Midline Shift Guide: Causes, Diagnosis & Correction

MIDLINE SHIFT – CAUSES &
CORRECTION
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com


Contents
 Etiology
 Diagnosis



1.Clinical examination
2. Radiographic examination
3. Localization of asymmetry
4. Differential diagnosis of midline discrepancies
Treatment
1. Functional shift
2. Dental midline shift
3. Skeletal midline shift

ETIOLOGY
DENTAL:
 Unbalanced

loss of deciduous canine , 1st molar &
possibly deciduous 2nd molar; the age of
extraction ; the degree of crowding & the tooth
extracted. { The more anterior , the greater the
effect on the extent of midline shift }
 Unilateral retained primary incisor, canine or
molar.
 Hypodontia of an incisor or premolar.

 Supernumerary

incisor or premolar.

 Oligodontia.
 Lateral

mandibular displacement on closure
producing unilateral buccal segment
crossbite ( often secondary to digit or thumb
sucking habit ).
 Premature contact or tooth guidance leading
to functional shift.

SKELETAL
 Early

unilateral condylar fracture leading to
deficient growth on the affected side.
 Rheumatoid arthritis of TMJ.
 Hemifacial microsomia.
 Hemimandibular hypertrophy ( condylar
hyperplasia ). Most likely in females between age
of 15 – 20 yrs.
 Neurofibromatosis
 Cleft lip and cleft palate especially unilateral
clefts.

DIAGNOSIS
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Clinical examination:
Functional analysis.
Frontal analysis.
Vertical Occlusal Evaluations.
Transverse and Anteroposterior occlusal Evaluations.
Radiographic examination:
Lateral Cephalometric Radiograph
Panoramic Radiograph
Posteroanterior projection
Localization of the asymmetry.
Submento-vertex view
Differential diagnosis of midline discrepancies.

Functional Analysis


It consists of observing the behavior of the
midline of the mandible as the teeth are brought
together from rest position to habitual occlusion.



2 types can be differentiated in crossbite cases
with a lateral shift of the mandibular midline:


1.

LATEROOCCLUSION
In postural rest , the
midlines are coincident
and well centered.
The mandible slides
laterally from the rest
position to habitual
occlusion.
This is called lateroocclusion or pseudo
crossbite.
It is caused by tooth
guidance

2.

LATEROGNATHY
Cases in which midline
shift is present in both
occlusion and rest
position.
True asymmetrical
facial skeleton


Frontal analysis
 Patients

frontal view photograph is useful in this
analysis.
 Facial landmarks such as nose,chin,philtrum are
used as references for maxillary midline
positioning.
 Analysis of facial midline is difficult in patients
with deviated nasal septum.
 Arnett and Bregman noted that the philtrum is a
reliable midline structure is the basis for midline
assesment.
 Commonly used technique of placing a piece of
dental floss vertically through the facial midline to
relate it to dental midline can be deceiving.

 Coronal

view taken from above the patient
enhances the ability to detect any
deviations.


 Ventral

view
taken from the
lower aspect of
the mandible can
complement the
analysis



Most practical guide
to locate the facial
midline is an
imaginary line
extending through
soft tissue nasion
and midpoint of
philtrum in the
upper lip.



This line not only
locates the facial
midline but also
determines the
direction of midline

 Maxillary

midline should be coincident with
facial midline. If not possible , the midline
between central incisors should be strictly
vertical and parallel to facial midline.

Vertical occlusal evaluation
 The

presence of a canted occlusal plane can
be readily observed by asking the patient to
bite on a tongue blade to determine how it
relates to the interpupillary plane.
 Canted occlusal plane could be due to
unilateral increase in the vertical length of
the condyle and ramus, condylar
hyperplasia or hypoplasia.

Transverse and Anteroposterior
occlussal evaluations
 Asymmetry

in the buccolingual relationship ( e.g,
a unilateral posterior crossbite ) should be
carefully diagnosed whether skeletal , dental , or
functional.
 After functional analysis, if there is a mandibular
deviation from centric relation to centric
occlusion, the lower dental midline and chin point
should be compared with other midsagittal dental,
skeletal and soft tissue landmarks in the open ,
initial contact , and closed mandibular positions.

 Starting

position of any evaluation of
asymmetric occlusion is centric relation.
 Centric relation can be obtained by
manipulation of mandible . The use of
splints where tight musculature prevents
mandibular manipulation.


Therapeutic diagnosis
 When a functional shift acquired for a prolonged
period is difficult to detect clinically, an occlusal
splint may need to be constructed for the patient to
wear.
 The appliance allows the musculature to freely
guide the mandible to its proper relationship
without the distracting influence of the occlusal
interferences.
 Some tooth movement is accomplished such as
crossbite correction by expansion or other minor
tooth movement before final treatment plan is
established.
 After initial tooth movement it is easier to
establish a correct centric relation.

 Examination

of overall shape of the
maxillary and mandibular arches from an
occlusal view discloses both side-to-side
asymmetries and buccolingual angulation
of the teeth.
 This is important in presence of skeletal
constriction where expansion of dental units
may adversely influence stability of
correction.
 Also moving already tipped posterior teeth
bucally to correct cross bite will cause
greater chances of relapse.

 Arch

asymmetry leading to midline shift
could also be caused by rotation of the
whole maxilla or mandible.

 The

diagnosis of a rotary displacement of
the maxilla can be accurately evaluated by
mounting the dental casts on an anatomic
articulator using face bow transfer.


Radiographic examination
Lateral cephalometric radiograph
• It provides little useful information on
asymmetries in ramal height , mandibular
length , and gonial angle.
• Limited use due to superimposition of right
and left structures on each other.


 Panoramic

radiograph
 To determine the presence of any gross
pathologic conditions , missing or
supernumerary teeth.
 Shapes of ramus and condyles on both sides
can be grossly compared.
 Limitations due to geometric distortions and
superimposition of spine in anterior region
limits its usage to determine midline shift .


Posteroanterior projection
 It is the most useful projection to study the
right and left structures because they are
located at relatively equal distances from
the film and x-ray source.
 This results in lesser distortion as the effects
of unequal enlargement by diverging rays
are minimized.
 Comparison between sides is therefore
more accurate as the midlines of the face
and dentition can be accurately recorded.

•

A PA view can also
be used to determine
functional deviation
by taking views in
both centric occlusion
and rest position
{mouth open}

•

Localization of
asymmetry

Bisection approach
2. Tiangulation
approach
1.


Submento-vertex
view
 It

helps more
precisely diagnose
the nature of the
asymmetry ,
particularly if it is a
mandibular
problem.

 Computerized

axial
tomography
scans can be
used to reval
anatomic details
of asymmetry
leading to
midline shift

Differential diagnosis of midline
discrepancies
- Charles .J.Burstone

1.

Limitations of different methods of diagnosis of
midline discrepancies:
Construction of various horizontal planes using
a PA head film . From these planes drawing
perpendiculars through crista galli and other
midline points. But these planes may not be
parallel to each other and are often difficult to
establish. Any deviation in the horizontal plane
and the perpendicular drawn can lead to
erroneous dental midline.

2.

3.

Bisection approach used in PA view:
Even in most symmetric individuals , there are
differences in width between right and left
sides . Hence , bisecting the distance between 2
corresponding points can lead to an erroneous
midline.
Lundstorm found that using the median palatal
raphe as a guide to determine symmetry in
lateral direction is not reliable. This is because
of error in establishing a perpendicular to the
raphe and many raphes are not linear , but
display a curvature.

4.

Using a dental floss to establish a dental
midline by connecting points like glabella,
nasion, subnasale and pogonion . This can
lead to erroneous results because of
inaccuracy in identifying points and
parallax required in visualizing the points.


 More

reliastic approach is that the plane on
which dental midlines should be placed is
namely , the facial midline and the apical
base discrepancy.
 The center of the philtrum is a good guide
to the placement of the maxillary dental
midline. The “ V ” at the vermillion border
forms a good landmark that is easily
identified by orthodontists and patients.
 Another guide is to look at the distance
between the canine or 1st premolar and the
corner of mouth.

 If

the midline is properly positioned , the
patient will see the same amount of tooth
exposure on the right and left side.
 To determine skeletal asymmetry a tracing
is made of the PA head film.
 A treatment occlusal plane is established ,
and to the occlusal plane , the midlines of
the maxilla and mandible are evaluated.
 Use teeth as markers to evaluate midline.


 Points

are identified
approximately at the
center of the roots of the
upper incisors. This
median point of the roots
is called the apical base
point. Perpendiculars are
drawn to the occlusal
plane from these points
to evaluate if any apical
base midline
discrepancy exists.


 Apical

base discrepancy imply some type of
skeletal asymmetry.
Clinical examination of study casts
 Axial inclinations of teeth can be used to
determine if apical base discrepancy exists.
 When there is an apical base discrepancy ,
treatment becomes more difficult because
translation of teeth across the midline
required.
 During translation anchorage loss can
produce rotation of the arches.

 Incisor

apical base
discrepancy between
upper and lower arches.
 Arbitrary skeletal
midsagittal plane passes
through the lower
apical base midline and
lower incisal midline.
 Upper apical base point
is to the patients right.


 When

there is apical
base discrepancy
treatment becomes
more difficult because
translation of teeth
across the midline
required.
 With translation ,
anchorage loss can
produce skewing or
rotation of arches.

 Upper

dental midline to
the right of the lower
midline.

 Skeletal

problem with
apical base
discrepancy.


A

skeletal discrepancy exists.
 Equalizing axial inclinations would not help
the dental midline.
 The midlines become further apart as the
teeth are uprighted.


 Upper

dental midline
to the right without
an apical base
discrepancy.
Upper incisors are
tipped toward right.
 Dental midline shift
with no apical base
discrepancy.


 By

mentally uprighting the incisors to
equalize their axial inclinations , midlines
would correspond and a dental midline
discrepancy therefore exists.

 Mechanics

is simple as a single tipping
force required to correct the midline.


 Dental

midlines
correspond.

 Apical

base
discrepancy is
masked by
compensatory tipping
of the upper incisors
to the left side.


 No

discrepancy between
dental midlines but apical
base discrepancy exists.
 Mentally uprighting these
teeth would produce a
midline discrepancy.
 In such cases compensatory
axial inclinations should be
maintained at least in part to
ensure proper correction in
apical base discrepancies
for which no surgery is
required.

 The

upper midline is to the patients right. No
apical base discrepancy.
 Incisors tipped to patients left after extraction
therapy.No wires or appliances were used on
incisors as the incisors followed the canines and
were self – correcting by means of the transeptal
fibers

 The

patient shown in above slides has an
extreme skeletal discrepancy that requires
orthognathic surgery.
 In preparation for surgery, for the bones to
be positioned correctly , the compensatory
axial inclinations should be equalized so
that there is no asymmetry at the end of
treatment.
 If treatment of choice is nonsurgical, it is
necessary to maintain the asymmetry of
axial inclinations.

 Why

is this maintainance of compensatory
axial inclinations necessary ?
 It should be understood that the compensation in
the form of axial inclinations of teeth as in arch
width has resulted from muscular activity.
 It would be an error to correct the axial
inclinations by placing symmetric brackets with
straight wires that produce torque on the
individual teeth.
 Such mechanics would lead to an iatrogenic
crossbite


 Idealistically

, orthodontists have been taught that
the most desirable arch form is symmetric.
 In these patients , it is necessary to maintain the
asymmetry of the axial inclinations.
 If the malocclusion presented with class II on one
side and class III on the other side is of dental
origin than tooth movement required is movement
around arch.
 Not an en masse movement but movement of the
teeth around the arch like pearls on a chain.
 To reach this goal distal movement or extraction
required.

 If

such asymmetric posterior occlusion is
tried to correct by the use of classIII elastics
on one side and class II on the other side, or
diagonal / criss-cross elastics, the
movement is an en-masse movement in
which the arch is rotated around its center
of resistance.
 This movement is difficult to achieve and
can also lead to crossbite and lack of arch
harmony.

 Correction

of right
and left mesiodistal
occlusal differences
requires movement
around the arch.
 Arrows show
possible direction of
tooth movement.


 Crisscross

/
diagonal elastics
and or combined
classII and class
III elastics can
produce rotation
of the entire
arch ,which is not
desirable.

 OCCLUSAL

PLANE CONSIDERATIONS
 A surgically treated patient should have
occlusal plane , as evaluated from frontal
view, parallel to facial structures such as
eyes.
 In a non surgical patient , there may be cant
to the plane of occlusion relative to face.
 This cant is not easily altered because of
mechanical difficulties in intruding entire
posterior segments.

 It

is desirable to treat a canted occlusal
plane if a skeletal asymmetry is present and
very undesirable in a skeletally symmetric
patient.

 One

of the undesirable effects of the use of
an anterior crisscross elastic is that it can
cant the plane of occlusion.


 An

anterior diagonal elastic produces the
undesirable side effect of canting the occlusal
plane .
 Patient showing an unaesthetic canting produced .
 Mechanics to move the incisors around the arch
should have been used.

Treatment of midline shift
 FUNCTIONAL

MIDLINE SHIFTS
 Functional shifts caused by premature contact,
unilateral posterior crossbite can be eliminated by
minor occlusal adjustments, expansion ,etc.
 Severe deviations need orthodontic treatment to
align the teeth.
 Occlusal splints necessary to properly evaluate
presence of functional shift by eliminating
habitual posture in tight musculature by
deprogramming the musculature.

 Skeletal

asymmetry leading to functional
shifts need rapid maxillary expansion ,
orthognathic surgery and orthodontic
treatment.









Centric occlusion of a patient in early mixed dentition with
unilateral posterior right crossbite.
Dental midlines coincide.
Middle fig of the same patient in centric relation. Note the
shift in lower midline .posterior occlusion was cusp to
cusp buccolingually.
Expansion was done in maxillary arch and alignment of
the mandibular incisors with a lingual arch.
Maxillary hawley with posterior bite plate and lingual
flange to maintain correction

 The

activator can correct lower midline shifts or
deviations only if actual lateral translation of the
mandible itself exists.
 If midline abnormality is caused by tooth
migration , no asymmetric relationship exists
between the maxilla and mandible.
 Any attempt to correct this type of dental problem
could lead to iatrogenic asymmetry.
 Functional crossbites in the functional analysis
can be corrected by taking proper construction
bite.

Herrens activator in asymmetrical
class II div 1case
 In

asymmetrical distoclusion, class II div 1
subdivision, molar relation is neutral on one
side and distal on the other with midline
discrepancy between maxilla and mandible.
 Mandible is deviated towards distocclusion
side. Maxilla is coincident with facial
midline.

 Treatment

plan
 Shift of the mandible into proper neutral molar
relationship on both sides will cause the midlines
of both the dental arches correspond exactly.
 Transverse expansion of both the dental arches
required for optimal interdigitation of teeth.
 Expansion is asymmetrical.
 Mandibular arch requires expansion on side of
distoclusion only.
 Maxillary arch requires expansion on the side of
neutroclusion.

 Principle

of overcompensating activator
applied in both sagittal and transverse
dimensions.
CONSTRUCTION BITE.
 Midline discrepancy is overcompensated .
The extent of overcompensation equals the
original midline discrepancy.
 On the side of distoclusion the molar
relationship is corrected 3-4mm beyond
neutroclusion.

 For

asymmetrical dental arch expansion,
Zehles modification is used.
 Expansion screw is incorporated in both
upper and lower segments of activator along
median line.
 Asymmetric saw cut will split the appliance
into 2 halves , but will free only the
segment that requires expansion.
 Maxillary segment includes the buccal teeth
and canine but no incisors.

 Dorsal

from upper
expansion screw the
appliance is cut along
median line , where as
mesial from the screw ,
the appliance is cut
along a oblique line
down the slope of
palatal vault to contact
point between lateral
incisor and cuspid.
 Horizontal cut between
arrow head clasps and
occlusal surfaces of
mandibular posterior
teeth frees the segment
for expansion.

 Mandibular

segment
to be expanded is
horizontally cut
running between
occlusal surfaces of
upper and lower
segment.
 Vertical cut in median
plane near mandibular
expansion screw

Dental midline shifts
 Two

approaches are most commonly used
for dental midline correction:
1. Asymmetric extraction, so that the
midline shifts in the desired direction as the
extraction spaces are closed.( major shifts )
2. Asymmetric elastics usually classII or
class III and diagonal elastics. ( minor shifts
).

 Fletcher’s

approach in Begg technique:
 Fletcher considers shifts of more than 2mm as
major shifts and 2mm and under as minor and self
correcting.
Minor centre line shifts correction
In stage 2 , the application of intra and inter
maxillary elastics will complete closure on the
side to which centre has shifted before closure on
the opposite side.
Solution : Intra-maxillary elastic on the side
which closes first can be discontinued.
Intermaxillary traction is continued bilaterally as
before.

 In

the lower arch the continuation of classII
mechanics brings forward the posterior
teeth, which movement, through existence
of interproximal contacts on the side of
closure induces a corrective swing of the
centre line.


 Minor

centre lines displacement
discrepancy existing even if all residual
spaces closed in stage 2.
 One side elastic direction changed from
classII to class III.
 This is one situation in which it is not
imperative to have space on the side to
which a centre is to be moved.


Minor centre line discrepancy confined to the
upper arch.
 In such instance close all space on the side to
which centre has shifted whilst some space still
exists on the opposing side.
 The class III elastic will move the upper posteriors
on that side mesially through interproximal
contacts, the teeth of the labial segment also.

 To

assist balance of anchorage while
correcting midline further auxiliaries can be
added.
 In case of unilateral class III elastic , an
uprighting spring for distal movement of
cuspid root on the same side as the class III
elastic can be given.
 The uprighting spring on upper cuspid is to
be allowed some mesial movement of
crown of that tooth , the intermaxillary hook
must be placed slightly mesially to the
cuspid bracket.

 Lower

cuspid on the same side can be
supported against retraction by placement
of an uprighting spring. This prevents
dislocation of lower centre line. This is
logical in case of only single upper arch
midline correction.


 Diagonal

elastic
 In cases where there is
mild centre line
discrepancy due to little
swing of upper and lower
dental arches .
 Inaccurate molar tube
alignment and failure to
correlate archwires is the
possible cause of such
swing.
 Anterior diagonal elastic
worn between upper and
lower cuspid hooks in a
desired direction.

 Major

centre line shifts:
 It will not be corrected by space closure
procedures of stage 2.
 Teeth of offending segment need
independent and individual movement, 1 or
2 at a time.
 The mechanics for the purpose will
temporary interrupt the general treatment
progress, which can be resumed after the
centre line problem is eliminated.

 On

the side to which centre is to be moved, cuspid
can be retracted with very light pressure using
very light intramaxillary elastic worn between
molar hook and tag of cuspid lock pin.
 Light pressure and free tilting of cuspid reduces
the chances of anchorage loss.

 The

opposite cuspid is equipped with an
uprighting spring for distal movement of apex,
which reciprocally drives the crown mesially.
 Once the 1st cuspid is retracted it is tied back with
central and lateral of same side moved into contact
by use of an elastic thread between tag of central
lock pin and intermaxillary hook.

 The

intended movement is not obstructed by
intermaxillary hooks and neither of these has been
tied to arch wire.
 The distal ends of wire through the buccal tube
must be turned or cinched to prevent arch wire
swing to left or right.
 An alternative system of correction is use of active
coil springs.

Midline correction in Pre-adjusted
technique
Discrepancy between upper and lower dental
midlines is most noticeable at the end of treatment.
It is also at this time this problem is most difficult
to correct.
Tipping is major type of tooth movement that can
be used to correct the midlines at the finishing
stage.
The range of correction for each arch at this stage
is approximately 1mm on each side.

 Use

of asymmetric intrusion arch
 Asymmetric intrusion arch placed to correct the
incisal cant leading to midline shift.
 0.0175 x 0.025 nickel titanium arch is tied to the
distal of the upper left lateral incisor.
 A continuous 0.0175 x 0.025 stainless steel
segment from upper right central to upper left
lateral incisor is tied into the brackets of these
teeth.

 Force

system produces
an intrusive force and
moment at center of
resistance of the
anterior segment.
 Correction seen 1
month later showing
correction of cant and
improvement in
midline discrepancy.


 Moment

produced by 2
cantilevers with the
same amount of force
in opposite directions.

 Moment

produced by a
cantilever with a single
couple tied to an
auxillary tube in the
anterior segment.

A

transpalatal arch is
used in both situations
for a solid anchor unit
to minimize side
effects.

Problems involving only
midline discrepancy and no
incisal cant rely mainly on
1.A combination of class II
elastics on one side and class
III on the other.
2. Diagonal / crisscross
elastics.
Side effects of long term
elastic use in this manner.
1. In the vertical direction
along x-axis , canting of
occlusal planes as a result
of anterior crisscross
elastic.




 In

the transverse
direction , rotation of
the dental arches
around the y-axis with
the use of class II
/class III elastics may
result in a crossbite
tendency on one
buccal segment and
Brodie bite tendency
on the other.


 Final

method to correct dental midline in
the finishing stages is the use of a cantilever
with the active force along the x-axis.
 The upper anteriors are treated as a segment
and a force is applied at bracket level of this
segment.
 The anchor unit is made of molars and
premolars. A palatal arch is used to prevent
rotational moment and lingual force
( mesial in ) on the anchor unit where the
cantilever (couple side ) is inserted.

 Treatment

of skeletal asymmetry leading
to midline shift
 In preadolescent children, 2 major problems
cause severe asymmetry leading to midline
shift : hemifacial microsomia and growth
deficiency secondary to trauma , especially
early fracture of the condylar process of
mandible.
 Hemifacial microsomia
missing soft
tissue
deficient growth potential

 Condylar

fracture
deficient growth
of mandible
distortion of alveolar
peocess
maxilla affected.
 Modify the expression of growth
To allow the child grow out of asymmetry


 Growth

modification with asymmetric
functional appliances
 Translation of condyle is important for
mandibular growth.
 Minimum 20 mm opening required.
 Growth possible on deficient side even if
mandibular deviates on opening but some
translation does occur.

 Construction

bite
 It is important to bring mandible forward to match
the midline and also open vertically more on
affected side.
 Wax soft on unaffected side
+
Wax hard on affected side
Ramus torqued downwards on the short side.
For more transverse expansion modification in the
appliance not the bite.

Hybrid appliance
 Frankel

type buccal shield on affected side to
create transverse expansion.
 Bite block to stabilize occlusion on the more
normal side and inhibit tooth eruption there.
 Lingual shield on the side where vertical
development desired to keep tongue away from
between the teeth on affected side .
 Lingual pad to posture the mandible forward and
to more normal side.

Surgical procedures :
 Surgically

assisted rapid palatal expansion.
 Distraction osteogenesis.
 Vertical ramus osteotomy.
 Sagittal split osteotomy.


Esthetically acceptable midline
deviations
 Kokich

et al recently reported an interesting
interaction between the maxillary central incisor
midline deviations and crown angulation.
 His survey showed that even a 4mm maxillary
midline deviation was not detected by dentists and
lay people as long as dental midline is parallel to
the facial midline.
 On the other hand a slanted dental midline with
canted incisal crown angulation ( 2mm deviation )
as easily noticeably unattractive.

Midline Shift Guide: Causes, Diagnosis & Correction

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