Tenet Healthcare Corp. is buying United Surgical Partners International, a portfolio company of Welsh, Carson, Anderson & Stowe, in a joint venture. Terms of the deal, call for Tenet to initially own 50.1 percent. Over the next five years, Tenet will have a path to full ownership through a put/call structure. Tenet said is paying $425 million cash to Welsh Carson and other existing shareholders. In a separate transaction, Tenet is buying Aspen Healthcare from Welsh Carson for approximately $215 million in cash. Aspen Healthcare will not be included in the new joint venture. Welsh Carson acquired Dallas-based USPI, a provider of a short-stay surgical facilities, in 2007. Aspen Healthcare Ltd., of London, operates nine private hospitals and clinics in the United Kingdom.