For decalcified rat tibia, or undecalcifed bone?

Decalcified is easy, just do that first and use a clean, new razor blade.
I prefer the teflon coated razor blades from Ted Pella, used for EM,
however a low profile microtome blade could be used, guard you fingers.
Be sure to slice and not press the blade through the bone in order
to prevent crushing.

For undecalcified, you may need to use a diamond cutoff blade, if you have
one available, other wise you will crush the bone/cartilage, etc. shattering
interfaces you wish to maintain.  Hand sawing could possibly be done, if
you are extremely careful, finding a tiny narrow saw is not always easy,
jewelers saws, wire type can work, but flexible.  This would not be terrible
easy if you want the full length of tibia, through the diaphysis.  Some
small devices, called Dremel tools may work, but getting a blade to saw
through the bone was discontinued years ago, however, a hobby shop had
blades for other type autotools, that might be adapted.  Try to keep the
bone stable, held down without crushing.

Good luck  and let us know what worked for you.
Gayle Callis