedia.org/wiki/Foot_drop