Orthodontic force creates pressure and tension zones in the periodontal ligament, and the bone responds by resorbing on the pressure side and forming on the tension side, so the tooth moves. Biomechanics describes how force, moment and the centre of resistance decide whether the tooth tips, translates or rotates.
Biology: how bone responds to force
When a force is applied, the periodontal ligament is compressed on one side and stretched on the other. Osteoclasts remove bone where it is compressed and osteoblasts lay down bone where it is stretched. Light continuous forces keep this process steady. Heavy forces can crush the ligament, delay movement and raise the risk of root resorption. Movement commonly averages in the region of one millimetre a month, with large individual variation, and some medicines affect the rate.
Biomechanics: force, moment and the centre of resistance
A bracket sits on the crown, while the centre of resistance of the tooth lies within the root. A force applied at the bracket therefore creates a moment about the centre of resistance, and the tooth tips. Different ratios of moment to force produce tipping, bodily movement or root movement. This is the reason rectangular wires, torque and couples matter.
Anchorage: Newton's third law at work
Every force has an equal and opposite reaction. When teeth are retracted, the reaction moves other teeth, which is anchorage loss. Anchorage planning decides which teeth should move and which should stay, and mini implants are one way to hold the reaction.
Why teeth sometimes move unexpectedly
The early levelling phase can proclinate lower incisors, canines can tip during retraction, and posterior teeth can extrude with vertical elastics. These are the predictable outcome of the forces applied and they can be anticipated and managed when you understand the mechanics. This is the content of the troubleshooting section of the course.
Key takeaways
- Light continuous forces give the most efficient and safest movement.
- A force at the bracket creates a moment about the centre of resistance.
- Plan anchorage before applying a force, since every force has a reaction.
Frequently asked questions
How much force is used in orthodontics?
Orthodontic forces are light and continuous. The ideal magnitude depends on the tooth and the type of movement, and heavy forces slow movement and raise the risk of root resorption.
Why do teeth move faster in some patients?
Age, bone density, force magnitude, medicines and individual biology all play a part, so movement rates differ between patients.