The term ‘functional appliance’ refers to a variety of removable appliances designed to alter the arrangement of various muscle groups that influence the function and position of the mandible to transmit forces to the dentition and basal bone. Typically, these muscular forces are generated by altering the mandibular position sagittally and vertically, resulting in orthodontic or orthopaedic changes.

The original concept of functional jaw orthopaedics encompasses growth modulation for the correction of mandibular retrognathia, that is, correction of skeletal class II malocclusion not by active forces of the appliance but by the forces generated by the muscles when the mandible is held forward. The functional forces indirectly induce growth modulation and bring about changes in jaw bones, hence the term functional jaw orthopaedics. The term has now been applied to any device that is used to displace and hold the mandible, including a variety of removable and fixed functional appliances.

It involves a variety of treatment modalities that are adapted to create harmony and balance of the teeth-bearing facial skeleton either through growth enhancement of the mandible, growth restraint on the maxilla or a combination of both. It also includes restraining the excessive growth of the mandible in skeletal class III situations, protraction of the maxillary complex and procedures such as rapid maxillary expansion that brings about transverse changes in the maxillary skeletal base. These therapeutic modalities include the use of orthopaedic forces of 450 g or more in combination with functional growth stimulation and orthodontic forces to affect skeletal and dental alterations.

Growing the short mandible to match the upper jaw dates to 1879 when Norman Kingsley described a bite plate ‘to change or jump the bite in the case of excessively retreating lower jaw’ in patients who exhibited excessive overjet and retrognathic mandibles. The term ‘jumping the bite’ describes the concept of getting the mandible to hold into a favourable occlusal position and inducing it to stay there. This treatment was often utilised after the expansion of the arches but often did not prove successful due to the mandible’s tendency to revert to its original position ( Fig. 62.1 A and B). ‘The fault of the old Kingsley appliance was the tendency, even with the bite guide, for all the teeth to return.’

(A) An anterior bite plate, which helps open the bite, can also assist in positioning the mandible forward. The anterior bite plate is the simplest form of a functional appliance. (B) The concept of ‘jumping the bite’ was introduced by Kingsley, where the mandible is held in a forward position and not allowed to return to its original place. (C) To prevent the mandible from slipping back, Andresen added lingual acrylic extensions in both the anterior and (D) buccal regions to support the mandible and maintain it in a forward position.

• Pierre Robin (1902): Pierre Robin, a French stomatologist, used a modified bite plate extending all along the lingual surfaces of lower gums in new-born children with micrognathia syndrome to influence muscle activity to prevent glossoptosis in early 1902.

• Viggo Andresen (1909) found his daughter’s left-over lingering problem of class II relation after fixed orthodontic treatment was corrected with a modified bite jumping type of appliance, in which he added a horseshoe-shaped lingual flange. He aimed to guide the mandible forward by 3–4 mm in occlusion and correct the tendency for a deep bite. These observations and clinical results were the fortuitous introductions of functional appliances in orthodontics.

  • The concept of adding lingual extensions to bite jumping plates eventually crystallised into an appliance called the ‘activator’ ( Fig. 62.1 C and D).

• Viggo Andresen successfully used this appliance in many other patients after correcting his daughter’s occlusion from class II to class I. In 1910, he reported about a new upper retention appliance. Andresen continued to use this appliance as a prophylactic tool for young patients. He modified the retainer into the appliance, using a wax bite to advance the mandible in the forward position.

• Andresen and Häupl activator (1936): Andresen, in collaboration with Karl Häupl, published a book, Funktionskieferorthopädie, a philosophy of treatment with the use of the activator, which he called the ‘Norwegian system’. He called it ‘muscle and circulation activators’ ( Figs 62.2 and 62.3 ). The book was later published in several editions in 1939, 1942, 1945 and the 5th edition in 1953 after the death of Andresen, edited by Häupl and Petrik. ^,

  • The use of an activator appliance became widespread in Europe and its philosophy influenced the profession so much that ‘… functional jaw orthopaedics became a profession of faith, religion, besides which no other opinion was tolerated’.

  

  Activator in the mouth showing its relationship with maxillary incisors, mandibular incisors, and buccal teeth.

  The activator in the mouth interacts with the maxillary incisors, mandibular incisors and buccal teeth. The inclines in the occlusal acrylic section are designed to promote the mesial and occlusal eruption of the mandibular molars, while the maxillary molars should be tipped distally. Additionally, relief is provided for the palatal tipping of the maxillary incisors. Arrows indicate the directions of dental eruption facilitated by the inclines in the activator, which are stimulated by the activation of muscle function in the orofacial region.

  

  Monoblock or Andresen–Häupl activator.

  Arrows point the direction of eruption guidance.

• Bimler’s appliance (1949): During those years, several modifications to the activator appliance and new concepts were introduced. These included cutting the bulk of acrylic, raising bite height and incorporating wire and spring components to initiate dental corrections. The idea was to create an appliance that is more comfortable and efficient in treating malocclusion. Hans Bimler (1949), a German orthodontist, incorporated elastic force into an orthopaedic appliance, which he called ‘Gebissformer’ by a German name. Later, he also called it an ‘adaptor’. However, this appliance is known by its inventor as the ‘Bimler’s appliance’. ^,

• Wilhelm Balters (1950) modified the activator by removing its bulk from the palate and substituting it with a coffin spring. He advocated three designs of bionator for classes I, II and III malocclusions ( Figs 62.4–62.7 ). ^,

• Leopold Petrik (1953) introduced an activator with a large interocclusal thickness to increase the vertical dimensions of the bite.

• Hugo Stockfisch (1953) An innovative modification of the activator was introduced by Hugo Stockfisch in 1953. The new appliance, which he termed ‘Kinetor’, consisted of two movable plates connected by wires, the buccinator loops. These loops kept the dental arches free from the abnormal pressure of cheek muscles. An unusual feature of the kinetor was the elastic tubes between the two plates that acted not only as shock absorbers but also as a means of broadening and optimising orofacial muscle pressures.

  • The incorporation of an additional force mechanism was a significant modification to the original philosophy of the activator mechanism.

  • With the publication of Functional Orthopaedics for the Masticatory System in 1952 by Eschler, the amalgamation of a combination of muscle stimuli and forces created by inherent elastic elements was starting to get accepted by the profession.

• Rolf Frankel (1957) recognised that the correction of the structural and functional deviations of the muscular system influences the stability of treatment. Frankel designed the function regulator (FR, 1957), making the oral vestibule the operational basis for his treatment. The appliance was designated as FR-1, FR-2, and FR-3, for treating classes I–III malocclusions. ^, An additional modification of FR-4 was later designed.

• Martin Schwarz (1956) modified the single block activator into split plates for the upper and lower halves. The idea was to take advantage of the orthopaedic correction of activator and active forces on teeth. Schwarz’s double plate eventually got crystallised into the popular twin block appliance.

• William J. Clark (1977) of Fife, Scotland, developed, used and promoted twin block appliance treatment. ^, Later, William Clark introduced bite blocks that can be inserted into the specially designed tubes welded onto molar bands called ‘fixed twin block’.

• Vardimon Functional orthopaedic magnetic appliance (FOMA) 1989: An innovative addition to the armamentarium on FA was FOMA II, introduced in 1989 by Alexander D. Vardimon and colleagues. FOMA II is an active appliance that directs its inherent magnetic forces to the jaws, thereby constraining the lower jaw in an advanced posture. Later in the subsequent year, an appliance was developed for the treatment of class III malocclusions which exhibit mid face sagittal deficiency with or without mandibular excess. ^,

  • Clinicians and scientists in the late 20th and beginning of the 21st centuries focused research on evaluating the true benefits of functional appliances and the relationship of treatment outcomes with facial morphology. Functional jaw orthopaedics is now an accepted mode of therapy in growing children with class II malocclusion.

  • The evolution of removable functional appliances has been summarised in Table 62.1 .

    TABLE 62.1

    Noteworthy events in the evolution of removable functional appliance

    Inventor	Appliance	Year	Remarks
    Norman Kingsley	Bite plate (‘bite-jumping’ appliance)	1879	Mandible reverted to its original position
    Pierre Robin	Monobloc	1902	Forerunner of removable functional appliances
    Viggo Andresen Karl Häupl	Activator	1909 1936	The first functional appliance to be widely accepted
    Hans Bimler	Gebissformer/Adaptor/Bimler appliance	1949	Reduced size made it possible to wear all-day
    Wilhelm Balters	Bionator	1950	Modified activator removed its bulk from the palate and substituted with a coffin spring
    Leopold Petrik	Vertical concept	1953	Activator has a greater occlusal thickness to increase the vertical dimensions.
    Hugo Stockfisch	Kinetor	1953	Two movable plates connected by wires buccinator loops
    Martin Schwarz	Double plate	1956	Resembled a monobloc or an activator constructed in two pieces
    Rolf Frankel	Function regulator	1957	Oral vestibule the operational basis for treatment
    William J Clark	Twin block	1977, 1982, 1988	Split into upper and lower halves. TWO-PIECE appliance
    Alexander D Vardimon	Functional Orthopaedic Magnetic Appliance (FOMA) II	1989	Directs its inherent magnetic forces to jaws and thereby constrains the lower jaw in an advanced posture

The ‘activator’ is a loose-fitting appliance made of heat-cured acrylic that holds the mandible forward due to the extended lingual flanges coming from the maxillary plate. It is a single-piece appliance. The only wire component is a labial bow.

Activator is indicated in growing young children to correct class II malocclusion, mainly due to the smaller mandible. This appliance is a loose device worn at night. Being a loose-fitting appliance, it drops in the mouth during sleep. Consequently, the mandible responds for closure to hold it in place. The muscles of the face and stomatognathic system are thus activated, hence the name Áctivator.

The appliance is worn at night, bringing about sagittal correction of the molar relationship and bite opening. The resultant changes induced are mainly dentoalveolar. These changes are accompanied by marked changes in facial features and correction of abnormal habits. ‘… the activator should be used only as a passive apparatus, that is, it should not produce any power but only receive and further transport power from the functional milieu to the teeth’. Loose passive appliance would shake and jolt the teeth and periodontal tissue, thereby causing tissue remodelling, the only source being ‘muscular forces’. These muscular forces are capable of growth remodelling and creating favourable tooth movements. ‘ The activators activate the appropriate muscles while the muscles, in turn, activate the appliances ’.

This philosophy was later modified with a recommendation to record the bite with greater vertical activation beyond freeway space and to add other wire components to induce further teeth movements. A high pull face-bow attached to the activator is indicated in patients with an increased vertical dimensions of the face. The combined headgear activator therapy provides greater cumulative skeletal benefits than a single appliance alone.

The steps involve making upper and lower impressions with deep flanges. The bite is recorded by asking the patient to bring the mandible forward and biting on a horseshoe-shaped wax roll softened in a water bath.

The activator bite is recorded ‘within the freeway space’. The sagittal forward positioning is 4–5 mm. The bite with upper and lower models is transferred to a hinge articulator. A labial bow of 0.8 mm (21 gauge) wire is constructed. A conventional or traditional activator has large lingual flanges extending up to the distal of the first or second molars. The upper labial bow is constructed from canine to canine. The lower incisors are free and do not have any acrylic caps on them. However, there is a trend to cap lower incisors now, which prevents labial tipping of lower incisors. Appliance wax-up is done, followed by routine dewax and acrylisation in heat cure acrylic.

The most critical aspect of activator finishing is trimming the inclined planes of the bite block. The inclined planes are trimmed with extreme caution and care to induce buccal and distal tooth movement of the maxillary buccal teeth and enhance mesial and occlusal movement of the mandibular buccal teeth.

Trimming is done with a sharp TC pear shape bur at a slow speed to avoid heating the acrylic. The patient is asked to bite on an acrylic inter-occlusal bite block. The distal cusp positions of the maxillary buccal teeth and mesial cusp positions of the mandibular buccal teeth are marked with a metal marker. The trimming is done for each tooth and checked in the mouth. It is a tedious and time-consuming process that can be a test of patience for both the orthodontist and the patient. At the completion of trimming, the inter-occlusal block of acrylic resembles a honeycomb appearance. The trimmed activator has inclines and space created for the upper buccal segment of teeth to distalise, while buccal teeth in the mandibular arch provide inclines and space to erupt vertically and mesially. The distal positioning of the maxillary teeth, mesial migration of mandibular teeth and their forward position leads to the correction of class II molar relation to class I. Palatal acrylic on maxillary anterior teeth is trimmed to induce intrusion, while mandibular incisors are capped nowadays to prevent undue flaring ( Figs 62.2 and 62.3 ).

The patient is motivated for its use, and those who wear the appliance have responded favourably, showing improvement in aberrant peri-oral muscle behaviour by optimising the lip seal and definite control of tongue thrust. The sagittal correction gradually shows up, settling the occlusion into a class I molar relationship. The treatment duration may vary from 18 months to longer.

Further trimming of inclined planes is required to settle the buccal teeth in good inter-cuspation.

The traditional view about retention after activator treatment was that no retention might be required since the appliance works through modification of muscle function. However, this is not the case since occlusal changes take longer to settle. The current thinking is to finish the case with a fixed orthodontic appliance, the second phase of the comprehensive orthodontic therapy, and retention with a bite plate with an incline to keep the mandible in a forward posture.

The activator treatment inhibits maxillary growth, moves the maxillary incisors and molars distally, and moves the mandibular incisors and molars mesially. Mandibular growth appeared not to be affected by activator treatment. Most treatment effects are of dentoalveolar in nature. Cooperation is a critical determinant of successful treatment, and the activator treatment is effective in the late mixed dentition rather than initial stages of occlusion development.