Mo. Rev. Stat. § 169.577
Any member of a retirement system subject to the provisions of this chapter, who is within five years of being eligible to retire with a retirement allowance as provided in this chapter, may elect to purchase additional creditable service of up to five-tenths of a year which shall, when so purchased, be included in the total of the member's years of creditable service, used to enable the member to achieve the minimum creditable service time required for a retirement allowance, and applied in the computation of the member's annual service retirement allowance. For any member of a retirement system established by sections 169.010 to 169.141 or 169.600 to 169.715, and notwithstanding any other provision within this section to the contrary, the purchase shall be effected in the same manner as provided in section 169.056. The request for purchase of the additional creditable service shall be made in writing to the board of trustees of the system in which the applicant is a member. The purchase shall be effected by the member paying to the retirement system the amount required by the rules and regulations established by the respective retirement system, or absent such rules and regulations, the amount, with interest, the member would have contributed thereto and the amount the member's employer would have contributed thereto had the person been employed in a position covered by the retirement system for the number of months for which the member is electing to purchase credit, and had the member's compensation during such period been the same as the annual salary rate at which the member is receiving at the time of application, and the contribution rate in effect on the date of election to purchase credit. The payment shall be completed prior to termination of membership with the retirement system with interest on the unpaid balance. Nothing in this section shall be construed to allow a member to vest in the retirement system by using the creditable service purchased pursuant to the provisions of this section to reach the time of vesting.
§ 169.577, RSMo