No, the cup isn’t deforming the positioning of their teeth.
But that is a good guess.
Most therapists know that the prolonged use of these covered cups slow down learning the mouth and jaw control needed to drink from an open cup. So that is a terrific reason to ditch the Sippy cup as soon as possible. But the big reason that Sippy cups can damage your child’s teeth is the same reason sipping frequently from your water bottle all day long could contribute to your own dental decay.
Kids are rinsing away the protective effects of their saliva.
Saliva is more than water. Anyone that has ever seen their granny spit on a tissue and clean something off successfully knows this. That looks gross, but Granny is aware of something that you might not realize. Saliva is filled with enzymes that degrade the harmful coating that bacteria leaves on your teeth. You simply cannot brush and floss all day long, but biology has provided you with a natural solution: saliva. Your spit is a key factor in protecting your teeth. People with disorders and diseases that reduce saliva know only too well what happens when this function is gone. They develop cavities, and progressively start to lose their teeth.
Dentists know this, but mine hasn’t said one word asking about whether I roam around with a water bottle all day. He should. Because limiting snacking AND not sipping constantly from a water bottle could protect my teeth. It is free, it is based in science, and it won’t be a lot of work to plan out my water and food consumption. It will save me the pile of money I would spend on expensive dental treatments, and it could prevent the pain of gum irritation and dental damage.
Of course, this means that your child needs to drink sufficient water throughout the day, and eat a decent amount at meals. That could require some thought. If you think it is less work to just fill a Sippy cup and be done with it, I would like to remind you that you have to take time off from work to bring a child in pain to the dentist, and you need to get up with them at night when they have dental pain. You just lost all the time you saved by handing them a Sippy cup. If you need another reason to ditch the Sippy, it is impossible to know when a kid with a Sippy cup who is also doing potty training needs to pee. Their bladder is always filling up and never empty. Oops.
But what about having to make sure they eat and drink during meals? Well, unless your child has a disorder or is a super-picky eater, they will be thirsty and hungry when mealtime and a single snack time between meals finally come around. A toddler that hasn’t been grazing from a bag of Goldfish crackers all morning is eager to eat up their snack and will wash it down. That is when their saliva gets to work on all that sticky Goldfish gunk between their teeth.