Twin block

Case selection criteria
1. Angles Class II division I malocclusion with
good arch form
2. Uncrowded or well aligned lower arch
3. Upper arch that is aligned or can be aligned
4. An overjet of 10-12 mm and a deep overbite
5. A full unit distal occlusion in the buccal
segments
6. Clinically good VTO
7. Patient in active phase of growth
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Construction
1. Good set of impressions
2. Accurate construction bite
3. Models mounted on an articulator

Bite registration
There are two types of bite gauges used to register
bite for twin block:
1. George bite gauge
2. Exactobite gauge

George bite gauge
Has a sliding jig
attached to a
millimeter scale
Designed to measure
the protrusion path of
the mandible and can
record a protrusive
bite of no more that
70% of the total
protrusion path.

Incisal portion has three
incisal grooves to be
positioned on the
incisal edge of the
upper incisor
A single groove on the
opposing side that
engages the incisal
edge of the lower
incisor.
The appropriate groove
is selected
Exactobite or Projet Bite GaugeExactobite or Projet Bite Gauge

• Designed to record a protrusion bite for
construction of twin blocks.
• Registers 2 mm vertical clearance between
the incisal edges of the upper and the
lower incisors.
• 5 or 6 mm of clearance in the first
premolar region and 2 mm of clearance
distally in the molar region
• Ensures that space is available for vertical
development of posterior teeth to reduce
the overbite.

Procedure
First rehearse the procedure of bite
registration with patient using a mirror.
The patient is instructed to close correctly into
the bite gauge before applying the wax.
The patient should be instructed to occlude
with the midlines coincident and the upper
incisors occluding in the appropriate groove to
reduce the overjet when the mandible closes
into the incisal guidance groove.
A relatively firm wax which is dimensionally
stable is used to register the occlusion.

Guidelines
Horizontal consideration:
According to the Roccabado (1992), the
position of maximal protrusion is not a
physiological position and the range of
physiological movement of the mandible is
only 70% of the total protrusive path. This
is also called freedom of movement.

 Total protrusion path is calculated by
measuring the overjet in most retruded
position and then in the most maximal
protrusion and finding the difference
between the two.
 The initial activation should not exceed
70% of the protrusive path.
 Average 5 – 10 mm on initial activation,
depending upon the freedom of movement
in protrusion function.
 This degree of activation allows an overjet
as large as 10 mm to be corrected.

Midline consideration:
Centre lines should be coincident
provided no dental asymmetry is
present

Vertical consideration:
Two factors determine the amount of
vertical clearance. They are:
1)Thickness of the bite block
Adequate vertical clearance must be
available between the cusps of the
upper and lower first premolars or
deciduous molars to accommodate
blocks of sufficient thickness to
activate the appliance.

2)The vertical activation must open the bite
beyond the freeway space to ensure that
the patient can not drop the mandible into
rest position and negate the
proprioceptive functional response of the
inclined planes.

Intergingival height
To establish the correct
vertical dimension
Measured from gingival
margin of upper incisor to
gingival margin of lower
incisor when teeth are in
occlusion.
Comfort zone for
intergingival height for
patients is generally found
to be 17-19 mm.
Height of upper & lower
incisors minus overbite

Horizontal Vs Vertical growth pattern
Horizontal growth pattern - maintain edge to
edge incisor relationship more easily (provided
the overjet is not excessive)
Vertical growth patterns - may not tolerate the
same degree of sagital activation.
A smaller initial activation is necessary
Gradual mandibular advancement

Clinical management
Two phases:
• Active phase
• Support phase
ACTIVE PHASE
Twin blocks are worn full time. The objective is to
correct arch relationships in the anterior-posterior,
vertical and transverse dimensions.
Normally overjet and overbite are corrected within 6
months and the lower molars have erupted into
occlusion into 9 months.
The average time is 6 – 9 months

Clinical management during active phase
Instructions to the patient
1st
visit
1. Ways to insert and remove the appliance.
2. Operation of screw – The screw that is turned
for the first time after the appliance has been
worn for one week.
3. Patient should be instructed to eat with the
appliance
4. Proper cleaning especially after eating

• 1st
visit
-The clinician should check that the patient bites
comfortably in a protrusive bite.
-Overjet is measured for future reference
-The lingual acrylic of the appliance must be
relieved.
- The clasps are adjusted.
- If a labial bow is present, it should be out of
contact with the upper incisors.

• 2nd
visit : after 10 days
– Should be comfortable with the appliance.
– If the patient is failing to posture forward
consistently, reduce activation by trimming
the inclined planes to achieve patient
compliance.
– Activation of screw under supervision in the
beginning.
– Trimming of upper block occlusodistally by
1mm in case of deep overbite. No trimming
should be done in case of reduced overbite or
open bite.

• 3rd
visit : after 4 weeks.
-review of progress – reduction of OJ &
correction of molar relationship.
-adjustment of labial bow to keep out of contact.
-check up for screw activation & its effects.
-trimming of upper block as needed.
• 4th
visit : after 6 weeks
-similar pattern of adjustment.

Reactivation of
twin blocks:
To increase the
forward posture -
by the addition
of the cold cure
acrylic to extend
the anterior
incline of the
upper twin block.

Reactivation is needed when:
a) Overjet is greater than 10mm
b) In vertical growth pattern when patient
cannot tolerate 10mm protrusion.
c) In adult treatment, when the muscles and
ligaments are less responsive to a sudden
large displacement of the mandible.
d) TMJ dysfunction

SUPPORT PHASE
 The aim is to retain the
corrected incisor
relationship until buccal
segment occlusion is
fully established.
 An upper removable
appliance with steep
anterior inclined guide
plane.

The lower appliance is left out at this
stage and the posterior bite blocks are
removed to allow the posterior teeth to
erupt into occlusion.
The upper and lower buccal teeth are
usually in occlusion within 4 -6 months.
Important phase. Stability is excellent
after twin block treatment; can be
attributed partly to the supportive phase.

Retention phase:
A normal period of retention follows
treatment after occlusion is fully
established.
During the retention period the appliance
wear can be gradually reduced to night
time wear.

Average treatment time
1.Active phase- 6-9 months
2.Support phase- 3-6 months
3.Retention phase- 9 months
Average treatment time 18 months

Monitoring condylar position
during Rx
• X-rays to evaluate:
Position of condyle in the glenoid fossa
– Before Rx with the teeth in contact
– Downward and forward position of the
condyle when the appliance is inserted
– After the overjet has been reduced
– On completion of Rx

Treatment of deep overbite:
 Trimming upper twin block
occlusodistally to encourage
eruption of the lower
molars
 1-2 mm clearance over the
lower molars.
 The inclined plane must
remain intact, however to
maintain the activation to
propel the mandible down
and forward.

 Vertical development slower than sagital
correction. Should therefore be made as
early as possible in treatment to allow
vertical development to proceed
concurrently with sagital correction.

Treatment of reduced overbite
Bite registration
Activation should not be more than 70% of the
total protrusive path
Yellow Projet or exactobite to register a 4mm
interincisl clearance with a 5mm clearance in
premolar region

Appliance designs
 All posterior teeth
must be in occlusal
contact with the
opposite bite blocks to
prevent the over
eruption
 Second molar eruption
should be controlled by
placing occlusal rests
or extending the upper
twin block distally
Pitfalls

Palatal spinner
 Application of intrusive orthopedic forces
may be used to help control vertical
growth

Intraoral elastics –
first used by Dr
Christine Mills

Magnetic force-
attracting or
repelling force on
the inclined plane

Treatment of Class II Div 2
Bite registration
Incisors in an edge to
edge occlusion
Cut off the handle of
exactobite later and
ask the patient to
close fully in an edge
to edge relation

• Sagital screws
• Three way screw
• Sagital and
transverse
Appliance designsAppliance designs

Springs

•Triple screw

Treatment of class III
Reverse Twin Blocks
Reverse the angulation of inclined planes to
advance the maxilla
Important that the patients condyles are
not displaced superiorly and or posteriorly
in the glenoid fossae at full occlusion

Bite registration
• Not the same degree
of activation because
of less scope for
distal displacement of
the mandible
• Downward and
backward forces
absorbed at the gonial
angle

Appliance design
• Three way
expansion screw

• Lip pads

• Reverse pull face
mask

Treatment during mixed dentition
Appliance modification
Limited retention in
deciduous teeth
Methods to improve
retention:
 Use of C clasp
 Bond composite on to
the buccal surface of
to create an undercut
or
 Bond C clasp directly to
deciduous molar

Synthetic crown
contours (Truax),
which are
bonded on buccal
surface to improve
retention
Grinding a concavity
for a ball clasp
Grinding retention
grooves into buccal
surface

Treatment of asymmetry
• Effective in correction of facial and dental
asymmetry
• Occlusal inclined plane ideal for unilateral
activation
Appliance design
Sagital screws
More frequent turning of screw on the side that
requires more distal movement
Use of magnets

Treatment of TMJ
• Indicated in-early click when condyle is
displaced distal to the disc.
Following objectives are attained:
– Immediate relief from pain
– Retraining of muscles to a healthy pattern &
relief of muscle spasm
– Recapturing of disc by downward & forward
posture of mandible
– Movement of teeth causing occlusal imbalance

• Sagital twin block –relieves
compression on the joint
• Important to maintain posterior
occlusal support at all times
• Full time commitment from patient

Advantages of Twin block
therapy
• Comfort – Patient wear twin blocks 24 hr per
day and eat comfortably.
• Aesthetics – Twin blocks can be designed with
no visible anterior wires without losing efficiency in
correction of arch relationships.
• Function - There is less interface with normal
functions because the mandible can move freely in
anterior and lateral excursion without being
restricted by a bulky one piece appliance.
1.

• Patient compliance – Twin blocks maybe fixed
to the teeth temporarily or permanently. Removable
twin blocks can be fixed in the mouth for the first
week or 10 days of treatment
• Facial appearance –the appearance is
noticeably improved when twin blocks are fitted.
Improvements in the facial balance are seen
progressively in the first three months of
treatment.

 Speech: Patients can learn to speak normally with
twin blocks. Do not distort speech by restricting
movements of the tongue, lips or mandible.
 Clinical management:
 Adjustment and activation is simple.
 The appliances are robust and not prone to
breakage.
 Chairside time is reduced in achieving major
orthopedic correction.

• Arch development:
• Twin blocks allow independent control of upper
and lower arch width.
• Appliance design is easily modified for
transverse and sagittal arch development.
• Mandibular repositioning:
Full time appliance wear consistently achieves
rapid mandibular repositioning that remains stable
out of retention.

 Vertical control: Twin blocks achieve excellent
control of the vertical dimension in treatment of
deep over bite and anterior open bite.
 Facial asymmetry: Asymmetrical activation
corrects facial and dental asymmetry in the growing
child.

 Safety : Twin blocks can be worn during sports
activities with the exception of swimming and
violent contact sports, when they may be removed
for safety.
 Efficiency: Twin blocks achieve more rapid
control of malocclusion compared to one piece
functional appliances because they are worn full
time.

 Age of treatment: Arch relationships can be
corrected from early childhood to adulthood.
However treatment is slower in adults but the
response is less predictable.
 Integration with fixed appliances:
Simultaneous skeletal correction and alignment.
During the support phase an easy transition can be
made to fixed appliances.

 Treatment of TMJ dysfunction: The twin
blocks may at times also be used as an effective
splint in treatment of patients who present TMJ
dysfunction due to displacement of the condyle
distal to the articular disc. Full time wear allows
the disc to be recaptured when disc reduction is
possible in early stage TMJ problems and at the
same time sagital,vertical and transverse arch
development proceeds to eliminate unfavourable
occlusal contacts.

Effects of Twin Block
Effects on hard tissue
Morris et al 1998 compared the skeletal and
dentoalveolar effects of three appliances :
Bass,Bionator,Twin Block.
Results showed :
– Greatest anterior movement of mandible in Twin
block group
– Greater restriction of the anterior movement of
point A
– Significant reduction in the inclination of upper
incisors to the maxillary plane.

McNamara et al 1999 studied the Rx effects
produced by Twin block and FR-II appliance compared
with an untreated control group.
1)Increase in mandibular length in
– Twin Block -3.0mm
– Frankel -1.9mm
2)Increase in lower anterior facial height was more in
Twin block group
3)More extensive dentoalveolar adaptation was
observed more with the tooth borne Twin block
appliance.

Mills & McCulloch 2000 evaluated the post
treatment changes after successful correction of
class II malocclusion with the Twin Block
appliance. Found that during the active Rx phase
increase in the mandibular length in:
Twin Block group -6.5 mm (14 months)
Control group- 2.3 mm (13 months)
In the post Rx phase
Twin Block grp-6.0 mm(36 months)
Control grp-6.7 mm (34 months)

Trenouth 2000 evaluated the Rx effects of
Twin Block as compared with the natural growth
changes. Following results were found:
• The twin block appliance reduced the class II
relation to a significant level
• Correction was found to be comparable to that of
Herbst appliance
• Some dentoalveolar compensation was also found
• Improved response was due to a 24 hr wear

Parkin et al 2001 compared the effects of 2
modifications of Twin block-
1. Twin block with labial bow
2. Twin block with high pull headgear and
torquing spurs
– Greater reduction in the ANB angle in 2nd
group
– Increased maxillary growth restriction
– Less retroclination of upper incisors due to
torquing spurs.

• O’Brien et al 2003 did a randomized clinical
trial to evaluate the effectiveness of Twin
block and Herbst appliance.
• Twin block with labial bow was used
• Changes in mandibular length :
Twin Block --3.46mm
Herbst ---3.36mm
Concluded : No difference in dental & skeletal
effects of Rx.

Effects on soft tissue
 Rapid changes in craniofacial musculature due to
altered muscle function
 As appliance is worn full time , even during eating,
rapid soft issue adaptation occurs.
 Significant facial changes within 2-3 weeks.
 Twin Block appliance increases the intermaxillary
space so difficult to form an anterior oral seal by
contact between the tongue and the lower lip, and
patients adopt a natural lip seal without instruction.
 Good lip seal is a functional necessity to prevent food
and liquid escaping from the mouth
So, no need for lip exercises.

• 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)
• Formation of a tension zone distal to the
condyle (Harvold)

Sampson et al(2000) evaluated whether the
protrusive muscles were responsible for
mandibular repositioning after Twin Block
therapy. Found that fatiguing these muscles did
not alter mandibular position in Twin block group
after 6 months of treatment
Hypothesized that Twin block therapy may have
shortened the protrusive muscles and
consequently slowed the increase of muscle force

• Conclusion
In the pursuit of ideals in orthodontics,
facial balance and harmony are of equal
importance to dental and occlusal
perfection.
We cannot afford to ignore the importance
of orthopedic techniques in achieving
these goals by growth guidance during the
formative years of facial and dental
development

• References
Twin Block Functional Therapy-Applications in
Dentofacial Orthopaedics.William J Clark
WJ Clark. The twin block technique. A functional
orthopedic appliance system.AJODO1988;93(1):1-
18
Illing et al. A prospective evaluation of
Bass,Bionator and Twin block appliances. Part I-
the hard tissues. EJO1998;20:501-516
Chintakanon et al. Effects of Twin block therapy on
protrusive muscle
functions.AJODO2000;118:392-6.

Baccetti et al. Treatment timing for Twin block
therapy.AJODO2000;118:159-70
Mills et al.Post treatment changes after successful correction
of class II malocclusion with Twin block appliance.
AJODO2000;118:24-33
Parkin et al.comparison of 2 modifications of the Twin block
appliance in matched class II samples.AJODO2001;119:572-
7.
Read et al .The intergation of functional and fixed appliance
treatment. J of Orthodontics2001;28:13-18
Singh et al.Localization of mandible changes in patients with
class II Division I maloclusions treated with twin block
appliances: Finite element scaling
analysis.AJODO2001;119:419-25
O’Brien et al.Effectiveness of Treatment for class II
malocclusion with the Herbst or Twin block appliances:A
randomised controlled trial.AJODO2003;124(2):128-137

Twin block

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