planning orthognathic surgery 20103501

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Workshop

Planning Orthognathic Surgery2010

Kamal F. Busaidy, BDS, FDSRCS,

Associate Professor,

Dept. Oral and Maxillofacial Surgery.

Overview of the Workshop• Setting goals

• Clinical evaluation

• Radiographic evaluation

• Cephalometric tracing and analysis

• Photographs

• Mounting of models

• Formulating the surgical plan

• Performing prediction tracings (The VTO)

• Model surgery and constructing splints

• The TMJ and orthognathic Surgery

• Planning for stability

• Pitfalls in planning and execution

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• Primary references:

– Modern Practice in Orthognathic Reconstructive Surgery (Edited by William H. Bell)

– Essentials of Orthognathic Surgery (Johan Reyneke)

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Goals in Orthognathic Surgery

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The Key to Successful Planning

• Find out where you are

• Determine your destination

• Plan your journey

• Allow for contingencies

• Communicate with the team

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What problem are we addressing?

• Inability to incise or chew

• Speech impediment

• Oral health (dental, periodontal)

• Poor esthetics• Facial soft tissue

• Facial hard tissue

• Dental

• OSA

• TMJ

• Primary versus secondary growth disturbance

• Psychological issues

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What is success?

• In the eyes of the patient success is measured by

– Addressing the original complaint

– Absence of adverse outcomes

– Stability of result

Assuming there is no underlying psychiatric issue!

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Clinical Evaluation of the Orthognathic Surgery Patient

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The Team Approach

• Orthodontist

• OMS

• General Dentist

• ENT

• Plastic surgeon

• Periodontist

• Prosthodontist

• Psychiatrist

• Pulmonologist/Sleep physician

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OMFS Evaluation

• Stage 1 Initial evaluation/Feasibility

• Stage 2 Pre surgical evaluation

• Stage 3 Post surgical evaluation

(Long term)

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Ortho Treatment

Finalization

Ortho 2nd Evaluation

OMFS: 2nd Evaluation

ENT / PRS etc

Referring Practitioner

OMFS:1st Evaluation Ortho:1st Evaluation

Ortho Treatment

OMFS: Surgery

OMFS: 3rd Evaluation Ortho 3rd Evaluation Perio / Pros etc

Coordination of

Care

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Patient Evaluation

1. Complaint + History

2. Health Status

3. Assessment of Facial Esthetics

4. Routine Dental Examination

5. Orthodontic Evaluation

6. Cephalometric Evaluation

7. Photos

8. Dental casts

* Psychological Assessment

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Facial Esthetics

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1/3

1/3

1/3

Facial Esthetics

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ULL 21mm (+/- 2 mm) Men

ULL 19 mm (+/-2 mm) Women

Incisor Show at Rest 2 - 4 mm

Note lip-tooth relationships at

rest and when active! 1/3

2/3

Facial Esthetics

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Facial Esthetics

• Nasofacial Angle 30o- 40

o

• Nasomental Angle 120o

-132o

• Mentocervical Line 80o

– 95o

to Vertical

• Mentocervical Line 110o

– 120o

to Nasomental Line

• Nasolabial Angle 100o

- 110o

Powell and Humphreys: Proportions of the

Aesthetic Face. New York, Thieme-Stratton,

1984

100

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http://dcealumni.com/gallery/aishwarya-rai-wallpapers/aishwarya_rai_wallpaper_6

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Dental Esthetics

Tooth Location (Midline)

Tooth Size

Tooth Shape

Tooth Number

Tooth Orientation

Emergence

Tooth Color

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Arch Form

Occlusal Plane

Occlusal Level

Overbite

Overjet

Buccal Corridor

Surrounding Tissues

Dental Esthetics

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Case Example

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Case Example

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12 mm 9 mm

SMILE REST

Case Example

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Case Example

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Case Example

Class II Skeletal Pattern

(*mandible)

Increased incisal show

No increased LFH!

Close bite (?traumatic)

Maxillary cant

Ocular dystopia

Unstable occlusion. Poor

bridges (shape/color)

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Radiographic Evaluation of the Orthognathic Surgery Patient

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Radiographs

• Lateral Cephalogram

• Panoramic Dental Xray

• Periapicals

• SMV

• PA Cephalogram

• Others (MRI/CT/Bone scan/Wrist Films)

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MRI/CT/Bone scan/Wrist Films

• TMJ meniscus position

• OSA

• Complex craniofacial deformities

• Local growth disturbance (Condylar Hyperplasia)

• Systemic growth disturbance (Excess growth

hormone)

• Autoimmune arthritis

• Assessment of completion of growth

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PA Cephalogram• Symmetry

(particularly gonial angles, symphysis)

• Position of proximal segment post op

• Position of internal fixation post op

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SMV• Thickness of mandible (Superseded by CBCT!)

• Flaring of rami (vertical ramus osteotomy)

• Position of proximal segment post op

• Position of internal fixation post op

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Periapicals

• Periodontal bone loss

• Proximity of apices (multi-piece segments)

• Periodontal bone loss post op

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Panoramic Radiograph

• Third Molars

• Inferior alveolar nerve position

• Intraosseus pathology (best screening tool)

• Position of fixation post op

• Position of condylar head post op

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Lateral Cephalogram

• Skeletal proportions

• Growth prediction

• Cessation of growth

• Soft tissue measurements

• Planning (primary tool)

• Position of fixation post op

• Baseline post op status***

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Cone Beam CT

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What is wrong with this Lateral Ceph?

Lateral Cephalogram

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Nasion

Pt point

Orbitale

ANS

A Point

Pm Point

Xi Point

Basion

Porion

Pogonion

Lateral Cephalogram

Gonion

PNS

GnathionMenton

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• Ba- Basion: the lowest point on the anterior margin of the foramen magnum, at the base of the clivus

• Po-Porion: the midpoint of the upper contour of the external auditory canal (anatomic porion); or, the midpoint of the upper contour of the metal ear rod of the cephalometer (machine porion)

• Pt- the point at about 11 0’clock on the outline of the pterygomaxillary fissure adjacent to the foramen rotundum

• Or-Orbitale: the lowest point on the inferior margin of the orbit• ANS-anterior nasal spine: the tip of the anterior nasal spine• Point A: the innermost point on the contour of the premaxilla between the anterior nasal

spine and the incisor tooth• Pog-Pogonion: the most anterior point on the contour of the chin• Pm-Suprapogonion: the point where the anterior curvature of the mandible changes from

concave to convex• Me- Menton: the most inferior point on the mandibular symphysis • Na-Nasion: the anterior point of the intersection between the nasal and frontal bones• Go- Gonion: the midpoint of the contour connecting the ramus to the body of the mandible• Gn-Gnathion: the most outward and everted point on the mandibular symphysis• PNS-Posterior nasal spine: the tip of the posterior nasal spine of the palatine bone, at the

junction of the hard and soft palate• Xi- The point in the middle of the ramus, approximately in line with the occlusal plane• FH-Frankfort Plane: the horizontal reference plane in the heads natural position extending

from the porion to orbitale

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Hands-on Exercise

•Lateral Ceph

•Pencil

•Protractor/Ruler

Identify the points marked in the previous slides, (then trace the outlines of the skeleton as described), and start measuring the pertinent angles using Rickett’s analysis.

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Pt pointOrbitale

ANS

A Point

Pm Point

Xi Point

Basion

Porion

Pogonion

Lateral Cephalogram

Gonion

PNS

GnathionMenton

NasionMARK THESE POINTS ON YOUR CEPHALOGRAM

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Facial Depth (Angle) 87o

+/- 3

Pogonion

Nasion

PorionOrbitale

87o

Frankfort Horizontal

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Mandibular Plane Angle: 26o

+/- 4

Pogonion

Gonion

Menton

26o

Mandibular Plane

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Facial Axis: 90o

+/- 3

90o

Basion

Skull Base

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Maxillary Depth: 90o

+/- 3

90o

A point

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Convexity at point A: 2mm +/- 2 mm

A point

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Lower incisor to APog: 1mm +/- 2 mm

A point

Pogonion

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Xi

Xi Point and Functional Occlusal Plane

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Xi

Lower Face Height : 47o

+/- 4

ANS

Pm

Point

47o

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Interincisal Angle: 130o

130o +/-6

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Other Analyses

90o+/-7

112o+/-6Approximately

Parallel32

o+/-5

112o+/-6

130o +/-6

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Evaluation of Soft Tissue on Lateral Ceph

130o

30-40o

UFH:

LFH:

85-95o120-132

o

100-110o

CHECK THAT THE PATIENT IS IN REPOSE, WHICH THIS PATIENT IS NOT KB 2010

Clinical Photography

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Clinical Photographs

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Clinical Photographs

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Mounting the Case

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• Take the impressions

• Interocclusal records

• Face bow record

• Mount the casts

• Measuring in 3 planes of space

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Impressions

• 2 sets of upper impressions

• 2 sets of lower impressions

• Block out brackets with wax to prevent distortion of the impression

• Avoid bubbles/voids in pour-up

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Interocclusal Record

• Record occlusion in centric relation (Potential

disparity with centric relation when asleep)

• Avoid displacement from premature contacts (Wax is not ideal for occlusal records)

• Alternatives:• Record occlusal relationship supine

• Deprogramming

• Short general anesthetic!

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Facebow Recording

• Find Frankfort Horizontal (Easier said than done!)

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A Common Reference Plane

The Frankfort plane identified

clinically should correlate with

the Frankfort plane on the

articulator AND the lateral Ceph KB 2010

True Frankfort versus Clinical

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Clinical FrankfortProjected Frankfort

Radiographic Frankfort

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Identifying True Frankfort

J Oral Maxillofac Surg. 2001 Jun;59(6):635-40; discussion

640-1.

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Identifying True Frankfort

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A Common Reference Plane

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Facebow Recording

• Find Frankfort Horizontal (Easier said than done!)

• Ensure the facebow is centered on the face

• Lock down the hinges to prevent distortion of record

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Midlines and occlusal

angulations/cants are consistent with

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A B

Mount Two Sets of Casts

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Erickson Model Block and Platform

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3 Planes of Measurement

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RIGHT SIDE DOWN!KB 2010


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Formulating the Surgical Plan and the VTO

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Is the position and

form of the chin

acceptable?

Is the position of the anterior

maxilla acceptable?

When I hand articulate the models can I get a good

occlusion?

Segmental maxilla /

(Segmental mandible) /

More Ortho

Proceed to Next

Maxillary osteotomyProceed to Next

Genioplasty Finished

No Yes

No Yes

Mandible

acceptable?

No No. There

is an AOBMaxillary osteotomy +/-

Mandibular osteotomyMandibular

osteotomy

No Yes

Yes

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Prediction Tracing: Exercise One

Visualized Treatment Objective (VTO) for Mandibular Sagittal Split Osteotomy

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Exercise 1: VTO for BSSO Setback

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Trace the cephalogram and indicate in the mandible where the osteotomy will be placed

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Take a new piece of tracing paper and trace over the original: only trace structures in the maxilla and above.

Trace the soft tissues of the nose and upper lip.

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Reposition the prediction tracing on the original such that the maxillary teeth of the prediction tracing meet the mandibular teeth on the original tracing in class 1

Trace the mandible ANTERIOR to the osteotomy line, including the teeth.

Trace the soft tissues of the lower lip and chin.

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Reposition the prediction tracing such that the skull bases and orbits coincide.

Rotate the prediction tracing around the axis of rotation in the condylar head until the inferior border of the proximal mandibular segment seems aligned with the inferior border of the distal segment.

Trace the proximal mandibular segment. Note the degree of overlap. This corresponds to the amount of mandibular setback.

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Exercise 2: VTO for Le Fort 1

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Prediction Tracing: Exercise Two

Visualized Treatment Objective (VTO) for Maxillary Le Fort 1 Osteotomy

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Trace the cephalogram and indicate in the maxilla where the osteotomy will be placed

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Take a second piece of tracing paper and trace again all the structures that will NOT move during the osteotomy (i.e. above the osteotomy cut).

Stop tracing the soft tissue of the nose at the supra-tip break.

Mark a horizontal line that corresponds to the level of desired maxillary incisal vertical height

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Rotate the top sheet around the axis of rotation in the condylar head until the tip of the lower incisors protrude above the horizontal line by 2 to 3 mm. This represents the overbite.

Trace the entire mandible and the soft tissue of the neck and chin up to the labiomental fold.

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Reposition the top tracing over the original such that the maxillary dentition occludes with the new mandibular dentition in class 1. Pay particular attention to the incisal relationship.

Trace the maxilla and the maxillary teeth.

Trace the remainder of the nose and upper lip, then complete the tracing of the lower lip.

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Reorient the prediction tracing on the original such that the skull bases and orbits coincide.

Examine the degree of movement of the maxilla in 2 planes. Make a note of these measurements.

Examine the degree of autorotation of the mandible.

Examine also the effect on the chin prominence and assess whether a genioplasty is required.

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Exercise 3: VTO for 2-Jaw Surgery

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Prediction Tracing: Exercise Three

Visualized Treatment Objective (VTO) for Bimaxillary Osteotomy

(Le Fort 1 and BSSO)

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Trace the cephalogram and indicate in the maxilla AND mandible where the osteotomies will be placed

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Take a new sheet of tracing paper and trace over the original: only trace structures that will NOT move in either the maxillary or mandibular osteotomies.

Stop tracing the soft tissue of the nose at the supra-tip break

Indicate the desired vertical height of the incisal edges of the maxillary teeth with a horizontal line.

Indicate with a vertical line the desired AP position of the incisal edge of the maxillary incisors

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Rotate the top sheet around the axis of rotation in the condylar head until the tip of the lower incisors protrude above the horizontal line by 2 to 3 mm. This represents the overbite.

Trace the mandible.

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Reposition the prediction tracing such that the maxillary incisal edge rests in the indicated ideal position. Align the maxillary occlusal plane with the occlusal plane of the mandibular teeth on the prediction tracing. (Note that the maxillary teeth NEED NOT be in class 1 occlusion with the mandibular teeth at this point!)

Trace the maxilla and the maxillary teeth.

Trace the remainder of the nose and the upper lip.

The degree of reverse overjet indicates the amount the mandible must be set back.

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Your prediction tracing should look like this now. Label this tracing “IPT” (Intermediate Prediction Tracing)

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Take a new sheet of tracing paper and trace over all hard structures on the first prediction tracing except the mandible. It is recommended that you use a different color pencil.

Trace soft tissues down to and including the upper lip.

Label this tracing “FPT” (Final Prediction Tracing)

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Place the Final prediction tracing (FPT) over the Intermediate Prediction Tracing (IPT) in such a way that the maxillary teeth on the FPT meet the mandibular teeth on the IPT in class 1.

Trace the mandible ANTERIOR to the mandibular osteotomy line. Trace the mandibular teeth.

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Reposition the FPT on the IPT such that the skull bases and orbits coincide.

Rotate the FPT around an axis of rotation on the condylar head until the inferior border of the proximal mandibular segment aligns with the inferior border of the distal mandibular segment.

Trace the proximal mandibular segment.

The overlap indicates the amount of mandibular setback.

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Place the FPT on the original tracing of the cephalogramsuch that the lower incisor and symphysis of both coincide. Estimate the predicted chin and lower lip shape.

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Your FPT should now look like this.

Measure the vertical and AP predicted movement of the maxilla and mandible and record the measurements.

Note that the post-surgical occlusal plane in this example was determined by the occlusal plane of the mandible after rotation; however the occlusal plane can be adjusted (within limits) to fit the needs of the individual case.

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Soft Tissue PredictionsMandible

• Advancement

– Chin 100%

– Lower Lip 70%

• Setback

– Chin 90%

– Lower Lip 90%

– Upper Lip 20%

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Soft Tissue PredictionsMaxilla

• Advancement– Nasal Tip 30%

– Upper Lip 50% at incisor level (70% - 90% with VY closure)

– Upper lip shortens 1-2 mm

• Setback– Upper Vermillion 50% - 60% (Less with VY)

– Subnasale 30% (Less with VY)

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Soft Tissue PredictionsMaxilla

• Inferior

– Lip length increases 10-15%

• Superior

– Subnasale 20% up

– Nasal Tip 20% up

– Lip 10% up (Less if VY)

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Predicting Chin PositionHorizontal distance to 0-Meridian

0-Meridian:Perpendicular to FH from soft tissue forehead. Chin should be 0-3mm ahead of this line

0-Meridian

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Predicting Chin PositionFH to Z Line

Z Line:Tangent to most protrusive lip and soft tissue chin

78o

+/- 10

Z Line

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Predicting Chin PositionH Line to NB

H Line:Tangent to most protrusive lip and soft tissue chin

8o

+/- 2

H Line

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Review of Process in Planning. Start with the Maxilla

1. Predict ideal A.P. position of maxilla form lateral ceph

2. Predict ideal superior/inferior position of anterior maxilla from clinical incisal show

3. Set occlusal plane: Use Xi point, Frankfort Horizontal and mandibular occlusal plane as primary guides

4. Find required lateral repositioning of maxilla from clinical assessment of midlines

5. Assess cant from clinical measurement and mounted casts

6. Assess maxillary arch width from models

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Detailed Process in Planning (continued)

7. Trace the new maxilla and mandible positions (VTO) as we did in the exercises.

8. Re-analyze using Ricketts to compare the VTO to cephalometric norms.

9. Record the intended changes in vertical, transverse, AP and arch width dimensions of the posterior and anterior maxilla and the intended amount of set back/push forward at the mandibular osteotomy.

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Detailed Process in Planning (Step Back)

10. Are the movement planned so far reasonable. If not start again and redistribute the movements between the maxilla and mandible, or change the plan entirely, (SARPE or more orthodontics)

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Detailed Process in Planning (Chin and Profile)

11. Assess the projected soft tissue profile, particularly the chin

12. Proceed to model surgery

13. Verify on the models that the movements are surgically feasible

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Model Surgery and Splint Construction

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Model Surgery1. Calculate the new measurements that would give the

desired new maxillary cast position (AP, Vertical and Transverse).

2. Segmentalize the upper segment if necessary and make occlusal adjustments to give best intercuspation

3. Mount maxillary model to new position using the Erickson model block and platform

4. Mount mandibular model to new position (in occlusion with upper model) on the articulator

5. Verify movements correlate with intention

6. Note magnitude of movements in all planes

7. Verify movements are surgically feasible

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Adjust Occlusal Surfaces

Segment maxillary

cast at this stage to

achieve best

occlusion if

performing multi-

piece Le Fort 1

Record where occlusal

adjustments are made so

that they can be duplicated

intraoperatively

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Remount Upper Cast to Desired Position in Space

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Maxillary Post op cast with Mandibular Post op cast

Final splint

ONLY

CONSTRUCT FINAL

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CONSTRUCT INTERMEDIATE

SPLINT SECOND

Maxillary Post op cast with Mandibular Pre op cast

Final splint AND

Intermediate

splint

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Final Splint

Final Splint capable of being wired into maxillary

dentition to support maxillary fixation

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Intermediate Splint

Intermediate Splint should locate positively in Final Splint

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Summary

• Take the records meticulously

• Verify that the “A” casts match the “B” casts

• Verify that the mounted casts match the clinical picture

• Perform the model surgery on one set of casts

• Construct the splints in correct sequence for the planned surgery.

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TMJ Considerations in Orthognathic Surgery

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The “Normal” TMJ

• What does a normal TMJ look like and how do we identify it?

– Clinically

– Radiographically

– MRI

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Goals of Orthognathic Surgery as Relate to the TMJ

• Restore/maintain “normal” range of opening

• Eliminate/avoid joint pain and noises

• Achieve stable condyle and meniscus position in fossa when teeth are in centric occlusion

• Where is the ideal

location for the condyle?

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Condylar Malposition

• Condylar sag:

Inferior displacement of the condylar head within the glenoid fossa

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Central Condylar Sag

• Condyle is positioned inferiorly in the fossa

• No contact between condylar head and articular fossa in centric occlusion

• Immediate malocclusion on release of fixation (assuming no hemarthrosis or joint edema is present)

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Central Condylar Sag

Peripheral Condylar Sag

• Contact between condylar head and articular fossa may support the inferiorly positioned condylar head

• Immediate or late relapse

• Late relapse associated with condylar resorption

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Peripheral Condylar Sag

Condylar Resorption

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Other Causes of Condylar Malposition

• Posterior positioning of condyle is associated with increased risk of post-operative symptoms of popping and locking.

• Limit that the condyle may be posteriorly positioned increased by – Supine, paralyzed state

– Improper surgical technique

– Condylar sag

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Other Causes of Condylar Malposition

• Uneven contacts between the proximal and distal segments may cause the condyle to become laterally or medially displaced when fixation is applied

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Minimizing Condylar Malposition

• Avoid creating intrarticular edema or hemarthrosis

– Support during split

– Support during mobilization

– Avoid rotating the condyle around its long axis

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• Avoid bad splits; they complicate condylar positioning!

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• Ensure adequate stripping of medial pterygoid to eliminate interference to distal movement of distal segment.

• Reduce bony interferences, especially on mandibular setback.

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• Eliminate uneven contact between osteotomized segments that prevent passive, even and stableapposition

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• Gentle use of clamps to hold segments whilst placing fixation

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• Use shims of bone to eliminate inter-segmental gaps

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• Avoid lag screw fixation

• Positional screws are fine

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• Plates can be adapted in order to provide passive fixation. More difficult to achieve with positional screws.

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• Positioning the condyle prior to fixation

– Direction of force

– Magnitude of force

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• Ensure adequate bone removal at posterior of maxilla in Le Fort 1 osteotomy

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• Avoid heavy post-op elastics as the effect on the occlusion may be more temporary than you think!

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Idiopathic Condylar Resorption

• Progressive alteration of the condylar shape with decreased mass bilaterally, intemporomandibular joints that previously exhibited normal growth patterns

• AICR (Adolescent Internal Condylar Resorption)

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Risk Factors for ICR

• Female

• Age 15-30

• Pre-op TMJ disease

• Mandibular hypoplasia

• High mandibular plane angle

• Small posterior face height

• Posterior inclination of condylar neck

• Large mandibular advancement

• Counterclockwise rotation

• IMF

• Posterior repositioning of condylar head in fossa

• Increase in ramus length

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Treatment and Prognosis

• Re-osteotomy alone has 50-100% failure rate

• Stabilization of occlusion with occlusal splint prior tore-osteotomy has similar failure rate

• Orthodontic occlusal compensation and stabilization achievable in some

• Advanced cases require condylectomy and joint reconstruction (alloplastic or costochondral)

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Effect of Orthognathic Surgery on the Symptomatic TMJ Patient

• Lack of consistency in terminology used to categorize TMJ disease

• Populations are often poorly described

• Outcomes are poorly defined

• Lack of information on the post-op condylar position in patients studied

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Concomitant TMJ and Orthognathic Surgery for Symptomatic TMJ Patients

• Pts without symptoms from TMJ pathology can become symptomatic after orthognathic surgery

• Pts with anterior disc displacement prior to BSSO will most likely not improve, and may get worse

• IVRO in a pt with ADD improves disc-condyle relationships and pain

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Concomitant TMJ and Orthognathic Surgery for Symptomatic TMJ Patients

• Goncalves et al (JOMS April 2008). Retrospective cohort study, looking at 51 pts with pre-op TMJ symptoms and compared concomitant TMJ + orthognathic surgery to orthognathic surgery alone. Demonstrated improved stability and relief of symptoms in the former group after 31 months follow up

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Summary

• Perform a baseline TMJ exam on every patient

• Avoid intra-operative trauma to the TMJ that might cause intra-articular edema

• Take care with positioning and fixation of the segments

• Orthognathic surgery may induce symptoms from the TMJ

• Consider treating the TMJ first if disease is present

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Stability Issues

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Instability

• Early: From the time of surgery up to week 8

• Late: After 8 weeks

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Long Term Stability in MaxillaryOsteotomies

• Impaction

• Setback

• Advancement

• Downgraft

• Expansion (**SARPE)

• Advancement with downgraft

MORE

STABLE

LESS

STABLE

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Long Term Stability in Mandibular Osteotomies

• Advancement***

(Proportional to advancement)

• Setback

***Idiopathic Condylar Resorption

MORE

STABLE

LESS

STABLE

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Limiting Long Term Instability

• Bone grafting especially when downgrafting a maxilla by 5mm or more

• Conservative moves, not ambitious. (*Cleft cases)

• Overcorrection especially when doing a mandibular setback (easier to correct a relapsing class II with ortho than a relapsing class III)

• ? Rigid fixation versus IMF. ? Positional Screws versus miniplates

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Pitfalls in Planning and Execution

• Leaving appliance activated at time of surgery

• Inadequate strength of arch wire at surgery

• Inadequate incisor decompensation (leads to inappropriate incisal relationship)

• Inaccurate pre-op occlusal record (condylar position)

• Inadequate root divergence before segmentalizing

• Hasty split (fracture or nerve damage)

• Occlusal splint too thick

• Poor condylar position during application of fixation

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Pitfalls in Planning and Execution (continued)

• Compromising blood supply– Gingivae during flap for segmental osteotomy

– Over-ambitious advancement Le Fort 1 level

• Tear of palatal mucosa during segmentalization

• Condylar sag (very difficult to plan for)

• Failure to check condylar position post-op

• Setback of mandible in presence of a flat chin-throat angle

• Planning for >6mm posterior maxillary impaction

• Weak brackets/hooks at time of surgery

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planning orthognathic surgery 20103501

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