IMO, it's all about a persons id or ego.
As long as it's 100% natural, he's got a clean aura and full essence. Doesn't matter if he's lost 2 limbs, an eye and had a kidney removed. Everything he is, is still pure and natural, his body still true to his id/ego.
But the moment something unnatural becomes part of his or her being, that being becomes something else, something less pure and natural and has deviated from his spiritual id/ego, which is resembled by essence loss and a visibly altered aura.