The removal of the Pisiform bone is an "Ostectomy" and/or "Carpectomy," removal of an "entire" bone from the hand, so I would look for that under the "Excision" or maybe the "Reconstruction" sections of the CPT codes of the wrist and hand. As an identifiable procedure, it will probably not show up because this is such a rare procedure, but is "comparable" to removal of a carpal bone such as the Navicular or Lunate, which are Carpectomies. You may have to think in terms of Tenosynovectomy of the Flexor Carpi Ulnaris with Carpectomy of the Pisiform, with Modifier 22 for a more extensive procedure.
I hope this helps. I am at a meeting and don't have my CPT book with me, so I can't come up with more accurate codes.
Respectfully submitted, Alan Pechacek, M.D.