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Newton’s third law of motion states that for every action there is an equal and opposite reaction. This concept in physics affects every aspect of Phoenix orthodontic biomechanics. Since the sums of the forces and the moments must equal zero in a state of equilibrium, unwanted side effects occur with almost any Phoenix orthodontic movement. For example, if a canine is retracted with an elastomeric chain, the posterior teeth will be protracted with the same amount of force.

In order to reduce these unwanted movements, anchorage requirements must be considered for each case. There are three classifications of anchorage for cases requiring extraction.

Group A Anchorage – The majority of the extraction space is used for retraction of the anterior teeth. Distal movement of the posterior teeth may be needed.
Group B Anchorage – The extraction space is shared equally between the anterior and posterior teeth. This is considered the easiest movement to accomplish.
Group C Anchorage - The majority of the extraction space is used for the protraction of the posterior teeth (Nanda, 1997).

Phoenix orthodontists have limited options to produce this anchorage. Even the father of modern Phoenix orthodontics, Edward Angle, discussed this problem, “According to the well-known law of physics, action and reaction are equal and opposite, hence it must follow that the resistance of anchorage must be greater than that offered by the tooth to be moved, otherwise there will be displacement of the anchorage and failure in the movement of the teeth to the extent, or, possibly, in the direction desired. The sources at our disposal for securing anchorage or resistance are, first, the teeth themselves, and second, sources external to the teeth, and anchorages may be classified as simple, stationary, reciprocal, intermaxillary and occipital”(Angle, 1907). Today, Phoenix orthodontists incorporate osseointegrated implants to utilize the bone as an anchor, creating a new term, absolute anchorage.