In a 2002 study, 75 patients ( 100 knee replacements ) were queried about whether they could kneel 6 months after a knee replacement.  Approximately two-thirds of the patients could kneel with little or no discomfort.  Approximately 25% could not kneel due to pain.  Over 50% of patients avoided kneeling because of uncertainty or recommendations from their surgeon or friends.  So to summarize, it is definitely possible to kneel after knee replacement and it should not be an activity that is avoided unless it is uncomfortable to do so.