orthodontics

Biomechanics of Space Closure (Group B Anchorage)

Anisha ValliLeave a comment

Types of Anchorage Based on Maximum Anchorage Demand

Anchorage is classified into three types based on the maximum anchorage required:

Type A: Maximum Anchorage

  • Definition: Anchorage demand is very high.
  • Space Utilization: Not more than 1/4th of the extraction space should be closed by forward movement of anchor teeth.
  • Mechanism: The extraction space is primarily closed by maximum incisor retraction.

Type B: Moderate Anchorage

  • Definition: Anchorage demand is moderate.
  • Space Utilization: Anchor teeth are allowed to move forward by up to half the extraction space.
  • Mechanism: The extraction space is closed by a combination of incisor retraction and posterior tooth protraction.

Type C: Minimum Anchorage

  • Definition: Anchorage demand is very low.
  • Space Utilization: More than half of the extraction space is closed by mesial movement of the anchor teeth.
  • Mechanism: The extraction space is primarily closed by protraction of posterior teeth with minimal incisor retraction.

Group B Anchorage: Biomechanics of Space Closure

Key Features

  • Archwire: A working archwire with a minimal curve of Spee (e.g., 0.019 × 0.025-inch SS wire) is engaged into the bracket slots and molar tubes.
  • Couples and Forces:
    • The wire generates a couple at the level of the bracket/molar tube at both ends, leading to the generation of a moment of couple (M_c).
    • Tiebacks create a force for space closure (F), generating a moment of force (M_F).
  • Force and Moment Interaction:
    • At either end of the system, the moments of couple and force are created in opposite directions, canceling each other.
    • This leaves only the translatory force (F) to exist, enabling space closure.

Translational Movement

  • When moments are balanced correctly:
  • No vertical component of force is produced.
  • No rotational tendency occurs in the system.
  • Occlusal plane and overbite remain unchanged.

  • Diagram: Depicts the working archwire (faded blue line) and the resulting forces and moments.
  • Outcome: Translation of anterior and posterior segments without altering the occlusal plane or overbite.

Planned Imbalance of Moments

  • In some scenarios, moments are deliberately imbalanced to achieve specific vertical movements:
  • Posterior Teeth: Extrusion (yellow arrows).
  • Anterior Teeth: Intrusion (yellow arrows).
  • Effect: Opens the deep overbite by modifying the vertical dimension.
  • Diagram: Illustrates the planned imbalance (thick red curved arrow) leading to controlled vertical movements.
  • Outcome: Deep overbite correction through extrusion of posterior teeth and intrusion of anterior teeth.

Summary

Group B anchorage allows for controlled space closure through balanced forces and moments. By adjusting the moments, orthodontists can achieve either translational movement or vertical adjustments, making it a versatile approach in clinical orthodontics.

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