(a) Once each year during the first five years after a member retires for disability, and once in each three-year period after that, the board of trustees may require a disability retiree who is less than 60 years old to undergo a medical examination by one or more physicians the board designates.
(b) If a disability retiree refuses to submit to a medical examination as provided by this section, the board of trustees shall discontinue the retiree's annuity payments until the retiree submits to an examination.