One of the most common orthodontic problems that we encounter in new patients is a small lower jaw. This may manifest itself as an overbite, a deep bite, or crowding (in either jaw). Wouldn’t it be great if we had some way to encourage the jaws to grow in the amount and direction needed to correct these issues? This has been the dream of orthodontists and their patients for over 100 years. Is there really a way to change the size a jaw without surgery? Can functional appliances actually “grow” the jaws?
Functional appliances are so named because they are designed to encourage patients to function (bite and chew) with their jaws in an improved relationship in hopes that the body will respond by growing the jaw in that direction. “Old school” functional appliances like the Bionator, the Frankel, and the Twin Block are removable devices made of plastic and wire that force the patient to bite into a corrected jaw position. The Herbst appliance was one of the first “fixed” functional appliances to gain popularity. Modern functional appliances also include the MARA appliance, Forsus Springs, the Myobrace appliance, and Jasper Jumpers. In every case, functional appliances use the teeth in one arch to push on the teeth in the other jaw in hopes of making the jaws grow differently than they would naturally.
The size and shape of our jaws is determined by genetics (DNA). Orthodontists can measure the size, shape, and position of the dental and skeletal structures of the face using an accurate technique called cephalometrics. This technique involves tracing and measuring standard structures on an x-ray of your head. Such analysis performed on siblings of the same parents reveals that their underlying skeletal structures are very similar and mostly inherited. Even if your teeth are arranged differently than your siblings, the size and shape of your jaws are primarily determined by your genealogy.
Another important use of cephalometrics is determining the effects that orthodontic devices have on the mouth during treatment. A tracing made before treatment can be compared to a tracing made afterwards to reveal how correction was really achieved. If there has been jaw growth, the tracings will indicate that. If the jaw sizes remain the same and only teeth have been moved, the analysis also shows this. University-based cephalometric studies conducted over the past several decades have all come to the same conclusion. The changes produced by functional appliances of all types are primarily DENTAL in nature. In other words, the size and shape of the jaws remains almost unchanged while the teeth in both arches are shifted so that they come together better.
The reasons that patients (and some orthodontists) mistakenly believe that functional appliances actually grow the lower jaw is that when they are used, overbites get better! If an overbite gets better, does it really matter how the functional appliance achieved it? Not really if there is enough bone and gum tissue to support the teeth in their new position. If a patient has a really short lower jaw however, they may not have enough bone to safely allow for the amount of tipping needed to correct the overbite. This is why orthodontists (orthodontic specialists) are trained to analyze jaw size and tooth position before functional appliances are prescribed. While they may work great in some patients, they may actually be harmful in others. There is nothing wrong with functional appliances when used in the right patients. I prescribe them all the time. Understanding that they primarily tip teeth and don’t “grow” jaws is critical in determining for whom they are appropriate.
NOTE: The author, Dr. Greg Jorgensen, is a board-certified orthodontist who is in the private practice of orthodontics in Rio Rancho, New Mexico (a suburb on the Westside of Albuquerque). He was trained at BYU, Washington University in St. Louis, and the University of Iowa in the United States. Dr. Jorgensen’s 25 years of specialty practice and nearly 10,000 finished cases qualify him an expert in two-phase treatment, extraction and non-extraction therapy, functional orthodontics, clear aligners (Invisalign), and multiple bracket systems (including conventional braces, Damon and other self-ligating brackets, Suresmile, and lingual braces). This blog is for informational purposes only and is designed to help consumers understand currently accepted orthodontic concepts. It is not a venue for debating alternative treatment theories. Dr. Jorgensen is licensed to diagnose and treat patients only in the state of New Mexico. He cannot diagnose cases described in comments nor can he select treatment plans for readers. Please understand that because he has over 40,000 readers each month, IT IS IMPOSSIBLE FOR HIM TO RESPOND TO EVERY QUESTION. Please read all of the comments associated with each article as most of the questions he receives each week have been asked and answered previously. The opinions expressed here are protected by copyright laws and can only be used with written permission from the author.