Mode of action

Functional appliances work by posturing the mandible forwards, which causes soft tissue stretching. This generates Class II intermaxillary traction forces (Figure 39.1A). The resultant correction in overjet is produced by combination of tooth movement (70%) and skeletal change (30%) (Figure 39.1B). The effects of functional appliances are:

•  Dentoalveolar changes - these include retroclination of the maxillary incisors and proclination of the mandibular incisors due to the Class II traction forces developed between the arches, and because the upper incisors come under the control of the lower lip with anterior mandibular posturing.

•  Increased mandibular length - due to downward and forward translation of the condyle which may encourage backward compensatory growth. In the short term, mandibular length may increase by 2-4 mm although there is great individual variation. There is evidence from long-term studies that the early growth benefit may be lost with time.

•  An increase in lower anterior face height (LAFH) - due to a combination of molar eruption and downwards mandibular growth. This is useful where there is a deep overbite (OB)/short LAFH and detrimental in those with a reduced OB/ increased LAFH.

•  Forward remodeling of the glenoid fossa - this occurs secondary to anterior condylar repositioning.

•  Restraint of maxillary growth - due to the Class II traction forces acting on the maxilla. Incorporation of headgear into functional appliance treatment increases this effect.

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