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 Table of Contents  
Year : 2017  |  Volume : 9  |  Issue : 4  |  Page : 276-281

Functional appliances

Department of Orthodontics and Dentofacial Orthopedics, Desh Bhagat Dental College and Hospital, Sri Muktsar Sahib, Punjab, India

Date of Web Publication1-Dec-2017

Correspondence Address:
Anil Prashar
H. No. NN 422, Gopal Nagar, Jalandhar - 144 008, Punjab
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/IJDS.IJDS_65_16

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The purpose of this review is to evaluate the previous studies that describe the effects of functional appliances and their efficiency in treating class II malocclusion. This review is limited to three appliances: activator, bionator, and twin-block appliances.

Keywords: Activator, bionator, twin block

How to cite this article:
Kaur S, Soni S, Prashar A, Bansal N, Brar JS, Kaur M. Functional appliances. Indian J Dent Sci 2017;9:276-81

How to cite this URL:
Kaur S, Soni S, Prashar A, Bansal N, Brar JS, Kaur M. Functional appliances. Indian J Dent Sci [serial online] 2017 [cited 2023 Sep 29];9:276-81. Available from: http://www.ijds.in/text.asp?2017/9/4/276/219627

  Introduction Top

Class II malocclusion is one of the most common orthodontic problems and it occurs in about one-third of population.[1],[2],[3] Class II malocclusion can result from many contributing factors, both dental and skeletal. Although maxillary protrusion and mandibular retrusion are both found to be possible causative factors, it has been reported that the most common component in a class II sample population is mandibular retrusion.[4] For class II patients in whom the mandible is retrognathic, the ideal treatment is to target the source and try to alter the amount or direction of growth of mandible. The primary treatment for this is functional appliance therapy.[5] Functional appliances include removable and fixed devices that are designed to alter the position of the mandible, both sagittally and vertically and to induce supplementary lengthening of the mandible by stimulating increased growth at the condylar cartilage.[6],[7],[8],[9]

Functional appliances have been used since the 1930s. Despite this long history, there is much controversy relating to their use, method of action, and effectiveness.[10]

Many authors feel there is little evidence to support the claim that functional appliances significantly affect mandibular growth. Bjork [11] and Pancherz [12] demonstrated only small changes in mandibular growth and concluded that it was not affected by treatment with functional appliances. By contrast Harris,[13] DeVincenzo,[14] and Windmiller [15] suggested significant influences on mandibular growth after timely intervention.

  Activator Top

The original monobloc designed by Robin in 1902 was a one-piece removable appliance.[16] This appliance positioned the mandible forward in patients with severe mandibular retrognathism. Later, Viggo Andresen in 1908 developed a mobile, loose-fitting appliance that transferred functioning muscle stimuli to the jaws, teeth, and supporting tissues; it is called biomechanic working retainer. Later, Andresen and Haupl called their appliance activator because of its ability to activate the muscle force.

According to Andersen and Haupl, the activator is effective in exploiting the interrelationship between function and changes in internal bone structure. Activator induces musculoskeletal adaptation by introducing a new pattern of mandibular closure. The adaptations in functional pattern caused by activator also affect condyles. Condylar adaptations consist of growth in an upward and backward direction to maintain the integrity of temporomandibular joint structures.[17] The appliance advances the mandible and generates a biomechanical force as the muscles attempt to return the mandible to its normal position.[18] Since it was designed to be loose fitting and required the patient to actively hold the appliance in place, it was often described as an exercise appliance.[10]

  Skeletal Effects of Activator Top

Activator inhibits the horizontal growth of the maxilla [19],[20],[21],[22] and results in increased growth of the mandible and anterior relocation of the glenoid fossa.[19] Kahl-Nieke and Fischbach [23] found that activator appliance therapy in hemifacial microsomia patients showed improvement of function and occlusion and facial asymmetry was reduced. When the construction bite is taken in such cases, the mandible is kept in a slightly forward and overcompensated position which caused change in muscle activity that can lead to enhanced bone apposition and optimal growth direction of the condyle. Horizontal activator produces increase in SNB angle, mandibular plane angle and reduces SNA angle, ANB angle, and increase in mandibular length.[24],[25] Mehta et al.[26] reported that activator corrects class II malocclusion by increasing condylar growth and mandibular base length. According to Luder's hypothesis, a great interocclusal height of an activator would lead to improvement in mandibular retrognathism, no change in maxillary prognathism, clockwise rotation of occlusal plane and low construction bite results in reduction in maxillary prognathism, clockwise mandibular rotation, anterior tipping of lower front teeth.[27],[28] Some clinical studies found no significant increase in mandibular length with the use of this device,[20],[22] but other authors reported a significant increase in the length or protrusion of mandible using the activator.[29],[30],[31],[32],[33] Treatment with activator headgear combination resulted in restricted maxillary growth reduced soft-tissue facial convexity with downward and forward mandibular growth.[29]

  Dental Effects Top

Bjork,[34] Calvert,[35] Pancherz,[22] and Wieslander and Lagerstrom [36] observed significant dentoalveolar change. Class I occlusion is achieved through distal tipping of the maxillary teeth and a mesial, vertical movement of the mandibular dentition. Overjet reduction also occurs mainly due to dentoalveolar changes that are retroclination of maxillary incisors and proclination of mandibular incisors.[22],[25] Pancherz [22] found that >70% of the overjet corrected by incisor tipping. Vargervik and Harvold [19] found that activator results in inhibition of mesial migration of maxillary teeth, inhibition of maxillary alveolar height increase and also causes extrusion of mandibular molars,[15] mesial movement of mandibular teeth. Harvold and Vargervik [37] also observed that the appliance causes 1.4 mm of maxillary incisor lingual tipping and 0.5 mm of mandibular incisor labial tipping. Appliance achieved a Class I occlusion by inhibiting maxillary dentoalveolar vertical development while encouraging mandibular dentoalveolar mesial and vertical development.[26] Activator with headgear combination resulted in upper incisor retrusion, upper molar distalization, and mesial movement of lower molars.[28],[29],[30],[38]

  Bionator Top

Balters developed the original appliance in the early 1950s. It is the prototype of a less bulky appliance. Its lower portion is narrow; upper part has only lateral extensions, with a crosspalatal stabilizing bar. The palate is free for proprioceptive contact with tongue and the buccinator wire loops hold away the potentially deforming muscular action.

  Principles of Bionator Top

According to Balters, the equilibrium between tongue and circumoral muscles is responsible for the shape of dental arches and intercuspation. The functional space for tongue is essential for normal development of orofacial system. A discoordination of its functions can lead to abnormal growth and actual deformation. Bionator establishes good coordination and eliminates these deforming and growth restricting aberrations.

The principle of treatment with bionator is not to activate the muscles but to modulate muscle activity, thereby enhancing normal development of inherent growth pattern and eliminating abnormal and potentially deforming environmental factors.

The bite cannot be opened and must be positioned in an edge to edge relationship. Balters reasoned that a high construction bite could impair tongue function and the patient can actually acquire a tongue thrust habit as the mandible dropped open and the tongue instinctively moved forward to maintain an open airway.

Balters [39],[40] is one of the most commonly used appliances for functional treatment of class II division 1 malocclusion associated with mandibular retrusion. The popularity of this appliance is due to a number of favorable characteristics that include the relative ease in the construction and clinical handling of the appliance and the high level of comfort for the patient, who usually shows positive acceptance and compliance.

  Skeletal Effects of Bionator Top

Bionator appliance therapy improved maxillomandibular relationship in class II patients as it increases mandibular length [41] and has slight restrictive effect on anteroposterior dimension of the maxilla,[41],[42] but some other studies found no significant restriction of maxillary growth with this appliance.[42],[43],[44] Freeman et al. reported that use of bionator and high-pull facebow combination followed by fixed appliance therapy in patients with hyperdivergent facial patterns resulted in increase in mandibular plane angle and larger inclination of Frankfort horizontal plane to occlusal plane in treated group than controls. Hence, they did not recommend this combination for growing patients with hyperdivergent facial patterns.[45] Bionator therapy resulted in increased anterior facial height [41],[44] and posterior facial height,[42],[45],[46],[47],[48],[49] forward movement of point B, and increased SNB angle.[45],[46] Bionator, when used during pubertal growth spurt, results in elongation of mandible,[50],[51] increase in mandibular ramus height, and significantly more backward direction of condylar growth.[52] Some studies also reported increase in posterior maxillary base width with bionator appliance.[53],[54],[55],[56],[57]

  Dental Effects of Bionator Top

Bionator appliance corrects molar relationship and overjet of class II patients mostly by dentoalveolar changes. Bionator treatment resulted in reduced overjet, labial tipping of lower incisors, and lingual inclination of upper incisors.[41],[43],[44],[51],[53] Class II molar relation is corrected by mesial movement of mandibular molars and distal movement of maxillary molars.[43],[51]

Almeida, Henriques, and Ursi concluded that bionator results in labial tipping and linear protrusion of the lower incisors and a lingual inclination and retrusion of the upper incisors, significant increase in mandibular posterior dentoalveolar height.[50] Bionator produced no extrusion of the upper molars.[42],[53] However, open-bite bionator resulted in reduced overjet, eruption of maxillary molars,[50],[58] less increase in facial height, and no change in eruption of lower molars,[58] but other studies showed extrusion of mandibular molars occurred with open-bite bionator.[47],[59]

Another study showed that bionator produced proclination of lower incisors and insignificant increase in inclination of upper incisors.[60]

  Twin Block Top

This appliance developed by Dr. William J. Clark in Scotland. Twin-block appliances are based on the same principle as the protrusive functional appliances used on monkeys by McNamara.[61] In normal development, cuspal inclined planes play important role in determining the relationship of teeth as they erupt into occlusion. Occlusal forces transmitted through dentition provide constant proprioceptive stimuli to influence the growth rate and adaptation of trabecular structure of the supporting bone.

Twin blocks are simple bite blocks that effectively modify the occlusal inclined plane. This appliance achieves rapid functional correction of malocclusion by transmitting favorable occlusal forces to the occlusal inclined planes covering the posterior teeth and guiding the mandible forward into correct occlusion. With the appliance in the mouth, the patient cannot occlude comfortably in former distal position, and the mandible is encouraged to adopt a protrusive bite with inclined planes in occlusion. Thus, unfavorable cuspal contacts of distal occlusion are replaced by favorable proprioceptive contacts of the inclined planes of the twin blocks, correcting the malocclusion and freeing the mandible from its locked distal functional position.

  Skeletal Effects of Twin Blocks Top

Twin-block appliances produce both skeletal and dentoalveolar changes for correction of class II malocclusion. Twin-block appliance treatment resulted in increased mandibular length,[43],[62] increased SNB angle, and no significant restraining effect on maxillary growth.[43],[62],[63] However, some studies observed some headgear effect resulting in slight inhibition of forward maxillary growth.[64] Singh and Hodge also concluded that twin-block appliances along with extraoral traction cause growth modulation in specific regions of midfacial complex and change in position of the mandible.[65],[66] Siara-Olds et al. demonstrated good vertical control on mandibular plane angle with twin-block appliance therapy as compared to Herbst, bionator, and MARA appliances.[67]

  Soft-Tissue Changes Top

There was found to be decreased skeletal convexity and H angle, increased mentolabial angle,[68] and reduction in the prominence of lower lip [69] after treatment. Lower lip, lower lip sulcus, and soft-tissue pogonion moved anteriorly after twin-block treatment.[68],[70] Singh and Morris et al. reported anterior and inferior movement of chin.[48],[71]

  Dental Effects of Twin Blocks Top

Overjet reduction with twin-block appliances is mostly due to dentoalveolar changes. Many attempts have been done to minimize tipping of lower incisors. The best results achieved using Southend clasps and acrylic cover for lower incisors.[72],[73] Various studies showed dental changes caused by this appliance that are proclination of lower incisors,[74],[75] retroclination of upper incisors,[62] lower molar eruption and mesial movement of lower molars,[43],[75] and headgear-like effect resulting in distal movement of upper molars. The proclination of mandibular incisors was probably due to mesial force on mandibular incisors induced by protrusion of the mandible.[49] McNamara concluded that lingual tipping of the maxillary incisors is due to the contact of the lip musculature during twin-block treatment and labial wire in bionator and twin-block appliances, which might come into contact with the incisors during sleeping, causing them to retract. However, Yaqoob et al. found that twin blocks showed similar results in terms of dentoalveolar and skeletal change when designed with or without a labial bow, indicating no effect of labial bow.[72]

  Summary and Conclusion Top

All removable functional appliances increase mandibular length so useful in correction of skeletal class II malocclusion.[76] Functional appliances such as bionator type I produced mesial positioning of mandible, mandibular sagittal growth, decreased sagittal maxilla-mandibular angle, and decreased overjet.[77],[78] Patients with obstructive sleep apnea-hypopnea syndrome (OSAHS) have increased upper and lower posterior facial height, inferiorly placed hyoid bone, and increased SNA angle, indicating class II cases.[79] Twin block, when used in patients with class II division 1 malocclusion on skeletal class II base, produces significant change in vertical position of hyoid bone after advancement of mandible. This resulted in opening of airway space and increase in inspiratory and expiratory total air volume. Therefore, it can be used for treating OSAHS.[80] Another study concluded that functional appliances may be considered only in specified cases as an adjunct in treatment of patients having craniofacial anomalies which are risk factors for apnea.[81],[82]

Both skeletal and dentoalveolar changes can be achieved in activator functional appliance therapy. Depending on timing, technique, and trimming, significant facial and occlusal changes can be achieved. In addition to the elimination of abnormal perioral muscle function, growth guidance is the major contribution of functional therapy. Activator therapy also has some limitations; it is less effective in treating maxillary prognathism and vertical growth patterns, inappropriate for achieving extensive bodily movement, torque, rotation, and intrusion of teeth. It also inhibits speech and lateral jaw movements. It is single-block appliance so cannot be used in cases with nasal obstruction.[23]

Bionator establishes a muscular equilibrium between forces of tongue and outer neuromuscular envelope which influence the form and shape of dental arches. It is useful in class II malocclusion with mandibular retrognathism, some open bite and class III cases. The main advantage of bionator is its reduced size, so it can be worn day and night time. Constant wear makes its action faster than activator and also results in more rapid sagittal adjustment of musculature to forward mandibular posture. Bionator is effective in treating functional type retrusions with relatively normal skeletal potential and sufficient growth increments.

Twin block has separate upper and lower appliances with occlusal bite blocks, so the appliance gives greater freedom of movement in anterior and lateral excursions and causes less interference in normal function. The patient can eat comfortably with the appliances in mouth, and the patient can learn to speak normally with twin blocks. Twin blocks can be designed with no visible anterior wires without losing its efficiency in correction of arch relationships. Twin blocks may be fixed to teeth temporarily or permanently to guarantee patient compliance. Adjustment and activation is simple and chairside time is reduced in achieving major correction.[73] It can also be used to correct transverse discrepancy by incorporating midline jackscrew. Therefore, the twin-block appliances due to its acceptability, adaptability, versatility, efficiency, and ease of incremental mandibular advancement without changing the appliance have become one of the most widely used functional appliances in correction of class II malocclusion.

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Conflicts of interest

There are no conflicts of interest.

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Twin Block
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