I would read the Operative Report very carefully, and talk to your surgeon as well. If the main problem for which the procedure was done was on the acetabular side, then the replacement of the femoral head (only) on the femoral side may have been "coincidental" in that it would have to be removed to access/expose the acetabulum for revision. If the original femoral head removed was not damaged, worn, etc. in such a way that it needed to be replaced by a new one, then putting in a "new" one would not necessarily be considered a part of the "Revison Arthroplasty" (27134), and the Acetabular Revision (only) Code 27137 would apply. However, even if the original femoral head component was not damaged as such, replacing it would be done by the vast majority of surgeons, and putting the original one back on is just "not going to happen," i.e. New Acetabulum = New Femoral Head. If however the original femoral head did show wear or damage, then replacing it with a new one would be appropriate/indicated, and 27134 with Modifier 52 for Reduced Services would be correct since the entire femoral component was not redone.
Hopefully this helps.
Respectfully submitted, Alan Pechacek, M.D.