• The teaching physician rules describe a payment method by which Medicare pays an attending physician or teaching physician for services performed jointly with an intern, resident, or fellow, in an approved graduate medical education program (GME).
• The teaching physician’s presence and participation is required.
• The rules regarding participation and documentation vary by the type of service.

From the CMS manual:

The teaching surgeon is responsible for the preoperative, operative, and postoperative care of the beneficiary. The teaching physician’s presence is not required during the opening and closing of the surgical field unless these activities are considered to be critical or key portions of the procedure. The teaching surgeon determines which postoperative visits are considered key or critical and require his or her presence.

The teaching physician must be present for all “critical and key portions of the procedure.”