In orthodontics, the center of resistance of a tooth is defined as what?

• A. The crown's centroid.
• B. The point about which a tooth would rotate if a pure moment were applied.
• C. The cusp tip.
• D. The apex of the root.

Answer: (B)

The center of resistance is the point in the tooth–periodontal ligament–bone system about which the tooth would rotate if a pure moment were applied. It represents the balance point of the resisting tissues so that, with only a moment (no net force), the tooth would rotate around this point rather than translate.

In practice, this point lies inside the root structure—roughly near the middle of the root for a single-root tooth, and near the center of mass of the root bundle for multi-rooted teeth (often closer to the furcation area). It is not at the crown, cusp tip, or the root apex. This concept helps explain why a force passing through the center of resistance tends to produce bodily movement, while forces away from it create tipping or rotational effects, and why a pure moment would cause rotation about this specific point.