Here is our answer: In the interior of the vas deferens we have a mucous. Because the vas is a kind of muscularly tissue which can provide the mucuos with blood, this mucous moisten the inside of the tube. So there will be no problem for the sperm to flow upside again, even after a long period of status “closed”. As a proof you may consider all the successful surgeries, which have been done to men who had vasecetomies more than a decade ago. Therefore we do not see any problem of a kind you mentioned. Thanx a lot DB