Patent Publication Number: US-2023149028-A1

Title: Mixed reality guidance for bone graft cutting

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 63/004,904, entitled “MIXED REALITY GUIDANCE FOR BONE-GRAFT HARVESTING,” and filed on Apr. 3, 2020, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Surgical repair procedures involve the repair and/or replacement of a damaged or diseased anatomical object. One example of a surgical repair procedure is bone grafting, wherein a surgeon harvests a bone graft from a donor site for repairing damaged or diseased bone, or for conditioning the damaged or diseased bone for implantation. For example, an arthroplasty is the standard of care for the treatment of shoulder joint arthritis. A reversed shoulder arthroplasty (RSA) may allow even better range of motion, limits notching, and corrects bone deficiency. 
     SUMMARY 
     This disclosure describes example techniques for guiding a physician through the harvest of a bone graft. A computing device may obtain a virtual model of a bone graft to be harvested from a donor bone, wherein the virtual model of the bone graft specifies a volume of the donor bone to be harvested as the bone graft; and output for display, via a visualization device, a graphical representation of the specified volume to be harvested relative to a portion of the donor bone viewable via the visualization device. 
     In this manner, the example techniques provide a technical solution for accurately harvesting a bone graft from a donor site. For instance, the example techniques provide for practical applications of preoperative and intraoperative planning utilizing image processing for facilitating bone graft harvesting from a donor site. 
     In one example, the disclosure describes a system for guiding bone graft harvesting, the system including a visualization device and processing circuitry configured to obtain a virtual model of a bone graft to be harvested from a donor bone, wherein the virtual model of the bone graft specifies a volume of the donor bone to be harvested as the bone graft; and output for display, via the visualization device, a graphical representation of the specified volume to be harvested relative to a portion of the donor bone viewable via the visualization device. 
     In another example, the disclosure describes a method for bone graft harvesting in orthopedic surgery, the method including obtaining a virtual model of a bone graft to be harvested from a donor bone, wherein the virtual model of the bone graft specifies a volume of the donor bone to be harvested as the bone graft; and outputting for display, via a visualization device, a graphical representation of the specified volume to be harvested relative to a portion of the donor bone viewable via the visualization device. 
     The details of various examples of the disclosure are set forth in the accompanying drawings and the description below. Various features, objects, and advantages will be apparent from the description, drawings, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of an orthopedic surgical system according to an example of this disclosure. 
         FIG.  2    is a block diagram of an orthopedic surgical system that includes a mixed reality (MR) system, according to an example of this disclosure. 
         FIG.  3    is a block diagram illustrating an example of a computing system configured to perform one or more examples described in this disclosure. 
         FIGS.  4  and  5    are conceptual diagrams illustrating examples of bone extraction for bone grafting. 
         FIG.  6    is a conceptual diagram illustrating an example of the extracted bone grafted to the glenoid bone. 
         FIGS.  7 A and  7 B  are conceptual diagrams illustrating examples of a bone graft template within a representation of a donor site. 
         FIG.  8    is an example graphical user interface (GUI) for using CT-scan data to create a virtual bone graft. 
         FIG.  9    is another example GUI for using CT-scan data to create a virtual bone graft. 
         FIG.  10    is another example GUI for using CT-scan data to create a virtual bone graft. 
         FIG.  11    is another example GUI for using CT-scan data to create a virtual bone graft. 
         FIG.  12    is a flowchart illustrating example methods of operations in accordance with one or more example techniques described in this disclosure. 
         FIG.  13    is a schematic representation of a visualization device for use in an MR system, according to an example of this disclosure. 
         FIG.  14    is a plan view of a visualization device configured to guide a bone graft harvest, in accordance with one or more techniques of this disclosure. 
         FIG.  15    is a plan view of an orthopedic surgical system that includes an MR system, according to an example of this disclosure. 
         FIGS.  16 A and  16 B  are example MR GUIs that may be generated and displayed on a visualization device in accordance with one or more techniques of this disclosure. 
         FIG.  17    is a plan view of an orthopedic surgical system that includes an MR system, according to an example of this disclosure. 
         FIGS.  18 A and  18 B  are example MR GUIs that may be generated and displayed on a visualization device in accordance with one or more techniques of this disclosure. 
         FIGS.  19 A and  19 B  are example MR GUIs that may be generated and displayed on a visualization device in accordance with one or more techniques of this disclosure. 
         FIGS.  20 A- 20 C  are example MR GUIs that may be generated and displayed on a visualization device in accordance with one or more techniques of this disclosure. 
         FIG.  21    is an example MR GUI that may be generated and displayed on a visualization device in accordance with techniques of this disclosure. 
         FIG.  22    is a flowchart illustrating example methods of operations in accordance with one or more example techniques described in this disclosure. 
         FIG.  23    is a flowchart illustrating example methods of operations in accordance with one or more example techniques described in this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Orthopedic surgery can involve implanting one or more prosthetic devices to repair or replace a patient&#39;s damaged or diseased joint. Orthopedic surgery also includes grafting bone from a donor site to a target bone. The target bone may be a damaged or diseased bone or may be bone that is offset or augmented using the graft. Autografting refers to bone being taken from a donor site within the patient for grafting to the target bone within the same patient. The example techniques are described with respect to autografting but are applicable to other types of grafting as well (e.g., from a donor site not within the patient). In some examples, the bone grafting may be used to augment or otherwise prepare the damaged or diseased bone for receiving the one or more prosthetic devices. In some examples, the bone graft is attached to a prosthetic device, and then the combined assembly (e.g., prosthetic device plus bone graft) are attached to the target bone. 
     Virtual surgical planning tools may use image data of the diseased or damaged joint to generate an accurate three-dimensional bone model that can be viewed and manipulated preoperatively by the surgeon. These tools can enhance surgical outcomes by allowing the surgeon to simulate the surgery, select or design an implant that more closely matches the contours of the patient&#39;s actual bone, and select or design surgical instruments and guide tools that are adapted specifically for repairing the bone of a particular patient. 
     Use of these planning tools typically results in generation of a preoperative surgical plan, complete with an implant and surgical instruments that are selected or manufactured for the individual patient. Oftentimes, once in the actual operating environment, the surgeon may desire to verify the preoperative surgical plan intraoperatively relative to the patient&#39;s actual bone. This verification may result in a determination that an adjustment to the preoperative surgical plan is needed, such as a different implant, a different positioning or orientation of the implant, and/or a different surgical guide for carrying out the surgical plan. In addition, a surgeon may want to view details of the preoperative surgical plan relative to the patient&#39;s real bone during the actual procedure in order to more efficiently and accurately position and orient the implant components. For example, the surgeon may want to obtain intra-operative visualization that provides guidance for positioning and orientation of implant components, guidance for preparation of bone or tissue to receive the implant components, guidance for reviewing the details of a procedure or procedural step, and/or guidance for selection of tools or implants and tracking of surgical procedure workflow. 
     As described above, for bone grafting, the surgeon extracts a bone graft from a donor site for grafting to the target bone. The surgeon may desire to determine characteristics of the donor site to determine whether the donor site is adequate for providing the bone graft. For example, the surgeon may desire to determine whether there is sufficient bone at the donor site, and as another example, whether the bone at the donor site is sufficiently dense. There may potentially be some negative impacts if there is insufficient bone at the donor site and/or if the bone at donor site is not sufficiently dense. For instance, there is potentially an increase in the probability of fracture at the donor site, there may be cosmetic deformities, and there may be damage to surrounding tissue at the donor site because the bone at the donor site is insufficient to properly support the tissue. 
     A computing device may generate information indicative of whether a donor site is suitable for being a donor for a bone graft. The surgeon may utilize the generated information to determine preoperatively whether to extract bone from the donor site. The surgeon may also utilize the generated information intraoperatively to confirm that the donor site is suitable, and as described in more detail, possibly utilize the generated information intraoperatively for surgical guidance. 
     For example, processing circuitry (e.g., processing circuitry of one or more computing devices) may be configured to determine a bone graft template for a bone graft to be connected to a first anatomical object (e.g., target bone). Bone graft template refers to a representation of size and shape of the bone graft that is to be extracted. The bone graft template may be a graphical representation that can be displayed, such as intraoperatively on a mixed reality (MR) visualization device. The bone graft template may be represented by shape equations that define a particular size and shape, or points within particular size and shape having assigned coordinates, as a few examples. 
     There may be various ways in which the processing circuitry determines the bone graft template. As one example, the processing circuitry may output for display image data showing the target bone, and the processing circuitry may receive input from the surgeon for what the bone graft template should look like. The processing circuitry may determine the bone graft template based on the received input. As another example, the processing circuitry may be configured to generate a premorbid construction of the target bone. The processing circuitry may determine a difference between the premorbid construction and the actual target bone to determine the bone graft template. 
     The processing circuitry may determine information indicative of placement of the bone graft template within a representation of a second anatomical object (e.g., donor site). For example, a memory may store image data for one or more images of anatomical objects. As one example, memory may store image data of computerized tomography (CT) scans of the patient. Part of the image data includes a representation of the donor site (e.g., images of the donor site). The processing circuitry may determine information indicative of how the bone graft template fits within the representation of the donor site based on the image data. 
     As one example, the surgeon may move (e.g., drag and place with a stylus) a displayed representation of the bone graft template within the representation of the donor site. In response to the movement of the bone graft template, the processing circuitry may determine information needed to move the representation of the bone graft template (e.g., information such as coordinates of where the bone graft template is to be displayed). The processing circuitry may then output information indicative of the placement of the bone graft template within the representation of the donor site (e.g., output graphical information used to render the bone graft template with the representation of the donor site). 
     As another example, the processing circuitry may be configured to utilize the points or shape equations of the bone graft template and the points in the representation of the donor site to determine how to place the bone graft template within the representation of the donor site. The processing circuitry may utilize certain criteria in determining information indicative of the placement such as information that defines boundaries within the donor site from where a bone graft can be extracted. For instance, the boundary may define certain configurations from which a bone graft can be extracted with comparative ease, where for other configurations of the bone graft template within the donor site, there may be complications in removing the bone graft (e.g., hard to access the bone graft). As another example, the boundary may define portions of the donor site where if the bone graft is removed from other portions there may be a possibility of cosmetic defect, injury, or susceptibility to injury. The processing circuitry may output the determined information (e.g., graphical information used to render the bone graft template within the representation of the donor site). 
     In some examples, in addition to generating information that can be used to determine whether a donor site is suitable for extracting a bone graft, the processing circuitry may be configured to generate information indicating whether the donor site is potentially suitable as a donor site for the bone graft (e.g., whether there is sufficient clearance, whether the bone graft extraction process can be performed with relative surgical ease, and the like). Also, in some examples, the processing circuitry may generate pre-operative planning information. 
     As one example, the processing circuitry may generate information indicative of a location from which the bone graft is to be extracted within the donor site (e.g., location that is to be cut for extracting the bone graft). In some examples, processing circuitry is configured to generate information indicative of an axis along which to the second anatomical object for extracting the bone graft. The processing circuitry may be configured to generate information indicative of a maximum depth at which to cut the second anatomical object for extracting the bone graft. As one example, the processing circuitry is configured to generate information indicative of a types of tool to utilize to cut the second anatomical object for extracting the bone graft. 
     There may be various ways in which the surgeon may preoperatively view image content such as bone graft template, placement of bone graft template in donor site, and surgical guidance information. Also, in some examples in accordance with this disclosure, the surgeon may be able to view the bone graft template, placement of bone graft template in donor site, and surgical guidance information during the operation. 
     For example, the surgeon may use a mixed reality (MR) visualization system to assist with creation, implementation, verification, and/or modification of a surgical plan before and during a surgical procedure. Because MR, or in some instances virtual reality (VR), may be used to interact with the surgical plan, this disclosure may also refer to the surgical plan as a “virtual” surgical plan. Visualization tools other than or in addition to mixed reality visualization systems may be used in accordance with techniques of this disclosure. 
     A surgical plan, e.g., as generated by the BLUEPRINT™ system or another surgical planning platform, may include information defining a variety of features of a surgical procedure, such as features of particular surgical procedure steps to be performed on a patient by a surgeon according to the surgical plan including, for example, bone or tissue preparation steps and/or steps for selection, modification and/or placement of implant components. Such information may include, in various examples, dimensions, shapes, angles, surface contours, and/or orientations of implant components to be selected or modified by surgeons, dimensions, shapes, angles, surface contours and/or orientations to be defined in bone or tissue by the surgeon in bone or tissue preparation steps, and/or positions, axes, planes, angle and/or entry points defining placement of implant components by the surgeon relative to patient bone or tissue. Information such as dimensions, shapes, angles, surface contours, and/or orientations of anatomical features of the patient may be derived from imaging (e.g., x-ray, CT, MRI, ultrasound or other images), direct observation, or other techniques. 
     In this disclosure, the term “mixed reality” (MR) refers to the presentation of virtual objects such that a user sees images that include both real, physical objects and virtual objects. Virtual objects may include text, 2-dimensional surfaces, 3-dimensional models, or other user-perceptible elements that are not actually present in the physical, real-world environment in which they are presented as coexisting. In addition, virtual objects described in various examples of this disclosure may include graphics, images, animations or videos, e.g., presented as 3D virtual objects or 2D virtual objects. Virtual objects may also be referred to as virtual elements. Such elements may or may not be analogs of real-world objects. In some examples, in mixed reality, a camera may capture images of the real world and modify the images to present virtual objects in the context of the real world. In such examples, the modified images may be displayed on a screen, which may be head-mounted, handheld, or otherwise viewable by a user. 
     This type of mixed reality is increasingly common on smartphones, such as where a user can point a smartphone&#39;s camera at a sign written in a foreign language and see in the smartphone&#39;s screen a translation in the user&#39;s own language of the sign superimposed on the sign along with the rest of the scene captured by the camera. In some examples, in mixed reality, see-through (e.g., transparent) holographic lenses, which may be referred to as waveguides, may permit the user to view real-world objects, i.e., actual objects in a real-world environment, such as real anatomy, through the holographic lenses and also concurrently view virtual objects. 
     The Microsoft HOLOLENS™ headset, available from Microsoft Corporation of Redmond, Wash., is an example of a MR device that includes see-through holographic lenses, sometimes referred to as waveguides, that permit a user to view real-world objects through the lens and concurrently view projected 3D holographic objects. The Microsoft HOLOLENS™ headset, or similar waveguide-based visualization devices, are examples of an MR visualization device that may be used in accordance with some examples of this disclosure. Some holographic lenses may present holographic objects with some degree of transparency through see-through holographic lenses so that the user views real-world objects and virtual, holographic objects. In some examples, some holographic lenses may, at times, completely prevent the user from viewing real-world objects and instead may allow the user to view entirely virtual environments. The term mixed reality may also encompass scenarios where one or more users are able to perceive one or more virtual objects generated by holographic projection. In other words, “mixed reality” may encompass the case where a holographic projector generates holograms of elements that appear to a user to be present in the user&#39;s actual physical environment. 
     In some examples, in mixed reality, the positions of some or all presented virtual objects are related to positions of physical objects in the real world. For example, a virtual object may be tethered to a table in the real world, such that the user can see the virtual object when the user looks in the direction of the table but does not see the virtual object when the table is not in the user&#39;s field of view. In some examples, in mixed reality, the positions of some or all presented virtual objects are unrelated to positions of physical objects in the real world. For instance, a virtual item may always appear in the top right of the user&#39;s field of vision, regardless of where the user is looking. 
     Augmented reality (AR) is similar to MR in the presentation of both real-world and virtual elements, but AR generally refers to presentations that are mostly real, with a few virtual additions to “augment” the real-world presentation. For purposes of this disclosure, MR is considered to include AR. For example, in AR, parts of the user&#39;s physical environment that are in shadow can be selectively brightened without brightening other areas of the user&#39;s physical environment. This example is also an instance of MR in that the selectively-brightened areas may be considered virtual objects superimposed on the parts of the user&#39;s physical environment that are in shadow. 
     Furthermore, in this disclosure, the term “virtual reality” (VR) refers to an immersive artificial environment that a user experiences through sensory stimuli (such as sights and sounds) provided by a computer. Thus, in virtual reality, the user may not see any physical objects as they exist in the real world. Video games set in imaginary worlds are a common example of VR. The term “VR” also encompasses scenarios where the user is presented with a fully artificial environment in which some virtual object&#39;s locations are based on the locations of corresponding physical objects as they relate to the user. Walk-through VR attractions are examples of this type of VR. 
     The term “extended reality” (XR) is a term that encompasses a spectrum of user experiences that includes virtual reality, mixed reality, augmented reality, and other user experiences that involve the presentation of at least some perceptible elements as existing in the user&#39;s environment that are not present in the user&#39;s real-world environment. Thus, the term “extended reality” may be considered a genus for MR and VR. XR visualizations may be presented in any of the techniques for presenting mixed reality discussed elsewhere in this disclosure or presented using techniques for presenting VR, such as VR goggles. 
       FIG.  1    is a block diagram of an orthopedic surgical system  100  according to an example of this disclosure. Orthopedic surgical system  100  includes a set of subsystems. In the example of  FIG.  1   , the subsystems include a virtual planning system  102 , a planning support system  104 , a manufacturing and delivery system  106 , an intraoperative guidance system  108 , a medical education system  110 , a monitoring system  112 , a predictive analytics system  114 , and a communications network  116 . In other examples, orthopedic surgical system  100  may include more, fewer, or different subsystems. For example, orthopedic surgical system  100  may omit medical education system  110 , monitor system  112 , predictive analytics system  114 , and/or other subsystems. In some examples, orthopedic surgical system  100  may be used for surgical tracking, in which case orthopedic surgical system  100  may be referred to as a surgical tracking system. In other cases, orthopedic surgical system  100  may be generally referred to as a medical device system. 
     Users of orthopedic surgical system  100  may use virtual planning system  102  to plan orthopedic surgeries. Users of orthopedic surgical system  100  may use planning support system  104  to review surgical plans generated using orthopedic surgical system  100 . Manufacturing and delivery system  106  may assist with the manufacture and delivery of items needed to perform orthopedic surgeries. Intraoperative guidance system  108  provides guidance to assist users of orthopedic surgical system  100  in performing orthopedic surgeries. Medical education system  110  may assist with the education of users, such as healthcare professionals, patients, and other types of individuals. Pre- and postoperative monitoring system  112  may assist with monitoring patients before and after the patients undergo surgery. Predictive analytics system  114  may assist healthcare professionals with various types of predictions. For example, predictive analytics system  114  may apply artificial intelligence techniques to determine a classification of a condition of an orthopedic joint, e.g., a diagnosis, determine which type of surgery to perform on a patient and/or which type of implant to be used in the procedure, determine types of items that may be needed during the surgery, and so on. 
     The subsystems of orthopedic surgical system  100  (i.e., virtual planning system  102 , planning support system  104 , manufacturing and delivery system  106 , intraoperative guidance system  108 , medical education system  110 , pre- and postoperative monitoring system  112 , and predictive analytics system  114 ) may include various systems. The systems in the subsystems of orthopedic surgical system  100  may include various types of computing systems, computing devices, including server computers, personal computers, tablet computers, smartphones, display devices, Internet of Things (IoT) devices, visualization devices (e.g., mixed reality (MR) visualization devices, virtual reality (VR) visualization devices, holographic projectors, or other devices for presenting extended reality (XR) visualizations), surgical tools, and so on. A holographic projector, in some examples, may project a hologram for general viewing by multiple users or a single user without a headset, rather than viewing only by a user wearing a headset. For example, virtual planning system  102  may include a MR visualization device and one or more server devices, planning support system  104  may include one or more personal computers and one or more server devices, and so on. A computing system is a set of one or more computing systems configured to operate as a system. In some examples, one or more devices may be shared between the two or more of the subsystems of orthopedic surgical system  100 . For instance, in the previous examples, virtual planning system  102  and planning support system  104  may include the same server devices. 
     In the example of  FIG.  1   , the devices included in the subsystems of orthopedic surgical system  100  may communicate using communication network  116 . Communication network  116  may include various types of communication networks including one or more wide-area networks, such as the Internet, local area networks, and so on. In some examples, communication network  116  may include wired and/or wireless communication links. 
     Many variations of orthopedic surgical system  100  are possible in accordance with techniques of this disclosure. Such variations may include more or fewer subsystems than the version of orthopedic surgical system  100  shown in  FIG.  1   . For example,  FIG.  2    is a block diagram of an orthopedic surgical system  200  that includes one or more mixed reality (MR) systems, according to an example of this disclosure. Orthopedic surgical system  200  may be used for creating, verifying, updating, modifying and/or implementing a surgical plan. In some examples, the surgical plan can be created preoperatively, such as by using a virtual surgical planning system (e.g., the BLUEPRINT™ system), and then verified, modified, updated, and viewed intraoperatively, e.g., using MR visualization of the surgical plan. In other examples, orthopedic surgical system  200  can be used to create the surgical plan immediately prior to surgery or intraoperatively, as needed. In some examples, orthopedic surgical system  200  may be used for surgical tracking, in which case orthopedic surgical system  200  may be referred to as a surgical tracking system. In other cases, orthopedic surgical system  200  may be generally referred to as a medical device system. 
     In the example of  FIG.  2   , orthopedic surgical system  200  includes a preoperative surgical planning system  202 , a healthcare facility  204  (e.g., a surgical center or hospital), a storage system  206  and a network  208  that allows a user at healthcare facility  204  to access stored patient information, such as medical history, image data corresponding to the damaged joint or bone and various parameters corresponding to a surgical plan that has been created preoperatively (as examples). Preoperative surgical planning system  202  may be equivalent to virtual planning system  102  of  FIG.  1    and, in some examples, may generally correspond to a virtual planning system similar or identical to the BLUEPRINT™ system. 
     In the example of  FIG.  2   , healthcare facility  204  includes a mixed reality (MR) system  212 . In some examples of this disclosure, MR system  212  includes one or more processing device(s) (P)  210  to provide functionalities that will be described in further detail below. Processing device(s)  210  may also be referred to as processor(s) or processing circuitry. In addition, one or more users of MR system  212  (e.g., a surgeon, nurse, or other care provider) can use processing device(s) (P)  210  to generate a request for a particular surgical plan or other patient information that is transmitted to storage system  206  via network  208 . In response, storage system  206  returns the requested patient information to MR system  212 . In some examples, the users can use other processing device(s) to request and receive information, such as one or more processing devices that are part of MR system  212 , but not part of any visualization device, or one or more processing devices that are part of a visualization device (e.g., visualization device  213 ) of MR system  212 , or a combination of one or more processing devices that are part of MR system  212 , but not part of any visualization device, and one or more processing devices that are part of a visualization device (e.g., visualization device  213 ) that is part of MR system  212 . 
     In some examples, multiple users can simultaneously use MR system  212 . For example, MR system  212  can be used in a spectator mode in which multiple users each use their own visualization devices so that the users can view the same information at the same time and from the same point of view. In some examples, MR system  212  may be used in a mode in which multiple users each use their own visualization devices so that the users can view the same information from different points of view. 
     In some examples, processing device(s)  210  can provide a user interface to display data and receive input from users at healthcare facility  204 . Processing device(s)  210  may be configured to control visualization device  213  to present a user interface. Furthermore, processing device(s)  210  may be configured to control visualization device  213  to present virtual images, such as 3D virtual models, 2D images, and so on. Processing device(s)  210  can include a variety of different processing or computing devices, such as servers, desktop computers, laptop computers, tablets, mobile phones and other electronic computing devices, or processors within such devices. In some examples, one or more of processing device(s)  210  can be located remote from healthcare facility  204 . In some examples, processing device(s)  210  reside within visualization device  213 . In some examples, at least one of processing device(s)  210  is external to visualization device  213 . In some examples, one or more processing device(s)  210  reside within visualization device  213  and one or more of processing device(s)  210  are external to visualization device  213 . 
     In the example of  FIG.  2   , MR system  212  also includes one or more memory or storage device(s) (M)  215  for storing data and instructions of software that can be executed by processing device(s)  210 . The instructions of software can correspond to the functionality of MR system  212  described herein. In some examples, the functionalities of a virtual surgical planning application, such as the BLUEPRINT™ system, can also be stored and executed by processing device(s)  210  in conjunction with memory storage device(s) (M)  215 . For instance, memory or storage system  215  may be configured to store data corresponding to at least a portion of a virtual surgical plan. In some examples, storage system  206  may be configured to store data corresponding to at least a portion of a virtual surgical plan. In some examples, memory or storage device(s) (M)  215  reside within visualization device  213 . In some examples, memory or storage device(s) (M)  215  are external to visualization device  213 . In some examples, memory or storage device(s) (M)  215  include a combination of one or more memory or storage devices within visualization device  213  and one or more memory or storage devices external to the visualization device. 
     Network  208  may be equivalent to network  116 . Network  208  can include one or more wide area networks, local area networks, and/or global networks (e.g., the Internet) that connect preoperative surgical planning system  202  and MR system  212  to storage system  206 . Storage system  206  can include one or more databases that can contain patient information, medical information, patient image data, and parameters that define the surgical plans. 
     For example, medical images of the patient&#39;s target bone typically are generated preoperatively in preparation for an orthopedic surgical procedure. The medical images can include images of the relevant bone(s) taken along the sagittal plane and the coronal plane of the patient&#39;s body. The medical images can include X-ray images, magnetic resonance imaging (MRI) images, computerized tomography (CT) images, ultrasound images, and/or any other type of 2D or 3D image that provides information about the relevant surgical area. Storage system  206  also can include data identifying the implant components selected for a particular patient (e.g., type, size, etc.), surgical guides selected for a particular patient, and details of the surgical procedure, such as entry points, cutting planes, drilling axes, reaming depths, etc. Storage system  206  can be a cloud-based storage system (as shown) or can be located at healthcare facility  204  or at the location of preoperative surgical planning system  202  or can be part of MR system  212  or visualization device (VD)  213 , as examples. 
     MR system  212  can be used by a surgeon before (e.g., preoperatively) or during the surgical procedure (e.g., intraoperatively) to create, review, verify, update, modify and/or implement a surgical plan. In some examples, MR system  212  may also be used after the surgical procedure (e.g., postoperatively) to review the results of the surgical procedure, assess whether revisions are required, or perform other postoperative tasks. To that end, MR system  212  may include a visualization device  213  that may be worn by the surgeon and (as will be explained in further detail below) is operable to display a variety of types of information, including a 3D virtual image of the patient&#39;s diseased, damaged, or postsurgical joint and details of the surgical plan, such as a 3D virtual image of the prosthetic implant components selected for the surgical plan, 3D virtual images of entry points for positioning the prosthetic components, alignment axes and cutting planes for aligning cutting or reaming tools to shape the bone surfaces, or drilling tools to define one or more holes in the bone surfaces, in the surgical procedure to properly orient and position the prosthetic components, surgical guides and instruments and their placement on the damaged joint, and any other information that may be useful to the surgeon to implement the surgical plan. MR system  212  can generate images of this information that are perceptible to the user of the visualization device  213  before and/or during the surgical procedure. 
     In some examples, MR system  212  includes multiple visualization devices (e.g., multiple instances of visualization device  213 ) so that multiple users can simultaneously see the same images and share the same 3D scene. In some such examples, one of the visualization devices can be designated as the master device and the other visualization devices can be designated as observers or spectators. Any observer device can be re-designated as the master device at any time, as may be desired by the users of MR system  212 . 
     In this way,  FIG.  2    illustrates a surgical planning system that includes a preoperative surgical planning system  202  to generate a virtual surgical plan customized to repair an anatomy of interest of a particular patient. For example, the virtual surgical plan may include a plan for an orthopedic joint repair surgical procedure, such as one of a standard total shoulder arthroplasty or a reverse shoulder arthroplasty. In this example, details of the virtual surgical plan may include details relating to at least one of preparation of glenoid bone or preparation of humeral bone. In some examples, the orthopedic joint repair surgical procedure is one of a stemless standard total shoulder arthroplasty, a stemmed standard total shoulder arthroplasty, a stemless reverse shoulder arthroplasty, a stemmed reverse shoulder arthroplasty, an augmented glenoid standard total shoulder arthroplasty, and an augmented glenoid reverse shoulder arthroplasty. 
     The virtual surgical plan may include a 3D virtual model corresponding to the anatomy of interest of the particular patient and a 3D model of a prosthetic component matched to the particular patient to repair the anatomy of interest or selected to repair the anatomy of interest. Furthermore, in the example of  FIG.  2   , the surgical planning system includes a storage system  206  to store data corresponding to the virtual surgical plan. The surgical planning system of  FIG.  2    also includes MR system  212 , which may comprise visualization device  213 . In some examples, visualization device  213  is wearable by a user. In some examples, visualization device  213  is held by a user, or rests on a surface in a place accessible to the user. MR system  212  may be configured to present a user interface via visualization device  213 . The user interface is visually perceptible to the user using visualization device  213 . For instance, in one example, a screen of visualization device  213  may display real-world images and the user interface on a screen. In some examples, visualization device  213  may project virtual, holographic images onto see-through holographic lenses and also permit a user to see real-world objects of a real-world environment through the lenses. In other words, visualization device  213  may comprise one or more see-through holographic lenses and one or more display devices that present imagery to the user via the holographic lenses to present the user interface to the user. 
     In some examples, visualization device  213  is configured such that the user can manipulate the user interface (which is visually perceptible to the user when the user is wearing or otherwise using visualization device  213 ) to request and view details of the virtual surgical plan for the particular patient, including a 3D virtual model of the anatomy of interest (e.g., a 3D virtual bone of the anatomy of interest) and a 3D model of the prosthetic component selected to repair an anatomy of interest. In some such examples, visualization device  213  is configured such that the user can manipulate the user interface so that the user can view the virtual surgical plan intraoperatively, including (at least in some examples) the 3D virtual model of the anatomy of interest (e.g., a 3D virtual bone of the anatomy of interest). In some examples, MR system  212  can be operated in an augmented surgery mode in which the user can manipulate the user interface intraoperatively so that the user can visually perceive details of the virtual surgical plan projected in a real environment, e.g., on a real anatomy of interest of the particular patient. In this disclosure, the terms real and real world may be used in a similar manner. For example, MR system  212  may present one or more virtual objects that provide guidance for preparation of a bone surface and placement of a prosthetic implant on the bone surface. Visualization device  213  may present one or more virtual objects in a manner in which the virtual objects appear to be overlaid on an actual, real anatomical object of the patient, within a real-world environment, e.g., by displaying the virtual object(s) with actual, real-world patient anatomy viewed by the user through holographic lenses. For example, the virtual objects may be 3D virtual objects that appear to reside within the real-world environment with the actual, real anatomical object. 
     As described above, in some examples, the techniques described in this disclosure further provide for ways in which to determine whether bone can be extracted from a donor site for grafting. For example, in orthopedics, autograft is commonly used for bony reconstruction. Surgeons extract a bone graft from various locations within the patient&#39;s body according to volume and shape of the bone graft needed for the given bony reconstruction. However, the desired bone graft may be too large for a donor site and its extraction (e.g., harvesting) may lead to additional complications at the donor site such as bone fractures, cosmetic deformities, injuries to surrounding tissue, and the like. 
     As an example, in reverse shoulder arthroplasty, a bone graft is extracted from the humeral head (e.g., the humeral head is the donor site) and used to augment or reconstruct the glenoid bone (e.g., for preparation to insert prosthesis). This technique is referred to as BIO-RSA (bony increased offset-reverse shoulder arthroplasty). If the bone graft extracted from the humeral head is too large, there may be possibility that the extraction results in fracture of the tuberosities, rotator cuff injury and/or excessive bone removal that may alter the quality of the fixation of the component for the BIO-RSA (e.g., stem or nucleus inserted into the humerus). 
     In accordance with one or more examples described in this disclosure, preoperative surgical planning system  202  may be configured to retrieve image data for one or more images of anatomical objects stored in storage system  206 . The images of the anatomical objects include representations (e.g., as image data) of anatomical objects such as a first anatomical object (e.g., diseased bone) and a second anatomical object (e.g., donor site). 
     Preoperative surgical planning system  202  may determine a bone graft template for a bone graft to be connected to the first anatomical object. For example, visualization device  213  may display a representation of the first anatomical object. The surgeon, viewing the representation of the first anatomical object, may determine the size and shape of the bone graft that is to be connected to the first anatomical object. The surgeon may interact with the displayed representation and draw a bone graft template based on the size and shape of the bone graft. Visualization device  213  may display the bone graft template with the first anatomical object for the surgeon to confirm that the bone graft template adequately represents the bone graft that is to be connected. 
     Processing device(s)  210  may output information of the bone graft template to preoperative surgical planning system  202 . This is one example way in which preoperative surgical planning system  202  may determine the bone graft template. 
     In some examples, storage system  206  may store a plurality of pre-generated bone graft templates of various size and shapes. Visualization device  213  may display the pre-generated bone graft templates, and the surgeon may select one of the pre-generated bone graft templates. Processing device(s)  210  may output information of the selected pre-generated bone graft template to preoperative surgical planning system  202 . This is another example way in which preoperative surgical planning system  202  may determine the bone graft template. 
     In some examples, preoperative surgical planning system  202  may be configured to determine the bone graft template for the bone graft, and possibly with little to no intervention from the surgeon. For example, preoperative surgical planning system  202  may be configured to determine a shape of the first anatomical object. There may be various ways in which preoperative surgical planning system  202  may determine the shape of the first anatomical object, such as by segmenting out the first anatomical object from the other anatomical objects. Example ways in which to segment out the first anatomical object are described in U.S. Provisional Application Ser. Nos. 62/826,119, 62/826,133, 62/826,146, 62/826,168, and 62/826,190 all filed on Mar. 29, 2019 and incorporated by reference in their entirety. There may be other example ways in which to segment out the first anatomical object, such as in U.S. Pat. No. 8,971,606, and incorporated by reference in its entirety. 
     As one example, for segmenting, preoperative surgical planning system  202  may utilize differences in voxel intensities in image data to identify separation between bony regions and tissue regions to identify the first anatomical object. As another example, for segmenting, preoperative surgical planning system  202  may utilize closed-surface fitting (CSF) techniques in which preoperative surgical planning system  202  uses a shape model (e.g., predetermined shape like a sphere or a shape based on statistical shape modeling) and expands or constricts the shape model to fit a contour used to identify separation locations between bony regions and tissue or between tissue. 
     Preoperative surgical planning system  202  may determine a premorbid shape of the target bone (e.g., prior to disease or damage in examples where the target bone is for diseased or damaged bone) of the first anatomical object. Example ways in which to determine the premorbid shape of the first anatomical object are described in U.S. Provisional Application Nos. 62/826,172, 62/826,362, and 62/826,410 all filed on Mar. 29, 2019, and incorporated by reference in their entirety. 
     As one example, for determining premorbid shape, preoperative surgical planning system  202  may align a representation of the first anatomical object to coordinates of an SSM of the first anatomical object. Preoperative surgical planning system  202  may deform the SSM to determine an SSM that registers to the representation of the aligned first anatomical object. The version of the SSM that registers to the representation of the first anatomical object may be the premorbid shape of the target bone. 
     Preoperative surgical planning system  202  may compare the shape of the first anatomical object to the premorbid shape of the first anatomical object. For example, preoperative surgical planning system  202  may determine a difference between the first anatomical object (e.g., how the first anatomical object appears after disease or damage) and the premorbid shape of the first anatomical object (e.g. how the first anatomical object appeared before disease or damage). Based on the comparison (e.g., difference), preoperative surgical planning system  202  may determine the bone graft template. For instance, preoperative surgical planning system  202  may determine a bone graft template that has the approximately the same size and shape as the difference between the shape of the first anatomical object and the premorbid shape of the first anatomical object. 
     In one or more examples, preoperative surgical planning system  202  may be configured to determine information indicative of placement of the bone graft template within a representation of a second anatomical object (e.g., donor site) based on the image data. For example, the image date includes representations of various anatomical objects within the patient, such as the humeral head and the humerus, the iliac crest, and the like. Using BLUEPRINT™ or using one or more the segmentation techniques described in U.S. Provisional Application Ser. Nos. 62/826,119, 62/826,133, 62/826,146, 62/826,168, and 62/826,190 all filed on Mar. 29, 2019 or U.S. Pat. No. 8,971,606, visualization device  213  may display a representation of the second anatomical object (e.g., a 3D representation of a potential donor site). Although described with respect to a 3D representation, in some examples, visualization device  213  may display 2D scans of the second anatomical object. 
     Using visualization device  213 , the surgeon may “drag and drop” the bone graft template (e.g., as drawn by the surgeon or as determined by preoperative surgical planning system  202 ) include the representation of the second anatomical object. In some examples, the surgeon may translate or rotate the bone graft template along the x, y, and/or z axis before or after dragging and dropping the bone graft template in the representation of the second anatomical object. 
     In some examples, preoperative surgical planning system  202  may be configured to perform the calculations of rotating the bone graft template and calculating the coordinates of the bone graft template for aligning the bone graft template to the coordinate space of the representation of the second anatomical object. For example, the bone graft template and the representation of the second anatomical object may be in different coordinate systems, and to move the bone graft template to the representation of the second anatomical object (e.g., donor site), preoperative surgical planning system  202  may determine a transformation matrix that provides for rotation, translation, scaling, and shearing, as needed so that the bone graft template and the second anatomical object are in the same coordinate system. One example way in which preoperative surgical planning system  202  may perform the rotation, translation, scaling, and shearing is using the OpenGL application programming interface (API); however, other ways in which to perform the rotation, translation, scaling, and shearing are possible. Also, once the bone graft template is in the coordinate system of the second anatomical object or before the bone graft template is in the coordinate system of the second anatomical object, the surgeon may rotate the bone graft template to view the bone graft template from different perspectives. Preoperative surgical planning system  202  performing the above example operations of aligning coordinate system, rotating, and moving the bone graft template into the representation of the second anatomical object are non-limiting examples of preoperative surgical planning system  202  determining information indicative of a placement of the bone graft template within a representation of the second anatomical object based on the image data. 
     In the above example of preoperative surgical planning system  202  determining information indicative of a placement of the bone graft template within a representation of the second anatomical object based on the image data, the surgeon performed “dragging and dropping” operations. In some examples, preoperative surgical planning system  202  may be configured to determine information indicative of placement of the bone graft template within a representation of the second anatomical object based on the image data with little to no intervention from the surgeon. 
     For example, preoperative surgical planning system  202  may align the bone graft template to the coordinate system of the second anatomical object. Preoperative surgical planning system  202  may then, based on the coordinates of the bone graft template (e.g., coordinates along the boundary of the bone graft template) and coordinates of the second anatomical object, move the bone graft template to be within the representation of the second anatomical object. For instance, preoperative surgical planning system  202  may rotate and shift the bone graft template so that the bone graft template fits within the representation of the second anatomical object. 
     Accordingly, preoperative surgical planning system  202  may compare a size and shape of the bone graft template to the representation of the second anatomical object and determine information indicative of the placement based on the comparison. In this manner, preoperative surgical planning system  202  may determine information indicative of placement of the bone graft template within a representation of the second anatomical object based on the image data. 
     In the above examples, the bone graft template is described as being aligned with the coordinate system of the second anatomical object. In some examples, the second anatomical object may be aligned with the coordinate system of the bone graft template. 
     In some examples, preoperative surgical planning system  202  may be configured with criteria that preoperative surgical planning system  202  uses when determining information indicative of placement of the bone graft template within the representation of the second anatomical object. For example, preoperative surgical planning system  202  may be configured to determine bone density of the second anatomical object such as based on U.S. Provisional Application No. 62/826,168 filed Mar. 29, 2019. If the bone density is too low (e.g., below a threshold) in certain portions, preoperative surgical planning system  202  may not determine whether the bone graft template can be placed in the portions having low bone density. 
     As another example, preoperative surgical planning system  202  may determine whether a particular placement of the bone graft template would cause the bone graft template to cross a cortical wall within the second anatomical object. If the particular placement would cause the bone graft template to cross the cortical wall, then preoperative surgical planning system  202  may determine that the particular placement is not a valid placement of the bone graft template. 
     As another example, preoperative surgical planning system  202  may determine whether a particular placement of the bone graft template would result in complicated surgery, preoperative surgical planning system  202  may determine that the particular placement is not a valid placement of the bone graft template. For example, if placement of the bone graft template in a particular location would result in the bone graft not being accessible or require complicated surgery (e.g., excessive shifting of bone, higher changes of complication, etc.) to access the bone graft, then preoperative surgical planning system  202  may determine that the such placement of the bone graft template is not valid. 
     There may be other criteria that preoperative surgical planning system  202  may utilize when determining information indicative of placement of the bone graft template within the representation of the second anatomical object. Preoperative surgical planning system  202  may be configured to use the above examples of the criteria and the additional examples of the criteria either alone or in any combination. 
     In some examples, preoperative surgical planning system  202  may be configured to output information indicative of whether the second anatomical object is potentially suitable as a donor site for the bone graft. For example, preoperative surgical planning system  202  may utilize the various criteria to determine whether the bone graft template can be placed in the second anatomical object. If there are no valid placements for the bone graft template, preoperative surgical planning system  202  may output information indicating that the second anatomical object may not be suitable as a donor site. If there are valid placements for the bone graft template, preoperative surgical planning system  202  may output information indicating that the second anatomical object is suitable as a donor site. 
     In some examples, there may be multiple ways in which the bone graft template can fit within the second anatomical object. Preoperative surgical planning system  202  may output the various valid options indicating from where the bone graft can be extracted from the second anatomical object. In some examples, preoperative surgical planning system  202  may rank the valid options. In some examples, preoperative surgical planning system  202  may determine the best of the valid options (e.g., the location within the second anatomical object from where the bone graft can be removed with the greatest ease while minimizing impact to patient). 
     Preoperative surgical planning system  202  may be configured to output information indicative of the placement of the bone graft template within the representation of the second anatomical object. As one example, preoperative surgical planning system  202  may generate information used by visualization device  213  to render the bone graft template within the representation of the second anatomical object at the determined placement. As another example, preoperative surgical planning system  202  may generate coordinate values of the location of the bone graft template. There may be other examples of the information that preoperative surgical planning system  202  generates for outputting that is indicative of the placement of the bone graft template within the representation of the second anatomical object (e.g., donor site). 
     In some examples, preoperative surgical planning system  202  may be configured to generate pre-operative planning information based on placement of the bone graft template within the representation of the second anatomical object. For example, the information indicative of the placement of the bone graft template may include information indicative of where the bone graft template is located within the representation of the second anatomical object. The bone graft template may therefore provide a visual indication of from where to extract the bone graft. 
     As one example, preoperative surgical planning system  202  may be configured to generate information indicative of a location within the second anatomical object from which the bone graft is to be extracted (e.g., that is to be cut for extracting the bone graft). Visualization device  213  may display the location that is to be cut for extracting the bone graft. Visualization device  213  may display the location preoperatively and/or intraoperatively. 
     As one example, preoperative surgical planning system  202  may be configured to generate information indicative of an axis along which to the second anatomical object for extracting the bone graft. Visualization device  213  may display the axis that is to be cut for extracting the bone graft. Visualization device  213  may display the axis preoperatively and/or intraoperatively. 
     As one example, preoperative surgical planning system  202  may be configured to generate information indicative of a maximum depth at which to cut the second anatomical object for extracting the bone graft. Visualization device  213  may display the maximum depth preoperatively and/or intraoperatively. 
     As one example, preoperative surgical planning system  202  may be configured to generate information indicative of a types of tool to utilize to cut the second anatomical object for extracting the bone graft. Visualization device  213  may display the types of tools preoperatively and/or intraoperatively 1. 
     In the above examples, preoperative surgical planning system  202  is described as performing various operations. In some examples, the operations of preoperative surgical planning system  202  may be performed by processing device(s)  210 . In some examples, some of the example operations described above may be performed by preoperative surgical planning system  202  and some of the example operations described above may be performed by processing device(s)  210 . 
     In this disclosure, processing circuitry may be considered as performing example operations described in this disclosure. The processing circuitry may be processing circuitry of preoperative surgical planning system  202  or may be processing device(s)  210 . In some examples, the processing circuitry refers to the processing circuitry distributed between MR system  212  and preoperative surgical planning system  202 , as well as other processing circuitry in system  200 . 
       FIG.  3    is a block diagram illustrating an example of computing system configured to perform one or more examples described in this disclosure.  FIG.  3    illustrates an example of computing system  300 , and preoperative surgical planning system  202  is an example of computing system  300 . Examples of computing system  300  include various types of computing devices, such as server computers, personal computers, smartphones, laptop computers, and other types of computing devices. 
     Computing system  300  includes processing circuitry  320 , data storage system  304 , and communication interface  306 . Computing system  300  may include additional components, such as a display, keyboard, etc., not shown in  FIG.  3    for ease. Also, in some examples, computing system  300  may include fewer components. For example, data storage system  304  may be similar to storage system  206  of  FIG.  2    and reside off of (e.g., be external to) computing system  300 . However, data storage system  304  may be part of computing system  300  as illustrated. Even in examples where data storage system  304  is external to computing system  300 , computing system  300  may still include local memory for storing instructions for execution by processing circuitry  302  and provide functionality for storing data used by or generated by processing circuitry  302 . When data storage system  304  is the local memory, the amount of storage provided by data storage system  304  may less than storage system  206 . 
     Examples of processing circuitry  302  include fixed-function processing circuits, programmable circuits, or combinations thereof, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Fixed-function circuits refer to circuits that provide particular functionality and are preset on the operations that can be performed. Programmable circuits refer to circuits that can programmed to perform various tasks and provide flexible functionality in the operations that can be performed. For instance, programmable circuits may execute instructions specified by software or firmware that cause the programmable circuits to operate in the manner defined by instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations that the fixed-function circuits perform are generally immutable. 
     Examples of data storage system  304  include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store data. In some examples, data storage system  304  may also store program code in the form of instructions or data structures and that can be accessed by processing circuitry  302  for execution. 
     Communication interface  306  refers to circuitry that allows computing system  300  to connect, wirelessly or with wired connection, with other components. For instance, communication interface  306  provides the circuitry that allows computing device  300  to transmit to and receive from network  208  of  FIG.  2   . 
     Processing circuitry  302  is an example of processing circuitry configured to perform one or more example techniques described in this disclosure. In some examples, such as where MR system  212  is configured to perform various operations of preoperative surgical planning system  202 , processing device(s)  210  may include processing circuitry  302 . Also, in some examples, the processing circuitry that is configured to perform the example operations described in this disclosure may include the combination of processing circuitry  302 , processing device(s)  210 , and possibly one or more other processing circuitry. For example,  FIG.  3    is described with respect to processing circuitry  302 . 
     For example, data storage system  304  may store image data for one or more images of anatomical objects, and processing circuitry  302  may access the image data from data storage system  304 . Utilizing one or more of the example techniques described above, processing circuitry  302  may be configured to determine a bone graft template for a bone graft to be connected to a first anatomical object, determine information indicative of placement of the bone graft template within a representation of a second anatomical object based on the image data, and output the information indicative of the placement of the bone graft template within the representation of the second anatomical object. 
       FIGS.  4  and  5    are conceptual diagrams illustrating examples of bone extraction for bone grafting. For example,  FIGS.  4  and  5    illustrate examples of bone extraction (e.g., harvesting) for bony increased offset-reverse shoulder arthroplasty (BIO-RSA). Other example procedures that may utilize the techniques described in this disclosure include the Norris technique. 
     As shown in  FIG.  4   , graft reaming tool  402  may be used to ream the surface of humeral head  404  of humerus  400 . The surgeon may connect graft reaming tool  402  to a drill or other instrument and MR system  212  may display virtual guidance to assist in reaming the surface of humeral head  404 . For instance, MR system  212  may display depth guidance to enable the surgeon to ream the surface of humeral head  404  to a target depth. As another example, MR system  212  may provide targeting guidance. For instance, MR system  212  may display one or both of a virtual marker that identifies a center point or prescribed axis of the reaming and/or an indication of whether graft reaming tool  402  is aligned with the prescribed axis. 
     In this example, graft reaming tool  402  may be a cannulated reaming tool configured to be positioned and/or guided by a guide pin, such as guide  406 . In other examples, graft reaming tool  402  may not be cannulated and may be guided without the assistance of a physical guide pin. For instance, MR system  212  may provide virtual guidance (e.g., depth guidance and/or targeting guidance such as a displayed virtual marker) to enable a surgeon to ream a graft from humeral head  404  without the use of guide  406 . 
     As shown in  FIG.  5   , a surgeon may attach mechanical guide  502  on humerus  500  prior to performing a resection of humeral head  504 . The surgeon may adjust one or more components of mechanical guide  502  such that top surface  506  of mechanical guide  502  is co-planar with anatomic neck  508  of humerus  500  (for purposes of illustration, anatomic neck  508  is illustrated as a broken line). After attaching mechanical guide  502  to humeral head  504  and adjusting the mechanical guide, the surgeon may perform the resection of humeral head  504  by guiding a cutting tool (e.g., a blade of an oscillating saw) along top surface  506 . 
       FIG.  6    is a conceptual diagram illustrating an example of the extracted bone  600  grafted to glenoid bone  602 . As shown in  FIG.  6   , bone graft  600 , possibly taken from humeral head  404  ( FIG.  4   ) or  504  ( FIG.  5   ), is grafted to glenoid  602 . Prosthesis  604  is inserted into bone graft  600 , as part of the BIO-RSA. Stem  606  is inserted into the humerus, as part of the BIO-RSA. 
       FIGS.  7 A and  7 B  are conceptual diagrams illustrating examples of a bone graft template  702  within a representation of a donor site  704 .  FIGS.  7 A and  7 B  illustrate bone graft template  702  and donor site (e.g., humeral head)  704 . For ease of illustration, bone graft template  702  and donor site  704  are illustrated as two-dimensional images, such as may be displayed on x-ray or CT-scan imagery. Visualization device  213  may be configured to display bone graft template  702  and donor site  704  as three-dimensional images. However, visualization device  213  may display bone graft template  702  and donor site  704  in two-dimensional images, such as images from different perspectives. 
     Processing circuitry (e.g., processing circuitry  302 , processing device(s)  210 , or a combination thereof) may be configured to determine bone graft template  702  for a bone graft to be connected to a first anatomical object (e.g., glenoid bone  602  of  FIG.  6   ). The processing circuitry may utilize the various example techniques described above to determine bone graft template  702 . The processing circuitry may be configured to determine information indicative of placement of bone graft template  702  within a representation of a second anatomical object (e.g., donor site  704 ) based on the image data utilizing the above example techniques. 
     The processing circuitry may output information indicative of the placement of bone graft template  702  within the representation of the second anatomical object (e.g., donor site  704 ). For example,  FIGS.  7 A and  7 B  illustrate the rendering of bone graft template  702  within donor site  704  from different perspectives. The graphical information used to render bone graft template  702  within donor site  704  is an example of the information that the processing circuitry outputs indicative of the placement of bone graft template  702  within donor site  704 . 
       FIGS.  8 - 11    depict four examples of graphical user interfaces (GUIs)  800 - 1100 , respectively, configured to allow a user to create (e.g., design) a customized virtual bone graft  600  for a patient based on imported x-ray data and/or CT-scan data of the patient. For example, GUIs  800 - 1100  may be generated by the BLUEPRINT™ system as described above with respect to  FIGS.  1  and  2   . As shown in  FIG.  8   , GUI  800  may receive CT-scan data (e.g., imagery)  802 , and x-ray data (e.g., imagery)  804 , to allow (e.g., facilitate) the determination and/or generation of a custom-shaped virtual bone graft  600  that conforms to the contours of glenoid  602 . In the example of  FIG.  8   , GUI  800  includes a virtual 3-D model of bone graft  600  and prosthesis  604  oriented with respect to a virtual 3-D model of glenoid  602 , which may be based on CT-scan data  802 . GUI  800  additionally includes a cross-sectional view of bone graft  600  and prosthesis  604  oriented with respect to CT scan imagery  804  depicting glenoid  802 . GUI  800  additionally includes a plurality of user inputs  806  configured to allow a user to modify (e.g., customize) a size, shape or relative orientation of bone graft  600  and or prosthesis  604  with respect to glenoid  602 . 
       FIG.  9    depicts another example GUI  900  configured to allow a user to create, design, customize, and/or modify a virtual bone graft  600  for a patient. GUI  900  includes a plurality of user input fields  902 A- 902 C (collectively, user inputs  902 ) configured to receive data from a user to customize the dimensions of virtual bone graft  600 , such as a minimum height  902 A, a maximum height  902 B, and a diameter  902 C (e.g., for bone grafts  600  having a circular cross-sectional area, such as a cylinder). 
       FIG.  10    depicts another example GUI  1000  configured to test a range of motion of a shoulder of a patient, as permitted by a customized virtual bone graft  600  and prosthesis  604 . For example, GUI  1000  may be generated by the BLUEPRINT™ system as described above and/or displayed on virtualization device  213  ( FIG.  13   ). In the example of  FIG.  10   , GUI  1000  includes a plurality of tools to customize the volume (e.g., size and shape) of virtual bone graft  600 , as well as virtually test its functionality. For example, as shown in  FIG.  10   , GUI  1000  includes a simulator configured to identify an impingement point  1016  between stem  606  and glenoid  602 . In these scenarios, virtual bone graft  600  may require modification to extend stem  606  farther away from glenoid  602 .  FIG.  11    depicts another example GUI  1100  for using CT-scan data to create a virtual bone graft  600 . In some examples in accordance with this disclosure, the computing system may be configured to output data indicative of virtual bone graft  600  for display on a visualization device  213  ( FIG.  13   ). 
       FIG.  12    is a flowchart illustrating example methods of operations in accordance with one or more example techniques described in this disclosure. The example techniques of  FIG.  8    may be performed with processing circuitry (e.g., processing circuitry  302 , processing device(s)  210 , or a combination thereof). 
     The processing circuitry may determine a bone graft template for a bone graft to be connected to a first anatomical object ( 1200 ). As one example, the processing circuitry may be configured to receive input of the bone graft template (e.g., based on a drawing from a surgeon or from a selection of pre-generated bone graft templates). As another example, the processing circuitry may be configured to determine a shape of the first anatomical object, compare the shape of the first anatomical object to a premorbid shape of the first anatomical object, and determine the bone graft template based on the comparison. 
     The processing circuitry may determine information indicative of placement of the bone graft template within a representation of a second anatomical object based on stored image data ( 1202 ). As one example, the processing circuitry may be configured to receive information indicative of placement of the bone graft template within the representation of the second anatomical object (e.g., based on “drag and drop” by surgeon). As one example, the processing circuitry may compare a size and shape of the bone graft template to the representation of the second anatomical object and determine information indicative of the placement based on the comparison. 
     The processing circuitry may output information indicative of the placement of the bone graft template within the representation of the second anatomical object ( 1204 ). As one example, processing circuitry is configured to output graphical information for rendering, for display, the bone graft template within the representation of the second anatomical object. For instance, the processing circuitry may output graphical information for rendering, for display, the representation of the second anatomical object. In such examples, to receive information indicative of the placement of the bone graft template, the processing circuitry may be configured to receive information indicative of the placement of the bone graft template within the displayed representation of the second anatomical object. To output the information indicative of the placement of the bone graft, the processing circuitry may be configured to output graphical information for rendering, for display, the bone graft template within the displayed represented of the second anatomical object based on the received information indicative of the placement of the bone graft template within the displayed representation of the second anatomical object. 
     In some examples, the processing circuitry may be configured to output information indicative of whether the second anatomical object is potentially suitable as a donor site for the bone graft for the first anatomical object. For example, the processing circuitry may be configured with criteria to use to determine whether a placement of the bone graft template within the second anatomical object is valid. If the processing circuitry determines that there is no valid placement of the bone graft template in the second anatomical object, the processing circuitry may output information indicating that the second anatomical object is not suitable for a donor site for the bone graft for the first anatomical object. 
     The processing circuitry, may in some examples, recommend an alternative site when no valid placement of the bone graft is found. For example, the processing circuitry may be preloaded with representations of different possible second anatomical objects (e.g., humeral head, iliac crest, etc.). The processing circuitry may determine whether there are valid placements of the bone graft in the different second anatomical objects, and identify potential locations that can be candidates for the second anatomical object. Rather than trying different possible second anatomical objects, in some examples, the processing circuitry may determine valid placements in the different second anatomical objects and identify which ones of the second anatomical objects can be donor sites. The processing circuitry may also provide a ranking of second anatomical objects indicating which ones could be the best donor sites (e.g., in terms of ease of surgery and most available bone). 
     In some examples, the processing circuitry may generate pre-operative planning information based on placement of the bone graft template within the representation of the second anatomical object ( 1206 ). As one example, the processing circuitry is configured to generate information indicative of a location within the second anatomical object from which the bone graft is to be extracted (e.g., that is to be cut for extracting the bone graft). As another example, the processing circuitry is configured to generate information indicative of an axis along which to the second anatomical object for extracting the bone graft. As another example, the processing circuitry is configured to generate information indicative of a maximum depth at which to cut the second anatomical object for extracting the bone graft. As another example, the processing circuitry is configured to generate information indicative of a types of tool to utilize to cut the second anatomical object for extracting the bone graft. 
       FIG.  13    is a schematic representation of visualization device  213  ( FIG.  2   ) for use in an MR system, such as MR system  212  of  FIG.  2   , according to an example of this disclosure. As shown in the example of  FIG.  13   , visualization device  213  can include a variety of electronic components found in a computing system, including one or more processor(s)  514  (e.g., microprocessors or other types of processing units) and memory  516  that may be mounted on or within a frame  518 . Furthermore, in the example of  FIG.  13   , visualization device  213  may include a transparent screen  520  that is positioned at eye level when visualization device  213  is worn by a user. In some examples, screen  520  can include one or more liquid crystal displays (LCDs) or other types of display screens on which images are perceptible to a surgeon who is wearing or otherwise using visualization device  213  via screen  520 . Other display examples include organic light emitting diode (OLED) displays. In some examples, visualization device  213  can operate to project 3D images onto the user&#39;s retinas using techniques known in the art. 
     In some examples, screen  520  may include see-through holographic lenses. sometimes referred to as waveguides, that permit a user to see real-world objects through (e.g., beyond) the lenses and also see holographic imagery projected into the lenses and onto the user&#39;s retinas by displays, such as liquid crystal on silicon (LCoS) display devices, which are sometimes referred to as light engines or projectors, operating as an example of a holographic projection system  538  within visualization device  213 . In other words, visualization device  213  may include one or more see-through holographic lenses to present virtual images to a user. Hence, in some examples, visualization device  213  can operate to project 3D images onto the user&#39;s retinas via screen  520 , e.g., formed by holographic lenses. In this manner, visualization device  213  may be configured to present a 3D virtual image to a user within a real-world view observed through screen  520 , e.g., such that the virtual image appears to form part of the real-world environment. In some examples, visualization device  213  may be a Microsoft HOLOLENS™ headset, available from Microsoft Corporation, of Redmond, Wash., USA, or a similar device, such as, for example, a similar MR visualization device that includes waveguides. The HOLOLENS™ device can be used to present 3D virtual objects via holographic lenses, or waveguides, while permitting a user to view actual objects in a real-world scene, i.e., in a real-world environment, through the holographic lenses. Although the example of  FIG.  13    illustrates visualization device  213  as a head-wearable device, visualization device  213  may have other forms and form factors. For instance, in some examples, visualization device  213  may be a handheld smartphone or tablet. 
     Visualization device  213  can also generate a user interface (UI)  522  that is visible to the user, e.g., as holographic imagery projected into see-through holographic lenses as described above. For example, UI  522  can include a variety of selectable widgets  524  that allow the user to interact with a mixed reality (MR) system, such as MR system  212  of  FIG.  2   . Imagery presented by visualization device  213  may include, for example, one or more 3D virtual objects. Details of an example of UI  522  are described elsewhere in this disclosure. Visualization device  213  also can include a speaker or other sensory devices  526  that may be positioned adjacent the user&#39;s ears. Sensory devices  526  can convey audible information or other perceptible information (e.g., vibrations) to assist the user of visualization device  213 . 
     Visualization device  213  can also include a transceiver  528  to connect visualization device  213  to a processing device  510  and/or to network  208  and/or to a computing cloud, such as via a wired communication protocol or a wireless protocol, e.g., Wi-Fi, Bluetooth, etc. Visualization device  213  also includes a variety of sensors to collect sensor data, such as one or more optical camera(s)  530  (or other optical sensors) and one or more depth camera(s)  532  (or other depth sensors), mounted to, on or within frame  518 . In some examples, the optical sensor(s)  530  are operable to scan the geometry of the physical environment in which user of MR system  212  is located (e.g., an operating room) and collect two-dimensional (2D) optical image data (either monochrome or color). Depth sensor(s)  532  are operable to provide 3D image data, such as by employing time of flight, stereo or other known or future-developed techniques for determining depth and thereby generating image data in three dimensions. Other sensors can include motion sensors  533  (e.g., Inertial Measurement Unit (IMU) sensors, accelerometers, etc.) to assist with tracking movement. 
     MR system  212  processes the sensor data so that geometric, environmental, textural, etc. landmarks (e.g., corners, edges or other lines, walls, floors, objects) in the user&#39;s environment or “scene” can be defined and movements within the scene can be detected. As an example, the various types of sensor data can be combined or fused so that the user of visualization device  213  can perceive 3D images that can be positioned, or fixed and/or moved within the scene. When fixed in the scene, the user can walk around the 3D image, view the 3D image from different perspectives, and manipulate the 3D image within the scene using hand gestures, voice commands, gaze line (or direction) and/or other control inputs. As another example, the sensor data can be processed so that the user can position a 3D virtual object (e.g., a bone model) on an observed physical object in the scene (e.g., a surface, the patient&#39;s real bone, etc.) and/or orient the 3D virtual object with other virtual images displayed in the scene. As yet another example, the sensor data can be processed so that the user can position and fix a virtual representation of the surgical plan (or other widget, image or information) onto a surface, such as a wall of the operating room. Yet further, the sensor data can be used to recognize surgical instruments and the position and/or location of those instruments. 
     Visualization device  213  may include one or more processors  514  and memory  516 , e.g., within frame  518  of the visualization device. In some examples, one or more external computing resources  536  process and store information, such as sensor data, instead of or in addition to in-frame processor(s)  514  and memory  516 . In this way, data processing and storage may be performed by one or more processors  514  and memory  516  within visualization device  213  and/or some of the processing and storage requirements may be offloaded from visualization device  213 . Hence, in some examples, one or more processors that control the operation of visualization device  213  may be within the visualization device, e.g., as processor(s)  514 . Alternatively, in some examples, at least one of the processors that controls the operation of visualization device  213  may be external to the visualization device, e.g., as processor(s)  210 . Likewise, operation of visualization device  213  may, in some examples, be controlled in part by a combination one or more processors  514  within the visualization device and one or more processors  210  external to the visualization device. 
     For instance, in some examples, when visualization device  213  is in the context of  FIG.  2   , processing of the sensor data can be performed by processing device(s)  210  in conjunction with memory or storage device(s) (M)  215 . In some examples, processor(s)  514  and memory  516  mounted to frame  518  may provide sufficient computing resources to process the sensor data collected by cameras  530 ,  532  and motion sensors  533 . In some examples, the sensor data can be processed using a Simultaneous Localization and Mapping (SLAM) algorithm, or other known or future-developed algorithm for processing and mapping 2D and 3D image data and tracking the position of visualization device  213  in the 3D scene. In some examples, image tracking may be performed using sensor processing and tracking functionality provided by the Microsoft HOLOLENS™ system, e.g., by one or more sensors and processors  514  within a visualization device  213  substantially conforming to the Microsoft HOLOLENS™ device or a similar mixed reality (MR) visualization device. 
     In some examples, MR system  212  can also include user-operated control device(s)  534  that allow the user to operate MR system  212 , use MR system  212  in spectator mode (either as master or observer), interact with UI  522  and/or otherwise provide commands or requests to processing device(s)  210  or other systems connected to network  208 . As examples, the control device(s)  534  can include a microphone, a touch pad, a control panel, a motion sensor or other types of control input devices with which the user can interact. 
       FIG.  14    is a plan view of a visualization device  213  configured to guide a bone graft harvest, in accordance with one or more techniques of this disclosure. Visualization device  213  (described further with respect to  FIG.  13   , above) may containing processing circuitry configured to either obtain (e.g., receive as input) or generate a virtual bone graft  600  ( FIGS.  6 ,  8 - 11   ) and output a graphical representation of virtual bone graft  600  as virtual bone graft  1402  for display on transparent screen  520  ( FIG.  13   ). For example, a wearer or user of visualization device  213  may observe real-world elements through transparent screen  520 , with virtual bone graft  1402  laid over top of the real world elements either alone or in combination with other additional virtual graphical objects. For example, as shown in  FIG.  15   , visualization device  213  may be configured to display virtual bone graft  1402  in a fixed position relative to a real observed bone structure, such as a head  504  of a humerus  500  of a patient undergoing arthroplasty. In particular, visualization device  213  may be configured to detect (e.g., identify) a patient&#39;s humeral head  504 , display virtual bone graft  1402  overtop of or “inside of” humeral head  504 , and “lock” virtual bone graft  1402  in place with respect to humeral head  504 . In other words, motion sensors  533  of visualization device  213  ( FIG.  13   ) may be configured to track a motion of visualization device  213  with respect to humerus  500 , and update the displayed position of virtual bone graft  1402  with respect to humeral head  504 , such that virtual bone graft  1402  retains its position relative to humeral head  504 , as viewed by the user or wearer. 
     As shown in  FIGS.  16 A and  16 B , in some examples in accordance with this disclosure, visualization device  213  may be configured to “lock” the displayed virtual bone graft  1402  with respect to an interior of the donor bone  504  (e.g., a real observed bone structure) through a process called “registration.” Visualization device  213  may perform the registration process in two steps: initialization and optimization (e.g., minimization). During initialization, the user of MR system  212  may use the visualization device  213  in conjunction with information derived from the preoperative virtual planning system  102  ( FIG.  1   ), the orientation of the user&#39;s head (which provides an indication of the direction of the user&#39;s eyes (referred to as “gaze” or “gaze line”), rotation of the user&#39;s head in multiple directions, sensor data collected by the sensors  530 ,  532  and/or  533  (or other acquisitions sensors), and/or voice commands and/or hand gestures to visually achieve an approximate alignment of the 3D virtual bone model  2002  with a corresponding observed bone structure (e.g., humeral head  504 ). 
     In some examples, preoperative planning system  102 , MR system  212 , and/or visualization device  213  may receive data indicative of virtual bone graft  1402  as well as a virtual donor bone model  2002 . As shown in  FIG.  16 A , the data may indicate a fixed location of the virtual bone graft  1402  with respect to the interior of the virtual donor bone model  2002 . Preoperative planning system  102  may identify a point  2004  or region of interest on the surface of the virtual donor bone model  2002  and a virtual normal vector  2006  to the point  2004  (or region) of interest on the surface of the region. MR system  212  may connect the identified point (or region) of interest to the user&#39;s gaze point (e.g., a central point in the field of view of visualization device  213 ). Thus, when the head of the user of visualization device  213  is then moved or rotated, the virtual bone model  2002  may also move and rotate in space. 
     In the example of a shoulder arthroplasty procedure, the point of interest  2004  on the surface of virtual bone model  2002  can be an approximate center of the virtual bone model that can be determined by using a virtual planning system  102 , such as the BLUEPRINT™ planning system. In some examples, the approximate center of the virtual bone model  2002  can be determined using a barycenter find algorithm, with the assistance of machine learning algorithms or artificial intelligence systems, or using another type of algorithm. For other types of bone repair/replacement procedures, other points or regions of the bone can be identified and then connected to the user&#39;s gaze line or gaze point. 
     In some examples, the ability to move and rotate virtual bone model  2002  in space about the user&#39;s gaze point alone may not be sufficient to orient virtual bone model  2002  with the observed bone (e.g., humeral head  504 ). Thus, as part of the initialization procedure, MR system  212  may also determine the distance between visualization device  213  and a point (or points)  2008  on the surface of the observed bone  504  in the field of view of visualization device  213  and the orientation of that surface using sensor data collected from the depth, optical, and motion sensors  530 ,  532 ,  533 . For example, the orientation of the virtual bone model surface can be approximated by determining a vector  2010  that is normal (i.e., perpendicular) to a point (e.g., a central point)  2008  on the surface. This normal vector is referred to herein as the “observed normal vector”  2010 . It should be understood, however, that other bones may have more complex surfaces. For these more complex cases, other surface descriptors may be used to determine orientation. 
     Regardless of the particular bone, MR system  212  may derive distance information from depth camera(s)  532  ( FIG.  13   ). MR system  212  may utilize this distance information to derive the geometric shape of the surface of an observed bone  504 . That is, because depth camera(s)  532  provide distance data corresponding to any point in a field of view of depth camera(s)  532 , MR system  212  may determine the distance to the user&#39;s gaze point on the observed bone  504 . With this information, either visualization device  213  can automatically, or the user can manually, move 3D virtual bone model  2002  in space and approximately align it with the observed bone  504  at a point or region of interest using the gaze point. That is, when the user shifts gaze to observed bone structure  504 , virtual bone model  1402  (which is connected to the user&#39;s gaze line) moves with the user&#39;s gaze. The user can then align 3D virtual bone model  2002  with observed bone structure  504  by moving the user&#39;s head (and thus the gaze line), using hand gestures, using voice commands, and/or using a virtual interface to adjust the position of virtual bone model  2002 . For instance, once 3D virtual bone model  2002  is approximately aligned with observed bone structure  504 , the user may provide a voice command (e.g., “set”) that causes MR system  212  to capture the initial alignment. The orientation (“yaw” and “pitch”) of the 3D model can be adjusted by rotating the user&#39;s head, using hand gestures, using voice commands, and/or using a virtual interface which rotate 3D virtual bone model  2002  about the user&#39;s gaze line so that an initial (or approximate) alignment of the virtual and observed objects can be achieved. In this manner, virtual bone model  2002  is oriented with the observed bone  504  by aligning the virtual normal vector  2006  and the observed normal vector  2010 . Additional adjustments of the initial alignment can be performed as needed. For instance, after providing the voice command, the user may provide additional user input to adjust an orientation or a position of virtual bone model  2002  relative to observed bone structure  504 . This initial alignment process is performed intraoperatively (or in real time) so that the surgeon can approximately align the virtual and observed bones. In some examples, such as where the surgeon determines that the initial alignment is inadequate, the surgeon may provide user input (e.g., a voice command, such as “reset”) that causes MR system  212  to release the initial alignment such that the central point is again locked to the user&#39;s gaze line. 
     When the user detects (e.g., sees) that an initial alignment of 3D virtual bone model  2002  with observed bone structure  504  has been achieved (at least approximately), the user can provide an audible or other perceptible indication to inform MR system  212  that a fine registration process (i.e., execution of an optimization (e.g., minimization) algorithm) can be started. For instance, the user may provide a voice command (e.g., “match”) that causes MR system  212  to execute a minimization algorithm to perform the fine registration process. The optimization process can employ any suitable optimization algorithm (e.g., a minimization algorithm such as an Iterative Closest Point or genetic algorithm) to perfect alignment of virtual bone model  2002  with observed bone structure  504 . Upon completion of execution of the optimization algorithm, the registration procedure is complete. The registration process may result in generation of a transformation matrix that then allows for translation along the x, y, and z axes of the 3D virtual bone model  2002  and rotation about the x, y and z axes in order to achieve and maintain alignment between the virtual and observed bones. 
     In some examples, once the registration of virtual bone model  2002  has been completed, the surgeon may elect to command MR system  212  (e.g., visualization device  213 ) to stop displaying virtual bone model  2002 . For example, as shown in  FIG.  16 B , visualization device  213  may be configured to directly display virtual bone graft  1402  “locked” in position (e.g., registered) with respect to observed head  504  of humerus  500 , without displaying virtual bone model  2002 . 
     While described above as being performed visually, it is noted that the techniques of this disclosure can be performed using any suitable registration technique. For instance, in some examples, the surgeon may attach one or more physical markers to the bone (e.g., humeral head  504 ) at known locations. MR system  212  may utilize data from one or more sensors to track positions/orientations of the physical markers and display virtual guidance (e.g., virtual bone model  2002  and/or virtual bone graft  1402 ) at positions/orientations relative to the positions/orientations of the physical markers. 
     By displaying virtual bone graft  1402  intraoperatively, the techniques of this disclosure may improve the harvest or extraction of a bone graft by allowing an orthopedic surgeon to harvest a bone graft that is customized to fit the specific patient. For example, instead of using generic bone-graft cutting tools configured to harvest standardized “cookie cutter” bone graft shapes, a surgeon may use more specialized tools to cut around the unique contours of virtual bone graft  1402 , producing a customized bone graft that is a better fit for the patient, improving patient outcomes. 
     In some examples, visualization device may be configured not only to identify and target-lock a humeral head of a patient, but also identify a removed (e.g., resected) portion of a humeral head and target-lock a virtual bone graft within the removed humeral head. For example, a surgeon may perform a resection of humeral head  504 , as described above with respect to  FIG.  5   . As shown in  FIG.  17   , the resection of humeral head may produce a semi-spherical bone “cap”  1706 . Visualization device  213  may be configured to identify and/or recognize the resectioned bone cap  1706  and output virtual bone graft  1402  for display overtop of, or “inside” of, bone cap  1706 . In this way, the surgeon or physician&#39;s assistant may more-easily harvest the bone graft from bone cap  1706  rather than from humeral head  504  as a whole. 
     For example, after resecting bone cap  1706  from the rest of the humeral head, the surgeon may bring bone cap  1706  to a customized bone-graft-extraction station within the operating room and used specialized cutting tools to extract the bone graft according to the locked-in virtual bone graft  1402 . In some examples, visualization device  213  may be configured to display an indication of a difference in geometry between bone cap  1706  and virtual bone graft  1402 . A surgeon may observe bone cap  1706  through visualization device  1706  and use a surgical tool to “chip away” at bone cap  1706  until the shape of bone cap  1706  accurately matches or mirrors the shape of virtual bone graft  1402 . 
     For example, as shown in  FIGS.  18 A and  18 B , visualization device  213  ( FIG.  17   ) may be configured to output for display additional surgical guidance to further facilitate the bone graft harvest. For example, visualization device  213  may output for display one or more surgical instructions  1806 , such as a recommended tool (e.g., cutting tool  402 ) that could effectively harvest the bone graft. Additionally or alternatively, visualization device  213  may output for display one or more cutting axes  1808  so as to facilitate the alignment of cutting tool  402  with bone cap  1706 . 
       FIGS.  19 A and  19 B  are example mixed reality graphical user interfaces (MR GUIs) that MR system  212  may generate and display on visualization device  213  ( FIG.  13   ) in accordance with one or more techniques of this disclosure. In the example of  FIGS.  19 A and  19 B , visualization device  213  has obtained (e.g., received) or generated a rectangular-prism-shaped virtual bone graft and output for display a corresponding rectangular-prism-shaped virtual bone graft  1902 . For example, as described above, visualization device  213  may be configured to “register” virtual bone graft  1902  by identifying humeral head  504  and displaying virtual bone graft  1902  on transparent screen  520  such that rectangular-prism-shaped virtual bone graft  1902  appears “locked” in place with respect to humeral head  504 . 
     As shown in  FIGS.  20 A- 20 C , the surgeon may then perform a resection of humeral head  504  (as described in  FIG.  5   , above) to remove bone cap  1706  and then use one or more cutting tools  1504  to extract the customized bone graft along the contours of the displayed virtual bone graft  1902 . For example, visualization device  213  may be configured to display an indication of a difference in geometry between bone cap  1706  and virtual bone graft  1902 . A surgeon may observe bone cap  1706  through visualization device  1706  and use a surgical tool  1504  to “chip away” at the perimeter of bone cap  1706  until the shape of bone cap  1706  accurately matches or mirrors the shape of virtual bone graft  1902 . 
     In another example, rather than registering virtual bone graft  1902  such that it appears to be located “inside” of a host bone, visualization device  213  may instead display a “floating” virtual bone graft  1902  for use as a visual aid in extracting the actual bone graft from the host bone. For example, as shown in  FIG.  21   , a surgeon may then perform a resection of humeral head  504  to remove bone cap  1706 . Visualization device  213  may be configured to display virtual bone graft  1902  at a fixed location (e.g., as appearing next to, but separate from, bone cap  1706 ). The surgeon may then use virtual bone graft  1902  as a visual reference (e.g., as a template) while using one or more tools, such as cutting tool  1504 , to extract the actual bone graft from bone cap  1706 . For example, the surgeon may use surgical tool  1504  to “chip away” at the perimeter of bone cap  1706  until the shape of bone cap  1706  accurately matches or mirrors the shape of virtual bone graft  1902 . 
     In some examples, the surgeon may move bone cap  1706  to the location at which virtual bone graft  1902  is being displayed. As such, the surgeon may be able to directly compare the current shape of bone cap  1706  with the shape of virtual bone graft  1902  (e.g., to determine whether any additional material should be removed from bone cap  1706  to cause the shape of bone cap  1706  to match the shape of virtual bone graft  1902 ). By displaying virtual bone graft  1902  at a fixed location, visualization device  213  may enable the surgeon to shape a bone graft from a piece of donor bone without having to maintain a registration between a virtual model and the donor bone. In this way, the techniques of this disclosure may reduce the complexity of bone graft shaping. 
       FIG.  22    is a flowchart illustrating example methods of operations in accordance with one or more example techniques described in this disclosure. Although the techniques of  FIG.  22    are described with respect to visualization device  213  of  FIG.  13   , the techniques may be performed by any appropriate virtual-reality display device. Processing circuitry within visualization device  213  may obtain data indicative of (e.g., describing dimensions of) a virtual model of a bone graft ( 2200 ). For example, the virtual bone graft may be generated using software incorporated within visualization device  213 , or alternatively, generated on a separate computing system as shown in  FIGS.  8 - 11   , above, and subsequently received by visualization device  213 . 
     Visualization device  213 , using one or more sensors, may intraoperatively detect (e.g., locate or identify) an exposed bone of a patient within the field-of-view of transparent screen  520  ( 2202 ). For example, visualization device  213  may receive sensor data indicating imagery within the field-of-view of transparent screen  520  and apply image-recognition software to the sensor data to identify an exposed humerus of a patient. 
     Visualization device  213  outputs for display on transparent screen  520  a graphical object depicting the obtained virtual bone graft ( 2204 ). Visualization device  213  may display the hologram in a relative position with respect to the position of the identified humerus of the patient such that the hologram appears directly over the top of, or in some examples, appears to be “inside” of the patient&#39;s humerus. Visualization device  213  may further track the motion of transparent screen  520  with respect to the patient&#39;s humeral head so that it may “update” the display of the bone-graft hologram so that the hologram appears “locked” in place with respect to the humerus. In some examples, visualization device  213  may output for display surgical guidance, such as audio-based or text-based surgical suggestions, or mixed-reality cutting guides. 
       FIG.  23    is a flowchart illustrating example methods of operations in accordance with one or more example techniques described in this disclosure. Although the techniques of  FIG.  23    are described with respect to visualization device  213  of  FIG.  13   , the techniques may be performed by any appropriate display device. A surgeon or other user or wearer of visualization device  213  may observe a hologram of a virtual bone graft displayed on transparent screen  520 , displayed such that the hologram appears directly over top of, or “inside” of, a patient&#39;s humeral head ( 2300 ). 
     The surgeon performs a resection of the patient&#39;s humeral head ( FIG.  5   ) to remove a semi-spherical bone cap from which a bone graft may be harvested ( 2302 ). The surgeon may use one or more specialized cutting tools to harvest a customized bone graft from the bone cap by cutting around the edges of the projected hologram, which remains “locked” in place within the removed bone cap ( 2304 ). The surgeon may perform an arthroplasty, such as a Bony Increased Offset-Reversed Shoulder Arthroplasty BIO-RSA™, on the patient by securing the harvested bone graft and prosthesis to the patient&#39;s glenoid ( 2306 ). 
     The following examples are described herein. Example 1: A system for guiding bone graft harvesting, the system comprising: a visualization device; and processing circuitry configured to: obtain a virtual model of a bone graft to be harvested from a donor bone, wherein the virtual model of the bone graft specifies a volume of the donor bone to be harvested as the bone graft; and output for display, via the visualization device, a graphical representation of the specified volume to be harvested relative to a portion of the donor bone viewable via the visualization device. 
     Example 2: The system of example 1, wherein the processing circuitry is further configured to determine a donor site within the donor bone. 
     Example 3: The system of example 2, wherein the processing circuitry is configured to determine the donor site by: receiving video data from a camera; recognizing a donor bone from the video data; and measuring a donor volume within the donor bone that fits the specified volume to be harvested. 
     Example 4: The system of any of examples 1-3, wherein the visualization device comprises a see-through holographic lens configured to display the one or more virtual model of the bone graft as a hologram. 
     Example 5: The system of any of examples 1-4, the processing circuitry further configured to: determine a change in position of the visualization device relative to the donor bone; and update the display of the graphical representation of the specified volume to be harvested in response to determining the change in position so as to maintain a position of the specified volume to be harvested relative to the portion of the donor bone. 
     Example 6: The system of any of examples 1-5, wherein the specified volume to be harvested comprises a cylinder within the donor bone. 
     Example 7: The system of any of examples 1-5, wherein the specified volume to be harvested comprises a rectangular prism within the donor bone. 
     Example 8: The system of any of examples 1-7, wherein the donor bone comprises a head of a humerus of a patient. 
     Example 9: The system of any of examples 1-8, the processing circuitry further configured to output for display additional surgical guidance information. 
     Example 10: The system of example 9, wherein the additional surgical guidance information comprises a recommended cutting tool. 
     Example 11: The system of example 9, wherein the additional surgical guidance information comprises a virtual cutting axis configured to guide an alignment of a cutting tool relative to the donor bone. 
     Example 12: The system of any of examples 1-11, wherein the system is configured to output for display the graphical representation of the specified volume to be harvested relative to the portion of the donor bone by registration of the graphical representation of the specified volume to be harvested. 
     Example 13: The system of any of examples 1-12, wherein the volume of the donor bone to be harvested as the bone graft comprises a first volume of the donor bone, and wherein the processing circuitry is further configured to: determine a second volume of the donor bone, wherein the first volume is configured to be extracted from the second volume; output for display, via the visualization device, surgical guidance information configured to guide a surgeon to resect the second volume from the donor bone; and output for display, via the visualization device, the graphical representation of the specified first volume to be harvested relative to the second volume viewable via the visualization device. 
     Example 14: A method for bone graft harvesting in orthopedic surgery, the method comprising: obtaining a virtual model of a bone graft to be harvested from a donor bone, wherein the virtual model of the bone graft specifies a volume of the donor bone to be harvested as the bone graft; and outputting for display, via a visualization device, a graphical representation of the specified volume to be harvested relative to a portion of the donor bone viewable via the visualization device. 
     Example 15: The method of example 14, further comprising determining a donor site within the donor bone. 
     Example 16: The method of example 15, wherein determining the donor site comprises: receiving video data from a camera; recognizing a donor bone from the video data; and measuring a donor volume within the donor bone that fits the specified volume to be harvested. 
     Example 17: The method of any of examples 14-16, further comprising: determining a change in position of the visualization device relative to the donor bone; and updating the display of the graphical representation of the specified volume to be harvested in response to determining the change in position so as to maintain a position of the specified volume to be harvested relative to the portion of the donor bone. 
     Example 18: The method of any of examples 14-17, wherein the virtual model comprises a cylinder within the donor bone. 
     Example 19: The method of any of examples 14-17, wherein the virtual model comprises a rectangular prism within the donor bone. 
     Example 20: The method of any of examples 14-19, wherein the donor bone comprises a head of a humerus of a patient. 
     Example 21: The method of any of examples 14-20, further comprising outputting for display additional surgical guidance information. 
     Example 22: The method of example 21, wherein the additional surgical guidance information comprises a recommended cutting tool. 
     Example 23: The method of example 21, wherein the additional surgical guidance information comprises a virtual cutting axis configured to guide an alignment of a cutting tool relative to the donor bone. 
     Example 24: The method of any of examples 14-23, wherein the system is configured to output for display the graphical representation of the specified volume to be harvested relative to the portion of the donor bone by registration of the graphical representation of the specified volume to be harvested. 
     Example 25: The method of any of examples 14-24, wherein the volume of the donor bone to be harvested as the bone graft comprises a first volume of the donor bone, the method further comprising: determining a second volume of the donor bone, wherein the first volume is configured to be extracted from the second volume; outputting for display, via the visualization device, surgical guidance information configured to guide a surgeon to resect the second volume from the donor bone; and outputting for display, via the visualization device, the graphical representation of the specified first volume to be harvested relative to the second volume viewable via the visualization device. 
     Example 26: A system for bone graft selection in orthopedic surgery, the system comprising: means for obtaining a virtual model of a bone graft to be harvested from a donor bone, wherein the virtual model of the bone graft specifies a volume of the donor bone to be harvested as the bone graft; and means for outputting for displaying graphical representation of the specified volume to be harvested relative to a portion of the donor bone. 
     Example 27: The system of example 26, further comprising means for performing the method of any of examples 15-25. 
     Example 28: A computer-readable storage medium storing instructions that when executed cause one or more processors to: obtain a virtual model of a bone graft to be harvested from a donor bone, wherein the virtual model of the bone graft specifies a volume of the donor bone to be harvested as the bone graft; and output for display, via the visualization device, a graphical representation of the specified volume to be harvested relative to a portion of the donor bone viewable via the visualization device. 
     Example 29: The computer-readable storage medium of example 28, further comprising instructions that cause the one or more processors to perform the method of any of examples 15-25. 
     While the techniques have been disclosed with respect to a limited number of examples, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations there from. For instance, it is contemplated that any reasonable combination of the described examples may be performed. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention. 
     It is to be recognized that depending on the example, certain acts or events of any of the techniques described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the techniques). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium. 
     By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc, where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     Operations described in this disclosure may be performed by one or more processors or processing circuitry, which may be implemented as fixed-function processing circuits, programmable circuits, or combinations thereof, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Fixed-function circuits refer to circuits that provide particular functionality and are preset on the operations that can be performed. Programmable circuits refer to circuits that can programmed to perform various tasks and provide flexible functionality in the operations that can be performed. For instance, programmable circuits may execute instructions specified by software or firmware that cause the programmable circuits to operate in the manner defined by instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations that the fixed-function circuits perform are generally immutable. Accordingly, the terms “processor” and “processing circuitry,” as used herein may refer to any of the foregoing structures or any other structure suitable for implementation of the techniques described herein. 
     Various examples have been described. These and other examples are within the scope of the following claims.