Fig. 3
From: Mechanical force system of double key loop with finite element analysis
Loading conditions of key loops. Archwire with key loops was fixed on the mesial end, and the distal end was constrained in the vertical component. a Single key loop subjected to horizontal force at the distal end. b DKL subjected to horizontal force at the distal end. c DKL subjected to retraction force from the distal key to the tube of the second molar. d DKL subjected to retraction force from the distal key to the tube of the second molar plus a spring between key loops generating a preactivation angle (θ)