Medicare paid out hundreds of millions of dollars in 2013 based on improper bills from chiropractors, according to an investigation by a federal watchdog agency.

More than 80 percent of payments to chiropractors should not have been made, investigators found. This isn't the first time the Centers for Medicare and Medicaid Services has been warned repeatedly that chiropractors are billing for services that don't comply with Medicare requirements. Yet CMS has yet to address vulnerabilities in the payment system, according to the report by the Office of Inspector General for the Department of Health and Human Services reported.

Medicare is only supposed to cover manual manipulation of the spine in some cases of subluxation - where spinal bones are misaligned. But in studying records on a sample of 43 services, subluxation wasn't present in 37, the OIG found. For the remaining six, subluxation was present but medical records didn't indicate it was treated by manual manipulation.

"Strong controls to prevent improper payments for chiropractic services are important to program integrity," the report says. CMS should consider limiting the number of chiropractic services a year that Medicare will cover, say to 30, and determine a reasonable length of time for chiropractic treatment.

Click here for an explainer on what chiropractic services are covered by Medicare.