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Sunday, March 2, 2008

Important Orthodontic Studies

For anyone practicing orthodontics, keeping up with the literature is essential. There are manyways 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 majororthodontic 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, butalso 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 OrthodonticDepartment of the Royal College of Dentistry in Copenhagen is the single most valuable studyever 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. Bjorkplaced 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. Todays medical ethics prevent researchers from placing implants forobservation 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 1950s than it is today, Bjork was

able to provide the orthodontic community with a valuable body of data.

So, whats 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 thedifference between growth and bone remodeling.

Interpretation of Bjorks data lead to some interesting conclusions. The driving force responsiblefor 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.

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If cellular proliferation is near the posterior surface of the head of the condyle, the mandiblerotates in a backward (clockwise) direction. See figure below.

As the mandible moves due to the cellular proliferation, the sling of muscles that encapsulate themandible 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.

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Because of the way the muscles act (as well as some other factors), forward rotators are referredto as strong muscled patients, and backward rotators are called weak muscled patients. Almostall 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 differenttreatment 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 openbite 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 determineif 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 Bjorks 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 somestrong 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 verticaldimension in borderline patients is one of the most important aspects of good treatment.

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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 fivecharacteristics 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 totreatment mechanics.

Lets 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 roundingis 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 themuscles 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 thecondyle.In strong muscled patients, the condyle will incline anteriorly, while in weak muscled

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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). Althoughweak 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 datawill 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 threetypes of intramatrix rotation, two which can occur in strong muscled patients, and one whichoccurs 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 theincisal 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.

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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 earlyexfoliation 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 theposterior 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 archlength 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.