I recently had a right knee replacement and wanted to print a set of the bones involved. Here they are!
These are downloaded from the life science 3D database at http://lifesciencedb.jp/bp3d/?lng=en
I only ran them through netfabb studio to clean up some errors and orient them correctly for 3D printing, any and all rights remain with the original source.
These are full size, so you will probably need to scale them for printing. I used 45% to fit the femur diagonally on my Printrbot Plus. They'll need to be printed with support.
Scale to fit your print platform, slice with support.