Patent Publication Number: US-2023144308-A1

Title: Implant for correcting a defect of a bone structure with navigation fiducials

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application is a continuation of International Application No. PCT/US2021/034907, filed May 28, 2021, which claims benefit of and priority to U.S. Provisional Patent Application Ser. No. 63/031,888, filed May 29, 2020, the entirety of each of which is incorporated by reference herein for all purposes. 
    
    
     BACKGROUND 
     Field of the Disclosure 
     This application relates to methods and apparatus for manufacturing and using implants with navigation fiducials. 
     Description of the Related Technology 
     Trauma, resection of tumor tissue, congenital anomalies, and other defects can lead to loss of bony tissue or otherwise disrupt bone structures, such as the complex and relatively thin bone structures surrounding and supporting the human eye. For example, such defects in the orbital walls and particularly the orbital floor can cause the eyeball to leave its natural position, potentially leading to blurry vision, an aesthetically unpleasing appearance, and in some cases inability to fully close the eyelids. Such defects present difficult internal bone repair and fixation problems in reconstructive surgery and in trauma surgery. 
     In some examples, implants can be used for internal repair of defects and fixation of fractures. Typically, an implant may take the form of thin plates. Implants can be manufactured flat or pre-bent to a particular shape, and can be further shaped intra-operatively to fit an individual patient&#39;s anatomy. In some examples, an implant may have a patient-specific form, where the shape, circumference, and surface profile are designed to match the individual patient&#39;s anatomy. Patient-specific implants generally provide a better fit to the patient anatomy and an easier implantation due to reduced intra-operative shaping. 
     However, as is typical in orbital implant surgeries, and many other surgeries, the shape and accessibility of the anatomy, such as the eye socket, is such that, upon implantation, certain regions of the implant may be located underneath soft tissue (e.g., and the eyeball), thereby obscuring these regions from view. In an example, if, during surgery, placement of the implant to restore the original position of the eyeball cannot initially be achieved, the surgeon may need to remove and reshape the implant, or possibly even stuff certain areas of the eye socket with graft material, resulting in a potentially lengthy trial-and-error surgery. 
     It is important to restore the original position of the eyeball as closely as possible, not only for aesthetic reasons, but also to avoid blurred vision. It is therefore important to increase the chance of a close-to-natural initial repositioning of the eyeballs. In view of these and other problems, apparatuses and methods that improve the likelihood of a successful implantation of an implant are described herein. 
     SUMMARY 
     Certain aspects are directed to an (e.g., orbital) implant for correcting a defect of a (e.g., orbital) bone structure. The implant includes a first surface comprising a shape configured to interface with a surface of the bone structure (e.g., a floor and/or a rim of the orbital bone structure). The implant also includes a second surface opposite the first surface and substantially conformal to the shape of the first surface, the second surface comprising three or more point features on the second surface, wherein the three or more point features comprise a first point feature, a second point feature, and a third point feature that form nodes of a triangle, wherein each of the three or more point features are recessed below the second surface. 
     Certain aspects are directed to a method for correcting a defect of a bone structure. The method includes positioning a first surface of an implant onto the defect of the bone structure, a first surface comprising a shape configured to interface with a surface of the bone structure (e.g., a floor and/or a rim of the orbital bone structure), and a second surface opposite the first surface and substantially conformal to the shape of the first surface, the second surface comprising three or more point features on the second surface, wherein the three or more point features comprise a first point feature, a second point feature, and a third point feature that form nodes of a triangle, wherein each of the three or more point features are at least one of raised above or recessed below the second surface. The method also includes tracing a stylus of a navigation system along the second surface until the stylus is received by each of the three or more point features to compare one or more of: (i) the shape of the implant relative to a planned shape of the implant, or (ii) a position of the implant in a patient relative to a planned position of the implant in the patient. 
     Certain aspects are directed to a method for generating an implant for correcting a defect of a bone structure. The method includes receiving a design of the implant, the design indicating a shape, size, and position of the implant relative to a build area corresponding to an additive manufacturing device, the design of the implant defining: a first surface comprising a shape configured to interface with a surface of the bone structure (e.g., a floor and/or a rim of the orbital bone structure); and a second surface opposite the first surface and substantially conformal to the shape of the first surface, the second surface comprising three or more point features on the second surface, wherein the three or more point features comprise a first point feature, a second point feature, and a third point feature that form nodes of a triangle, wherein each of the three or more point features are at least one of raised above or recessed below the second surface. The method also includes causing manufacturing of the implant using additive manufacturing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating an example of a computer environment suitable for the implementation of three-dimensional (3D) object design, build simulation, and manufacturing. 
         FIG.  2    is a diagram illustrating a functional block diagram of one example of a computer of  FIG.  1   . 
         FIG.  3    is a diagram illustrating a process for manufacturing a 3D object or device. 
         FIG.  4 A  is a diagram illustrating an exemplary additive manufacturing apparatus for generating a 3D object. 
         FIG.  4 B  is a diagram illustrating a recoating mechanism (e.g., a leveling drum/roller) that may be used instead of the recoating mechanism of  FIG.  4 A . 
         FIGS.  5 A- 5 E  are diagrams illustrating an example orbital implant installed over an orbital floor of a patient&#39;s right eye socket. 
         FIGS.  6 A and  6 B  are diagrams illustrating an example off-the-shelf orbital implant, and the same implant in a bent shape for implantation, according to aspects of the disclosure. 
         FIGS.  7 - 23    are diagrams illustrating patient-specific orbital implants according to aspects of this disclosure. 
         FIG.  24    is a diagram illustrating various examples of elevated ridges according to aspects of this disclosure. 
         FIG.  25    is a flow diagram illustrating example operations for using an optical implant. 
         FIG.  26    is a schematic diagram illustrating a conceptual example of a navigation system. 
     
    
    
     DETAILED DESCRIPTION 
     Apparatuses and methods disclosed herein include techniques for producing implants with navigation fiducials, and using the implants for improved surgical implantation and defect resolution procedures. The aspects of the present invention are illustrated by means of orbital implants, but a person skilled in the art will readily appreciate that they also apply to other types of implants, particularly implants that are partially or entirely hidden from view during installation in the patient&#39;s body or implants that may be shaped to better match the patient&#39;s anatomy. In certain aspects, the orbital implants comprise thin plates (sometimes perforated) or meshes. While features of the present disclosure may be discussed relative to certain embodiments and figures below, all embodiments of the present disclosure can include one or more of the advantageous features discussed herein. In other words, while one or more embodiments may be discussed as having certain advantageous features, one or more of such features may also be used in accordance with various other embodiments discussed herein. In similar fashion, while exemplary embodiments may be discussed below as device, instrument, or method embodiments it should be understood that such exemplary embodiments can be implemented in various devices, instruments, and methods. Unless explicitly mentioned, all method steps disclosed herein may be entirely performed by a computing device, partly performed by a computing device, or partly or entirely performed by a skilled user, such as a medical professional or non-medical professional, such as a technician or engineer. Accordingly, the word “specialist” refers to such a skilled medical or non-medical user. For example, all method steps described for the design and production of implants, including but not limited to image segmentation, defect reconstruction, implant design, implant bending and implant manufacturing, may be entirely performed by a computing device, partly performed by a computing device, or partly or entirely performed by a skilled user, such as a medical professional or non-medical professional, such as a technician or engineer. 
     In this disclosure, the terms “distal” and “proximal” in the context of an implant refer to positions relative to a specialist placing the implant when the implant is in its planned position with respect to the patient&#39;s anatomy. For example, the distal edge of an implant is the edge farthest away from the specialist when the implant is in its planned implanted position. In this disclosure, the terms “anterior” and “posterior” in the context of an implant refer to positions relative to the patient&#39;s anatomy as the implant is in its planned position with respect to the patient&#39;s anatomy. For example, the anterior edge of an implant is the edge most towards the anterior side of the patient when the implant is in its planned implanted position. 
     In certain aspects, an orbital implant may be designed on a computing system using any suitable computer-aided design (CAD) software, and/or any suitable system for designing, simulating, and/or manufacturing 3D objects. Turning to  FIG.  1   , an example of a computer environment suitable for the implementation of 3D object design, build simulation, and manufacturing is shown. The environment includes a system  100 . The system  100  includes one or more computers  102   a - 102   d,  which can be, for example, any workstation, server, or other computing device capable of processing information. In some embodiments, each of the computers  102   a - 102   d  can be connected, by any suitable communications technology (e.g., an internet protocol), to a network  105  (e.g., the Internet). Accordingly, the computers  102   a - 102   d  may transmit and receive information (e.g., software, digital representations of three dimensional (3D) objects, commands or instructions to operate an additive manufacturing device, etc.) between each other via the network  105 . 
     The system  100  further includes one or more additive manufacturing devices (e.g., 3D printers)  106   a - 106   b.  As shown the additive manufacturing device  106   a  is directly connected to a computer  102   d  (and through computer  102   d  connected to computers  102   a - 102   c  via the network  105 ) and additive manufacturing device  106   b  is connected to the computers  102   a - 102   d  via the network  105 . Accordingly, one of skill in the art will understand that an additive manufacturing device  106  may be directly connected to a computer  102 , connected to a computer  102  via a network  105 , and/or connected to a computer  102  via another computer  102  and the network  105 . 
     It should be noted that though the system  100  is described with respect to a network and one or more computers, the techniques described herein also apply to a single computer  102 , which may be directly connected to an additive manufacturing device  106 . 
       FIG.  2    illustrates a functional block diagram of one example of a computer of  FIG.  1   . The computer  102   a  includes a processor  210  in data communication with a memory  220 , an input device  230 , and an output device  240 . In some embodiments, the processor is further in data communication with an optional network interface card  260 . Although described separately, it is to be appreciated that functional blocks described with respect to the computer  102   a  need not be separate structural elements. For example, the processor  210  and memory  220  may be embodied in a single chip. 
     The processor  210  can be a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any suitable combination thereof designed to perform the functions described herein. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The processor  210  can be coupled, via one or more buses, to read information from or write information to memory  220 . The processor may additionally, or in the alternative, contain memory, such as processor registers. The memory  220  can include processor cache, including a multi-level hierarchical cache in which different levels have different capacities and access speeds. The memory  220  can also include random access memory (RAM), other volatile storage devices, or non-volatile storage devices. The storage can include hard drives, flash memory, etc. 
     The processor  210  also may be coupled to an input device  230  and an output device  240  for, respectively, receiving input from and providing output to a user of the computer  102   a.  Suitable input devices include, but are not limited to, a keyboard, buttons, keys, switches, a pointing device, a mouse, a joystick, a remote control, an infrared detector, a bar code reader, a scanner, a video camera (possibly coupled with video processing software to, e.g., detect hand gestures or facial gestures), a motion detector, or a microphone (possibly coupled to audio processing software to, e.g., detect voice commands). Suitable output devices include, but are not limited to, visual output devices, including displays and printers, audio output devices, including speakers, headphones, earphones, and alarms, additive manufacturing devices, and haptic output devices. 
     The processor  210  further may be coupled to a network interface card  260 . The network interface card  260  prepares data generated by the processor  210  for transmission via a network according to one or more data transmission protocols. The network interface card  260  also decodes data received via a network according to one or more data transmission protocols. The network interface card  260  can include a transmitter, receiver, or both. In other embodiments, the transmitter and receiver can be two separate components. The network interface card  260 , can be embodied as a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any suitable combination thereof designed to perform the functions described herein. 
       FIG.  3    illustrates a process  300  for manufacturing a 3D object or device. As shown, at a step  305 , a digital representation of the object is designed using a computer, such as the computer  102   a.  For example, two dimensional (2D) or 3D data may be input to the computer  102   a  for aiding in designing the digital representation of the 3D object. Continuing at a step  310 , information corresponding to the 3D object is sent from the computer  102   a  to an additive manufacturing device, such as additive manufacturing device  106 , and the device  106  commences a manufacturing process for generating the 3D object in accordance with the received information. At a step  315 , the additive manufacturing device  106  continues manufacturing the 3D object using suitable materials, such as a polymer or metal powder. Further, at a step  320 , the 3D object is generated. 
       FIG.  4 A  illustrates an exemplary additive manufacturing apparatus  400  for generating a 3D object. In this example, the additive manufacturing apparatus  400  is a laser sintering device. The laser sintering device  400  may be used to generate one or more 3D objects layer by layer. The laser sintering device  400 , for example, may utilize a powder (e.g., metal, polymer, etc.), such as the powder  414 , to build an object a layer at a time as part of a build process. 
     Successive powder layers are spread on top of each other using, for example, a recoating mechanism  415 A (e.g., a re-coater blade). The recoating mechanism  415 A deposits powder for a layer as it moves across the build area, for example in the direction shown, or in the opposite direction if the recoating mechanism  415 A is starting from the other side of the build area, such as for another layer of the build. After deposition, a computer-controlled carbon dioxide (CO2) laser beam scans the surface and selectively binds together the powder particles of the corresponding cross section of the product. In some embodiments, the laser scanning device  412  is an X axis and Y axis moveable infrared laser source. As such, the laser source can be moved along an X axis and along a Y axis in order to direct its beam to a specific location of the top most layer of powder. Alternatively, in some embodiments, the laser scanning device  412  may comprise a laser scanner which receives a laser beam from a stationary laser source, and deflects it over moveable mirrors to direct the beam to a specified location in the working area of the device. During laser exposure, the powder temperature rises above the material (e.g., glass, polymer, metal) transition point after which adjacent particles flow together to create the 3D object. The device  400  may also optionally include a radiation heater (e.g., an infrared lamp) and/or atmosphere control device  416 . The radiation heater may be used to preheat the powder between the recoating of a new powder layer and the scanning of that layer. In some embodiments, the radiation heater may be omitted. The atmosphere control device may be used throughout the process to avoid undesired scenarios such as, for example, powder oxidation. 
     In some other embodiments, such as shown with respect to  FIG.  4 B , a recoating mechanism  415 B (e.g., a leveling drum/roller) may be used instead of the recoating mechanism  415 A. Accordingly, the powder may be distributed using one or more moveable pistons  418 ( a ) and  418 ( b ) which push powder from a powder container  428 ( a ) and  428 ( b ) into a reservoir  426  which holds the formed object  424 . The depth of the reservoir, in turn, is also controlled by a moveable piston  420 , which increases the depth of the reservoir  426  via downward movement as additional powder is moved from the powder containers  428 ( a ) and  428 ( b ) in to the reservoir  426 . The recoating mechanism  415 B, pushes or rolls the powder from the powder container  428 ( a ) and  428 ( b ) into the reservoir  426 . Similar to the embodiment shown in  FIG.  4 A , the embodiment in  FIG.  4 B  may use the radiation heater  416  alone for preheating the powder between recoating and scanning of a layer. 
     Example Orbital Implants 
       FIGS.  5 A- 5 E  are diagrams illustrating various stages of implanting an orbital implant in a patient. 
       FIG.  5 A  illustrates an example computer-generated three-dimensional (3D) model of a defect  508  in the orbital floor of a patient&#39;s right eye socket  500 . A skilled user may generate a model of the defect  508  in order to better model an ideal placement and shape of an implant. For example,  FIG.  5 B  illustrates an example of the model of  FIG.  5 A , in which the healthy anatomical surface surrounding defect  508  is married with a reconstructed anatomical surface  510  (e.g., wherein the reconstructed anatomical surface  510  is a mirror image of the patient&#39;s orbital floor in the left eye socket). This model of a desired bone structure can be used to produce (e.g. by means of additive manufacturing) a physical model of the desired bone structure, which may serve as a bending mold for pre-bending an off-the-shelf implant, or can be used for designing and manufacturing a patient-specific implant. 
       FIG.  5 C  is a diagram illustrating an example of a model off-the-shelf orbital implant  520  installed over the defect  508  in the orbital floor of the patient&#39;s right eye socket  500 . Here, the 3D model of  FIGS.  5 A and  5 B  provides a basis for determining the extent to which portions of the off-the-shelf orbital implant  520  should be bent or reshaped in order to best imitate the healthy anatomical surface  510  of  FIG.  5 B . The off-the-shelf orbital implant  520  includes three extensions  512  configured to be attached to a rim  506  of the orbital bone structure by means of fixation elements, such as screws. A 3D model of the bent or reshaped off-the-shelf implant can be used to design and produce a bending mold, such as the one shown in  FIG.  5 E :  FIG.  5 E  is a diagram illustrating an example bending mold  550  for pre-operatively or intra-operatively bending an off-the-shelf implant. 
       FIG.  5 D  is a diagram illustrating an example of a model patient-specific orbital implant  502  installed over an orbital floor  504  of a patient&#39;s right eye socket  500 , and attached to a rim  506  of the orbital bone structure. Here, the 3D model of  FIGS.  5 A and  5 B  provides a basis for designing a shape of the patient-specific orbital implant in order to best imitate the healthy anatomical surface  510  of  FIG.  5 B . 
     As discussed, patient-specific orbital implants  502  may be designed based on information about the patient&#39;s orbital bone structure. Such information can be found in, for example, computerized tomography (CT) scans, magnetic resonance imaging (MRI) scans, ultrasound images, or images from other suitable medical imaging modalities. Through a process called “segmentation,” the information in these medical images can be used to develop a virtual 3D model of the patient&#39;s orbital bone structure (e.g., by means of the Materialise Mimics software). With the use of CAD software, the orbital implant  502  can be specifically designed to mate with a portion of the patient&#39;s orbital bone structure to resolve a structural defect. The orbital implant  502  can then be manufactured by using the additive manufacturing technology discussed above, such as selective laser sintering or selective laser melting with any suitable material, such as titanium, titanium alloy, stainless steel, polyether ether ketone (PEEK), etc. 
     In some examples, a defect in an orbital bone structure may prevent a specialist from developing a 3D model that accurately models the desired bone structure. In such a case, the specialist may instead model an orbital bone structure of a contralateral side if that side is intact. Thus, a mirror image of at least a part of the virtual 3D model containing the contralateral eye socket may be registered with the healthy tissue surrounding the defect and used as a template for the design of the orbital implant  502 . Alternatively, a statistical shape model (SSM) of the orbital bone structure may be used to model the orbital bone structure based on the healthy or intact tissue and/or bone surrounding the defect. For example, an SSM may be constructed based on segmentation or partitioning of a digital image or stack of digital images into multiple segments (e.g., sets of pixels or voxels). Next, the segmentation changes the representation of the image into something relatively easier to analyze, such as a three-dimensional model such as a wireframe model or a surface model, enveloping one or more of such sets of pixels or voxels. In some examples, the digital images are analyzed to locate objects and/or boundaries, such as anatomical areas of interest. In this case, the anatomical area of interest may include the orbital bone structure. Based on a statistical analysis of a training set (e.g., a number of such three-dimensional models from specimens of a population), an SSM may be created, which is a three-dimensional deformable representation that encodes the correlations between shape variations among members of training set, which is assumed to be representative for the entire population. An SSM is characterized by a vector of limited length. By assigning values to the coordinates of this vector, the SSM may be deformed in a way that is consistent with the shape variations witnessed in the training set. Once an SSM is created based on a training set of intact orbital bones, the SSM can be fitted to the healthy or intact tissue and/or bone surrounding the defect. The instance of the SSM thus obtained may then comprise a section spanning the defect and exhibiting a shape representative of non-defective tissue. It may therefore be used to model a desired bone structure (i.e. reconstruct the defect). The desired bone structure may be used as a template for designing the orbital implant  502 . The benefit of such a process is that it increases the likelihood of the eyeball being pushed back in its original position, which is one of the main benefits of patient-specific orbital implants  502 . In some aspects, the present disclosure relates to a method as described above for designing and/or manufacturing any of the patient-specific implant described below. 
     Pre-operative planning can transfer many of the advantages of patient-specific orbital implants  502  to off-the-shelf implants (e.g., see the off-the-shelf orbital implant  600  described in more detail in  FIGS.  6 A and  6 B  below). For example, information about the patient&#39;s bony anatomy may be obtained (step  1 ) from medical images and used to develop (step  2 ) a 3D model (e.g., see  FIG.  5 A ) as described above (e.g., model of orbital bone structure). A target shape for an eyeball-facing surface of the orbital implant may then be designed (step  3 ) using the 3D model, a mirror image of the contralateral side, and/or an SSM to model a desired bone structure as described above. The target shape may then be offset towards the bone over a distance equal to the thickness of the implant. The target shape or the resulting offset target surface may then be married to the 3D model of the patient&#39;s bone structure surrounding the defect to obtain (step  4 ) a 3D model of a desired bone structure (e.g., see  FIG.  5 B ) to serve as a bending mold. This bending mold, i.e. a physical model of the desired bone structure, may be manufactured (step  5 ) from the 3D model using an additive manufacturing technology in any suitable material, such as polyamide. A skilled user, such as a surgeon or a non-medical professional, such as a technician or an engineer, may then use this mold to select the most appropriate off-the-shelf implant (e.g., see  FIG.  6 A ) type and size, and bend (step  6 —see  FIG.  5 C ) the structure of the selected implant to fit the patient anatomy and the target shape. In some aspects, the present disclosure relates to a kit of such a bending mold and an off-the-shelf implant. In some aspects, the present disclosure relates to a method as described above for designing and/or manufacturing a physical model of the desired bone structure to serve as a bending mold as described above and/or for selecting and/or bending an off-the-shelf implant. 
     In some examples, the bending of the selected off-the-shelf orbital implant may first be simulated using a 3D model of the selected off-the-shelf implant and the 3D model of the patient&#39;s bone structure. The bending may be simulated such that the eyeball-facing surface of the orbital implant closely follows the target shape, and such that the bone-facing surface of the implant is well seated on the bone structure surrounding or adjacent to the defect (e.g., see  FIG.  5 C ). Simulated bending may allow the specialist, such as a medical professional or a non-medical professional, such as a technician or engineer, to choose an appropriate position for the implant. This can ensure that fixation points (e.g., attachment points where the orbital implant is attached to the bone structure) are in locations with good bone stock, and that the implant is not in a position where it could disturb or harm delicate anatomical features, such as nerves. Simulated bending may allow said specialist to ensure that the defect is well covered, and that there is little excess implant material. In some examples, a simulation of bending the orbital implant may include a selection of the most appropriate implant type, shape, and/or size from a library. 
     In some examples, the resulting shape of a virtually bent orbital implant may be used to generate a set of instructions for a sheet-metal bending machine configured to bend the implant according to the instructions. Alternatively, the resulting shape may be used to design a bending mold (e.g., see  FIG.  5 E ) mating with either the eyeball-facing surface or the bone-facing surface of the simulated bend of the implant. For example, the bending mold may have any shape suitable for easy manipulation, such as a prismatic shape with a flat lower surface, and may have a top surface configured to mate with either the eyeball-facing or the bone-facing surface of the virtually bent implant. The bending mold may include markings to delineate the outline of the bent implant, or protrusions  552  to be received in one or more perforations of the implant or notches along the edge of the implant such as the apertures of the mesh or any apertures for receiving fixation elements. The bending mold may be manufactured from the design using the additive manufacturing described above using any suitable material, such as polyamide. A skilled user can then bend the implant until it fits on the bending mold (see  FIG.  5 E ), using any markings and protrusions as aides. The bent implant may then be shipped to the hospital, optionally together with the mold. Alternatively, the mold may be shipped to the hospital, together with the unbent implant. The surgeon or a technician can then bend the implant until it fits on the bending mold, using any markings and protrusions as aides. In some aspects, the present disclosure relates to a kit of such a bending mold and an off-the-shelf implant, either in its original state or in a pre-bent state. In some aspects, the present disclosure relates to methods as described above for selecting and/or virtually bending an off-the-shelf implant, for providing a bending mold and/or for bending an off-the-shelf implant. 
     Whether an orbital implant is manufactured directly in a patient-specific form or bent according to a pre-operative planning procedure using an off-the-shelf implant, correct positioning of the implant in the patient is important for restoring the natural position of the eyeball. The shape and accessibility of the eye socket is such that it is often only possible to fixate the orbital implant to the bone on one side of the implant, generally along a proximal edge of the implant. A slight deviation from the planned position along this edge may lead to a large deviation along the eyeball-facing surface and the distal edge of the implant. Thus, in some cases, navigation systems may be used during a surgical procedure to verify the position of the orbital implant with respect to the patient anatomy, and to compare this position with one or more of a planned position or with a planned target implant shape. 
     Surgical navigation systems (e.g. see  FIG.  26   ) may provide functionality, including: (i) loading (e.g. onto electronic processing device  2606 ) and displaying (e.g. on display unit  2602 ) images or image feeds from intra-operative imaging equipment (e.g., digital/film camera, radiography, magnetic resonance imaging (MRI), computed tomography (CT), fluoroscopy, ultrasound, echocardiography, nuclear medicine, such as positron emission tomography (PET), etc.); (ii) loading (e.g. onto electronic processing device  2606 ) and displaying (e.g. on display unit  2602 ) pre-operatively made virtual 3D models of patient anatomy; (iii) loading (e.g. onto electronic processing device  2606 ) and displaying (e.g. on display unit  2602 ) pre-operatively made virtual 3D models of any hardware, such as implants; (iv) tracking (e.g. using infra-red sensor  2610 ) the location and orientation of instruments (e.g., stylus  2608 ) to determine their position in the navigation system&#39;s coordinate system; (v) sweeping surfaces of the anatomy or indicating certain anatomical landmarks with a tracked instrument, such as a probe or a stylus (e.g. stylus  2608 ) to determine their position in the navigation system&#39;s coordinate system; (vi) sweeping surfaces of any hardware, such as implants, or indicating identifiable features (e.g., navigation fiducials) of such hardware, such as points, lines, curves, contours, with a tracked instrument, such as a probe or a stylus (e.g. stylus  2608 ) to determine their position in the navigation system&#39;s coordinate system; and (vii) establishing a relationship between loaded virtual 3D models and (a) images or image feeds from intra-operative imaging equipment, and/or (b) data gathered with tracked instruments to correctly place the virtual 3D models in the navigation system&#39;s coordinate system. For example, the information gathered under (v) may be used to locate the patient in the navigation system&#39;s coordinate system and/or to bring the patient and pre-operatively made virtual 3D models of patient anatomy into a common coordinate system. For example, a relationship may be established between a “real” coordinate system and a “virtual” coordinate system, and one way of doing this is by sweeping surfaces of the patient&#39;s anatomy to establish a correspondence between the patient&#39;s anatomy and a loaded pre-operatively made virtual 3D model of the patient&#39;s anatomy. 
     As discussed, the 3D model may be generated from medical images using topological and geometrical information gleaned from the images. In some examples, the relationship between the coordinate system of the 3D model and the real coordinate system is established by matching corresponding anatomical landmarks in both the patient and the virtual 3D model. In other examples, the relationship between the coordinate system of the 3D model and the real coordinate system is established by generating a trajectory along an axis of the geometric model that matches the same trajectory along an axis of the patient&#39;s anatomy, and conducting moving trihedron modeling along the trajectory. 
     Once the patient has been located in the navigation system&#39;s coordinate system, any further movement of the patient with respect to the navigation system may be tracked by means of a rigidly attached tracker  2612  attached to the patient anatomy  2616 , and optionally another tracker  2614  attached to the stylus  2608  and the spatial correspondence between the actual patient and the virtual 3D model of the anatomy may be maintained. For example, the information gathered under (vi) may be used to locate any hardware in the navigation system&#39;s coordinate system and/or to bring the hardware and pre-operatively made virtual 3D models of the hardware into a common coordinate system. For example, the same sweeping procedure discussed above may be performed to establish a relationship between a 3D model of an orbital implant (and a corresponding coordinate system) with the actual physical orbital implant. 
     Thus, surgical navigation systems may be used to verify the position of an orbital implant during a procedure to install the implant within the patient (e.g., a virtual 3D model of the patient anatomy and a virtual 3D model of the orbital implant (e.g., either in patient-specific form, or off-the-shelf form with or without a planned simulated bend) in its planned position relative to the virtual 3D model of the patient anatomy). The virtual 3D model of the patient anatomy may be used to correctly position the 3D model of the orbital implant in the navigation system&#39;s coordinate system. 
     In some examples, a specialist may use a tracked instrument of a navigation system, such as a probe or a stylus, to trace particular features of the orbital implant. By tracing the features of the implant, the position of these features can be collected and the location of the features in the navigation system&#39;s coordinate system may be determined and displayed on the navigation system. By locating the features of the actual implant in a coordinate system, the position of the implant may be compared to a planned position (e.g., a position of the 3D model of the orbital implant relative to the 3D model of the patient anatomy). The navigation system can measure any deviation between the actual physical position of the implant and the planned position of the implant, and report the deviation to the specialist, optionally with instructions on how to modify the position of the implant to match the planned position. 
     Accordingly, a surgical navigation system may determine an extent to which the planned bending of an optical implant has been achieved, and/or an extent to which the location of the optical implant matches the planned position, by utilizing identifiable features positioned at certain locations of the implant to establish the implant&#39;s position in space. 
     Example Techniques for Manufacturing and Using Orbital Implants with Navigation Fiducials 
       FIG.  6 A  is a diagram illustrating an example off-the-shelf orbital implant  600  according to aspects of the disclosure. The desired shape of an off-the-shelf implant  600  may be determined either pre-operatively or intra-operatively, and the implant may be bent pre-operatively or intra-operatively to fit the patient anatomy. Any of the bending molds as described above may be used while bending the implant. 
     As shown, the off-the-shelf orbital implant  600  includes bending locations in the form of linear slots  602  that perforate the implant, or narrowed connections  604 . Such bending locations locally reduce the implant&#39;s  600  resistance to deformation, while also subdividing the implant into several major sections  606  (e.g., a first major section  606   a,  a second major section  606   b,  and a third major section  606   c ). Here, the narrowed connections  604  connect multiple holes  608  through which the implant  600  can be fastened to a patient&#39;s bone structure. 
     The orbital implant  600  also includes a plurality of point features  610  that a specialist can use to assess a location of the implant  600  in a coordinate system. For example, the point features  610  may be configured to receive a stylus so that the stylus can feedback location information to the navigation system. In some examples, the point features  610  may be positioned on a section&#39;s surface and/or along its edges, such as the edges it shares with adjacent sections. In certain aspects, the point features  610  may include a hole at an end of a linear slot  602 . This may allow a specialist using the stylus to trace the linear slot  602  to the point feature  610 . In some examples, the linear slot  602  may be a path feature that provides the stylus location information in addition to the point feature  610 .  FIG.  6 B  illustrates an example bend of the orbital implant  600  of  FIG.  6 A , wherein the implant  600  is bent to imitate a healthy anatomical orbital floor of a patient. 
     By locating the features of the actual implant in a common coordinate system with the patient anatomy, the position of the implant may be compared to a planned position (e.g., a position of the 3D model of the orbital implant relative to the 3D model of the patient anatomy). The navigation system can measure any deviation between the actual physical position of (a feature or portion of) the implant and the planned position of (said feature or portion of) the implant, and report the deviation to the specialist, optionally with instructions on how to modify the position of the implant to match the planned position and/or to perform additional bending of the implant to match the planned implant shape. Alternatively, a desired anatomical shape may be reconstructed pre-operatively as described above (e.g. by utilizing a mirror image of the intact contralateral orbital cavity, or an SSM of intact orbital cavities). By intra-operatively locating features of the implant as preliminarily placed on the patient, the navigation system may compare the position of these features with the desired anatomical shape of the orbital cavity, and report the deviation to the specialist, optionally with instructions on how to modify the position of the implant and/or to perform additional bending of the implant to better match the desired anatomical shape of the orbital cavity. 
     Typically, patient-specific implants are designed specifically for individual patients and are manufactured according to a design developed pre-operation. Although any number of point features may be used, patient-specific implants may require fewer point features relative to off-the-shelf orbital implants because fewer or no implant shape adjustments may be required by a patient-specific implant. 
       FIG.  7    is a diagram illustrating a patient-specific orbital implant  700  according to aspects of this disclosure. As illustrated, the top surface of the implant  700  is an eyeball-facing surface  704 , with a bone-facing surface  710  on the opposite side of the implant  700 . The bone-facing surface  710  is configured to interface with a portion of the patient&#39;s anatomy (e.g. a portion of the floor and/or anterior rim of the orbital cavity). To this end, the bone-facing surface  710  may be partly or in its entirety shaped to mate with a surface of the of the patient&#39;s anatomy. Holes (e.g., a first hole  706   a  and a second hole  706   b ) are provided on a proximal edge of the implant  700  to accommodate screws or other attaching elements configured to fix the implant  700  to the bone structure of an eye socket. The implant  700  includes an elevated ridge  708  along a distal edge of the implant  700 . The implant  700  also includes three point features (e.g., a first point feature  702   a,  a second point feature  702   b,  and a third point feature  702   c ), in this example in a non-isosceles triangle formation. In some examples, and to increase robustness of implant-position determination, point features may be spread as widely apart as possible. 
     In this example, the second point feature  702   b  and the third point feature  702   c  take the form of dimples, or recesses, in the eyeball-facing surface. These two point features are near the proximal edge of the implant  700 , and thus, are easier to reach with the tracked stylus compared to the first point feature  702   a  which is closer to the distal edge of the implant  700 . Here, when using a navigation system to determine a location of the implant  700 , a specialist may insert a distal tip of a stylus of the navigation system into the point features  702  to map the implant&#39;s location. 
     As noted, the first point feature  702   a  is located closer to the distal edge of the implant  700 , and is thus further away from the surgeon implanting the implant  700 . During implantation, the distal edge of the implant  700  may sit underneath soft tissue and the eyeball, which may obscure the first point feature from the surgeon&#39;s vision. Thus, in order to guide the stylus, the elevated ridge  708  is configured to direct the stylus into the first point feature  702   a.  In this example, the elevated ridge  708  has at least one side wall that sits at an angle  2402  from the eyeball-facing surface. In some examples, the angle may be greater than 45°, (e.g., an angle between 60° and 90°, see the first cross section  2400   a  of  FIG.  24   ). The height of the elevated ridge  708  may be high enough to prevent the tip of the stylus from slipping over the ridge, but low enough so as not to cause irritation of the soft tissue covering the implant  700  while in an implanted state (e.g., a height between 0.5 mm and 1.5 mm). As discussed below, the elevated ridge  708  may include one or more other shapes and sizes. 
     To verify the position of the implant intra-operatively, the surgeon may place the tip of the stylus in the second point feature  702   b  and the third point feature  702   c  to indicate the proximal point features. The surgeon may also trace the tip of the stylus over the eyeball-facing surface in a distal direction until it hits the elevated ridge  708 . By then sliding the stylus in a lateral direction along the elevated ridge  708 , the surgeon can guide the stylus tip to the first point feature  702   a.  Accordingly, no visual access of the distal region of the implant is necessary. 
     The stylus may include an angled tip, or alternatively, a straight tip. However, a stylus with a straight tip may have difficulties entering into a dimple or hole, because of the narrow access between the implant and the soft tissue and because of its consequently sharp angulation with respect to the eyeball-facing surface. Accordingly, the stylus may need to be held in a position close to tangential to the surface (e.g. at an angle between the shaft of the stylus and the surface of 30 degrees or less). Thus, as illustrated in  FIG.  7   , the first point feature  702   a  is a wedge-shaped depression that opens towards the proximal side of the implant  700  to be able to receive the stylus. While pushing the tip of the stylus along elevated ridge  708 , the surgeon may sense that the stylus drops into the depression. Thus, once in the depression, the surgeon may continue to push the stylus in a distal direction to keep the tip of the stylus in the apex of the depression until the stylus meets the elevated ridge  708 . It should be noted that any of the point features described herein may include one or more of a dimple, a depression, a hole, or any other suitable physical marking in the eyeball-facing surface to indicate that the stylus has reached a particular point or location of the implant. In certain embodiments, at least three point features may be used, as three points may be sufficient to accurately locate the implant in a coordinate system. In certain aspects, the three points form a triangle (such as, a non-isosceles triangle) such that each edge length between the points is different so that each edge can be differentiated to better determine a position and orientation of the implant in a coordinate system. 
       FIG.  8    is a diagram illustrating a patient-specific orbital implant  800  according to aspects of this disclosure. The implant  800  includes several characteristics that are similar to those shown in  FIG.  7   , including a first point feature  802   a  located near a distal edge of the implant  800 , and a second point feature  802   b  and a third point feature  802   c  near a proximal edge. However, in this example, an elevated ridge  808  at the distal end of the implant  800  includes a notch  804  partially surrounding the first point feature  802   a.  The notch  804  may be configured to indicate to the surgeon manipulating the stylus when the tip of the stylus has reached the first point feature  802   a  in addition to the depression of the first point feature  802   a.  In some examples, the notch  804  may be used as an alternative to the depression characteristic. As an alternative to the notch  804 , an interruption of the elevated ridge  808  may be used. For example, the elevated ridge  808  may include a gap or a notch configured to act as a first point feature, wherein the gap is smaller than the diameter of the tip of the stylus so that it “catches” the stylus. 
       FIG.  9    is a diagram illustrating a patient-specific orbital implant  900  according to aspects of this disclosure. The implant  900  includes several characteristics that are similar to those shown in  FIGS.  7  and  8   , including a first point feature  902   a  located near a distal edge of the implant  900 , and a second point feature  902   b  and a third point feature  902   c  near a proximal edge. In this example, the implant  900  includes an elevated ridge  904  that does not follow the distal edge of the implant  900 , but rather follows a V-shaped trajectory. Here, when a surgeon pushes the stylus against the elevated ridge  904 , it will cause the stylus to be guided towards the V-shape&#39;s apex, where the first point feature  902   a  is located. The surgeon may sense the change in direction of the elevated ridge  904  and understand that the first point feature  902   a  has been reached. If more than one point feature is arranged along the elevated ridge  904 , the elevated ridge&#39;s trajectory may be shaped as a concave and/or polyline, with segments (e.g., straight elevated ridges) between the point features. The first point feature  902   a  may optionally include a depression region to help “catch” the stylus. As in the examples above, the elevated ridge may include a notch at least partially surrounding the first point feature  902   a,  or an interruption at the first point feature  902   a.    
       FIG.  10    is a diagram illustrating a patient-specific orbital implant  1000  according to aspects of this disclosure. The implant  1000  includes several characteristics that are similar to those shown in  FIGS.  7 - 9    that may not be discussed further for the purposes of brevity. In this example, an elevated ridge  1004  does not run near the distal edge of the implant  1000  as in the previous examples, but from a third point feature  1002   c  to a first point feature  1002   a.  After indicating the third point feature  1002   c  (e.g., a point feature that is visible to the surgeon) with the stylus, the surgeon may slide the tip of the stylus along the elevated ridge  1004  until it reaches the first point feature  1002   a.  The elevated ridge  1004  has a hook shape or hockey-stick shape that curves around the first point feature  1002   a  to trap the stylus at the first point feature  1002   a  and signal to the surgeon when the stylus has reached that point feature. 
       FIG.  11    is a diagram illustrating a patient-specific orbital implant  1100  according to aspects of this disclosure. The implant  1100  includes several characteristics that are similar to those shown in  FIGS.  7 - 10    that may not be discussed further for the purposes of brevity. Here, the implant  1100  includes an elevated ridge  1104  that follows a path between a third point feature  1102   c  and a first point feature  1102   a.  The elevated ridge  1104  also has a second leg continuing from the first point feature  1102   a  to a second point feature  1102   b.  Because the elevated ridge  1104  is continuous, the stylus may be guided through the entire sequence of point features. 
       FIG.  12    is a diagram illustrating a patient-specific orbital implant  1200  according to aspects of this disclosure. The implant  1200  includes several characteristics that are similar to those shown in  FIGS.  7 - 11    that may not be discussed further for the purposes of brevity. Here, the implant  1200  includes an elevated ridge  1204  to guide the stylus to one or more point features. However, because the point features are arranged to one side of the elevated ridge  1204 , the elevated ridge  1204  includes a series of alternating indentations and protrusions  1206  configured to notify the surgeon when the stylus is along the wrong side of the elevated ridge  1204  by preventing the stylus from moving and/or making progression of the stylus more difficult along that side of the ridge  1204 . Such warning features may be combined with the elevated ridges of any of the other examples. 
       FIG.  13    is a diagram illustrating a patient-specific orbital implant  1300  according to aspects of this disclosure. The implant  1300  includes several characteristics that are similar to those shown in  FIGS.  7 - 12    that may not be discussed further for the purposes of brevity. Here, a first point feature  1302   a  is a depression at an apex of an elevated ridge  1304 . The depression has an elongated shape so as to receive the tip of the stylus held in a position close to tangential to the eyeball-facing surface. The elevated ridge  1304  includes a side that makes a smooth or gradual transition  1306  from the top of the elevated ridge  1304  to the surface of the implant  1300 . The smooth transition  1306  prevents the stylus being guided along the wrong side of the elevated ridge, and instead allows the stylus to be guided back to the correct side without having to lift the stylus from the surface of the implant  1300 . Such a smooth transition may be combined with the elevated ridges of any of the other examples. 
       FIG.  14    is a diagram illustrating a patient-specific orbital implant  1400  according to aspects of this disclosure. The implant  1400  includes several characteristics that are similar to those shown in  FIGS.  7 - 13    that may not be discussed further for the purposes of brevity. Here, the implant has an elevated ridge  1406  configure to guide a stylus toward point features  1402 . In contrast to previous examples, the elevated ridge  1406  is positioned inside of a triangle formed by the point features  1402 . In this example, the elevated ridge  1406  forms a plateau  1406  in the shape of a triangle. However, in some examples, the center of the triangle may include an elevation that is the same as the surface of the implant  1300  outside of the triangle. The implant  1400  also includes a plurality of perforations  1404  that pass through the eyeball-facing surface to the opposite side. The size of the perforations  1404  may be relatively smaller than the point features  1402  and/or the stylus to prevent the surgeon from wrongly indicating point features. Note that in this example there are no perforations  1404  along paths between point features  1402 . This prevents the stylus from being caught by a perforation or falsely indicating a point feature  1402 . Such perforations  1404  may be combined with any of the other examples. The plateau-style elevated ridge  1406  may also be used without perforations  1404 . 
     The following examples demonstrate how features described above may be freely combined. They are not limitative to the scope of the present disclosure. 
       FIG.  15    is a diagram illustrating a patient-specific orbital implant  1500  according to aspects of this disclosure. The implant  1500  includes several characteristics that are similar to those shown in  FIGS.  7 - 14    that may not be discussed further for the purposes of brevity. Here, the implant  1500  includes perforations  1504  and an elevated ridge  1506  configured to guide a stylus between point features  1502 . 
       FIG.  16    is a diagram illustrating a patient-specific orbital implant  1600  according to aspects of this disclosure. The implant  1600  includes several characteristics that are similar to those shown in  FIGS.  7 - 15    that may not be discussed further for the purposes of brevity. Here, the implant  1600  includes perforations  1604  and an elevated ridge  1606  configured to guide a stylus between point features  1602 . In this example, a first point feature  1602   a  is a depression, while a second point feature  1602   b  and a third point feature  1602   c  are holes or dimples. 
       FIG.  17    is a diagram illustrating a patient-specific orbital implant  1700  according to aspects of this disclosure. The implant  1700  includes several characteristics that are similar to those shown in  FIGS.  7 - 16    that may not be discussed further for the purposes of brevity. Here, the implant  1700  includes perforations  1704  and an elevated ridge  1706  configured to guide a stylus between point features  1702 . Because the point features are arranged to one side of the elevated ridge  1706 , the elevated ridge  1706  includes a series of alternating indentations and protrusions  1708  configured to notify the surgeon that the stylus along the wrong side of the elevated ridge  1706  by preventing the stylus from moving and/or making progression of the stylus more difficult along that side of the ridge  1706 . In this example, a first point feature  1702   a,  a second point feature  1702   b,  and a third point feature are holes or dimples. 
       FIG.  18    is a diagram illustrating a patient-specific orbital implant  1800  according to aspects of this disclosure. The implant  1800  includes several characteristics that are similar to those shown in  FIGS.  7 - 17    that may not be discussed further for the purposes of brevity. Here, a first point feature  1802   a  is a depression at an apex of an elevated ridge  1806 . The elevated ridge  1806  includes a side that makes a smooth transition  1808  from the top of the elevated ridge  1806  to the surface of the implant  1800 . The smooth transition  1808  allows the stylus to be guided back to a correct position without having to lift the stylus from the surface of the implant  1800  if the stylus is guided too far away from the point features. 
       FIG.  19    is a diagram illustrating a patient-specific orbital implant  1900  according to aspects of this disclosure. The implant  1900  includes several characteristics that are similar to those shown in  FIGS.  7 - 18    that may not be discussed further for the purposes of brevity. Here, the implant includes a plurality of perforations  1904 , and an elevated ridge  1906  connecting three point features  1902 . 
       FIG.  20    is a diagram illustrating a patient-specific orbital implant  2000  according to aspects of this disclosure. The implant  2000  includes several characteristics that are similar to those shown in  FIGS.  7 - 19    that may not be discussed further for the purposes of brevity. Here, the implant  2000  includes an elevated ridge  2004  with a smooth transition  2006  connecting three point features  2002  characterized by dimples. 
       FIG.  21    is a diagram illustrating a patient-specific orbital implant  2100  according to aspects of this disclosure. The implant  2100  includes several characteristics that are similar to those shown in  FIGS.  7 - 20    that may not be discussed further for the purposes of brevity. Here, the implant  2100  includes an elevated ridge  2104  with a smooth transition  2106  connecting three point features  2102  characterized by holes. 
       FIG.  22    is a diagram illustrating a patient-specific orbital implant  2200  according to aspects of this disclosure. The implant  2200  includes several characteristics that are similar to those shown in  FIGS.  7 - 21    that may not be discussed further for the purposes of brevity. Here, the implant  2200  includes an elevated ridge  2204  connecting three point features  2202 . The first point feature  2202   a  is a depression, while the second point feature  2202   b  and the third point feature  2202   c  are dimples. 
       FIG.  23    is a diagram illustrating a patient-specific orbital implant  2300  according to aspects of this disclosure. The implant  2300  includes several characteristics that are similar to those shown in  FIGS.  7 - 22    that may not be discussed further for the purposes of brevity. Here, the implant  2300  includes an elevated ridge  2304  connecting three point features  2302 , wherein each point feature is a hole. 
       FIG.  24    is a diagram illustrating six different example forms of elevated ridges that that may be used in an optical implant. A first elevated ridge  2400   a  is a “top surface” ridge, meaning the elevated ridge runs along the edge of the implant (e.g., see the elevated ridge  708 / 808  of  FIGS.  7  and  8   ). A second elevated ridge  2400   b  is shaped like a wave, having a concave portion of the ridge more able to accept the tip of a stylus. A third elevated ridge  2400   c  is an elevated ridge that is formed a distance from the edge of the implant (e.g., see the elevated ridge of  FIGS.  10 - 13  and  15 - 23   ). A fourth elevated ridge  2400   d  is shaped like a wave, having a concave portion of the ridge more able to accept the tip of a stylus, and is formed a distance away from the edge of the implant. A fifth elevated ridge  2400   e  is a plateau type ridge, as discussed above in  FIG.  14   . And a sixth elevated ridge  2400   f  is an elevated ridge with a smooth transition from the top of the ridge to the surface of the implant (e.g., see the elevated ridge having a smooth transition from the ridge to the eyeball-facing surface of the implant in  FIGS.  18 ,  20 , and  21   ). 
     In certain aspects, elevated ridges may be replaced by gutters (e.g., grooves or indentions). Accordingly, an implant according to aspects herein may have non-planar guiding features, which refer to elevated ridges and/or gutters. However, elevated ridges offer the benefit that they do not locally weaken the implant. In other words, they do not reduce the mechanical strength of the implant, or the implant&#39;s resistance to deformation. In addition, as described above, in distal areas of the implant, where guidance of the stylus is most needed, the stylus may be angled almost tangentially to the eyeball-facing surface of the implant, which may increase the risk of the tip of the stylus leaving the gutter or may altogether prevent the tip of the stylus from staying in and following a gutter. For that reason, elevated ridges may be preferred. 
     It is also possible to combine two elevated ridges spaced with a distance between them to establish a channel for guiding the tip of the stylus. However, as with gutters, in distal areas of the implant, the angulation of the stylus with respect to the implant surface may increase the risk of the tip of the stylus leaving the channel or may altogether prevent the tip of the stylus from staying in and following the channel. For that reason, it may be preferred to provide the implant with an elevated ridge or a series of elevated ridges on only one side of a stylus “path”—e.g., the trajectory or range of trajectories that the tip may be expected to follow on its way to a point feature, or the trajectory establishing a path feature—such as the distal side. 
       FIG.  25    is a flow chart illustrating operations  2500  for using an orbital implant in accordance with aspects disclosed herein. The operations  2500  may be implemented as software components that are executed and run on one or more processors (e.g., electronic processing device  2606  of  FIG.  26   ) in conjunction with one or more medical instruments (e.g., stylus  2608  of  FIG.  26   ). 
     At a first block  2501 , the operations  2500  include loading a virtual model comprising a design or planned shape of an orbital implant into a navigation system and bringing it into a common coordinate system with the actual patient. For example, a virtual 3D model of a portion of the patient&#39;s anatomy may be loaded, wherein the virtual model of the implant and the virtual model of the anatomy are such that the implant is in its planned position with respect to the anatomy of the patient. The actual patient and the virtual model of the anatomy may then be registered (e.g., brought into a common coordinate system). For example, a stylus of the navigation system may be used to indicate anatomical landmarks on the patient anatomy. The locations of these anatomical landmarks may then be registered with the locations of corresponding anatomical landmarks in the virtual model of the anatomy, while maintaining the relative position of the virtual model of the implant with respect to the virtual model of the anatomy. 
     At a second block  2502 , the operations  2500  include positioning a first surface of an orbital implant onto the defect of the orbital bone structure, a first surface comprising a shape configured to interface with a floor and/or a rim of the orbital bone structure, and a second surface opposite the first surface and substantially conformal to the shape of the first surface, the second surface comprising three or more point features on the second surface, wherein the three or more point features comprise a first point feature, a second point feature, and a third point feature that form nodes of a triangle, wherein each of the three or more point features are at least one of raised above or recessed below the second surface. 
     For example, as discussed with regards to  FIGS.  5 A- 5 D , the shape of the first surface may roughly mimic the shape of a surface of a floor and/or a rim of the orbital bone structure, so as to allow a more or less stable positioning of the implant onto the bone structure. Alternatively, as described above for patient-specific implants, a part or the entirety of the shape of the first surface may be designed to closely follow the shape of the bone structure so as to mate with it. 
     For example, as illustrated in  FIGS.  6  and  7   , the orbital implant  600 / 700  may include a plurality of point features  610 / 702 . Specifically, in  FIG.  6   , the point features  610  may be recessed to the extent that they are perforations in the implant  600 . In  FIG.  7   , the first point feature  702   a  is a wedge-shaped depression or recess below the second surface, while the second point feature  702   b  and third point feature  702   c  are dimples recessed below the second surface. 
     At a third block  2504 , the operations  2500  include tracing the stylus of the navigation system along the second surface until the stylus is received by each of the three or more point features to compare one or more of: (i) the shape of the orbital implant relative to a planned shape of the orbital implant, or (ii) a position of the orbital implant in a patient relative to a planned position of the orbital implant in the patient. For example, a surgeon may trace the stylus along the second surface (e.g., eyeball-facing surface) of the implant, making contact between the stylus and each of the three or more point features. In this way, the physical act of sweeping, or making contact between the stylus and the point features, provides the navigation system with a position of these features in the navigation system&#39;s coordinate system. In some examples, making contact between the stylus and a point feature, particularly a point feature near the distal edge of the implant, may comprise sliding the stylus along the second surface in a distal direction until a tip of the stylus hits an elevated ridge of the implant, and then sliding the tip of the stylus along the elevated ridge until it is received in the point feature. 
     At a fourth block  2506 , the operations  2500  may optionally include receiving an indication of a difference between one or more of: (i) the shape of the orbital implant relative to the planned shape of the orbital implant, or (ii) the position of the orbital implant in the patient relative to a planned position of the orbital implant in the patient. For example, once the position of the point features in the navigation system&#39;s coordinate system are determined, the navigation system may compare the position of the point features determined by touching the implant with the stylus, with a position of point features of the virtual 3D model of the planned shape of the implant, which was brought into the navigation system&#39;s coordinate system in the first block  2501 . The navigation system may then determine any differences, such as differences between the constellation of point features in the actual implant and the constellation of point features in the virtual 3D model of the planned shape of the implant, or—derived from such differences—differences between the shape of the actual implant and the planned shape of the implant, and report the differences to the user. In another example, the navigation system may compare the position of the point features determined by touching the implant with the stylus, with a position of point features of the virtual 3D model of the implant relative to the virtual model of the patient&#39;s anatomy (e.g., orbital bone structure). In this way, the navigation system may determine a difference between an actual placement of the physical implant relative to the patient&#39;s anatomy, and a planned placement of a model of the implant relative to a model of the patient&#39;s anatomy. 
     At a fifth block  2508 , the operations  2500  may optionally include adjusting, intra-surgery, one or more of the shape of the orbital implant or the position of the orbital implant according the indication of the corresponding difference. For example, based on the information provided at the fourth block  2506 , the user may adjust the implant to more closely match the planned model of the implant, and/or the position of the implant to more closely match the planned position (e.g., virtual positioning) of the implant. 
       FIG.  26    is a schematic diagram illustrating a conceptual example of a navigation system  2600  with a stylus  2608 . The system  2600  can be used in conjunction with preoperative images from an imaging process (e.g., a computed tomography (CT) scan, 3D and/or 2D fluoroscopy, ultrasonic (US) imaging, magnetic resonance imaging (MRI)), and/or the like to perform an image-guided surgical procedure. The system  2600  includes at least an electronic processing device  2606 , a display unit  2602 , at least one tracked instrument  2608 , such as a stylus, and an infra-red sensor  2610 . The system  2600  further includes one or more trackers (e.g. a rigidly attached tracker  2612  attached to the patient  2616  anatomy, and optionally another tracker  2614  attached to the stylus  2608 ). Instruments  2608  and trackers  2612 / 2614  may comprise markers, which can be easily detected by the infra-red sensor  2610  and which allow the navigation system to locate the instruments  2608  and trackers  2612 / 2614  in space. 
     The electronic processing device  2606  can be, for example, a personal computer, or the like. The electronic processing device  2606  includes at least a processor and a memory (e.g., a non-transitory computer-readable medium). The memory (not shown) can be, for example, a random access memory (RAM), a memory buffer, a hard drive, a read-only memory (ROM), an erasable programmable read-only memory (EPROM), and/or so forth. In some embodiments, the memory of the electronic processing device  2606  stores instructions to cause the processor to execute operations (e.g., the operations  2500  of  FIG.  25   ) and/or functions associated with using a personal computer application, controlling one or more medical instruments such as a stylus, displaying and updating an image on display unit  2602  according to movement of the stylus, and/or the like. 
     The processor (not shown) of the electronic processing device  2606  can be any suitable processing device configured to run and/or execute a set of instructions or code. For example, the processor can be a general purpose processor, a central processing unit (CPU), an accelerated processing unit (APU), or the like. In some embodiments, the processor of the electronic processing device  2606  can be included in, for example, an application specific integrated circuit (ASIC). The processor can be configured to run and/or execute a set of instructions (e.g., the operations  2500  illustrated in  FIG.  25   ) or code stored in the memory associated with using a personal computer application, a mobile application, an internet web browser, telephonic or cellular communication, and/or the like. 
     The display unit  2602  is configured to be in electronic communication with the electronic processing device  2606 . The display  2602  unit can be any suitable display configured to provide a user interface to the electronic processing device  2606 . For example, the display unit  2602  can be a cathode ray tube (CRT) monitor, a liquid crystal display (LCD) monitor, a light emitting diode (LED) monitor, a head-mounted device and/or the like. The display unit  2602  can be configured to provide the user interface for a personal computer application or the like. For example, the display unit  2602  can be configured to graphically represent a medical image of an anatomical structure. In some embodiments, the display unit  2602  can graphically represent the position of one or more medical instruments (e.g., the stylus  2608  and/or any other suitable device) as the medical instrument positioned with respect to a target tissue (e.g., an organ and/or bone structure) of a patient and relative to a preoperative image or model of the target tissue. In some embodiments, the processing device  2606  can be configured to map the movement of the stylus  2608  relative to a preoperative image of the target tissue, and the display unit  2602  can graphically represent a virtual position of the stylus  2608  relative to the image of the target tissue. The processing device  2606  can determine the position of the stylus  2608  with respect to the target tissue. 
     The electronic processing device  2606  may be configured to be in electronic communication with the stylus  2608  (e.g., via a wireless connection, an Ethernet cable, universal serial bus (USB), SATA cable, eSATA cable, or the like). The stylus  2608  can be any suitable instrument configured to be tracked by a tracking system (e.g., optical tracking or other modality). For example, an infrared emitting diode (IRED) may be used to map the position of the stylus  2608  in physical space onto a virtual coordinate system and preoperative image. 
     Example Aspects 
     Implementation examples are described in the following numbered clauses. The numbered clauses related to implants and methods involving implants. A person skilled in the art will readily appreciate that they may relate to orbital implants or other types of implants. 
     1. An implant for correcting a defect of a bone structure, comprising: a first surface comprising a shape configured to interface with a surface of the bone structure; and a second surface opposite the first surface and substantially conformal to the shape of the first surface, the second surface comprising three or more point features on the second surface, wherein the three or more point features comprise a first point feature, a second point feature, and a third point feature that form nodes of a triangle, wherein each of the three or more point features are recessed below the second surface. 
     2. The implant of aspect 1, wherein at least one of the three or more point features comprises a hole through the implant from the first surface to the second surface. 
     3. The implant of any of aspects 1 and 2, wherein at least one of the three or more point features comprises a depression configured to receive a tip of a stylus angled close to tangentially to the second surface. 
     4. The implant of aspect 3, wherein the depression is a wedge-shaped or elongated depression. 
     5. The implant of any of aspects 1-4, further comprising an elevated ridge on the second surface partially surrounding at least one of the three or more point features. 
     6. The implant of aspect 5, wherein the elevated ridge surrounds the at least one of the three or more point features on a distal side. 
     7. The implant of any of aspects 5 or 6, wherein the elevated ridge follows a distal edge of the second surface. 
     8. The implant of any of aspects 5-7, wherein the elevated ridge comprises at least one of a corner, a hook, a notch or a gap at the at least one of the three or more point features. 
     9. The implant of aspect 8, wherein the corner, hook, notch or gap is configured to receive a tip of a stylus angled substantially tangential to the second surface. 
     10. The implant of any of aspects 5-9, wherein the elevated ridge extends from a first point feature of the three or more point features to a second point feature of the three or more point features. 
     11. The implant of any of aspects 5-10, further comprising another elevated ridge on the second surface, the other elevated ridge extending from the second point feature to a third point feature of the three or more point features. 
     12. The implant of aspect 11, wherein the elevated ridge and the other elevated ridge form a continuous ridgeline on the second surface. 
     13. The implant of any of aspects 5-12, wherein the elevated ridge comprises a plurality of protrusions at a side of the elevated ridge. 
     14. The implant of aspect 13, wherein the side is a distal side of the elevated ridge. 
     15. The implant of any of aspects 5-14, further comprising a gradual transition at a side of the elevated ridge from a top of the elevated ridge to the second surface. 
     16. The implant of aspect 15, wherein the side is a distal side of the elevated ridge. 
     17. The implant of any of aspects 5-16, wherein the elevated ridge comprises a concave side for receiving a tip of a stylus. 
     18. The implant of aspect 17, wherein the concave side is a proximal side of the elevated ridge. 
     19. The implant of any of aspects 1-4, further comprising one or more elevated ridges extending along a path between a first point feature of the three or more point features and a second point feature of the three or more point features, wherein each of the one or more elevated ridges are located on a first side of the path and not a second side of the path. 
     20. The implant of aspect 19, wherein the first side of the path is a distal side of the path. 
     21. The implant of any of aspects 1-20, further comprising a plurality of perforations through the implant from the first surface to the second surface. 
     22. The implant of aspect 21, wherein a path between a first point feature of the three or more point features and a second point feature of the three or more point features does not include the plurality of perforations. 
     23. A method for correcting a defect of a bone structure of a patient, the method comprising: positioning a first surface of an implant onto the defect of the bone structure, the first surface comprising a shape configured to interface with a surface of the bone structure, and a second surface opposite the first surface and substantially conformal to the shape of the first surface, the second surface comprising three or more point features on the second surface, wherein the three or more point features comprise a first point feature, a second point feature, and a third point feature that form nodes of a triangle, wherein each of the three or more point features are at least one of raised above or recessed below the second surface; and tracing a stylus of a navigation system along the second surface until the stylus is received by each of the three or more point features to compare one or more of: (i) the shape of the implant relative to a planned shape of the implant, or (ii) a position of the implant in the patient relative to a planned position of the implant in the patient. 
     24. The method of aspect 23, further comprising loading a virtual 3D implant model into the navigation system and bringing the virtual 3D implant model and the bone structure into a common coordinate system. 
     25. The method of any of aspects 23 and 24, further comprising loading a virtual 3D anatomy model into the navigation system, wherein bringing the virtual 3D implant model and the bone structure into a common coordinate system comprises: registering anatomical features of the patient with anatomical features of the virtual 3D anatomy model, and maintaining a relative position of the virtual 3D implant model and the virtual 3D anatomy model. 
     26. The method of any of aspects 23-25, further comprising receiving an indication of a difference between the one or more of: (i) the shape of the implant relative to the virtual 3D implant model, or (ii) the position of the implant in the patient relative to a position of the virtual 3D implant model with respect to the patient. 
     27. The method of aspect 26, further comprising adjusting, intra-surgery, one or more of the shape of the implant or the position of the implant according the indication of the difference. 
     28. The method of any of aspects 23-27, wherein at least one of the three or more point features is configured to receive a tip of the stylus angled close to tangentially to the second surface. 
     29. The method of any of aspects 23-28 wherein the at least one of the three or more point features comprises a wedge-shaped or elongated depression. 
     30. The method of any of aspects 23-29 wherein the at least one of the three or more point features is partially surrounded by an elevated ridge. 
     31. The method of aspect 31, wherein the elevated ridge at least partially surrounds the at least one of the three or more point features on a distal side of the at least one of the three or more point features. 
     32. The method of any of aspects 30 and 31, wherein the elevated ridge comprises a corner, hook, notch or gap at the at least one of the three or more point features for receiving a tip of the stylus of the navigation system. 
     33. The method of any of aspects 23-32, wherein the implant comprises an elevated ridge, and wherein tracing the stylus of the navigation system along the second surface until the stylus is received by each of the three point features further comprises: sliding a tip of the stylus along the second surface until it hits the elevated ridge; and sliding the tip of the stylus along the elevated ridge until the tip of the stylus is received in at least one of the three point features. 
     34. A method for generating an implant for correcting a defect of a bone structure, the method comprising: receiving a design of the implant, the design indicating a shape, size, and position of the implant relative to a build area corresponding to an additive manufacturing device, the design of the implant defining: a first surface comprising a shape configured to interface with a surface of the bone structure; and a second surface opposite the first surface and substantially conformal to the shape of the first surface, the second surface comprising three or more point features on the second surface, wherein the three or more point features comprise a first point feature, a second point feature, and a third point feature that form nodes of a triangle, wherein each of the three or more point features are at least one of raised above or recessed below the second surface; and causing manufacturing of the implant using additive manufacturing. 
     35. The method of aspect 34, wherein at least one of the three or more point features comprises a hole through the implant from the first surface to the second surface. 
     36. The method of any of aspects 34 and 35, wherein at least one of the three or more point features comprises a depression configured to receive a tip of a stylus angled close to tangentially to the second surface. 
     37. The method of aspect 36, wherein the depression is a wedge-shaped or elongated depression. 
     38. The method of any of aspects 34-37, wherein the design of the implant further defines an elevated ridge on the second surface partially surrounding at least one of the three or more point features. 
     39. The method of aspect 38, wherein the elevated ridge partially surrounds the at least one of the three or more point features on a distal side. 
     40. The method of any of aspects 38 and 39, wherein the elevated ridge follows a distal edge of the second surface. 
     41. The method of any of aspects 38-40, wherein the elevated ridge comprises at least one of a corner, a hook, a notch or a gap at the at least one of the three or more point features. 
     42. The method of aspect 41, wherein the corner, hook, notch or gap is configured to receive a tip of a stylus angled substantially tangential to the second surface. 
     43. The method of any of aspects 38-42, wherein the elevated ridge extends from a first point feature of the three or more point features to a second point feature of the three or more point features. 
     44. The method of any of aspects 38-43, wherein the design of the implant further defines another elevated ridge on the second surface, the other elevated ridge extending from the second point feature to a third point feature of the three or more point features. 
     45. The method of aspect 44, wherein the elevated ridge and the other elevated ridge form a continuous ridgeline on the second surface. 
     46. The method of any of aspects 38-45, wherein the design of the implant further defines a plurality of protrusions at a side of the elevated ridge. 
     47. The method of aspect 46, wherein the side is a distal side of the elevated ridge. 
     48. The method of any of aspects 38-47, wherein the design of the implant further defines a smooth transition at a side of the elevated ridge from a top of the elevated ridge to the second surface. 
     49. The method of aspect 48, wherein the side is a distal side of the elevated ridge. 
     50. The method of any of aspects 34-37, wherein the design of the implant defines one or more elevated ridges on one side of a path between a first point feature of the three or more point features and a second point feature of the three or more point features. 
     51. The method of aspect 50, wherein the one side of the path is a distal side of the path. 
     52. The method of any of aspects 34-37, wherein the design of the implant further defines an elevated ridge comprising a concave side for receiving a tip of a stylus. 
     53. The method of aspect 52, wherein the concave side is a proximal side of the elevated ridge. 
     54. The method of any of aspects 34-37, wherein the design of the implant further defines a plurality of perforations through the implant from the first surface to the second surface. 
     55. The method of aspect 54, wherein the design of the implant does not define any of the plurality of perforations along a path between a first point feature of the three or more point features and a second point feature of the three or more point features. 
     56. The method of any of aspects 34-37, further comprising: receiving a digital file comprising the design of the implant; and loading the digital file onto a navigation system. 
     57. The method of aspect 56, wherein the digital file further comprises data indicative of a position of each of the three or more point features. 
     58. The method of aspect 57, wherein the data indicative of a position is data indicative of a planned position of the implant with respect to a patient&#39;s anatomy. 
     59. The method of aspect 58, wherein the data indicative of the planned position of the implant comprises a virtual 3D representation of at least part of the patient&#39;s anatomy and data indicative of a position of each of three or more point features of the implant with respect to the patient&#39;s anatomy. 
     Additional Considerations 
     Various embodiments disclosed herein provide for the use of a computer control system. A skilled artisan will readily appreciate that these embodiments may be implemented using numerous different types of computing devices, including both general purpose and/or special purpose computing system environments or configurations. Examples of well-known computing systems, environments, and/or configurations that may be suitable for use in connection with the embodiments set forth above may include, but are not limited to, personal computers, server computers, hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments (e.g., networks, cloud computing systems, etc.) that include any of the above systems or devices, and the like. These devices may include stored instructions, which, when executed by a microprocessor in the computing device, cause the computer device to perform specified actions to carry out the instructions. As used herein, instructions refer to computer-implemented steps for processing information in the system. Instructions can be implemented in software, firmware or hardware and include any type of programmed step undertaken by components of the system. 
     A microprocessor may be any conventional general purpose single- or multi-chip microprocessor such as a Pentium® processor, a Pentium® Pro processor, a 8051 processor, a microprocessor without interlocked pipelined stages (MIPS®) processor, a Power PC® processor, or an Alpha® processor. In addition, the microprocessor may be any conventional special purpose microprocessor such as a digital signal processor or a graphics processor. The microprocessor typically has conventional address lines, conventional data lines, and one or more conventional control lines. 
     Aspects and embodiments disclosed herein may be implemented as a method, apparatus or article of manufacture using standard programming or engineering techniques to produce software, firmware, hardware, or any combination thereof. The term “article of manufacture” as used herein refers to code or logic implemented in hardware or non-transitory computer readable media such as optical storage devices, and volatile or non-volatile memory devices or transitory computer readable media such as signals, carrier waves, etc. Such hardware may include, but is not limited to, field programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), complex programmable logic devices (CPLDs), programmable logic arrays (PLAs), microprocessors, or other similar processing devices.