I would seek legal help here on a consultation basis.  You may need to sue the HMO to get them to pay for the operation and that may be more expensive than the operation itself.  Consumers of HMO's have certain rights.  That have the right to be advised of all forms of treatment regardless of whether or not their plan will pay for it; they have the right to appropriate treatment, ask questions, file complaints and file lawsuits.  So if you think that a letter from an attorney would make them cringe a bit and possible give in, do it.  Try and negotiate a flat fee with the attorney to write the letter and inquire on your behalf.  If it does not work then you will need to discuss the suit and that will be hourly (but ask if the fee is recoverable if you will).  Good luck.