Patent Publication Number: US-2022218452-A1

Title: Automated creation of tooth restoration dental appliances

Description:
TECHNICAL FIELD 
     The present disclosure relates to dental restoration appliances for re-shaping teeth. 
     BACKGROUND 
     Dental practitioners often utilize dental appliances to re-shape or restore a patient&#39;s dental anatomy. The dental appliance is typically constructed from a model of the patient&#39;s dental anatomy, augmented to a desired dental anatomy. The model may be a physical model or a digital model. Designing the dental appliance is often a manual, time-consuming and inexact process. For example, a practitioner typically designs a model of the dental appliance by trial and error. For instance, the practitioner may add, remove, rearrange, and/or re-size features until the practitioner is satisfied with the model of the dental appliance. 
     SUMMARY 
     The disclosure relates to techniques for automating the design and manufacture of a dental restoration appliance for restoring the dental anatomy of a patient. For example, a computing system is described that automatically analyzes a digital representation (model) of a future (i.e., desired) dental anatomy of a patient to generate a custom model of a patient-specific dental restoration appliance, which is used for manufacturing a corresponding physical appliance. As described herein, the computing system utilizes one or more digital libraries of predefined appliance “features,” where each feature represents a digital 3D mesh defining a particular geometric shape to be utilized as one portion within an overall dental appliance. The computing system automates selecting and positioning/orienting a set of appliance features, based on detected landmarks of the dental anatomy, to define an overall dental restoration appliance. Moreover, the computing system may generate additional custom features (i.e., additional 3D meshes) at particular positions and orientations for the dental restoration appliance based on the landmarks. The computing system combines the custom features and the selected pre-defined features to automatically generate an overall 3D digital custom model which can be used to manufacture the patient-specific dental restoration appliance, such as by 3D printing the restoration appliance from the 3D custom model. 
     The techniques and practical applications described herein may provide certain advantages. For example, by automatically determining the shape and placement of various features (e.g., predefined features automatically selected from a library and automatically generated custom features) and combining those features to form an overall model of the dental appliance, a computing device may more quickly and/or more accurately generate a complete digital model of a patient-specific dental restoration appliance, than would be possible by a manual trial and error-based process. Creating a more accurate digital model of the dental appliance may improve the functionality and efficacy of the dental appliance, which may enable the dental practitioner to restore the patient&#39;s dental anatomy more quickly, more accurately, and/or more predictably. Restoring the patient&#39;s dental anatomy more quickly and/or more accurately may improve the functionality (e.g., reducing grinding or interference between teeth), which may improve the patient&#39;s quality of life, for example, by reducing pain caused by poor dental anatomy. In some examples, restoring the patient&#39;s dental anatomy more accurately may improve the appearance of the patient&#39;s dental anatomy, which may further improve the patient experience and/or quality of life. Further, by creating a quick and predictable process for restoring dental anatomy procedures with the custom device become efficient for a wider range of dental practitioners and affordable for a wider set of patients. 
     In one example, a system includes: a digital three-dimensional (3D) model of a future dental anatomy of a patient, the future dental anatomy representing an intended shape of at least one tooth of the patient; a landmark identifier configured to automatically compute, based on the digital 3D model of the future dental anatomy of the patient, one or more landmarks of the future dental anatomy of the patient; a custom feature generator configured to automatically generate, based on the one or more landmarks, one or more custom appliance features for a dental appliance for restoring the at least one tooth of the patient; and a memory device configured to store a digital model of the dental appliance, the digital model of the dental appliance including the one or more custom appliance features and one or more pre-defined appliance features. 
     In another example, a method includes: receiving, by a landmark identifier of a computing device, a digital three-dimensional (3D) model of a future dental anatomy of a patient, the future dental anatomy representing an intended shape of at least one tooth of the patient; automatically computing, by the landmark identifier, based on the digital 3D model of the future dental anatomy of the patient, one or more landmarks of the future dental anatomy of the patient; and storing, at a memory of the computing device, a digital model of the dental appliance, the digital model of the dental appliance including the one or more custom appliance features and one or more pre-defined appliance features. 
     In yet another example, a computer-readable storage medium includes instructions that, when executed, cause at least one processor of a computing device to: receive a digital three-dimensional (3D) model of a future dental anatomy of a patient, the future dental anatomy representing an intended shape of at least one tooth of the patient; automatically compute, based on the digital 3D model of the future dental anatomy of the patient, one or more landmarks of the future dental anatomy of the patient; and generate a digital model of a dental appliance for restoring the at least one tooth of the patient based on the one or more custom appliance features and one or more pre-defined appliance features. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram illustrating an example system for automatically designing and manufacturing a dental appliance for restoring the dental anatomy of a patient, in accordance with various aspects of this disclosure. 
         FIG. 2  is a flow diagram illustrating an example technique for generating a digital model of a dental appliance, in accordance with various aspects of this disclosure. 
         FIG. 3  is a flow diagram illustrating an example technique for generating a mold parting surface, in accordance with various aspects of this disclosure. 
         FIG. 4  is a conceptual diagram illustrating an example technique for generating a digital model of a dental appliance, in accordance with various aspects of this disclosure. 
         FIG. 5  is a conceptual diagram illustrating a plurality of slices of an example digital model of a dental anatomy, in accordance with various aspects of this disclosure. 
         FIG. 6  is a conceptual diagram illustrating midpoints of a plurality of teeth, in accordance with various aspects of this disclosure. 
         FIG. 7  is a conceptual diagram illustrating points between adjacent teeth, in accordance with various aspects of this disclosure. 
         FIGS. 8A-8B  are conceptual diagrams illustrating convex hulls, in accordance with various aspects of this disclosure. 
         FIG. 9  is a conceptual diagram illustrating example splines, in accordance with various aspects of this disclosure. 
         FIGS. 10A-10B  are conceptual diagrams illustrating example mold parting surfaces, in accordance with various aspects of this disclosure. 
         FIG. 11  is a conceptual diagram illustrating an example gingival trim surface, in accordance with various aspects of this disclosure. 
         FIG. 12  is a conceptual diagram illustrating an example facial ribbon, in accordance with various aspects of this disclosure. 
         FIG. 13  is a conceptual diagram illustrating an example lingual shelf, in accordance with various aspects of this disclosure. 
         FIG. 14  is a conceptual diagram illustrating example doors and windows, in accordance with various aspects of this disclosure. 
         FIG. 15  is a conceptual diagram illustrating example rear snap clamps, in accordance with various aspects of this disclosure. 
         FIG. 16  is a conceptual diagram illustrating example door hinges, in accordance with various aspects of this disclosure. 
         FIGS. 17A-17B  are conceptual diagrams illustrating example door snaps, in accordance with various aspects of this disclosure. 
         FIG. 18  is a conceptual diagram illustrating an example incisal ridge, in accordance with various aspects of this disclosure. 
         FIG. 19  is a conceptual diagram illustrating an example center clip, in accordance with various aspects of this disclosure. 
         FIG. 20  is a conceptual diagram illustrating example door vents, in accordance with various aspects of this disclosure. 
         FIG. 21  is a conceptual diagram illustrating example doors, in accordance with various aspects of this disclosure. 
         FIG. 22  is a conceptual diagram illustrating an example diastema matrix, in accordance with various aspects of this disclosure. 
         FIG. 23  is a conceptual diagram illustrating an example manufacturing case frame and an example dental appliance, in accordance with various aspects of this disclosure. 
         FIG. 24  is a conceptual diagram illustrating an example dental appliance including custom labels, in accordance with various aspects of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram illustrating an example system for designing and manufacturing a dental appliance for restoring the dental anatomy of a patient, in accordance with various aspects of this disclosure. In the example of  FIG. 1 , system  100  includes clinic  104 , appliance design facility  109 , and manufacturing facility  110 . 
     Practitioner  106  may treat patient  102  at clinic  104 . For example, practitioner  106  may create a digital model of the current dental anatomy of patient  102 . The dental anatomy may include any portion of crowns or roots of one or more teeth of a dental archform, gingiva, periodontal ligaments, alveolar bone, cortical bone, implants, artificial crowns, bridges, veneers, dentures, orthodontic appliances, or any structure that could be considered part of the dentition before, during, or after treatment. In one example, the digital model of the current dental anatomy includes a three-dimensional (3D) model of the current dental anatomy of the patient. The 3D model may be generated using an intra-oral scanner, Cone Beam Computed Tomography (CBCT) scanning (i.e., 3D X-ray), Optical Coherence Tomography (OCT), Magnetic Resonance Imaging (MRI), or any other 3D image capturing system. In some examples, computing device  190  stores a digital model of a current dental anatomy of patient  102 . 
     Computing device  190  of clinic  104  may store a digital model of a future dental anatomy for the patient. The future dental anatomy represents the intended shape of the dental anatomy to be achieved by application of a dental appliance  101 . In one example, practitioner  106  may create a physical model of the future dental anatomy and may utilize an image capturing system (e.g., as described above) to generate the digital model of the future dental anatomy. In another example, practitioner  106  may modify the digital model of the current anatomy of patient  102  (e.g., by adding material to a surface of one or more teeth of the dental anatomy) to generate the digital model of the future dental anatomy. In yet another example, computing device  190  may modify the digital model of the current dental anatomy to generate a model of the future dental anatomy. 
     In one scenario, computing device  190  outputs the digital model representing the dental anatomy (e.g., current and/or future) of patient  102  to another computing device, such as computing device  150  and/or computing device  192 . As illustrated in  FIG. 1 , in some examples, computing device  150  of design facility  108 , computing device  190  of clinic  104 , and computing device  192  of manufacturing facility  110  may be communicatively coupled to one another via network  114 . Network  114  may include a wired or wireless network, such as via WIFI®, BLUETOOTH®, 3G, 4G LTE, 5G, and the like. 
     In the example of  FIG. 1 , design facility  108  includes computing device  150  configured to automatically design a dental appliance for re-shaping the dental anatomy of patient  102 . In one example, computing device  150  includes one or more processors  172 , one or more user interface (UI) devices  174 , one or more communication units  176 , and one or more storage devices  178 . 
     UI device  174  may be configured to receive user input and/or output information, also referred to as data, to a user of computing device  150 . One or more input components of UI device  174  may receive input. Examples of input are tactile, audio, kinetic, and optical input, to name only a few examples. For example, UI device  174  may include a mouse, keyboard, voice responsive system, video camera, buttons, control pad, microphone  316 , or any other type of device for detecting input from a human or machine. In some examples, UI device  174  may be a presence-sensitive input component, which may include a presence-sensitive screen, touch-sensitive screen, etc. 
     One or more output components of UI device  174  may generate output. Examples of output are data, tactile, audio, and video output. Output components of UI device  174 , in some examples, include a display device (e.g., a presence-sensitive screen, a touch-screen, a liquid crystal display (LCD) display, a Light-Emitting Diode (LED) display, an optical head-mounted display (HMD), among others), a light-emitting diode, a speaker, or any other type of device for generating output to a human or machine. 
     Processor  172  represents one or more processors such as a general-purpose microprocessor, a specially designed processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a collection of discrete logic, or any type of processing device capable of executing the techniques described herein. In one example, storage device  178  may store program instructions (e.g., software instructions or modules) that are executed by processor  172  to carry out the techniques described herein. In other examples, the techniques may be executed by specifically programmed circuitry of processor  172 . In these or other ways, processor  172  may be configured to execute the techniques described herein. 
     One or more storage devices  178  may store data for processing by processors  172 . In some examples, storage device  178  is a temporary memory, meaning that a primary purpose of storage device  178  is not long-term storage. Storage device  178  may be configured for short-term storage of data as volatile memory and therefore not retain stored contents if deactivated. Examples of volatile memories include random access memories (RAM), dynamic random access memories (DRAM), static random access memories (SRAM), and other forms of volatile memories known in the art. 
     Storage device  178  may, in some examples, also include one or more computer-readable storage media. Storage device  178  may be configured to store larger amounts of data than volatile memory. Storage device  178  may further be configured for long-term storage of data as non-volatile memory space and retain data after activate/off cycles. Examples of non-volatile memories include, solid state drives (SSDs), hard disk drives (HDDs), flash memories, or forms of electrically programmable memories (EPROM) or electrically erasable and programmable (EEPROM) memories. Storage device  178  may store program instructions and/or data associated with software components  182 - 188  and/or operating system  180 . 
     In the example of  FIG. 1 , storage device  178  includes appliance feature library  164 , models library  166 , and practitioner preferences library  168 . Libraries  164 ,  166 , and  168  may include relational databases, multi-dimensional databases, maps, and hash tables, or any data structure that stores data. In one example, models library  166  includes 3D models of the patient&#39;s current and/or future dental anatomy. In some instances, libraries  164 ,  166 , and  168  may be stored locally at computing device  150  or may be accessed via a networked file share, cloud storage, or other remote datastore. 
     Computing device  150  may execute software components  182 - 188  with one or more processors  172 . Computing device  150  may execute any of components  182 - 188  as or within a virtual machine executing on underlying hardware. In one example, any of components  182 - 188  may be implemented as part of operating system  180 . 
     In accordance with the techniques of this disclosure, computing device  150  automatically or semi-automatically generates a digital model of dental appliance  101  for restoring the dental anatomy of patient  102  based on a digital model of the patient&#39;s future dental anatomy. Pre-processor  181  may pre-process the digital model of the future dental anatomy of patient  102 . In one example, pre-processor  181  performs pre-processing to identify one or more teeth in the future dental anatomy of patient  102 . In some instances, pre-processor  181  identify a local coordinate system for each individual tooth and may identify a global coordinate system that includes each tooth of the future dental anatomy. As another example, pre-processor  181  may pre-process the digital model of the future dental anatomy to identify the root structure of the dental anatomy. In another example, Pre-processor  181  may identify the gingiva. In this way, pre-processor  181  may determine portions of the future dental anatomy that include gingiva and portions of the future dental anatomy that include tooth. As yet another example, pre-processor  181  may pre-process the digital model of the future dental anatomy by extending the roots to identify the top surface of the root of each respective tooth. 
     Landmark identifier  182  may determine one or more landmarks of the future dental anatomy. Example landmarks include a slice, a midpoint, a gingival boundary, a closest point between two adjacent teeth (e.g., a point of contact between adjacent teeth or a point of closest approach (or closest proximity), a convex hull, a center of mass, or other landmark. A slice refers to a cross section of the dental anatomy. The midpoint of a tooth refers to a geometric center (also referred to as a geometrical midpoint) of the tooth within a given slice. The gingival boundary refers to a boundary between the gingiva and one or more teeth of the dental anatomy. A convex hull refers to a polygon whose vertices include a subset of the vertices in a given set of vertices, where the boundary of the subset of vertices circumscribes the entire set of vertices. The center of mass of a tooth refers to a midpoint, center point, centroid, or geometric center of the tooth. In some instances, landmark identifier  182  determines the landmarks in the local coordinate system for each tooth. 
     In some examples, landmark identifier  182  determines a plurality of slices of the patient&#39;s future dental anatomy. In one example, the thickness of each slice is the same. In some instances, the thickness of one or more slices is different than the thickness of another slice. The thickness of a given slice may be pre-defined. In one instance, landmark identifier  182  automatically determines the thickness of each slice. In another instance, the thickness of each slice may be user-defined. 
     Landmark identifier  182  determines, in some examples, a midpoint for each tooth. In one example, landmark identifier  182  determines a midpoint of a particular tooth by computing the extrema of the particular tooth&#39;s geometry based on the entirety of the particular tooth (e.g., without dividing the dental anatomy into slices) and determine the midpoint of the particular tooth based on the extrema of the tooth geometry. 
     In some examples, landmark identifier  182  determines a midpoint for each tooth for each slice. Landmark identifier  182  may determine the midpoint for a particular slice of a particular tooth by calculating the center of mass of a constellation of vertices around the edge of the particular tooth for that particular slice. In some instances, the midpoint of the particular tooth for the particular slice may be biased toward one edge of the tooth (e.g. in the case that one edge has more points than another edge). 
     In another example, landmark identifier  182  may determine the midpoint of a particular tooth in a particular slice based on a convex hull of the particular tooth for the particular slice. For example, landmark identifier  182  may determine a convex hull of a set of edge points of the tooth for a given slice. Landmark identifier  182  determines, in some instances, a geometric center from the convex hull by performing a flood-fill operation on the region circumscribed by the convex hull and computing a center of mass of the flood-filled convex hull. 
     In some examples, landmark identification module  182  determines a closest point between two adjacent teeth. The closest point between two adjacent teeth may be a point of contact or a point of closest approach. In one example, landmark identification module  182  determines a closest point between two adjacent teeth for each slice. In another example, landmark identification module  182  determines a closest point between two adjacent teeth based on the entirety of the adjacent teeth (e.g., without dividing the dental anatomy into slices). 
     Responsive to determining the landmarks of the future dental anatomy, in some examples, custom feature generator  184  generates one or more custom appliance features for dental appliance  101  based at least in part on the landmarks. For example, custom feature generator  184  may generate the custom appliance features by determining the characteristics of the custom appliance features, such as a size, shape, position, and/or orientation of the custom appliance features. Examples of custom appliance features include a spline, a mold parting surface, a gingival trim surface, a shell, a facial ribbon, a lingual shelf (also referred to as a “stiffening rib”), a door, a window, an incisal ridge, a case frame sparing, a diastema matrix wrapping, among others. 
     A spline refers to a curve that passes through a plurality of points or vertices, such as a piecewise polynomial parametric curve. A mold parting surface refers to a 3D mesh that bisects two sides of one or more teeth (e.g., separates the facial side of one or more teeth from the lingual side of the one or more teeth). A gingival trim surface refers to a 3D mesh that trims an encompassing shell along the gingival margin. A shell refers to a body of nominal thickness. In some examples, an inner surface of the shell matches the surface of the dental arch and an outer surface of the shell is a nominal offset of the inner surface. The facial ribbon refers to a stiffening rib of nominal thickness that is offset facially from the shell. A window refers to an aperture that provides access to the tooth surface so that dental composite can be placed on the tooth. A door refers to a structure that covers the window. An incisal ridge provides reinforcement at the incisal edge of dental appliance  101  and may be derived from the archform. The case frame sparing refers to connective material that couples parts of dental appliance  101  (e.g., the lingual portion of dental appliance  101 , the facial portion of dental appliance  101 , and subcomponents thereof) to the manufacturing case frame. In this way, the case frame sparing may tie the parts of dental appliance  101  to the case frame during manufacturing, protect the various parts from damage or loss, and/or reduce the risk of mixing-up parts. 
     In some examples, custom feature generator  184  generates one or more splines based on the landmarks. Custom feature generator  184  may generate a spline based on a plurality of tooth midpoints and/or closest points between adjacent teeth (e.g., points of contact between adjacent teeth or points of closest proximity between adjacent teeth). In some instances, custom feature generator  184  generates one spline for each slice. In one instance, custom feature generator  184  generates a plurality of splines for a given slice. For instance, custom feature generator  184  may generate a first spline for a first subset of teeth (e.g., right posterior teeth), a second spline for a second subset of teeth (e.g., left posterior teeth), and a third spline for a third subset of teeth (e.g., anterior teeth). 
     Custom feature generator  184  generates, in some scenarios, a mold parting surface based on the landmarks. The mold parting surface may be used to split an encompassing shell for molding without undercuts. In some examples, custom feature generator  184  generates additional copies of the mold parting surface. For example, custom feature generator  184  may place one or more copies of a mold parting surface at small offsets to the main parting surface for the purpose of creating an interference condition when the appliance is assembled (which may, for example, improve shape adaptation and sealing when applying a tooth restoration material to the teeth). 
     Appliance feature library  164  includes a set of pre-defined appliance features that may be included in dental appliance  101 . Appliance feature library  164  may include a set of pre-defined appliance features that define one or more functional characteristics of dental appliance  101 . Examples of pre-defined appliance features include vents, rear snap clamps, door hinges, door snaps, an incisal registration feature, center clips, custom labels, a manufacturing case frame, a diastema matrix handle, among others. Each vent is configured to enable excess dental composite to flow out of dental appliance  101 . Rear snap clamps are configured to couple a facial portion of dental appliance  101  with a lingual portion of dental appliance  101 . Each door hinge is configured to pivotably couple a respective door to dental appliance  101 . Each door snap is configured to secure a respective door in a closed position. In some examples, an incisal registration feature comprises a male and female tab pair that falls on the incisal edge of dental appliance  101  (e.g., along the midsaggittal). In one example, the incisal registration feature is used to maintain vertical alignment of a facial portion of dental appliance  101  and a lingual portion of dental appliance  101 . Each center clip is configured to provide vertical registration between the lingual portion of dental appliance  101  and the facial portion of dental appliance  101 . Each custom label includes data identifying a part of dental appliance  101 . The manufacturing case frame is configured to support one or more parts of dental appliance  101 . For example, the manufacturing case frame may detachably couple a lingual portion of dental appliance  101  and a facial portion of dental appliance  101  to one another for safe handling and transportation of dental appliance  101  from manufacturing facility  110  to clinic  104 . 
     Feature manager  186  determines the characteristics of one or more pre-defined appliance features that are included in pre-defined appliance feature library  164 . In one example, the pre-defined appliance features are configured to perform functionality of dental appliance  101 . The characteristics of the pre-defined appliance features may include the size, shape, scale, position, and/or orientation of the pre-defined appliance features. Feature manager  186  may determine the characteristics of the pre-defined appliance features based on one or more rules. The rules may be pre-programmed or machine generated, for instance, via machine learning. 
     Feature manager  186  determines, in some instances, a placement of a rear snap clamp based on the rules. In one example, feature manager  186  positions two rear snap clamps along the archform on opposite ends of the archform (e.g., a first snap clamp at one end and a second snap clamp at another end). In some examples, feature manager  186  positions the rear snap clamps one tooth beyond the outer-most teeth to be restored. In some examples, feature manager  186  positions a female portion of the rear snap clamp on the lingual side of the parting surface and positions a male portion of the rear snap clamp on the facial side. 
     In some examples, feature manager  186  determines a placement of a vent based on the rules. In one example, feature manager  186  positions the vent at the midline of a corresponding door on the incisal side of dental appliance  101 . 
     In some scenarios, feature manager  186  determines a placement of a door hinge based on the rules. In one scenario, feature manager  186  positions each door hinge at the midline of a corresponding door. In one scenario, feature manager  186  positions the female portion of the door hinge to anchor to the facial portion of dental appliance  101  (e.g., towards the incisal edge of a tooth) and positions the male portion of the door hinge to anchor to the outer face of the door. 
     In one instance, feature manager  186  determines a placement of a door snap based on the rules by positioning the door snap along a midline of a corresponding door. In one instance, feature manager  186  positions the female portion of the door snap to anchor to an outer face of the door and extends downward toward the gingiva. In another instance, feature manager  186  positions the male portion of the door snap to anchor to the gingival side of the facial ribbon. For instance, the door snap may secure the door in a closed position by latching the male portion of the door snap to the facial ribbon. 
     Feature manager  186  may determine the characteristics of a pre-defined appliance feature based on preferences of practitioner  102 . Practitioner preferences library  168  may include data indicative of preferences of various practitioner  102 . In one example, practitioner preferences directly affect the characteristics of one or more appliance features. For example, practitioner preferences library  168  may include data indicating a preferred size of various appliance features, such as the size of the vents. In such examples, larger vents may enable the pressure of the dental composite or resin to reach equilibration faster during the filling process but may result in a larger nub to finish after curing. 
     As another example, practitioner preferences indirectly affect the characteristics of appliance features. For example, practitioner preferences library  168  may include data indicating a preferred stiffness of the appliance or a preferred tightness of the self-clamping feature. Such preference selections may also affect more complex design changes to section thickness of the matrix and or degree of activation of the clamping geometry. Feature manager  186  may determine the characteristics of the appliance features by applying the practitioner preferences to one or more rules, a simulation (e.g. Monte Carlo) or finite element analysis. Features characteristics also may be derived from properties in the materials to used with the matrix, such as type of composite that the dentist prefers to use with the appliance. 
     Model assembler  188  generates a digital 3D model of dental appliance  101  used to re-shape the dental anatomy (e.g., to the future dental anatomy) in response to determining the characteristics of the custom and pre-defined appliance features. The digital model of dental appliance  101  may include a point cloud, 3D mesh, or other digital representation of dental appliance  101 . In some instances, model assembler  188  stores the digital model of dental appliance  101  in models library  166 . 
     Model assembler  188  may output the digital model of dental appliance  101 . For example, model assembler  188  may output the digital model of dental appliance  101  to computing device  192  of manufacturing facility  110  (e.g., via network  114 ) to manufacture dental appliance  101 . In another example, computing device  150  sends the digital model of dental appliance  101  to computing device  190  of clinic  104  for manufacturing at clinic  104 . 
     Computing device  192  may send the digital model of dental appliance  101  to manufacturing system  194 . Manufacturing system  194  manufactures dental appliance  101  according to the digital model of dental appliance  101 . Manufacturing system  194  may form dental appliance  101  using any number of manufacturing techniques, such as 3D printing, chemical vapor deposition (CVD), thermoforming, injection molding, lost wax casting, milling, machining, laser cutting, among others. 
     Practitioner  106  may receive dental appliance  101  and may utilize dental appliance  101  to re-shape one or more teeth of patient  102 . For example, practitioner  106  may apply a dental composite to the surface of one or more teeth of patient  102  via one or more doors of dental appliance  101 . Excess dental composite may be removed via one or more vents. 
     In some examples, model assembler  188  generates a digital model of dental appliance  101  based on an existing digital model (e.g., stored in models library  166 ). In one example, models library  166  may include data indicative of appliance success criteria associated with each completed dental appliance  101 , the appliance success criteria indicating a manufacturing print yield, practitioner and/or customer feedback or ratings, or a combination thereof. For example, model assembler  188  may utilize an existing digital model to generate a new or updated digital model of a dental appliance  101  in response to determining the appliance success criteria for the previous dental appliance  101  satisfy a threshold criteria (e.g., a threshold manufacturing yield, or a threshold practitioner rating). In one example, the existing digital model is a template or reference digital model. In such examples, model assembler  188  may generate a digital model of a dental appliance  101  based on the template digital model. For example, the template digital model may be associated with different characteristics of a potential patient&#39;s dental anatomy, such as the patient having small teeth or being unable to open the mouth widely. 
     In one example, model assembler  188  generates a digital model of a dental appliance  101  based on an existing digital model by utilizing one or more morphing algorithms. For example, model assembler  188  may utilize morphing algorithms to interpolate appliance feature geometries. In one instance, model assembler  188  may generate a new digital model of a dental appliance  101  based on the design of the existing digital model. In one instance, the design feature of an existing digital model may include a window inset from the perimeter, such that model assembler  188  may morph the geometry of the existing digital model based on landmarks for a different dental anatomy. 
     Techniques of this disclosure may enable a computing device to automatically determine the shape of dental appliance  101  and the placement of various appliance features. In this way, the computing device may more accurately and more quickly generate a digital model of a dental appliance  101 . More accurately determining the shape of dental appliance  101  and the placement of the appliance features may increase the efficacy of dental appliance  101  and the tooth restoration. Determining the shape of dental appliance  101  and placement of the appliance features more quickly may enable the practitioner to correct a patient&#39;s teeth more quickly, which may improve the appearance and/or functionality of the patient&#39;s teeth, thereby potentially improving the patient experience. Additionally, reducing the time required to generate the digital model of a dental appliance  101  may reduce the cost of production and making treatment affordable for a wider set of patients. 
     While computing device  150  is described as automatically generating a digital model of dental appliance  101  based on a digital model of a future dental anatomy of the patient, in some examples, computing device  150  may utilize a digital model of the current, unrestored state of the dental anatomy of the patient to generate all or part of the digital model of dental appliance  101 . For example, computing device  150  may utilize a digital model of the current dental anatomy to determine the position of snap clamps (which may be placed on teeth that are not to be restored) or generate the facial ribbon (e.g., as the gingival margin may not change during restoration). 
       FIG. 2  is a flow diagram illustrating an example technique for generating a digital model of a dental appliance, in accordance with various aspects of this disclosure.  FIG. 2  is described below in the context of system  100  of  FIG. 1 . 
     Computing device  150  receives a digital 3D model of a future (i.e., desired) dental anatomy for a patient  102 . In one example, computing device  150  receives the digital model of the future dental anatomy from another computing device, such as computing device  190  of clinic  104 . The digital model of the future dental anatomy of the patient may include a point cloud or 3D mesh of the future dental anatomy. A point cloud includes a collection of points that represent or define an object in 3-dimensional space. A 3D mesh includes a plurality of vertices (also referred to as points) and geometric faces (e.g., triangles) defined by the vertices. In one example, practitioner  106  creates a physical model of the future dental anatomy and utilizes an image capturing system to generate the digital model of the future dental anatomy. In another example, practitioner  106  modifies the digital model of the current anatomy of patient  102  (e.g., by adding material to a surface of one or more teeth of the dental anatomy) to generate the digital model of the future dental anatomy. In yet another example, computing device  190  may modify the digital model of the current dental anatomy to generate a model of the future dental anatomy. 
     In some examples, pre-processor  181  pre-processes the 3D model of the future dental anatomy to generate a modified model by digitally extending the roots of the initial digital model of the future dental anatomy according to the projected root extension determined by pre-processor  181 , thereby more accurately modeling the complete anatomy of the patient&#39;s teeth ( 204 ). In some examples, because the tops (e.g., the area furthest the gingival emergence) of the roots may be at different heights, pre-processor  181  may detect the vertices corresponding to the tops of the roots and then project those vertices along a normal vector, thereby digitally extending the roots. In one example, pre-processor  181  groups vertices into clusters (e.g., using a k-means algorithm). Pre-processor  181  may compute the average normal vector for each cluster of vertices. For each cluster of vertices, pre-processor  181  may determine a sum of residual angular differences between the average normal vector for the cluster and the vector associated with each of the vertices in the cluster. In one example, pre-processor  181  determines which cluster of vertices is the top surface of the roots based on the sum of the residual angular differences for each cluster. For example, pre-processor  181  may determine that the cluster with the lowest sum of residual angular differences defines the top surface of the roots. 
     Further, landmark identifier  182  processes the 3D model of the future dental anatomy to automatically detect a set of one or more landmarks of the future dental anatomy, where each landmark represents an identifiable geometric construct within the 3D model that is useful for determining the position and orientation with respect to one or more tooth surfaces ( 206 ). In some examples, the landmarks computed by landmark identifier  182  include a plurality of slices of the dental anatomy and each slice of the dental anatomy may include one or more additional landmarks. For example, landmark identifier  182  may divide the 3D mesh of the future dental anatomy into a plurality of slices. Responsive to dividing the digital model of the dental anatomy into slices, in one example, landmark identifier  182  computes one or more landmarks for each slice, such as a midpoint for each tooth in the slice, a closest point between two adjacent teeth (e.g., a point of contact between two adjacent teeth or a point of closest approach between two adjacent teeth), a convex hull for each tooth in the slice, among others. 
     Custom feature generator  184  automatically generates, including determining the particular size, shape, position and orientation, one or more custom appliance features for dental appliance  101  based on the landmarks, where each “feature” represents a digital 3D mesh defining a particular geometric shape to be utilized as one portion (i.e., a sub-mesh) within a 3D model defining overall dental appliance ( 208 ). Examples of custom appliance features include a 3D mesh for a spline, a mold parting surface, a gingival trim surface, a shell, a facial ribbon, a lingual shelf, a door, a window, among others. In one example, custom feature generator  184  generates one or more digital meshes representing splines for each slice of the dental anatomy. Custom feature generator  184  may generate a spline for a given slice based on a plurality of tooth midpoints of teeth within the slice and/or closest points between adjacent teeth within the slice (e.g., points of contact between adjacent teeth within the slice or points of closest proximity between adjacent teeth within the slice). In other words, in this example, custom feature generator  184  accumulates a set of points (e.g., tooth midpoints, points of contact between adjacent teeth, points of closest approach between adjacent teeth, or a combination thereof) for each slice to generate features representing a spline for each digital slice. 
     In some examples, custom feature generator  184  automatically generates a mold parting surface as one example feature to be incorporated within an overall 3D model of a dental restoration appliance. Custom feature generator  184  may generate the mold parting surface based on the plurality of midpoints and/or closest points between adjacent teeth. For example, custom feature generator  184  may accumulate a plurality of the points for each spline for each slice to generate the mold parting surface. As one example, in an example where custom feature generator  184  divides the dental anatomy into four slices and generates a single spline for each slice, custom feature generator  184  aggregates the points of each of the four splines to generate the mold parting surface. 
     In one scenario, feature manager  186  receives data indicative of practitioner preferences ( 210 ). For instance, feature manager  186  may query practitioner preferences library  168  to determine preferences for practitioner  106 . Examples of data stored within practitioner preferences library  186  include a preferred size or orientation of a pre-defined appliance feature for a particular practitioner. 
     Feature manager  186  receives data indicative of pre-defined appliance features, such as by accessing and retrieving the data from one or more libraries (e.g., datastore or other electronic repository) of 3D meshes representing pre-defined features for incorporation within an overall 3D model ( 212 ). For example, feature manager  186  may receive data by querying appliance feature library  164 . Appliance feature library  164  stores data defining 3D meshes for a plurality of pre-defined appliance features, such as vents, rear snap clamps, door hinges, door snaps, an incisal registration feature (also referred to as a “beak”), among others. 
     In one example, feature manager  186  selects one or more pre-defined appliance features of a plurality of pre-defined appliance features stored within appliance feature library  186 . For example, appliance feature library  186  may include data defining a plurality of different pre-defined appliance features of a given type of pre-defined appliance feature. As one example, appliance feature library  164  may include data defining different characteristics (e.g., size, shape, scale, orientation) for a given type of pre-defined appliance feature (e.g., data for differently sized and/or differently shaped doors, windows, hinges, etc.). In other words, appliance feature library  164  may determine the characteristics of a pre-defined appliance feature and select a feature from the pre-defined appliance library that corresponds to the determined characteristics. In some scenarios, feature manager  186  selects a pre-defined appliance feature (e.g., a particularly sized door hinge) from appliance feature library  186  based on landmarks for a corresponding tooth, characteristics (e.g., size, type, location) of the corresponding tooth (e.g., a tooth for which the appliance feature will be used to restore when the dental appliance is applied to the patient), practitioner preferences, or both. 
     In another example, appliance feature library  164  includes data defining a set of required pre-defined appliance features, such that feature manager  186  retrieves data for the 3D meshes representing the pre-defined features for each of the required pre-defined features. In such examples, feature manager  164  may transform the 3D mesh for including in the patient specific dental appliance. For example, feature manager  164  may rotate or scale (e.g., re-size) a 3D mesh for a particular feature based on the landmarks for a corresponding tooth, characteristics of the tooth, and/or practitioner preferences. 
     Model assembler  188  operates to construct an overall 3D mesh for the dental appliance by, for example, determining the characteristics of one or more custom appliance features and one or more pre-defined dental appliance features based at least in part on the patient-specific landmarks ( 214 ). For example, based on the landmarks for the particular patient, model assembler  188  may determine example characteristics such as a size, position, and/or orientation of each 3D mesh corresponding to each of the appliance features (e.g., custom or pre-defined appliance features) for the overall appliance. In one example, modeler assembler  188  may determine the position of a custom appliance feature based on the midpoint of a particular tooth. For example, modeler assembler  188  may align or otherwise position a 3D mesh of a window and/or door (as example features) based on a midpoint of the tooth. In this way, model assembler  188  may determine the position of a pre-defined appliance feature based on the landmarks. As one example, model assembler  188  may determine the position of a rear snap clamp based on the position of the teeth. In some instances, modeler assembler  188  determines the position of a pre-defined appliance feature based on the position of a custom appliance feature. For instance, modeler assembler  188  may align a door hinge, door snap, and/or vent with a midline of a door. Further, the model assembler  188  may adjust the feature geometry, scale or position based analysis of the overall model, such as performing a finite element analysis to adjust the active clamping forces of snap clamp. The model assembler  188  may also make adjustments based on subsequent expected manufacturing tolerances, such as providing suitable clearance between features. Similarly, the model assembler  188  may make adjustments based on the properties of the material used in the creation of the physical appliance, such as increasing thicknesses when using more flexible materials. 
     Model assembler  188  generates the full digital 3D model of the dental appliance  101  based on the custom dental appliance features and the pre-defined dental appliance features and their determined characteristics ( 216 ). The digital model of dental appliance  101  may include a point cloud, 3D mesh, or other digital representation of dental appliance  101 . 
     Computing device  150  stores, transmits and/or outputs the digital 3D model of dental appliance  101  ( 218 ). For example, computing device  150  may output the digital 3D model of dental appliance  101  to computing device  192  of manufacturing facility  110 . Manufacturing system  194  generates dental appliance  101  ( 220 ) based on the digital 3D model of dental appliance  101 . For example, manufacturing system  194  may generate the physical dental appliance  101  via 3D printing, CVD, machining, milling, or any other suitable technique. 
     In some examples, computing system  150  receives feedback on dental appliance  101  from practitioner  106  ( 222 ). For example, after practitioner  106  receives the physical dental appliance  101 , practitioner  106  may utilize computing device  190  to send feedback to computing device  150 . As one example, computing device  150  may receive data indicating a request to adjust a characteristic (e.g., size, relative position) of a pre-defined appliance feature. In some examples, computing system  150  updates the practitioner preferences library  168  based on the feedback ( 224 ). 
       FIG. 3  is a flow diagram illustrating an example technique for generating a mold parting surface, in accordance with various aspects of this disclosure.  FIG. 3  is described below in the context of system  100  of  FIG. 1 . 
     Pre-processor  181  receives a digital 3D model of a future dental anatomy for a patient  102 . The digital model of the future dental anatomy of the patient may include a point cloud or 3D mesh of the future dental anatomy. 
     In some examples, pre-processor  181  of computing device  150  analyzes the digital model of the future dental anatomy to identify individual teeth and roots of the future dental anatomy ( 302 ). Pre-processor  181  may analyze the digital model of the future dental anatomy to extend the roots by finding the vertices on the top surface of the roots and then projecting those vertices along a normal vector, as described above. 
     Computing device  150  receives, in the example of  FIG. 3 , data indicative of practitioner preferences ( 304 ). Examples of practitioner preferences include a preferred size of a pre-defined appliance feature for a particular practitioner. For instance, feature manager  186  may receive data from practitioner preferences library  168 . 
     In some scenarios, landmark identifier  182  receives data indicative of landmarks of the future dental anatomy ( 306 ). For example, computing device  180  may pre-determine one or more landmarks, for example, and may send data indicative of the landmarks to computing device  150  when sending the digital model of the future dental anatomy of patient  102 . 
     Landmark identifier  182  automatically computes one or more landmarks of the future dental anatomy. In some examples, landmark identifier  182  divides the digital model of the future dental anatomy into a plurality of slices ( 308 ). In one example, the thickness of each slice is the same. In one example, the thickness of one or more slices is different than the thickness of another slice. The thickness of a given slice may be pre-defined or user-defined. In one example, landmark identifier  182  dynamically determines the thickness of each slice. 
     Landmark identifier  182  computes a geometric midpoint of each tooth for each slice of the plurality of slices ( 310 ). Landmark identifier  182  may determine the midpoint of a tooth for a particular slice by calculating the center of mass of the constellation of points around the edge of a tooth. In another example, landmark identifier  182  determines the midpoint of a tooth for a particular slice based on a convex hull of the tooth for that particular slice. In such examples, landmark identifier  182  determines a geometric center from the convex hull by performing a flood-fill operation on the region circumscribed by the convex hull and computing a center of mass of the flood-filled convex hull. 
     Landmark identifier  182  may cleave a portion of one or more teeth of the future dental anatomy ( 314 ). In one example, landmark identifier  182  cleaves one or more teeth (e.g., posterior and/or anterior teeth) to remove part (e.g., part of the lingual portion) of one or more teeth. Cleaving part of the lingual portion of the teeth may reduce or eliminate lingual cusp tips in the posterior teeth. In some examples, cleaving part of a tooth for a given slice may create additional vertices that define the edge of the tooth for that slice of the tooth. 
     Custom feature generator  184  re-computes the midpoint of each tooth for each slice ( 316 ). For example, custom feature generator  184  may re-compute the midpoint of each tooth in a manner similar to the techniques described above. In one example, custom feature generator  184  re-computes or updates the convex hull for a slice of a tooth based on the additional vertices created by cleaving the geometry of the tooth along a specified plane. For example, the updated convex hull may include the vertices created by cleaving the tooth, which may result in a smoother convex hull. In other words, the updated convex hull is less likely to have a degenerate shape, such that the slice of the tooth is more likely to be valid. In some instances, custom feature generator  184  re-computes or updates the midpoint of the tooth for the slice based on the updated convex hull. 
     In some examples, landmark identifier  182  computes a closest point between two adjacent teeth (e.g., a point of closest approach or a point of contact between each set of adjacent teeth) within each slice of the plurality of slices ( 318 ). For example, landmark identifier  182  may determine a distance between each point defining an edge of a first tooth and each point defining an edge of a second tooth adjacent to the first tooth. In such examples, landmark identifier  182  determines the closest point between the first and second teeth based on the distances between each set of points. For example, landmark identifier  182  may determine the closest point between adjacent teeth by calculating the center between a point on the edge of the first tooth and a point on the edge of the second tooth. 
     Custom feature generator  184  automatically generates one or more custom appliance features for dental appliance  101  based on the landmarks. In the example of  FIG. 3 , custom feature generator  184  generates one or more coarsely-grained splines for each slice of the future dental anatomy ( 320 ) based on the midpoints of each tooth and the closest points between adjacent teeth (e.g., a point of closest approach or a point of contact between adjacent teeth) for each slice. For example, custom feature generator  184  may accumulate a set of points (e.g., tooth midpoints, closest points between adjacent teeth, or a combination thereof) for each slice to generate a spline for each slice. In one example, custom feature generator  184  generates the spline by curve fitting the plurality of midpoints and/or closest points between adjacent teeth to a polynomial function. Responsive to determining the polynomial function that fits the midpoints and/or closest points, custom feature generator  184  may generate the points of the coarsely-grained spline by inputting a first series of values into the polynomial function to determine a corresponding output point (e.g., x, y coordinate) for each input value. In some examples, the difference between any two consecutive input values in the first series of input values is a fixed amount. In other words, the density of points in the coarsely-grained spline is a first density. Said another way, custom feature generator  186  determines a polynomial function that includes a plurality of landmark points (e.g., tooth midpoints, closest points between adjacent teeth) and interpolates between the landmark points using the polynomial function to generate a coarsely-grained spline defined by a first density of points. 
     In the example of  FIG. 3 , custom feature generator  184  generates a finely-grained spline for each slice based on the corresponding coarsely-grained spline ( 322 ). For example, custom feature generator  184  may generate the finely-grained spline by inputting a second series of values into the polynomial function defining the spline. In one example, the difference between each input value for the finely-grained spline is less than the difference between each input value for the coarsely-grained spline. In other words, the space between the points of the finely-grained spline is less than the space between points of the coarsely-grained spline, such that the density of the finely-grained spline is a second density that is greater than the density of the coarsely-grained spline. 
     Custom feature generator  184  generates, in some examples, accumulates the points for all of the finely-grained splines across all of the slices ( 324 ). In other words, custom feature generator  184  aggregates the points for each of the splines. 
     According to some scenarios, custom feature generator  184  generates a 3D mesh of the mold parting surface based on the accumulated points across all of the slices ( 326 ). In other words, custom feature generator  184  may utilize each of the points for each of the splines to create a 3D mesh of the mold parting surface. 
     Responsive to generating the mold parting surface, custom feature generator  184  may output data indicative of the mold parting surface ( 328 ). For example, custom feature generator  184  may output data indicative of the 3D mesh to model assembler  188  for constructing the digital model of dental appliance  101 . 
       FIG. 4  is a conceptual diagram illustrating an example technique for generating a digital model of a dental appliance, in accordance with various aspects of this disclosure.  FIG. 4  is described below in the context of system  100  of  FIG. 1 . 
     Model assembler  188  of computing device  150  may receive data indicative of one or more custom appliance features, one or more pre-defined appliance features, and the digital 3D model of the future dental anatomy of patient  102 . 
     Model assembler  188  may identify an outer surface (B) of a dental appliance  101  based on the digital model of the future dental anatomy ( 402 ). For example, model assembler  188  may identify an input mesh (A) of the future dental anatomy based on the digital model of the future dental anatomy and apply an offset to the input mesh (A) to identify an outer surface (B) of dental appliance  101 . 
     In the example of  FIG. 4 , model assembler  188  applies the one or more custom appliance features (C) and one or more pre-defined appliance features (D) and (E) to the outer surface (B) of dental appliance  101  ( 404 ). For example, model assembler  188  positions and orients features (C), (D), and (E) according to one or more rules. The rules may be pre-programmed or machine generated, for example, via machine learning. 
     Responsive to applying the custom appliance features (C) and pre-defined appliance features (D) and (E) to the outer surface (B) of dental appliance  101 , model assembler  188  unions the outer surface (B) and appliance features (C), (D), and (E) to generate a unioned mesh (F) of dental appliance  101  ( 406 ). 
     Model assembler  188  subtracts the input mesh (A) from the unioned mesh (F) to generate the final digital model (G) of dental appliance  101  ( 408 ). Subtracting the input mesh (A) after generating unioned mesh (F) may enable model assembler  188  to automatically generate a digital model of dental appliance  101  such that none of appliance features (C), (D), or (E) protrude through dental appliance  101 . 
       FIG. 5  is a conceptual diagram illustrating a plurality of slices of an example digital model of a dental anatomy, in accordance with various aspects of this disclosure.  FIG. 5  is described below in the context of system  100  of  FIG. 1 . 
     Landmark identifier  182  computes one or more landmarks of the future dental anatomy of patient  102  based on the digital model  500  of the future dental anatomy. In the example of  FIG. 5 , landmark identifier  182  computes the landmarks by dividing the digital model  500  into a plurality of slices  502 A- 502 D (collectively, slices  502 ). In one example, the thickness of each slice is the same. In one example, the thickness of one or more slices is different than the thickness of another slice. The thickness of a given slice may be pre-defined or user-defined. In one example, landmark identifier  182  dynamically determines the thickness of each slice. While  FIG. 5  is illustrated with four slices  502 , landmark identifier  182  may divide the digital model into any number of slices  502 . 
     Responsive to dividing the digital model of the future dental anatomy into slices, landmark identification  182  may determine whether the slice is valid. In some examples, landmark identification  182  determines whether the slice for the particular tooth is valid based on the area of a given slice of a particular tooth. In one example, landmark identification  182  determines the slice is a valid in response to determining the tooth slice area (e.g., an area of the tooth for a given slice) is one of the largest tooth slice areas for the slices of that tooth (e.g., the area of the tooth for a given slice is one of the three largest slices for that tooth). As another example, landmark identification  182  determines whether the slice for the particular tooth is valid based on the area of the particular slice and the area of the largest slice for the particular tooth. For example, landmark identification  182  may determine the slice is valid in response to determining the area of a particular tooth for the particular slice satisfies (e.g., is greater than or equal to) a threshold percentage (e.g., forty percent) of the area of the largest slice for that particular tooth. 
     In some instances, landmark identification  182  determines whether the slice is valid based on a length of respective portions of a convex hull of a particular tooth for a given slice. In one instance, landmark identification  182  determines a length of a line segment for a particular convex hull. Landmark identification  182  may determine the slice is invalid in response to determining the length of the particular line segment satisfies (e.g., is greater than or equal to) a threshold length (e.g., fifty percent of the length of the perimeter of the convex hull). 
       FIG. 6  is a conceptual diagram illustrating midpoints of a plurality of teeth, in accordance with various aspects of this disclosure.  FIG. 6  is described below in the context of system  100  of  FIG. 1 . 
     Landmark identifier  182  determines a midpoint  604  for each tooth  602 A- 602 N (collectively, teeth  602 ) for each slice. In other words, where landmark identifier  182  divides the digital model into four slices, landmark identifier  182  determines four midpoints for each tooth of the digital model of the future dental anatomy. In some examples, midpoints  604  for a given tooth of teeth  602  may be substantially planar with one another. For example, as shown in  FIG. 6 , midpoints  604  for tooth  602 B appear to be substantially stacked on top of one another. In some examples, midpoints  604  for different slices of the same tooth may be offset from one another. For instance, in the example of  FIG. 6 , midpoints  604  of tooth  602 A are offset from one another. Similarly, as shown in  FIG. 6 , in some examples, midpoints  604  of the anterior teeth (e.g., teeth  602 E- 602 J) may be offset from one another. 
     Landmark identifier  182  may determine the midpoint of a tooth for a particular slice by calculating the center of mass of constellation of points around the edge of a tooth. In another example, landmark identifier  182  determines the midpoint of a tooth for a particular slice based on a convex hull of the tooth for that particular slice. In such examples, landmark identifier  182  determines a geometric center from the convex hull by performing a flood-fill operation on the region circumscribed by the convex hull and computing a center of mass of the flood-filled convex hull. 
       FIG. 7  is a conceptual diagram illustrating points between adjacent teeth, in accordance with various aspects of this disclosure.  FIG. 7  is described below in the context of system  100  of  FIG. 1 . 
     Digital model  700  of a patient&#39;s dental anatomy include teeth  702 A- 702 L (collectively, teeth  702 ). In some examples, landmark identifier  182  determines a closest point  704  between a pair of adjacent teeth  702 . Each of points  704  may be a point of closest approach or a point of contact between adjacent teeth. In one example, landmark identifier  182  determines a closest point  704  between a pair of adjacent teeth  702  for each slice of a plurality of slices. In another example, landmark identifier  182  determines the closest point  704  between adjacent teeth  702  based on the entirety of the adjacent teeth (e.g., without dividing the dental anatomy into slices). 
       FIGS. 8A-8B  are conceptual diagrams illustrating convex hulls, in accordance with various aspects of this disclosure.  FIGS. 8A-8B  is described below in the context of system  100  of  FIG. 1 . 
       FIG. 8A  illustrates a single convex hull for a single slice of a digital model of the future dental anatomy. Landmark identification  182  determines, in some examples, a convex hull of a particular tooth for a particular slice. In one example, landmark identification  182  identifies a set of points that reside within a narrow delta of a given value of the Z-axis. For example, for a slice at Z=1.0, landmark identification  182  may identify all of the vertices  802  in a tooth mesh  800  that lie in the particular slice defined by values of Z between 0.8 and 1.0. Landmark identification  182  may examine the XY coordinates of vertices  802 . For example, landmark identification  182  may identify the convex hull  804  of the particular tooth for a particular slice by identifying a subset of vertices of a given slice that circumscribe the entire set of vertices  802  for that slice. The number of vertices in a slice may depend on the thickness of the slice. For instance, the number of vertices for a slice of a tooth increases as the thickness of the z-axis for that slice increases. In some examples, cleaving part of a tooth for a given slice may create additional vertices for that slice of the tooth. 
       FIG. 8B  illustrates a plurality of convex hulls that are each associated with a respective slice of a plurality of slices of a digital model of the future dental anatomy. In one example, a first slice includes vertices  812  that define a first convex hull  814  and a second slice includes vertices  822  that define a second convex hull  824 . 
       FIG. 9  is a conceptual diagram illustrating example splines, in accordance with various aspects of this disclosure.  FIG. 9  is described below in the context of system  100  of  FIG. 1 . 
     Custom feature generator  184  generates one or more custom appliance features. In the example of  FIG. 9 , custom feature generator  184  generates spline  902 . Custom feature generator  184  may generate spline  902  by accumulating a plurality of points  904 AA- 904 NN (collectively, points  904 ) for all of the slices of the future dental anatomy. Points  904  may include midpoints of each tooth and points between adjacent teeth (e.g., points of contact or points of closest approach). In one example, coordinate symbol  906 A- 906 B illustrates the local coordinate system for each respective tooth. 
       FIGS. 10A-10B  are conceptual diagrams illustrating example mold parting surfaces, in accordance with various aspects of this disclosure.  FIGS. 10A-10B  are described below in the context of system  100  of  FIG. 1 . 
     Custom feature generator  184  automatically generates one or more custom appliance features for dental appliance  101  based on the landmarks. In one example, custom feature generator  182  generates a mold parting surface  1002  based on landmarks such as a midpoint of each tooth and points between adjacent teeth (e.g., points of contact between adjacent teeth and/or points of closest approach between adjacent teeth) for each slice. For example, custom feature generator  184  may generate one or more splines for each slice based on the midpoints and points between adjacent teeth for a respective slice, and may generate the mold parting surface based on the splines for all of the slices. In one example, custom feature generator  184  generates a coarsely-grained spline based on the midpoints and points between adjacent teeth. 
     In another example, custom feature generator  184  generates a finely-grained spline for each slice based on the coarsely-grained spline or splines for each slice. In such examples, custom feature generator  184  accumulates the points for all of the finely-grained splines across all of the slices. In other words, custom feature generator  184  aggregates the points for each of the splines to generate a 3D mesh for a mold parting surface  1002 . Said yet another way, custom feature generator  184  may utilize each of the points (e.g., midpoints, points of closest approach, and/or points of contact) for each of the splines to create a 3D mesh of the mold parting surface  1002 . 
       FIG. 11  is a conceptual diagram illustrating an example gingival trim surface, in accordance with various aspects of this disclosure. Gingival trim surface  1102  may include a 3D mesh that trims an encompassing shell between gingiva  1104  and teeth  1106 . 
       FIG. 12  is a conceptual diagram illustrating an example facial ribbon, in accordance with various aspects of this disclosure. Facial ribbon  1202  is a stiffening rib of nominal thickness that is offset facially from the shell. In some instances, the facial ribbon follows both the archform and the gingival margin. In one instance, the bottom of the facial ribbon falls no farther gingivally than the gingival trim surface. 
       FIG. 13  is a conceptual diagram illustrating an example lingual shelf  1302 , in accordance with various aspects of this disclosure. Lingual shelf  1302  is a stiffening rib of nominal thickness on the lingual side of the mold appliance, inset lingually and following the archform. 
       FIG. 14  is a conceptual diagram illustrating example doors and windows, in accordance with various aspects of this disclosure. Windows  1404 A- 1404 H (collectively, windows  1404 ) includes an aperture that provide access to the tooth surface so that dental composite can be placed on the tooth. A door includes a structure that covers the window. The shape of the window may be defined as a nominal inset from the perimeter of the tooth when viewing the tooth facially. In some instances, the shape of the door corresponds to the shape of a window. The door may be inset to create clearance between the door and window. 
       FIG. 15  is a conceptual diagram illustrating example rear snap clamps, in accordance with various aspects of this disclosure.  FIG. 15  is described below in the context of system  100  of  FIG. 1 . 
     Feature manager  186  may determine one or more characteristics of rear snap clamps  1502 A- 1502 B (collectively, rear snap clamps  1502 ). Rear snap clamps  150  may be configured to couple a facial portion of dental appliance  101  with a lingual portion of dental appliance  101 . Example characteristics include size, shape, position, and/or orientation of the rear snap clamps. In one example, feature manager  186  determines the position of rear snap clamps  150  based on the position of the outer-most teeth to be restored. For example, manager  186  may position rear snap clamps along the archform on opposite ends of the archform (e.g., a first snap clamp at one end and a second snap clamp at another end). In some examples, feature manager  186  positions a female portion of the rear snap clamp on the lingual side of the parting surface and positions a male portion of the rear snap clamp on the facial side. 
       FIG. 16  is a conceptual diagram illustrating example door hinges, in accordance with various aspects of this disclosure.  FIG. 16  is described below in the context of system  100  of  FIG. 1 . 
     Feature manager  186  may determine one or more characteristics of door hinges  1602 A- 1602 F (collectively, door hinges  1602 ). Door hinges  1602  may be configured to pivotably couple a door to dental appliance  101 . Example characteristics include size, shape, position, and/or orientation of the door hinge  1602 . 
     In one example, feature manager  186  determines the position of door hinges  1602  based on a position of another pre-defined appliance feature. For example, feature manager  186  may position each door hinge  1602  at the midline of a corresponding door. In one scenario, feature manager  186  positions the female portion of a door hinge  1602 A to anchor to the facial portion of dental appliance  101  (e.g., towards the incisal edge of a tooth) and positions the male portion of the door hinge  1602 A to anchor to the outer face of the door. 
       FIGS. 17A-18B  are conceptual diagrams illustrating example door snaps, in accordance with various aspects of this disclosure.  FIG. 17A-17B  is described below in the context of system  100  of  FIG. 1 . 
     Feature manager  186  may determine one or more characteristics of door snaps  1702 A- 1702 F (collectively, door snaps  1702 ). Example characteristics include size, shape, position, and/or orientation of the door snaps  1702 . 
     In one example, feature manager  186  determines the position of door snaps  1702  based on a position of another pre-defined appliance feature. For example, feature manager  186  may position each door snap  1702  at the midline of a corresponding door. In one instance, feature manager  186  positions the female portion of the door snap to anchor to an outer face of the door and extends downward toward the gingiva. In another instance, feature manager  186  positions the male portion of the door snap to anchor to the gingival side of the facial ribbon. 
       FIG. 18  is a conceptual diagram illustrating an example incisal ridge, in accordance with various aspects of this disclosure. Incisal ridge  1802  provides reinforcement at the incisal edge. 
       FIG. 19  is a conceptual diagram illustrating an example center clip, in accordance with various aspects of this disclosure. Center clip  1902  aligns the facial portion and the lingual portion of the dental appliance with one another. 
       FIG. 20  is a conceptual diagram illustrating example door vents, in accordance with various aspects of this disclosure. Door vents  2002 A- 2002 B (collectively, door vents  2002 ) transport excess dental composite out of the dental appliance. 
       FIG. 21  is a conceptual diagram illustrating example doors, in accordance with various aspects of this disclosure. In the example of  FIG. 20 , a dental appliance includes door  2102 , door hinge  2104 , and door snap  2106 . 
       FIG. 22  is a conceptual diagram illustrating an example diastema matrix, in accordance with various aspects of this disclosure. Diastema matrix  2202  includes handle  2204 , body  2206 , and wrapping portion  2208 . Wrapping portion  2208  is configured to fit in the interproximal region between two adjacent teeth. 
       FIG. 23  is a conceptual diagram illustrating an example manufacturing case frame and an example dental appliance, in accordance with various aspects of this disclosure. Manufacturing case frame  2302  is configured to support one or more parts of a dental appliance. For example, the manufacturing case frame  2302  may detachably couple a lingual portion  2304  of a dental appliance, a facial portion  2306  of the dental appliance, and a diastema matrix  2308  to one another via case frame sparring  2310 . In the example of  FIG. 23 , case frame sparring  2310  ties or couples the parts  2304 ,  2306 , and  2308  of the dental appliance to the manufacturing case frame  2302 . 
       FIG. 24  is a conceptual diagram illustrating an example dental appliance including custom labels, in accordance with various aspects of this disclosure. Custom labels  2402 - 2408  may be printed on various parts of the dental appliance and includes data (e.g., a serial number, a part number, etc.) identifying a respective part of the dental appliance. 
     Various examples have been described. These and other examples are within the scope of the following claims.