Twin block /certified fixed orthodontic courses by Indian dental academy

INDIAN DENTAL ACADEMY
Leader in continuing Dental
Education
www.indiandentalacade
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CONTENTS
• Introduction to Twin block
• Occlusal inclined plane
• Development of twin block
• Diagnosis and treatment
planning
• Construction bite of
Functional appliance
• Bite registration in Twin Block
• Method of Bite Registration
• Evolution of Appliance Design
• Standard Twin Block
• Indications &
Contraindications
• Advantages
• Stages of Treatment
• Treatment of class II division 1
malocclusion deep overbite
• Studies On Twin Block
• Conclusion
• Bibliography
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INTRODUCTION TO TWIN BLOCKS
• The goal in developing the Twin Block approach to treatment was to produce a
technique that could maximise the growth response to functional mandibular
protrusion by using an appliance system that is simple, comfortable and aesthetically
acceptable to the patient.
• Twin blocks are constructed to a protrusive bite that effectively modifies the occlusal
inclined plane by means of acrylic inclined planes on occlusal bite blocks.
• The purpose is to promote protrusive mandibular function for correction of the
skeletal Class II malocclusion.
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• It is designed for full time wear
• they achieve rapid functional correction of malocclusion by modifying the occlusal
inclined plane, guiding the mandible forward into correct occlusion
• it uses forces of occlusion & mastication to correct the malocclusion.
• Upper & lower bite blocks interlock at a 700 angle when engaged in full closure.
• Bite blocks are similar in feel to wearing dentures & patients can eat comfortably with
the appliance in place.
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• Early stages of their evolution, TB were conceived as simple removable appliances
with Interlocking occlusal bite blocks designed to posture the mandible forward to
achieve functional correction of a class II division I malocclusion.
• In the treatment of class II division 2 malocclusion, appliance design is modified by
the addition of sagittal screws to advance the upper anterior teeth.
• Twin blocks satisfy both the patient & the operator as one of the most “patient
friendly” of all the functional appliances.
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OCCLUSAL INCLINED PLANE
• It 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.
• If mandible occludes in distal relationship to maxilla, the occlusal forces acting on
mandibular teeth during function will have a distal component of force i.e.
unfavourable to normal forward development of mandible.
• It act as a guiding mechanism displacing the mandible downward & forward.
• With the appliance in the mouth patient cannot occlude in the former relation. So it
aims at intervening treatment at earlier stage of development
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DEVELOPMENT OF TWIN BLOCK
• It is true that “necessity is the mother of invention”.
• Twin block appliance was developed in 1977 as a two piece appliance resembling a
Schwarz- double plate & split activator.
• Twin Blocks was evolved in response to a clinical problem when a dental colleague
son , fell down, & the upper right central incisor was avulsed. Within few hours of
the trauma the tooth was reimplanted and a temporary splint was constructed to
hold the tooth in position.
• The occlusal relationship was class II division 1 with an overjet of 9 mm and the
lower lip was trapped lingual to the upper incisors.
• To prevent the lip from trapping in the overjet it was necessary to design an appliance
that could be worn full time to posture the mandible forward.
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• After 6 mons. with stabilising splint, the tooth had partiaaly reattached but there was
evidence of severe root resorption.
• At that time, simple bite blocks were designed.
• The appliance mechanism was designed to harness the forces of occlusion to correct
the distal occlusion and also to reduce the over jet without applying direct pressure to
the upper incisors.
• The upper and lower bite blocks engaged mesial to the first permanent molars at 900
to the occlusal plane when the mandible postured forward.
• This positioned the incisors edge-to-edge with 2 mm vertical separation to hold the
incisors out of occlusion.
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• The patient had to make positive effort to posture his mandible forward to occlude
the bile blocks in a protrusive bite.
• The first Twin Block appliances were fitted on 7 September 1977, when the patient
was aged 8 years 4 months.
• T he bite blocks proved comfortable to wear and treatment progressed well as the
distal occlusion corrected and the overjet reduced from 9 mm to 4mm in 9 months.
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Angulation of the Inclined Plane:
• During evolution of the technique, the angulation of the inclined plane varied from
90 to 450 to the occlusal plane, before arriving at an angle of 70°.
• 900 angle: patient had to make a conscious effort to occlude in a forward position.
Difficult to maintain a forward posture and, therefore, would revert to retruding the
mandible back to its old distal occlusion position
• Occluding the bite blocks together on top of each other on their flat occlusal
surfaces- posterior open bite
• This was experienced in approximately 30% of the earliest Twin Block cases.
• It was resolved by altering the angulation of the bite blocks to 45° to the occlusal
plane in order to guide the mandible forwards
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• An angle of 450 to the occlusal plane :applies an equal downward and forward
component of force to the lower dentition.
• Encourages a corresponding downward & forward stimulus to growth.
• After using a 450 angle on the blocks for 8 yrs., the angulation was finally changed to
the steeper angle of 700 to the occlusal plane to apply a more horizontal component
of force.
• It was reasoned that this may encourage more forward mandibular growth.
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DIAGNOSIS & TREATMENT PLANNING
• Clinical examination is a fundamental guideline for a proper case selection. Clinical
diagnosis has the advantage of providing an accurate prediction of 3- dimensional
change in facial contour.
• If the facial profile improves when the mandible is advanced with the lips tightly
closed, then functional mandibular advancement is the treatment of choice.
• The change in facial appearance is a preview of the anticipated result of functional
treatment
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• Photographs:
• Profile and frontal photographs with the mandible in retrusive and advanced
position, are used to assess the changes that can occur during treatment.
• Study models:
• Occlusal changes can be checked by sliding the lower model forward and observing
the articulation of the mandibular dental arch with that of the upper model.
• Radiographs:
• OPG is vital to study the dentition and condition of alveolar bone and periodontium.
• TMJ X-rays may also be required to assess the joint condition before treatment.
• Hand wrist film may be taken to assess the developmental status of the patient.
• Lateral cephalograms to support and confirm the clinical diagnosis.
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• ARCH LENGTH DISCREPANCY: defines the amount of crowding present in the
dental arch by compairing the space available with the space required to
accommodate all the teeth in the arch in correct alignment.
• Richter scale : helpful in treatment planning to classify the degree of difficulty of
the malocclusion as mild, moderate or severe. In arch length discrepancy:
Mild crowding 1-3mm
Moderate Crowding 4-5mm
Severe crowding 6mm or more
• This is a sliding scale expressing degree of difficulty for dental correction by non
extraction therapy.
• The higher the value, the more difficult it is to resolve crowding permanently without
extractions.
• The Richter scale can also be applied when the measure of convexity is used to
determine the skeletal discrepancy
 A skeletal convexity of 1-3mm is within the range of normal
 4-5mm convexity is moderate class II skeletal discrepancy
 6mm or more in severe class II
 The higher the convexity the more likely that functional orthopaedics is
indicated to improve the skeletal relationship
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CONSTRUCTION BITE OF FUNCTIONAL
APPLIANCE
• Determines the degree of activation built into the appliance, aiming to reposition the
mandible to improve jaw relationship.
• The degree of activation should stretch the muscles of mastication sufficiently to
provide a positive proprioceptive response.
• At the same time, activation must be within the physiologic range of activity of the
muscles of mastication and the ligamentous attachments of the temporomandibular
joint.
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• According to Woodside (1977) in construction of the activator as described by
Andresen (1910):
• “A bite registration used commonly throughout the world registers the mandible in a
position protruded approximately 3.0mm distal to the most posterior position that
the patient can achieve, while vertically the bite is registered within the limits of the
patient’s freeway space”
• In North America, a similar protrusive bite registration is made, except that the
vertical activation is 4mm beyond rest position.
• Roccabado quantifies normal physiological TMJ movement as 70% of total joint
displacement.
• Hence, the maximal forward positioning of the mandible should not exceed 70% of
the total protrusive path of the patient.
• Beyond this position, the medial capsular ligament begins to displace the disc by
pulling the disc medially & distally off the condyle.
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BITE REGISTRATION IN TWIN BLOCK
TECHNIQUE
• overjet of up to 10 mm : single activation to an edge-to-edge incisor relationship with
2mm interincisal clearance
• If the overjet > 10mm, initial advancement of 7 -8mm is done followed by
reactivation later.
• Some patients had difficulty in maintaining the forward posture and occluding
correctly on the inclined planes.
• These patients usually had a vertical growth pattern with weak musculature and were
unable to maintain the forward mandibular posture consistently.
• To overcome this problem the activation of the appliance was reduced slightly by
trimming the inclined planes until the patient occluded comfortably and consistently
in the forward position.
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• There are two types of bite gauges used to register bite for twin block:
• George bite gauge
• Exactobite gauge/ Project bite gauge (name differs in the USA & UK)
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• GEORGE BITE GAUGE: Has a
sliding jig attached to a
millimeter scale designed to
measure the protrusion path of
the mandible
• To determine accurately the
amount of activation
registered in the construction
bite.
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• Total protrusive movement is
calculated by first measuring the
overjet in centric occlusion & then
in the position of maximum
protrusion.
• The protrusive path of the
mandible is the difference
between the two measurements.
• Functional activation within
normal physiological limits should
not exceed 70% of the protrusive
path.
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• EXACTOBITE OR PROJECT
BITE GAUGE:
• Incisal portion has three incisal
grooves to be positioned on the
incisal edge of the upper incisor.
• A single groove on the opposing
side that engages the incisal edge
of the lower incisor. The
appropriate groove is selected
• Designed to record a protrusion
bite
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• Registers 2 mm vertical
clearance between the incisal
edges of the upper and the
lower incisors.
• 5 or 6 mm of clearance in the
first premolar region and 3
mm of clearance distally in the
molar region
• Ensures that space is available
for vertical development of
posterior teeth to reduce the
overbite.
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• Vertical Activation: determined by 2 factors.
• Firstly, adequate vertical clearance must be available between upper
and lower teeth to accommodate blocks of sufficient thickness to
activate the appliance.
• Secondly, the vertical activation must open the bite beyond the
freeway space to ensure that the patient cannot posture out of the
appliance when the mandible is in rest position.
• Class II division 1 deep bite : blocks are not less than 5mm thick in
the first premolar or first deciduous molar region with 2mm of
interincisal clearance.
• In CIass II division 2 malocclusion: edge to edge bite without 2mm
interincisal clearance
• Anterior openbite: bite is registered with greater interincisal
clearance.
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• At bite registration a judgement should be made according to the amount of
vertical space between the cusp tips of first premolar or deciduous molars to
achieve the correct degree of bite opening to accommodate blocks of at least
5mm thickness.
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• Single or Progressive activation: Petrovic et al (1981) found that stepwise activation is
the best procedure to promote orthopaedic lengthening of the mandible on the basis
of this Falke & Frankel (1989) reduced initial activation for mandibular advancement
to 3mm.
• Concept of progressive activation for functional correction to achieve the optimum
growth response : investigated ( De Vincenzo & Winn 1989; Falke & Frankel, 1989)
with differing result & require further investigation
• Later on occlusal bite blocks was used to investigate the relative effects of
progressive activation compared to a single large activation
• Concluded that there is no difference in either orthodontic or orthopaedic variables
between progressive 3 mm advancement and a single advancement averaging 5-
6mm.
• Continous advancement by progressive 1mm activations shows a diminished but still
significant response
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• .Progressive activation is found to be time consuming with no measurable
improvement in the response.
• large activation is more efficient than smaller progressive activations.
• Carmichael, Banks & Chadwick : described a screw advancement
mechanism for progressive activation of twin blocks.
• Stepwise advancement may be beneficial in correction of large overjets, or
in the treatment of vertical growth patterns, where smaller adjustments
may improve patient tolerance
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METHOD OF BITE REGISTRATION:
• The centric position is checked and the desired degree of activation decided.
• The patient is then trained to bite in the desired position by giving him a
mirror.
• The wax is softened in a water bath and adapted.
• The patient is instructed to bite into the desired position.
• After the wax has hardened sufficiently, it is removed and chilled.
• The models with the bite are articulated and the twin block is constructed.
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CONTROL OF THE VERTICAL
DIMENSION
• The mechanism of control of the vertical dimension differs in fixed and
functional therapy.
• fixed mechanics: the teeth remain in occlusion during the course of
treatment, and the effect is limited to intrusion or extrusion of individual
teeth to increase or decrease overbite and level the occlusal plane.
• Functional appliances are designed to influence development in the
anteroposterior and vertical dimensions simultaneously, control of the
vertical dimension is achieved by covering the teeth in the opposing arches
& controlling the intermaxillary space.
• The management of the appliance differs according to whether the bite is to
be opened or closed during treatment.
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• Opening the bite:
• It is necessary first to check that the profile is improved when the patient
postures the mandible downwards and forwards,
• This confirms that the bite should be opened by encouraging the eruption
of the posterior teeth to increase the vertical dimension of occlusion.
• Achieved by placing an occlusal table between the teeth to encourage
increased development of posterior facial height by growth of the vertical
ramus.
• At the same time the occlusion is freed between the posterior teeth to
encourage selective eruption of posterior tooth to increase the vertical
dimension of occlusion in the posterior quadrants.
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• Closing the bite:
• Reduced overbite or anterior open bite is often related to a vertical facial
growth pattern.
• The lower facial height is already increased and the vertical dimension must
not be encouraged to increase during treatment.
• An acrylic occlusal table is designed into the appliance to maintain contact
on the posterior teeth throughout treatment.
• This results in a relative intrusion of the posterior teeth while the anterior
teeth are free to erupt, thereby reducing the anterior open bite
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• In treatment of reduced overbite it is very important that the opposing
acrylic occlusal bite block surfaces are not trimmed.
• All posterior teeth must remain in contact with the blocks through out
treatment to prevent eruption of posterior teeth
• By separating the posterior teeth it is possible to adjust the dimensions of
the intermaxillary space anteroposteriorly & vertically to correct skeletal
discrepancies.
• The mechanics can be reversed, applying the same principles for correction
of class III malocclusion.
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Establishing vertical dimension: The
Intergingival Height
• Intergingival height-used to establish correct vertical dimension.
• Measured from the gingival margin of upper incisor to the gingival margin of lower
incisor when the teeth are in occlusion
• Help as a restorative approach to rebuild the occlusion in treatment of patients with
TMJ dysfunction
• Comfort zone-17-19 mm for adult patients & 15-17mm for young patients-equivalent
to the combined heights of the upper & lower incisors minus overbite
• Measured by using a millimeter ruler or dividers with a vernier scale
• This is used as a guide to establish the correct vertical dimension during treatment
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• EVOLUTION OF APPLIANCE.
It is important to design
appliances that are “patient
friendly” to remove any obstacles
to compliance & to motivate the
patient to cooperate in treatment.
• The earliest twin blocks were
designed
1. Occlusal bite blocks
2. Midline screws to expand the
upper arch.
3. Clasps on upper molar and
premolar.
4. Clasps on lower premolars
5. Inter dental clasps on lower
incisors.
6. Springs to move individual
teeth and improve the arch form
as required
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STANDARD TWIN BLOCK
• Labial bow:
• In its earlier stages all twin blocks
incorporated a labial bow to
retract the upper anteriors.
• Labial bow engaged the upper
incisor, it tended to overcorrect
incisor angulations-- retracting
upper incisors prematurely and
limiting the scope of functional
correction with mandibular
advancement.
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• This led to the conclusion that a labial bow is not always required unless it
is necessary to upright severely proclined incisors and even then it must not
be activated until full functional correction is complete and a class I buccal
segment relationship is achieved.
• In twin block treatment, a good lip seal is achieved naturally without
additional lip exercises. The lips act like a labial bow and lip pressure is
effective in uprighting upper incisors making a labial bow superfluous.
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• Clasps:
• Though the early design of twin blocks incorporated Adam's clasps
(modified arrowhead clasps, 1970 ), Clark introduced the Delta Claps in
1985 to enhance appliance fixation.
• It is similar in principle to the modified arrowhead clasp but includes new
features to improve retention, minimize adjustment and reduce metal
fatigue, thereby reducing breakage.
• Adam's clasp : designed to fit individual teeth and incorporates interdental
tags and mesial and distal retentive loops that are directed lingually into
undercuts and joined by a buccal bridge.
• The slope and position of the crown heads allows the clasp to open slightly
with repeated insertion and removal, thus it requires routine adjustment at
every visit to maintain retention. This increases the risk of metal fatigue and
breakage.
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Delta clasp (constructed-:0.70-0.75 SS wire)
• The Delta clasp retains the basic
shape of the Adams clasps with its
interdental tags, retentive loops,
and buccal bridge.
• However, the difference is in the
retentive loops which are shaped
as a closed triangle (from which
the name delta clasp is derived)
instead of the open V shaped loop
of the Adams clasp.
• Modifications has produced
circular loops which are easier to
construct
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• Permanent dentition: placed on upper
first molars & on lower first
premolars, may also be used on
deciduous molars.
• Clark has evaluated that the breakage
rate of Delta clasp (1 %) was
significantly less than that of Adam's
Clasp (10%)
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• Ball ended clasps: Ball shaped
interdental clasps may be
placed for increased retention.
• Routinely employed mesial to
lower canines & in upper
premolar or deciduous molar
region to gain interdental
retention from adjacent teeth.
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• The Delta clasp can be adjusted in 2 ways
• ->By placing pliers on the wire as it emerges from the acrylic. A slight
adjustment extends the retentive loop of the clasp into the gingival or
interdental undercut.
• ->By grasping the arrowhead from the buccal aspect and twisting the
retentive loop inwards towards the tooth to adjust into the mesial and distal
undercut.
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• Base Plate
• The base plate and occlusal bite blocks: made from heat cure or cold cure
acrylic.
• Advantage of heat cure acrylic is additional strength and precision (as
blocks are first made in wax)
• Cold cure acrylic: advantage of speed and convenience but strength is less.
• Preformed bite blocks made of good quality heat cure acrylic are being
manufactured for incorporation into cold cure appliances to combine
convenience with strength and accuracy.
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• POSITION OF THE INCLINED PLANE:
• Determined by the lower block.
• It is important that the inclined plane is clear of mesial surface contact with the lower
molar, which must be free to erupt unobstructed in order to reduce the overbite.
• The inclined plane on the lower bite block is angled from the mesial surface of the
second premolar or deciduous molar at 700 to the occlusal plane.
• Lower block should extend distally to the buccal cusp of the lower second premolar
or deciduous molar, stopping short of the distal marginal ridge , this allows the leading
edge of the inclined plane on the upper appliance to be positioned mesial to the lower first
molar so as not to obstruct eruption.
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• Buccolingually: lower block covers the
occlusal surfaces of the lower
premolars or deciduous molars to
occlude with the inclined plane on the
upper twin block.
• Flat occlusal bite block passes
forwards over the first premolar to
become thinner buccolingually in the
lower canine region.
• The upper inclined plane is angled
from the distal surface of the upper
second premolar to the mesial surface
of the lower first molar.
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• The flat occlusal portion then
passes distally over the
remaining upper posterior
teeth , reducing in thickness as
it extends distally.
• Only the lingual cusps of the
upper posterior teeth should
be cover rather than full
occlusal surface as it makes
the clasps more flexible &
allows adjustment of the
clasps.
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INDICATIONS & CONTRAINDICATIONS
• Indications :
• Indicated for treatment of uncrowded permanent dentition with Class II
division 1 malocclusion.
• It is designed to correct Class II skeletal relationship, to correct molar
relationship & to correct overjet.
• Patient should be in growing age for favourable skeletal change
achievement.
o Treatment of Class II division 1 in mixed dentition period
o Treatment of Class II division 1 with anterior open bite
o Treatment of Class II division 1 with deep overbite
o Treatment of Class II division 2 malocclusion
o Treatment of Class III malocclusion
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• Contraindication:
• Cases with vertical growth pattern
• Crowding that may require extraction
• When VTO is not positive
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By:
STUTI MOHAN
(contd . . .. . . . )
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ADVANTAGES
• Comfort
• Aesthetics
• Function
• Patient compliance
• Facial appearance
• Speech
• Clinical management
• Arch development
• Vertical control
• Facial asymmetry
• Safety
• Efficiency
• Treatment of temporomandibular joint dysfunction
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TWIN BLOCK TECHNIQUE-STAGES OF TREATMENT
• Twin block Functional therapy is divided into three stages:
1.Active Phase
2.Support Phase
3.Retention Phase
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• ACTIVE PHASE:
• correction of anteroposterior relationship & establishment of the correct
vertical dimension.
• Achieve rapid functional correction of mandibular position from a skeletally
retruded class II to class I occlusion using occlusal inclined planes over the
posterior teeth to guide the mandible into correct relationship with the
maxilla.
• In all functional therapy sagittal correction is achieved before vertical
development of the posterior teeth is complete.
• The vertical dimension is controlled by adjustment of the occlusal bite
blocks.
• At the end of active phase the aim is to achieve correction to Class I
occlusion & control of vertical dimension by a three- point occlusal contact
with the incisors & molars in occlusion.
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• Appliance fitting: it is first necessary to check that the patient bites
comfortably in a protrusive bite with the inclined planes occluding
correctly. To avoid irritation, it is important to relieve the lower appliance
slightly over the gingivae lingual to the lower incisors.
• Initial adjustment after 10 days:
• The patient should now be wearing the appliances comfortably & eating
with them in position. The initial discomfort of a new appliance should be
resolved.
• The patient should now be turning the upper midline screw one quarter
turn per week
• Deep overbite: the upper bite block should be trimmed clear of the lower
molars leaving a clearance of 1-2mm to allow these to erupt.
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• If patient is failing to posture forwards consistently to occlude correctly on the
inclined planes then this shows that appliance is activated beyond the patient’s
tolerance level so the angulation of the inclined plane reduced to 450
• Adjustment visit – after 4 weeks:
• The first monthly visit positive progress should already be evident with respect to
better facial balance.
• Progress can also be confirmed by noting the amount of reduction in overjet, as
measured intraorally with the mandible fully retracted , this also helps in monitoring
the progress.
• Check that the screw is operating correctly, & adjust the clasp if necessary to improve
retention , if the appliance include labial bow , adjust it so as to out of contact with
the upper incisors.
• In the treatment of deep overbite ensure that the lower molars are not in contact
with the upper block. The upper block is trimmed occlusodistally to clear the
occlusion.
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• Routine adjustment- time interval 6 weeks
• A similar pattern of adjustment continues with steady correction of distal
occlusion & reduction of overjet.
• The upper arch width is checked at each visit, until the sufficient expansion
to accommodate the lower arch in its corrected position .
• Trimming of the upper block continues until all the occlusal cover is
removed from the upper molars to allow the lower molars to erupt
completely into occlusion
• The overjet, overbite & distal occlusion should be fully corrected by the end
of the twin block phase.
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• SUPPORT PHASE:
• maintain the corrected incisor
relationship until the buccal segment
occlusion is fully interdigitated for this
an upper removable appliance with an
anterior inclined plane
• Vertical control is essential during the
support phase after reduction of
overbite.
• For this :a flat occlusal stop of acrylic
extends forwards from the inclined
plane to engage the lower incisors.
• This maintains the intergingival
height as the posterior teeth erupt into
occlusion.
• The upper & lower buccal teeth should
normally settle into occlusion within
2-6 months, depending on the depth
of the overbite
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• RETENTION PHASE:
• Treatment is followed by retention with the upper anterior inclined plane
appliance.
• Appliance wear is reduced to night time only when the occlusion is fully
established.
• A good buccal segment occlusion is important for stability after correction
of arch –to- arch relationship.
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TIME TABLE OF TREATMENT-AVERAGE
TREATMENT TIME
• Active phase: average time 6-9 months to achieve full reduction of overjet
to a normal incisor relationship & to correct the distal occlusion.
• Support phase: 3-6 months for molars to erupt into occlusion and for
premolars to erupt after trimming the blocks.
• Retention phase: 9 months, reducing appliance wear when the position is
stabilised.
• An average estimate of treatment time is 18 months, including retention.
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MANAGEMENT OF DEEP OVERBITE:
• The upper bite block is trimmed
occluso distally to allow the lower
molar to erupt and reduce the deep
bite with increase in lower facial
height.
• occlusion is cleared over the lower
molars progressively at each visit by
1 to 2 mm only, to facilitate
eruption.
• At each subsequent visit for
appliance adjustment the occlusion
is cleared by sequentially trimming
the upper block occluso distally to
allow further eruption of lower
molars.
• The lower molars will erupt into
occlusion normally within 6-9
months
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• End of the active phase: incisors and molars are in correct occlusion and
deep bite corrected.
• However the presence of bite blocks leads to openbite in the premolar
region. The lower block is then trimmed slightly to allow the premolars to
erupt with the appliance.
• Active eruption of lower molars may be encouraged by applying vertical
elastics from the upper appliance to hooks on the lower molars. This is
especially useful in older patients in whom eruption by natural forces tends
to be slower.
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• FUNCTIONAL REGULATION OF CONDYLAR CARTILAGE
GROWTH RATE
• A fundamental study of the relationship between form & function WAS
carried out in animal experiments at the University of Michigan, and the
results were summarised by McNamara (1980).
• The studies evaluated changes in muscle function and related changes in
bone growth in the rhesus monkey by a comparison of experimental and
control animals as monitored by EMG, cephalometric & histological studies
• Concluded: the findings were based on the use of fixed occlusal inclined
planes that were designed to cause a forward postural displacement of the
mandible in all active and passive muscle activity.
• The pattern of muscle behaviour during the experimental period showed a
cyclical change in response to functional mandibular propulsion
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• Initial placement of the appliance produced an increase in the
overall activity of the muscles of mastication as the animal sought
to find a new occlusal position.
• A distinct change in muscle activity occurred within 1 - 7 days,
characterised by a decrease in the activity of the posetior head of
temporalis, an increase in activity OF MASSETER MUSCLE and
increase in function of the superior head of the lateral pterygoid
muscle
• After 3 weeks , muscle activity was reached at a higher level of
activity than the pre treatment record
• This level of activity persisted for 4 weeks before a further decline in
muscle activity over a period of 4 weeks to the level recorded before
treatment.
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• The cycle of changes was completed in a 3 month period
• These changes are consistent with an equilibrium of muscle activity
before treatment which is disturbed by placement of the appliance.
• T he level of muscle activity increases accordingly until, after a
period of adjustment, a new equilibrium is reached at a higher level
activity.
• Further adaptations within the muscles over a period of time results
in a reduction of muscle activity when a new equillibrium is again
established at the same level that existed before tratment.
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• Muscle Respose to Twin Block appliance – An EMG study
• India- Aggarwal et al -1999:Research on a group of patients Treated with
Twin blocks
•
• Provides important information on the adaptive changes during treatment.
• Bi lateral EMG activity of elevator muscles Of the mandible (i.e. anterior
temporalis and masseter) was monitored longitudinally to determine
changes in postural, swallowing and maximum voluntary clenching activity
during an observation period of 6 months .
• The muscle activity was measured at the start of treatment, within 1 month
0f Twin block insertion, at the end of 1 months and at the end of 6 months.
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• Results: significant increase in postural and maximum clenching
EMG activity in masseter and in anterior Temporalis activity
during the 6 month period of treatment
• The increased activity can be attributed to an enhanced stretch
(myotatic) reflex of the elevator muscles, contributing to isometric
contractions.
• The main corrective force for Twin Block treatment appears to be
provided through increased active tension in the stretched muscles
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• The increased EMG activity during posture and maximum voluntary
clenching supports active reflex contractions to play a dominant
role in the neuromuscular changes with Twin Block treatment.
• The results of this study reaffirm the importance of full-time wear
for functional appliances to exert their maximum therapeutic effect
by way of neuromuscular adaptation.
• This study supports that repeated contact between the inclined
planes during posture & clenching leads to uninterrupted stretch on
the muscles spindle & repeated stimulation of stretch receptors
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• THE APPLIANCE :
• A modification of the Twin Block appliance described by Clark was used.
• Adams clasps:maxillary and mandibular first premolars
• First molars and ball clasps to the lower labial segment to maximize
retention.
• A labial bow was also used that was soldered to the Adams clasp on the
maxillary premolars.
• The jaw registration was taken with approximately 7 to 8 mm protrusion
and the blocks 6 to 7 mm apart in the buccal segments.
• The steep inclined planes interlocked at about 70° to the occlusal plane.
• Compensatory lateral expansion of the upper arch was achieved by means
of an upper midline expansion screw that was turned once a week.
• Reactivation of the blocks was carried out when necessary after 4 or 5
months therapy.
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The twin block functional appliance used. (a) anterior, (b) lateral, (c)
upper occlusal, and (d) lower occlusal views.
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• After completion of functional appliance treatment , changes were
evaluated by means of cephalometric analysis.
• Result :Skeletal changes as a result of Twin Block therapy:
• 1. A mean forward growth/repositioning of the mandible of 2.4 mm,
measured at Ar-Pog, with some forward movement of Pogonion
demonstrated after Twin Block therapy. But it was not possible to
determine whether the increase in Ar-Pog was due to an increase in
mandibular length or a repositioning of the mandible
• 2. The most noticeable skeletal change was an increase in the angle SNB.
• 3. There was an increase in lower anterior facial height.
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• Dental changes as a result of Twin Block therapy
• 1. The mean overjet reduction of 7.5 mm involved a net 10.8°
retroclination of the upper incisors and 7.9° proclination of the
lower incisors.
• 2. Buccal segment correction occurred by distal movement of the
upper molars and lower molar eruption in an anterior and superior
direction.
• Conclusion :This study demonstrates that the Twin Block appliance
is a very effective and efficient tool with which overjets can be
reduced
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CONCLUSION
• In the pursuit of ideals in Orthodontics, facial balance and harmony
are of equal importance to ideal and occlusal perfection. The role of
functional jaw orthopedic techniques is widely acknowledge in
achieving these goals by growth guidance during the formative years
of facial and dental development.
• Twin blocks are extremely patient and operator friendly functional
appliances. They have the gift of versatility of design, which allows
their use in a variety of clinical situations to effectively correct
different types of malocclusions.
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BIBLIOGRAPHY
• William J Clark: Twin block functional therapy, applications in dentofacial
Orthopaedic Mosby Company 2nd edition.
• T.M. Graber ; Thomas Rakosi ;Alexander .G. Petrovic ;Dentofacial
orthopedic with functional appliance;2nd edition, mosby, 1997; pgs. 268-
298
• William . J. Clark. Twin block technique. AmJ Orthod 1988 January;1-18
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