Bowing effect in sliding space closure
Bowing is:
- Extrusion of upper incisors
- Retroclination of labial segment – uncontrolled tipping
- = Increase in overbite extend treatment time
Possible solution to bowing effect investigated using finite model analysis:
- Using a powerarm in retraction – distal to upper lateral incisors
- 6mm powerarm – Good control of anterior torque but still bowing effect
- 10mm powerarm with 17×22 archwire in 018 slot – Good control of anterior torque, no bowing but intrusion of anterior segment
- 12 mm powerarch with 19×25 archwire in 022 slot – Good control of anterior torque and little change in occlusal plane
Difference in archwire slot size due to differences in slop, recommend the following:
- 018 slot = 9mm powerarm with 17×22 archwire
- 022 slot = 12mm powerarm with 19×25 archwire
Not possible with 022 slot to prevent any anterior loss of torque or bowing
Solutions to preventing bowing effects
- Compensatory curve anteriorly 2-6mm
- High torque prescription in the labial segment
Compensatory curve details
- The greater the compensatory curve = greater friction produced
- Friction = coef of friction x normal force = greater friction in the later stages of space closure
Reduction of friction
- Archwire with rectangular anterior cross section, and round posterior cross section = reduced friction as less binding
Forces in space closure using power arm
- Forces unequally distributed in conventional en-mass retraction
- High force canine 60g, lateral and central 10-20g of force
Solution to unequal forces on anterior segment
- Retract canine and lateral incisors with different space closing mechanics
Rowboat effect – posterior anchorage loss during space closure due to trying to maintain anterior torque
- Use of TADs
- Loop space closure with = loss of anterior torque
- Incorporating a 30 degree gable bend does not significantly help with anterior torque loss
