FSA balances are generally “use-it-or-lose-it” although some companies allow a grace period for filing claims.  Because of the “use-it-or-lose-it” nature of the account, it is helpful to plan ahead for larger medical expenditures so that you have a proper FSA balance to utilize. You can't use FSA dollars saved in a future year to pay off a procedure completed in a prior year, even if the procedure was financed and you are still making payments.
HSA balances do have a carry-over feature, so patients can continually save on a pre-tax basis and use the funds when needed. This means funds put into your HSA account in past years can be used for procedures performed in future years. This is a great feature for smoothing out health care related costs.