Orthopedists routinely recommend recumbent or stationary bike use after knee replacement but I've never read of anyone approve competitive cycling, and for good reason. First off, artificial knees can withstand injury but damage to your knee area can be catastrophic because multiple operations can leave you with permanent damage to muscle cover and structural bone / quad support. Second, while biking is generally seen as a low impact activity, peddling while standing can place tremendous stress on the knee due to a combination of lateral bike movement and constant weight shifts. I don't do criteriums but would guess the risk of injury is significant; I'll leave evaluation of the risk to others. It may be, though, that knee replacement and criteriums don't mix well.