The open coil spring is activated by compression and exerts a net pushing force in two directions away from its center. Open coil springs are mainly used for opening spaces to crowding or for the distalization of the molars. Providing heavier teeth moving forces when compressed, compared to nickel-titanium coils.

A length of the coil that is one to three millimeters longer than the interbracket distance is cut, inserted over the archwire, and tied into place with steel ligatures on either end to resist the rotational moments created.

The open coil may be "packed" or compressed in the anterior segment on Nickel-Titanium archwires, but should only be used on a stainless steel archwire (which is more rigid) when used in the posterior.

Open Coil may be compressed in any part of the arch, anterior or posterior with 18x25N as the supporting archwire. 

Activation of nickel chrome / stainless steel coils should be 25% of the original length. Therefore, if compressing nickel chrome / stainless steel open coils between brackets 8mm apart, a 10mm piece of the coil would be used. 

You would also use nickel chrome / stainless steel open coils to move "several teeth", such as when opening a space in the middle of the arch.

Morelli open coil springs are made from the finest material and retain constant pushing forces for a long time without deforming a loss of strength.

Advantages:

• Heavier teeth moving forces, compared to nickel-titanium coils.

• Made from the finest material.

• Retain constant pushing forces for a long time.

• Without deforming.

• Without loss of strength.