Knees do not necessarily wear out evenly, sometimes one part of the knee is perfectly fine while another part is completely destroyed. If only part of the knee joint is worn out, why replace all of it?
Dual head-to-head racing is the most exciting thing you can watch in competitive skiing. It began in the 1960's with Henri Moreault in Aspen, was expanded into a pro racing tour by the late Bob Beattie in the 1970's, continued by Ed Rogers then fizzled out in the 1990's.
It is now back, under the guidance of Ed Rogers—and it rocks.