Twin blocks are simple
bite blocks with
occlusal inclined planes
DESIGN OF THE TWIN BLOCK
The occlusal inclined plane
The occlusal inclined plane is the
fundamental functional mechanism of
Cuspal inclined planes play an important
part in determining the relationship of the
If the mandible occludes in a distal
relationship to the maxilla (in class
II) the occlusal forces acting on the
mandible in normal function have a
distal component of force that is
unfavorable to normal forward
Twin-blocks constructed in a
protrusive bite effectively
modifies the occlusal inclined
planes by means of bite-blocks
The bite blocks acts as a guiding mechanism
causing the mandible to be displaced
downward and forward.
The unfavorable cuspal contacts of a distal
occlusion are replaced by favorable
proprioceptive contacts on the inclined
planes of twin-blocks to correct the
malocclusion & to free the mandible from its
locked distal functional position.
STANDARD TWIN BLOCKS
Standard twin blocks are essentially for
treatment of an uncrowded class II div 1
malocclusion with a good arch form.
Clark’s Twin Block appliance consists of
1. Base plate
2. Occlusal inclined plane or bite blocks
3. Retentive components - Delta and ball end
4. Active components- screw ,springs and bows
Base plate/bite blocks
Appliances may be heat-cure or self cure.heat
cure is preferred because of strength
Routinely placed on upper first molars and on
lower first premolars.
Routinely employed mesial to lower
canines and in the upper premolar or
deciduous molar region.
Additional c-clasps may be placed to improve
Occlusal inclined planes
Initially the angulation between the blocks were
made at 90 degrees
since it was difficult to hold the mandible forward
at this angle, the angulation was changed to a 45
the angulation was changed to 70 degrees to the
occlusal plane to apply a more horizontal force
encouraging a more forward mandibular growth.
The inclined plane on lower bite block is angled
from the mesial surface of the second premolar or
deciduous molar whichever present.the lower bite
block does not extend distally to the marginal ridge
on the lower second premolar.
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.
Buccolingually the lower bite block covers the
occlusal surfaces of the lower premolars .
In canine region it has to be thinner.
Upper inclined plane is angled from the mesial
surface of the upper second premolar to the
mesial surface of the first molar.
Since the upper arch is wider than the lower,it is
necessary to cover only lingual cusps of upper
posterior teeth rather than the full occlusal
Mistakes in the appliance can lead to treatment
CASE SELECTION FOR CLASS II
1. Actively growing patients; M: 13+2; F:
2. Class II skeletal relationship with
retrognathic mandibular position
3. Increased overjet
4. Deep overbite
5. None to very mild crowding in U/L
6. Low to average mandibular plane angle
7. Good VTO
In growing patients, overjet up to 10mm can
be corrected on the initial activation by
posturing the mandible in edge-to-edge
The amount of mandibular protrusion
depends on the ease with which the patient
can posture forward.
As a general rule, the initial activation
should reduce the overjet by 5 to 7
mm leaving 3 to 5 mm interocclusal
clearance in the first premolar region.
In case there is no adequate posterior
clearance for the construction of the bite
block then a 2 mm of anterior opening is
Larger overjet can be corrected by
progressively reactivation of the appliance
during the course of treatment.
summary of bite registration
Inter incisal clearance
In first premolar region
THE TWIN BLOCK TECHNIQUE
Twin Block is described in two stages.
The first phase is an active phase in which
Twin Block appliance is used followed by
the support phase
In the first phase of twin block therapy, the
appliance is used to achieve correction of sagittal
jaw position. After correction vertical discrepancy
is corrected by selectively trimming the posterior
The aim of the first phase is to achieve correction
to class I occlusion and control of the vertical
dimension by a three-point contact with the
incisors and the molars. At this stage the overjet
overbite and sagittal relationship is full corrected.
Time required is 6-9 months
The aim of the support phase is to maintain the
corrected incisor relationship until the buccal
relationship is fully interdigitated.
To achieve this objective an upper removable
appliance is fitted with an anterior inclined plane
with a labial bow to engage the lower incisors and
Time required is 4-6 months
Twin block bio finisher
Extruding lower molars by vertical traction to stabilize the
Advantages of twin blocks
The twin block is the most comfortable ,
the most aesthetic and the most efficient
of all the functional appliances .
Twin blocks have many advantages
compared to other functional appliances.
Patient can wear twin blocks 24 hours per
day &can eat comfortably with the
appliances in place.
It is to take full advantage of all functional
forces applied to the dentition , including
forces of mastication.
Twin blocks can be designed with no visible
anterior wires without loosing efficiency
The occlusal inclined plane is the most natural
of all the functional mechanisms.there is less
interferences with normal function because
the mandible can move freely in anterior and
lateral excursion without being restricted by a
Twin blocks may be fixed to the teeth
temporarily or permanently to guarantee
From the moment twin blocks are fitted the
appearance is noticeably improved.The
absence of lip,cheek or tongue pads ,places no
restriction on normal function & does not
distort the facial appearance during treatment
Patients can learn to speak normally with the
Adjustments and activation is simple.The
appliance is robust and not prone for
breakage.chair side time is reduced in
achieving major orthopedic correction.
Twin blocks allow independent control of
upper and lower arch width.appliance
design is easily modified for transverse
and sagittal arch development.
Twin blocks achieve excellent control of
the vertical dimension in treatment of deep
overbite and anterior open bite
Asymmetrical activation corrects facial and
dental asymmetry in a growing child
Twin blocks can be worn during sports
activities with the exception of swimming &
violent contact sports
Age of treatment
Arch relationships can be corrected from
early childhood to adulthood.However
treatment is slower in adults & the response
is less predictable
Integration with fixed appliances
Integration with conventional fixed appliance is simpler than
with any other.
to detail the occlusion
Twin blocks for TMJ dysfunction
Effective as splints
Un favorable occlusal contacts eliminated
Simultaneously sagittal,vertical ,transverse arch
Do functional appliance have
Functional appliances have been used for
over a century in the treatment of Class II
Division 1 malocclusions.
Although few clinicians deny their
clinical efficacy, proof of their growth
modifying effect remains elusive.
The effects of Twin Blocks: A prospective controlled study David Ian Lund
There is little evidence to support the claim that
functional appliances significantly affected
Björk and Pancherz demonstrated only small
changes in mandibular growth and said that it
was not affected by treatment with functional
There may be significant influences on
mandibular growth after timely intervention.
As suggested by Harris, DeVincenzo, and
(The effects of Twin Blocks: A prospective controlled study David Ian Lund 1988
Little scientific evidence exists as to
the effect of functional appliances
appliance on growth of the jaws in
The effects of Twin Blocks: A prospective controlled study David
Ian Lund, 1998 Jan
BRODIE’S pattern concept
Face may get bigger ,but its form never
Then is it
possible to grow
Studies done on effects
of functional appliances
Studies on humans
Well-controlled animal studies at the
University of Michigan-Ann Arbor and
the University of Toronto have shown
large amounts of downward and
forward glenoid fossa relocation in
appliances worn 24 hours a day.
(Do functional appliances have an orthopedic effect? Donald G. Woodside 1998 AJO)
The effects of Twin Blocks: A prospective controlled study
David Ian Lund 1998 AJO
This study investigated the net effects of the Twin Block
functional appliance taking into account the effects of normal
growth in an untreated control group.
The treatment group consisted of 36 subjects, mean age of
The control group consisted of 27 subjects with a mean
age of 12.1 years.
These patients were observed for a mean time of 1.2 years
Is mandibular growth increased?
There was a statistically significant increase in
mandibular length measured from Articulare-Pogonion
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.
Baumrind and Korn and Haynes found similar
changes in Ar-Pog. (1986 AO,AJO 1981)
No actual measurement of fossa adaptation or
relocation was made in this study.
Can a functional appliance (FA)
stimulate mandibular growth?
Animal studies, with some exceptions, have found
significant increases in length
In human beings conflicting results have been
reported with use of the Herbst and the Fränkel
appliances and the activator.
(AJO1991 Changes in mandibular length with a
functional appliance – DeVincenzo)
Do Twin Blocks restrain maxillary
When forward growth of the maxilla was assessed
by means of conventional angular measurement
little change in SNA was observed thus indicating
little maxillary restraint.
So no headgear like effect as said by DeVinzenzo
it was postulated that some degree of
might have occurred but was not detected
because of dentoalveolar remodeling
disguising the skeletal effects of the
Is there a beneficial sagittal change?
Despite the fact a restraining effect on the maxilla
could not be demonstrated,
the forward growth/repositioning of the mandible
does result in a significant change in ANB thus
the severity of the Class II skeletal pattern is
Does tooth tipping contribute greatly to
There was a significant amount of tipping of the
labial segment teeth in both arches.
The maxillary incisors were retroclined,
mandibular incisors were proclined as a result
of treatment, which greatly contributed to correction
of the overjet.
Does anteroposterior molar movement aid
correction of the malocclusion?
A restraining effect on the upper molars was
demonstrated to the extent that there was
slight distalization along with a statistically
significant forward movement of the lower
This change in molar position aids the
correction of the disto-occlusion.
Do Twin Blocks control the vertical
position of the teeth?
There was a significantly increased eruption of
the lower molars during treatment after judicious
trimming of the bite blocks.
This differential lower molar eruption is an
important feature in Twin Block therapy as it not
only contributes to overbite reduction and closure
of lateral open bites but also helps with Class II
Also called as tension zone by HARVOLD.
These are clinical signs after fitting functional
The patient experiences adaptation of muscle
function immediately on insertion of the appliance,
in response to altered occlusal function..
Within few days the patient experiences pain behind
the condyle when the appliance is removed.
From the studies of histological changes in animal
experiment , it may be deduced that retraction of the
condyle results in compression of connective tissue &
blood vessels and that ischeamia is the principal cause
A new pattern of muscle behavior is quickly
established whereby patient finds it difficult and later
impossible to retract the mandible into its former
After a few days it is comfortable to wear
the appliance than to leave it out.
This change in muscle action has been
described by McNAMRA as the pterygoid
response & results from the altered activity
of the medial head of the lateral pterygoid
The lateral pterygoid muscle hypothesis
Suggests that both postural and functional
activity in the masticatory muscles increases after
functional appliance insertion. This increased
activity, especially in the superior head of the lateral
pterygoid muscle, then acts as a stimulus to
(McNamara JA. Neuromuscular and skeletal adaptations to altered
function in orofacial region. AJO 1973)
SUMMARY OF TREATMENT EFFECTS
Skeletal changes as a result of Twin Block therapy
A mean forward growth/repositioning of the
mandible of 2.4 mm, measured at Ar-Pog, was
demonstrated after Twin Block therapy.
The most noticeable skeletal change was an
increase in the angle SNB.
No significant maxillary restraint could be
There was an increase in lower anterior facial
Dental changes as a result of Twin Block
retroclination of the upper incisors
proclination of the lower incisors.
Buccal segment correction occurred by distal
movement of the upper molars
lower molar eruption in an anterior and superior
Robertson suggested that the principal changes that
occurred with functional appliance therapy were
distalization of the upper buccal teeth
retroclination of the upper labial segments,
mesial movement of the lower buccal segments
proclination of the lower labial segments.
delay of eruption of the upper maxillary molars
enhanced eruption of the mandibular molars.
Effects of force on bone
The challenge of functional therapy is to maximize the
genetic potential of growth & guide the growing face &
developing dentition towards a pattern of optimal
In the dentition the force of occlusion of the teeth is the
most natural functional mechanism that can be used to
influence the structure of the supporting bone.
This natural process of bony remodelling forms the basis
of functional correction with the twin-block technique
(W, J CLARK IN GRABER, RAKOSI, PETROVIC)
Wolff’s law of transformation of
The internal & external structure
of bone is modified by functional
demands to withstand the physical
demands made on it with the greatest
degree of economy of the structure
functional-appliance therapy can achieve
correction of Class II malocclusion through the
(1) dentoalveolar changes
(2) restriction of forward growth of the midface
(3) stimulation of mandibular growth beyond that
which would normally occur in growing children
(4) redirection of condylar growth from an upward
and forward–directed growth to a posterior direction,
Do functional appliances have an orthopedic effect?
Donald G. Woodside 1998 JAN AJO
(5) horizontal expression of mandibular growth
from downward and forward to horizontal.
changes in neuromuscular anatomy and
function that would induce bone re-modeling
(7) adaptive changes in glenoid fossa location to a
more anterior and vertical position.
He concludes that ,
There is still convincing evidence
supporting the concept that functional
appliances do create an orthopedic
effect in specific individuals
Do functional appliances have an orthopedic effect? Donald G. Woodside
Treatment effects produced by the Twin-block appliance and the
FR-2 appliance compared with an untreated Class II sample
Linda Ratner Toth, and James A. McNamara, Jr AJO 99
cephalometric study compares the treatment effects
40 patients treated with the Twin-block appliance
40 children treated with the FR-2 appliance
40 untreated Class II controls
significant increases in mandibular length
were observed in both treated groups.
The Twin-block achieved an additional 3.0
mm of mandibular length, whereas the
Fränkel 1.9 mm more than did the controls.
No restriction of midfacial growth in either
appliance group relative to controls
A increase in lower anterior facial height
in both treatment groups.
more dentoalveolar adaptation was
observed in tooth-borne Twin-block
appliance than with the tissue-borne
The Twin-block and FR-2 samples both
showed significant retroclination and
extrusion (eruption) of the maxillary incisors.
The Twin-block patients exhibited distal
movement of the upper molars; however,
there was no extrusion.
Slight lower incisor proclination was noted
& greater in the Twin-block group compared
with the other 2 samples.
Treatment effects of the twin block appliance
Christine M. Mills, and Kara J. McCulloch
A clinical study was done to investigate the
treatment effects of a modified Twin Block
Pretreatment and posttreatment cephalometric
records of 28 consecutively treated patients with
Class II malocclusions were evaluated and
compared with untreated Class II control subjects.
Results indicated that mandibular growth in
the treatment group was on average 4.2 mm
greater than in the control group over the 14month treatment period
In addition, some dentoalveolar effects in
both arches contributed to the overjet
Muscle response to the Twin-block appliance: An EMG stu
Preeti Aggarwal, Kharbanda,, Rashmi Mathur,AJO 1999
An EMG study was performed on 10 young
growing girls in the age group of 9 to 12 years
with Class II Division 1 malocclusion who were
under treatment with Twin-block appliances.
Bilateral EMG activity of elevator muscles of the
mandible (ie, anterior temporalis and masseter)
was monitored for 6 months.
The changes were noted
at the start of treatment
within 1 month
end of 3 months,
end of 6 months.
The results revealed increase EMG
activity in masseter anterior temporalis
activity during the 6 month period of
The increased electromyographic activity
can be attributed to an enhanced stretch
(myotactic) reflex of the elevator muscles,
contributing to isometric contractions.
functional appliances that are
worn full-time elicits a
greater and more rapid
neuromuscular response than
those worn only part-time.
orthodontists still are searching for
the most effective means of stimulating
mandibular growth preferentially.
Barring surgical correction, functional
appliances seems to be the most direct
approach to treatment of a mandibular
I would like to thank my
guide Dr. triveni for
helping me out with this
Leader in continuing dental education