The effect of retention on orthodontic relapse after the use of small continuous or discontinuous forces. An experimental study in beagle dogs.

Relapse is a major concern in orthodontics for which avoidance retention is the general procedure. However, the effect of retention on relapse after active tooth movement with different force regimes has never been studied in a standardized experimental setting. Mandibular third premolars were extracted in 19 young adult beagle dogs. Three months later, the second premolars were bodily moved distally with forces of 10 cN or 25 cN. The forces were applied for 24 h d-1 or for 16 h d-1. After 4 months, relapse was allowed in half of the animals, while in the others relapse was preceded by retention for 90 d. Statistical analyses were performed on the relation between force regime, active tooth movement, retention, and relapse. Force magnitude had no effect on relapse, while continuous forces resulted in a longer-lasting and more pronounced relapse than did discontinuous forces. A significant positive correlation was found between the amount of active tooth movement and both the rate and the total amount of relapse, but not between the amount of active tooth movement and the duration of the relapse. Retention had no effect on the duration of the relapse, but it strongly decreased its total amount. Finally, the effect of retention on the amount of relapse was strongly correlated with the amount of active tooth movement.