The human permanent teeth can be divided as front (anterior) teeth and back (posterior) teeth. The anterior teeth are namely the central incisors, lateral incisors and canines. The posterior teeth are the premolars and molars. The proper harmony of these teeth are maintained when they are properly aligned and are met together in proper bite. The milk teeth shed to pave the way for permanent teeth. However, when this shedding-eruption pattern is disturbed by genetic or environmental causes, certain bony changes do not allow the teeth to erupt in their normal position. Some of the reasons that explain this abnormality includes abnormal tongue forces and patients exhibiting mouth breathing who have a lower positioned tongue.

The upper and lower jaw arches need to be sufficient in length and width to compensate all the permanent teeth. A proper harmony between the upper and lower teeth indicates a normal bite of the person. When any tooth moves out of the arch, it hampers its relation with the antagonist tooth in the upper or lower arch. This leads to the affected tooth or teeth move in a sideways direction. The tooth can drift either towards the cheek or towards the palate/tongue. When such a scenario affects the back teeth of either arch, it leads to the development of a posterior crossbite. This condition leads to an abnormal locking of the upper and lower back teeth with each other.

A posterior crossbite can be of multiple types depending on the tooth involved and the direction in which it has been displaced. The severity can also depend on whether the crossbite is present on one side or both sides. The most common complication of a posterior crossbite is asymmetry of the face. The tissues and bones are in abnormal strain due to the posterior crossbite which results in warping of the tissue systems. The upper jaw constricts whereas the lower jaw bones distort towards the cheek region.

The treatment of posterior crossbite is started only after recognizing the total width of the upper and lower jaw arches. Based on this measure, and the teeth involved, it is decided whether the upper jaw needs to be expanded or constricted in a transverse dimension (sideways when viewed from the front). Posterior crossbites at times are caused because of improper relation of the upper and lower deciduous (milk) canines. Thus, one of the treatment approaches to posterior crossbite involves recontouring of these canines in order to ensure the bite is corrected. This solves the posterior crossbite in 2 to 4 weeks.

Posterior crossbite can also occur because of abnormally excessive growth of the lower jaw on one side. Another factor is prevalence of thumb sucking habit since childhood. The treatment in this case should involve breaking the habit by an appliance and later rectifying the posterior crossbite.

The dentist usually corrects posterior crossbite by making the patient wear elastics, hooks or bands in order to move the tooth or teeth in crossbite. The forces generated by these components can push the teeth into the desired area. Expansion of the upper arch is attained by fitting an appliance with a screw-based mechanism in the centre. This screw needs to be rotated at certain angulation at regular intervals of time (1 or 2 weeks) in order to gradually expand the arch. When the arch expands, the tooth or teeth in crossbite properly meet (in bite) their adjacent counterpart of the other jaw.

The treatment of posterior crossbite should be carried out in children between 4 to 10 years of age. The treatment time varies from 2 weeks to 12 weeks. However, there are certain precautions that must be taken while resolving posterior crossbite. Most of these measures should be such that the forces generated by the appliances do not have an adverse effect on the bony architecture. The most common side-effect during expansion occurs when the suture in the centre of the upper jaw begins to stretch and open. Such abnormal forces should not be generated.

It should by no means understood that generation of higher forces for rapid treatment should not be done. In severe cases the rapid expansion of maxilla can correct the crossbite. However, whenever such a treatment modality is concerned, the dentist is likely to replace the expansion appliance with a retention appliance that keeps/retains the teeth in their new position.

Posterior crossbite must be recognized at the earliest as the constriction of the upper arch might result in upper airway obstruction. Moreover, improper bite by the abnormal closure of the lower jaw can cause disorders of the temporomandibular joint (TMJ), which connects the lower jaw to the skull. Thus, considering all the factors including age, severity of the crossbite and the underlying mechanism, the necessary treatment modality is considered to attain a proper occlusion.