Left knee is about 50% metal and has been for 25 years. They take some out and put some in about every ten years. No problem with riding, just keep your legs strong.
My brother rides an R1 with bilateral hip and knee replacements with no problems. Kurt

 

my dad had a knee replaced, they told him it was fully healed in 30 min after the adhesive cured.. that leg is better then his good one now..he couldnt ride before thou.. droped my hondas several times when i was growing up..

 

+1 I hardly ever feel the weight of my 850 pound bike.

 

Total replacement left knee 2003, fairly severe arthritis in right knee - balance is critical as has been mentioned; and I have shorter than average legs, so being able to flat-foot easily makes a big difference - at least for me.

Basically it's just a matter of making the necessary adjustments associated with getting longer in the tooth.

 

No fake ones yet, but the 2 I got aren't worth a damn. Between arthritis and missing/torn cartilage I won't be playing wide receiver any time soon.