Once of the features we look for when doing a facial aesthetic assessment is symmetry. It is generally believed that symmetry is good and is essential for aesthetics. However, the human face is never perfectly symmetrical. In fact, the human body is not symmetrical. The right side of the body is not a mirror image of the left. The differences between the right and left side give a face its unique character. It is only when the asymmetry becomes the overwhelming feature of the face that it becomes unaesthetic. How do we draw the line?
First, there is no absolute line to cross. Aesthetics is a very personal perception. Afterall, beauty lies in the eye of the beholder. Very often, a new patient will ask me what I think is wrong with her face and to recommend treatment. My usual response is to point out the functional deformities that when corrected can improve function in a tangible way. The difficulty with facial asymmetry is that most cases do not entail a functional deficit. As such, I do not always highlight the facial asymmetry unless it is part of the patient’s presenting complaint. If it is not something that the patient notices and is bothered by, I usually do not recommend surgery.
I do take objective measurements to compare the right side of the face with the left. This is done both physically on the face as well as on radiographs. Physical measurements in person gives the soft tissue dimensions which can be quite different from the skeletal readings obtained from computerized measurement of the skull radiograph. The two sets of data are analyzed together to arrive at a diagnosis. Facial asymmetry is measure from the frontal view.
The eyes, or more specifically, the orbital sockets, are not usually treated in non-pediatric maxillofacial surgery. I take them as reference landmarks on which to base the measurement of the parts that we can change, the upper and lower jaws. The horizontal distance between landmarks to the facial midline is compared between the right and left sides. A faux plane is placed in the mouth to look for canting, i.e. whether the left and right side of the upper jaw is at the same level or one side is higher placed than the other, something called canting. The vertical distance between the inner, middle and outer corners of the eye and the faux plane is also measured and compared.
Next I look at the chin point. The chin being at the outermost path of any rotational track is likey to be the most conspicuously affected by any asymmtrically development that happens. Somehow, the magical number seems to be 5mm. If the chin is more than 5mm to the left or right of the facial midline, it will be quite noticeable. The chin itself can also be irregular in shape, with more bulk on one side than the other.
While still looking at the face for asymmetry, I will evaluate the angles of the lower jaw, particularly the thickness of the masseter muscles covering the angles. The masseters are powerful muscles and a person who chews predominantly on one side will tend to develop larger masseters on that side, much like a right-handed tennis player have larger right arms. The underlying bone is a reactive tissue and when stimulated constantly, will respond with increased size as well.
The cheek bones, also known as the zygoma, is next. This zygomatic arch contributes to the broadness of the face when viewed from the front. A more pronounced arch on one side will result in significant asymmetrical appearance. The body of the zygoma, on the other hand, contributes to the frontal projection of the midface. Some of these facial features can be corrected with orthognathic surgery, i.e. repositioning of the jaw bones, while others need recontouring surgery. In some cases, the repositioning and recontouring can be done in one sitting while in others, they need to be staged.
Orthognathic surgery must be done with the teeth in consideration. A thorough intraoral examination is essential. While some patients seeking orthognathic surgery are primarily interested in changing their facial, others may be seeking an improvement in their bite. Regardless of intention, both facial features and dental occlusion must be considered together in deriving a treatment plan.
Dental occlusion is classified according to Angle’s Classification which basically groups the different types of bite into three types, Class I is the ideal and is what we seek to achieve through surgery. Class II is often referred to as an Overbite, where the upper teeth are positioned further forward of the lower teeth and Class III is the Underbite, meaning, the lower front teeth are positioned in front of the upper. Superimposed on each Class of occlusion is the degree of crowding. Severely crowded teeth needs to be corrected orthodontically prior to surgery. For assessment of asymmetry, I compare the dental midline, ie the line between the upper left and right central incisors, to the facial midline. If they are not coincident, treatment to correct them may be in the form of braces, for mild midline shift, or surgery, for more significant deviation. The upper dental midline also has to be checked against the lower dental midline, which in turn needs to be assessed for its relationship to the chin point.
Most cases of asymmetry will require a double jaw surgery. Upper jaw surgery is often needed to correct the canting while the lower jaw surgery is needed to achieve a proper bite against the upper as well as to correct the chin point back to the centre. Sometimes, after shifting the jaws into the correct position, contouring surgery may be need to reduce the enlarged side. If the asymmetry is due to muscles, Botox injection is a convenient and safe way to reduce the muscles bulk.
Ultimately, in the absence of a functional problem, facial asymmetry is primarily an aesthetic problem and the choice of whether to correct is subject to the aesthetic sense and preferences of the patient.