A method of fabricating a plurality of orthodontic shell appliances, comprising: determining geometries of a plurality of orthodontic shell appliances , the appliances each comprising a second layer and a first layer , such that the first layer and the second layer form a plurality of cavities shaped to receive teeth; determining positions of at least one discontinuity in the first layer on each geometry of the plurality of orthodontic shell appliances to facilitate generating of tooth-moving forces to reposition a patient's teeth from an initial position towards a target position; fabricating the plurality of orthodontic shell appliances based on the determined geometries; wherein fabricating the plurality of orthodontic shell appliances comprises: fabricating each of the first layers of the plurality of orthodontic shell appliances; forming the at least one discontinuity on each of the first layers of the plurality of orthodontic shell appliances based on the determined positions; and fabricating each of the second layers of the plurality of orthodontic shell appliances; wherein each of the second layers is a more elastic layer compared to each of the first layers and is positioned to interact with a respective one of the discontinuities to_facilitate the generating of the tooth-moving forces to reposition the patient's teeth from the initial position towards the target position; wherein at least a portion of one of the plurality of orthodontic shell appliances is fabricated directly using a direct fabrication technique based on a corresponding digital model of the portion; wherein the direct fabrication technique comprises a rapid prototyping fabrication technique; and wherein the plurality of orthodontic shell appliances are fabricated prior to a patient wearing any of the plurality of orthodontic shell appliances. determining geometries of a plurality of orthodontic shell appliances , the appliances each comprising a second layer and a first layer , such that the first layer and the second layer form a plurality of cavities shaped to receive teeth; determining positions of at least one discontinuity in the first layer on each geometry of the plurality of orthodontic shell appliances to facilitate generating of tooth-moving forces to reposition a patient's teeth from an initial position towards a target position; fabricating the plurality of orthodontic shell appliances based on the determined geometries; wherein fabricating the plurality of orthodontic shell appliances comprises: fabricating each of the first layers of the plurality of orthodontic shell appliances; forming the at least one discontinuity on each of the first layers of the plurality of orthodontic shell appliances based on the determined positions; and fabricating each of the second layers of the plurality of orthodontic shell appliances; fabricating each of the first layers of the plurality of orthodontic shell appliances; forming the at least one discontinuity on each of the first layers of the plurality of orthodontic shell appliances based on the determined positions; and fabricating each of the second layers of the plurality of orthodontic shell appliances; wherein each of the second layers is a more elastic layer compared to each of the first layers and is positioned to interact with a respective one of the discontinuities to_facilitate the generating of the tooth-moving forces to reposition the patient's teeth from the initial position towards the target position; wherein at least a portion of one of the plurality of orthodontic shell appliances is fabricated directly using a direct fabrication technique based on a corresponding digital model of the portion; wherein the direct fabrication technique comprises a rapid prototyping fabrication technique; and wherein the plurality of orthodontic shell appliances are fabricated prior to a patient wearing any of the plurality of orthodontic shell appliances. <EOS>