I've got two replacement knees . Left one five years ago , right one a month ago and still healing . Your wife aint gonna be doing hair scrambles or MX the torque is a real problem until the prothesetic (fake knee parts) grows into the bones , A lot of MD's don't use cement for the thigh part they pound it on with a mallet and let the bone grow around it ! Not joking here that's how it's done . You will be at least a year before you can do serious off road ridin , you aint gonna ever race but you can eventually get out there and ride/thrash that beast .
With that said , look at the bright side at least you can still walk and after a while you can ride trails , and there is what I'll be doing all summer therapy and ridin my street bikes ! it's also great fun in airports cause you will set off the metal detectors every time !
maybe later this summer I'll get to try the 450X on some easy trails but it won't be for a couple of months