Any participant who has at least 5 years of service credit toward retirement under the System (excluding military and naval service) and who becomes totally disabled or incapacitated for useful and efficient service by reason of disease, illness, or injury (not due to vicious habits, intemperance, or willful conduct of the participant) shall, upon his or her own application or upon order of the Secretary, be retired on an annuity computed as prescribed in section
4046 of this title. If the disabled or incapacitated participant has less than 20 years of service credit toward retirement under the System at the time of retirement, his or her annuity shall be computed on the assumption that the participant has had 20 years of service, except that the additional service credit that may accrue to a participant under this sentence shall in no case exceed the difference between his or her age at the time of retirement and age 60.
However, if a participant retiring under this section is receiving retired pay or retainer pay for military service (except that specified in section
8332
(c)(1) or (2) of title
5) or Department of Veterans Affairs pension or compensation in lieu of such retired or retainer pay, the annuity of that participant shall be computed under this part excluding extra credit authorized by this subsection and excluding credit for military service from that computation. If the amount of the annuity so computed, plus the retired or retainer pay which is received, or which would be received but for the application of the limitation in section
5532 of title
5 or the Department of Veterans Affairs pension or compensation in lieu of such retired pay or retainer pay, is less than the annuity that would be payable under this part in the absence of the previous sentence, an amount equal to the difference shall be added to the annuity computed under this part.