ABO Orthodontic Biomechanics 2 — Questions and Answers
Question 1: A force applied at a distance from the center of resistance of a tooth produces which type of movement?
- Pure translation
- Tipping combined with rotation (Correct answer)
- No movement
- Pure intrusion only
Correct answer: Tipping combined with rotation
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?
- At the root apex
- About 33–40% of the root length from the alveolar crest (Correct answer)
- At the incisal edge
- Outside the tooth in the bone
Correct answer: About 33–40% of the root length from the alveolar crest
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?
- 0:1
- 5:1
- 10:1 (Correct answer)
- 17:1
Correct answer: 10:1
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?
- Crown tipping
- Root movement (torque) (Correct answer)
- Extrusion
- Pure rotation about the long axis
Correct answer: Root movement (torque)
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:
- A single force through the center of resistance
- Two equal and opposite non-collinear forces producing pure rotation (Correct answer)
- A force plus friction
- Any force causing translation
Correct answer: Two equal and opposite non-collinear forces producing pure rotation
A couple consists of two equal, opposite, parallel forces that create pure rotation without net translation.
Question 6: Which factor most directly determines the magnitude of the moment created by a force on a tooth?
- Force magnitude times perpendicular distance to the center of resistance (Correct answer)
- Tooth color
- Patient age alone
- Bracket material only
Correct answer: Force magnitude times perpendicular distance to the center of resistance
Moment equals force multiplied by the perpendicular distance between the force line and the center of resistance.
Question 7: Optimal orthodontic force for tooth movement is best described as:
- The heaviest force tolerable
- Light continuous force that does not exceed capillary blood pressure (Correct answer)
- Intermittent maximum force
- Force applied only once weekly
Correct answer: Light continuous force that does not exceed capillary blood pressure
Light, continuous forces that avoid occluding the periodontal capillary blood supply produce efficient, safe movement.
ABO Orthodontic Biomechanics 2
A force applied at a distance from the center of resistance of a tooth produces which type of movement?