A protruding lower jaw is a fairly common facial deformity in Singapore. While it may not be the most common, it is one which for which surgical correction is most sought after. Underbite, as it is commonly called by many patients, poses significant functional challenges, depending on the degree of deformity. When the lower jaw grows disproportionately more than the upper jaw, the lower teeth are brought forward and causing an open bite. This disproportionate growth also cause the facial profile to resemble what some people call the “shoe horn”. Many patients coming for consultation usually assume that the problem can be solved by surgery to set the lower jaw back. However, in reality, most patients actually need something else.
The bite of an underbite deformity can be corrected by surgery to set the lower jaw back alone. However, such single jaw surgery may create a different deformity or a different functional problem. Sometimes, the main cause of an underbite may not be an excessively long lower jaw. It may be due to an underdeveloped upper jaw. As such, a thorough assessment of the facial dimensions is critical to establishing the proper diagnosis. If the problem lies with the upper jaw, then surgery to advance the upper jaw is the correct thing to do. If the lower jaw is set back instead, it will result in a very retruded lower jaw which not only make the face appear very flat but may cause obstructive sleep apnea. Obstructive sleep apnea is a condition whereby breathing during sleep is obstructed due to tissues in the airway from the nose to the throat blocking the flow of air. Repositioning the lower jaw too far back reduces the space for the tongue, pushing it into the throat. During sleep, the tongue in its relaxed state falls backwards and blocks the flow of air downwards into the lungs. Obstructive sleep apnea may give rise to long term medical problems such as hypertension and heart failure due to the excessive work that the heart needs to do to compensate for the lack of breathing. The flat face effect caused by wrong surgery is also detrimental to the patient’s psyche. Even if the patient is not seeking jaw surgery for cosmetic reasons, replacing one facial deformity with another can be most disappointing.
The converse can be true as well. If the problem truly lies with the lower jaw being too long, advancing the upper jaw forward to achieve a proper bite will create an unaesthetic result too. An upper jaw that is repositioned too far forward will give an ape-like appearance. Granted, this will not cause obstructive sleep apnea but another functional problem, velopharyngeal incompetence, can arise. This is due to the soft palate being brought too far forward from the throat when the upper jaw is advanced. Patients with cleft palate often have this problem. In its normal position, the soft palate is responsible for separating the oral part of the throat from the nasal part. When the soft palate, which is made up of muscles, contracts, it comes into contact with the back wall of the throat, thereby creating a separation of the nose and mouth. This prevents drinks and food from being regurgitated up the nose when we eat. It is also responsible for speech. If the upper jaw is brought too far forward to correct the bite to the extent that the soft palate is unable to contact the back wall of the throat, food and drinks may get into the nose during meals and some words cannot be pronounced properly in addition to a very nasal speaking tone.
In reality, the majority of cases are not simple single jaw growth anomalies. There is always a bit of both i.e. the lower jaw is long and the upper jaw is short. Doing a single jaw surgery, whether upper or lower, will bring about compromises both functionally as well as aesthetically. Patients presenting for surgical correction of underbite are young and usually parental consent is needed. Parents are usually concern about the morbidity of surgery and would like to have as simple a surgery as possible, and rightfully so. However, if a simpler surgery that does not achieve the objective is definitely more invasive than a less simple surgery that does. Worse, if in the process of solving one problem, we may create new problem.
Some patients or parents may claim that their aesthetics is not important and that all they want is a functional correction. However, in this day and age, I do not believe that any patient will want to change from a long face to a flat face. As such, if the patient only wants a single jaw surgery when a double jaw procedure is needed, I will not proceed with the surgery. Very often, patients are not able to visualize the final outcome even with the help of simulation software. By the time the postoperative swelling subsides and they see themselves in the mirror, it will be quite difficult to change.
The objective of corrective jaw surgery is to correct the bite and harmonize the facial appearance while maintaining a proper airway for breathing, mastication and speech. A good plan should aim to achieve all of the above and sometimes, a double jaw surgery is needed while at other times, a single jaw surgery may suffice.