ABO Orthodontic Biomechanics 2

Question 1

A force applied at a distance from the center of resistance of a tooth produces which type of movement?

Accepted Answer

Tipping combined with rotation

Answer

A single force offset from the center of resistance creates a moment, causing tipping (rotation) plus translation.

Question 2

The center of resistance of a single-rooted tooth is generally located at what position along the root?

Accepted Answer

About 33–40% of the root length from the alveolar crest

Answer

For a single-rooted tooth, the center of resistance lies roughly one-third to two-fifths down the root from the alveolar crest.

Question 3

What is the moment-to-force (M/F) ratio approximately required to produce pure translation (bodily movement) of a tooth?

Accepted Answer

10:1

Answer

An M/F ratio of about 10:1 counteracts tipping and produces bodily (translational) tooth movement.

Question 4

Increasing the M/F ratio beyond that needed for translation results in which movement?

Accepted Answer

Root movement (torque)

Answer

M/F ratios higher than ~10:1 (near 12:1) move the root more than the crown, producing root torque.

Question 5

A couple is defined as:

Accepted Answer

Two equal and opposite non-collinear forces producing pure rotation

Answer

A couple consists of two equal, opposite, parallel forces that create pure rotation without net translation.

American Board of Orthodontics (ABO) Practice Test