Separators are placed mesial and distal to the upper first molars, and mesial and distal to the lower first and second premolars. Second Visit Bands are selected for the upper first molars and the lower premolars. The acrylic blocks are attached to the bands by 3D lingual and buccal tube assemblies .These are welded to the lingual surface of the lower second premolar band and the palatal surface of the upper first molar band. Upper and lower impressions are taken over the bands in alginate impression material
Current status of twin block
CURRENT STATUS OF
INDIAN DENTAL ACADEMY
Leader in continuing dental education
Clarks Twin Block
Modifications in TBA
Retrospective studies on TBA
Prospective studies on TBA
Clip on fixed appliance ( M. Read )
Cohort study proving efficiency
Modified TB for treatment of Class II div 2
Modified TB for treatment of Class III
Finite element scaling analysis
Optical surface scanning for assessment of effect of
( TB ) on soft tissues Animal study
A wide range of functional appliances is
available for the correction of Class II skeletal
and occlusal disharmonies (eg, Bionator,FR-2 of
Frankel, fixed and removable Herbst appliances
Among these, the Twin-block originally
developed by William Clark has gained
increasing popularity during the last decade.
The appliance is indicated for the correction of
Class II malocclusions characterized in part by
mandibular skeletal retrusion. In the dentition
the force of occlusion of teeth is the most
natural functional mechanism that can be used
to influence the structure of the supporting
bone. This natural process of bony remodeling
forms the basis of functional correction with the
Twin Block technique.
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
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 and are designed for full-time wear to take
advantage of all functional forces applied to the dentition
including the forces of mastication.
TWIN BLOCK APPLIANCES
Twin blocks may be either removable or fixed
REMOVABLE TWIN BLOCK
The upper appliance is retained by modified arrowhead clasps.
Adding ball-ended clasps in the labial or buccal segments may
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.
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 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.
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.
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
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
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 theoverbite by increasing the
lower facial height. www.indiandentalacademy.com Exactobite
The Excatobite or Projet Bite Gauge is designed to record
a protrusive bite for construction of Twin Blocks. The blue
bite gauge registers 2mm vertical clearance between the
incisal edge of the upper and lower incisors, which is an
appropriate interincisal clearance for bite registration in
most Class II division 1 malocclusion with increased
The incisal portion of the bite guage has three incisal
grooves on one side that are designed to be positioned on
the incisal edge of the upper and a single groove on the
opposing side that engages the incisal edge of the lower
incisior. The appropriate groove in the bite guage for bite
registration is selected depending on the ease with which
the patient can posture the mandible forwards.
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.
The technique is described in two stages, an
active phase with twin blocks and a support
phase with a guide plane after correction of arch
STAGE 1— ACTIVE PHASE
Twin blocks are removed for eating for the first 3
days until the initial discomfort from appliance
wear has been resolved. Thereafter, the
appliances are worn continuously.
The upper midline screw is turned a one-quarter
turn every week to 10 days until the arch width is
adequate to accommodate the lower arch in its
Clinical response in active phase
Within a few days of fitting the appliances, the position 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 (Harvold). It is rare for
such a response to be observed with functional appliances
that are not worn full-time.
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 dentoskeletal structures adapt to restore a
functional equilibrium to support the altered position of
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
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.
Management of overbite
Overcorrecting the incisors to an edge-toedge relationship before reducing the height
of the bite-blocks reduces deep overbite.
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.
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
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
It is important to maintain adequate
interlocking wedges to maintain
anteroposterior correction of arch
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 Hawleytype removable appliance with an inclined guide plane to
retain the sagittal relationship.
The upper and lower buccal teeth are usually in
occlusion within 4 to 6 months and the support phase is
continued for a further 3 to 6 months to allow functional
reorientation of the trabecular system before the position
is retained (Harvold).
CLASS II BITE REGISTRATION:
In class II division 1 malocclusion a protrusive bite is registered to
reduce the overjet and the distal occlusion on average 5-1-mm on initial
activation, depending on the freedom of movement in protrusive
funtion.. The activation should not exceed 70% of the protrusive path.
Larger overjets invariably require partial correction, followed by
reactivation after the intial correction is complete.
In the vertical dimension a 2mm interincisal clearance is equivalent to
an approximately 5to6mm in the first premolar region. This is usually
leaves 3mm clearance distally in the molar region, and ensures that
space is available for vertical development of posterior teeth to reduce
It is very important to open the bite slightly beyond the clearance of the
free-way space to encourage the patient to close the appliance rather
than allow the mandible to drop out of contact into rest position, which
is one of the disadvantages of making the blocks too thin.
CLASS III BITE REGISTRATION:
It is not possible to build-in the same degree of
activation in the construction bite for functional
correction of Class III malocclusion compared to
Class II correction, because there is less scope
for displacement of the mandible.
The Blue exacobite is used to register a
construction bite with the teeth closed to the
position of maximum retrusion, leaving sufficient
clearance between the posterior teeth for the
occlusal bite blocks. This is normally achieved in
the fully retruded postion.
The saggital design is used
to advance the upper
incisors to correct the lingual
occlusion in treatment of
Class III malocclusion.
In many cases, the maxilla
is contracted laterally in
addition to occluding in a
distal relationship to the
mandible. This is an
indication for combined
sagittal and transverse
expansion using a threescrew sagittal appliance
which includes a midline
screw to complement the
action of the saggital
An alternative design uses
a three-way expansion
screw to combine
transverse and sagittal
expansion. This is also
effective in expanding a
contracted maxilla and in
occlusion if used in
combination with reversed
Lip pads may be added to
support the upper lip clear
of incisors to enhance
forward movement of
upper labial segment.
The reverse pull facial mask can be attached to the
upper Twin Block to maximize the forward component of
force on the maxilla. A sagittal appliance is used in the
upper arch. The elastic force is increased gradually from
the time the facial mask is fitted and as the patient
adapts to the pressure.
A starting pressure of 8oz using bilateral 3/8 in elastics is
recommended for 1st 2 weeks.
The force can be later increased to 14oz by using 1/2inch elastics. A maximum of 14 oz with 5/16 inch elastic
can then be delivered
CORRECTION OF ANTERIOR OPEN BITE
Reduced over bite or anterior open bite is often related to
unfavorable vertical growth. Throughout treatment, all
posterior teeth must remain in contact with the opposing
bite blocks to prevent over eruption. Intrusion of posterior
teeth helps reduce anterior open bite and encourages a
favorable mandibular rotation to close the mandibular
plane angle. The second molar need to be avoided from
over eruption by including occlusal rests even before
these teeth erupt in order to control their eruption.
These patients benefit from phased progressive
activation to allow their weak muscles to adapt more
gradually to mandibular advancement.
Vertical Elastics or attracting magnets help to solve this
BITE REGISTRATION FOR ANTERIOR OPEN
Exactobite designed to register a 4mm
interincisal clearance resulting in approximately
5mm clearance between the cusps of the first
premolars or deciduous molars is used.
Blocks of thickness sufficient to open the bite
beyond the freeway space to intrude the teeth,
and to make it difficult for the patient to
disengage the blocks are used.
APPLIANCE DESIGN IN TREATING ANTERIOR OPEN BITE
The lower appliance extends distally to the lower molar region with
clasps on the lower first molars and occlusal rests on the second molars
to prevent their eruption. It is not necessary to extend acrylic to contact
the upper and lower anterior teeth so that they are free to erupt to
reduce the anterior open bite.
A palatal spinner can be added to the upper appliance to help control an
anterior tongue thrust. The spinner is an acrylic bead that is free to
rotate round a transpalatal wire positioned in the palate.
Intra Oral Traction to Close Anterior Open Bite
Intraoral elastics can be used to accelerate bite closure as
an efficient alternative to high-pull extraoral traction.
This method introduced by Dr. Christine Mills in Vancouver
was used mainly to maintain occlusal contact on the
Running vertical elastic between upper and lower appliances
or to brackets or bands with gingival hooks reinforces the
intrusive effect of the bite blocks. The elastics are worn at
night to maintain occlusal contact of all the posterior teeth on
the bite blocks to intrude posterior teeth.
Magnetic force in correction of Anterior Open bite
Magnets are incorporated in the inclined planes on the
posterior bite blocks to increase the intrusive focus.
Dellinger in 1986 investigated the effect of repelling
magnets and found that they increased the opposing
forces in the occlusal bite blocks to intrude opposing
Attracting magnets increased the frequency of occlusal
contacts on the inclined planes. These accelerated both
anteroposterior and vertical correction
TREATMENT OF CLASS II DIV 2 MALOCCLUSION
Retroclined upper incisors are responsible for holding the
mandible in a distal position in Angle's Class II Div. 2
Twin blocks have the effect of unlocking the
malocclusion by releasing the mandible from an
entrapped position of distal occlusion and thereby
encouraging a rapid transition to class I arch
Releasing the mandible downwards, forwards, and
encouraging the lower molars to erupt achieve correction
of class II div 2 malocclusion. At the same time, the
upper incisors are advanced to achieve a normal upperto-lower incisor relationship that is cleared far enough
forwards to accommodate the advancing mandibular
arch of closure.
The construction bite in Class II div 2
malocclusion is registered with the incisors in
edge-to-edge occlusion. When the overbite is
excessive, the clearance between the
posterior teeth is correspondingly increased.
The occlusal bite blocks are made thicker in
the premolar region, to allow the clearance of
the upper and lower incisors.
The amount of mandibular advancement is
limited in class II Div. 2
The upper twin block incorporate two sagittal screws set
in the palate for antero-posterior development. The
screws expand the arch by advancing the upper incisors
and at the same time, drive the upper buccal segments
distally and buccally along the line of the arch.
This sagittal design is suitable for lower expansion also.
If the upper arch is constricted as well in such a case, the
triple screw sagittal twin block is useful to improve the
arch form in AP and transverse directions.
FIXED TWIN BLOCKS
Twin block appliances may be designed
for direct fixation to the teeth by bonding.
Preformed wedge attachments are being
designed at present for direct fixation to
molar bands to allow simpler application in
fixed appliance technique.
Magnetic Twin Blocks
The role of magnets in Twin block therapy is to
accelerate correction of arch relationships.
Two types of rare earth magnets (samarium cobalt &
neodynium boron) are used.
Attracting magnets: It pulls the appliances together and
encourages the patients to occlude actively and
consistently in forward position. It increases the
frequency and force of contact on the inclined planes. It
can be used in the patients having weak musculature.
Repelling magnets : They deliver additional forward
mandibular posture without reactivation of blocks.
Magnets are recommended where speed of treatment is
FIXED TWIN BLOCKS
The advantages of fixed appliances compared to removable
appliances are, the increased control by the operator, which
does not rely on the compliance of the patient.
The functional components of fixed twin blocks are :
1. The twin block Transpalatal arch
This consists of a transpalatal arch with occlusal inclined
planes over the posterior teeth that can be cemented in
place. Occlusal wire tags that are extension of the
transpalatal wire secure the inclined planes. The entire
assembly is secured on Wilson lingual tubes on upper molar
bands by the standard Wilson plug-in attachment.
2. The twin block lingual arch
The occlusal inclined plane component in the lower arch is
combined with the Wilson 3D lingual arch and extends over
the occlusal surfaces of the lower deciduous molars, or
premolars, depending on stage of development.
Fitting the appliance
Correct arch forms of both the arches are achieved before fitting the
appliance. After achieving this, the lingual arches are removed and
impressions are taken over molar bands to construct the fixed twin block
The occlusal components of Twin Blocks are designed to be removable
from the lingual
tubes on molar bands together with the Wilson lingual attachment.
It can be fitted by assembling the twin block appliances and band as one
piece & cementing it, or the molar bands are cemented 1st, followed by
attaching the occlusal and lingual attachments.
Light cured material is used for additional working time.
Twin Block Hyrax appliance
Transverse development can be combined simultaneously with
mandibular advancement by adding twin blocks to a RPE appliance like
hyrax screw. The lower appliance can be anchored on lower second
deciduous molars and may incorporate a lower lingual arch
Mills and McCulloch
McNamara and Baccetti ( Treatment
Parkin and sandler - Comparison of 2
Mills & McCulloch : 1998 Ajo
28 treated patients with skeletal Class II
malocclusions were compared cephalometrically
to a control group of untreated individuals
The mean age was 9 years 1 month.
The mean observation period was 14 months
It was a modification of Clarks Twin Block
An acrylic labial bow was added to the lower
incisor area to improve retention.
Elastic hooks were soldered onto the delta clasps
of the upper and lower members of the appliance.
These hooks allow vertical elastics to be worn at
night to encourage the patient to keep the
mandible closed forward into the appliance
while sleeping, thus increasing the effective
wearing time in a 24-hour period.
Like Clarks design :
1. The maxillary appliances had a midline expansion
screw that was used to correct the transverse
relationships as the mandible came forward with growth.
2. Labial bows were not used on the maxillary
appliances of any of the patients in this design keeping
with the recommendations of Clark to minimize lingual
movement of the upper incisors during treatment
The initial wax construction bite was
taken with the mandible protracted
approximately 6 mm and opened
vertically by about 5 mm. In the patients
with slight asymmetries of the mandible,
the construction bite was taken with the
upper and lower midlines coincident in
an effort to correct the asymmetry by
encouraging differential growth of the
A. Skeletal :
1. Effect on maxilla:
The TB group experienced on average a slight inhibition of
forward maxillary growth. In TB group SNA reduced by 0.9
° as compared to +0.1° increase in control .
2. Effect on mandible:
The mandibular unit length (measured from Co to Gn)
increased by 6.5 mm in the Twin Block group in contrast to
only a 2.3 mm increase in the control group.
Ramus height increased 4.1 mm in TB group as
compared to 1.2 mm in control.
Mandibular body length (Go-Gn) increased by 3.0 mm in
TB group which was almost twice as that of the control
The mandibular growth accounts for the 1.9°
increase in angle SNB in the treatment group
as compared with almost no change in the
3. Vertical changes:
Both anterior facial height and posterior facial
height increased significantly in the Twin
Block group during treatment (5.6 mm and
4.3 mm), respectively. So, In spite of the
change in vertical facial height, there was no
net increase in the angle of SN to mandibular
plane in the treatment group.
Dentoalveolar changes were evident in the
Maxillary teeth :
An average up righting effect of 2.5° on the upper
incisors was observed during Twin Block
treatment, but almost no change occurred in the
Molars: Twin Block group experienced a
distalization effect on the upper molars of 1.6 mm.
Control group: Upper first molars came forward
1.5 mm in the control group relative to the vertical
reference plane, and only about 0.3 mm was
attributable to tooth movement.
2. Mandibular teeth :
The lower incisors were proclined thrice in the Twin
Block group (5.2° compared with only 1.4 ) during
The lower molars erupted on average almost four times
as much (2.3 mm compared with 0.6 mm) in the Twin
Block group as in the control group.
The net dentoalveolar change for the lower molars is a
mesial movement of 1.4 mm in the treatment group as
compared with only 0.2 mm for the control group.
When the distance from the lower molars to the vertical
reference plane (through S perpendicular to PP) was
measured, a total increase of 5.2mm (1.4 mm dental
and 3.8 mm skeletal) was noted for the Twin Block
group compared with only 1.9 mm for the control group.
The overjet was decreased in total 5.6 mm in the
treatment group. Nearly two thirds of this decrease could
be accounted for by the forward growth of the mandible
4. Molar relation
In the Twin Block treatment group, molar overjet reduced
by 6.2 mm as compared with a reduction of only 0.4 mm
in the control group.
Mandibular contribution = 3.8 mm skeletal + 1.4 mm
dental = 5.2 mm
Maxillary contribution = 1.6 mm dental - .6mm skeletal =
Thus in the treatment group, approximately 50% of the
molar correction was accomplished by skeletal
improvement in the lower jaw and 50% by dentoalveolar
change in the upper and lower molars.
Posttreatment changes after successful correction
of Class II malocclusions with the Twin Block
Mills and Mcculloch (AJO 2000 July) study was
done 3 years after the phase I Twin Block
Do functional appliances place a "mortgage" on
mandibular growth? In other words, if enhanced
increments of mandibular growth are achieved
during the active phase of treat-ment with a
functional appliance, is this treatment effect
negated by diminished increments of mandibular
growth in the posttreatment phase?
The mean post retention period (during which the
patient had no Twin Block wear) was 14 months.
(Art-Go) increased less in the Twin Block group (2.5 mm) than in the
controls (3.0 mm) These differences indicate a tendency for some
"rebound" effect after treatment with the Twin Block appliance
Maxillary Changes Posttreatment
There was a trend for slightly less
forward maxillary growth in the Twin
Block treatment group from T2- T3 than
in the untreated controls.
The saddle angle opens slightly (0.9°)
The mandibular plane angle increases
Greater increase in anterior facial height
(1.2 mm) seen in the Twin Block group.
The posttreatment incisor overjet measurement
increased in the Twin Block group (1.0 mm),
whereas it tended to decrease slightly in the
untreated controls (-0.1 mm).
The molar relationship as measured in the sagittal
plane showed a mean relapse of 1.2 mm in the
Twin Block group as compared with almost no
change (0.1 mm) in the control group .
Although there was a trend toward slightly smaller
mandibular growth increments in the treatment
group than in the controls, most of the positive gain
in mandibular size achieved during the active
treatment phase were present 3 years posttreatment.
Timing for TB treatment
Baccetti & McNamara AJO 2000 AUG In their study
evaluated skeletal and dentoalveolar modifications
produced by Twin block appliance in 2 samples of
subjects with class II disharmony treated at different
stages of mandibular skeletal maturity, as determined
by cervical vertebral maturation.
It was a retrospective study and 35 out of 79 patients
were evaluated after Twin block therapy.. The treated
sample were divided according to skeletal maturity at
the start of treatment by CV maturation.
Mean age of ETG was 9 years at T1 ( stage 1 or 2 ).
Mean age of LTG was 12 years 11 months. CVM
stage was stage 3 to 5.
Clarks TB appliance was used.
Optimum treatment timing for Twin-block therapy of Class II disharmony
appears to be during or slightly after the onset of the pubertal peak in
Major favorable effects induced by functional therapy in
LTG in comparison with earlier phases are:
Greater skeletal contribution to the correction of the
molar relation(67 %)
Skeletal contribution to overjet correction was 54 %.
Larger and clinically significant increments in total
mandibular length and in ramus height
More posterior direction of condylar growth, a biological
mechanism enhancing supplementary mandibular
lengthening and reducing the amount of forward
condylar displacement in favor of elective mandibular
growth and reshaping
Trenouth (AJO 2000 Jan) evaluated the TB appliance in
the treatment of Class II div 1 malocclusion ON 30
patients. It was a retrospective study.
1. The overjet was reduced during treatment by over 7 mm.
The treatment effect was greater than 2ME. ( Method
2. The angle ANB(- 2.6° showed a statistical significant
reduction with a treatment effect greater than 2ME,
which was mainly due to a statistically significant
increase in angle SNB (+ 2°). There was a small but
statistically significant reduction in angle SNA, but the
treatment effect was less than 2ME.
3.Linear changes :
A.) No significant maxillary change (Ar-A, Co-A)
B.) Clinically significant increase in mandibular length (ArB, Co-B, Ar-Po, Co-Po).
4. The upper incisor angulation was significantly reduced
with the interincisal angle correspondingly increased,
both treatment effects greater than 2ME.
There was no significant change in the lower incisor
5. The MM angle remained virtually unchanged during
treatment but decreased in the control group and the
difference was statistically significant.
Parkin & Sandler (AJO 2001 June) in their retrospective study
compared the skeletal and dental changes contributing to Class II
correction with 2 modifications of the Twin-block appliance
The aim of this study was to
enhance the skeletal effects of
the Twin-block appliance in
the hope that dentoalveolar
effects would make less of a
contribution to the correction
of the malocclusion
TB appliances that use a labial
TB appliances that incorporate
high pull HG and torquing
spurs on the maxillary central
Despite the addition of torquing springs (TB2) to the
maxillary central incisors, average of 6.9° retroclination
In the mandibular arch of the TB2 group, there was 6.4°
of proclination of the mandibular incisors, as compared to
8.2° in the TB1 group.
Increased distal movement of the maxillary molars in the
TB2 sample. Their vertical eruption appears to have been
restricted when compared with the TB1 sampl
The TB2 group demonstrated restraint in the
anteroposterior position of the maxilla.
The SNA angle was significantly reduced in the TB2
sample, despite slight tipping of the maxillary plane in an
The ANB value was significantly more reduced in the
TB2 sample, suggesting that this appliance was more
effective in reducing the sagittal Class II discrepancy.
The LFH / TAFH ratio increased in the TB1 sample and
remained unchanged in the TB2 sample. This was the
most significant difference between the 2 groups
The results demonstrated that the addition of headgear to
the appliance resulted in effective vertical and sagittal
control of the maxillary complex and thus maximized the
Class II skeletal correction in the TB2 sample. Use of the
torquing springs resulted in less retroclination of the
maxillary incisors in the TB2 sample when compared with
the TB1 sample; however, this difference did not reach
the level of statistical significance
Lund and Sandler
RCT of Twin block
Psychological effects of Twin Block
Effects on Protrusive muscle function
MRI study to evaluate the effects on TMJ
Lund and Sandler(AJO1998) .
The Lund and Sandler group also experienced
slightly more labial proclination of the lower
incisors (mean of 8.2°) as compared with the
Mills study where there was a mean proclination
of 5.2°. This difference may be related to the
difference in the appliance design. In Mills study,
an acrylic labial bow was used for retention
purposes on the lower anterior teeth whereas
the Lund and Sandler group had only ball clasps
on the labial of the lower incisors
Chintakanon et al (AJO Oct 2000) investigated Protrusive
mandibular function, including maximum protrusive force
and fatigue time, was investigated in 66 children
displaying Class II Division 1 malocclusion. 32 children
were treated with the Clark Twin-block appliance and the
other 34 children served as untreated controls. Crosssectional data based on pretreatment records showed
that maximum protrusive force ranged from 18.5 N to 160
N, with a mean of 80.3 ± 30.7 N. Maximum protrusive
force was significantly higher in males than in females
Maximum protrusive force in the group of children with
disk displacement was not significantly different from that
of the group without disk displacement.
Maximum protrusive force increased significantly
in untreated group because of growth.
The measured change in the Twin-block-treated
children was not significant.
Fatiguing the protrusive muscles did not alter
mandibular position in the Twin-block group after
6 months of treatment.
This study does not support the lateral pterygoid
hypothesis, as there was no evidence of an
increase in mandibular protrusive function after
treatment with the Twin-block functional
A prospective study of Twin-block appliance therapy
assessed by magnetic resonance imaging
Chintakanon, ( AJO 2000 Nov )
40 children displaying Class II Division 1 malocclusion
were involved in a prospective magnetic resonance
image investigation to evaluate the effects of Twin-block
functional appliances on the temporomandibular joints.
None of these children had clinical signs or symptoms of
temporomandibular disorders. 19 children were treated
with a Clark Twin-block appliance for 6 months; the other
21 children received no treatment and served as controls
Comparison between control and Clark Twinblock groups suggested that
Reduction of the condylar axial angle represents
a feature of untreated Class II growth patterns,
whereas axial angle stability with Clark Twinblock therapy may suggest alteration of condylar
Condyles that were positioned at the crest of the
articular eminence by the Clark Twin-block at the
beginning of treatment had reseated back into
the glenoid fossa after 6 months. However, 75%
of the condyles were more anteriorly positioned
in successfully treated Clark Twin-block cases.
None of the subjects demonstrated dual bite or
Sunday bite as a result of treatment with CTB.
There was no clear evidence of remodeling of the
glenoid fossa at the eminence as a result of Clark
The initial prevalence of asymtomatic disk
displacements for the combined groups was 7.5%
anterior, 5% medial, and 12.5% for lateral disk
Clark Twin-block therapy had neither positive nor
negative effects on disk position, and there was
no convincing evidence that the disk was
Kevin O’Brien, AJO (2003 sept)
In this study a randomized Controlled trial was done that
studied the effectiveness of early orthodontic treatment
with the Twin-block ap-pliance. Phase I was treatment
with a functional appliance, to correct the Class II skeletal
discrepancy, when the child is 7 to 10 years old. This was
followed by phase II treatment with fixed appliances.
Appliance used was
A modification of the Clarks Twin-block appliance.
A passive maxillary labial bow was used to aid anterior
retention and retrocline the maxillary incisors if they were
In narrow maxillary arch compensatory lateral expansion
of the arch was achieved with expansion screw.
Skeletal versus dental change
Early intervention with a Twin-block appliance
successfully reduced dental overjet, molar
discrepancies, and severity of malocclusion. This was
achieved by a combination of dental and skeletal
Skeletal changes :
The amounts of overjet and molar change that were
attributable to skeletal change were 27% and 41 %,
Skeletal change was statistically significant; it amounted
to only 1.9 mm, which might not be considered to be
clinically significant or useful.
The most important changes resulting from treatment
Twin-block showed small restraining effect on maxillary
growth. This might have occurred because of the labial
bow on the Twin-blocks. This design was adopted
because all the operators used labial bows on their Twinblocks to increase retention and to control the maxillary
incisors. As a result, the labial bow might have retroclined
the maxillary incisors, and the position of A point might
have been influenced..
Clark suggests that a labial bow should not be used
because, by retroclining the maxillary incisors, the amount
of potential skeletal change is reduced.
In this study, Class II div 1 patients on average had
proclined maxil-lary incisors, and, at the end of Twin-block
treatment, the incisor angulation was normal and not
Prospective study VS retrospective study
2 retrospective investigations ( Millls & MCulloch , Baccetti &
Mcnamara ) that have evaluated the effect of early Twin-block
treatment showed the amount of skeletal change that contributed to
overjet correction as 50%, and this does not reflect our finding.
Most likely reason for higher skeletal changes is that retrospective
investigations, because of the inherent selection bias, tend to
overestimate the effects of treatment.
In these studies maxillary labial bow was not used, whereas in RCT a
passive maxillary labial bow was used to aid anterior retention and
retrocline the maxillary incisors if they are proclined.
Based on the findings of the several randomized trials, we can
conclude that early treatment with a functional appliance does not, on
average, change the Class II skeletal pattern of a child to a clinically
Kevin O ‘Brien in his multicenter randomized controlled
trial ( AJO Nov 2003 ) reported higher self-concepts and
more positive childhood experiences than did the controls
who received no orthodontic intervention. However, both
groups, at baseline and at 15 months, had self-concept
scores that exceeded population norms
Their findings of enhanced self-concept for treated
patients do not support past findings. For example, in the
study by Dann et al, (ANGLE 1995 ) the investigators did
not seek to reduce the overjets, and labial bows were not
placed on the bionator appliances. Dan reduced the
overjet by a mean of 2 mm, it could be that the esthetic
appearance of the children’s teeth was not changed
enough to have an impact on self concept.
In Kevin O’Brien’s multicenter RCT, if the incisors were
proclined, an active labial bow was used which reduced
the overjet, leading to a mean reduction in overjet for the
treatment group of mm. This, of course, would result in
an improvement to the the appearance of the dental
malocclusion and could have influenced psychosocial
responses in a more dramatic way. .
With respect to negative social experiences, authors
finding suggest that children who received the Twinblock intervention reported significantly fewer negative
social experiences posttreatment. that could positively
influence self-esteem including "feeling bet-ter about
themselves" and "improvement in appear-ance
In evaluating the effects of any treatment, patient
compliance must be considered, and non cooper-ation
rate of 16% was similar to that in other investigations
A major problem with most forms of removable
functional appliance therapy for treating Class II malocclusions is that much patient cooperation is required.
However, although the morphological effects of fixed
functional appliances have been investigated, only 1
study has evaluated their coopera-tion rates: A
multicenter randomized controlled trial (AJO 2003
Aug) that compared the effectiveness of the Herbst and
the Twin-block appliances. The authors concluded that
both appliances had similar effects on the dentition and
skeletal pattern, but the non completion rates were 12%
with the Herbst and 33% with the Twin-block.
Unfortunately, these authors also found that the trade-off
for the greater completion rate with the Herbst was more
visits to repair fractured appliance components or
After this study, Michael Read decided that a
problem with the Herbst appliance was that
the link between the mandibular and maxillary
dentitions was fixed. This might lead to high
levels of stress in the components, resulting in
fracture or debonds. They thought that a
method of reducing this was to develop a fixed
functional appliance in 2 separate parts. As a
result, they decided to adapt the Twin-block
appliance so that it could be fixed to the teeth.
Michael Read BJO 2001 march
Clip on fixed appliance
Appliance design :
In this appliance acrylic blocks were attached to
bands on the lower premolar and upper molar
teeth. The blocks of acrylic rest on and cover the
The way in which the appliance is constructed
allows the, fixed appliance to be placed at the
same time as the Class II relationship is being
corrected so there is an overlapping of the phases.
This ensures a seamless transition to the fixed
phase and, there-fore, significantly reduces
Separators are placed mesial and distal to the upper first
molars, and mesial and distal to the lower first and second
Bands are selected for the upper first molars and the lower
premolars. The acrylic blocks are attached to the bands by
3D lingual and buccal tube assemblies .These are welded
to the lingual surface of the lower second premolar band
and the palatal surface of the upper first molar band.
Upper and lower impressions are taken over the bands in
alginate impression material
The construction bite is taken with the mandible advanced
to the desired position
The bands are removed
from the mouth and
seated accurately in the
construction bite are sent
to the laboratory for the
fabrication of the
The blocks are
constructed on the model
in the laboratory. A tube
is soldered to the lower
wings of this first premolar
bracket to accept the
buccal retaining wire of
the lower block and a
lingual arch also is fitted.
The block is inserted into
the buccal tube and
rotated to seat into the
lingual tubes. Bands are
cemented in position and
the blocks are located
and checked for fit. There
should be an even
contact between the
blocks on one side and
blocks on both sides
should also meet at the
This visit should be
scheduled for a week or 10
days after the appliance is
fitted. The patient should be
asked at any discomfort from
the teeth or
temporomandibular joint and
The appliance cannot be
removed by the patient, and
the small gap between the
blocks and the occlusal
surface is filled with glass
ionomer cement. The blocks
are standard Twin-block
design with steeply inclined
planes interlocked at about
70° to the occlusal plane.
Timing of the treatment:
A major requisite of this appliance is the full
eruption of the lower premolars to enable a well
fitting bands placement. The most favorable time
to treat patients with this appliance is during the
peak of the pubertal growth.
Advantages of the appliance:
Patient co-operation is not required.
It works for 24 hours a day.
A full fixed appliance can be placed at the same
time as the class II correction is being carried out.
Treatment time is short because of full time wear
There is no transitional phase between functional phase
and the fixed phase. So the treatment time is reduced
because of overlap of functional and fixed phase.
It is less bulky than other functional appliances.
Breakage of appliance.
Appliance construction needs a skilled technician
Oral hygiene problem
The repair is fairly straightforward; this frequently
involved replacing loose blocks or bands. If a band had
split, it was replaced.
Clip -on fixed functional appliance Study proving
M. Read & Kevin O'Brien (AJO 2004 April) evaluated the
effec-tiveness of a modified version of this appliance in a
Kevin and Read in their study showed that the clip-on
fixed functional appliance is an effective and rapid
method of treating Class II malocclusion.
It was well accepted by patients.
However, if these data are compared with those from a
randomized controlled trial of the effectiveness of Herbst
and Twin-block appliances, it appears that the treatment
times obtained with this appliance are slightly less.
When we compare the cephalometric measures, it also
appears that this appliance produces somewhat similar
changes to both the removable Twin-block and the Herbst
appliance; these, of course, included normal mandibular
growth. The only factor that influenced the final
discrepancy was the pretreatment discrepancy. It appears
that treatment contributes to reducing the discrepancy but
does not totally eliminate it. Finally the results of treatment
as recorded by the PAR index show that this treatment is
Most important clinically relevant finding was the treatment
completion rate. This was substantially more than authors
previous study in which they had non- completion rates of
13% and 33% for the Herbst and the Twin-block
appliances, respectively. It therefore appears that this
appliance is well tolerated, and its use seems to obtain
high levels of cooperation.
Importantly, the mean number of visits needed to repair
the Herbst appliances in Kevin O’Brien’s previous study
was 4.3; in this study, the mean number of additional
visits was 2.3. Therefore, we can assume that the clip-on
fixed functional appliance fractures less than the version
of the Herbst appliance that they evaluated.
Reasons for this:
First, in the Herbst appliance, the pistons connect the
maxillary and mandibular appliance, and this might lead
to stresses on the appliance components, particularly in
Furthermore, orthodontic bands and standard wire
components retain the appliance, and this might lead to
increased flexibility of the appliance and more resistance
to flexural forces. In addition, the acrylic blocks might
distribute the vertical forces of occlusion to the occlusal
surfaces of the teeth and minimize the possibility of
This was a prospective cohort study, without a control.
This study design was adopted to evaluate the feasibility
of the appliance.
This prospective cohort study showed that
This modification of the Twin-block appliance is an
effec-tive method of treating Class II malocclusion in
terms of the morphological effects on the dental and
The main theoretical advantages of this appliance over
the removable Twin-block are that patient cooperation is
enhanced and the appliance is active for 24 hours a
day, there is no transition phase between the functional
and fixed appliance phases, and it is less bulky.
A randomly allocated prospective study with long- term
evaluation is required to fully evaluate the efficacy of this
MODIFIED TWIN BLOCK IN TREATMENT
OF CLASS II DIV 2
The bite registration is taken with the buccal
segment relationship in an over corrected
position, this may result in an edge-to-edge
incisor position or a slight reversed overjet. However, by ensuring that there is 7-8
mm of separation in the buccal segments,
there should be no incisal interference as
the upper labial segment is proclined.
The appliance design
Appliances are modifications of the Clark Twin Block.
The additional modifications for each are as follows:
Addition of one anterior screw with torquing spurs to both
upper central incisors
Double cantilever spring behind the upper labial
segment followed by bonding of the upper labial
segment with preadjusted Edgewise fixed
Sectional fixed appliance can be placed from the
outset and commence correction of axial
inclination early in treatment. So the complete
transition into fixed appliances is also enhanced
as alignment has already occurred during
Class III Twin Blocks:
Kidner & DiBiase evaluated a modified version
of Clarks Twin Block (BJO Sept 2003 )
Fourteen subjects under 12 years ( mean age
11 years ) of age with a Class III malocclusion
and reverse overjet were included in case
The indications for treatment with the reverse
Twin Block are those cases in the mixed
dentition with a reverse overjet associated with
a mild sagittal skeletal discrepancy and an
average or reduced anterior vertical dimension.
Bite registration was taken in maximum retrusion with
about 2 mm inter-incisal clearance.
A modified version of the Clark Class III Twin Block was
constructed from heat-cured acrylic resin with inclined
planes at 70 degrees directing occlusal force downwards
A midline palatal screw was incorporated for expansion
of the upper arch where this was required.
A lower labial bow and Adams clasps or ball-ended
clasps on upper and lower first molars and premolars
(0.7 mm diameter stainless steel wire) retained the
appliance. If premolars were not present then Adams
clasps were placed on deciduous molars or C clasps on
There were significant changes in the inclination of the
incisors with the upper incisors being proclined the lower
Angle SNB decreased and the anterior vertical dimension
and maxillary mandibular planes angle increased.
The appliance was effective at correcting reverse overjet
during the mixed dentition as an alternative to the Frankel
FR III appliance or an upper removable appliance alone.
Changes occur rapidly with a mean treatment time of only
6.6 months, which compares favourably with the FR III
appliance that was shown to have a mean treatment time
of 3.1 years and achieved similar results.
Changes were mainly dento-alveolar, due to proclination
of the upper incisors and retroclination of the lower
incisors. Skeletal change is limited to slight downward and
backward rotation of the mandible, with an associated
increase in anterior, vertical dimension.
Herbst VS TB
Kevin O'Brien et al (AJO 2003 Aug) reported the results
of a randomized clinical trial that evaluated the
effectiveness of orthodontic treatment with either a
Herbst or a Twin-block functional appliance.
One disadvantage of removable functional appliances is
that extensive cooperation is needed, and discontinuation
rates can vary between 9% and 15% with the Twin-block.
One solution to noncompliance is to use fixed functional
appliances, such as the Herbst appliance.
Appliance used in this study
Twin block: Clarks Twin Block was modified. Maxillary
labial was incorporated in the design.
Herbst appliance: Cast Cobalt Chromium design as
described by Panchrez.
Twin Block group showed 33.6 %.
Herbst Group showed 12.9 % didn’t complete the
functional appliance phase of treatment.
So non completion rate with TB was 2.4 times more
than Herbst appliance.
But, Herbst appliance was prone to damage resulting in
debonding and fractured components. It could be
suggested that the trade-off for the increased
compliance rate is that the patient must return to the
clinician for several appliance repairs during the
functional phase of treatment
Evaluation of the morphologic effects of the appli-ances shows that
most of the changes were dental; the maxillary incisors were
retracted and the mandibular incisors were proclined. In addition, the
skeletal changes were less than those reported in retrospective
Patient cooperation with the Herbst appliance was better than that
with the Twin-block.
Phase I treatment was more rapid with the Herbst appliance, but
overall duration of treatment was similar to that with the Twin-block.
The Herbst appliance is prone to debonding and component
There are no differences in the dental and skeletal effects of
treatment between the 2 appliances, but there was a marked sex
effect: girls responded to treatment better than boys. There was more
residual maloc-clusion for boys than for girls
The Herbst appliance has some advantages over the Twin-block,
mostly concerning increased compliance. Nevertheless, the trade off
for these benefits is the additional cost of appliance construction and
the extra visits for appliance repair
Finite element scaling analysis
Optical surface scanning for assessment
of effect of ( TB ) on soft tissues Animal
Localization of effect of TBA by Finite element scaling
analysis- A morhometric study
( AJO 2001 April) GD Singh and Clark
Previous studies, on Cephalogram only were unable to
localize the effects of TBA on mandibular growth. In this
study geometric morphometric techniques was used that
have become available for the assessment of allometry
(size-related shape change) in patients undergoing
Cephalometric studies demonstrate generic mandibular
lengthening, but the actual sites of putative mandibular
bone growth are not determinable
Thirty mandibular landmarks were digitized from
cephalographs of 46 children (prepubertal, =10 years old)
and 53 adolescents (pubertal=13 years old) to determine
mandibular morphological changes in patients with Class II
Division 1 malocclusions treated with Twin-block
Posttreatment changes prepubertal patients
Prepubertal male configuration.
Condylar neck shows 12% increase in local size.
Apex of coronoid process and antegonal notch
show 17% decrease in local size.
Ramus and corpus show 10% increase in local
Symphyseal region shows decrease in local size
B. Prepubertal female configuration.
Condylar neck shows 3% increase in local size.
Apex of coronoid process 4% increase in local size.
Coronoid process and angle of mandible 5% decrease in
Ramus show increase in size more anteriorly,
Symphyseal region indicates no change in local size.
Pubertal male and female configurations are similar
except that the pubertal male, show negative allometry in
the ramus, gonial angle, or the symphysis.
Mandibular length (articulare–gnathion)
increased overall by 5 mm, from 110.8 mm to
116.2 mm, after treatment.
The corpus length (gonion–gnathion) increased
by3 mm, from 71.5 mm to 74.6 mm.
The ramus height (articulare–gonion) increased
by 3 mm, from 45.5 mm to 48.8 mm.
Positive allometry was noted in the mandibular
corpus, posterosuperior area of the ramus, the
midregion of the corpus, and the dentoalveolar
process, which are not related to muscle
Areas exhibiting isometry and negative allometry may be
related to muscle attachment. Specifically, negative
allometry at the gonial angle and the antegonial notch
relates to the attachments of the masseter muscle,
whereas isometry extends over its area of insertion on
the ascending ramus. Similarly, areas of negative
allometry on the coronoid process relate to the insertion
of the tendon of the temporalis muscle. The area of the
mental protuberance and the symphysis exhibit a
negative allometry that may be associated with the
insertion of the mentalis muscle. In the pubertal male,
however, no negative allometry is observed in the ramus,
gonial angle, or the symphysis.
Distal aspect of condylar neck consistently
showed positive allometry at all stages examined
in line with the LPM hypothesis.
Localization of growth in the condylar neck with
concomitant remodeling of the coronoid process
may reflect the correction of mandibular form
achieved with TBA.
TBA therapy may involve developmental
modulations at the condylar cartilage, remodeling
of the ramus and corpus, and osteogenic
deposition in dentoalveolar regions.
McDonagh, & J. P. Moss, ( EJO April
2001) evaluated the effect of different
functional appliances on the soft tissues as
assessed by cephalometry and optical
42 patients were randomly allocated to
Bass, Twin Block (TB), and Twin Block +
Headgear (TB +Hg) groups.
Lateral cephalograms and optical surface
scans were recorded before and after the
10-month study period.
2 &3 TB appliance. Clarks TBA was used. The
first molars were also clasped in high angle
cases to allow attachment of occlusal stops to
prevent eruption of the second molars. Flyover
tubes were included in the region of the second
premolars to facilitate the attachment of highpull headgear.
Optical surface technique: precision of this
technique is better than 0.5mm
Bass group :
Chin point and sulcus inferioris was positioned forward
by 3 – 5mm.
The vertical dimension was increased by 5 – 7mm at
Chin point moved forward by 1 – 3mm.
LAFH increased by 5 – 7 mm, which was larger than
TB + HG group (without incisal coverage)
Retraction of upper lip 1 – 3mm occurred.
LAFH increased by 7 – 9 mm.
All three groups showed forward positioning of Sulcus
Superioris compared with Labrale Superioris suggesting
a straightening of the upper lip.
The optical surface scanning and cephalometric
results were consistent in the sagittal dimension.
In the vertical dimension, however, the optical
surface scans consistently recorded a greater
increase compared with cephalometric values.
However, the Bass appliance produced greater
forward positioning of soft tissue pogonion as
assessed by optical surface scanning.
The Effect of Continuous Bite-Jumping in Adult Rats: A
( Angle 2004 Feb ) Rabie and Hagg
The growth of condyle and glenoid fossa of growing rats
could be enhanced by bite-jumping appliances. TMJ
growth is regulated by factors endogenously expressed by
cells in the condyles, as well as in glenoid fossa. Forward
mandibular positioning led to a change in the biophysical
environment of TMJ that led to the release of key
regulatory factors that enhanced condylar growth.
Because these factors were endogenously expressed by
cells in the condyles and glenoid fossa in response to
mechanical strain, a similar effect might occur in adult rats,
regardless of their growth status.
This was a morphological study, to ensure a continuous
forward advancement in adult rats, the incline bite plane
was inserted on the upper incisors; a lower crown with an
anterior incline plane was also bonded to the lower
Changes observed were
Changes in size and angle, the appearance of the
condyle surface were also changed. In the control, the
surface of the condyle looked more like a bone. On the
contrary, the surface of the condyle on days 30 and 60 of
the experimental groups showed translucence, especially
in the posterior part, indicating formation of cartilage.
The apposition of bone was differential and did not occur
on the anterior surface of the condylar head but only on
the posterior and superior surfaces, the size as well as
the shape of the condylar head were affected, which was
also supported by the reduction of the angle of the
condylar process to the mandibular plane.
Results demonstrated that condyles of the experimental
group animals were elongated. The longitudinal growth of
the condyle was thus due to increased bone apposition in
the posterior part of the condyle and superior part of the
Bite-jumping appliances can improve proliferation of
mesenchymal cells in the condylar cartilage in young
rats. The same mechanism may also exist in adult
species. When more mesenchymal cells transform into
chondrocytes, there will be more bone formation in the
condyle. Thus, mandibular advancement could also
stimulate the adaptive growth of the condyle in adult rats.
Because there was no increase in the length of
the mandibular base, the remodeling of the
condyle ultimately resulted in the increase in
mandibular length .This finding supports the
opinion that the length of mandible is not entirely
predetermined by genetic factors.
This study demonstrated that adaptive
morphological changes could be achieved by 30day continuous mandibular advancement in adult
rats. Because of the new bone apposition in the
posterior condylar head, the angulation of the
condylar process was significantly affected, as
well as the length of mandible and condylar
Based on the prospective RCT of TB appliance by Kevin
O’Brian et al it may be concluded that functional
appliances could result in statistically significant
improvements for skeletal Class II patients. These results
indicate that early traetment increases a child’s self –
concept and reduces negative social experience.
Gianelley (AJO 1995) advocates the use of full banding
one-stage treatment with maxillary molar distalizing
techniques used in the late mixed dentition stage.
Hopefully, the severe skeletal Class II patients involved
would qualify to be included in that 9% to 10% of patients
whom He considers suitable candidates for functional
appliance treatment. If not successfully corrected while
actively growing, these patients ultimately may require
orthognathic surgery to correct their mandibular deficiency
or multiple years of fixed edgewise treatment to achieve a
"camouflage" correction. In addition, they also have
increased risk of incisor fracture due to lip incompetence.
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