Improved orthodontic appliances, along with related systems and methods, are provided. In one aspect, an appliance includes a shell having a plurality of cavities shaped to receive teeth, a discontinuity formed in the shell, and an elastic member positioned to interact with the discontinuity. In another aspect, an appliance includes a plurality of discrete shell segments joined by an elastic material to form a single appliance shell. In another aspect, an appliance includes a shell having an exterior layer and an interior layer having a stiffness less than a stiffness of the exterior layer, and a discontinuity formed in the exterior layer. In another aspect, an appliance includes a shell comprising an interior surface and an exterior surface, and an elastic coating covering at least a portion of one or more of the interior surface or exterior surface of the shell.