Applications for disability retirement shall be referred by the director to the medical board. The medical board shall investigate all essential statements and certificates by or on behalf of the member in connection with the application for disability retirement, and shall pass upon, conduct, or cause to be conducted, all medical examinations which in its opinion are necessary to determine the cause, extent, and permanence of the member's disability. The medical board shall make and file with the director a written report of its conclusions and recommendations.