Morphogenesis and the development of the face is one of the most complex processes in human development. The role of nature versus nurture has long been debated. Proponents of genetic theory claim that growth patterns may arise from the activation and repression of genes that regulate the metabolic activity of differentiated cells, while proponents of epigenetic theories believe that growth control factors may lie in sutures,¹ cartilage,² or functional matrices³ surrounding the craniofacial structures. Others believe that it is the confluence of genetics and external environmental factors that ultimately determines the size, shape, and location of craniofacial structures.⁴

The role of heredity in the development of one’s phenotypical appearance has been examined through observational studies of monozygotic or dizygotic twins, who have greater similarity in appearance than unrelated individuals.⁵ The question always has been whether and to what degree the external environment can influence the genetically predisposed size, shape, and direction of growth of craniofacial structures. While some studies⁶ suggest that the presence of malocclusion can be greatly influenced by environmental factors, others believe its cause is multifactorial.⁵ Even though it is reported that both Class II and Class III malocclusions may have strong familial tendency,^7,8 it may be difficult to estimate the influence of environmental factors in modifying craniofacial growth because the heritability studies of occlusion are typically based on twins and siblings who did not receive orthodontic treatment.

Class II malocclusion is one of the most common problems presenting to orthodontists. Skeletally, a prognathic maxilla, a retrognathic mandible, or a combination of the two is a possible etiology for this malocclusion. Studies in the literature^9–12 report that Class II malocclusion is more commonly associated with mandibular retrognathism than with maxillary prognathism. Stahl et al.¹³ reported that features of a skeletal Class II occlusion develop early in the deciduous dentition and do not tend to self-correct with age, implying that some sort of intervention is necessary to achieve correction. Depending on the patient’s growth status, treatment options commonly include use of functional appliances or fixed functional appliances (FFAs) to enhance mandibular growth, headgear to restrict maxillary growth, camouflage by extraction of upper and/or lower premolars, or surgical correction of the underlying skeletal discrepancy in patients in whom facial growth has been completed.¹⁴ The goal of either orthodontic intervention or orthognathic surgery is generally to improve the patient’s dental and facial esthetics, dental function, self-image, and quality of life.

The term functional appliance refers to a group of appliances that posture the mandible forward in an attempt to stimulate mandibular growth. Historically, the use of functional appliances began with Kingsley’s vulcanite biteplate but probably was first propagated by Anderson with the use of the Activator appliance.¹⁵ Since then, numerous functional appliances have been reported in the literature, with the most popular being the Activator, Bionator, and the Twin Block appliance. However, despite the use of functional appliances for more than a century, there is still considerable debate over the effectiveness of such mandibular anterior repositioning appliances in stimulating skeletal mandibular growth.¹⁶ Studies^17–19 on experimental animals such as rats, mice, and primates depict a clear increased mandibular growth potential with the use of such appliances. However, similar experiments with these bite-jumping appliances to enhance mandibular skeletal growth in human subjects did not yield comparable results. The results of randomized clinical trials^20–22 have shown that with the use of functional appliances there is an initial increase in the mandibular growth response that is greater than that seen in the control sample; however, in the long-term, when patients treated with functional appliances were compared to untreated control subjects, the difference in the overall mandibular length was statistically insignificant.

Typically a functional appliance is advocated for a minimum of 12 hours per day for 6 to 9 months to observe dentoskeletal changes. Thus patient compliance is critical to the success of removable functional appliances.^23,24 In general, the factors associated with patient compliance are age, nature of the treatment delivered, and psychosocial characteristics of the individual patient. It has been reported that compliance with treatment is better in adults than in children and that younger children are more compliant with treatment than are adolescents.²⁵ Since most growth modulation procedures are undertaken in the adolescent phase, proficient patient compliance may be difficult to obtain in certain patients. Removable appliances such as headgear and functional appliances are usually rejected by patients due to discomfort, pain, restriction, and constraints on physical activity and movement.²⁶ In a study on patient compliance with functional appliances monitored microelectronically, Sahm et al.²⁷ reported that the patients wore the functional appliances far less than recommended to obtain dentoskeletal changes.

To eliminate patient compliance for Class II correction, the use of non–patient-compliance-based FFAs was first propagated by Emil Herbst in 1905 with the Herbst appliance. The appliance never really gained popularity until Pancherz revived it in the 1970s. Since then there have been numerous modifications of the rigid Herbst appliance²⁸ and various other FFAs^29–32 have also been reported in the literature. These FFAs may be broadly classified as rigid, flexible, and semirigid fixed functional appliances. The major difference between functional appliances and FFAs is that the mandible is forcefully postured in an anterior position with FFAs with the help of inter-arch anchorage using the maxillary denture base as the anchor unit.

Using the concept of equilibrium,³³ it can be understood that when a rigid FFA is placed in the mouth, the appliance is in static equilibrium. This essentially means that when there is a mesial or forward directed force on the lower arch, there exists an equal and opposite distal or backward directed force on the upper arch. This effect has been proved by cephalometric studies as the “headgear effect”^34,35 of FFAs. Apart from the “skeletal” effects on the denture bases, effects on the dentition such as retroclination of maxillary incisors,³⁶ proclination of mandibular incisors,^36–38 distalization of upper molars and mesialization of lower molars,^37,38 and clockwise rotation of the occlusal plane³⁸ have also been observed with FFAs. Thus Class II correction with FFAs is reported to be due to a combination of skeletal and dental effects.