Hi and welcome to YT! It's great that you found us.
You will find lots of information on the site about luxating patellas. Unfortunately LP is a common yorkie problem as it is with other toy breeds. You can do a search here, and will have more than enough information to read. I'm guessing either Buffy or Willow suffers from LP? My little Ruby does, but not severe enough for any surgery yet. I've been giving her joint supplements and have hopes of avoiding surgery as long as possible.
I probably should be able to guess why you picked the names you did, but really don't know. You'll have to tell me, sorry I couldn't figure it out!
Congratulations on your early retirement - I'm sure you are enjoying it immensely!