Mandibular 1st Molar to Central Incisor ─ Burstone's Principle
Question
You want to intrude L1 (mandibular central incisor). Where should the V-Bend be placed?
A: Near incisor
Incisor extrudes, Molar intrudes
Molar → Intrusion Incisor → Extrusion
B: Near molar ★Correct
Molar extrudes, Incisor intrudes
Molar → Extrusion Incisor → Intrusion
C: Midpoint
Zero vertical force, Moment only
Both → Moment only
Burstone's Principle
Tooth near the V-Bend = Extrusion (pulled out)
Tooth far from the V-Bend = Intrusion (pushed in)
∴ To intrude the incisor → Place the bend at B (near the molar), far from the incisor
Why does this happen? ─ Think of a Seesaw
Wire = Seesaw board, V-Bend = Fulcrum
Place the fulcrum near the molar. The nearby molar springs up (extrusion), and the distant incisor sinks (intrusion).
If you place the seesaw fulcrum right next to yourself, you spring up, while the distant partner stays down.
Same thing with a Clothesline pole
What if you push up the pole near the molar side?
The stand near your finger (molar) rises, and the distant stand (incisor) is pressed down.
① Straight = Zero force
② Push up near the molar = V-Bend
③ Near side rises, far side sinks
Tooth near the V-Bend = Extrusion
Tooth far from the V-Bend = Intrusion
Seesaw or clothesline pole — same principle. That's all there is to it.
Application ─ Anti-Spee Curve (Reverse Curve) follows the same principle
V-Bend = Single bend point → Reverse Curve = Collection of continuous bend points
Placing a reverse curve in the wire to correct the Curve of Spee is essentially the elastic restoring force of the wire. It is equivalent to multiple V-Bends distributed continuously near the molar region.
V-Bend (single point) vs Reverse Curve (continuous)
V-Bend = Single bend point → Pinpoint control between 2 teeth Reverse Curve = Continuous bend points → Force distributed across the entire arch
Both rely on "the elastic force of the wire trying to return to its straight form." Same principle.
When a reverse curve is placed in a case with a deep Curve of Spee: Molar region → Extrusion (the curve is strongest near the molars = near the V-Bend) Incisor region → Intrusion (far from the curve = far from the V-Bend)
As a result, the Curve of Spee is leveled.
Supplementary ─ U-arch vs Continuous Arch
The V-Bend principle holds true directly in segmented arches
Burstone advocated segmented arches to simplify the force system and make it controllable.
U-arch (Segmented) = 2-point constraint → Force is predictable and controllable Continuous = Multi-point constraint → Force is dispersed and unpredictable due to friction
The V-Bend principle holds true directly in segmented arches. This is why Burstone advocated the segmented arch approach.