twin block

based on important functional appliance that is used mostly nowadays "twin block"

1. THE TWIN BLOCK TECHNIQUE: A FUNCTIONAL ORTHOPEDIC APPLIANCE SYSTEM W. J. CLARK, B.D.S., D.D.O. KIRKCALDY, SCOTLAND

2. THE OCCLUSAL INCLINED PLANE The occlusal inclined plane is the fundamental functional mechanism of the natural dentition . Cuspal inclined planes play an important part in determining the relationship of the teeth as they erupt into occlusion . Occlusal forces transmitted through the dentition provide a constant proprioceptive stimulus to influence the rate of growth and the trabecular structure of the supporting bone .

3. TWIN BLOCKS Twin blocks are bite-blocks that effectively modify the occlusal inclined plane to induce favorably directed occlusal forces by causing a functional mandibular displacement Upper and lower bite-blocks interlock at a 45” angle D esigned for full-time wear to take advantage of all functional forces applied to the dentition including the forces of mastication . give greater freedom of movement in anterior and lateral excursion cause less interference with normal function .

4. Appliance design

5. ORTHOPEDIC TRACTION In cases in which the skeletal discrepancy is severe,the addition of an orthopedic traction system to support the action of occlusal inclined planes provides a versatile appliance technique that is effective in the treatment of a wide range of malocclusions. The indications for treatment include maxillary protrusion,mandibular retrusion , and vertical growth discrepancies A functional orthopedic approach eliminates the uncertainty of treatment response that is sometimes associated with purely functional techniques The technique achieves rapid correction of malocclusion even in cases with severe malocclusions that are unfavourable for conventional fixed or functional appliance therapy .

6. THE CONCORDE FACE-BOW The twin block technique uses a new method of applying inter-maxillary traction. The Concorde facebow combines intermaxillary and extraoral traction by the addition of a recurved labial hook to a conventional face-bow . Intermaxillary traction is applied as a horizontal force from the labial hook to the lower appliance, eliminating the unfavorable upward component of force associated with conventional intermaxillary traction. The traction components are worn only at night to reinforce the action of the occlusal inclined plane . .

7. If the patient fails to posture the mandible to the corrected occlusal position during the night, the intermaxillarytraction force is automatically increased to compensateso that favorable intermaxillary forces are applied continuously Extraoral traction is added when orthopedic force to the maxilla is indicated-for example, in correction of maxillary protrusion A vertical component of extraoral traction applies an intrusive force to the upper posterior teeth and the palate via the upper appliancen to limit downward maxillary growth This facilitates correction of Class II arch relationships in vertical growth discrepancies .

8. The Concorde face-bow effectively combines extraoral and intermaxillary traction in the treatment of severe skeletal discrepancies. A recurved labial hook is added to a conventional face-bow. Outer bow is 1.5 mm; inner bow is 1.13 mm reinforced with tubing; labial hook is 1.13 mm

9. Intraoral views showing twin blocks in open and closed positions . The Concorde face-bow is illustrated with detail of the recurred labial hook and intraoral attachment of the intermaxillary elastic .+

10. Simple twin block appliances are shown in conjunction with fixed appliances . This combination produces rapid correction of arch relationships that can counteract a slow response to treatment in cases with unfavorable growth patterns

11. Twin blocks may be either removable or fixed to the teeth APPLIANCE DESIGN Removable twin blocks The upper appliance is retained by modified arrowhead clasps. The clasps incorporate a coiled tube for attachment of a face-bow if traction is to be applied . Retention may be increased by adding ball-ended clasps in the labial or buccal segments . A midline expansion screw provides compensatory lateral expansion of the upper arch to accommodate a functional protrusion of the lower arch from its retruded position . Labial and lingual bows (as needed) are included to control upper incisor angulation .

12. In the lower arch, retention is often obtained by l-mm interdental ball clasps in the lower incisor region combined with clasps in the buccal segments . The delta clasp was specifically designed bythe author to extend the area of contact of the clasp in the undercut and to improve retention with a closed triangle to increase resistance to fatigue. This combination of clasps gives excellent retention and is very effective in limiting proclination of lower incisors during the twin block stage

13. In mixed dentition treatment, clasps are placed on the lower incisors and on deciduous molars or first permanent molars. The lower appliance may be split anteriorly with the addition of a screw or helical spring to expand and develop the lower arch, if desired. The upper bite-blocks cover the lingual cusps of the upper posterior teeth, extending to the mesial ridge of the upper second premolar . This allows the clasp to be more flexible and improves retention of the appliance

14. Full occlusal cover is necessary in the lower premolar region to compensate for the discrepancy in arch width and to allow the inclined planes to interlock in occlusion . The lower bite-block extends to the distal marginal ridge of the lower second premolar For correction of deep overbite, it is an advantage to leave the lower molars free of the appliance, allowing their eruption to be controlled in relation to the overbite It is very important to prevent molar eruption in cases in which there is reduced overbite or anterior openbite . All erupted posterior teeth must occlude on the bite-block to prevent overeruption .

15. , Diagram showing details of twin blocks combined with fixed appliances. Occlusalinclined planes are outlined in black. Clasps on 655 55 6 7 are shown in black. In this example the upper fixed appliance is confined to 3211123 at this stage until arch relationships are corrected, allowing A easy transition into a full fixed appliance. In the lower arch, a utility arch accommodates clasps in the buccal segments. The fixed appliances are shown in red.

16. BITE REGISTRATION For Class II problems, the proper construction bite is taken and the models are articulated with mandibular protrusion . The amount of mandibular protrusion depends on the ease with which the patient can posture forward . As a general rule, the initial activation should reduce the overjet by 5 to 7 mm leaving 3 to 5 mm interocclusal clearance in the first premolar region. The interocclusal clearance is increased where there is increased overbite and the bite-blocks are designed to allow the free eruption of the lower molars to reduce the overbite by increasing the lower facial height

17. The registration bite should allow for correction of midlines in cases in which they are displaced by functional occlusal interference or guidance into habitual occlusion . Twin blocks may be activated unilaterally to correct postural mandibular displacement with center line displacement and asymmetric buccal segment relationships . The occlusal inclined plane is particularly well suited to the correction of functional abnormalities associated with asymmetric mandibular development . For correction of asymmetry, the lower appliance requires maximum retention in the lower arch to minimize dental movement and to encourage asymmetric compensatory growth .

18. Intraoral views before treatment Intraoral views after treatment

19. FIXED TWIN BLOCKS Twin block appliances may be designed for direct fixation to the teeth by bonding . The appliances resemble the Herbst appliance, substituting occlusal inclined planes for telescopic tubes to provide the functional component to guide the mandible into a protrusive po sition . Preformed wedge attachments are being designed at present for direct fixation to molar bands to allow simpler application in fixed appliance technique

20. CLINICAL MANAGEMENT This report refers to the treatment of an uncrowdedClass II, Division 1 malocclusion primarily, using removabletwin blocks . The patient had a combination of maxillary protrusion and mandibular retrusion . The technique is described in two stages, an ,active phase with twin blocks and traction attachments, and a support phase with a guide plane after correction of arch relationships .

21. STAGE 1 -ACTIVE PHASE Twin blocks are combined with intermaxillary and extraoral traction for rapid correction of arch relationships The initial activation is checked when twin blocks are fitted to confirm that the patient can comfortably maintain the altered postural position . Twin blocks are removed for eating for the first 3 days until the initial discomfort from appliance wear has been resolved

22. Thereafter, the appliances are worn avoid soft-tissue irritation. The upper midline screw is continuously . It is important for the patient to understand that wearing twin blocks for eating increases the orthopedic forces and improves the response to treatment ; this makes it a true functional appliance. It may be necessary to trim or relieve the flange the lower appliance, lingual to the lower incisors, to avoid soft tissue irritation The upper midline screw is continuously turned a one-quarter turn every week to 10 days until the the arch width is adequate to accommodate the lower arch in its corrected position. It is important to check the expansion of the upper arch at each visit to avoid excessive expansion

23. The Concorde face-bow is adjusted so that it lies just below the level of the upper lip at rest, with the ends of the outer bow sloping slightly upward above the level of the inner bow. The resulting extraoral traction applies an upward component of force that helps to retain the upper appliance The intermaxillary elastic is attached under the ball clasps in the lower labial segment and passes to the labial hook on the face-bow (Fig. 1, C). If lower incisors are already proclined , less elastic traction will be tolerated. Avoid prolonged elastic traction in slow vertical-growing patterns. The retention of the appli ance must be checked after the traction assembly is fitted Extraoral traction is applied by a straight pull toa conventional headcap worn every night for 8 to 10 hours using 200 g distal extraoral force on each side and approximately 150 g intermaxillary force. At each monthly visit, the occlusion is checked for correction of arch relationships

24. Clinical response in active phase Within a few days of fitting the appliances, theposition of muscle balance is altered so that it becomes painful for the patient to retract the mandible. This has been described as the “ pterygoid response ” (McNamara*) or the formation of a “tension zone” distal to the condyle (Harvold5). It is rare for such a responseto be observed with functional appliances that are not worn full-time.

25. The rapid clinical response confirms the summary of adaptive responses in functional protrusion experiments with fixed inclined planes by McNamara . The placement of appliances results in an immediate change in the neuromuscular proprioceptive response. Provided all phasic and tonic muscle activity is affected, the resulting muscular changes are very rapid, and can be measured in terms of minutes, hours and days. Structural alterations are more gradual and are measured in months, whereby the dento skeletal structures adapt to restore a functional equilibrium to support the altered position of muscle balance.’

26. The patient’s rate of growth should be taken into account in timing the reactivation of the bite-blocks by the addition of cold cure acrylic to the mesial inclined plane of the upper blocks . However, an overjet of up to 10 mm can be corrected without reactivating the biteblocks if the rate and direction of mandibular growth are favorable . If the patient’s rate of growth is slow or the direction of growth is vertical rather than horizontal, it is advisable to advance the mandible more gradually over a longer period of time to allow compensatory mandibular growth to occur . Full correction of sagittal arch relationships can be achieved in as little as 2 to 6 months, giving a normal incisor relationship with the buccal segments out of occlusion due to the presence of the bite-blocks . It is a consistent feature in functional techniques that sagittal correction of arch relationships is achieved before compensatory vertical development in the buccal segments is complete

27. Management of overbite Deep overbite is reduced by overcorrecting the incisors to an edge-to-edge relationship before reducing the height of the bite-blocks. Vertical development of the lower molars is encouraged from the beginning of the active phase of treatment by progressively trimming the upper bite-block occlusodistally to allow the lower molars to erupt. At the end of the active phase, the incisors and the molars should be in correct occlusion. At this stage an open bite is still present in the premolar region because of the presence of the biteblocks . The lower block is trimmed over a period of 2 or 3 months to reduce the open bite in the premolar region . It is important to maintain adequate interlocking wedges to maintain anteroposterior correction of arch relationships. This method of reducing overbite by controlled eruption of posterior teeth supported by occlusal biteblocks results in favorable changes in facial balance by increasing lower facial height .

28. Conversely, if the overbite is reduced before treatment, it is important to prevent overeruption of posterior teeth , which would further reduce the overbite . All erupted teeth must then be in occlusal contact with the bite-blocks . If second molars erupt during the active phase , occlusal cover or occlusal rests must be extended to prevent overeruption of these teeth . When the overbite is reduced, clasps are placed on the posterior teeth and the appliances are left clear of the anterior teeth to encourage eruption of the incisors . In addition, a vertical-pull headgear may be used to apply an intrusive force to the upper molars to reduce the vertical component of growtha

29. STAGE 2-SUPPORT PHASE The aim of the second stage of treatment is to retain the corrected incisor relationship until the buccal segment occlusion is fully established, using an upper Hawley-type removable appliance with an inclined guide plane to retain the sagittal relationship .

30. CASE SELECTION The technique has a wide application in those cases in which anteroposterior correction of arch relationship . For the neophyte, the technique should be used to treat intially class 2 div 1 with an uncrowded lower arch. In cases with crowding arch relationships arecorrectd first with twin block before crowdin is relieved . In the treatment of Class II, Division 2 , twin blocks are designed to procline upper incisors and align the labial segments while correcting the sagittal malrelationship . The inclined planes are shaped to encourage molar eruption and the labial segments are aligned while correcting the distal mandibular occlusion

31. An integrated approach with fixed appliances allows alignment , intrusion, and torque to be carried out for the maxillary labial segments during the active phase of treatment when arch relationships and skeletal discrepancies are treated simultaneously . The presence of bite-blocks prevents traumatic occlusion on the fixed attachments and avoids breakage as a result of excessive overbite . The necessity to combine a functional orthopedic phase of treatment with a subsequent orthodontic phase is recognized in cases in which additional dental correction is required .

32. Reverse twin blocks may be used to correct sagittal arch relationships in Class III malocclusion. The upper twin block is designed to procline upper incisors and Class III intermaxillary or extraoral traction may also be applied . The technique is effective in the mixed or permanent dentition and may also be successful in adult treatment except that only dentoalveolar correction occurs as opposed to skeletal adaptation in the growing child. The response to treatment is always related to the patient’s growth pattern

33. DISCUSSION In many respects the occlusal inclined plane is a significant improvement on existing appliance mechanisms in the functional guidance of facial growth and development. Significant changes in facial appearances are seen within 2 or 3 months of starting treatment with twin blocks as a result of altered muscle balance and continuous wear, even during eating. Rapid soft-tissue adaptation occurs in response to an improved occlusal relationship .

34. Soft-tissue compensation occurs to assist the primary functions of mastication and swallowing, which require an effective anterior oral seal . The twin block appliance positions the mandible downward and forward, increasing the intermaxillary space. As a result it is difficult to form an anterior oral seal by contact between the tongue and the lower lip, and patients spontaneously adopt a natural lip seal for deglutition without exercises when twin blocks are fitted .

35. The lip seal is maintained throughout treatment and improved facial balance is evident within a few months of starting treatment . Twin blocks have been described by patients as the most comfortable of all the functional appliances . Although the appliances are removable, they produce rapid improvements in facial appearance that encourage good patient motivation .

36. CEPHALOMETRIC ANALYSIS Changes during the active phase of treatment. Reduction in the anteroposterior apical base discrepancy on angular assessment of ANB angle Increase in effective mandibular length ( articulare to gnathion ) Increase in length of the facial axis (cc to gnathion ) Increase in facial height ( nasion to menton ). The majority of patients in the control sample had deep overbite and the aims of treatment were consistent with increasing facial height . Reduction in facial convexity (A point to facial plane) Reduction in the distance from the distal outline of the upper first molar to the pterygoid vertical

37. LOWER INCISOR ANGULATION In the study of 70 consecutively treated cases, the lower incisor was shown to procline in the active phase of treatment and to upright in the support phase . After treatment the angulation of the lower incisor to the mandibular plane had decreased slightly by a mean value of less than 1”. This had no statistical significance, but is important in evaluating the stability of the lower anterior segment after treatment .

38. REFERENCES 1. Charlier JP, Petrovic A, Herman- Stutzman J. Effects of mandibular hyperpropulsion on the prechondroblastic zone of young rat condyle . AM J ORTHOD 1969;55:71-4. 2. McNamara JA Jr. Functional determinants of crania-facial size and shape. Eur J Orthod 1980;2:131-59. 3. Howe RP. The bonded Herbst appliance. J Clin Orthod 1982;10:663-7. 4. Howe RP, McNamara JA Jr. Clinical management of the bonded Herbst appliance. J Clin Orthod 1983;7:456-63. 5. Harvold EP. Bone remodelling and orthodontics. Eur J Orthod 1985;7:217-30. 6. Harvold EP. Primate experiments on oral sensation and morphogenesis. Transactions of 3rd International Congress of the European Orthodontic Society. 1973;43 1:4. 7. Clark WJ. The twin block traction technique. Eur J Orthod 1982;4: 129-38. 8. McNamara JA, Bookstein FL, Shaughnessy TG. Skeletal and dental changes following functional regulator therapy on Class II patients. AM J ORT~IOD 1985;88:91-110. 9. Riolo ML, Moyers RE, McNamara JA Jr , Stuart Hunter W. An atlas of craniofacial growth. Cephalometric standards from the University School Growth Study, the University of Michigan. Ann Arbor : 1974. Center for Human Growth and Development, University of Michigan. 10. Prahl Andersen B, Kowalski CJ, Heydendael PHJM. Mixed longitudinal interdisciplinary study of growth and development. University of Nijmegen. San Francisco: Academic Press, 1979. Reprint