What happened to his face?

Uhh what do you mean?

Spoiler - it's from from wearing some sort of head since he was 14 (if you meant physically wrong).

Hmm I was pretty sure those were lobotomy scars....but who knows.

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I thought they were just marks left from the head. I am assuming that the head had some kind of band for support so it wouldn't fall off easily. I mean theres times where in the movie clearly if the head was not strapped in some how it should have been taken off a bunch of times but it was stuck on when pulled by people and when he fell the head never came off so to me it was obvious that it was strapped in somehow. Then you have to take into account how much and how long he wears it and I am sure the only time he does take it off is if he switches heads or changes his clothes but rest of the time when hes in the shower or sleeping he wears it. So when a strap is worn for days straight it will start to leave marks on your head and most likely it will kill your hair so since he wore it for so long the marks were kinda permanent because he had worn the head for so long and so much it was just one of those things and it may be possible if he decided to ditch the head from that point on that the marks over time will clear up and he will get it hair back.

I use to play baseball and sometimes I wore my hat real tight and after a long game you take your hat off you see similar marks Frank had in the movie so I took that and just multiplied the magnitude of how long he wore it for.