Orthodontic spring are used in a variety of ways.  In our profession we are very thankful for Hooke's Law,                                                    the spring equation that states Fs=kx where Fs is the force of the spring and K is the spring stiffness                                                         constant and x is the amount a spring is compressed.  In this case potential energy in the spring is                                                                 transferred to the teeth which loosens the ligaments that hold them in place allowing the teeth to                                                                 move through bone.  Another key to making sure he teeth move where you want them is to be precise                                                             with the spring placement.  There are many ways to do this.  Springs can be attached to the brackets                                                                 or wires allowing constant force on the teeth (w=fd).  They can be placed from a tooth to the bone                                                             using a TAD (temporary anchor device) that is a small screw placed into the bone. This allows for torque                                                         or pressure to be placed on one tooth in particular without needing a counter force of other teeth that                                                         we do not want moved. A very common spring used in the open coil spring which is placed around the                                                             wire between teeth creating pressure on both teeth it is in between allowing them to be pushed aside                                                         making room for a tooth pushed out of alignment and otherwise blocked by other teeth.  Closed coil                                                             springs are used to attach to brackets that have excess space that needs to be closed.  The last                                                                     example of springs used I would like to talk about is used on an appliance such as a retainer.  In the                                                               example to the bottom left this hawley retainer has springs that fit behind the left central incisor that                                                         will push it forward.  This is used if post-treatment a tooth has slid out of its ideal position. In each of                                                             these cases placement is very important depending on the direction the tooth is moved.  Springs are                                                              very often used to realign a "crooked" tooth by rotating it based on the placement of force on the                                                                     tooth, this can be calculated using the formula for force and direction W=Fdcosð.  In this calculation                                                             we have the magnitude of Force times the magnitude of direction times the angular distance between the direction of F and d.