For anyone practicing orthodontics, keeping up with the literature is essential. There are many ways to do this, but one of the easiest methods is to subscribe to Practical Reviews in Orthodontics. ( http://www.cmeonly.com/programdetails.cfm/2/44/2) Each month you receive an audio CD and a written synopsis of the most germane orthodontic articles from all the major orthodontic journals. The reviewers do a great job of summarizing all that is new and important in the orthodontic literature. Try this service; you will not be disappointed.
The orthodontic practitioner should not only keep abreast of current orthodontic literature, but also be aware of the studies that have shaped how orthodontics is practiced today. I believe the study performed by Professor Arne Bjork while he was the chairman of the Orthodontic Department of the Royal College of Dentistry in Copenhagen is the single most valuable study ever done in the field of orthodontics.
Professor Bjork practiced orthodontics for about 20 years before accepting the previously mentioned teaching position in 1950. For the next 15 years, he worked on this study. Bjork placed titanium implants in the maxillas and mandibles of 240 children. He then took yearly records, performing no other treatment on these patients. This research is valuable because it can never be duplicated. Today’s medical ethics prevent researchers from placing implants for observation only. In addition it is now unethical to watch and not treat severe malocclusions. Because the scope of medical ethics was so different in the 1950’s than it is today, Bjork was able to provide the orthodontic community with a valuable body of data.
So, what’s the big deal? Why is this information so precious? Well, by superimposing cephalometric x-rays on the implants, Bjork was accurately able to determine how faces changed with growth. When superimposing cephs without implants, it is nearly impossible to discern the difference between growth and bone remodeling.
Interpretation of Bjork’s data lead to some interesting conclusions. The driving force responsible for facial growth seems to be the condyles. If cellular proliferation is near the anterior surface of the head of the condyle, the mandible rotates in a forward direction (counter- clockwise, if one views the chin in profile). See figure below.
If cellular proliferation is near the posterior surface of the head of the condyle, the mandible rotates in a backward (clockwise) direction. See figure below.
As the mandible moves due to the cellular proliferation, the sling of muscles that encapsulate the mandible are responsible for pressures and tension directed onto the bone. These forces result in apposition and resorption of mandibular bone. Therefore, mandibular morphology is different for forward and backward mandibular rotation.
Because of the way the muscles act (as well as some other factors), forward rotators are referred to as strong muscled patients, and backward rotators are called weak muscled patients. Almost all orthodontic mechanics result in extrusive forces on the teeth. Strong muscled patients resist this extrusive tendency, while weak muscled patients tend to not resist this tendency. This leads us to a very important concept: the same brackets, bands and wires will produce different treatment results in different patients. Muscle strength (which can vary by a factor of 6 between strong and weak muscled patients) is the main reason for these variable treatment responses.
So, how do we use this knowledge to improve treatment? Weak muscled patients tend to be open bite patients; the extrusive component of orthodontic mechanics is often expressed. Conversely, it is often very difficult to open the bite in strong muscled patients (who tend to be deep bite patients). By looking at the shape (morphology) of the mandible, the practitioner can determine if bite opening or closing will be a problem. A specific treatment plan for the individual patient can then be devised.
Some other facts stemming from Bjork’s work are very important. First, the distribution of growth cells on the head of the condyle follows a bell shaped curve. That is, not all patients are entirely strong or weak muscled. About 85% of patients are predominately strong muscled (good thing, because weak muscled, open bite patients are difficult to treat). Many patients have some strong and some weak muscled characteristics. The most difficult cases are the very strong, and especially very weak muscled patients. These cases are often easy to pick out because the mandibular morphology is very diagnostic. The difficult part is to monitor the borderline cases to see if vertical control becomes problematic. Graber states in his textbook that controlling vertical dimension in borderline patients is one of the most important aspects of good treatment.
Second, forward or backward rotation is a highly genetic phenomenon. Condylar growth direction depends on the location of the growth cells; this is an inherited trait. However, growth patterns can be affected by the environment. For example, airway blockage, habits, allergies, etc. can change the normal position of the mandible, allowing different parts of the growth center to be more fully expressed. So, according to Bjork, environment influences growth while genetics controls it.
Bjork used knowledge of apposition and resorption of bone based on muscular pressures and tensions to determine muscle strength based on mandibular morphology. I like to use five characteristics to point out the morphological differences between strong and weak muscled patients. Not all these characteristics are visible on all patients, and previous growth direction does not insure that future growth will continue in the same direction. But despite these limitations, mandibular morphology is a useful predictor of both future growth and response to treatment mechanics.
Let’s explore the specific morphological characteristics I use. First,
the gonial angle will be more acute in strong muscled patients and more obtuse in weak muscled patients. Second, the shape of the lower border of the mandible is a good predictor. In weak muscled patients, apposition below the symphysis and resorption anterior to the gonial angle produces a concavity throughout the lower border. In strong muscled patients, anterior rounding is absent. In addition, notching occurs anterior to the gonial angle. This results in an "S" shaped curve on the lower border. The third predictor I like to use is the density of bone at the symphysis. A thick symphysis indicates strong muscles, while a thin symphysis means the muscles are weak. Fourth, the inclination of the symphysis is a reliable predictor of muscle strength. In strong muscled patients, the inclination is relatively acute, while the norm for weak muscled patients is a more obtuse inclination. The final indicator I use is the inclination of the condyle.In strong muscled patients, the condyle will incline anteriorly, while in weak muscled patients, the condyle will have a posterior inclination. This trait is not always visible on the ceph because of superimposition of structures over the condyle on ceph x-rays.
There are many other predictors of mandibular growth rotation.Many clinicians rely solely on mandibular plane angle to predict muscle strength (and, hence, treatment response). Although weak muscled patients usually have higher mandibular plane angles than do strong muscled patients, this measurement can be deceiving. If the clinician uses more than one measurement to arrive at a diagnosis, the diagnosis will probably be more accurate. Using all the available data will help insure that the patient will receive the best diagnosis possible.
In addition to maxillary and mandibular growth rotation (the maxilla follows the same basic rotational pattern as the mandible), Bjork also described the intramatrix rotation. He defined the intramatrix as the maxillary and mandibular teeth and alveolar processes. Bjork described three types of intramatrix rotation, two which can occur in strong muscled patients, and one which occurs in weak muscled patients. To understand intramatrix rotation, one must understand Bjork's definition of the fulcrum. The fulcrum is simply the most anterior contact point of teeth in occlusion.
Type I intramatrix rotation occurs in strong muscled patients when the fulcrum exists at the incisal edges of the maxillary and mandibular anterior teeth. This combination of mandibular and intramatrix rotation leads to normal downward and forward growth of the cranio-facial complex. This results in the best possible growth for the patient.
Type II intramatrix rotation occurs when mandibular rotation is forward without an incisal edge fulcrum. This lack of incisal edge fulcrum often results from tongue or lip habits, or from early exfoliation of primary teeth.The fulcrum now exists in the middle of the arch. This pattern leads to over eruption of maxillary and mandibular anterior teeth, a deep bite, and collapse (lingual movement) of the maxillary anterior segment-a classic Class II, Division II malocclusion.
Type III intramatrix rotation occurs in weak muscled patients where the fulcrum is on the posterior teeth. If sufficient eruption occurs in the anterior segments, the result is a long face with good occlusion. If something (tongue, lip, fingers) interferes with anterior eruption, an open bite results.
Understanding cranio-facial growth rotation leads to many interesting diagnostic conclusions. In Type I and Type II intramatrix rotation, teeth move forward and laterally on the alveolar processes. The opposite occurs in Type III intramatrix rotation. Therefore, expansion and arch length gaining treatment may be more successful in Type I and II intramatrix rotation than in Type III intramatrix rotation. Crowding that can be corrected by expansion in a strong muscled patient may require extractions in a weak muscled patient. In fact, every decision you make regarding a patient's treatment will be influenced by the patent's muscle strength. Extraction vs. non-extraction, bracket position, composition of arch wires used, and type of retainer used are all greatly influenced by a patent's muscle strength. It is clear that an understanding of Bjork's research will change the way you look at orthodontic diagnosis.
