If it's really about "physical trauma"—which itself is debatable—then the problem would be that lots of past chances to unblock it had occurred that the writers ignored for the sake of Screwtherulesihaveplot: first, Aang getting poked in the back generally during those months before he faced Ozai (Toph alone did this all over his back with rocks); second, applications of healing water (even from the Spirit Oasis, once) which normally can fix the harms of physical trauma quite well; and third, Aang's own natural healing process, which could arguably have occurred during those months.