Effects of functional appliances
For class II with deep overbite
• Retroclination of the upper incisors and proclination of the lower incisors are usual, although the latter is not found consistently and is best minimised by placing acrylic capping on the lower anterior teeth.
• Inhibition of lower incisor eruption and promotion of eruption of the posterior teeth leads to levelling of the curve of Spee. This process is facilitated by lower incisor capping on the appliance.
• Guidance of eruption of the lower posterior teeth in an upward and forward direction while preventing eruption and forward movement of the upper posterior teeth encourages correction of a class II buccal relationship.
• Arch expansion is intended through the buccal shields of the Frankel appliance and the buccal wire of the Bionator. Long-term stability of these changes is unsubstantiated.
• Enhancement of mandibular growth is brought about by movement of the mandibular condyle out of the fossa, promoting growth of the condylar cartilage and forward migration of the glenoid fossa. This effect is very variable.
• Inhibition of forward maxillary growth occurs.
• An increase in lower facial height is mediated by the alterations in the eruption of the posterior teeth, as described above.
For class III
• Proclination of the upper and retroclination of the lower incisors can be achieved.
• The appliance facilitates downward and forward eruption of the upper posterior teeth while inhibiting eruption of mandibular teeth.
There is no evidence to substantiate an anteroposterior skeletal effect, but lower face height is increased.