Understanding the basis of space closure in Orthodontics for a more efficient orthodontic treatment

Abstract

Introduction: Space closure is one of the most challenging processes in Orthodontics and requires a solid comprehension of biomechanics in order to avoid undesirable side effects. Understanding the biomechanical basis of space closure better enables clinicians to determine anchorage and treatment options. In spite of the variety of appliance designs, space closure can be performed by means of friction or frictionless mechanics, and each technique has its advantages and disadvantages. Friction mechanics or sliding mechanics is attractive because of its simplicity; the space site is closed by means of elastics or coil springs to provide force, and the brackets slide on the orthodontic archwire. On the other hand, frictionless mechanics uses loop bends to generate force to close the space site, allowing differential moments in the active and reactive units, leading to a less or more anchorage control, depending on the situation.

Objective: This article will discuss various theoretical aspects and methods of space closure based on biomechanical concepts.

Figures

Figure 1. Anchorage classification: Group A space closure includes, on average, 25% of posterior anchorage loss and 75% of anterior retraction; Group B space closure includes more equal amounts of anterior and posterior tooth movement; Group C space closure includes, on average, 75% posterior protraction and 25% of anterior retraction. Absolute anchorage includes practically 100% of anterior retraction.

Figure 2. A force that does not pass through the center of resistance produces a rotational movement (moment of force) as well as s linear movement.

Figure 3. Types of tooth movement: A) Uncontrolled tipping; B) Controlled tipping; C) Bodily movement; D) Root movement. The red arrows represent the force applied to teeth and the moment of force. The blue arrows represent the force of a wire into the bracket and the moment of a couple. The green arrow is the resultant moment (moment of force minus moment of a couple).

Figure 4. Differential moment reduces the moment/force ratio on one segment while increasing the moment/force ratio on another. Vertical forces occur due to difference in alpha and beta moments.

Figure 5. As the canine tips distally during retraction, the orthodontic wire binds against the edge of the bracket slot ("binding effect"), increasing friction.

Figure 6. Force system generated by a closed coil spring applying force bellow de center of resistance of the segments. Due to linear distance between the force application and center of resistance, moments occur, and the dumping effect with vertical forces will take part of the space closure.

Figure 7. Force system generated by a closed coil spring applying forces at the level of the center of resistance by means of extension hooks (power arms). No moments and vertical forces occur.

Figure 8. A) Closing loop with bends in the winding-direction. This configuration presents more resistance to permanent deformation during activation; B) Closing loop with bends in unwinding-direction.

Figure 9. Tear drop loop asymmetrically placed (closer to anterior than posterior segments) provides a very low moment/force ratio with inadequate root control. The advantage of this loop position is the possibility of numerous activations on the same wire as the space closes.

Figure 10. Space closure in a clinical case with non-extraction treatment: A) Initial phase; B) Beginning of the space closure phase; C) End of treatment.

Figure 11. Space closure in a clinical case with extraction treatment: A) Initial phase; B) Beginning of the space closure phase; C) Gable bends: D) End of treatment.

Figure 12. Clinical case with maxillary first premolar extractions and congenitally missing mandibular second premolars. A) End of the alignment and leveling phase; B) Beginning of space closure; C) End of treatment.

Figure 13. Clinical case with all four first premolar extractions. A) Initial; B) Partial canine retraction; C) Beginning of space closure using a T-loop design; D) Progress of space closure; E) Management of canine relationship; F) End of the case.

Figure 14. Management of space closure in a surgical case. A) Initial phase; B) Space closure phase; C) Class III elastics to create a differential anchorage control and decompensation of the incisors; D) End of treatment.

Figure 15. Clinical case with maxillary and mandibular first premolar extractions. A) Initial phase; B) Beginning of space closure; C) Headgear to provide greater anchorage on maxillary molars; D) Frictionless mechanics on maxilla and friction mechanics associated with miniscrew anchorage on the mandible; E) End of treatment.

Figure 16. Clinical case without extraction. A) Space closure using miniscrew as anchorage in the maxilla; B) End of the space closure phase.

Figure 17. Most common space closure loop designs used by orthodontists: A) reverse vertical loop, B) open vertical loop, C) closed vertical loop, D) bull loop, E) reverse vertical loop with helix, F) open vertical loop with helix, G) closed vertical loop with helix, H) tear drop loop, I) helical loop, J) T-loop.