Functional appliances are appliances that utilise, eliminate or guide the forces of muscle function, tooth eruption and growth to correct malocclusion.
Functional appliances are usually used in patients with a mild to moderate class 2 skeletal discrepancy. They are not effective at correcting tooth irregularities and improving arch alignment.
Functional appliances should be used when the patient is growing, treatment should coincide with pubertal growth spurt. Two types of treatment are in the early mixed or late mixed dentition.

  • Early mixed dentition (patient <10 years old): Initial phase – functional appliances. After the adult dentition erupts, secondary phase of fixed appliance.
  • Late mixed dentition: Functional appliance given during late mixed dentition, following the termination of functional appliance stage of treatment, the adult dentition has erupted, therefore fixed appliances can be applied straight away.
Treatment Using Functional AppliancesHow Do Functional Appliances WorkCommon Types Of Functional AppliancesClinical Management Of Functional AppliancesKey Facts About Functional AppliancesReferences

Functional appliances have been used to treat class 2 malocclusion. They typically are used to treat class 2 division 1 malocclusions but with minor alterations they can be used to adjust class 2 division 2 malocclusions.

Treatment Of Class 2 Div 1:

  • If arches are well aligned at the start of treatment and there is only anteroposterior discrepancy between the arches, functional appliance alone may be sufficient. Tell the patient to wear at night every day until growth has completed. Wear for longer to account for any relapse.
  • This is called growth modification or dentofacial orthopaedics
  • By correcting anteroposterior problems with functional appliance it reduces the amount of anchorage needed with fixed appliance stage
  • Following the functional stage, patient is reassessed for need of extractions/ fixed appliances

Treatment Of Class 2 Div 2:

  • Difficult to treat due to increased overbite
  • Using functional appliances followed by fixed is better than trying to treat this malocclusion with fixed alone.
  • Class 2 div 2 incisor relationship converted to class 2 div 1 then a fixed appliance is used.
  • Retroclined upper incisors can be proclined using pre-functional removable appliance/ sectional fixed appliance on upper labial segment
  • Some functional appliances can be modified to procline the upper incisors as part of the functional appliance phase of treatment

A modified twin block and FR3 Frankel appliance have been described to be used for class 3 malocclusions however there is no evidence of skeletal correction.

  • Principally work due to dento-alveolar changes: distal movement of upper dentition, mesial movement of lowers, tipping of upper incisors palatally, and lower incisors labially.
  • Some skeletal changes: degree of mandibular growth and maxillary restraint (1-2mm) too small to predictably replace the need for orthognathic surgery in severe skeletal discrepancies
  • Patients incisor relationship corrected allowing patient to comfortably obtain competent lips at rest.
  • It causes growth modification but it is minimal: doesn’t correct severe deformity
  • Improves malocclusion to a point where orthodontic camouflage rather than orthognathic surgery can be used to complete the treatment
  • Major difficulty: whether or not to give a child with a severe discrepancy growth modification. If the parents and child are happy that its only 20-30% improvement then its fine, if not then orthognathic surgery when pt is older should be considered.

Twin Block appliance

  • Constructed as 2 separate upper and lower parts which have interlocking posterior bite blocks which posture the mandible forward
  • Easy to modify
  • Needs to be 5mm high
  • Does create a (transient) bilateral posterior open bite at the end of the functional phase.
  • Trimmed as molars erupt
  • Compliance is a large problem

Side effect: lateral posterior open bite especially if they initially presented with a deep overbite. Some clinicians will trim away some of the occlusal surface allowing the lower molars to erupt.

Herbst appliance

  • Fixed functional appliance
  • Sections of the upper and lower buccal segments are joined by a rigid arm that postures the mandible forward
  • Easier to wear than the bulkier twin block
  • Higher costs
  • Breaks easier

Medium Opening Activator

  • One piece removable appliance
  • Consists of palatal acrylic which extends with 2 acrylic posts lingually to create a lower labial segment, leaving a breathing hole anteriorly.
  • No molar capping therefore it avoids creating a posterior open bite and is also useful in reducing deep anterior overbite (molars erupt more) as palatal acrylic acts as an anterior bite plane.

Bionator

  • Another one piece appliance
  • Similar to MOA except no palatal coverage
  • Useful in concomitant arch expansion (if want to expand arch at same time)

Frankel appliance

  • Tissue borne removable functional appliance
  • Corrects both class II and class III
  • Cheeks are held away from the teeth using buccal shields
  • Difficult to wear, expensive to make and very difficult to repair therefore rarely used
  1. Prepare the functional appliance:
    • Upper and lower alginate impression and bite recording
  2. Fitting the functional appliance
  3. Reviewing the functional appliance
  4. End of functional appliance treatment- aim to overcorrect to reduce the risk of relapse.
  • Success rate of approximately 80%, failure rate is often due to poor patient compliance
  • Potential causes of unsuccessful functional appliance treatment:
    • Poor compliance
    • Lack of growth/ unfavourable growth rotation
    • Problems with design/ fit of appliance
  • They produce mainly dentoalveolar effects and a small amount of skeletal change
  • Individual patient response to functional appliances is variable
  • Functional appliances posture the mandible and are useful in growing patients
  • Usually used for correction of mild to moderate class 2 skeletal problems
  • In most cases they are followed by a second phase of fixed appliances
  • They can be used alone to treat class 2 div 1 malocclusions if the arches are well aligned
  • They are used in the late mixed dentition providing the patient is still growing
  • They can be difficult to wear initially and require encouragement and motivation from the clinician.
  1. Littlewood, S.J. and Mitchell, L., 2019. An introduction to orthodontics. Oxford university press.
  2. Macey-Dare, L.V. and Nixon, F., 1999. Functional appliances: mode of action and clinical use. Dental update, 26(6), pp.240-246.
  3. Dowsing, P., Murray, A. and Sandler, J., 2015. Emergencies in orthodontics part 2: management of removable appliances, functional appliances and other adjuncts to orthodontic treatment. Dental update, 42(3), pp.221-228.
  4. Littlewood, S.J., Tait, A.G., Mandall, N.A. and Lewis, D.H., 2001. The role of removable appliances in contemporary orthodontics. British dental journal, 191(6), pp.304-310.

See also Class II Malocclusion