The Glasgow Orthognathic Model Surgery Technique

©West of Scotland Regional Maxillofacial Prosthetics and Technology Service

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Table of contents.

Page 1. CoverPage 2. Table of ContentsPage 3. Articulator Selection.Page 4. Articulator Selection cont.Page 5. Face Bow Selection.Page 6. Face Bow Selection cont.Page 7. Face Bow Recording ProcedurePage 8. Impressions for Orthognathic Model Surgery.Page 9. Articulator Preparation.Page 10.Trimming The CastsPage 11. Mounting The Maxillary Cast.Page 12. Mounting The Mandibular Cast.Page 13. Marking The Casts.Page 14. Marking The Casts.Page 15. Marking The Casts.Page 16. Model Surgery.Page 17. Model Surgery.Page 18. Model Surgery.Page 19. Occlusal Wafer ConstructionPage 20. Assessing The Results.Page 21. Assessing The Results.Page 22. Assessing The Results.Page 23. Assessing The Results.Page 24. Assessing The Results.Page 25. Areas Of Error. Appendices.Appendix 1. Associates Skeletal Reference Planes.Appendix 2. Planning Sheet.Appendix 3. Literature.Appendix 4. Contact Details.

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The Glasgow Model Surgery Technique

Orthognathic Model Surgery Procedures.

Model surgery has become an essential procedure for planning surgical outcome for patients

requiring the correction of a dentofacial deformity. The technique described in the following

pages has evolved over many years with input from numerous surgeons and technologists and

has been modified many times. It should be noted that the use of semi-adjustable articulators for

orthognathic model surgery has inherent inaccuracies which can only be partially over come.

This is not criticism of the instruments themselves they are after all being used for a purpose for

which they were not designed. The Glasgow Model Surgery Technique allows us to provide the

information required to successfully plan orthognathic surgery however is not offered as the

solution to all planning problems.

The selection of articulator is the first step in preparation for effective model surgery. The

following list indicates the criteria for articulator selection.

Articulator Selection.

Plain line or simple hinge articulator (Fig.1.), this can be used satisfactorily in the following

applications.

1. Maxillary advancement with no height change of the Maxilla i.e.: no impaction / no down

graft.

2. Mandibular advancement as a single jaw procedure.

3. Mandibular set back as a single jaw procedure.

4. Segmental surgery with no height change.

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Fig.1. Plain line articulator.

Semi-adjustable articulator and face bow (Fig.2.), this can be used satisfactorily in the following

applications.

1. Maxillary osteotomies with height changes i.e.: impaction or downgraft.

2. Bi-Maxillary procedures.

3. Segmental or multi-part maxillary osteotomies.

4. Cases of facial asymmetry.

Fig.2. Semi-adjustable articulator (Dentatus).

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Face Bow Selection.

The function of the face bow recording is to mount the maxillary cast on the articulator to

reproduce the anatomical position of the maxilla in its relation to the base of the skull. Once the

maxillary cast is mounted the face bow is removed and the mandibular cast is mounted using a

wax bite recording to relate the mandibular position to that of the mounted maxilla.

There are several types of face bow which are used in conjunction with semi-adjustable

articulators. The most popular of these fall into two categories, auricular and condylar. These

face bows are classed as average value and record three points of reference.

Taking each in turn, the auricular average value face bow records (Fig.3.)

1. The external auditory meati.

2. The maxillary occlusal plane (recorded by the wax bite on the bite fork)

3. The third point differs from manufacturer to manufacturer, commonly used points are alar

tragal line (Campers plane), nasion, orbitale.

Fig.3. Auricular average value face bow (Denar).

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Condylar average value face bow records (Fig.4.).

1. The condylar head (identified by palpation).

2. The maxillary occlusal plane (recorded by the wax bite on the bite fork)

3. The third point differs from manufacturer to manufacturer, commonly used points are

nasion or orbitale.

Fig.4.Condylar average value face bow (Dentatus).

It is a popular miss-conception that face bow registrations are accurate. In most cases the face

bow will overestimate the maxillary occlusal plane angle with dire consequences. It is

ESSENTIAL to record the maxillary occlusal plane angle accurately otherwise the model

surgery will be inaccurate and rendered useless.

This seems to be a severe criticism of semi-adjustable articulators and face bows however it must

be noted that as dental instruments they perform their function well and we are using them for a

purpose for which they were not designed.

To address this situation a simple modification can be made to the face bow and the registration

technique. As can be seen in Fig.5 the orbital pointer has been replaced with a bull’s eye spirit

level which has been adapted to fit the face bow. This modification will allow us to register the

patients natural head position which has proved to provide a more accurate reference plane when

mounting the casts for orthognathic model surgery. Although the Dentatus face bow is shown the

principle can be adapted to most face bow systems.

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Fig.5 Adapted Dentatus face bow.

Face Bow Recording Procedure.

The patient is seated on a stool with no back in front of a full length mirror. The stool should be

at a distance of 170 cm from the mirror in accordance with the Moorrees and Kean protocol for

adopting the natural head position.

The patient’s condylar heads are located by palpation and their position is marked on the soft

tissue with a marker. Wax is adapted to the face bow bite fork and the patient is asked to bite into

the wax to record the maxillary dentition .The bite fork is located on the face bow in its clamp

which is left lose at this stage. The condylar rods are placed over the previously marked

condylar heads and are adjusted to register the same measurement on the sliding scales on the

left and right sides. With the help of an assistant the face bow is carefully positioned on the

previously located and marked condylar heads and held in this position while the bite fork clamp

is tightened.

The patient is now asked to look directly into the reflection of their own eyes in the mirror and

the bull’s eye spirit level is adjusted until the bubble is in the centre. The clamp holding the spirit

level is tightened and the face bow recording is complete.

The condylar rods are loosened and the face bow is carefully removed from the patient ensuring

there is no movement of the bite fork or the spirit level.

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The face bow should be carefully hung on a hook until required (Fig.6.). The face bow should

never be laid on a bench as it is most likely the bite fork will bear the weight of the face bow

frame with the possibility of it moving. Should this happen the recording must be repeated.

Fig.6. Face bow hung on hook

Impressions for Orthognathic Model Surgery.

The impression technique for orthognathic model surgery is very technique sensitive. Perforated

metal trays provide the best results. If the impression has parted from the tray at any point it must

be rejected and re-taken. If a defective impression is accepted the distortion may not be noticed

on the plaster model but the occlusal relationship may be altered and subsequently, particularly

with segmental procedures, a position which is not surgically achievable may be predicted. A

further complication would be the inaccurate fit of the intra operative occlusal positioning

wafers.

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Articulator Preparation

Prior to mounting the maxillary cast the articulator should be set to average values as follows

(Figs.7 a, 7b, 7c, 7d.).

Fig.7 a.Condylar post set to 15° to Fig.7 b.Condylar inclination set to 30°replicate medio-lateral condylar angle. to replicate antro-posterior condylar

plane.

Fig.7 c.Incisal pin 0° Fig.7 d.Incisal table 0°

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Trimming the Casts.

The maxillary cast is trimmed square (Fig.8.).

Fig.8.Trimmed maxillary cast.

The procedure for trimming the mandibular cast differs from that of the maxillary cast as

additional information is required. As with the maxillary cast the mandibular cast is trimmed

square. In addition the base of the cast is trimmed with a flat plane placed across the occlusal

surface of the teeth. The base of the cast is then trimmed until it is parallel with the flat plane.

( Fig.9.).

Fig.9.Mandibular cast trimmed to occlusal plane.

When mounted on the articulator the base of the mandibular cast will represent the mandibular

occlusal plane. The reason for this will be explained in the results section.

Mounting the Maxillary Cast.

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Flat plane

Base trimmed parallel to flat plane

The face bow is attached to the articulator and the condylar rods are set to the same measurement

on both right and left sides, this duplicates the procedure carried out on the patient at the time of

the clinical face bow recording. The face bow is adjusted vertically to centre the bubble

( Fig.10.). The maxillary cast is seated in the wax record on the bite fork ensuring the cast is

stable and there is no obstruction from orthodontic attachments or similar, should the cast be

unstable in the wax it will be necessary to trim the bite until stability is achieved (Fig.11). The

maxillary cast is now attached to the articulator disc with plaster of Paris completing the

articulation of the upper cast (Fig.12.). The face bow is now detached from the articulator in

preparation for the mounting of the mandibular cast.

Fig.10. Face bow attached to the articulator.

Fig.11. Cast in wax registration.

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Fig.12. Completed mounting of the maxillary cast.

Mounting the Mandibular Cast.

The mandibular cast is mounted using the centric wax record which is taken at the same clinical

appointment as the face bow registration. The wax bite record is placed on the mounted

maxillary cast and the mandibular cast is engaged with the wax bite, they are held securely

together and attached to the articulator with plaster of Paris using the same method as mentioned

in the procedure for mounting the maxillary cast. The wax registration is important in the success

of model surgery and there are a few procedures which should be followed. In all cases of

overclosure of bite especially class III malocclusion the jaw registration should be taken in the

rest position as opposed to centric occlusion. This allows the accurate identification of

anatomical landmarks on the maxillary and mandibular dentition. These points are required for

cephalometric tracing and are often obscured by each other on the cephalometric x ray if the

jaws are in centric occlusion. It is crucial the orthognathic model surgery and the cephalometric

planning start from the same position. Once the mandibular cast is attached to the mounting base

with plaster of Paris the articulated models are ready to proceed with marking out.

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Marking the Casts.

All markings on the casts should be scribed using a scalpel or similar instrument. Indelible pencil

should be avoided as when dampened the width of the lines drawn on the cast will double in

thickness thus increasing the possibility of taking inaccurate measurements during the model

surgery. It is also useful to use a colour coding system for the model markings. The example

shown uses red marker for all pre-op positions. The first line marked on the articulated casts is a

horizontal line scribed 5mm from the base of the articulated model (Fig.13.arrow B.). A second

line is scribed on the white mounting plaster (Fig.13.arrowA.). The distance between the two

lines should be sufficient to allow trimming if maxillary impaction is required. The master casts

should not be trimmed as this will make returning the casts to the starting position difficult in the

event of a plan change. Any plaster to be removed for a maxillary or segmental impaction should

be limited to the white mounting plaster. The distance between the two horizontal lines is not

critical however it must be scribed on the cast to ensure being able to re-measure the model

surgery at a later date. A suggested measurement would be 15mm (5mm on the master cast and

10mm on the mounting plaster.) this has proved to accommodate the most extreme of

movements.

Fig.13. Scribed lines on the mandibular and maxillary cast.

in addition a further line is scribed on both sides of the maxillary and mandibular casts. These

lines can be used to re orientate the casts to the start position should a change in plan be

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A

BC

necessary. The centric occlusion (start position) is marked at tooth level with a line which is

scribed on the posterior teeth. This line is coincidental with both maxillary and mandibular teeth

(Fig.13.arrow C.).

Maxillary and mandibular centre lines are scribed on the casts (Fig.14.arrow A.)

Fig.14.Vertical repositioning lines scribed on mandibular and maxillary casts.

Further lines are scribed on both sides of the maxillary and mandibular casts. These lines can be

used to reorientate the casts to the start position should a change in plan be necessary (Fig.14

arrow B.). Two vertical lines positioned in the molar region are scribed on the posterior of the

maxillary cast (Fig.15.).

Fig.15. Posterior maxilla rotation lines scribed on the cast.

These lines will be used to ensure the maxilla is not rotated at the centre of the palate when the

anterior midline needs to be shifted. This would often tend to incorporate a posterior movement

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A

A

B

B

of the maxilla should a correction of the maxillary centre line be required. This movement

should be avoided

Fig.16.Advancment measurement device.

A measurement is taken from the incisal pin on the articulator to the upper incisors and similarly

to the lower incisors. In Fig16 a simple device was made consisting of square tube which was

attached to the incisal pin of the articulator and secured by a screw. The pin is a sliding fit and

can be adjusted to touch the teeth. The point of contact on the selected tooth should be marked

with an indelible pencil. A measurement can then be taken and used to evaluate the change in

position of the maxilla. This measurement can also be taken using a vernier calliper and

measuring the distance between the teeth and the pin. These measurements are recorded on the

cast for future reference.

The mounting and marking of the casts is now completed and model surgery can be started.

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Incisal pin

Pin measured at these points to evaluate movements

Model Surgery

A prescription sheet similar to appendix 2 is used to record the skeletal and dental landmarks

which will be relevant to the process of model surgery, the sheet will also have a proposed plan

for the correction of the patient’s deformity. It is necessary for the Technologist to be present at

the initial patient assessment as this will be an opportunity to visualise both hard and soft tissues

of the patients face.

The case used as an example has a class III skeletal deformity. Correction will require a 3 part

segmental maxilla to narrow the maxillary arch with a maxillary advancement. The mandible

will require a bi-lateral sagittal split osteotomy to set the lower jaw back to the best occlusion.

The first stage in the planning procedure is to cut a duplicated set of models in order to assess

what occlusion is achievable. This is not an anatomical procedure at this stage and is only an

occlusal assessment (Fig.17.).

Fig.17.Hand held plan.

On completion of this assessment the face-bow recording is taken and the upper and lower dental

casts are articulated on the semi-adjustable articulator as previously described.

A set of study casts are taken and must be kept as a record of the pre-op occlusion.

The maxillary cast is separated from the mounting plaster. This can be done by placing a plaster

knife at the join of the mounting plaster and the maxillary cast, with a sharp blow to the knife the

cast and mounting base will separate (Fig.18.).

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Fig.18.Maxillary cast being separated from the mounting plaster.

The cast is now segmented to duplicate the cuts made in the preliminary plan (Fig 17.). The

segments of the maxillary cast are repositioned in optimum occlusion using the lower cast and

are sealed together using sticky wax (Fig.19.)

Fig.19.Occluded segmented cast.

The maxillary cast is then attached to the upper mounting plaster using sticky wax and is re-

positioned using the prescribed movements obtained from the cephalometric planning (Fig.20.).

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Fig.20.Maxillary cast re attached to the mounting plaster.

The mandibular cast is separated from the mounting plaster in the same way as described for the

maxillary cast. The mandible is repositioned to the prescribed final occlusion (Fig.21.).

Fig.21.Arrows indicating the direction of movement of the casts.

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Occlusal Wafer Construction.

The final position intra-operative wafer is now constructed. There are several methods of wafer

design and construction which is a matter of operator choice. This case shows a self curing

acrylic wafer (Fig.22.) which was adapted directly to the repositioned mounted casts.

Fig.22.Self cure acrylic intra-operative positioning wafer.

The upper and lower casts had a coat of separating medium applied (cold mould seal). The

acrylic was rolled into a cylindrical shape at the dough stage and adapted to the lower teeth. The

upper cast was then rotated into occlusion the excess acrylic was trimmed with scissors and the

acrylic wafer was left to cure. The wafer was then trimmed and polished. The final position

wafer is now completed.

The mandibular cast is separated from the articulated mounting base and repositioned to the pre-

surgical position (Fig.23.).

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Fig.23.Mandibular cast repositioned to the intermediate position.

This is the intermediate position (maxillary surgery completed, no mandibular surgery). An

acrylic wafer is constructed using the previously described method. It is good practice to use

different colours of acrylic for wafer construction this allows easy identification in theatre. The

system employed for this purpose is the final position wafers are always clear and the

intermediate wafers are always ivory this prevents confusion of wafer selection during surgery.

Once completed the casts are returned to the final position for reference in the operating theatre.

Assessing The Results of Cast Movements.

Maxillary Advancement.

Maxillary advancement is calculated by measuring the alteration in length of the horizontal pin

resting on the labial surface of the upper central incisor (arrows Fig.24.).

Fig.24.Measuring the pin to evaluate the degree of maxillary advancement.

Example: Antero-posterior incisal pin measured in the starting position 50mm.

Antero-posterior incisal pin measured in the final position (maxillary cast moved to

prescribed position) 53mm. Maxillary advancement = 3mm.

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Correction of Maxillary Centre Line.

Fig.25. Casts moved to correct the maxillary centre line.

The maxillary centre line is assessed at the final clinical appointment. Correction (if necessary)

is carried out by rotating the maxillary cast using the scribed centre line as the point of

measurement (Fig.25.).

Fig.26.Maxillary rotation lines aligned.

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On completion of the correction of the maxillary centre line the two lines on the posterior surface

of the maxillary cast must be coincidental with the lines on the mounting plaster. This ensures

the maxilla has not been rotated at the centre of the palate (Fig.26).

If the maxilla rotates in its centre there will be a posterior shift of one side of the maxilla this will

then indicate a posterior shift which is not surgically possible as it will hit the pterygoid plate. It

is desirable to advance the maxilla by 1mm to accommodate any centre line alteration.

Maxillary Impaction.

Maxillary impaction is measured between the horizontal lines scribed on the cast and the

mounting plaster (Fig.27.).

Fig.27.Measuring the movements.

The distance between the horizontal lines (indicated by the red arrow) is measured. This

measurement is compared with the start position prior to repositioning of the cast. In this case

20mm (as indicated by the black arrow). The increase or decrease in measurement indicates the

impaction or down graft of the maxilla.

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Maxillary Width.

Segmental maxillary procedures or palatal midline splits are measured as indicated by the

arrows. This measurement is compared with the study cast and the difference in measurement

indicates the amount of expansion or closing of the maxillary arch (Fig.28.).

Fig.28.Planned correction in the transverse plane.

Advancement of posterior segments can be measured by comparing the measurement of the teeth

on either side of the cut with the study cast the difference will indicate advancement of the

segment (Fig.29.).

Fig.29.Measuring planned movements.

Assessment of Mandibular Movement.

Mandibular advancement is measured in the same way as the maxilla using the antero-posterior

pin. This procedure was described in the maxillary advancement section.

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Fig.30.Measurement of mandibular movements.

If the mandible is occluded with the maxilla and the anterior section of the cast has lifted from

the plaster mounting base (Fig.31.) there is an indication of the possibility of an unstable

procedure. This situation indicates a downward movement of the ramus placing an unacceptable

stress on the ptreygo-masseteric sling in addition there will be a downward pull of the hyoid

muscles indicating an almost certain relapse of the predicted mandibular position.

Fig.31.Models indicating an unstable mandibular procedure.

The trimming of the casts section explained the procedure for trimming the mandibular cast

ensuring the base of cast was parallel with the mandibular occlusal plane. This once the

mandibular cast is mounted transfers the mandibular occlusal plane angle to the mounting

plaster. This then allows the lower cast to be moved anteriorly or posteriorly on the mandibular

occlusal plane therefore any anterior lift of the cast warns of a potentially unstable surgical

outcome which needs to be taken account of.

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Areas of Error.

There are some simple precautions which can be taken to ensure errors are not incorporated into

the technique.

1. Impressions should be taken in perforated impression trays ensuring all teeth (including

8s) are recorded.

2. The impression must be attached to the tray at all points. No tearing of the impression or

separation from the tray should be accepted.

3. The impressions should not be rested on their heels either in the surgery or the laboratory.

4. The wax jaw registration must be taken with extreme care. This is the most common area

of error in the model surgery procedure. The model surgery and the cephalometric

assessment must start from the same jaw position.

5. Face bow registrations must be carefully treated both in the surgery and laboratory. Face

bows are easily moved rendering them useless. Should a face bow recording be knocked

or rested on the bite fork it should be repeated (Fig.32.).

Fig.32.Face bows suspended prior to mounting.

6. When mounting the maxillary cast the bite fork on the face bow should be supported to

ensure the weight of plaster does not distort its position.

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Appendix 1.

Skeletal reference planes.

1. Frankfort horizontal plane. A plane passing through the left orbitale (most inferior point of the orbit) and the highest point of each external auditory meatus.2. Axis orbital plane. This plane is an imaginary line joining orbitale and the axis of mandibular rotation (the most prominent palpable area of the condylar head).3Maxillary occlusal plane. This plane is determined by placing a flat plate over the surfaces of the maxillary teeth. Usually the most prominent anterior tooth and the most prominent molar cusp will form this plane.

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Appendix 2.

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Appendix 3.

Sharifi.A, Jones.R, Ayoub.A, Moos.K, Walker.F, Khambay.B, McHugh.S, (2008) How accurate

is model planning for orthognathic surgery. International Journal of Maxillofacial

Surgery,37,1089-1093.

Walker.F, Ayoub.A, Moos.K.F, Barbenel. J, (2008) Face bow and articulator for planning

orthognathic surgery: 1 face bow. British Journal of Oral and Maxillofacial Surgery, 46,567-572.

Walker.F,Ayoub.A,Moos.K.F,Barbenel. J, (2008) Face bow and articulator for planning

orthognathic surgery: 2 articulator. British Journal of Oral and Maxillofacial Surgery, 46,573-

578.

Barbenel. J.C, Paul P.E, Khambay.B.S, Walker.F.S, Moos. K.F, Ayoub.A.F, (2010) Errors in

orthognathic surgery planning: the effect of inaccurate study model orientation. International

Journal of Oral and Maxillofacial Surgery, 39,1103-1108.

Paul. P.E, Barbenel. J.C, Walker.F.S, Khambay.B.S, Moos. K.F, Ayoub.A.F (2012) Evaluation

of an improved orthognathic articulator system: 1. Accuracy of cast orientation. International

Journal of Oral and Maxillofacial Surgery, 41, 150-154

Paul. P.E, Barbenel. J.C, Walker.F.S, Khambay.B.S, Moos. K.F, Ayoub.A.F (2012) Evaluation

of an improved orthognathic articulator system:2.Accuracy of occlusal wafers. International

Journal of Oral and Maxillofacial Surgery, 41,155-159

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Appendix4.

Contact Details

Regional Maxillofacial Prosthetics and Technical Service

Southern General Hospital

1345 Govan Road

Glasgow

G51 4TF

Phone 0141 232 7325

E-Mail maxfaxlab@ggc.scot.nhs.uk

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