Have you ever stretched a rubber band between your two hands before? Did each hand feel the same amount of “pull” back towards the other hand? This principle is explained by Newton’s Third Law of Motion: For every action there is an equal but opposite reaction.

This Law of Motion also applies to the forces that are used by your orthodontist to move teeth. When a spring is placed between two teeth, each tooth feels an equal (but opposite) force. This situation is acceptable except in cases where it would be desirable for some teeth to not move and other teeth to move a great distance.

Consider the case of a patient with a large “overbite,” whose front teeth stick out too far forward. Ideally, the back teeth would stay in place while the front teeth were pulled back to the appropriate position. Unfortunately, according to Newton’s Third Law of Motion, when a force is placed between the front and back teeth, both will feel an equal pressure, and thus would be subject to moving. This situation results in one of the most challenging biomechanical issues for orthodontists.

Orthodontic Anchorage
So what is your orthodontist going to do to control how much each tooth moves? Is there a way to overcome Newton’s Third Law of Motion? To explain how this is accomplished, we need to discuss the concept of anchorage.

What do you think of when you hear the word anchor? You may think of a heavy object that is lowered into the water to keep a boat in position. When the concept of anchorage is applied to orthodontics, it is defined as: Resistance to unwanted tooth movement.

Since orthodontic anchorage is a key determinant of treatment success, many approaches have been developed over the decades. These techniques can be divided based on whether or not they are based in bone (skeletal anchorage) or not (traditional).

Traditional Anchorage Techniques
There are a number of techniques that have provided orthodontists with a measure of anchorage that are quite straight forward.

It is possible to create a favorable anchorage situation by using a large group of teeth to pull against a smaller group. This is similar to a “tug-of-war” match, with the smaller group moving more than the larger group of teeth would.

Another way orthodontists control the anchorage situation is by using special appliances. There are various appliances that can be used to engage either the soft tissues or other teeth to provide anchorage.

Finally, there are auxillaries, such as elastics and headgear that can be worn by the patient to prevent unwanted tooth movement

As a group, these techniques are predictable, low in cost, and effective to a degree. There are disadvantages to consider as well such as the side effects of unwanted tooth movement, a high degree of patient compliance, and a relatively low level of anchorage.

Skeletal Anchorage Techniques
Skeletal anchorage would be like pulling a rubber band that is connected to a hook on the wall. Both your hand and the wall would feel an equal amount of pressure (according to Newton’s Third Law of Motion), but the wall would not move.

Employing the underlying jaw bones for anchorage is not a new concept. Early attempts are recorded in the literature and date back to the 1940’s. With the development of dental implants through the late 1980’s and early 1990’s, a reliable strategy for using skeletal anchorage became available and was used widely.

Subsequently, additional skeletal anchorage approaches were introduced such as the retromolar implant, the palatal implant, the onplant, zygomatic ligature wires, and mini-plates.

While these techniques could provide maximum amounts of anchorage, eliminate the unwanted dental side effects of traditional techniques, and reduce the need for patient compliance, they were not without some drawbacks.

Relatively high cost, invasive surgical procedures for placement and removal of the devices, limitations on placement locations, and a prolonged latency period prior to utilizing the anchor all affected the overall acceptance of these skeletal anchorage methods.

The ideal skeletal anchorage unit would be cost effective, easy to place and remove, have the flexibility to be placed in multiple locations, and could be used immediately to produce maximum anchorage for quicker treatment results.

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Dr. Garfinkle is a Board Certified Orthodontist practicing in Portland, Oregon. Read more about Dr. Garfinkle or visit his practice online at www.garfinkleortho.com.