Unilaterial posterior crossbite
Skeletal Class III crossbite
In a normal dentition, primary or permanent, the buccal cusps of the upper posterior teeth occlude on the buccal aspect of the lower teeth, the lingual cusps of the upper posterior teeth align with the occlusal surface of the lower teeth, and the upper anterior teeth are positioned to the labial surface of the lower anterior teeth.
When these relationships do not exist in fully erupted teeth, one or more teeth will be in a “crossbite”. If this mal-relationship occurs with anterior teeth, the crossbite is referred to as an “anterior crossbite”. If it occurs with posterior teeth, it is a “posterior crossbite”. When a crossbite only occurs on one side of the dental arches, it is referred to as a “unilateral crossbite”. When it occurs on both sides, it is called a “bilateral crossbite”.
If the upper and lower cusp tips of the teeth occlude tip-to-tip, but demonstrate a 'functional shift' to the right or left upon full closure to achieve maximum intercuspation, the resulting crossbite is referred to as a “pseudocrossbite”, and not considered a “true” crossbite.
While a crossbite may be due to a mal-posed tooth or teeth, i.e. a “dental crossbite”, a crossbite may also be the result of a severely narrow or a horizontally mal-positioned maxilla or mandible, i.e. a “skeletal crossbite”.
Depending on the extent of the dental crossbite, a dental expansion procedure may be used in conjunction with movement of the teeth with a treatment appliance, like clear aligners or braces, to correct the crossbite. Often, dental crossbites can be completely corrected with clear aligners alone.
Removable aligners are especially effective in the predictable correction of dental crossbites, unilateral or bilateral. This is true because there is a layer of plastic (the aligner) that separates the upper and lower arches during the corrective tooth movement, allowing the tooth/teeth in crossbite to move without conflict from any opposing teeth.
When the crossbite correction is being made with fixed braces, turbos or bite blocks may be adhered to selected teeth to separate the arches during the correction to reduce occlusal collisions of the moving teeth.
Correction of a skeletal crossbite must include a skeletal expansion and/or surgical procedure to optimize the defective skeletal foundation before re-arranging the teeth.