In designing custom jawline implants for the ‘high angle’ patient, the question is always how much should the jaw angle be lowered. While there are some aesthetic guidelines for placement of jaw angle position, it is first important to establish the diagnosis…is the jaw angle really high and, if so, how high? Such an assessment is based on two types of angular measurements, the mandibular plane angle and the gonial angle.
The mandibular plane angle, technically the Frankfort mandibular plane angle, is created by the intersection of the Frankfort horizontal plane line and the mandibular plane line on a cephalometric x-ray tracing. In this tracing the high angle patient has a 30 degrees or more plane angle while the low angle patient is 20 degrees or less. Short of a cephalometric x-ray and using the patient’s 3D CT scan, which is how custom implants are designed, I use an inversion of this classic measurement. Using a horizontal line at the bottom of the 3D image the mandibular plane line can intersect with it….creasing virtually the same mandibular plane angle measurement. This also has more clinical relevance since this is what the patients sees.
Conversely the gonial angle is formed by the intersection of the vertically inclined posterior border and the more horizontally inclined inferior border of the mandibular ramus. This is more a measurement of the direct shape of the jaw angle rather than necessarily determining if it is high, normal, or low. Such gonial angle measurements are more aesthetic in nature and do not relate to any occlusal pathology per se. Gonial angle measurements can range from 110 to 140 degrees as a normal aesthetic range. Where one chooses to fall within that range is a matter of personal preference.
By definition there are correlations between the gonial and mandibular plane angles. The higher the mandibular plane angle the wider the gonial angle will usually be. The same is true for a low mandibular plane angle and the more obtuse the gonial angle will be. There are also different implications to these measurements as a high mandibular plane angle patient will likely want the angles lowered versus the normal mandibular plane angle patient who will be more focused on the gonial angle (jaw shape) only.
I find these angular measurements iin custom jawline implant design ncreasingly useful as clinically it can be very hard to tell from the outside what the patient’s jaw angles look like. In many patient’s the jaw angle bone curves inward back to the angle from the external oblique line near the teeth. This can make the jaw angle look high as it is obscured by the overlying soft tissues when it is really lower than one would think.
Dr. Barry Eppley