If you are worried about the knot having come undone then I think your husband needs to speak to his doctor to be on the safe side. When my fiance had his vasectomy he had to take a semen sample in to the doctor a few months after to make sure there were no sperm present,has your hubby done that yet? I'm not sure whether you are supposed to be able to feel the knots so I had to ask my fiance whether he could detect his,he thinks i've gone mad as he had the operation 10 years ago and i've only just asked him.lol