Herbst appliance is one of the most popular fixed functional appliances to treat orthodontic patients with small lower jaws. Since small lower jaws are reported to be one of the main reasons to induce sleep disordered breathing(SDB) of children, Herbst appliance can be used to treat SDB children. Previously no study compared the effects of Herbst appliance therapy with removable functional appliance on airway dimension, so it still remains unclear which kinds of functional appliance is more suitable for improving airway dimension. This study will assess the effect of Herbst appliance on airway dimension and compare the effects with those of twin block appliance (a popular 24 hours used removable functional appliance, and also commonly used to treat adult sleep apnea).

Sleep disordered breathing (SDB) is an important paediatric problem. It is estimated that 3.2% to 12.1% of children have habitual snoring and 0.7% to 10.3% have obstructive sleep apnea syndrome (OSAS). Children OSAS is associated with a series of daytime and nighttime signs and symptoms. Nighttime symptoms include heavy snoring, restless sleep, heavy seating and enuresis, etc. Daytime symptoms include excessive daytime sleepiness and abnormal behavior such as aggressiveness and hyperactivity, etc. Untreated OSAS was reported to result in serious morbidity, such as failure to thrive, poor learning, attention deficit and hyperactivity disorder, etc.

There is an increasing concern related to sleep disordered breathing(SDB) of children by orthodontists. Since not only the tonsilar and adenoid hypertrophy is the main cause of paediatric OSA, but also craniofacial anomalies such as retrognathic mandible, narrow maxillary palatal arch and increased lower facial height can also contribute to it, orthopedic treatment such as rapid maxillary expansion (RME) and mandibular advancement device (MAD) may potentially permanently cure the children OSA. RME which is used to widen maxilla, thus increasing the nasal cavity and improving the nasal flow, has been thoroughly researched. However, only a few previous studies focused on the treatment effect of MAD for treating OSA children. These studies showed sorts of MAD such as jaw-positioning appliance, mono-block, and Herbst appliance were effective to OSA children, but it remains unclear how to choose a functional appliance, and if there is any treatment difference between removable functional appliance and fixed functional appliance for improving airway dimension.

Inclusion Criteria:

1. Class II malocclusion

Exclusion Criteria:

1. Cleft lip and palate
2. Craniofacial syndrome