LoopThis page contains detailed instructions and screen shots for use with the article:
- Halazonetis DJ. Understanding orthodontic loop preactivation. Am J Orthod Dentofacial Orthop 1998;113:237-41.
- (click here to see all publications)
The first example is a simple T-loop.
Start the Loop program and follow these instructions to activate the T-loop:
1. Select File > Open from the program's menu. The Open dialog box appears.
2. Select the Tloop1.llp file and click Open. The T-loop will be displayed:
3. Select Edit > Options to open the Options dialog box.
4. Select a wire cross-section of 0.017 x 0.025 and TMA wire material and click on Apply to Wire.
5. Click the OK button to close the Options dialog box.
6. Select Run > Run or click on the Run button on the Toolbar.
The T-loop will be activated until it is inserted into the right bracket:
To view the values of the forces and moments at the two brackets, select Edit > Force... or click on the Force button on the Toolbar. The Edit Force dialog box will open. Click on Set to Zero and then close the dialog box, to return the loop to the non-activated state.
Next, let's see what happens when we do not insert the free-end of the loop into the right bracket slot, but just activate it until it reaches the bracket. To do this, select Edit > Options... to open the Options dialog box. Click on the Calculation tab and select Target - Point contact. Close the dialog box by clicking OK. (alternatively, you can double click on the Slot / Angle / Point area of the Status bar at the lower part of the Loop window, until Point is displayed). Activate the loop again, by clicking on Run:
As shown above, only a force is applied to the free-end of the wire, in order to bring it to the right bracket. Notice that the free-end of the wire is not parallel to the bracket slot. The angle between the wire and the bracket slot is displayed if you select Edit > Wire Info.
The second example is a preactivated T-loop.
Open the preactivated T-loop: Select File > Open, select the tloop2.llp file and click on Open:
Change the Slot / Point / Angle option back to Slot and activate the loop by clicking on Run:
Larger forces and moments are produced by this loop. The much larger than expected forces are due to improper preactivation, which results in constriction of the loop at the neutral position. Let's see what the neutral position looks like:
Set the forces to zero: Select Edit > Force..., click Set to Zero and then OK.
Change Slot/Point/Angle option to Angle: Select Edit > Options..., click the Calculation tab and select Same Angulation, or double click the Slot/Point/Angle area of the Status bar until Angle is displayed.
Activate the loop: Click on Run.
The neutral position is the activated position of the loop, where only a moment is applied, so that the free-end of the wire is parallel to the bracket slot:
Notice how the free-end of the wire moves away from the right bracket. This is not what we want, if our purpose is to increase the moment applied by the loop without increasing the force. We can arrive at the proper preactivation shape by starting in the opposite way, i.e. from the ideal neutral position. At the ideal neutral position, the shape of the loop should be identical to the shape of the non-preactivated T-loop. As explained in the article, the required activating moment is 980 gf.mm. Therefore, the preactivated shape is arrived at, by using the following procedure:
Open the tloop1.llp file: Select File > Open, select the tloop1.llp file and click on Open.
Set the moment to 980: Select Edit > Force..., enter -980 (negative number) in the Moment edit box and click OK. The loop should assume this shape:
This is the required preactivated shape.
But the wire is now under the influence of a moment. What we need is a passive wire of the same shape. Select Edit > Freeze Wire and answer Yes when asked to reset forces to zero. You now have a correctly preactivated T-loop. You can activate the loop and check that it delivers a Moment / Force ratio of 10 mm:
Please read the Help file of the Loop program to learn how to use the Plier, Cutter and Spool tools to design loops of any shape. If you need further assistance, drop me a note at my e-mail address, email@example.com.