925H
Fig. 14.12 Unilateral posterior cross-bite with lateral mandibular displacement. (a)
Initial contact on closure. (b) Lateral displacement of the mandible on closure into
maximum intercuspal position causing unilateral posterior cross-bite. (c) Upper
expansion appliance. (d) Displacement has been eliminated after upper arch
displacement.
14.4.3 Increased overjet
The incidence of trauma to the upper incisors is greater where the overjet is increased,
to the extent that among 13 year olds twice as many children with overjets of 10 mm
or more have traumatized upper incisors compared with children with overjets of less
than 5 mm. Boys are at greater risk than girls. Reducing the risk of trauma is a good
reason for early reduction of a large overjet, even without cosmetic considerations.
In the mixed dentition this is usually done with a functional appliance. Details of the
management and effects of these appliances can be found in orthodontic texts, but
they induce correction of the incisor and molar relationships by a combination of
dentoalveolar and skeletal changes. This is not done by active components such as
springs, but instead the appliances harness forces generated by the masticatory and
facial musculature. They achieve this by holding the mandible in a forward postured
position, and all designs of functional appliance are similar in that they engage both
dental arches and cause mandibular posturing and displacement of the condyles
within the glenoid fossae (926HFig. 14.13 (a)-(f)).
Functional appliances have two main limitations: they only work in growing children,
most effectively during periods of rapid growth; and, while they change the occlusion
between the arches, they cannot treat irregularities of arch alignment such as
crowding.
In practice, these limitations mean that functional appliance treatment can become