Norwalk, Conn.-based Oxford Health Plans Inc. said more than 700 radiologists, orthopedists and other specialists agreed to treat its patients for a per-case fee, instead of billing for individual visits and treatments. Oxford Health, one of the largest U.S. health maintenance organizations, said it began this new offer to specialists in April. Although HMOs already pay general practice physicians flat fees for treating patients, the move by Oxford to enlist specialists appears to be an innovation, analysts said. The new method could cut the costs of specialists' treatment by 15% to 20%, said Edward Keaney, an analyst with Volpe Brown Whelan & Co. Another advantage is that specialists will be able to send patients for additional treatments and referrals to other doctors without first checking with Oxford. Oxford shares fell $3.13 to close at $73.75 on Nasdaq.