Biomechanics and Orthodontic Tooth Movement
Forces and moments
Understand the difference between force, moment, and couple, and how a moment-to-force ratio determines the center of rotation. Know that a pure force moves the tooth bodily through the center of resistance, while a moment produces rotation. Know the centers of resistance of single-rooted and multi-rooted teeth and how they shift with root length and bone level.
Tissue response
Know the pressure-tension theory and the role of the periodontal ligament in mediating tooth movement. Understand the phases of tooth movement: initial, lag, and post-lag, and the concept of optimal force. Know that excessive force causes hyalinization, undermining resorption, and delayed movement, while light continuous force produces frontal resorption and more efficient movement. Understand the role of blood flow, prostaglandins, and cytokines.
Traps
A classic trap is confusing the center of resistance with the center of rotation, or assuming that a heavier force always moves teeth faster. Another trap is ignoring anchorage loss: every action has an equal and opposite reaction, so questions often test whether you can identify the anchorage unit and how to reinforce it. Watch for questions that conflate tipping and translation.
Worked example
A canine needs 4 mm of distal translation without tipping. Rule of thumb: to translate a single-rooted tooth, apply a force through the center of resistance, or apply a force plus a counterbalancing moment with an appropriate moment-to-force ratio. If the crown tips, the center of rotation is near the root apex; if it translates, the center of rotation is at infinity.