BRISTOL PALIN is admitting that she had work done on her face . . . but she says the surgery was CORRECTIVE, not cosmetic.

She says, quote, "Yes, it improved the way I look, but this surgery was necessary for medical reasons . . . so my jaw and teeth could properly realign.  I don't obsess over my face."

But she adds, quote, "I'm absolutely thrilled with the results.  I look older, more mature and don't have as much of a chubby little baby face."

She added that she wouldn't have elective plastic surgery unless, quote, "I got in an accident or something terrible got disfigured."

Bristol says she went under the knife last December, after she got the boot from "Dancing With the Stars".

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