Short-term and long-term clinical and cephalometric investigations have been reported on groups of class II division 1 patients treated with various functional appliances. Several prospective studies, including randomised control trials, have also been reported.

Clinical and research studies based on successfully treated groups have exhibited significant clinical improvement in facial profile, overjet reduction and occlusion normalisation.

It is postulated that in patients with class II division 1 malocclusion, the inclines of teeth in malocclusion hold the mandible in a distal position under the cranium. Jumping the bite unlocks the mandible, which helps it to grow unhindered and thereby improves the patient’s profile and occlusion.

Therefore, FA therapy’s effects combine a forward mandible posture and growth modification of the mandible in young children. Dentofacial changes include skeletal, dentoalveolar, dental, soft tissue and airway volume.

Table 66.1 summarises these changes and illustrates them in Fig. 66.1 .

The skeletal and dental changes with functional appliance therapy.

The pre-and post-treatment lateral cephalogram tracings are superimposed at the S-N plane, registration at S. Black: pre-treatment, Red: post-treatment.

The condyle glenoid fossa complex (CGFC) undergoes remodelling ( Fig. 66.2 ). The ratio of the contribution of skeletal versus dental changes varies with the type of appliance used, the method of bite recording, compliance, elimination or persistence of aberrant environmental factors, the patient’s age and sex, the duration of treatment, remaining growth and the individual’s biologic/physiologic response to therapy.

Effect of FA therapy on CGF complex.

Schematic representation showing condyle glenoid fossa complex remodelling.

Source: Reproduced with permission from Meikle MC. Remodeling the dentofacial skeleton: the biological basis of orthodontics and dentofacial orthopedics. J Dent Res 2007; 86 (1): 12–24 Jan Review. PubMed PMID: 17189458. DOI: 10.1177/154405910708600103.

The quantum of increase in the ‘length’ of the mandible remains the most controversial and debated aspect of FA therapy, as does the long-term adaptation of the CGFC in the acquired functional position. The question, ‘Can the mandible be altered beyond its genetic potential?’ has been controversial since FA therapy was established.

Current thinking on the effectiveness of the FA is not limited to the measurement of the enhanced length of the mandible measured on 2D cephalograms. Current thinking on the benefits of FA includes improvement in the overall volume of the oral cavity that houses the dentition and oral structures, such as the position of the tongue and soft tissue drape around the face. ^,

The anterior positioning of the mandible may also have been a secondary response to alteration in oral volume caused by appliance and the subsequent alteration in tongue posture .

Forward mandible placement is also being viewed as an improvement in the lip seal and improved breathing pattern. Following the insertion of the sagittal repositioning appliance, the mandible is lowered to facilitate improved airway and altering tongue position. The forward repositioning of the mandible also stretches the muscles attached to the mandible, causing a horizontal pull of the hyoid bone, which increases the oropharyngeal space. ‘As adaptive functional changes occurred, the tongue, as well as the mandible, may have been positioned inferiorly, not only to establish a more efficient functional position for mandibular movements but also to maintain an adequate oral volume for lingual function’. Many patients with mild to moderate sleep apnoea benefit from FA therapy. More evidence of the benefits of FA therapy is being reported in the literature.

Cone beam computed tomography (CBCT) studies have reported an increase in airway volume of the nasopharynx (568–1123.5 mm ³ ), oropharynx (1167–14,697.6 mm ³ ), total airway volume (2590–10,442.6 mm ³ ) and minimum cross-sectional area (63.6–112.8 mm ² ) after removable or fixed FA therapy, with removable appliances showing greater improvement in airway dimensions.

A systematic review evaluating pharyngeal airway dimensions following FA therapy included eight studies fulfilling the criteria and reported significant improvement in oropharynx dimensions (1.54–2.12 mm). However, a minimum effect was observed in nasopharynx dimensions. There was an increase in the distance of hyoid bone to vertebrae (1.81 ± 2.50 mm), which could further improve the airway dimensions. Another systematic review evaluating 472 patients for oropharyngeal dimensions in skeletal class II malocclusion reported increased oropharyngeal volume after removable FA treatment (1167–5537 mm ³ ). Mandibular anterior repositioning appliance (MARA) had the highest volume increment, followed by Twin block and Herbst appliance.

Histological and histochemical studies on condyle and CGFC have been conducted following forced mandibular propulsion by experimental studies in rats, rabbits, sheep, ^, pigs and non-human primates. ^,

Breitner was the first to report histological evidence of extended effects of orthodontic treatment in the mandible and remodelling of the condyle, as well as glenoid fossa in experimental animals. Other investigators ^, have reported similar trends in the enhanced activity of condylar cartilage. Some of these studies were supported with serial cephalometric and electromyography (EMG) recordings. ^,

In growing animals, forward displacement of the mandible initiates compensatory adaptive changes at the condylar head, marked by its enhanced growth and change in the direction of growth. The facial skeletal adaptations occur through a composite of adaptations in the mandible, condyle, glenoid fossa and naso-maxillary complex to a new acquired position of the mandible. The extent and direction of growth of the mandible are altered while the horizontal and vertical displacement of the maxillary complex is affected. Experimental protrusions in Rhesus monkeys also showed increased chondrogenic activity at the head of the mandibular condyle. A temporary increase in proliferative activity in the mandibular condyle in rabbits and changes in the thickness of the pre-chondroblastic and chondroblastic layers were the most marked changes in the superior part of the condyle after propulsion. In a long-term follow-up experimental study on Rhesus monkeys, McNamara and Bryan have shown that an overall increase in mandibular length by an average of 5–6 mm at the end of 144 weeks occurred in infants and juvenile monkeys.

Histological and cephalometric evidence on enhanced mandibular growth and CGF adaptation in postural forward position paved the way for using the functional appliance in clinical practice beyond the boundaries of Europe. It was unfounded, once it was said that the final length of a mammalian mandible could not be increased orthopaedically or orthodontically beyond a genetically pre-determined value. It is pertinent to note that most adaptive changes of CGF were reported in young growing animals ( Figs 66.3–66.6 ).

The temporomandibular joint region of a 2-week experimental animal Macaca mulatta whose mandible was held forward with a cast ticonium onlay.

Note the increased thickness of the condylar cartilage and the proliferation of bone along the posterior border of the ramus. (Sagittal section. Haematoxylin and eosin stain. Magnification, ×5).

Source: Reproduced with permission from McNamara JA, Carlson DS. Quantitative analysis of temporomandibular joint adaptations to protrusive function. Am J Orthod 1979; 76(6):593–611 Dec. DOI: 10.1016/0002-9416(79)90206-9.

Condylar cartilages in a 2-week experimental Macaca mulatta.

Note the increased proliferation of the cells in the pre-chondroblastic and chondroblastic zones. (Sagittal section. Haematoxylin and eosin stain. Magnification, ×40).

Source: Reproduced with permission from McNamara JA, Carlson DS. Quantitative analysis of temporomandibular joint adaptations to protrusive function. Am J Orthod 1979;76 (6):593–611 Dec. DOI: 10.1016/0002-9416(79)90206-9.

Temporomandibular joint regions in a 12-week experimental Macaca mulatta.

A slight amount of increased proliferation along the posterior border of the condyle is still evident in this animal. (Sagittal section. Haematoxylin and eosin stain. Magnification, ×5).

Source: Reproduced with permission from McNamara JA, Carlson DS. Quantitative analysis of temporomandibular joint adaptations to protrusive function. Am J Orthod 1979; 76(6):593–611 Dec. DOI: 10.1016/0002-9416(79)90206-9.

The temporomandibular joint region of the 24-week experimental animal.

Although the overall thickness of the condylar cartilage is not different from that of the control animals; there is an apparent increase in the number of cells in the pre-chondroblastic zone. This observation has been verified by radioautographic analysis. (Sagittal section. Haematoxylin and eosin stain. Magnification, ×40).

Source: Reproduced with permission from McNamara JA, Carlson DS. Quantitative analysis of temporomandibular joint adaptations to protrusive function. Am J Orthod 1979; 76(6):593–611 Dec. DOI: 10.1016/0002-9416(79)90206-9.

Research works by Voudouris et al. ^, have provided further evidence for the above observations. All six non-human primates who received fixed FAs developed large super class I malocclusion. The adaptations resulted from many factors, including the posterior movement of the maxilla and the maxillary teeth, an increased horizontal component of condylar growth and anterior displacement of the mandible and the mandibular teeth. The glenoid fossa in experimental animals showed growth modification in an inferior and anterior direction. At the same time, a restriction on the downward and backward growth of the fossa was observed in the control animals. Differences in the area and the maximum thickness of new bone formation in the glenoid fossa and condylar growth were statistically significant.

Petrovic and coworkers ^, have demonstrated that the increased activity of the lateral pterygoid muscle (LPM) was associated with enhanced condylar growth response. These observations were also supported by the classical experiments conducted by McNamara and other investigators who reported an initial decrease in the activity of LPM from 6 to 18 weeks following the insertion of the mandibular propulsion appliance.

James A. McNamara, Jr. conducted experimental studies on 64 Macaca mulatta Rhesus monkeys that involved a duration of 26 weeks (13 weeks observation and 13 weeks experimental). The monkeys were in different age groups: infant (I), juvenile with complete deciduous dentition (II), adolescent with full permanent dentition (III) and adult (IV). The monkeys received gold cast intraoral appliances, which prompted all occlusal functions in 2 mm forward position and similar vertical displacement. The study involved serial cephalometric data taken at the beginning of the control period, the beginning of the experimental period and the conclusion of the experiment. EMG studies of temporalis, orbicularis oris, supra-hyoid muscles and the superior head of the LPM were taken at monthly intervals during observation. During the experimental period, EMG was recorded just before the cementation of an appliance, at the cementation of the appliance and after 1 h. After that, eight records were made at succeeding intervals of 6 h, 1 day, 4 days, 1 week, 2 weeks, 4 weeks, 8 weeks and during the 12th week. Histological studies of the temporomandibular joint of sacrificed animals were also obtained.

The findings suggested that changes in the maxillo-mandibular relationship resulted from adaptations occurring in the entire craniofacial complex. Histological studies have shown that anteroposterior alteration in the molar relationship was seen in 10 of 12 experimental monkeys. Mandibular adaptation was primarily skeletal in nature in the young animals, while compensatory movements of the dentition became more significant with increased maturation. The primary effect of the experimental conditions on the middle face was on the extent and vector of growth of the skeletal components of the maxillary arch. This occurred at all age levels. The mandibular condyles in juvenile animals showed an average increase in growth by 51% higher in increment compared to controls.

The superior head of the lateral pterygoid is the principal muscle that functions as a forwarding positioner of the mandible, which is evident by enhanced EMG activity. At the end of the experiment, EMG activity is less, reduced or normalised.

Authors have further reported that the increase in lateral pterygoid activity is associated with a forward positioning of the lower jaw. This new functional pattern first appeared associated with phasic activities such as swallowing and then during tonic functions such as maintenance of mandibular postural position. However, as the experiment progressed, there was a gradual return towards pre-appliance levels of muscle activity. This gradual change in the level of muscle activity was correlated in time with the skeletal and dental adaptations observed in the same animals.

However, animal studies carried out at the University of Toronto with chronically embedded implants have shown that LPM activity remains depressed for 18 weeks following the insertion of a FA device. Sessle et al. have demonstrated that with two types of FAs to induce mandibular protrusion, the activity of superior and inferior heads of the lateral pterygoid, anterior digastric and superior masseter muscles remain significantly decreased up to 6 weeks and then gradually returned to normal recorded through chronically implanted electrodes.

Traditionally, EMG studies have been used to measure the electrical potential of muscles of mastication, which indirectly reflects the muscle activity. EMG studies have been carried out mainly to record the activity of the anterior temporalis, masseter and digastric muscle following the insertion of various FAs.

EMG studies on class II patients treated with an activator, bionator, Herbst appliance and twin block appliance have been carried out at AIIMS New Delhi. The findings from the above studies reveal that EMG activity of anterior temporalis and masseter muscles is decreased immediately following the insertion of a FA. As time progresses, the new pattern of occlusal contacts is gradually established, which is evident by the normalisation of the EMG pattern in about 3 months. Thereafter, EMG activity shows higher values compared to pre-treatment values at 6 months and is maintained after that or normalised ( Fig. 66.7 ). The insertion of twin block results in an increase in the vertical dimension of occlusion and so the masseter muscle initially elongates which shows reduced thickness measured with ultrasonography or MRI; however, it then gradually normalises to original dimensions at 6 months or more ( Fig. 66.8 ).

EMG adaptations with twin block appliance.

(A) Representative sections of EMG during the postural position of the mandible without twin block. (B) Representative sections of EMG were present during the postural position of the mandible with a twin block. (A and B: Tracings 1, 2, 3 and 4 represent raw EMGs, and 5, 6, 7 and 8 are integrated EMGs.)

Source: Reproduced with permission from Aggarwal P, Kharbanda OP, Mathur R, Duggal R, Parkash H. Muscle response to the twin-block appliance: an electromyographic study of the masseter and anterior temporal muscles. Am J Orthod Dentofacial Orthop 1999;116(4): 405–14 Oct; PubMed PMID: 10511668. DOI: 10.1016/S0889-5406(99)70225-8 .

Effects of twin block FA therapy on masseter muscle.

Forces of twin block therapy on the masseter muscle cross-sectional area (MCSA) and muscle volume (MV) measured with MRI over 18 months of an observation period. Note that CSA and MV reduced up to 3 months (T1) at a significant level (P<0.05) and after that a gradual normalisation is observed.

Source: Dongre S. Long term changes in the masseter muscle dimensions following twin block appliance therapy [Thesis]. New Delhi: AIIMS; 2011 .

A 6-month EMG study on removable appliances has reported the mean voluntary contraction (MVC), showing higher activity in the temporalis muscles than in the masseter muscles. The maximum effort test indicated greater fatigue in the masseter than in the temporalis muscles.

• 1.

  The anatomical evidence of the superior head of the LPM being associated with the intra-articular disc and the head of the condyle has been debated. LPM is found to be attached to the outer border of the fibrous capsule.

• 2.

  There are reports on the increased activity of the superior head of the LPM on EMG recordings and contradictory reports on reduced EMG activity with chronically implanted electrodes.

• 3.

  The increased activity of the LPM following the protraction of the mandible is not expected since the new position shortens the length of the LPM, and increased hyperactivity from a shortened muscle is not expected.

• 4.

  Experimental studies by Voudouris et al. ^, have reported CGFC remodelling in laboratory animals at the condyle and the glenoid fossa, which correlated with decreased postural electromyography activity during the experimental period. ‘Results from permanently implanted electromyographic sensors demonstrated that lateral pterygoid muscle hyperactivity was not associated with condyle glenoid fossa growth modification with functional appliances. Other factors, such as reciprocal stretch forces and subsequent transduction along the fibrocartilage between the displaced condyle and fossa, might play a more significant role in new bone formation’.