I think about the implications behind Tomura’s teeth a lot....
Like... as a child, we see him vomiting in response to extreme stress multiple times. Hori had a finite number of pages to work with during MVA, and despite that, he dedicated three separate instances to Tomura losing his lunch while under extreme stress. When he wears his family’s hands, he explicitly states that it makes him feel like he’s going to vomit. And again, after he’s goaded into murder by AFO, he explicitly mentions how ill he feels-- “I feel sick.” “Like I’m going to hurl.” The overall implication being that Tomura has basically vomited from stress -so- much and -so- frequently that his teeth are ruined beyond repair. Frequent vomiting wears down enamel and gum tissue, vastly increasing a person’s risks for cavities even if they do otherwise brush their teeth regularly.
As an aside: AFO’s grooming rears its ugly mug even when the narrative isn’t explicitly drawing attention to it, as usual-- with adult Tomura reframing his nausea as something “awesome.” Yeah, it’s totally awesome that you feel like vomiting whenever you kill people, Tomura! I’m sure there’s no underlying reason for that! :’)
There’s also the fact that he’s so malnourished you can see his spinal cord and clavicles protruding from beneath his skin during his first appearance. And there’s so little body fat that the tendons/bones in his hands are also sharply visible.
Honestly, Hori is excellent when it comes to using his art to convey what’s going on behind the scenes and between the lines-- and that’s especially true in Tomura’s case. There is so much of Tomura’s story that’s conveyed through art alone, and dissecting that art really is necessary if you want to understand the full scope of his abuse at the hands of AFO. This is also why I don’t feel that Tomura being an unambiguous victim in need of rescuing was a “last minute” decision on Hori’s part-- because the evidence that he was being mistreated was always there.