midline shift / orthodontic courses by indian dental academy

MIDLINE SHIFT – CAUSES & MIDLINE SHIFT – CAUSES & CORRECTIONCORRECTION

INDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com

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Contents 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


ETIOLOGYETIOLOGYDENTAL:

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.


DIAGNOSISDIAGNOSIS 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.


FunctionalFunctional AnalysisAnalysis

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. LATERO-OCCLUSION

In postural rest , the midlines are coincident and well centered.

The mandible slides laterally from the rest position to habitual occlusion.

This is called latero-occlusion 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 analysisFrontal 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 evaluationVertical 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 Transverse and Anteroposterior occlussal evaluationsocclussal 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 examinationRadiographic 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 radiographTo 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 projectionIt 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

1. Bisection approach2. Tiangulation

approachwww.indiandentalacademy.com

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 Differential diagnosis of midline discrepanciesdiscrepancies

- - Charles .J.BurstoneCharles .J.Burstone Limitations of different methods of diagnosis of

midline discrepancies:1. 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. 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.

3. 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 castsAxial 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 CONSIDERATIONSA 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 shiftTreatment 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 correctionwww.indiandentalacademy.com

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 Herrens activator in asymmetrical class II div 1caseclass 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 shiftsDental midline shiftsTwo 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 Midline correction in Pre-adjusted techniquetechnique

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 applianceHybrid 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 :Surgical procedures :

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


Esthetically acceptable midline Esthetically acceptable midline deviationsdeviations

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.



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