A method of fabricating a plurality of orthodontic shell appliances, comprising: determining geometries of a plurality of orthodontic shell appliances (300, 400, 500), the appliances each comprising a second layer (306, 408, 508) and a first layer (304, 406, 506), 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 (312, 316, 320, 326, 328, 334, 402, 502) 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 (300, 400, 500), the appliances each comprising a second layer (306, 408, 508) and a first layer (304, 406, 506), 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 (312, 316, 320, 326, 328, 334, 402, 502) 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.