HOW DOES THE LOWER JAW NORMALLY GROW?
The lower jaw (aka the Mandible) has a very unique growth pattern and potential. When a child grows from the age of an infant to a full-grown adolescent, their lower jaw has typically made some significant strides! Intuitively, you might imagine that the lower jaw grows like a balloon that expands outward; getting bigger on all sides. It turns out this isn't true. The lower jaw has a very distinct growth pattern that involves adding bone to some sides and holding or even removing bone from other sides. The base of the lower jaw also expands in a "V-shaped" pattern while the connection with the skull (aka the Temperomandibular Joint or TMJ) also changes. The portion of the lower jaw that articulates with this jaw-joint is unique in that depending on the *direction* it grows, it can have positive or negative effects for the lower jaw's position and the overall profile appearance. The following diagram shows the general direction (red arrows) of the upper and lower jaws as well as some of the rotational factors (green arrows) that can effect the overall relationship between the upper and lower jaws.
OKAY - SO HOW CAN WE HELP THE LOWER JAW GROW?
The classic way to help a lower jaw grow forward was to address the upper jaw with "headgear". Most people hear the word headgear and conjure up embarrassing memories of their peers in Elementary school teasing them or some adolescent "coming of age" movie from the 80's. While the stigma of this appliance has been unfairly earned due to its very visible nature, it can be surprisingly effective in the right scenario. The appliance works by holding the upper jaw in place while "allowing" the lower jaw to grow downward and forward. In effect, by not allowing the upper jaw to interfere with the lower jaw's forward growth pattern (by causing it to swing backwards instead of forwards), a patient can achieve their optimal lower jaw position and growth potential. In addition, the upper molars can be guided backwards to assist in correcting the bite-relationship (dental effects). The sum of these treatment effects has been described as the "headgear effect" and can be found in greater or lesser amounts of each of the other appliances described, here.
We tend to use this appliance very infrequently. It's not that the appliance doesn't work (it certainly does), but rather we have other ways of helping our patients achieve similar treatment outcomes in shorter time-periods and with less burden on our patients and their parents.
THE HERBST-STYLE OF APPLIANCE
Another common type of appliance used to encourage the lower jaw to grow is known as the Herbst appliance. This appliance rose to popularity when orthodontic clinicians wanted to help their patients achieve the same type of effects as headgear provides but with an inside-the-mouth (intraoral) appliance that removes the visible portion of the headgear appliance. This appliance is generally considered "functional appliance" because the initial versions were thought to exert their effects by influencing the "functional environment" (skeletal structures and chewing muscles and even the tongue). These doctors also knew that not every patient wants to wear the headgear appliance throughout the day and they may intentionally or unintentionally fail to replace the headgear (after eating dinner for example). Orthodontists began considering other types of fixed (or glued in) functional appliances (like the Herbst style of appliance) that would provide the same treatment effects but remove the need for a patient or parent to remember to wear the headgear. In contrast with headgear, the Herbst style of appliance uses push-rods to move the upper teeth backward and the lower teeth forward. Those push rods are usually connected to some type of crown or band that encircles the upper and lower first molars. At the same time this appliance can encourage lower jaw growth with or without the presence of a positive changes in the position of the temperomandibular joint. The sum total of these effects is a better lower jaw position and corrected bite using an inside-the-mouth (intraoral) appliance that can at the same time allow for the use of braces to straighten the remaining teeth.
THE MARA (MANDIBULAR ANTERIOR REPOSITIONING APPLIANCE)
Sometimes our younger patients will benefit from the ability to encourage the lower jaw to grow but they may need an appliance that is very comfortable and a bit easier to adapt to. In this case, we may consider a MARA appliance. MARA stands for "Mandibular Anterior Repositioning Appliance" and has very similar effects as the Herbst style appliance. The difference is that this appliance is a bit more comfortable for most patients due to its protective "shield" and lack of moving parts. Just like the Herbst style of appliance, braces can be worn at the same time to straighten the front teeth while the back teeth are being shifted and the lower jaw is encouraged to grow.
These are only three different types of appliances that can be used to encourage the lower jaw to grow. There are many other styles and classes of similar appliances. Most of these appliances are worn for at least one year. The typical minimum protocol is to work on shifting the bite and posturing the lower jaw forward for 6 months and then holding things in place while the lower jaw finishes growing into position. Once the lower jaw position is confirmed as being stable, the appliance is typically removed and retainers are provided until the adult teeth have fully erupted into place. If the adult teeth are already present, treatment is completed by replacing the missing brackets and "dialing in" the tooth position for a beautiful and functional bite and smile!