WASHINGTON (AP) – Medicare says it’s expanding a major experiment that strives to keep seniors healthier by coordinating medical services to prevent common complications that lead to hospitalization.

Officials Monday announced 121 new “accountable care organizations.” They are networks of doctors and hospitals collaborating to better serve patients with chronic medical conditions.

The so-called ACOs work to improve quality and lower costs. Part of their Medicare payment is based on how well they meet those goals.

Monday’s announcement means 8.9 million beneficiaries will get their care through ACOs. That’s close to 1 in 4 seniors with traditional Medicare.

And Medicare is also opening the door to seniors picking an ACO, a choice officials say is best made in consultation with their doctors. Twenty-one new ACOs will be allowed to recruit patients.