Let’s get some definitions out of the way. Upper teeth that bite inside lower teeth are called:
An anterior crossbite involves the front teeth and a posterior crossbite involves the back teeth.
Rarely, the upper posterior teeth bite completely outside the lower teeth in a condition called a scissor bite.
A posterior crossbite can involve either one side, called a unilateral crossbite, or both sides, called a bilateral crossbite.
Now that all the definitions are out of the way, lets look into why they are a problem.
All crossbites cause a shifting of normal chewing patterns. The lower jaw works best to chew food if it is free to move not just up and down but side to side and front to back. Think of a cow chewing its cud with the exaggerated jaw movements and you will get the idea.
When the upper jaw has one or more teeth that hang down inside the lower teeth, the full movement is not possible and a more up and down chewing movement results.
Upper front teeth can also look bad if they are in crossbite and the lower teeth rub the front of the upper teeth.
Most seriously, the lower front teeth are much more likely to have gingival recession or other gum and periodontal problems if they are pushed forward by upper teeth in crossbite.
How can crossbites be corrected?
Anterior crossbites are corrected with an appliance that attaches to the permanent molars and pushes or pulls the upper front teeth forward. Usually this is a rather quick procedure that might take a few months.
Once an anterior crossbite is corrected, it will usually stay corrected if the upper teeth cross over the lower teeth enough vertically.
Posterior crossbites are usually corrected by expanding the palate, but sometimes by just moving one or two teeth. See the article on Expanding Your Palate for more on this.
Correcting scissor bites are a bit more difficult and usually requires some braces, at a minimum on the involved teeth and the opposing teeth with a rubber band stretched vertically. More often, braces on all the top and bottom teeth in the area are more effective.
As you can see, crossbites can be a problem but can also be fixed with relatively little work.
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