Springs for removable appliances must be fabricated from steel wire of more than 0.5 mm diameter to provide adequate strength and resilience. Wires thinner than 0.5 mm lack sufficient stiffness, making the springs prone to deformation and loss of force over time. Thicker wires ensure stable, controlled force application necessary for effective tooth movement. Additionally, using wire of appropriate thickness minimizes the risk of fracture during activation or function. Therefore, selecting steel wire >0.5 mm optimizes the balance between flexibility and durability in removable appliance springs.

Reference: Graber's Orthodontics, 7th Edition.