Even if the policy change was made, Medicare would still pay for patients to get traditional inpatient surgery. But with the agency also paying for the bulk of outpatient procedures, there would be a huge shift in money — out of hospitals and into surgery centers. Medicare could save hundreds of millions of dollars if it no longer needed to pay for multiple-day stays at the hospital. Investors at the outpatient centers could profit greatly, as could some surgeons, because doctors often have an ownership stake in the outpatient centers where they operate.

Whether the shift would be beneficial for patients remains an open question. Medicare patients tend to spend nearly three days in a hospital, data shows. Forty percent of Medicare patients also spend time in a rehabilitation facility for further recovery. The data, which reflects knee replacement operations from 2014, suggests that Medicare patients are taking advantage of the post-operation support at hospitals and aftercare centers. Given that, it is unclear what percentage of eligible patients would choose outpatient care.

But improvements in surgery — from new medicines to control bleeding to better pain management techniques — mean that, for some patients, the days of close medical supervision are no longer necessary.

Dr. Kirschenbaum, who is in favor of the change, acknowledged that outpatient surgery would be the right move for only a small subset of his Medicare patients — perhaps 10 to 15 percent — who have good caretaking at home and few chronic health issues. But it would not be for the people who are frail, live alone or in a dwelling with stairs, he said. The decision about whether surgery should be outpatient or inpatient tends to be made by the physician and patient.

“We want to make sure patients — when they go home, they’re safe, no question,” said Dr. Kirschenbaum, the chairman of orthopedics at Bronx-Lebanon Hospital Center and a founder of SwiftPath, a company that offers technical support to outpatient joint replacement centers.