RSMo Section 287.020.3 (1) In this chapter the term "injury" is hereby defined to be an injury which has arisen out of and in the course of employment. An injury by accident is compensable only if the accident was the prevailing factor in causing both the resulting medical condition and disability. "The prevailing factor" is defined to be the primary factor, in relation to any other factor, causing both the resulting medical condition and disability.
(2) An injury shall be deemed to arise out of and in the course of the employment only if:
(a) It is reasonably apparent, upon consideration of all the circumstances, that the accident is the prevailing factor in causing the injury; and
(b) It does not come from a hazard or risk unrelated to the employment to which workers would have been equally exposed outside of and unrelated to the employment in normal nonemployment life.
(3) An injury resulting directly or indirectly from idiopathic causes is not compensable.
(4) A cardiovascular, pulmonary, respiratory, or other disease, or cerebrovascular accident or myocardial infarction suffered by a worker is an injury only if the accident is the prevailing factor in causing the resulting medical condition.
(5) The terms "injury" and "personal injuries" shall mean violence to the physical structure of the body and to the personal property which is used to make up the physical structure of the body, such as artificial dentures, artificial limbs, glass eyes, eyeglasses, and other prostheses which are placed in or on the body to replace the physical structure and such disease or infection as naturally results therefrom. These terms shall in no case except as specifically provided in this chapter be construed to include occupational disease in any form, nor shall they be construed to include any contagious or infectious disease contracted during the course of the employment, nor shall they include death due to natural causes occurring while the worker is at work.
RSMo Section 287.220.1. All cases of permanent disability where there has been previous disability shall be compensated as herein provided. Compensation shall be computed on the basis of the average earnings at the time of the last injury. If any employee who has a preexisting permanent partial disability whether from compensable injury or otherwise, of such seriousness as to constitute a hindrance or obstacle to employment or to obtaining reemployment if the employee becomes unemployed, and the preexisting permanent partial disability, if a body as a whole injury, equals a minimum of fifty weeks of compensation or, if a major extremity injury only, equals a minimum of fifteen percent permanent partial disability, according to the medical standards that are used in determining such compensation, receives a subsequent compensable
injury resulting in additional permanent partial disability so that the degree or percentage of disability, in an amount equal to a minimum of fifty weeks compensation, if a body as a whole injury or, if a major extremity injury only, equals a minimum of fifteen percent permanent partial disability, caused by the combined disabilities is substantially greater than that which would have resulted from the last injury, considered alone and of itself, and if the employee is entitled to receive compensation on the basis of the combined disabilities, the employer at the time of the last injury shall be liable only for the degree or percentage of disability which would have resulted from the last injury had there been no preexisting disability. After the compensation liability of the employer for the last injury, considered alone, has been determined by an administrative law judge or the commission, the degree or percentage of employee's disability that is attributable to all injuries or conditions existing at the time the last injury was sustained shall then be determined by that administrative law judge or by the commission and the degree or percentage of disability which existed prior to the last injury plus the disability resulting from the last injury, if any, considered alone, shall be deducted from the combined disability, and compensation for the balance, if any, shall be paid out of a special fund known as the second injury fund, hereinafter provided for. If the previous disability or disabilities, whether from compensable injury or otherwise, and the last injury together result in total and permanent disability, the minimum standards under this subsection for a body as a whole injury or a major extremity injury shall not apply and the employer at the time of the last injury shall be liable only for the disability resulting from the last injury considered alone and of itself; except that if the compensation for which the employer at the time of the last injury is liable is less than the compensation provided in this chapter for permanent total disability, then in addition to the compensation for which the employer is liable and after the completion of payment of the compensation by the employer, the employee shall be paid the remainder of the compensation that would be due for permanent total disability under section 287.200 out of a special fund known as the "Second Injury Fund" hereby created exclusively for the purposes as in this section provided and for special weekly benefits in rehabilitation cases as provided in section 287.141. Maintenance of the second injury fund shall be as provided by section 287.710. The state treasurer shall be the custodian of the second injury fund which shall be deposited the same as are state funds and any interest accruing thereon shall be added thereto. The fund shall be subject to audit the same as state funds and accounts and shall be protected by the general bond given by the state treasurer. Upon the requisition of the director of the division of workers' compensation, warrants on the state treasurer for the payment of all amounts payable for compensation and benefits out of the second injury fund shall be issued.