I'm not so sure about that. I would expect that the actual hardware represents only a small portion of the total cost of getting a knee replacement. I wouldn't be surprised if the cost savings of going with used hardware doesn't really justify the shorter expected lifespan of the prosthesis.

Also, the wide variation in the price of a knee replacement, even within a single market, suggests that using used hardware is probably not the low-hanging fruit when it comes to reducing the cost. e.g., https://www.bcbs.com/news/press-releases/blue-cross-blue-shi...