Bodily tooth movement occurs when the tooth translates without tipping. Which condition must the force system satisfy to achieve translation?

• A. The line of action passes through the center of resistance and the moment-to-force ratio is optimized to produce translation.
• B. The line of action passes through the crown and the moment-to-force ratio is large.
• C. A pure moment is applied with no resultant force.
• D. The force is confined to the crown region to limit movement.

Answer: (A)

Bodily movement happens when the overall force system acts through the tooth’s center of resistance. The center of resistance is the point where a single force would move the tooth without tipping, so if the resultant force line passes through that point, there is no rotational moment to cause tipping. The moment-to-force ratio must be adjusted so there’s no net turning effect about the center of resistance, allowing translation rather than rotation. If the force passes through the crown, or if you apply a pure moment with no resultant force, or if the force is confined to the crown region, tipping occurs instead of translation. Hence, the condition that the line of action passes through the center of resistance and the force system is tuned to yield translation is the correct description.