Unilateral Posterior Crossbite

Unilateral Posterior Crossbite

   Fig 1. Unilateral Posterior Crossbite

Unilateral Posterior Crossbite  is manifested as a normal upper palatal cusps in lower central fossa of posterior teeth on one side; but on the other side - the upper buccal cusps of posteriors occlude in the central fossa of the lower posteriors. This is clearly seen on the picture above where there is normal relationship on the left side but cross bite on the right side.

Causes:  The main reason is constriction of the maxilla commonly caused due to -

• Thumb sucking and other non-nutritive digit sucking or long term use of pacifiers.
• ENT issues such as deviated nasal septum, nasal polyps, enlarged adenoids, etc. which all make the patient a mouth or oral breather rather than a nasal or oro-nasal breather.
• Any other developmental issues or syndromes

Is the Constriction Unilateral in Unilateral Posterior Crossbite ?

The Answer Is A Resounding NO

Then what causes a Unilateral Posterior Crossbite.....

In most cases the constriction occurs bilaterally but this results in an unstable 

upper buccal cusps to lower buccal cusps; and 

upper palatal cusps to lower palatal cusps relationship.

There is a Functional Shift of the Mandible towards one side to result in a stable maximum intercuspation position - upper buccal cusps in the central fossa on the side where it shifts and upper palatal cusps in the lower central fossa on the other side (resembling the normal relationship) as seen in the picture below.

 Fig. 2 - Functional Shift of the Mandible👆

 This Functional Shift of the Mandible manifests as a Midline Shift at the centre of the oral cavity as seen   above.

Fig. 3 - Observe the midline discrepancy due to the functional shift of the mandible to the right.👆

The constriction of the maxilla is bilateral but the manifestation is unilateral due to the functional shift of the mandible towards the cross bite side as evidenced by a midline discrepancy.

Clinical Significance : 

• This condition has been associated with Temporo-Mandibular Disorders. Although TMD is multifactorial, and the unilateral posterior crossbow does not cause TMD, it is a co-factor or as associated finding.
• Functional shift of the mandible is better to be corrected as it represents a significant CR- CO shift which is unhealthy.
• The constriction also manifests as clinical crowding in several cases due to decreased arch perimeter.

Treatment :

     The treatment is always bilateral expansion by means of devices such as rapid or slow palatal expansion plates, quad helix and its modifications, NiTi palatal expanders, MARPE or micro-implant assisted rapid palatal expansion, etc.

      The best time for treatment is the Mixed Dentition stage if the patient reports at an appropriate age.

Conclusion:

      The article is meant to dispel all notions about occurrence of a unilateral disorder which is actually a bilateral constriction.