Issues with Face-Bow Transfer

The ability of the surgeon to transfer the desired surgical plan to the patient during orthognathic surgery depends mainly on the accuracy of the surgical splint. The fabrication of an accurate splint requires that the models be mounted to replicate the position of the patient’s dentition. However, Ellis et al.²⁴ demonstrated a significant difference between the inclination of the occlusal plane on the mounted models and the actual occlusal plane as measured on the cephalograms. Another study²⁷ shows that the average occlusal plane inclination using the SAM Anatomical Face-Bow was 7.8 ± 4.2 degrees, statistically significant greater than the actual. The mean occlusal plane inclination of the models obtained using the Erickson Surgical Face-Bow was 4.4 ± 2.2 degrees statistically significant greater than the actual.

The advantage of using models that are accurately oriented to Frankfort horizontal becomes evident once the implications of using inaccurately mounted models are understood. Fig. 25-1 illustrates the results of using inaccurate models to fabricate an intermediate splint. Fig. 25-1, A, depicts the cephalometric tracing of a hypothetical patient with mandibular prognathism and maxillary hypoplasia. The surgical plan for this patient calls for a 10 mm maxillary advancement and a 4 mm mandibular setback. In this case, the axis-orbital plane (face-bow) is 12 degrees off the Frankfort horizontal. Fig. 25-1, A, depicts the articulator on which the models have been mounted using the conventional system. The occlusal plane inclination of the mounted models (Fig. 25-1, B) is 12 degrees greater than that on the cephalometric tracing (see Fig. 25-1, A). In Fig. 25-1, C, the maxillary model has been advanced 10 mm forward, and the intermediate splint has been fabricated. Fig. 25-1, D, depicts the planned position for the maxilla and the actual position of the maxilla at the time of surgery. In this hypothetical case, the position of the maxilla at surgery is 1.5 mm behind the planned position, producing a maxillary advancement of only 8.5 mm, or 15% less than desired.

FIG 25-1 Face-bow transfer in a hypothetical patient. A, Prediction tracing in a hypothetical patient. The horizontal line in black is Frankfort horizontal. The red line is the axis-orbital plane, which is 12 degrees off Frankfort horizontal. The plan calls for a 10 mm maxillary advancement (blue line). B, The models mounted on an articulator. Note that the occlusal plane inclination of the mounted models is 12 degrees steeper than the actual occlusal plane. C, Model surgery. The maxillary model has been advanced 10 mm, and the intermediate splint (red) has been fabricated. D, The maxillary position at surgery. The blue outline represents the desired position for the maxilla. The red outline represents the actual position at surgery. Note that the actual position is 1.5 mm behind the desired position.

In order to solve this problem, researchers and clinicians have developed various techniques. Ellis et al.²⁴ developed a modified mounting technique used with Hanau articular. Gateno et al.²⁷ also developed face-bow transferring technique used with SAM articulator that takes the individual anatomic variations among subjects into consideration.

Issues with Mandibular Autorotation

Predictable outcomes in double-jaw surgery depend on precisely positioning the maxilla. This is important, not only because an ideal maxillary position is necessary to achieve good mid-facial aesthetics, but also because the ultimate position for the mandible depends on it.

The first step in double-jaw surgery is to place an external reference marker (e.g., a K-wire or a screw) in the nasal bones. The vertical distance between this marker and the edges of the maxillary incisors is recorded. This measurement will help establish the correct vertical position for the maxilla. In double-jaw surgery, the maxilla is usually cut first. In order to position the maxilla at the time of the surgery, surgeons use an intermediate surgical splint that determines the AP and transverse position of the maxilla as well as any desired rotations in space. The intermediate splint is temporarily placed between the osteotomized maxilla and the mandible, and the teeth are wired in this position. The vertical position of the maxilla is then determined by autorotating the maxillo-mandibular complex to a predetermined vertical position, using the pre-determined distance between nasal marker and incisor edges as reference. Once the maxilla is in place, the dental fixation is released and the mandible is osteotomized and positioned according to the position of the maxilla. The splint is fabricated prior to surgery using plaster dental models, which have been mounted on an articulator. Articulators are mechanical devices that simulate the movement of the mandible. They are based on the principle that during opening and closing of the mandible in its most retruded position (centric relation), the condyles rotate around a hinge axis (hinge axis theory).^28–32 However, recent studies show that during mandibular opening, translation and rotation are always continuous and combined.^33–35 Even for the first millimeter of mouth opening, the condyles actually follow a path (combined rotation and translation), rather than rotating around a single point.^30,36,37 Because the mandible also slides forward during autorotation, the maxillo-mandibular complex will also move forward accordingly. Unfortunately, current dental articulators are unable to simulate these complex movements. Therefore, traditional surgical planning, which uses semiadjustable articulators to simulate surgery and fabricate the intermediate surgical splint, f/>