Mandibular Model Surgery for Orthognathic Surgery: The Perth ...

J Oral Maxillofac Surg 69:950-953, 2011

Mandibular Model Surgery for Orthognathic Surgery: The Perth Technique to Improve Planning G. Alastair Wylie, LOTA, MIMPT,* Sean Laverick, BDS, FDS, RCS, FRCS, Dip Fod,† Grant T. McIntyre, BDS, FDS, MOrth, PhD, FDS(Orth),‡ and Bruce N. Epker, DDS, MSD, PhD§ cases of facial asymmetry. The rotational movements required to correct the asymmetry can lead to large, sometimes unexpected effects on the proximal segment and condyle. Adjusting these bony interferences between the proximal and distal osteotomized mandible can be difficult at the time of surgery. Assessment of the “yaw” at the model surgery stage can be used to predict and plan actual surgery accordingly. Additionally, when unfavorable changes are noted during planning, it is possible to reassess the model surgery and compromise with a less than ideal occlusion, which would lead to a more acceptable proximal/distal segment relationship. This can be done when it is possible to correct any tooth/arch discrepancies with postsurgical orthodontics. We present a new method of conducting mandibular model surgery, which provides an improved 3D assessment of the prescribed movements, particularly mandibular yaw. This method greatly enhances the presurgical knowledge of the proximaldistal mandibular relationship at the site of the sagittal split.

There are several basic model surgery techniques currently being used in the transfer of measurements from the planning stage to the patient at the time of surgery. These include the “anatomically oriented” technique,1-3 the “key spacer” technique,4 and the 3-dimensional (3D) virtual surgery computer-based methods.5-9 The articulated plaster model methods give precise assessment of movements in the vertical, anteroposterior, and horizontal planes when properly done and this allows assessment of the more complex movements of “pitch” and “roll” (Fig 1A,B). However, the lack of a static reference point on the model base during rotational movements, within the horizontal plane, means that “yaw” (Fig 1C) (lateral motion when viewed in the axial plane) is very difficult to assess.10 This can lead to unwanted flare of the mandibular angle and potentially excessive rotational movements of the condyle in the glenoid fossa after rigid fixation. In essence, at the site of the sagittal split surgical site, this does not afford optimal proximal/ distal mandibular relationships. 3D computer planning programs allow the yaw to be visualized but as yet, there is no method of detecting this in model surgery. Although this new technique is useful for any mandibular osteotomy, it is particularly pertinent in

Technique Once the models have been mounted on an anatomical articulator via a face bow transfer and trimmed anatomically, measurements are taken from the lateral cephalogram and the mandibular model is marked as follows:

*Senior Chief Technician, Department of Dentistry and Maxillo Facial Surgery, Perth Royal Infirmary, Perth, Scotland. †Consultant, Oral and Maxillo Facial Surgeon, Department of Dentistry and Maxillo Facial Surgery, Perth Royal Infirmary, Perth, Scotland. ‡Consultant Orthodontist, Department of Dentistry and Maxillo Facial Surgery, Perth Royal Infirmary, Perth, Scotland. §Director, Facial Cosmetic and Surgical Center, Weatherford, TX. Address correspondence and reprint requests to Dr Wylie: Department of Dentistry and Maxillo Facial Surgery, Perth Royal Infirmary, Jeanfield Rd, Perth, Perth, PH1 1NX, Scotland, UK.

1) The depth of the mandibular inferior border is measured at the incisors and also at the molars. These measurements are transferred to the model and a horizontal line is drawn. 2) At the posterior aspect of the model, a referent mark is made lingually and from this point dividers are used to measure the distance to the incisal edge of the anterior teeth (Fig 2).

© 2011 American Association of Oral and Maxillofacial Surgeons

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3) A transverse line is drawn behind the last molars and coincides with an angular line, which simulates the osteotomy cut on the lingual of the ascending ramus. 4) A perpendicular line, which joins the horizontal and transverse lines, is drawn on the model.

FIGURE 2. Measurement from lingual referent mark to incisal edge of anterior teeth. Wylie et al. Mandibular Model Surgery For Orthognathic Surgery. J Oral Maxillofac Surg 2011.

FIGURE 1. (A) “Pitch.” (B) “Roll.” (C) “Yaw.”

FIGURE 3. Horizontal and transverse cuts are made by cutting disk.

Wylie et al. Mandibular Model Surgery For Orthognathic Surgery. J Oral Maxillofac Surg 2011.


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FIGURE 4. Referent markings each side of vertical cut. Wylie et al. Mandibular Model Surgery For Orthognathic Surgery. J Oral Maxillofac Surg 2011.

FIGURE 6. Patient with mandibular asymmetry. Wylie et al. Mandibular Model Surgery For Orthognathic Surgery. J Oral Maxillofac Surg 2011.

FIGURE 5. Referent lines showing changes in position at posterior of model.

FIGURE 7. Model surgery for patient in Figure 6.



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3) Anteroposterior change between maxillary and mandibular lines. 4) Any change between referent lines at the posterior of the model surgery as this could affect condylar position (Fig 5). Evaluation of the proximal/distal segment relationship particularly in cases of mandibular asymmetry (Fig 6) can allow potential unwanted effects manifesting at the angle of the mandible to be visualized (Figs 7, 8). The cutting of the mandibular plaster model in a method mirroring the anatomy of the operative procedure, ie, the bilateral sagittal split osteotomy, while having static reference lines in the horizontal plane allows a better assessment of yaw. Potentially, unwanted flare of the mandibular angle and excessive condylar rotation can be predicted. FIGURE 8. Model surgery for patient in Figure 6. Wylie et al. Mandibular Model Surgery For Orthognathic Surgery. J Oral Maxillofac Surg 2011.

This is perpendicular to the distal of the terminal molar and in an identical location to the actual cut during surgery. The model surgery is done using a cutting disk and coping saw. 1) The horizontal and transverse cuts are made by a cutting disk (Fig 3). 2) The vertical cut joining these 2 cuts is made by a coping saw. When the cutting is finished, the “free” distal segment of the mandible is positioned against and located to the maxilla in the desired position. The new position of the distal segment is secured with plaster to the articulator and referent markings can now be analyzed for changes in measurement and orientation, ie, 1) Lingual referent mark to incisal tip. 2) Distance between referent markings on each side of vertical cut (Fig 4).

Acknowledgments We would like to thank Brian Lawson and Nicky Goodfellow, Medical Illustration Services, Perth Royal Infirmary.

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