The twin blocks were initially designed to hold the mandible forward to relieve upper lip pressure in a child whose traumatised tooth was re-implanted in the maxilla. One of Clark’s colleague’s sons reported to his practice with luxation of the right upper central incisor. The tooth was re-implanted and held with temporary splinting. After 6 months, he noticed that the tooth became partially stable, but there was significant bone loss around the tooth. The patient had a class II division 1 pattern of occlusion with a lip trap and overjet of 9 mm. Clark wanted to design an appliance by which he could relieve the pressure of the upper lip on an upper incisor. He invented an acrylic device with two plates, one upper and a lower, inclined at 90 degrees. While closing the mouth, the patient had to posture the mandible forward and close teeth following the guidance of the inclined planes with an inter-incisal clearance of 2 mm. After 6 months, he observed that the mandible got sagittally advanced, and the molar relation became class I with a reduction of overjet to 4 mm. That was the beginning of a new concept of the two-piece functional appliance. Subsequently, Clark used this device to correct developing class II malocclusion in patients with small mandibles and continued to improve the design based on his clinical experience with patients.

It was designed so that treatment effects are ensured irrespective of patient compliance. Fixed twin blocks use the preformed component as an effective guidance mechanism for mandibular advancement. The system can be combined with the conventional fixed appliance or a TransForce lingual appliance for arch development.

Three distinct objectives of approach with fixed twin block include:

• 1.

  Interceptive treatment and arch development.

• 2.

  Mandibular advancement with fixed twin block.

• 3.

  Detailing of occlusion with fixed appliance.

Development of fixed twin block

Dr. Clark continued to develop and evolve the design of twin blocks for efficacy and different clinical situations of malocclusion, including patient cooperation. He initiated the concept of twin blocks early in 1990s. However, it was around 2008 that the first preformed block was created, which could be used as an attachment to molar bands using a blade attachment inserted into a lingual sheath. It was challenging to fit blocks correctly as an attachment to the molar band, which was a major limiting factor. By 2010, a new version attempted to eliminate attachment to the molar band.

Upper and lower blocks are bonded directly to the teeth. The blocks are designed to cover the lingual and occlusal surfaces of the teeth, leaving the buccal surfaces clear for attachment of bonded brackets. The upper block covers the second premolar and extends distally to the second molar region. The lower block covers the premolars with lingual extensions on the canine and first molar.

By 2014, the ultimate solution to fixed twin blocks was found. B uccal extensions are added to preformed blocks to improve stability and retention. The blocks fit over the teeth and are filled with Triad (visible light cure material by Dentsply-Sirona, North Carolina, USA) material for an accurate fit. The technique is similar to temporary crown and bridge and may be used as a direct or indirect technique after first checking the fit of blocks on a working model ( Fig. 63.2.ii ).

Components of a fixed twin block appliance.

(A and B) Pre-formed blocks have buccal and lingual extensions. (C–F) Blocks are integrated with fixed appliance treatment.

Preformed occlusal blocks cover the lingual, occlusal and buccal surfaces of upper and lower teeth; they will be available in three sizes—large, medium and small.

Digital scanning and 3D printing has been utilised to fabricate twin block making it a viable option without plaster and acrylisation process. Digitally printed twin block appliance can be fabricated with the help of intra-oral scans and computer-aided design and computer-aided manufacturing (CAD/CAM) printing using thermoplastic bases. The appliance has superior aesthetics, is relatively small and saves multiple visits hence cost effective. Upper and lower models are 3D printed followed by a protrusive bite transfer on an articulator. After the bite is registered, the models are scanned in protrusive position for digital designing and 3D printing of the twin block appliance with satisfactory accuracy and retention of the appliance. A 3D printed twin block is shown in Fig. 63.2.iii .

3D printed twin block appliance.

Source: From Chan El, Song YL, Foong KW, Chew MT. 3D printed twin block: A feasibility study. Ann 3D Print Med. 2023;11:100118.

Another innovation involved bonded twin block using a digital workflow. The appliance comprises of four blocks bonded to permanent/deciduous dentition. Occlusion is recorded at maximal intercuspation and planned mandibular advancement by an intra-oral scanner. The resin for customised imprint tray fabrication is placed intra-orally for bite registration and cured at the position of planned advancement of the mandible. Four blocks are then prepared digitally with 60 degrees inclined plane angulation for final appliance fabrication and is bonded using light cured compomer ( Fig. 63.2.iv ).

Digitally bonded twin block appliance.

(A) Intra-oral scan of the patient. (B) Resin is placed one side at a time, and the mandible is advanced to the desired position for bite registration. (C) Four blocks, namely upper posterior right and left blocks and lower anterior right and left blocks. (D1) Finished 3D printed twin blocks maxillary occlusal view. (D2) Finished 3D printed twin blocks mandibular occlusal view.

Source: From Nucera R, Barbera S, Militi A, Portelli M, Peditto M, Oteri G, et al. Digital Bonded Twin Block a new no-compliance device to treat skeletal class II malocclusion in mixed dentition: Design, fabrication, and clinical management. Semin Orthod. 2023;29(2):243−258 . doi: https://doi.org/10.1053/j.sodo.2023.05.006 .

Patients having the following features of occlusion/malocclusion are considered suitable for FA:

• 1.

  Growing children, preferably just before the pubertal growth spurt.

• 2.

  Normal physical and mental growth, normal weight and height/age.

• 3.

  A child who is free from any systemic disease which may affect skeletal/dental/neuromuscular behaviour and response.

• 4.

  A cooperative patient.

• 5.

  Normal eruption of teeth according to age.

• 6.

  Square face with an average or low mandibular plane angle.

• 7.

  Dental and skeletal class II malocclusion, class II molar and/or canine relationship.

• 8.

  Normal or retrognathic maxilla with a small mandible.

• 9.

  Maxillary dentoalveolar protrusion with a small mandible.

• 10.

  Upright mandibular incisors on basal bone or those with mild proclination.

• 11.

  Deep curve of Spee, supra-erupted mandibular incisors or infra-occlusion of the mandibular molars.

• 12.

  Deep bite.

• 13.

  Large overjet contributed by a small/retrognathic mandible.

• 14.

  Narrow maxilla may show a transverse deficiency.

• 15.

  Maxillary arch with minimal crowding of not more than 4 mm which could be resolved with arch alignment and maxillary expansion.

• 16.

  Aligned anterior teeth or minimal crowding in the mandibular arch.

• 17.

  Subjects with low caries risk and good oral hygiene practice.

A case of class II malocclusion who responded favourably to treatment is depicted in Fig. 63.5 .

Case study treatment with twin block.

(A) Case AJ, 10-year-old young boy with normal mid face, proclined maxillary incisors with spacing and convex profile due to retrognathic mandible. He has well-aligned upper and lower arches with full cusp class II molar and canine relations on both the sides. The lower incisors are upright and he is an average grower. (B) Profile and occlusion after 16 months of TB appliance therapy. The maxillary arch is wide and parabolic in shape, the buccal occlusion in class I relationship. A buccal open bite will be normalised with the eruption of lower buccal segment facilitated by sequential trimming of the upper block.

(C) Following completion of fixed appliance therapy. The facial and skeletal profile is orthognathic. The occlusion has all the features of normal occlusion.

(D) Follow-up after 2 years of completion of therapy. Excellent maintenance of the treatment results is evident.

(E–F) Lateral cephalograms showing growth and treatment changes. (i) Pre-treatment; (ii) post-functional (8 months); (iii) immediately post-debond; (iv) follow-up. Note the improved inter-incisal angle and sagittal jaw relations. The improvement to orthognathic skeletal profile C is maintained D. (F) Lateral cephalometric superimposition (i) Pre- and post-twin block therapy. (ii) Post-treatment superimposition. Note growth of the upper face, nose, middle and lower face. The upper incisors are stable nearly in pre-treatment position. The mandible is positioned forward and so are the lower incisors. (iii) Follow-up indicates further growth of the nose and face. The orthognathic skeletal profile and dental relations are maintained.

G Cephalometric comparative table showing skeletal and dental changes of twin block appliance. The net changes are more evident for sagittal advancement of the mandible (decreased ANB angle, angle of convexity). Retroclination of upper incisor, mild proclination of lower incisor, marked changes in position of the upper and lower lip.

• 1.

  Patients with poor oral hygiene and a higher risk of caries should optimise their dental health before they can be considered for functional appliance therapy.

• 2.

  Diseases of the skeletal system and neuromuscular disturbances contraindicate treatment with a functional appliance.

• 3.

  Children with a poor record of performance in school, stubbornness, and psychological problems and those on drugs for attention deficit hyperactivity disorder (ADHD) are less likely to wear the appliance, which can result in failure or poor treatment outcome.

• 4.

  A half-cusp class II molar relationship that is purely dental in nature, often due to a mesial migration of the upper molar(s) into the extraction space of the second deciduous molar, should be considered for maxillary molar distalisation.

• 5.

  Patients with a very narrow maxilla, long face and vertical growth pattern associated or superimposed with deleterious oral habits such as oral breathing are not considered favourable for treatment with removable functional appliances alone. These patients may require additional orthopaedic manipulation of the growing maxilla with either Kloehn’s headgear or a maxillary splint appliance.

• 6.

  Children with gross skeletal anomalies and extreme vertical growers may be better treated with surgical orthodontics.

In addition, following clinical situations contraindicate functional therapy:

• •

  Vertical growth pattern with a high mandibular angle

• •

  Pathologies associated with temporomandibular joint (TMJ)

• •

  Bimaxillary dental protrusion

• •

  Severe tongue thrust

• •

  Limited growth response, the late grower

• •

  Neuromuscular conditions and diseases of skeleton system such as juvenile arthritis

Multiple assessments are necessary to analyse mastication, respiration, speech, posture and the status of each component involved in accomplishing the functional activity. These include:

• •

  Determination of postural rest position of the mandible and interposed freeway space or inter-occlusal clearance.

• •

  Examination of TMJ function and dysfunction and condylar movement in performing the stomatognathic system’s task.

• •

  Assessment of the functional status of lips, cheek and tongue.

• The postural rest position of the mandible and interposed freeway space or inter-occlusal clearance is determined in natural head position (NHP) ( Fig. 63.6 ). The postural rest position of the mandible can be obtained using one or a combination of the following techniques:

  

  Vertical dimensions of face.

  (A) At rest. (B) Vertical dimension at occlusion. The difference between rest position and occlusion should be the freeway space. Dimension at rest position = Dimension at occlusion + Freeway space

• •

  Phonetic exercise: Child repeats word Mississippi 5–10 times.

• •

  Command method: Wet the lips with tongue and swallow.

• •

  Non-command method: Careful observation of the patient.

• •

  Combination of the above.

The patient is made to feel relaxed, asked to lick the lips, swallow and then hold still the mandible. At this stage, sub-mental muscles are evaluated; a lack of any activity is indicative of a relaxed state. It is pertinent that the orthodontist makes special efforts to create an office ambience that is conducive to stress-free work, which lets the child feel relaxed and attain a rest position of the mandible. The postural rest position is measured with a pair of callipers, kept at two reference points from soft tissue nasion to menton anterior facial height.

The path of the closure of the mandible should be evaluated from the postural rest position to the centric occlusion. Particular attention is given to locating any functional interferences leading to distal positioning, the common cause being a palatally erupting or position of the maxillary lateral incisor. The occlusion is evaluated for overjet and overbite. A deep bite, associated with the curve of Spee and supra-erupted mandibular incisors and reduced lower anterior face height, is better managed with a functional appliance, whereas children with deep bite with a gummy smile and long face are not suitable for FA treatment. Occlusion should be evaluated for any lateral cross-bite due to functional interferences. Should the dental midline coincide with the rest position and become shifted to closure in occlusion, interference during the closure should be suspected, identified and corrected. A distinction should be made on the midline shift of skeletal aetiology, which may contraindicate FA therapy.

Detailed TMJ analysis is an integral part of the clinical assessment in orthodontics, and it is more relevant to functional appliance therapy. TMJ examination should include evaluation of function, dysfunction and condylar movements in performing the functions of the stomatognathic system.

Abnormal swallowing habits and the posture of the tongue are contributor to malocclusion. Moyers, and Woodside illustrated the possible role of the nasal and pharyngeal blockade and compensatory tongue posture in malocclusion. A flat, low-lying, anteriorly postured tongue leads to the development of class III malocclusion. Abnormal tongue posture is considered more significant than merely the function.

A child should be assessed for symptoms of recurrent throat infection, allergies, deviated nasal septum and a history of snoring. Frank nasal obstruction or sleep disordered breathing (SDB) is not uncommon in children. The child may not tolerate the bulky oral appliance. Airway assessment can be done clinically by observations on enlargement of tonsils. Radiographically, the upper airway and lower pharyngeal airway can be assessed on a lateral cephalogram. In habitual mouth breathing with little respiratory resistance, functional therapy improves the lip seal and promotes nasal breathing. Physical obstruction to nasal breathing because of excessive adenoid tissue, allergies, deviated nasal septum, enlarged turbinate or such conditions necessitate ENT consultation and management prior to the institution of a bulky functional appliance in the mouth. Clark’s twin block is not a contraindication in most instances since it does not impede breathing.

Speech evaluation is an integral component of orthodontic clinical evaluation. It should be performed to assess how much overjet has contributed to distorted articulation due to a lack of lip seal. The expected benefits following successful therapy should be explained to the child and parents.

A case that is suitable for functional appliance therapy for malocclusion type and severity, age and skeletal maturation is further evaluated by performing a clinical endeavour of visual treatment outcome (VTO). A diligent evaluation of the nasolabial angle, mental sulcus, lower lip trap under the large overjet, and facial heights is necessary. In particular, the lower anterior face height is evaluated in reference to total face height. Reduced lower anterior face height and lip traps with near normal nasolabial angle are conducive to a positive VTO. The child is asked to bring the lower jaw forward in a state of normal or near normal overjet to evaluate improvement in the facial profile. A case with a normal maxilla and small mandible may show instant improvement in facial profile.

In the position of VTO, transverse, sagittal and vertical relations of dental arches are evaluated. Overjet is measured. Clinical judgement is made on the severity of protrusion/proclination of maxillary anterior teeth and the subsequent position they may occupy after their retraction. The maxillary arch is often narrow, and when the mandible is postured forward in class I molar relationship, maxillary canines often fall in a situation of cross-bite. Such a situation calls for expansion of the maxillary arch either in a separate phase of pre-functional therapy or provision should be made for the same in the appliance for it. With regards to vertical relations on arches, the greater the depth of the bite, the more inter-occlusal space is created in the position of VTO. The quantum of vertical tooth movements of the buccal segment needed to flatten the curve of Spee can be judged in the position of VTO.

Instant improvement in the facial profile, with the chin and maxilla in alignment with the upper face, leading to an orthognathic profile on posturing the mandible forward, is considered +VTO. Upper lip strain and nasolabial angle should be near normal or less than normal but not acute, which will suggest a dental protrusion. In the absence of spacing in the maxillary dentition, an acute nasolabial angle is indicative of poor prognosis. The mild protrusion can be accommodated with maxillary expansion ( Fig. 63.4 A).

A negative VTO will show a poor profile, increased lower face height with a poor proportion to upper face height, significant lip strain and an acute nasolabial angle ( Fig. 63.4 B).

Chronological age is the age of the subject in years from the time of birth. It has been proposed that under normal circumstances, the skeletal age would be within 10% of the chronological age. However, the same does not hold true for people who are early or late maturing or obese individuals. Chronological age is suggestive of the status of maturation and not a true indicator of skeletal age.

Removable FAs are indicated in young patients who are yet to reach peak height velocity (PHV) or pubertal growth spurt. The chapter on growth ( Chapter 11 ) discusses this subject in detail.

The dental age of a subject is determined by observing the stage of dental development and comparing it to known population standards. The following points are used for the assessment of dental age:

• 1.

  Number of erupted and unerupted teeth.

• 2.

  Stage of dentition (deciduous, mixed and permanent).

• 3.

  Stage of crown formation of developing teeth.

• 4.

  Stage of root formation of all erupted teeth.

Despite all the efforts, attempts to relate dental age to skeletal maturation have been found with a weak association and dental age indices are not considered valid indicators of skeletal maturation. Precise information in relation to the timing of the onset of the growth spurt may not be provided by dental maturation indices. They are, however, more accurate than skeletal age for the estimation of chronological age. This is because chronic systemic disturbances like undernutrition and hormonal imbalances have more effect on bone metabolism than dental development. The effort has been directed to ascertain the significance of the second molar in the prediction of skeletal maturation. Dental development has significant variation among different ethnic groups and races and is affected by a host of environmental influences; therefore, it is no longer used as a reliable indicator of skeletal maturity.

It can be assessed with hand-wrist radiographs and cervical vertebrae maturation.

Hand-wrist radiographic methods

Hand-wrist radiographic method is the most commonly used method for the assessment of skeletal age in medical clinics. Many methods have been described, including the Greulich–Pyle method and the Tanner–Whitehouse method. A few investigators, including Fishman, Hagg-Taranger and Singer, have correlated facial growth maturation with skeletal growth using skeletal maturity indicators. The Greulich–Pyle method and the Tanner–Whitehouse method are the most used methods in medical practice.

With respect to facial growth and its association with skeletal growth in general, Fishman correlated and provided 11 stages of skeletal maturation on hand-wrist radiographs. To an orthodontist, the appearance and development of the adductor sesamoid seem most relevant ( Figs 63.7 ). The onset of ossification of the sesamoid takes place at the time of an adolescent growth spurt in stature and height. The duration of later coincides with sesamoid development. (See red circle in the Fig. 63.7.i ).

Hand-wrist radiograph showing the various parameters which are considered for assessment for skeletal age.

Source: Based on Fishman LS. Radiographic evaluation of skeletal maturation. A clinically oriented method based on hand-wrist films. Angle Orthod. 1982 Apr;52(2):88–112. doi: 10.1043/0003-3219(1982) 052<0088:REOSM>2.0.CO;2. PMID: 6980608.

Radiographic identification of skeletal maturity indicators.

(A) The epiphysis is equal in the width of the diaphysis. (B) Appearance of adductor sesamoid of the thumb. (C) Capping of epiphysis. (D) Fusion of epiphysis.

Source: Based on the concept of Fishman LS. Radiographic evaluation of skeletal maturation. A clinically oriented method based on hand-wrist films. Angle Orthod. 1982 Apr;52(2):88–112. doi: 10.1043/0003-3219(1982)052<0088:REOSM>2.0.CO;2. PMID: 6980608.

Cervical vertebral maturation index (CVMI)

Cervical vertebral maturation has been found to be accurate in the assessment of the skeletal age; its accuracy was approaching that of hand-wrist radiographs. In the current scenario, cervical vertebral maturation is being increasingly used for the assessment of skeletal maturation status, replacing the hand-wrist radiographs, especially in orthodontics. The method suggested by Baccetti et al. involves evaluation of the morphology of the three cervical vertebrae (C2, C3 and C4) by visual inspection on a lateral cephalogram. A child in CVMI stage 2 and 3 are considered favourable for treatment ( Fig. 63.8 ).

Radiographs showing stages of cervical vertebra maturation from CS1 to CS6.

• 1.

  Cervical stage 1 (CS1) • Lower borders of all the three vertebrae (C2–C4) are flat. • Bodies of both C3 and C4 are trapezoid in shape. • Peak in mandibular growth will occur, on an average, 2 years after this stage.

• 2.

  Cervical stage 2 (CS2) • Concavity is present at the lower border of C2. • Bodies of both C3 and C4 are still trapezoid in shape. Peak in mandibular growth will occur 1 year after this stage (average).

• 3.

  Cervical stage 3 (CS3) • Concavities at the lower borders of both C2 and C3 are present. Bodies of C3 and C4 are either a trapezoid or rectangular horizontal in shape. • Peak in mandibular growth will occur within 1 year of this stage.

• 4.

  Cervical stage 4 (CS4) • Concavities at the lower borders of C2, C3 and C4 are present. • The bodies of C3 and C4 are rectangular horizontal in shape. The peak in mandibular growth has occurred within 1 or 2 years before this stage.

• 5.

  Cervical stage 5 (CS5) • Concavities at the lower borders of C2, C3 and C4 are still present. • At least one of the bodies of C3 and C4 is square shaped. If not square, the body of the other cervical vertebra still is rectangular horizontal. • The peak in mandibular growth has ended at least 1 year before this stage.

• 6.

  Cervical stage 6 (CS6) • Concavities at the lower borders of C2, C3 and C4 are evident. • At least one of the bodies of C3 and C4 is rectangular vertical in shape. If not rectangular vertical, the body of the other cervical vertebra is square shaped. • The peak in mandibular growth has ended at least 2 years before this stage.

A cephalogram should be visually evaluated before undertaking its comprehensive analysis. It is possible to make a judgement on the severity of sagittal discrepancy, more so the chin in profile, dentoalveolar protrusion of the maxillary teeth, the pattern of facial growth, the position of mandibular incisors and growth rotation of mandible and inclination of the palatal plane on a diligent visual inspection of a lateral cephalogram.

Cephalometric analysis

The very purpose of cephalometric analysis for a case of FA is to obtain information on the nature and severity of skeletal and dental dysplasia contributing to class II skeletal relationships and dental malocclusion. The analysis includes the following:

• 1.

  Cranial base variables

• 2.

  Face height variables

• 3.

  Anteroposterior variables

• 4.

  Mandibular variables

• 5.

  Variables of mid face

• 6.

  Dental variables

• 7.

  Soft-tissue variables

Cephalometric variables of high significance about functional appliances include measurements of cranial base length (S-N or Ba-N) and length of the maxilla and mandible and facial heights. It is important to know mid-facial and mandibular length as suggested by McNamara’s analysis ( Fig. 63.9 and Table 63.3 ). The norm of effective mandibular length measured at Co-Gn is 114.0 mm (SD ± 4.3), and effective mid-facial length Co-A is 85.0 mm (SD ± 4.8).

The cephalometric landmarks and definitions by McNamara.

S indicates sella (the centre of sella turcica); N, nasion (the most anterior limit of suture nasofrontalis); Ba, basion (the posterior inferior point on the occipital bone at the anterior margin of the foramen magnum); ANS, anterior nasion spine (the apex of the anterior nasal spine); A, subspinale (the most posterior point on the concave anterior border of the maxillary alveolar process); Pg, pogonion (the most anterior point on the mandibular symphysis); Gn, anatomical gnathion (the most anteroinferior point of the mandibular symphysis); cGn, constructed gnathion (the intersection of the facial plane and the mandibular plane; facial plane is the line from the nasion to the pogonion); Me, menton (the lowermost point on the shadow of the mandibular symphysis); Go, gonion (the most outward point on the angle formed by the junction of the ramus and body of the mandible on its posterior, inferior aspect); Co, condylion (the most posterior point on the outline of the mandibular condyle); P, porion (the superior aspect of the external auditory meatus); Or, orbit (the lower border of the orbit of the eye); PTM, pterygomaxillary fissure (the most posterosuperior aspect of the pterygomaxillary fissure).

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