We used to tell patients to wait as long as possible prior to considering a knee replacement, given the extensive nature of the procedure and the protracted recovery period.   According to a recent study... that advise might be off base. 

Women with osteoarthritis of the knee wait longer than men before pursuing knee replacement surgery, according to a study published in the November 2007 issue of The Journal of Bone and Joint Surgery. This delay in treatment may result in poorer post-operative outcomes for female patients.

 It seems that the possibility exists--- that the longer we wait and the more deterioration that occurs in the arthritic joint--- the poorer the outcome might be following a knee replacement.