Neutral buffered formalin should work fine.  Remove skin, and
dissect extra muscle away if it is not needed.  If you need knee joint,
do this carefully so as to not disturb the knee joint capsule and cartilage
of knee.  It takes longer to fix a mouse knee joint than one thinks, we
did ours for 3 - 4 days, up to a week, with a change of NBF after 24 hours
or so.  Be sure to use 20x volume of fixative to tissue.  The NBF is depleted
by too much surrounding tissue, and mouse bone is denser than one thinks.
If you do not fix totally, decalcification and subsequent processing will
be poor, for both paraffin and plastic (PMMA) technics.  Always felt more
fixative is better than a small volume, be generous!

Good luck,
Gayle Callis