The Modified Arnold Expansion Appliance has been an industry standard for many years. The use of compressed “open-coil” spring as part of a fixed appliance has helped facillitate arch development in those patients who simply will not wear a removable appliance. T Modified Arnold can produce both lateral and AP development -- and the introduction of NiTi coil spring to this design added a new dimension by exerting a lighter, yet more continuous force over a greater range of motion. Due to the continuous force provided by the coil, the patient must be monitored carefully to prevent over-development in both planes. Hint: It is always difficult to cement a pre-activated, spring-loaded appliance. To make cementation easier, ask the lab to return the appliance to you on the model. This allows you to tie the appliance together with floss and control the spring action prior to cementation.