The Missouri Department of Insurance has imposed $536,000 in fines on two sister health insurers in connection with allegations that they violated a state chiropractic benefits mandate.

The companies have offices in St. Louis and are subsidiaries of the health insurance arm of UnitedHealth Group Inc., Minnetonka, Minn.

A 2004 Missouri law requires state-regulated health insurers to cover the cost of at least 26 chiropractic visits per year without prior authorization. Coverage can be denied only if the treatment is determined not to be medically necessary, officials say.

Missouri regulators have reviewed the UnitedHealth subsidiaries’ treatment of chiropractic claims filed since 2004.

In some cases, the subsidiaries denied claims without evaluating the medical necessity of treatment, and, in other cases, the subsidiaries imposed other limits that violated the Missouri chiropractic benefits law, officials say.

The UnitedHealth subsidiaries have agreed to reexamine about 50,000 claims filed by chiropractors who treated the company’s policyholders, officials say.

If the subsidiaries denied claims improperly, they will have to reimburse the chiropractors for the claims and also pay interest, officials say.

A UnitedHealth spokesman said the company was seeking additional information about the Missouri announcement and could not answer questions about it.