Introduction
Edward H. Angle described class III malocclusion as an ‘abnormal relation of the jaws, all the lower teeth occluded mesial to the normal width of one bicuspid or even more in extreme cases’. With the advent of cephalometric radiography, Angle’s classification was extended to include abnormal skeletal jaw relationships where class III malocclusion may exhibit a small maxilla, prognathic mandible or a combination thereof ( Fig. 69.1 ).
The skeleton from the remains of Al Qusais
(1500–1200 BC). It shows midface hypoplasia combined with mandibular prognathism.
Jean Delaire, a French orthodontist, who worked extensively on the growth of maxilla, class III malocclusion and its interception named this condition as ‘syndrome prognathique mandibulaire’.
The clinical presentation of class III malocclusion has a broad spectrum, ranging from an edge-to-edge bite to a large negative overjet, with extreme variations of underlying skeletal jaw bases and a great diversity of craniofacial forms. Class III facial forms are usually associated with a deformity that extends deep into the cranial structures. The soft tissue profile may show compensation in milder forms of class III malocclusion, appearing as orthognathic to mild concave. The facial profile would be truly concave in severe forms of mandibular prognathism. The unbalanced skeletal growth of the face, disturbance in functional occlusion and unique facial pattern could lead to enormous psychosocial problems in patients suffering from this deformity. Many young patients may be ignorant of the deformity, while others may seek treatment affected by the adverse impact on their quality of life ( Fig. 69.2 ).
(A) Anterior cross-bite in deciduous dentition and mesial step, the first indicator of a developing class III malocclusion. (B) Anterior cross-bite in permanent anterior teeth and mesial step indicates of developing class III malocclusion.
Prevalence of skeletal class III malocclusion
A systematic review reported a significant variation in the prevalence of skeletal class III malocclusion across various geographic regions and ethnic groups. Chinese and Malaysian groups show higher mean prevalence rates of 15.69% and 16.59%, respectively. The estimates of anterior cross-bite in the Japanese population are 2.3%–13%, and edge-to-edge relationships are 2.7%–7.4%. If these two manifestations occur frequently, a significant portion of the Japanese population may have class III malocclusion characteristics. European countries demonstrate a prevalence rate of 2%–6%. Indian population show a relatively lower prevalence rate (1.19%) in comparison with the other racial groups, while high prevalence rates ranging from 9.48% to 11.38% are reported in Middle Eastern countries.
The prevalence of class III malocclusion in the North Indian population was reported to be 2.5% (Leh, age group of 10–18 years) and 3.4% (Delhi, age group of 10–13 years). In the South Indian population, it was reported to be 2.1% (Karnataka, age group of 13–17 years) and 4.1% (Kerala, age group of 10–12 years). A scoping review assessing the global prevalence of class III malocclusion found an average prevalence of 7.2% across different geographical regions. The lowest prevalence was reported in Jordan (1.4%), while the highest prevalence was observed in Puerto Rico (19.4%) ( Table 69.1 ).
TABLE 69.1
Prevalence of class III malocclusion in different geographical areas
| Author | Population | Prevalence rate |
|---|---|---|
| Hardy et al. (2012) | Chinese | 15.69% |
| Malaysian | 16.59% | |
| Middle eastern | 1.3%–15.2% | |
| Europeans | 2%–6% | |
| Miyajima et al. (1997) | Japanese |
Anterior cross-bite: 2.3%–13%
Edge-to-edge bite: 2.7%–7.4% |
| Singh et al. (2015) | Indians | South Indians: 2.1%–4.1% |
| Kharbanda et al. (1995) | Indians | North Indians: 2.5%–3.4% |
| Centazo N et al. (2021) | Global prevalence |
Average 7.2%
(Jordan 1.4% Puerto Rico 19.4%) |
Despite a lower prevalence of class III malocclusion in the Caucasian population, it has been reported that approximately one-third of patients undergoing orthognathic surgery present with this type of malocclusion. Early identification and possible orthopaedical correction should not be overlooked.
Aetiology of class III malocclusion
The aetiology of class III malocclusion is complex, multifactorial and diverse, encompassing genetic, environmental and gene–environment interactions ( Table 69.2 ).
TABLE 69.2
Aetiology of class III malocclusion
| Genetic factors | Environmental local factors | ||
|---|---|---|---|
| Both monogenic (commonly autosomal dominant with incomplete penetrance) and polygenic modes of inheritance | Skeletal | Dental | Functional |
| Maxillary transverse discrepancy |
Ectopic eruption of the maxillary central incisors
Early loss of the deciduous molars |
Macroglossia and abnormal tongue position
Nasal obstruction Mouth breathing Neuromuscular condition |
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Growth is affected by both genetic and environmental interaction, producing a class III phenotype. Mandibular prognathism is mainly genetic, and maxillary retrognathism results from midface deficiency, which is mainly environmental.
Genetics
It has been known for many years that mandibular prognathism follows familial transmission. A well-known example of a familial inheritance pattern is the Hapsburg family, where 33 out of 40 members showed mandibular prognathism. Class III malocclusion associated with mandibular prognathism is assumed to have a multifactorial and polygenic transmission trait. However, monogenic transmission by autosomal dominant inheritance with incomplete penetrance has also been reported. A genetic study conducted among parents of 24 children affected by severe class III pattern showed that one-third of the group had a parent who presented with the same malocclusion, and one-sixth had an affect ed sibling. Similar to the heritability of class III malocclusion, inheritance of the long face pattern was also found between family members.
Environmental factors
The local epigenetic factors include macroglossia, mouth-breathing habit and enlarged tonsils, leading to a forward shift of the mandible in response to functional demands , ( Fig. 69.3 ).
Respiratory obstruction resulting in a skeletal class III malocclusion.
(A-D) A young boy with class III molar and canine relations, (E) Enlarged tonsils obliterating oro-nasal passages. (F) The clinical examination shows an edge-to-edge bite; however, he tends to occlude teeth in a most convenient way resulting in a forward positioning of the mandible and deviation towards the right side. (G) Lateral cephalogram shows class III skeletal pattern with large mandible and small maxilla. (H) Posteroanterior shows discrepancy in the transverse widths of the maxilla and mandible exhibiting bilateral crossbite in the buccal region.
Normal development of the maxilla results from the translation of the constituent skeletal units, structural remodelling and the development of the anterolateral region, which depends on the orofacial functions. Excessive mandibular growth is also thought to result from the forward mandibular posture, leading to condylar distraction and growth. The natural head position (relationship of the head to the exact vertical) affects the cranial base orientation. Raised natural head position produced a class III effect, and it is related to the degree of maxillary retrognathism. Although mandibular prognathism is related to natural head position, the severity of prognathism is augmented by the closing or opening rotation of the mandible, which is closely associated with natural head posture (relationship of the head to the cervical column). Premature occlusal contacts and interferences during early mixed dentition may lead a child to adopt an anterior crossbite posture. If not treated in time, this could result in class III malocclusion.
Systemic factors
The systemic factors include hormonal disturbances syndromes associated with premature synostosis of the cranial sutures restricting maxillary growth. Apert’s and Crouzon’s syndrome are common examples.
Maxillary hypoplasia seen in operated patients with cleft lip and palate is due to the scarring effect of surgical repair that interferes with the anteroposterior, transverse and vertical development of the maxillary complex.
Epigenetic aetiology
Class III malocclusion develops due to multiple factors that interact during the growth period of the mandible. The skeletal morphology of the dentofacial complex is likely dependent upon susceptibility genes involved in gene–environment interactions, resulting in a class III phenotype. However, it is postulated that class III malocclusion due to mandibular prognathism is mainly influenced by genetic control. In contrast, those associated with maxillary hypoplasia result from environmental induction, which may also include the restraining effect of an inherited prognathic mandible inhibiting maxillary forward growth ( Fig. 69.4 ).
Representation of gene–environmental interaction.
Mandibular growth is induced by both genetic and environmental mechanisms, which interact with each other to produce the class III phenotype.
Source: Reproduced with permission from: Xue F, Wong RW, Rabie AB. Genes, genetics, and Class III malocclusion. Orthod Craniofac Res 2010;13(2):69–74 .
Components of class III malocclusion
A complex combination of skeletal and dentoalveolar components is responsible for the compound nature of the class III trait. Ellis and McNamara found a combination of maxillary retrusion and mandibular protrusion to be the most common skeletal relationship (30%), followed by maxillary retrusion (19.5%) and mandibular protrusion only (19.2%). However, studies have reported that the mandible is the most substantial contributor to the skeletal manifestation of class III malocclusion (45.2%–47.4%). , It is further noted that mandibular prognathism is mainly due to the positional deviation of the mandible relative to the cranial base. In contrast, maxillary retrognathism is primarily caused by inadequate length. There is a compensatory backwards rotation of the mandible, which appears necessary to coordinate the occlusion relative to the small maxilla. Different morphological studies on subjects with class III malocclusion illustrate several common features like a short cranial base with an acute saddle angle, a retrusive maxilla, a protrusive mandible, proclined maxillary incisors and retroclined mandibular incisors. , Four different class III facial profiles have been described ( Fig. 69.5 ).
Four different types of class III facial skeletal profiles.
(A) Normal maxilla and mandibular prognathism. (B) Maxillary retrusion and normal mandible. (C) Normal maxilla and mandible. (D) Maxillary retrusion and mandibular prognathism. N , Nasion; FH , Frankfurt horizontal plane; A , point A; Pog , pogonion.
Source: Based on Ngan P, Hägg U, Yiu C, Merwin D, Wei SH. Soft tissue and dentoskeletal profile changes associated with maxillary expansion and protraction headgear treatment. Am J Orthod Dentofacial Orthop 1996;109(1):38–49 Jan, Erratum in: Am J Orthod Dentofacial Orthop 1996 Apr;109(4):459 .
A new classification of class III malocclusion was proposed based on cephalometric data and facial photographs of 120 subjects belonging to Korean ethnicity. The patients were classified into three categories by Park et al.
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Type A is true mandibular prognathism, which means that the maxilla is normal, but the mandible is overgrown.
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Type B is characteristic of the overgrown maxilla and mandible with anterior cross-bite.
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Type C indicates a hypo-plastic maxilla with anterior cross-bite.
Multivariate analyses such as discriminant analyses, principal component analyses (PCA) and cluster analyses can identify different phenotypic subgroups specific to the population. , These cluster analyses of different phenotypes can guide treatment planning and the evaluation of treatment effects and find their application in genetic studies.
Ethnicity and gender differences
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Ethnicity: Heterogeneity in craniofacial morphology between various ethnic groups has been reported and is most likely determined by genetic factors. Chinese subjects exhibit a shorter anterior cranial base, a large posterior cranial base, a small gonial angle and an increased mandibular length compared to Caucasians. Korean population show smaller anterior cranial base and midface dimensions, exacerbated by a large and less favourable mandibular morphology compared to European American children. European American children exhibit less flattening of the cranial base and early closure of spheno-occipital synchondrosis, resulting in less apparent class III facial morphology. Japanese tend to have maxillary skeletal retrusion, large posterior cranial base length and increased lower anterior facial height. In contrast, Americans show mandibular prognathism, larger anterior cranial base length and facial depth.
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Gender differences exist in craniofacial growth due to variations in the concentration of sex hormones, particularly during the pubertal growth spurt. A high testosterone/oestrogen ratio in puberty causes a lateral growth of the mandible and chin and lengthens the lower face. Male subjects with class III malocclusion present significantly larger linear dimensions of the maxilla, mandible and anterior facial heights than female subjects during the circum-pubertal and post-pubertal periods. Sexual dimorphism was also reported in growth patterns. The faces of class III females show a tendency for development in a more horizontal direction compared to class III males, who show a more vertical direction of facial growth.
Diagnosis of class III malocclusion
A systematic approach to evaluating class III malocclusion aims to identify the aetiology and nature of dysplasia and other critical considerations in formulating a treatment plan ( Table 69.3 ).
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Extraoral features: Facial evaluation requires analysis of the overall profile, chin position and maxillary and mandibular position to differentiate between maxillary retrognathism, mandibular prognathism or a combination of both.
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Profile evaluation: Skeletal class III patients with midface deficiency present with a straight-to-concave profile. Parameters on profile photographs can also aid as a diagnostic tool. A soft tissue A’N’B’ angle of 6 degrees is the critical value below, which implies the presence of skeletal class III malocclusion. Reduced lower anterior facial height suggests over-closure or forward closure of the mandible, which is evident in pseudo-class III malocclusion ( Fig. 69.6 ).
Figure 69.6 (A) A patient with class III skeletal deformity exhibiting exaggerated severity due to the habitual forward mandibular shift. (B) When the mandible is guided into a centric relationship, an end-to-end incisor relationship is present and represents the true skeletal base relationship.
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Chin position: The position of the chin relative to the nose and upper face is evaluated by blocking out the upper and lower lips. The position of the chin is also assessed from a vertical line extending from soft tissue nasion. It is important to remember that facial convexity decreases as the patient matures.
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Maxillary position: The lower lip and chin are blocked out to accentuate the midface. There should be a convexity to the shadow extending from the inferior border of the orbit, through the alar base of the nose, down to the corner of the mouth. A straight vertical shadow indicates a midface deficiency. Upper lip length and upper lip support are evaluated, which indicates midface deficiency. The relationship of the anterior cheek mass to the anterior corneal plane indicates bony support along the malar eminence.
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Mandibular position: The presence, amount and direction of any mandibular shift are significant in determining whether the class III malocclusion is due to the maxilla or mandible. In cases displaying a significant anterior mandibular shift, the same facial evaluation should be performed with the mandible in centric relation.
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Facial asymmetry: It is frequently seen in patients with class III deformity. It is often associated with posterior cross-bites. A posterior cross-bite can be a consequence of constriction of the maxilla or the result of lower jaw deviation. Posterior cross-bite with jaw deviation is usually accompanied by an asymmetry of the shape of the temporomandibular joint (TMJ) and should be ruled out.
TABLE 69.3
Critical factors in diagnosis of growing class III malocclusion
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Orthopantomogram can be used to evaluate the symmetry of the condyles. In class III deformity, the lower jaw presents with more asymmetry than the upper jaw and left-sided facial laterality occurs more often than right-sided deviation.
Traditionally, the combined uses of posteroanterior, lateral and sub-mento vertex views are advocated for 3D evaluation of the maxillo-mandibular structures. CBCT offers excellent value in 3D assessment and is a substitute for multiple X-rays. 3D photographs taken in a natural head position allow for more precise measurement and analysis of facial soft tissues.
Dental features
The patient presents with class III molar relation with or without negative overjet. A compensated class III malocclusion is suspected if a positive overjet or end-to-end incisor relationship is found, along with retroclined mandibular incisors. The diagnostic criteria of at least two incisors in cross-bite or edge-to-edge with each other and a mean canine mesiocclusion of at least a half-cusp width correlate relatively higher with true class III sagittal molar relationship. If a negative overjet is present, the functional examination is performed to rule out pseudo-class III malocclusion.
Functional examination
The presence of a dental anterior cross-bite is not necessarily indicative of an underlying skeletal class III problem specially in mixed dentition and growing children. This anterior dental relationship may result from a pre-maxillary dentoalveolar deficiency or a functional problem where occlusal interferences create an anterior shift of the mandible during closure. After eliminating these interferences, mainly due to maxillary incisor retroclination, the maxilla and mandible return to a class I relationship. However, should it be left unattended, a functional class III malocclusion will likely develop into a skeletal malocclusion. Diagnostic characteristics of pseudo-class III malocclusion were identified in the southern Chinese population after comparing 36 subjects with pseudo-class III malocclusion and 31 subjects with class I malocclusion ( Table 69.4 ). A diagnostic scheme was proposed to differentiate between a true and pseudo-class III ( Fig. 69.7 ).
TABLE 69.4
Diagnostic criteria of pseudo-class III malocclusion
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Diagnostic scheme to differentiate between true and pseudo-class III malocclusion.
CO , Centric occlusion; CR , centric relation.
Cephalometric evaluation
Intermaxillary relations and maxillary and mandibular incisors contributing to class III malocclusion can be confirmed using cephalometric evaluation. However, during the diagnosis of class III malocclusion, cephalometric values can only provide the relative contributions of skeletal and dental components to the malocclusion, and neither jaw may be identified as the obvious contributing factor to a class III malocclusion, particularly in a young child. Because of this variability, other factors, such as the overall facial profile, chin position, maxillary position and mandibular functional shift, should also be considered. The important cephalometric parameters to evaluate class III malocclusion are presented in Fig. 69.8 . In cases of class III malocclusion associated with anterior functional shift, two lateral cephalograms may be taken, that is, one at centric occlusion and another at centric relation position as shown in Fig. 69.6 . The rotational component and sliding component are evaluated using these lateral cephalograms.
Linear and angular cephalometric parameters used in the diagnosis of class III malocclusion.
Prediction of class III skeletal growth
Class III growth pattern is established early in life, even before the pre-pubertal stage. The class III discrepancy worsens with age and does not stop later in life. , Due to the uncertainty of the long-term stability of treated skeletal class III malocclusion, it is crucial to identify potential predictors of class III growth patterns. If prediction could be made before treatment, then the type and timing of orthopaedic treatment could be modified, and the patient could be informed regarding the future need for orthognathic surgery. The positive family history and hereditary nature of malocclusion are good indicators for potential severe class III patterns .
Growth prediction is difficult due to the greater individuality and variation in craniofacial growth. Several methods to predict growth have been reported, including Ricketts’ arcial method, computerised growth prediction and Johnston’s forecast grid. ,
Bjork identified the seven structural signs of extreme mandibular growth rotation related to the inclination of the condylar head, the curvature of the mandibular canal, the shape of the lower border of the mandible, the width of the symphysis, the interincisal angle, the inter-molar angle and the anterior lower face height. Later, the use of symphyseal morphology to predict the direction of mandibular growth was proposed.
Mandibles that grew in an anterior direction were associated with reduced height, increased depth, a small ratio (height/depth) and a large angle of the symphysis. Symphysis dimensions may continue to change until adulthood, with male subjects experiencing greater and later changes than female subjects.
Schulhof et al. calculated the sum of the deviations of molar relationship, cranial deflection, porion location and ramus position from the norms with the Rocky Mountain data system. If the sum of deviations exceeds 4, it indicates increased mandibular growth.
Over time, several cephalometric parameters have been developed to assess mandibular growth quantitatively.
Franchi et al. used three variables: the inclination of the condylar head, maxillary-mandibular vertical relationship (MP-PP) and mandibular inter-molar width. Ghiz et al. used four variables, namely the position of the condyle to cranial base, ramal length, mandibular length and gonial angle, that can predict successful outcomes with a 95% degree of accuracy. However, these variables can only predict successful outcomes with a 70% accuracy.
Musich proposed the growth treatment response vector (GTRV) analysis to predict excessive mandibular growth after early orthopaedic treatment. Ngan detailed using a growth treatment response vector to predict whether patients who have had early protraction facemask therapy in the mixed dentition will require either a second phase of orthodontic camouflage or orthognathic surgery. He suggested using serial cephalometric radiographs of patients taken a few years (3–4 years) apart after facemask treatment.
GTRV analysis
GTRV analysis consists of the ratio of horizontal growth changes of the maxilla to the horizontal growth changes of the mandible measured along the occlusal plane. The mean GTRV ratio for the successful group was 0.49 ± 0.14 with a range of 0.33–0.88 and represented a group that can be successfully camouflaged with orthodontic treatment.
The mean GTRV ratio for the unsuccessful group was 0.22 ± 0.10, with a range of 0.06–0.38. The GTRV ratio below 0.38 should be warned of the need for future orthognathic surgery ( Fig. 69.9 ).
Growth prediction of class III malocclusion using GTRV method.
Source: Ngan P, Wei SH-Y. Early treatment of class III patients to improve facial aesthetics and predict future growth. Hong Kong Dent J 2004; 1: 24–30 .
A systematic review that analysed 38 different predictors of the treatment outcome (35 cephalometric parameters and 3 on dental casts) concluded that gonial angle was identified most frequently, combined with other predictors among the different studies. The possibility of the existence of a universal predictor and accurate prediction of the treatment outcome of class III malocclusion is questionable.
Management
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Interception of the problem through dentofacial orthopaedics:
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Appliances to restrain the growth of the mandible
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Chin cup with headgear
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Appliances primarily directed for orthopaedic effect on maxilla
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Protraction facemask
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Appliances that affect both the jaws by altering growth
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Functional appliances:
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Frankel’s FR III
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Reverse twin block
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Class III bionator
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Camouflage treatment
Interception of malocclusion
Until the 1970s, treatment of class III malocclusions was mainly directed towards surgical correction, for it was believed that these malocclusions were beyond the boundaries of orthodontic and dentofacial orthopaedic treatment.
Compared to the relative convex class II face profile favoured in the 1960s and 1970s, fuller profiles have become more desirable in contemporary society. This change in perception significantly influences orthodontic and orthognathic surgery treatment objectives in the management of classes II and III cases. As such, orthodontics and oral surgery specialities now prefer to bring the maxilla forward in class III cases and the mandible forward in class II cases over a retrusive profile.
Studies, however, have shown significant benefits of early treatment in class III maxillary deficient patients with the use of protraction headgear. In contrast, the long-term outcome of chin cup therapy for mandibular prognathism was found to be unsatisfactory. ,
The rationale for early treatment of developing class III malocclusion
The goals of early class III treatment are :
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To prevent progressive, irreversible soft tissue and hard tissue changes like abnormal wear of the lower incisor and thinning of the lower anterior alveolar bone.
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To improve skeletal discrepancies and provide a favourable environment for future growth.
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Improve occlusal function, eliminate CR-CO discrepancy and avoid adverse growth potential.
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To simplify phase II comprehensive treatment and minimise the need for orthognathic surgery.
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To provide more pleasing facial aesthetics, thus improving the child’s psychosocial development.
A recent multi-centric randomised clinical trial (RCT) study reported that early class III protraction facemask treatment reduces the need for orthognathic surgery from two-thirds (control group) to one-third (facemask treated group), and early protraction facemask treatment does not seem to confer a clinically significant psychosocial benefit.
Turpin outlined the positive and negative factors that decide the need to intercept a developing class III ( Table 69.5 ). Early treatment should be considered in patients with positive factors, and treatment can be delayed in patients with negative factors. However, patients instituted with early treatment should be made aware of the fact that surgery is the potential final treatment option and may be required later due to an unpredictable growth pattern or relapse of early intervention.
TABLE 69.5
Factors to be considered to intercept class III malocclusion
| Positive factors | Negative factors |
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Management of pseudo-class III malocclusion
Pseudo-class III malocclusion is associated with anterior cross-bite due to mandibular displacement. This functional shift of the mandible accentuates the degree of the class III deformity. The primary objective in such cases is to eliminate the functional shift. The negative forces on the maxilla are minimised or eliminated by removing the CR-CO discrepancy. Correcting the functional forward-positioned mandible into normal overjet facilitates normal growth of both the maxilla and mandible.
Treatment aims to correct premature occlusal contacts, particularly the anterior dental cross-bite, which can occur in deciduous and permanent dentition. Intraoral appliances used to correct non-skeletal cross-bites include:
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Catalan’s appliance (lower inclined bite plane) and tongue blade.
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Removable appliance using Z spring or expansion screw exerting labial force on maxillary incisors.
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Lingual arch with finger spring.
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Fixed appliances in the maxilla, most commonly 2 × 4 appliances.
In some cases, due to premature contact, the patient deviates from the mandible in a forward posture to obtain a convenient occlusal bite. If left untreated during the growth period, this condition could permanently alter the path of closure, facial asymmetry and skeletal class III malocclusion.
Karwetzky modified U bow activator
Removable functional appliances can be used to redirect the altered path of closure. These appliances provide proprioceptive stimuli that restrict the forward growth of the mandible and simultaneously assist in normalising the path of closure and lateral deviation of the mandible. One such appliance is the Karwetzky modified U bow activator, which has a delicate influence on the dentition and TMJ. Three types of modification of this appliance are used in different clinical situations. A young boy with a functional forward and left shift of the mandible resulting in an anterior crossbite and a midline shift to the left side was successfully treated using a Karwetzky activator. ( Fig. 69.10 ).
A young boy with functional forward and left shift of the mandible resulting in anterior cross-bite and midline shift to the left side.
Karwetzky U loop activator appliance.
The bite was recorded in centric relation and Karwetzky U loop activator appliance was given which rapidly corrected the crossbite and centre line shift.
Post-treatment photographs and radiographs.
Pre- and post-treatment cephalometric values are shown in the box.
Chin cup appliance
Attempts to control excessive lower jaw growth in patients with mandibular prognathism have been a part of orthodontic practice since its inception. The first chin cup was used in 1802 by Cellier to correct jaw dislocations. Joseph Fox used a chin cup to correct mandibular protrusion 1 year later. Kingsley and Farrar used appliances to restrain mandibular growth that resembles a modern chin cup. The early failure of treatment with the chin cup appliance was due to low force values that could not influence the condylar growth. The lack of a clinical concept of growth guidance led to the use of intermaxillary elastics to correct the skeletal class III malocclusion. In the early 1950s, the conceptual change of the use of orthopaedic forces in the range of 400–800 g reintroduced the chin cup appliance.
Clinical aspects of chin cup therapy
The chin cup is advised in growing patients with true skeletal class III malocclusion who lack maxillary recession, which is associated with acute cranial base angle. Chin cup therapy is attempted when orthognathic surgery is not an option. It can also be used with an upper removable appliance to procline maxillary incisors where substantial overbite can be achieved and mild to moderate skeletal discrepancy can be camouflaged. Chin cup is also used to retain functional occlusion after the first phase of treatment for growing class III patients.
The chin cup appliance can be used as an occipital pull for patients with mandibular protrusion and non-dolichofacial patients. In contrast, a high pull or combination of occipital and high pull chin cup can be used in dolichofacial patients exhibiting steep mandibular plane and excessive anterior facial height ( Fig. 69.11 A). The occipital pull chin cup appliance is available in two forms.
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The soft chin cup consists of a padded band extending coronally and a soft cup on the chin with an elastic traction band connecting both components. In this type, the position of the head cap determines the direction of the force.
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The hard chin cup type consists of padded bands extending coronally and cervically and a hard chin cup anchoring on the chin. These are connected using a Hickman-type headgear (force guide) placed in front of the ear. The hard cup can be custom-made to fit the appliance better. The direction of the force is adjustable according to the placement of the elastics between the chin cup and the Hickman-type force guide.
Various appliances to intercept the developing class III malocclusion.
(A) Chin cup appliance. (B) Frankel functional regulator III (FR-III). (C) Reverse twin block appliance with lip pads. (D) Class III bionator appliance with lip pads.
Evidence suggests that the morphologic pattern of the prognathic face with excessive forward mandibular growth is most likely to be established early in life. Hence, the treatment of mandibular protrusion is more successful when it starts in the primary or early mixed dentition stage. The treatment time varies from 1 year to as long as 4 years, depending on the severity of the malocclusion. , Gender should be considered since females mature earlier than males. The light force (125–250 g) does not produce significant orthopaedic change in the mandible.
The suggested force at the centre of the chin cup ranges from 300 to 600 g/side for 12–14 h/day. It is recommended to be worn particularly during sleep to avoid injury to the inter-articular disc. Otherwise, there is a higher risk of dislocating the disc at the joint if worn during functional activity like speech and mastication. ,
Biomechanics of chin cup therapy
The mechanism by which chin cup treatment acts on growing mandible has always remained controversial. The following mechanisms are reported in the literature :
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Redirection of mandibular growth at the chin.
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Backwards repositioning of the mandible.
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Retardation of mandibular growth at the condyle.
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Remodelling of mandibular morphology at the gonial angle and symphysis.
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Concomitant remodelling at TMJ.
There are two main approaches to the occipital pull chin cup therapy according to the direction of force against the mandible. In the first approach, the force is aimed directly at the condylar area to impede mandibular growth. In the second approach, the force is aimed below the condyle to produce a clockwise rotation of the mandible, causing a decrease in the prominence of the chin exchanged with an increase in the anterior facial height.
The finite element analysis (FEM) studies showed that the force vector passing through the condylar head induced little or no stress with clockwise rotation of the mandible to produce favourable changes for most class III patients with neutral or anterior growth rotation. The force vectors passing through the coronoid process or anterior to coronoid process produced higher stress levels and induced displacements in the counterclockwise direction that might be favourable mainly for open bite patients with no class III tendency.
Treatment effects on craniofacial structures and TMJ
Short-term application of occipital pull-chin cup results in a significant decrease in SNB angle, improving ANB angle values. This is contributed by a clockwise rotation of the mandible and, to some extent, restriction/redirection of the mandibular growth. There is an increase in lower anterior facial height due to the clockwise mandibular rotation. The closure of the gonial angle is widely reported in the literature due to remodelling effects, making it more obtuse. Dental effects include upper incisor protrusion, lower incisor retrusion and improving overjet. Soft tissue changes accompany the underlying skeletal and dentoalveolar tissue changes. Significant reductions in the facial convexity angle and lower lip inclination cause improvement in the soft tissue facial profile. ,
A short-term study has shown no adverse effects on pharyngeal airway passage, which is expected to reduce with a backward and downward rotation of the mandible. Long-term treatment with a chin cup resulted in significant inhibition of the vertical growth of the ramus and body length of the mandible, remodelling of the mandible and TMJ, and closure of the gonial angle. There are no changes observed in the maxilla and the cranial base with the use of the chin cup appliance.
Chin cup therapy is most frequently cited as a cause of temporomandibular disorder (TMD). It is claimed that the retracting force on the TMJ directed from the chin to the condyle may cause internal derangement of the TMJ. Stresses from the chin cup radiate in a postero-superior direction and concentrate at the neck of the condyle, which is most responsive to orthopaedic forces on the mandible. The condylar head angle (angle between the condyle and collum) decreases due to the condylar head’s forward bending. , The expected upward and backwards movement of the condyle is opposed by the horizontal portion of the temporomandibular ligament that acts as a ‘safety belt’ mechanism. A systematic review concluded that despite the craniofacial adaptations induced by chin cups in patients with class III malocclusion, it does not constitute a risk factor for the development of TMD. There may be other factors that should be taken into consideration. Age-related peak (20 and 45 years of age) in the occurrence of TMD is reported particularly in females due to emotional factors and stressful lifestyle.
Stability after chin cup therapy
The soft tissue shows general improvement in the facial profile, along with accompanying skeletal and dentoalveolar changes, but with an uncertain long-term stability. Animal studies have shown a decrease in the activity of the pre-chondroblastic layer of the condylar cartilage, resulting in a decreased bone formation at the condyle. It was suspected that the release of compressive forces before growth completion stimulated condylar growth, , which shows that the mandible attempts to recover the size initially determined morphogenetically until the growth terminates.
The pre-treatment facial and skeletal profile, severity of anteroposterior jaw discrepancy, type of mandibular rotation and displacement and degree of forward growth of the mandible determine treatment stability. A steeper mandibular plane, occlusal plane, a more vertical profile and obtuse gonial angle contributing to vertical characteristics of the facial profile were observed in the relapse group.
Clinical recommendations
Chin cup should be advocated only after correctly diagnosing the growth status, growth pattern and patient compliance. It is advisable to continue the chin cup therapy until growth is complete. The temporomandibular joint function should be assessed before and during active treatment. The remaining mandibular growth may lead to skeletal class III relations, which could necessitate orthognathic surgery.
Current thoughts on chin cup therapy are controversial. Chin cup therapy is considered ineffective in the long term and is no longer considered a primary treatment modality in the interception of the growing mandible.
Functional appliances to correct class III malocclusion
Functional appliances are used in orthopaedic and orthodontic correction of class III malocclusion. The effects of functional appliances in class III corrections are primarily dentoalveolar. A favourable case for a functional appliance should be a mild to moderate skeletal class III relationship, average to reduced anterior facial height, pseudo-class III with a functional shift of mandible and minimal incisor compensation. A functional appliance is also used as a retention appliance following facemask therapy during the growing stage ( Fig. 69.11 B–D).
Frankel’s functional regulator III appliance
The functional regulator type III (FR III) by Rolf Frankel appliance is indicated in growing patients, particularly during the late deciduous or early mixed dentition stage (CS 1 in skeletal maturation). Patients with mild maxillary deficiency and/or mandibular forward shift and willing to wear the appliance for a long time are expected to benefit from it.
FR III design.
The FR III appliance comprises wire and acrylic, with four acrylic parts. Two upper labial pads are positioned in the labial vestibule above the maxillary incisors, which are used to eliminate the restrictive pressure of the upper lip on the underdeveloped maxilla. Two vestibular shields extend from the depth of the mandibular vestibule to the height of the maxillary vestibule and stimulate labial alveolar bone apposition by stretching the adjacent periosteum. The vestibular shields are placed approximately 2 mm away from the alveolar buccal plates of the maxilla to eliminate the restrictive forces created by the buccinators and the associated facial muscles and fitted as closely to the alveolar process of the mandible as possible to hold or redirect growth posteriorly ( Fig. 69.11 B).
Bite recording.
The wax bite is recorded with the mandible gently guided posteriorly to the centric relation position. Allowing 1–2 mm of interocclusal clearance in the molar region is necessary to accommodate occlusal rests and crossover wire. In cases with an anterior open bite, only 1 mm of vertical bite-opening in the posterior region is necessary.
Treatment effects.
FR III appliance is reported to cause a decrease in SNB and an increase in ANB angle along with dental effects like linguoversion of the mandibular incisors. The downward and backward rotation of the mandible is primarily attributed to the correction of class III malocclusion. Long-term studies showed a significant change in maxillary size and position with modification in mandibular morphology, closure at the gonial angle and associated closure of the mandibular plane angle. The forward displacement of the maxillary complex has not been shown consistently in the literature. This was further supported by the systematic review, which concluded that the FR III appliance restricts mandibular growth and weak evidence against the forward movement of the maxilla.
Reverse twin block appliance (RTB)
William Clark described a modification of twin block appliance for correction of skeletal class III malocclusion in which the lower block occludes distal to the upper block. The proposed mode of action of the class III twin block is that the reverse angulation of blocks harnesses the occlusal forces to advance the maxilla and maxillary dentition while using the mandible as an anchorage and restricting its development ( Fig. 69.11 C).
Bite recording.
A construction bite is recorded with the mandible in a maximum retrusive position, leaving sufficient clearance between the posterior teeth for the occlusal bite blocks ( Fig. 69.11 D). Two millimetres interincisal clearance is given for this purpose. In patients with reduced lower anterior facial height, the bite is recorded with 4 mm interincisal clearance. The appliance is constructed with heat cure acrylic with Adams’ clasps placed on the first molars and interproximal ball-ended claps for retention. The upper and lower inclined bite planes are at 70 degrees to the occlusal plane configured in reverse of the conventional twin block. A midline expansion screw can be incorporated into the upper component in case of a constricted upper arch and to permit arch coordination. An alternative design uses a three-way expansion screw to combine transverse and sagittal expansion. Opening the screw applies a distal force on the upper molars, which are resisted by occlusion of the lower bite blocks on the reverse inclined planes. Therefore, the net effect of opening the screws is a forward driving force on the upper dental arch. A lower labial bow can be incorporated to control the position of the lower anterior segment. Adding acrylic to the inclined planes may be necessary to increase the forces over the maxilla and mandible to establish a positive overjet. Lip pads may be added to support the upper lip to enhance the forward movement of the upper labial segment, an action similar to that of the Frankel III appliance. ,
Treatment effects.
The primary effect of the RTB appliance appears to be dentoalveolar. Analysis of cases following full-time wear of RTB appliance for 6 months showed proclination of the maxillary incisors and retroclination of the mandibular incisors, downward and backward mandibular rotation with a concomitant increase in lower facial height and a relative reduction of mandibular prognathism. Comparison between protraction facemask and RTB therapy revealed RTB appliance-induced greater dentoalveolar changes. Compared with untreated controls, RTB was found to exert no skeletal effects on the maxilla.
Maxillary protraction appliances
Historical review
The use of a protraction facemask was first described in 1875 by Potpeschnigg. In 1944, Oppenheim reported that when the growth of the mandible was uncontrollable, it was possible to bring the maxilla forward to compensate for mandibular overgrowth. Haas demonstrated a forward and downward movement of the maxilla due to palatal expansion. In 1971, Delaire revived the interest in maxillary protraction and designed an appliance that incorporated a chin cup, forehead support and an inter-labial bow with spurs for attachment of elastics. Petit utilised this concept by using heavier force and reducing the treatment time. In 1987, McNamara used a bonded expansion appliance with acrylic coverage in conjunction with a facemask appliance ( Fig. 69.12 ).
Types of facemask appliance.
(A) Delaire’s facemask. (B) Petit facemask.
Experimental studies on primates demonstrated that applying an orthopaedic force to the maxilla caused its separation from the pterygoid plates, and the maxilla was repositioned anteriorly due to sutural modification. It was also observed that force variables play an essential role in attaining the desired change in the position of mid-facial bones.
Appliance design and manipulation
Many varieties of maxillary protraction appliances differ in the biomechanics of force application and the area of anchorage reinforcement ( Table 69.6 ).
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•
Delaire’s facemask consists of two pads that take anchorage from the forehead and chin region. An adjustable square metal framework connects the pads. An adjustable anterior wire with hooks is also connected to the framework to accommodate a downward and forward pull on the maxilla with elastics ( Fig. 69.12 A).
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•
Petit’s facemask comprises two pads that contact the soft tissue in the forehead and chin regions. The pads are connected by a midline framework made of stainless steel, and in the centre of the midline framework is a crossbar that has attachments for engaging elastics ( Fig. 69.12 B).
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•
Duane Grummons claimed that reverse pull headgear might have harmful effects on the TMJ and supported disengagement of the mandible during maxillary protraction. He introduced a new appliance that uses the zygomatic region as an anchorage unit for maxillary protraction. It has a forehead support, two sub-orbital pads and a metal frame.
TABLE 69.6
Different types of maxillary protraction appliances
| Anchorage area | Types of facemask appliances |
|---|---|
|
|
|
Grummons facemask |
|
Hickham reverse pull headgear |
|
Mini maxillary protraction device |
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