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TWIN BLOCKTWIN BLOCK
DIAGNOSIS, TREATMENT PLANNING,DIAGNOSIS, TREATMENT PLANNING,
APPLIANCE DESIGN ITS MODIFICATIONSAPPLIANCE DESIGN ITS MODIFICATIONS
AND ITS TREATMENT EFFECTSAND ITS TREATMENT EFFECTS
→ PHILOSOPHY OF TWIN BLOCK APPLIANCE
→ EVOLUTION OF TWIN BLOCK
→ DEVELOPMENT OF TWIN BLOCK
→ BITE REGISTRATION
→ ESTABLISHING CORRECT VERTICAL
→ STANDARD TWIN BLOCK APPLIANCE
→ FITTING TWIN BLOCK : INSTRUCTIONS TO THE
→ STAGES OF TREATMENT
→ MODIFICATIONS OF TWIN BLOCK
→ CLINICAL RESPONSE TO TREATMENT
PHILOSOPHY OF TWIN BLOCK
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.
If the mandible occludes in a distal relationship to the
maxilla, the occlusal forces, acting on the mandibular teeth in
a normal function have a distal component of force that is
unfavourable to normal forward mandibular development.
The inclined planes formed
by cusps of the upper and the
lower teeth represent a servo
mechanism that locks the
mandible in a distally occluding
functional position.Thus the
inclined plane mechanism plays
important role in determining the
cuspal and relationship of the teeth
as they erupt into occlusion.
The proprioceptive sensory feedback mechanism
controls muscular activity and provides a functional stimulus
or deterrent to the full expression of mandibular bone growth.
This unfavourable cuspal contacts of distal occlusion
represent an obstruction to normal forward mandibular
translation in function, and as such do not encourage the
mandible to achieve its optimum genetic growth potential.
A functional equilibrium is established under
neurological control in response to repetitive tactile stimulus.
Occlusal forces transmitted through the dentition provide a
constant proprioceptive stimulus to influence the rate of
growth and trabecular structure of the supporting bone.
Malocclusion is frequently associated with
discrepancies in arch relationships due to underlying skeletal
and soft tissue factors, resulting in unfavourable cuspal
guidance and occlusal function.
Functional appliance therapy aims to improve the
functional relationship of the dentofacial structures by
eliminating unfavourable developmental factors and
improving the muscle environment that envelops the
developing occlusion. By altering the position of the teeth
and supporting tissues, a new functional behaviour pattern is
established that can support a new position of equilibrium.
Hence, twin blocks are
constructed to a protrusive bite that
effectively modifies the occlusal
inclined planes by means of acrylic
incline planes on occlusal bite
blocks. The occlusal inclined plane
acts as a guiding mechanism to be
displaced downwards and forwards.
With the appliance in mouth,
patient cannot occlude comfortably
in the former distal position and
mandible is encouraged to adopt a
protrusive bite with the inclined
plane engaged in occlusion.
The unfavourable cuspal contacts of the distal
occlusion are replaced by a favourable proprioceptive
contacts on the inclined planes of the twin block to correct
the malocclusion and to free the mandible from into locked
distal functional position.
EVOLUTION OF TWIN BLOCK
After the evolution of functional appliances therapy,
many variations in the appliances design occurred after
original development of MONOBLOCK at turn of century.
Early appliances were bulky and cumbersome for the
patients and only suitable for night time wear. Subsequent
modifications of design reduced the bulk of these appliances
as clinicians sought to increase the day time wear by
reducing palatal acrylic and replacing acrylic with some wire
components. The labiolingual appliances of BIMLER (1949)
and vestibular appliances of FRANKEL (1967) exemplified
this approach. These modifications improved patients
acceptance of the appliance for day time wear in many cases
and therefore improved the consistency of response to
treatment. The addition of wires also increased the flexibility
of appliances but also made them more vulnerable to
fracture and distortion and difficult to repair.
However all these functional appliances shared the
common disadvantage of being made in one piece to fit the
teeth in both the jaws. The upper and lower components
were joined together, patient could not speak, eat or perform
other oral functions with appliance in the mouth. In
additions some variations were not esthetic and proved
uncomfortable and full time wear of appliance was not
Twin block free the patient from these restrictions.
The patient can function normally in twin blocks and is able
to eat and speak without restriction of normal movements of
tongue, lips and mandible and also esthetically more
acceptable. This freedom enable the appliance to be worn
DEVELOPMENT OF TWIN BLOCK
The Twin Block appliance evolved in response to a
clinical problem when a young patient, the son of a dental
colleague, fell and completely luxated an upper and central
incisors. The incisor was reimplanted and a temporary splint
was constructed to hold the tooth in position.
After 6 months with a stabilizing splint, the tooth had
partially reattached, but there was evidence of severe root
resorption and the long term prognosis for the reimplanted
incisor was poor.
The occlusal relationship was Class II division 1 with
an overjet of 9mm and lower lip was trapped lingual to the
upper incisors. Adverse lip action on the reimplanted incisor
was causing mobility, and root resorption.
To prevent the lip form trapping in the overjet it was
necessary to design an appliance that could be worn full time
to posture the mandible forward. At that time no such
appliance was available and simple bite blocks were
therefore, designed to achieve this objective. The appliance
mechanism was designed to harness, the forces of occlusion
to correction they distal occlusion and also the reduce the
overjet without applying direct pressure to the upper
incisors. The first Twin Block appliances were fitted on 7th
September 1977. The upper and lower bite blocks engaged
medial to the 1st
permanent molar at 90° to the occlusal plane,
when the mandible postured forward. This positioned the
incisors edge-to-edge with 2mm vertical separation to hold
the incisors out of occlusion. The patient had to make a
positive effort to posture the mandible forward to occlude
the bite blocks in protrusive bite.
EVOLUTION OF APPLIANCE DESIGN
The earliest twin block were designed with the
following basic components.
• A midline screw to expand the upper arch.
• Occlusal bite blocks.
• Clasps on upper molars and premolars.
• Clasps on lower premolars and incisors.
• A labial bow to retract the upper incisors.
• Springs to move the individual teeth and to improve
• Provision for extra oral traction in some cases.
Twin block appliances are tooth and tissue borne.
The appliances are designed to link teeth together as
anchor units to limit individual tooth movement, and to
maximise the orthopedic response to treatment.
CASE SELECTION FOR TWIN BLOCK APPLIANCE
Case selection for initial clinical use of Twin Block
should, display the following criteria :
• Angle's class II division 1 malocclusion with good arch
• A lower arch that is uncrowded or decrowded and aligned.
• An upper arch that is aligned or can be easily aligned.
• An overjet of 10 - 12mm and a deep overbite.
• A full unit distal occlusion in the buccal segments.
• On examination of models in occlusion with the lower
model advanced to correct the increased overjet, the distal
occlusion is also corrected and it can be seen that a
potentially good occlusion of the buccal teeth will results.
• On clinical examination the profile should be noticeably
improved when the patient advances the mandible
voluntarily to correct the overjet.
• To achieve a favourable skeletal change, during treatment,
patient should be actively growing. A more rapid growth
response may be observed when the treatment coincides
with the potential growth sport.
BITE REGISTRATION FOR CONSTRUCTION OF
TWIN BLOCKS FOR CLASS II DIVISION 1
Exactobite or project bite Gauge is designed to
record a protrusive bite for construction of twin blocks.
The blue bite gauge registers 2mmm vertical clearance
between the incisal edges of the upper and lower incisors,
which are in appropriate inter incisal clearance for bite
registration in most class II division 1 malocclusions with
In Class II division 1
malocclusion a protrusive bite is
registered to reduce the overjet
and distal occlusion on average 5
- 10mm on initial activation,
depending on the freedom of
movement in protrusive
functions. The length of the
patient's protrusive path is
determined by recording the
overjet in centric occlusion and
fully protrusive occlusion. The
activation should not exceed 10%
of protrusive path.
In growing child, with an overjet of upto 10mm,
provided the patient can posture comfortably forwards, bite
may be activated upto edge-to-edge on the incisors with a
2mm inter incisal clearance. Larger overjet require partial
It is best, first to rehearse the procedure of bite
registration with the patient using a mirror. The patient is
instructed to close correctly into the bite gauge before
applying the wax. Once the patient understands, what is
required, softened wax is applied to the bite gauge from a
hot water bath.
The clinician can then place the bite gauge in the
patient's mouth to register the bite. Central lines should be
coincident when provided no dental asymmetry is present.
In a vertical dimension a 2mm Inter incisal clearance
is equivalent to an approximately 5-6mm clearance in the 1st
premolar region and usually leaves 3mm clearance distally
in the molar region, and ensures that space is available for
vertical development of posterior teeth to reduce the
After removing the registration bite from the patient's
mouth, wax is chilled in cold water.
It is also very important to open the bite slightly
beyond the clearance of the free way space to encourage the
patient to close into the appliance rather than allow the
mandible to drop out of contact into rest position.
ESTABLISHING THE CORRECT VERTICAL
DIMENSION - THE INTERGINGIVAL HEIGHT
A simple guidance is used to establish the correct
vertical dimension during the twin block phase of treatment.
The intergingival height is measured from the gingival
margin of the upper incisor to the gingival margin of the
lower incisor when the teeth are in occlusion.
The 'comfort zone' for intergingival height for adult
patients is about 17-19mm. This is equivalent to combined
heights of the upper and lower incisors minus an overbite
within the range of normal. Patients whose intergingival
height varies significantly from comfort zone are at a greater
risk of developing T.M.D. This applies both to patients with
a deep overbite whose intergingival height is reduced, and to
patients with an anterior open bite who have an increased
The intergingival height is a useful guideline to check
progress and to establish the correct vertical dimensions
during treatment. Measurement of intergingival height is
done by using a mm ruler and divider or with a vernier scale
to measure the distance between the upper and lower
In twin block treatment the correct intergingival
height is achieved with great consistency. Deep overbite
may be corrected to an intergingival height of 20mm to allow
for a slight settling in with a resultant overbite increase after
In the younger patient's a range of 15 - 17 mm is
normal and allowance should be made for the diminutive
height of the clinical crowns.
STANDARD TWIN BLOCK APPLIANCE
Once the bite has been registered, any excess wax
extending over the buccal surfaces of the teeth should be
removed to allow the models to seat correctly into the
construction bite. The models are then mounted on an
1. Labial Bow, 2. Delta Clasps, 3. Ball and Clasps,
4. Base Plate, and 5. Occlusal Inclined Planes
1. Labial Bow
In early stages of development, the upper twin block
invariably incorporated a labial bow. It was observed that if
the labial bow engaged the upper incisors during functional
correction, it tended to overcorrect incisor angulation. It
was, therefore routinely adjusted out of contact with the
Retracting the upper incisor prematurely limits the
scope for functional correction by mandibular advancement.
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 completed and a class I buccal
segment relationship is achieved.
Delta clasps were designed
by Clark and is similar to Adam's
clasp in principle, but incorporates
new features to improve retention,
reduce metal fatigue and minimize
the need for adjustment. The
retentive loops, may be triangular
in shape (from which the name
'delta' is derived) or alternatively
the loops may be circular both
having equal retentive properly.
The main difference between Delta and Adam's clasp
is that the retentive arms are shaped as a closed triangle or
circle as opposed to an open U shaped arrowhead as in the
According to the area retention, there are two
methods for construction for the delta clasp. The first is
similar to the Adam's clasp with retentive loop placed in the
mesial and distal undercuts. This design is appropriate if the
tooth is favourably shaped, with good undercuts mesially
If the individual tooth/teeth are not favourably
shaped, the loop of the clasps may be directed interdentally.
Placement - In permanent dentition delta clasps are
placed routinely on upper first molars and on lower first
premolars deciduous molars.
Adjustment - The delta clasp may be adjusted
gingivally into an interdental undercut by placing pliers on
wire as it emerges from the acrylic interdentally.
Advantages of delta clasp
Main advantage of the closed loop is the that clasp does
not open with repeated insertion and removal, and therefore
maintain its shape better and requires less adjustment, and is
subjected less to breakage.
3.Ball and Clasps
These are employed usually mesial to lower canine and
in the upper premolar or deciduous molar region to gain
interdental retention from adjacent teeth.
4. Base Plate
Appliance can be either made with heat cure or cold
Heat cure acrylic has the
added advantage of additional
strength and accuracy. Making the
appliance first with wax allows the
blocks to be formed with greater
Cold cure acrylic has the advantage of speed and
convenience, but sacrificing something in strength and
accuracy. It is essential to use a top quality cold cure acrylic
to avoid problems with breakage. The inclined planes can
loose their definition as a result of wear if a soft acrylic is
The disadvantage of cold cure acrylic can be overcome
by using preformed blocks made from a good quality heat
5. Occlusal Inclined Planes
The position and angulation of the occlusal plane is
crucial to efficiency in correcting arch relationship.
In most cases the inclined planes are
angled at 70° to the occlusal plane,
although the angle may be reduced
to 45° if the patient fails to posture
forward consistently and thereby to
occlude the blocks correctly.
The position of the inclined plane is determined by the
lower block and is critical in treatment of deep overbite. It is
important that the inclined plane is clear of the 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
70° to the occlusal plane. The lower block does not extend
distally to the marginal ridge on the lower second premolar
or deciduous molar. 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 its
Buccolingually the lower
block covers the occlusal surfaces of
the lower premolars or deciduous
molars to occlude with inclined
plane on the upper twin block.
The flat occlusal bite block passes forwards over the
first premolars to become thinner buccolingually in the lower
canine region. The full thickness of the blocks need not be
maintained in canine region, thus allowing more freedom for
movement of tongue and improving the speech.
The upper inclined plane is angled from the mesial
surface of upper second premolar to the mesial surface of
upper first molar. The flat occlusal plane then passes distally
over the remaining upper posterior teeth in a wedge shape as
it reduces in thickness.
Since the upper arch is wider than the
lower, it is only necessary to cover only
the lingual cusps of the upper posterior
teeth than the fill occlusal surface. This
has the added advantage of making the
clasps more flexible and allows access
to the interdental wires of the clasps
FITTING TWIN BLOCKS : INSTRUCTIONS TO
Patient motivation is an important factor in all
removable appliance therapy. The process of patient
education and motivation continues when the patient attends
to have twin block fitted. It is often helpful to the patient if
the clinician demonstrates twin blocks on models to confirm
that is simple appliances system and is easy to wear with no
visible anterior wires.
Simply biting the blocks together guides the lower jaw
forwards to correct the bite. The appliance system is easily
understood even by young patients, who seen that biting the
blocks together corrects the jaw position. It is important to
emphasise positive factors and to motivate the patient before
The patient is shown how to insert the twin blocks
with the help of a mirror, pointing out the immediate
improvement in facial appearance when the twin blocks is
inserted and explaining that the appliance will produce this
change, in a few months, provided they are cover in full
time. A removable appliance only corrects the teeth when its
is in mouth, and not in the pocket. Both appliances must be
worn full time, especially during eating with sole exception
being removed for cleaning and during swimming and
At first the appliance may feel large in the mouth, but
within a few days, it will be very comfortable and easy to
wear. Twin blocks cause much less interference to speech
than other, one piece functional appliance. For a first few
days, speech will be affected, but will improve and should
return normal within a week.
As with any new appliances it is normal to expect a
little initial discomfort. But it is important to encourage the
patient to persevere and keep the appliance in mouth at all
times except for hygiene process.
The patient should be advised to remove the
appliance during eating for first few days. Then it is
important to learn to eat with the appliance. The force of
biting on the appliance corrects the jaw position, and
learning to eat with the appliances is important to accelerate
the treatment. In a few days patient should be eating with
the twin block and within a week should be more
comfortable with the appliance in the mouth than they are
It is necessary to check the initial activation and
confirm that the patient closes consistently on the inclined
plane with the mandible protruded in new position. The
overjet is marked with a mandible fully retruded and this
measurement should be recorded and checked at every visit
to monitor progress.
THE TWIN BLOCK TECHNIQUE
- STAGES OF TREATMENT
- Stage 1 : Active Phase
- Stage 2 : Support Phase
Twin block treatment is
described in two stages. Twin blocks
are used in the active phase to correct
the anteroposterior relationship and
establish the correct vertical
dimension. Once this phase is
completed, the twin block are replaced
with an upper Hawley's type of
appliance with an anterior inclined
plane which is then used to support
the corrected position as the posterior
teeth settle fully into the occlusion.
STAGE 1 : ACTIVE PHASE
Twin blocks achieve rapid
functional correction of mandibular
position from a skeletal retruded class II to
class I occlusion using occlusal inclined
planes over the posterior teeth to guide
mandible into correct relationship with the
maxilla. In all functional therapy, sagittal
correction is achieved before vertical
development of posterior teeth is
The upper block is trimmed
occlusodistally to leave the lower molars
1-2mm clear of the occlusion to encourage
lower molars to erupt and reduce the
overbite. By maintaining a minimal
clearance between the upper bite block
and the lower molars, the tongue is
prevented from spreading laterally
between the teeth.
This allows molars to erupt more quickly. At each
subsequent visit, the upper bite block is reduced
progressively to clear the occlusion with the lower molars to
allow these teeth to erupt, until finally all the acrylic has been
removed over the occlusal surfaces of the upper molars
allowing the lower molars to erupt fully into occlusion.
Throughout this trimming sequence, it is important
not to reduce the leading edge of the inclined plane, so that
adaptive functional occlusal support is given until a 3 point
occlusal contact is achieved with molars in occlusion.
STAGE 2 : SUPPORT PHASE
The aim of the support phase is to maintain the
corrected incisor relationship until the buccal segment
occlusion is fully interdigitated. To achieve this objective an
upper removable appliance is fitted with an anterior inclined
plane to engage the lower incisors and canine.
The lower twin block is left out at
this stage and removals of the posterior
bite blocks allow the posterior teeth to
erupt. Full time appliance wear to
necessary to allow time for internal bony
remodeling to support the occlusion as
the buccal segments settle into occlusion.
Treatment is followed by
retention with the upper anterior
inclined plane appliance. Appliance
wear is reduced to night time wear only
once the occlusion is fully established. A
good buccal segment occlusion is the
cornerstone of stability after correction
of arch-to-arch relationship. The
appliance affected advanced mandibular
position will not be stable until the
functional support of a full buccal
segment occlusion is well established.
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 and to
correct the distal occlusion.
• SUPPORT PHASE - 3-6 months for molar to erupt into
occlusion and premolar to erupt after trimming the blocks.
The objective is to support the corrected mandibular position
after active mandibular translation while buccal teeth settle
• RETENTION - 9 months, reducing appliance wear when
the position is stabilised.
An average estimate of treatment time is 18 months,
MODIFICATIONS OF TWIN BLOCK
- FIXED TWIN BLOCK
- TWIN BLOCK TRACTION TECHNIQUE
- REVERSE TWIN BLOCK
- MAGNETIC TWIN BLOCK
- TWIN BLOCK IN TEMPOROMANDIBULAR JOINT
- MODIFICATION TO ENABLE CONTROLLED
PROGRESSIVE ADVANCEMENT OF TWIN BLOCKS
1. FIXED TWIN BLOCK
i) Twin blocks have the unique advantage compared to other
functional appliances in that they can be fixed to the teeth.
Such temporary fixation guarantees full time wear, 24hours
per day, and excellent cooperation is established at the start
of the treatment.
The technique for fixing the appliance is simple. The
teeth should be fissure sealed and treated with topical
fluoride as a preventive measure prior to fixation. There are
two alternative methods of fixation of twin blocks.
• Appliance may be fixed by spreading cement on the teeth
borne areas of appliance but not on the gingival areas. The
appliance is then inserted and secured in place with cement
adhering to the teeth. Zinc phosphate or Zinc oxide cement
is suitable for temporary fixation.
Alternatively, a small quantity of Glass Ionomer
cement can be used taking care to ensure that the appliance
can be freed easily from the teeth.
• Twin block may also be bonded directly to the teeth by
applying composite around the clasps. This is a useful
approach in mixed dentition when the ball clasps may be
bonded directly to deciduous methods to improve fixation.
After a few days, when the patient had adapted to the Twin
Block and is wearing it comfortably the appliance can be
removed by freeing the clasps with a sickle scaler. Sharp
edges of the composite can be smoothed over, leaving some
composite attached to the teeth. The altered contour of
deciduous teeth will improve the retention of the appliance.
ii) Integration with Wilson Lingual appliance:
The integration of the Twin Block with Wilson 3D
modular appliances result in a fixed appliance system that is
adaptable to the requirements of combined orthopedic and
The Wilson 3D modular system is suitable for early
intervention and arch development. The use of Wilson 3D
lingual tube as a retentive element on molar bands provides
a means of attachment for occlusal Twin Block components,
whish may be fixed or fixed/removable under direct control
iii) The Twin Block transpalatal arch
This component consists of a transpalatal arch with
occlusal inclined planes over the posterior teeth that can be
cemented in place. The inclined planes are secured by
occlusal wire tags that are an extension of the transpalatal
wire. The entire assembly is secured in Wilson lingual tubes
on upper molar bands by the standard Wilson plug in
iv) 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 upon the stage of
development. The appliance may be used in the mixed
dentition, when functional correction can be controlled with
interceptive treatment for arch development by the lingual
arch. The Omega loop should be omitted to provide a mere
v) The Twin Block Hyrax Appliance
The Hydrax screw is commonly used with a fixed
appliance for rapid maxillary expansion. Transverse
development can be combined simultaneously with
mandibular advancement by adding Twin Blocks to a rapid
maxillary expansion appliance. The lower appliance may be
anchored on lower second deciduous molars and may
incorporate lower lingual arch.
2. TWIN BLOCK TRACTION TECHNIQUE
In most cases, full functional correction of occlusal
relationship can be achieved with Twin Blocks without the
addition of any orthopedic or traction forces. In case the
response to functional correction is poor the addition of
orthopedic fore, may be considered.
In early stages of development of Twin Block
technique, a method was devised to combine functional
therapy with orthopedic traction. This approach should be
limited to treatment of severe malocclusions, where growth
is unfavourable for conventional fixed or functional therapy.
Functional therapy combined with traction achieved rapid
correction of malocclusion.
1. In treatment of severe maxillary protrusion.
2. To control a vertical growth pattern by addition of a
vertical traction to intrude the upper posterior teeth.
3. In adult treatment, where mandibular growth cannot
assist the correction of a severe malocclusion.
In the early years using Twin Block, tubes were added
to clasps for extraoral traction on the upper application to be
worn at night so as to reinforce the functional components
for correction of a class II buccal segment relationship.
The Concorde face bow is a new means
of applying intermaxillary and extra oral
traction to restrict maxillary growth, and
at the same time, to encourage
mandibular growth in combination with
functional mandibular protrusion.
A conventional facebow is adapted by soldering a
recurved labial hook to extend forwards to rest outside the
lip as an anchor point to combine intermaxillary and extra
oral traction. Patient comfort and acceptance is similar to
conventional facebow. Intermaxillary traction was added to
the appliance system to ensure that if the patient postured
out of the appliance during the night the intermaxillary
traction force would increase, thus ensuring appliance was
effective 24 hours a day.
The Labial hook is positioned extra orally 1mm clear
of the lips in the midline. This enables an elastic back to pass
intraorally to attach anteriorly to the lower appliance to
apply intermaxillary traction as a horizontal force vector.
This has the added advantage of eliminating the
unfavourable upward component of the force in
conventional intermaxillary traction, which can extrude
lower molars and cause tipping of the occlusal plane.
When the distal extra oral traction is applied to a
removable appliance, the outer bow of the face bow should
be adjusted to lie slightly above the inner bow in order to
apply a slight upward component of force to help retain the
upper appliance. Fixation of the appliance must be excellent
before any orthopedic force is applied to a removable
appliance. The traction component is to be worn at night
only to reinforce the action of the occlusal inclined plane.
If the patient fails to posture the
mandible to the corrected occlusal
position during the night, the
intermaxillary traction force is
automatically increased to
compensate and to ensure that
favourable intermaxillary forces are
Careful case selection is essential before using a
combination of Twin Blocks with orthopedic traction. Thus
is a very powerful mechanism for maxillary retraction and as
the majority of class II malocclusion are due to mandibulars
retrusion, it is contra indicated in most of the cases. The
headgear effect tends to tip the occlusal plane and palatal
plane down anteriorly and to retrocline the upper incisors,
which may cause unfavourable auto rotation of the
Occasionally, high traction pull may be indicated to
intrude the upper posterior teeth in cases with a severe
vertical growth pattern, in an effort to achieve a forward
mandibular rotation by intruding upper molars.
3. REVERSE TWIN BLOCKS FOR
TREATMENT OF CLASS III
Functional correction of class III
malocclusion is achieved in Twin Block
technique by reversing the angulation of
the inclined planes, harnessing occlusal
force as the functional mechanism to
correct arch relationship by maxillary
advancement, while using the lower arches
as means of anchorage. The position of the
bite blocks is reversed compared to twin
blocks for class II treatment.
Reverse Twin Blocks are designed to encourage the
maxillary development by the action of reverse occlusal
inclined planes out at a 70° angle to drive the upper teeth
forwards by forces of occlusion and at the same time, to
restrict the forward mandibular development.
Prior to initiation to class III twin
block treatment, it is important to ensure
that the patient's condyles are not
displaced superiorly and / or
posteriorly in the glenoid fossa at full
In treatment with the reverse
twin block, the occlusal forces are
exerted on the mandible is directed
downwards and backwards by the
reverse inclined planes. No damaging
force is exerted on the condyles because
the bite is hinged open with the
condyles down and forward in the
fossae and the inclined planes are
directed downwards and backwards on
the mandibular teeth.
The force vector in the
mandible passes from the lower
molar towards the gonial angle. This
is the area of mandible best able to
absorb occlusal forces.
The blue exactobite 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 occlusal bite
blocks. This is normally achieved by
recording a construction bite with
2mm interincisal clearance in the
fully retruded position.
The sagittal design is used to
advance the upper incisors to correct
the lingual occlusion in the 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 3-screw sagittal
appliance which include a midline
screw to complement the action of the
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
correcting lingual occlusion if used in combination with
reverse inclined planes.
Management of Reverse Twin Block
The screws should be positioned
so that both are opened by turning in the
same direction. This is less confusing for
the young patient. The lower appliance
is retained with clasps on the lower
molars and additional interdental clasps
Opening the screws has reciprocal
effect of driving the upper molars distally
and advancing the incisors. Distal
movement of the molars (upper) is
restricted by the occlusion of the lower
bite blocks on the reverse inclined planes.
Therefore, the net effect of opening the screws, is a
forward driving forces on the upper dental arch. The
position of the cuts will influence their action on individual
teeth. The cuts may be portioned distal to the lateral incisors
to advance only the four incisors.
To enhance the forward movement of the upper labial
segment, lip pads may be added to support upper lip clear of
incisors with an action similar to that of Frankel III. It is
important to attach the lip pads to the anterior segment of
the appliance so that they advance when the screws are
opened, otherwise pads may become compressed against the
gingiva in the labial segment.
4. MAGNETIC TWIN BLOCKS
The role of magnets in twin block therapy is
specifically to accelerate corrections of arch relationships, the
purpose of the magnets is to encourage the increased
occlusal contacts on the bite blocks to maximize the
favourable functional forces applied to correct the
2 types of Rare Earth Magnet
- Samarium cobalt and
- Neodynium Boron
have been used to examine the response to attracting
magnetic forces in twin block treatment.
Both are effective, but Neodynium Boron deliver a
greater force from a smaller magnet.
Attracting magnets incorporated in occlusal inclined
planes may be effective in maintaining forward mandibular
posture when the patient is asleep.
Magnetic force is a new factor under investigation as
an activating mechanism in orthodontic and orthopedic
treatment. Animal; experiments in mandibular advancement
(VARDIMON et al, 1989, 1990) indicated an improved
mandibular response to magnetic functional appliance
compared to non magnetic appliance of similar design.
CLARK modified twin blocks by addition of
attracting magnets to occlusal inclined planes, using
magnetic forces as an activating mechanism to maximize
orthopedic response to treatment.
DARENDELIER & JOHO (1993)
have described similar appliances
which are based on the magnetic
The attracting magnetic force pulls the appliance
together and encourages the patient to occlude actively and
consistently in a forward position. The functional
mechanism of the twin block stimulates a proprioceptive
response by repeated contacts on the occlusal inclined plane.
Attracting magnets may accelerate the progress by increasing
the frequency and force of contact on the inclined planes,
thus enhancing the adaptive response to functional
Attracting or Repelling Magnets
There are logical reasons to
support use of both systems. The
advantages of both methods have
been summarized below.
5. TWIN BLOCK IN TEMPOROMANDIBULAR JOINT
Occlusion is inevitably related to the health and
function of the temporomandibular joint. No dental
condition is more distressing than a chronic T.M. joint pain.
A rationale of treatment is therefore important in dental and
orthodontic practice. This is a litigious area of dental
practice and second opinion should always be sought before
embarking on any treatment which may worsen an already
established pathological condition.
The dental profession is increasingly aware of a
multidisciplinary approach, recognizing the role of
chiropractors and craniosacral osteopaths in the diagnosis
and resolution of T.M. joint dysfunction.
Freedom of mandibular movement
It is essential to diagnose any limitation of movement
relative to the normal range of movement.
• Normal opening is 48mm (3 finger test)
• Transverse movement is 12mm to each side measuring the
lower midline displacement in maximum lateral movement.
The Reciprocal Clicks
A clicking joint is a symptom
of displacement of the articular disc
off the head of the condyle. A
reciprocal click describes the
condition where a click is heard when
the disc is recaptured by the head of
the condyle on forward translation
and a reciprocal click is heard when
the condyle is displaced again off the
articular disc on closing.
Although the clicking joint may otherwise be
asymptomatic, is nevertheless already compromised
internally and liable to present pathology at a later date due
to the chronic displacement of the articular disc.
The timing of the click on
opening is significant in the prognosis
• Early opening clicks : upto 22mm
opening are usually easy to resolve.
• Mid opening : 22 - 35mm opening
are moderate to resolve.
• Late opening clicks : Over 35 mm
opening are difficult to resolve.
Case selection for anterior
repositioning of the mandible to
relieve T.M. joint dysfunction is based
on the severity of the symptoms and
condylor position at full occlusion.
The prognosis is better for recapturing the disc for an
early opening click. It becomes progressively more difficult
for the mid and late opening click. When the pathological
oseoarthritic change is likely to have occurred in the joint.
SPAHL (1993) stresses that, nevertheless, disc
recapture is not the main goal of treatment for patient with
functionally induced T.M. joint pain dysfunction problems.
The true goal is the reduction of symptoms via the condylor
decompression procedures followed by reconstruction of the
occlusion in some manner to support the mandible / condyle
in that advanced position.
The Closed Lock
Limitation of the movement on opening is diagnostic
of a disc which is displaced usually anteromedially to the
condyle, and is not recaptured on opening. In the initial
stages, the patient maybe pain free and may complain only
of restriction of movement. This may be an episodic
experience where the disc is displaced from time to time and
patient may be periodically able to recapture the disc until
the displacement becomes more severe. If not detected and
treated, the disc may gradually folded inwards and not
recapturable, leading eventually to painful function and
restricted opening due to osteoarthritis.
The 3 stages of T.M. Internal derangement are :
• Stage 1 : Painless clicking caused when an anteriorly
displaced disc is recaptured on condyle during opening
• Stage 2 : Locking - persistent displacement of the disc
whish arrests condyle at mid opening.
• Stage 3 : disc displacement through all phases of jaw
function. Disc remains permanently uncaptureble.
Stage 1 and 2 respond to splint therapy and anterior
repositioning to recapture the disc.
Treatment should be affected to
create vertical space in the joint by
positioning the condyles downwards
and forwards in the glenoid fossae, and
to establish balanced occlusal support.
6. A MODIFICATION OF ENABLE CONTROLLED
PROGRESSIVE ADVANCEMENT OF TWIN BLOCK
One of the drawbacks of original design of Clark's
twin blocks was the inconvenience of reactivating the
appliance to achieve an edge-to-edge protrusive position in
some patient to enable full overjet reduction, or to allow a
degree of overcorrection. In the treatment of patients with
large initial overjets, the appliances usually requires block
augmentation or even appliance remakes, since many
patients are unable to tolerate mandibular protrusion greater
than 7mm. Patients with dolichofacial patterns tend to have
a weak craniomandibular musculature and are less able to
tolerate large initial protrusions, sometimes showing a
tendency to bite the occlusal blocks one upon the other,
instead of in correct protrusive position.
In such cases, CLARK (1995) recommends gradual
bite advancement. In class III treatment using twin block,
difficulty may be experienced with reactivating the
appliance. In these cases, gradual reactivation of the bite
would facilitate better patient treatment progression.
There has been more recent evidence that gradual
incremental advancement of the working bite during
treatment of class II division 1 malocclusions may give a
more favourable response to the growth modification
process with a reduced effect on incisor tilting (PETROVIK
et al 1981; FLACK AND FRANKEL , 1989; DE VINCENZO
AND WINN, 1989).
It has been suggested that this improves patient
comfort and speech (BASS, 1996) with greater likelihood of
maintaining the correct appliance position during sleep.
Currently, reactivation of the twin block appliance
requires the addition of cold curing acrylic at the chairside or
alternatively, time consuming laboratory modification. The
former has following disadvantages.
1. Unpleasant taste and smell for the patient
2. Inaccurate due to polymerization shrinkage
3. The use of methyl methacrylate monomer and its potential
4. Increased chairside time and inconvenience.
Light cured acrylic maybe used as an alternative as
this reduces chair side time. But the appliance still requires
finishing and dust extraction facilities are required during
trimming for operator's protection. Using laboratory
readvancement of the appliance means that the patient is
without the appliance for some time.
The advantages of chair side advancement of the twin
block appliance with an adjustable screw mechanism are as
• Accurate measurable advance
• Quickly adjustment at the chair side
• Avoids the used of free monomer.
• Laboratory support unnecessary.
• Asymmetric advancement facilitated.
• Smaller adjustments possible to allow stepwise
advancement and improve patient tolerance.
• Gradual advancement, particularly appropriate in class III
twin block treatments.
• Reversible if over - advancement is produced.
• More gradual mandibular advancement may be more
physiological and therefore, produce improved mandibular
Principal of Twin Block
The appliance modification
consists of insertion of a screw into the
mesial face of each block of the upper
appliance. The 3mm diameter 18/8
M3 stainless steel screw have slotted
pan heads which are machined to a
cone shape giving an included angle of
140°. This means that when positioned
longitudinal in the block the screw had
will always present a 70° angle with a
face of the lower block regardless of
the rotational position of the screw.
The screws are available in 12 and 16mm lengths (R.S.
components U.K.). The longer screws are used for
advancements greater than 5mm. Advancement is achieved
by adding cylindrical polyacetal co-polymer spacers, which
have been used in 1-, 2-, 3-, 4- and 5- mm length.
Original Prototype Appliance
These were made by
drilling a 2-3 mm diameter hole
along the length of the maxillary
appliance blocks followed by
cutting a square seating for the
screw head and tapping using a
3mm diameter plug tap. All later
appliance were made by forming
the blocks directly around the
screw thread with the head of the
screw retained by wax during
build up of the baseplates.
These techniques had a number of disadvantages.
1. It was difficult to achieve a consistent friction fit on the
thread and some screws were very tight as a result.
2. Positioning had to be done with eye, thereby increasing
the likelihood of an error.
3. Occasional cracking of the blocks occurred along the line
4. No grinding of the blocks should be carried out without
severely weakening the acrylic around the screws.
Screw Thread Housing
The above problems were overcome by the
development of an injection moulded screw thread housing
made from acetal resin (R.S. components, U.K.). The
housings have extended lateral tags moulded which serve
both as means of retaining the screw during processing and
facilitate screw alignment rods during waxing up. A block
height of 6mm between the second premolar will be
required to accommodate the screws. The following
construction procedure had been adopted successfully.
The advancement screws are positioned using wax,
either by inserting the tags through a wax dam on the buccal
aspect for self curing acrylic, or by waxing up directly onto
the occlusal surface before building up the blocks in the
usual way for heat cured acrylic. The screw are aligned
relative to the occlusal plane and midline it is preferable to
position the screws as parallel to the midline as possible, thus
ensuring that they will act against the centre of lower block
Housing tag extensions
all cut off during finishing and
the ends are polished flush with
the buccal surface of the blocks.
After finishing the maxillary
appliance the screw faces and
blocks are waxed over leaving
the working facet exposed. The
lower appliance is then formed
against this and resulting facets
serve as guide for finishing the
block faces to the correct angle.
The Advancement Spacers
These measure about 6mm
in diameter and are constructed
from polyacetal co-polymer resin
(also known as Acetal Resin or
polyoxymethylene) This is a
modern thermoplastic produced
by polymerization of
formaldehyde. It has been in the
use for dental application of about
10 years but its use has been
restricted due to the complexity of
processing and cost of equipment
required and its use was limited to
esthetic clasps in chrome partial
The following properties of Acetal resin are of significance.
• It is non-toxic (CANDINI et al, 1994) and non allergenic
(CARAFFINI and CALANDRA, 1990)
• It is upto 10 times stronger than acrylic resin with a high
• It has a high resistance to wear, but may be trimmed using
dental rotary instrument.
Clinical use of Advancement Mechanism
Initially the screws are inserted into the upper blocks
without the addition of any advancement spacers. Once the
appliance has been worn sufficiently to achieve the initial
degree of overjet reduction, additional advancements can be
made at subsequent visits by simply removing the screws
and inserting spacers between the screw head and block
faces. In most cases, 2-3mm advancement is suitable. For
patients with mandibular dental asymmetry, it is possible to
advance one side more than the other to obtain centre - line
The main disadvantage of this system is the reduced
facility for block trimming in the retention phase. This can
be overcome in a number of ways, and the advantages of this
system appear to outweigh the disadvantages.
Thus it is a simple effective and reliable method to
facilitate incremental chair-side advancement of the twin
CLINICAL RESPONSE TO TREATMENT
The clinical response observed after fitting twin blocks
is analogous to changes observed and reported in animal
experiments using fixed inclined planes. The rapid clinical
response is similar to adaptive responses described by
McNAMARA (1980) in animal experiments that studied
functional protrusion with fixed inclined planes.
McNAMARA findings (1980) may be summarised as
'The placement of appliance results in an immediate
change in the neuromuscular propioceptive response.
Provided all phasic and tonic muscle activity is affected, the
resulting muscular changes are very rapid, and can be
measured in terms of minutes, hours and days. Structural
alterations are more gradual and are measured in months,
whereby the dento-skeletal structures adapt to restore a
functional equilibrium to support the altered position of
In comparison within a few day of fitting of the twin
block appliance, the position of the muscle balance is altered
so greatly, that the patient experiences pain when retracting
the mandible. This has been described as 'PTERYGOID
RESPONSE' (MCNAMARA, 1980, PETROVIC), or the
formation of 'TENSION ZONE' distal to the condyle
(HARVOLD AND WOOSIDE). Such a response to rarely
observed with functional appliances that are not worn full
HARVOLD (1973) and PETROVIC et al described
the histologic changes that occur during the fitting of the
fixed inclined planes during animal experiments. As noted,
the area distal to the condyle is described as tension Zone.
No vacuum is created behind the condyle. Rather the
response is one of tissue proliferation to fill in the area
behind the condyle. Connective tissue and blood vessels
apparently proliferate in the retrodiscal attachments within
minutes or hours of fitting a full time functional mechanism
activated to advance the mandible.
The clinical signs in twin block treatment are
consistent with discussed research findings and are evident
after first few days of treatment. The patient experiences
discomfort in the condylar region when the appliance is
removed. On removal of the appliance the mandible is
retracted and the condyle compresses connective tissue and
blood vessels that the proliferated in the tension zone behind