The ability to consistently fabricate dimensionally accurate, custom-made, removable orthodontic appliances in large quantities is a manufacturing challenge that has only recently been met through advances in scanning and automation technology. The production of the Invisalign orthodontic appliance (Align Technology, Inc, Santa Clara, Calif) is a complex process that uses innovative solutions to accurately and efficiently fabricate orthodontic appliances. Most removable orthodontic appliances, including retainers and positioners, are made from plaster reference models; individual teeth on these models can be manually sectioned and repositioned with wax. An orthodontic product like Invisalign requires from 6 to 40 sequential appliances per arch; manual fabrication of this many appliances would be prohibitively expensive, and it would be difficult to maintain the required accuracy. Instead, Align Technology uses stereolithography technology to create its reference models, making the orthodontic industry the first to bring on-demand digital manufacturing to commercial production. Each set of aligners begins with a series of positive plastic resin models made from photoactivated polymer. From these plastic resin models, clear removable appliances (aligners) are made, each corresponding to a 2-to-3 week interval of treatment. Unique to the Invisalign process is the ability for the clinician to view, modify, and approve the actual arrangement of all appliances before they are fabricated. Before any of this can happen, the patient’s polyvinyl siloxane (PVS) impressions and bite registration must first be converted into dimensionally accurate 3-dimensional electronic study models. Scanning techniques have evolved over the years, from the relatively simple laser scan to complex computed tomography (CT) scanning. In a laser scan, a positive model is first created, and laser light is then reflected from the surface of the model. The resulting scatter pattern is captured by an optical sensor, and the original shape is reconstructed with mathematical algorithms. Laser scanning is relatively inexpensive, but the process is slow, and the resolution is limited to about 300 microns. Only 1 object can be scanned at a time; this limits the number of models that can be processed per day. Laser scanning cannot be used to accurately scan impressions directly, because undercuts in the impression create hidden surfaces that are inaccessible by the laser beam. In a destructive scan, a positive model is created and encased in a contrasting urethane resin. Paper-thin slices of the encased object are incrementally removed by a computer numerical-controlled machine cutter. After each increment is removed, a digital picture of the exposed area is captured. The scanned layers are electronically combined to recreate the original geometry. Destructive scanning technology is accurate to about 50 microns, but the object is destroyed in the process (hence the name). The preprocessing step (encasing) can be messy, but many objects can be encased together and scanned at once for efficiency. Impressions and bite registrations are not typically scanned directly because (1) rubbery materials such as PVS are difficult to slice cleanly in the scanner, (2) the wide variety of PVS colors available makes calibration difficult, and (3) the destructive process allows only 1 chance to correctly acquire the scan. A white-light scan is similar to a laser scan, but white-light interference patterns (moire patterns) are reflected instead of laser light; this improves resolution Private practice, San Francisco, Calif; Director of Product Development, Align Technology, Inc, Santa Clara, Calif. Private practice, Sunnyvale, Calif; Chief Clinical Officer, Align Technology, Inc, Santa Clara, Calif. Drs Kuo and Miller have financial interests in Align Technology, Inc. Editor’s note: Although this article focuses on techniques used by Align Technology, several other companies rely on scanning technology, including Cadent (OrthoCAD, Fairview, NJ), Geodigm (eModel, Chanhassen, Minn), Orametrix (Sure Smile, Dallas, Tx), and Ormco (Insignia, Orange, Calif). We hope that this explanation of how one company used CT scanning and robotic cutters will help readers to evaluate other systems that use scanning of dental arches in their processes. Reprint requests to: Eric Kuo, DDS, MS, 2001 Union St, Suite 550, San Francisco, CA 94123; e-mail, eric_kuo_dds@yahoo.com. Am J Orthod Dentofacial Orthop 2003;123:578-81 Submitted and accepted, December 2002 Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 0 doi:10.1016/S0889-5406(03)00051-9