Arch Wire Selection for Open Coil Spring Applications
When planning to use an open coil spring in orthodontics, you need a rigid rectangular arch wire, specifically 0.018 × 0.022 inch stainless steel, to provide adequate support and prevent unwanted tooth movements during space opening or distalization.
Wire Size and Shape Requirements
Rectangular arch wires (0.018 × 0.022 inch) are superior to round wires (0.016 inch) when using open coil springs, particularly with larger lumen coil springs, as they provide a greater linear range of force application and better control of tooth movement 1.
The rectangular wire shape provides better torque control and prevents unwanted rotations during space opening, which is critical when applying forces through open coil springs 1.
Material Selection
Stainless steel arch wires are the standard choice for use with open coil springs because they provide the necessary rigidity to resist deformation under the compressive forces generated by the spring 1, 2.
The arch wire must be sufficiently rigid to act as a stable platform for the coil spring to generate its intended force without bending or distorting the wire itself 1.
Force Mechanics Considerations
Open coil springs generate forces that vary based on their wire diameter and lumen size—with constant lumen size, larger wire diameters produce greater forces, while with constant wire size, larger lumen sizes produce lower forces 1.
The arch wire size directly affects the working range of the coil spring: smaller arch wires (0.016 inch round) show no significant difference in spring performance with smaller lumen coils, but rectangular wires demonstrate superior performance with larger lumen springs 1.
Clinical Application Guidelines
For space opening procedures, select nickel-titanium open coil springs that deliver forces between 0.25 N to 1.3 N at 25% compression, mounted on a 0.018 × 0.022 inch stainless steel rectangular arch wire 3.
Most open coil springs exhibit linear force-deflection behavior rather than true superelastic properties, meaning you must carefully calculate the compression percentage to achieve desired force levels 3.
Common Pitfalls to Avoid
Do not use undersized or round arch wires with open coil springs, as this compromises torque control and allows unwanted tooth movements perpendicular to the intended direction of space opening 1.
Avoid relying on manufacturer force specifications without considering the actual compression percentage—the force delivered varies significantly with the degree of spring activation 3.
Do not use open coil springs on flexible or undersized arch wires, as the wire may deform under the spring's compressive forces, negating the intended tooth movement 1, 2.