The gonial or jaw angle is an important aesthetic element of lower facial aesthetics particularly for men. While what constitutes an attractive jaw angle is influenced by each individual’s personal preference, most know it when they see it. But despite the emotional reaction to an appealing jaw shape it can be measured and quantified and has been done so for a long time. The three most common used measurements of the jaw angles are the gonial angle, the mandibular plane angle and bizygomatic width. Each evaluates jaw angle shape but does so from a different dimensional perspective.
The gonial angle measures the jaw angle shape directly by intersecting the posterior and inferior borders of the mandible. Desired or attractive jaw angles are in the 115 to 130 range. Jaw angles at 110 degrees or less may be considered too strong/prominent (too low) and those above 130 to 140 degrees would be too indistinct or lacking a visible shape (too high).
The mandibular plane angle measures the inclination or slope of the jawline using the anterior skull base as a reference. The skull base line can be either the sella-nasion (SN) or the Frankfort horizontal line (inferior orbital to porion). The gonion-menton line then intersects with the skull base reference line to measure the plane angle. The normal mandibular plane angle is 25 to 30 degrees. While historically used in cephalometric analyses to measure facial growth from an aesthetic standpoint it represents the lowest horizontal plane on the face and creates a clear separation between the face and the neck. The higher the mandibular plane angle the less distinct this separation is and a more narrow lower facial shape is seen. The lower it is the more profound this separation becomes but if it gets too low then the face can end up having a boxy appearance.
In patients seeking jaw augmentation they most commonly have high mandibular plane angles and more obtuse gonial angles. They may also have more normal angle values but lack/ of a good visible shape may be width related.
While the value of these angular measurements has proven utility in cephalometric, orthodontics and jaw osteotomy surgery can they be applied to aid in jawline implant designs? The answer is yes but they must be taken into context with the rest of the patient’s face, their aesthetic goals and whether they increase the risks of surgical implant complications.
Many patients seeking custom jawline implants have normal range gonial and mandibular plane angles and no vertical changes in the jaw angle shape is needed. The one caveat to the gonial angle is that it may have a sharp angle at the intersection of the two lines but rarely should an implant design have a corner that square. Squaring out the gonion increases the risk significantly of masseteric muscle dehiscence resulting in implant show/reveal. That is a risk of any implant design that extends the bony angle. But it is particularly so when needing the detached ligaments to stretch in a non-anatomic obliquely backward direction.
The mandibular plane angle can be one guide to determining the need for vertical jaw angle lengthening. A high mandibular plane angle in and of itself it can be misleading. But when combined with a high gonial angle as well as the need for vertical jaw angle lengthening (drop the jaw angle lower) is most assuredly aesthetically beneficial.
There are other compelling reasons for lowering the jaw angles such as a prior vertical lengthening bony genioplasty, to create a more linear jawline when a prominent antegonial notch exists and to vertically lengthen a short face to name a few.
Not uncommonly in an otherwise patient with normal angle values a small amount of vertical angle lengthening is added for the purposes of making its augmented shape more visible. It may also be done to ensure a linear jawline shape when a prominent antegonial notcu exists.
When considering widths in custom jawline implants the primary consideration is to determine jaw angle or gonial width needed. Often quoted is to use bizygomatic width as a reference. This is a horizontal linear measurement from the peak of the zygomatic bony arch width from side to side. While typical ranges are in the mid 140mms there is great diversity in this width based on gender, race, body size etc. Thus it is best used as a reference to compare mid- to lower facial widths. Patients often tell me that bigonial width should be equal to bizygomatic width. But that is incorrect as it does not take into consideration the thickness of the overlying soft tissues. The cheek soft tissues over the zygomatic arches are considerably thinner than that over the jaw angles. The thickness of the masseter muscle alone, which can be as much as 15mms or more in some patients, makes the soft tissue over the bone 2 to 3X thicker at the jaw angles than the cheek arches. Thus making the bizygomatic width equal the bigonial width would make for a boxy or even an inverted facial triangle with the jaw angles protruding beyond the cheeks. As a result implant jaw angle width should always stay inside the bizygomatic width. How much can be debated but the guideline I use is never exceed 5mms inside the zygomatic arch width. This will avoid the potential for a too boxy lower facial shape.
The use of bony landmarks and their well known cephalometric measurements are established guidelines in jaw osteotomies with predictable outcomes. But when it comes to using bony landmarks to create an aesthetic implant design, whose effect is seen externally by soft tissue displacement/expansion, the outcomes are nor as predictable. This doesn’t mean they are not useful, just that the overlying soft tissues very greatly amongst patients and introduce variables that bony guidelines alone do not take into account. Thus they have value as general guidelines but should be judiciously applied with the patient’s soft tissue anatomy factored into the implant design.
Dr. Barry Eppley
World-Renowned Plastic Surgeon