I would agree that your problem is not cosmetic, but reconstructive. If your doctor has written to the insurance company for pre-authorization for this procedure and surgery has been denied, then I would take steps to directly contact your carrier and obtain information about the basis for their decision. Also ask for the criteria they use to make the distinction between cosmetic and reconstruction surgery. Review this information with your doctor.
Finally, if all else fails, you may consider obtaining legal help to threaten a bad faith legal action. Another option would be to take the issue up with your state's insurance commission.