Moment to force ratios with the Phoenix orthodontist

A tooth’s center of resistance is typically at about 1/3 the distance from the apex. A force whose line of action goes through the center of resistance would result in translation. Phoenix orthodontics, however, does not usually allow the force to be placed directly at the center of resistance since the bracket is placed on the tooth’s crown. By placing a force at the bracket, the result is movement of the tooth around a center of rotation. This center of rotation can be altered by changing the M/F ratio (putting torque in the wire). The Phoenix orthodontist best explains the relationship between M/F ratio and the center of rotation.

As the M/F ratio approaches zero (Point A), the center of rotation approaches the center of resistance with never actually reaching it. This results in uncontrolled tipping of the tooth and is unconsidered undesirable by the Phoenix orthodontist.

As the M/F ratio increases (M/F≈7), the center of rotation moves to the root apex (Point B). This results in controlled tipping of the tooth.

As the M/F ratio increases (M/F≈10), the center of rotation approaches infinity in an apical direction (Point C). This results in translation (bodily movement) of the tooth.

A slight additional increase in M/F ratio (M/F≈10) causes the center of rotation to approach infinity in an incisal direction (Point D). This still results in translation of the tooth by the Phoenix orthodontist.

As the M/F ratio continues to increase (M/F≈12), the center of rotation moves to the incisal edge of the tooth (Point E). This results in controlled root movement by the Phoenix orthodontist.

As the M/F ratio continues to increase (M/F≈20), the center of rotation approaches the center of resistance once again (Point F). This results in uncontrolled tipping of the tooth.