User:Globalorthodontist/Functional Mandibular Shift

Functional shift of mandible occurs in patients with unilateral posterior crossbite. The shift has been reported to occur in 80-90% of unilateral posterior crossbite cases. This shift occurs in primary dentition about 8.4% and 7.2% in mixed dentition.

Etiology
Posterior crossbite is know as where the lower molar teeth are more outwards than the upper molar teeth. Small width of maxillary arch, is known to cause either unilateral or bilateral crossbite, leading to a functional shift of mandible. When a person bites down in their Centric relation bite, their midlines are matching and they have a cusp-to-fossa relationship. However, their centric relation bite is not coincident with the Centric occlusion bite. When they bite further from their CR position, their jaw tends to move to either side and thus causing a shift in the mandible. This manifests itself with following characteristics:
 * Mandibular dental midline deviation
 * Mandibular chin deviation
 * Posterior unilateral crossbite
 * CO not coincident with CR position

Treatment
It's been recommended to correct functional crossbites as early as possible. Usually, the treatment is not started until the 1st molar permanent tooth has been erupted.

Self-Correction
Self-correction of a shift of mandible leading to a posterior unilateral crossbite has been reported to be 0-9% successful.

Expansion
Main Article: Rapid palatal expansion

Rapid Palatal expansion has been known to correct functional crossbite in mixed dentition 84-100% of the time. Expansion through other devices such as W arch, quad helix, Haas expander, Hyrax expander or superscrew have also been described in the literature. The crossbite is expected to be corrected within 6-12 weeks after a quarter-turn is done on the screw every third day. Other methods and protocols of expansion have been described in the literature in the past.

Temporomandibular Disorders
Early correction of the functional crossbite may prevent temporomandibular disorders. Studies have been published which favor this relationship but others do not, so the causal relationship is still not properly understood.