Helmets aren't mysterious. They are typically a piece of expanded polystyrene (EPS) foam covered with polycarbonate. EPS is essentially "styrofoam" -- like a styrofoam cup, but because it's thicker, it's more like one of those cheap white styrofoam coolers or a piece of white styrofoam that protects something in a shipping box. I'm sure everyone has broken a piece of that foam before. If the drop or hit wasn't enough to crack or break a piece of foam like that and there's no evidence the foam is cracked or broken, then it is probably just as good as new -- unless it isn't new. That foam degrades over time also, so it should probably be replaced after several years time and maybe sooner if it has endured a lot of wear and tear from frequent use. Some race sanctioning bodies will expire helmets after 2 or 3 years. I feel pretty good about my helmets out to 5 years. After that, the materials become suspect. I can't claim they're ruined, but I don't have a practical way to verify they're not.