Patent Publication Number: US-2023146371-A1

Title: Mixed-reality humeral-head sizing and placement

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 63/017,428, entitled “MIXED-REALITY HUMERAL-HEAD SIZING AND PLACEMENT,” and filed on Apr. 29, 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, such as with a prosthetic implant device. 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 a joint replacement surgery. A computing device may identify a resected bone surface; determine an implant size and alignment to match the resected bone surface; and output for display, via a visualization device, a graphical representation of the implant relative to the resected bone surface viewable via the visualization device. 
     In this manner, the example techniques provide a technical solution for accurately guiding a surgeon through a joint replacement surgery, such as an arthroplasty. For instance, the example techniques provide for practical applications of preoperative and intraoperative planning utilizing image processing for facilitating accurate implant sizing and alignment. 
     In one example, the disclosure describes a system for guiding a joint replacement surgery, including a visualization device comprising one or more sensors; and processing circuitry configured to: determine, based on data generated by the one or more sensors, one or more size parameters of a bone resection surface viewable via the visualization device; select, based on the one or more size parameters of the bone resection surface and from a plurality of implants, an implant; and output for display, via the visualization device, a graphical representation of the selected implant relative to the bone resection surface. 
     In one example, the disclosure describes a method for guiding a joint replacement surgery, including determining one or more size parameters of a bone resection surface viewable via a visualization device; selecting, based on the one or more size parameters of the bone resection surface and from a plurality of implants, an implant; and outputting for display, via the visualization device, a graphical representation of the selected implant relative to the bone resection surface. 
     In some examples, a computer-readable storage medium includes instructions to cause one or more processors to determine one or more size parameters of a bone resection surface viewable via a visualization device; select, based on the one or more size parameters of the bone resection surface and from a plurality of implants, an implant; and output for display a graphical representation of the selected implant relative to the bone resection surface. 
     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. 
         FIG.  4    is a schematic representation of a visualization device for use in an MR system, according to an example of this disclosure. 
         FIG.  5    is a conceptual diagram of an MR system including a visualization device configured to guide a joint replacement surgery, in accordance with one or more techniques of this disclosure. 
         FIG.  6    is a conceptual diagram of an orthopedic surgical system that includes an MR system, according to an example of this disclosure. 
         FIGS.  7 - 9    are conceptual diagrams depicting one or more example overlaid MR graphical user interface (GUI) elements that may be generated and displayed on a visualization device, in accordance with one or more techniques of this disclosure. 
         FIG.  10    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 (e.g., “implants”) to repair or replace a patient&#39;s damaged or diseased joint. Prosthetic devices may be manufactured in a variety of different sizes. Selection of a “correct” size for a prosthetic device may be critical for patient outcomes. For example, a well-fit implant may improve a range-of-motion for the repaired joint. Further, a well-fit implant may improve the contact between the repaired joint and the surrounding tissue. In some examples, improved contact between the repaired joint and the surrounding tissue may help to shorten post-surgical recovery times. 
     Virtual surgical planning tools may use image data of the diseased or damaged joint to generate an accurate three-dimensional bone model and/or an implant model that can be viewed and manipulated preoperatively and/or intraoperatively 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/or 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 may result 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. 
     In accordance with one or more techniques of this disclosure, a computing device may generate information indicative of a respective size and fit for an implant to be coupled to a target site, such as a resected bone surface. The surgeon may utilize the generated information to select a particular implant from among a plurality of differently sized implants. The surgeon may also utilize the generated information intraoperatively for surgical guidance, such as to assist in precise alignment of the selected implant. In this way, the techniques of this disclosure may improve patient outcomes by improving the range-of-motion for the repaired joint, improve the contact between the repaired joint and the surrounding tissue, and shorten post-surgical recovery times. 
     For example, processing circuitry (e.g., processing circuitry of one or more computing devices) may be configured to determine at least one implant model for an implant to be connected to a first anatomical object (e.g., target bone). “Implant model” refers to a representation of size and shape of the prosthetic devices that is to be coupled to a target site. The implant model may be a graphical representation that can be displayed, such as intraoperatively on a mixed reality (MR) visualization device. The implant model 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 implant model. 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 implant model should look like. The processing circuitry may determine the implant model 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 implant model. 
     The processing circuitry may determine information indicative of placement of the implant model relative to a representation of an anatomical object (e.g., target 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 target site (e.g., images of the target site). The processing circuity may determine information indicative of how the implant model fits relative to the representation of the target 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 implant model relative to the representation of the target site. In response to the movement of the implant model, the processing circuitry may determine information needed to move the representation of the implant model (e.g., information such as coordinates of where the implant model is to be displayed). The processing circuitry may then output information indicative of the placement of the implant model relative to the representation of the target site (e.g., output graphical information used to render the implant model with the representation of the target site). 
     As another example, the processing circuitry may be configured to utilize the points or shape equations of the implant model and the points in the representation of the target site to determine how to place the implant model relative to the representation of the target site. The processing circuitry may utilize certain criteria in determining information indicative of the placement such as information that defines boundaries within the target site to where an implant may be coupled. For instance, the boundary may define certain configurations in which an implant may be coupled so as to substantially align with the implant site, where for other configurations of the implant model relative to the target site, there may be discrepancies in alignment of the implant. For example, the target site may include a substantially planar resected bone surface, configured to be coupled to a substantially planar surface of the implant. However, because the resected bone surface may include a different size and/or shape than the planar surface of the implant, the resected bone surface may exhibit undesired portions of “overhang” or “underhang” between the two surfaces. For example, as shown in  FIG.  7 A  below, an “overhang” may indicate a region wherein an edge of the prosthetic device extends past a corresponding edge of the resected bone surface. Conversely, an “underhang” may indicate a region where an edge of the prosthetic device falls short of the corresponding edge of the resected bone surface, or equivalently, where the edge of the resected bone surface extends past the corresponding edge of the prosthetic device. In some examples, these types of unmatched portions may present a possibility of cosmetic defect, injury, or susceptibility to injury, for example, due to contact between the misaligned edges and the surrounding tissue. The processing circuitry may output the determined information (e.g., graphical information used to render the implant model relative to the representation of the target site). 
     There may be various ways in which the surgeon may preoperatively view image content such as the implant model, placement of the implant model at the target site, and additional surgical guidance information. Also, in some examples in accordance with this disclosure, the surgeon may be able to view the implant model, placement of the implant model at the target site, and additional 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, MM, 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 (MM) 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  200  that may include a preoperative surgical planning system  202  and a mixed reality system  212  to guide or otherwise assist a surgeon to repair an anatomy of interest of a particular patient. For example, a surgical procedure may include an orthopedic joint repair surgical procedure, such as one of a standard total shoulder arthroplasty or a reverse shoulder arthroplasty. In these examples, the surgical procedure may include 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/or 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 a size and/or alignment for an implanted prosthetic device. For example, in orthopedics, a prosthetic implant is commonly used for joint reconstruction. Surgeons may select a particular implant size according to the available area and shape of the target site, such as a resected bone surface. However, the desired implant may be either too large or too small for the target site and its implantation (e.g., position and/or alignment with the target site) 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 a shoulder arthroplasty, a prosthetic humeral head implant is coupled to a resected surface of the humerus (e.g., the humerus is the target site). If the implant coupled to the resected humeral surface is too small (e.g., a portion of the prosthetic humeral head implant underhangs the resected bone surface), there may be possibility that the implantation 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 (e.g., stem or nucleus inserted into the humerus). 
     In accordance with one or more techniques described in this disclosure, mixed reality system  212  (MR system  212 ) may determine, based on image data for one or more images of anatomical objects, at least one virtual implant model for an implant to be connected to the anatomical object depicted in the image data. MR system  212  may receive the images via one or more image sensors, such as one or more cameras included in a visualization device worn by a surgeon. The images of the anatomical objects may include representations (e.g., as image data) of anatomical objects, such as a resected bone surface. 
     MR system  212  may analyze the image data to determine one or more size parameters of the resected bone surface depicted in the image data. Based on the determined size parameters, MR system  212  may determine at least one virtual implant model for an implant to be connected to the anatomical object depicted in the image data. For example, visualization device  213  may be configured to display a representation of a plurality of differently sized or differently shaped virtual implant models, each virtual implant model displayed relative to the resected bone surface viewable through the device  213 . The surgeon, viewing the representation of each of the plurality of implant models, may determine the size and shape of the implant that is to be connected to the resected bone surface. The surgeon may interact with the displayed representation to resize, position, and align an implant model based on the size and shape of the target site (e.g., the resected bone surface and/or an implant stem implanted within the resected bone surface). 
     In some examples, storage system  206  may store a plurality of pre-generated implant models of various size and shapes. Visualization device  213  may display the pre-generated implant models, and the surgeon may select one of the pre-generated implant models. Processing device(s)  210  may output information of the selected pre-generated implant model to preoperative surgical planning system  202  and/or MR system  212 . 
     In some examples, preoperative surgical planning system  202  and/or MR system  212  may be configured to determine the bone implant model for the implant, and possibly with little to no intervention from the surgeon. For example, preoperative surgical planning system  202  may be configured to determine a size and/or shape of a first anatomical object, such as the resected bone surface. 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 a statistical shape model (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 implant model to the premorbid shape of the first anatomical object. For example, preoperative surgical planning system  202  may determine a difference between each of a plurality of implant models 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 implant model, for example, by selecting the implant model that would be most similar to the premorbid shape of the first anatomical object with respect to size and/or position. For instance, preoperative surgical planning system  202  may determine an implant model that has the approximately the same size and shape as 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 implant model relative to a virtual representation of the anatomical object (e.g., target site) based on the image data. For example, the image data 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 3D virtual representation of the anatomical object, such as the target site. Although described with respect to a 3D representation, in some examples, visualization device  213  may display 2D scans of target site. 
     Using visualization device  213 , the surgeon may “drag and drop” one or more virtual implant models (e.g., as drawn by the surgeon or as determined by preoperative surgical planning system  202 ) onto the virtual representation of the target site. In some examples, the surgeon may translate or rotate the implant model along the x, y, and/or z axis before or after dragging and dropping the implant model onto the representation of the target site. 
     In some examples, preoperative surgical planning system  202  may be configured to perform the calculations of rotating the implant model and calculating the coordinates of the implant model for aligning the implant model to the coordinate space of the representation of the anatomical object. For example, the implant model and the representation of the anatomical object may be in different coordinate systems, and to move the implant model to the representation of the anatomical object (e.g., target site), preoperative surgical planning system  202  may determine a transformation matrix that provides for rotation, translation, scaling, and shearing, as needed so that the implant model and the 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 implant model is in the coordinate system of the anatomical object or before the implant model is in the coordinate system of the anatomical object, the surgeon may rotate the implant model to view the implant model from different perspectives. Preoperative surgical planning system  202  performing the above example operations of aligning the coordinate system, rotating, and moving the implant model into the representation of the anatomical object are non-limiting examples of preoperative surgical planning system  202  determining information indicative of a placement of the implant model relative to a representation of the 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 implant model relative to a representation of the 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 implant model relative to a representation of the 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 implant model to the coordinate system of the anatomical object. Preoperative surgical planning system  202  may then, based on the coordinates of the implant model (e.g., coordinates along the boundary of the implant model) and coordinates of the anatomical object, move the implant model to be aligned with the representation of the anatomical object. For instance, preoperative surgical planning system  202  may rotate and shift the implant model so that the implant model aligns with the representation of the anatomical object. 
     Accordingly, preoperative surgical planning system  202  may compare a size and shape of the implant model to the representation of the 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 implant model relative to a representation of the anatomical object based on the image data. 
     In the above examples, the implant model is described as being aligned with the coordinate system of the anatomical object. In some examples, the anatomical object may be aligned with the coordinate system of the implant model. 
     As another example, preoperative surgical planning system  202  may determine whether a particular placement of the implant would result in complicated surgery, preoperative surgical planning system  202  may determine that the particular placement is not a valid placement of the implant model. For example, if placement of the implant in a particular location would result in the implant not being accessible or require complicated surgery (e.g., excessive shifting of bone, higher changes of complication, etc.) to access the implant, then preoperative surgical planning system  202  may determine that the such placement of the implant model is not valid. 
     There may be other criteria that preoperative surgical planning system  202  may utilize when determining information indicative of placement of the implant model relative to the representation of the 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 anatomical object is potentially suitable as a target site for the implant. For example, preoperative surgical planning system  202  may utilize the various criteria to determine whether the implant model can be placed in the anatomical object. If there are no valid placements for the implant model, preoperative surgical planning system  202  may output information indicating that the anatomical object may not be suitable as a target site. If there are valid placements for the implant model, preoperative surgical planning system  202  may output information indicating that the anatomical object is suitable as a target site. 
     In some examples, there may be multiple ways in which the implant model can fit relative to the anatomical object. Preoperative surgical planning system  202  may output the various valid options indicating where the implant model can be coupled to (e.g., aligned with) the 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 on the anatomical object from where the implant may be coupled with the greatest ease while minimizing overhang and/or underhang between coupled planar surfaces). 
     Preoperative surgical planning system  202  may be configured to output information indicative of the placement of the implant model relative to the representation of the anatomical object. As one example, preoperative surgical planning system  202  may generate information used by visualization device  213  to render the implant model relative to the representation of the anatomical object at the determined placement. As another example, preoperative surgical planning system  202  may generate coordinate values of the location of the implant model. 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 implant model relative to the representation of the anatomical object (e.g., target site). 
     In some examples, preoperative surgical planning system  202  may be configured to generate pre-operative planning information based on placement of the implant model relative to the representation of the anatomical object. For example, the information indicative of the placement of the implant model may include information indicative of where the implant model is located relative to the representation of the anatomical object. The implant model may therefore provide a visual indication of where to couple the implant. 
     As one example, preoperative surgical planning system  202  may be configured to generate information indicative of a location relative to the anatomical object where the implant is to be coupled. 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 types of a tool to utilize to couple the implant to the target site. Visualization device  213  may display the types of tools preoperatively and/or intraoperatively. A tool may include, for example, an offset adaptor configured to couple the implant to an implant stem implanted within the target site. Visualization device  213  may indicate one or more offset values, such as a size, an offset distance, and an offset orientation of the offset adaptor in order to couple the implant to the desired location at the target site. 
     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 an implant model for an implant to be connected to an anatomical object, determine information indicative of placement of the implant model relative to a representation of the anatomical object based on the image data, and output the information indicative of the placement of the implant model relative to the representation of the anatomical object. 
       FIG.  4    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.  4   , 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.  4   , 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.  4    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.  5    is a conceptual diagram of a mixed reality system including a visualization device  213  configured to guide a joint repair or replacement surgery in accordance with one or more techniques of this disclosure. In some examples, visualization device  213  (described further with respect to  FIG.  4   , above) may contain processing circuitry configured to at least identify a target implant site on an anatomical object for a prosthetic implant. For example, visualization device  213  may include one or more cameras  530 ,  532  ( FIG.  4   ) configured to capture image data depicting a bone, such as humerus  500 , in the field-of-view (FOV)  508  of the cameras  530 ,  532 . Processor(s)  514  ( FIG.  4   ) of visualization device  213  may be configured to receive the image data from the cameras and identify an anatomical object within the image data. For example, processor(s)  514  may be configured to execute image-recognition software to identify an anatomical object within the image data. As shown in  FIG.  5   , the anatomical object may include a substantially planar resected bone surface  502 , configured to receive (e.g., match with) a corresponding planar surface of a prosthetic implant  700  ( FIG.  7   ). 
     In some examples, processor(s)  514  may be configured to recognize one or more colors of an exposed bone surface from the image data. For example, processor(s)  514  may be configured to recognize a first color of an outer cortical layer of an exposed bone surface, and/or a second color of an inner cancellous section of an exposed bone surface. In some examples, processor(s)  514  may be configured to identify an exposed bone surface from the image data by searching for a particular shape. For example, processor(s)  514  may be configured to recognize a substantially spherical intact humeral head or a substantially circular resected humeral head from within the image data. In some examples, processor(s)  514  may additionally be configured to identify, from the image data, an implant stem  504  implanted within the resected bone surface  502 , as well as the respective center  506  of a taper connection  510  of implant stem  504 . 
     As shown in  FIG.  6   , once visualization device  213  has identified an anatomical object such as a resected bone surface  502  and a center  506  of an implant stem  504 , visualization device  213  may further be configured to determine one or more size parameters of the respective bone surface. For example, visualization device  213  may be configured to determine a size (e.g., diameter  601 ) and relative position of a largest inscribed circle (e.g., an “incircle”)  602  that fits entirely within the boundaries of the area of resected bone surface  502 . Based on the size and the position of incircle  602 , visualization device  213  may additionally identify the center  604  of the incircle (e.g., the “incenter”) relative to resected bone surface  502 . Visualization device  213  may additionally identify one or more offset values, such as an offset distance  606  and an offset orientation (e.g., angle) between center  506  of implant stem  504  and incenter  604 . 
     Similarly, visualization device  213  may be configured to determine a size (e.g., diameter  607 ) and relative position of a smallest circumscribed circle (e.g., a “circumcircle”)  608  that fits entirely outside the boundaries of the area of resected bone surface  502 . Based on the size and the position of circumcircle  608 , visualization device  213  may additionally identify the center  610  of the circumcircle (e.g., the “circumcenter”) relative to resected bone surface  502 . Visualization device  213  may additionally identify one or more offset values, such as an offset distance  612  and an offset orientation (e.g., angle) between center  506  of implant stem  504  and circumcenter  610 . 
     In some examples, visualization device  213  may be configured to display one or more virtual user input devices (e.g., selectable widgets  524  of  FIG.  4   ) through which a user may indicate one or more user preferences to MR system  212 . For example, selectable widgets  524  may include virtual devices such as virtual buttons, virtual slider bars, etc., with which an orthopedic surgeon may select or indicate one or more preferences for a relative size, shape, position, and/or orientation for a prosthetic implant device (e.g., implant  700  of  FIGS.  7 A and  7 B ). As one example, a surgeon may use selectable widgets  524  to adjust the size of circumcircle  608  in order to indicate a preferred maximum implant size. As another example, a surgeon may use selectable widgets  524  to adjust the size of incircle  602  in order to indicate a preferred minimum implant size. In some examples, incircle  602  and/or circumcircle  608  may include virtual objects with which the user may directly interact. For example, a user may adjust the size, position, and/or orientation of incircle  602  and/or circumcircle  608  with one or more hand gestures, such as by “pinching” or “spreading” the respective circle. In some examples, a surgeon may use selectable widgets  524  to indicate a preferred relative weighting or ranking for a medial-lateral position and/or an anterior-posterior position for a prosthetic implant. In some examples, a surgeon may use selectable widgets  524  or another virtual interface to indicate preferences for other parameters, such as an amount of overhang  702  or underhang  704  ( FIGS.  7 A and  7 B ) between bone surface  502  and a prosthetic implant, or an alignment between the implant and bone surface  502  along a particular direction. In some examples, a surgeon may indicate these parameters via a manual input device, such as a keyboard, mouse, touchscreen, etc. 
     Based on the one or more size parameters of resected bone surface  502 , as well as the determined incircle  602  and circumcircle  608  and/or additional surgeon preferences, MR system  212  may be configured to determine (e.g., select, create, or identify) and output for display on visualization device  213  at least one prosthetic implant configured to match resected bone surface  502 . For example, as shown in  FIGS.  7 A and  7 B , the prosthetic implant may include a semi-spherical prosthetic humeral head, having a substantially planar circular surface configured to be coupled to (e.g., aligned with) planar resected bone surface  502 . As one specific example, processor(s)  514  of visualization device  213  may be configured to retrieve from memory  516  a set of data describing one or more differently sized implants having dimensions constrained to “fit” resected bone surface  502 . For example, the diameter of the circular planar surface of the implant may be constrained between the diameters  601  and  607  of incircle  602  and circumcircle  608 , respectively. 
     In some examples, visualization device  213  may be configured to automatically select a “best fit” implant from among a plurality of implants stored in memory  215 , such as based on indicated surgeon preferences and/or additional parameters. For example, visualization device  213  may be configured to select an implant that reduces a discrepancy between resected bone surface  502  and planar implant surface  706 . For example, a discrepancy between resected bone surface  502  and planar implant surface  706  may include one or more “overhang” regions  702 , wherein the implant  700  “hangs over” or extends past resected bone surface  502 , as well as one or more “underhang” regions  704 , wherein resected bone surface  502  hangs over or extends past implant  700 . 
     In some examples, visualization device  213  may be configured to select an implant  700  that most-closely approximates the native (e.g., premorbid) bone structure. For example, visualization device  213  may determine one or more size dimensions of the native bone structure from received image data, such as from historical x-ray, CT scan, or MRI image data, and select an implant  700  from memory having similar size dimensions. 
     In some examples in accordance with this disclosure, MR system  213  may be configured to select from memory  215  a plurality of differently sized implants  700 , and output a virtual graphical representation of each prosthetic implant for display on transparent screen  520  ( FIG.  4   ) of visualization device  213 . For example, a wearer or user of visualization device  213  may observe real-world elements through transparent screen  520 , with the virtual implant  700  laid over top of the real world elements either alone or in combination with other additional virtual graphical objects. For example, as shown in  FIGS.  7 A and  7 B , visualization device  213  may be configured to display virtual implant  700  in a fixed position relative to a real observed bone structure, such as the resected surface  502  of humerus  500  of a patient undergoing arthroplasty. In particular, visualization device  213  may be configured to detect (e.g., identify) resected bone surface  502 , display virtual implant  700  overtop of resected bone surface  502 , and “lock” virtual implant  700  in place with respect to resected bone surface  502 . In other words, motion sensors  533  of visualization device  213  ( FIG.  4   ) may be configured to track a motion of visualization device  213  with respect to humerus  500 , and update the displayed position of virtual implant  700  with respect to resected bone surface  502 , such that virtual implant  700  retains its position relative to humerus  500  as viewed by the user or wearer. For example, visualization device  213  may be configured to “lock” the displayed position of virtual implant  700  with respect to resected bone surface  502  (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  uses 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 virtual implant  700  with an observed bone structure (e.g., resected bone surface  502 ). 
     In some examples, preoperative planning system  102 , MR system  212 , and/or visualization device  213  receives data indicative of virtual implant  700  as well as a virtual model of the target implant site (e.g., resected surface  502  of humerus  500 ). The data may indicate a fixed location of the virtual implant  700  with respect to the surface  502  of humerus  500 . Preoperative planning system  102  identifies a point or region of interest on the surface of the virtual target implant site and a virtual normal vector to the point (or region) of interest on the surface of the region. MR system  212  connects 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 target implant site also moves and rotates in space. 
     In the example of a shoulder arthroplasty procedure, the point of interest on the surface of virtual target implant site can be an approximate center of the resected bone surface  502  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 target implant site 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. 
     The ability to move and rotate virtual target implant site in space about the user&#39;s gaze point alone generally is not sufficient to orient virtual target implant site with the actual observed bone (e.g., humerus  500 ). Thus, as part of the initialization procedure, MR system  212  also determines the distance between visualization device  213  and a point (or points) on the surface of the observed bone surface  502  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 observed bone surface  502  can be approximated by determining a vector that is normal (i.e., perpendicular) to a point (e.g., a central point) on the surface. This normal vector is referred to herein as the “observed normal vector.” 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, distance information can be derived by MR system  212  from depth camera(s)  532  ( FIG.  4   ). This distance information can be used to derive the geometric shape of the surface of an observed bone  502 . That is, because depth camera(s)  532  provide distance data corresponding to any point in a field of view of depth camera(s)  532 , the distance to the user&#39;s gaze point on the observed bone  504  can be determined. With this information, either visualization device  213  can automatically, or the user can manually, move the virtual target bone model in space and approximately align it with the observed bone  502  at a point or region of interest using the gaze point. That is, when the user shifts gaze to observed bone structure  502 , the virtual bone model (which is connected to the user&#39;s gaze line) moves with the user&#39;s gaze. The user can then align the virtual bone model with observed bone structure  502  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 the virtual bone model. For instance, once the virtual bone model is approximately aligned with observed bone structure  502 , 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 the virtual bone model 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, the virtual bone model is oriented with the observed bone  502  by aligning the virtual normal vector and the observed normal vector. 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 the virtual bone model relative to observed bone structure  502 . 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 the virtual bone model with observed bone structure  502  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 the virtual bone model with observed bone structure  502 . 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 virtual bone model 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 the combined virtual implant model  700  and virtual bone model has been completed, the surgeon may elect to command MR system  212  (e.g., visualization device  213 ) to stop displaying the virtual bone model, and instead, only display the virtual implant model  700  fixed relative to the actual observed bone  502 , as shown in  FIGS.  7 A and  7 B . For example, visualization device  213  may be configured to directly display virtual implant model  700  “locked” in position (e.g., registered) with respect to observed resected surface  502  of humerus  500 , without displaying the virtual bone model. 
     By displaying virtual implant model  700  intraoperatively, the techniques of this disclosure may improve the alignment of a prosthetic implant by allowing an orthopedic surgeon to select and align an implant that is customized to fit the specific patient. For example, once virtual implant model  700  is registered with (e.g., fixed with respect to) observed resected bone surface  502 , visualization device  213  may be configured to allow a user, such as a surgeon, to customize (e.g., adjust) an alignment of each virtual implant model  700  relative to resected bone surface  502 . For example, visualization device  213  may be configured to receive user input, such as by detecting a hand gesture or receiving verbal cues, indicating a change in position of virtual implant model  700  relative to resected bone surface  502 . After adjusting for the indicated change, visualization device  700  may re-register the virtual implant model  700  to lock virtual implant model  700  in place with respect to resected bone surface  502 . In doing so, visualization device  213  may allow the surgeon to observe and select a preferred customized position for virtual implant model  700 . 
     In some examples, visualization device  213  may be configured to display both virtual implant model  700 , as well as a virtual representation of the patient&#39;s native or premorbid bone structure, including the resected bone surface  502 . For example, based on data, such as CT scan data, x-ray data, or other imaging data, MR system  212  may be configured to generate a virtual model of the patient&#39;s premorbid bone structure and output the virtual premorbid bone model for display on visualization device  213 . Using a side-by-side comparison of the virtual premorbid bone model and virtual implant model  700 , a surgeon may visually determine (e.g., select or confirm) a particular virtual implant model  700  that is most similar to the patient&#39;s premorbid bone structure. 
     In some examples, visualization device  213  may be configured to display additional surgical guidance information configured to guide a surgeon through performing the surgical operation, including coupling the prosthetic implant to the resected bone surface  502 . For example, upon adjusting the relative position of virtual implant model  700  with respect to resected bone surface  502  based on user input, visualization device  213  may additionally determine an offset between the center of the new position of virtual implant model  700  and the center  506  of implant stem  504 . For example, as described with respect to  FIG.  6    above, visualization device  213  may determine both an offset distance and an offset orientation between the respective centers. As shown in  FIGS.  8 A and  8 B , based on the offset distance and the offset orientation, visualization device  213  may further determine (e.g., select) a particular size for an offset adaptor  800  configured to affix the prosthetic implant to the implant stem  504 . Visualization device  213  be configured to output a visible and/or audible indication  801  of the selected sized offset adaptor  800  (e.g., as a recommendation). In some examples visualization device  213  may further output indications  806 ,  808  that may assist the surgeon to properly orient the offset adaptor  800  with respect to resected bone surface  502 . 
     In some examples, offset adaptor  800  may be configured to rotate about the center  506  of implant stem  504 , and offset adaptor  800  may include a notch  802 , or any other suitable indicator, to indicate a relative alignment angle of offset adaptor  800 . In some examples, but not all examples, the indicator of the relative alignment angle of offset adaptor  800  can be provided by a structural feature of offset adapter  800 , such as notch  802 . In other examples, the indicator of relative alignment angle can be provided in other ways, such as via a marking on the surface of offset adaptor  800 , or, in some examples, via a virtual marking output by visualization device  213  that can appear to the user as if the virtual marking is on, at, or part of offset adapter  800 . In some of such examples, visualization device  213  may output one or more graphical elements to indicate an alignment status of offset adaptor  800 . As one example, visualization device  213  may output a visual indicator, such as arrow  804 , indicating a “correct” alignment angle for offset adaptor  800 . The offset adaptor may be considered to be in the “correct” alignment when arrow  804  points directly at notch  802 . As another example, visualization device  213  may output a visual indication when offset adaptor  800  is in an “incorrect” alignment, such as the “X” shape  806  ( FIG.  8 A ), as well as a visual indication when the offset adaptor  800  is in the “correct” alignment, such as the “check mark” indicator  808  ( FIG.  8 B ). Once the surgeon has aligned offset adaptor  800  in the correct orientation according to the additional surgical guidance information, the surgeon may affix the selected prosthetic implant to offset adaptor  800 . 
       FIG.  9    is a conceptual diagram including one or more example overlaid graphical user interface (GUI) elements that MR system  212  may generate and display on visualization device  213  ( FIG.  4   ), in accordance with one or more techniques of this disclosure. In particular,  FIG.  9    depicts a humerus  500  undergoing a reversed shoulder arthroplasty (RSA), as detailed further above. 
     As shown in  FIG.  9   , in some examples, visualization device  213  may be configured to intraoperatively display one or more graphical elements relative to humerus  500  during an RSA. For example, visualization device  213  may be configured to display a virtual implant  906  relative to humerus  500  viewable via a transparent screen  520  of visualization device  213 . In the example of  FIG.  9   , virtual implant  906  includes offset tray  916 , insert  918  (such as a polyethylene insert) and taper connection  912 . Similar virtual planning techniques to those described above with respect to standard or “anatomical” shoulder arthroplasty procedures may also apply with regard to RSA procedures. For example, visualization device  213  may be configured to identify, from received image data depicting an exposed surgical site, a target implant site (e.g., a resected bone surface  502  of humerus  500 ), register a virtual prosthetic implant  906  to the target implant site as detailed further above, and output for display virtual prosthetic implant  906  in a fixed position relative to the target implant site. For example, the fixed position may include both a relative location and a relative orientation with respect to the target implant site. 
     In some examples, MR system  212  may receive user input allowing a surgeon to adjust (e.g., customize) the size, shape, position, orientation, or alignment of any or all of the virtual elements displayed on visualization device  213 . For example, MR system  212  may receive user input, such as by detecting hand gestures, virtual input devices, etc., allowing a surgeon to adjust a position of virtual implant  906  along the plane of resected bone surface  502 . For example, taper connection  912  may be offset from the center of tray  916 , such that a rotation angle of taper connection  912  adjusts an alignment of implant  906  relative to resected bone surface  502 . In some examples, MR system  212  may select a rotation angle for implant  906  that causes implant  906  to be approximately centered relative to resected bone surface  502  (e.g., a rotation angle that does not result in a substantial overhang or underhang between resected bone surface  502  and tray  916  along any one particular circumferential region). 
     In some examples, MR system  213  may receive user input enabling a surgeon to customize a “height” of virtual implant  906  relative to resected bone surface  502 . For example, the surgeon may adjust one or more size and/or position parameters of offset tray  916  and/or insert  918  (e.g., along the anterior-posterior plane). In some examples, visualization device  213  may display a graphical element indicating a reconstruction distance or height h between high point  902  of virtual implant  906  and high point  910  on the greater tuberosity of humerus  500 , allowing a surgeon to further virtually plan the RSA procedure. For example, a surgeon may indicate a preference to reduce or minimize the reconstruction height h such that the high point  902  on reverse tray  916  is approximately aligned (e.g., along a horizontal axis) with greater tuberosity  910 . MR system  212  may be configured to automatically determine and indicate a rotation angle for implant  906  that reduces or minimizes this parameter h. 
     In some examples, MR system  212  may be configured to register (as detailed further above) a virtual humeral stem  914  to a physical humeral stem already implanted within humerus  500  and display the virtual humeral stem  914  as a further visual aid for determining a size and/or alignment of virtual implant  906 . For example, based on a visible portion of the physical humeral stem (e.g., a planar portion viewable along resected bone surface  502 ) and/or additional user input, MR system  212  may determine an approximate location of the physical humeral stem and output for display on visualization device  213  a corresponding virtual implanted humeral stem  914  relative to humerus  500 , e.g., with humeral stem  814  displayed “inside of” humerus  500  as though humerus  500  were transparent. The displayed virtual humeral stem  914  may further enable the surgeon to customize a location, size, and/or orientation for virtual implant  906  by indicating the approximate location of virtual taper connection  912  within virtual stem  914 . 
     In some examples, MR system  212  may generate and output for display on visualization device  213  additional surgical guidance information in order to assist a user to select a respective size and relative position for virtual implant  906 . For example, as shown in  FIG.  9   , MR system  212  may determine and output for display a virtual center of rotation  908  of the repaired joint, which may inform the user (e.g., the surgeon) of a projected range of motion of the repaired joint based on the selected relative size, position, and/or orientation of virtual implant  906 . 
       FIG.  10    is a flowchart illustrating an example method of operation  1000  in accordance with one or more techniques described in this disclosure. Although the techniques of  FIG.  10    are described with respect to MR system  212  of  FIG.  2    and visualization device  213  of  FIG.  4   , the techniques may be performed by any appropriate system and/or virtual-reality display device. 
     MR system  212  may receive image data, such as from one or more cameras, depicting a target site to affix a prosthetic implant. For example, the target site may include one or more anatomical objects, such as a resected bone surface  502 . Using image-recognition software, visualization device  213  may identify the target site within the image data ( 1002 ). 
     Based on the identified target site, MR system  212 , using one or more sensors, may determine one or more size parameters or other measurements of the target implant site ( 1004 ). For example, visualization device  213  may determine a length or width of the target implant site, or may determine a size and relative position for a circle that either fits entirely within (e.g., an incircle) or outside of (e.g., a circumcircle) the target implant site. 
     Based on the determined size parameters, MR system  212  may determine at least one prosthetic implant device configured to “fit” or match the target implant site ( 1006 ). For example, visualization device  213  may retrieve from memory  215  a plurality of differently sized prosthetic implants each having size parameters within a predetermined range based on the determine size parameters of the target implant site. 
     For each of the selected prosthetic implants, MR system  212  may be configured to output for display a virtual model  700  of the respective implant. The virtual implant model  700  may be displayed on a transparent display screen  520  and “fixed” in a position relative to the target implant site viewable through the display screen ( 1008 ). For example, visualization device  213  may display virtual implant model  700  in a relative position with respect to the position of the target implant site of the patient such that the hologram appears directly over the top of the target site. Visualization device  213  may further track the motion of transparent screen  520  with respect to the target implant site so that it may “update” the display of virtual implant model  700  so that the virtual model appears “locked” in place with respect to the implant site. While each virtual implant model  700  is displayed on transparent screen  500 , visualization device may receive user input indicating an intended change in position of virtual implant model  700  relative to the target implant site. For example, a surgeon or other user of MR system  212  may “customize” the alignment according to personal preferences (e.g., professional opinions). 
     In some examples, MR system  212  may output additional surgical guidance information ( 1010 ). For example, based on a selected prosthetic implant size and alignment, visualization device  213  may output visible and/or audible indications to assist the surgeon to precisely align the selected implant to the selected alignment. For example, visualization device  213  may output an indication of a recommended offset adaptor size and orientation configured to connect the selected prosthetic implant to a stem implanted within the target implant site. 
     The following examples are described herein. Example 1: A system for guiding a joint replacement surgery, the system comprising a visualization device comprising one or more sensors; and processing circuitry configured to determine, based on data generated by the one or more sensors, one or more size parameters of a bone resection surface viewable via the visualization device; select, based on the one or more size parameters of the bone resection surface and from a plurality of implants, an implant; and output for display, via the visualization device, a graphical representation of the selected implant relative to the bone resection surface. 
     Example 2: The system of example 1, wherein, to select the implant, the processing circuitry is further configured to: determine, based on the one or more size parameters, a diameter of the implant and a position of the implant relative to the bone resection surface; identify, based on data generated by the one or more sensors, a center of a taper connection of an implant stem implanted within the bone resection surface; and determine, based on the position and the identified center, one or more offset values for the selected implant. 
     Example 3: The system of example 2, wherein the one or more offset values comprise an offset distance and an offset orientation. 
     Example 4: The system of example 2 or example 3, wherein the one or more offset values comprise an offset adaptor size. 
     Example 5: The system of any of examples 1-4, wherein the visualization device comprises a see-through holographic lens configured to display the graphical representation as a hologram. 
     Example 6: The system of any of examples 1-5, the processing circuitry further configured to output for display, via the visualization device, a graphical representation of a native resected bone, including the bone resection surface, relative to the graphical representation of the implant. 
     Example 7: The system of any of examples 1-6, the processing circuitry further configured to determine a change in position of the visualization device relative to the bone resection surface; and update the display of the graphical representation of the selected implant in response to determining the change in position so as to maintain a position of the graphical representation relative to the bone resection surface. 
     Example 8: The system of any of examples 1-7, wherein the bone resection surface comprises a humeral resection surface; and the plurality of implants comprise prosthetic humeral heads. 
     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 graphical element indicating a correct offset orientation of an offset adaptor. 
     Example 11: The system of example 10, wherein the graphical element comprises an arrow having a color indicative of the correct offset orientation. 
     Example 12: A method for guiding a joint replacement surgery, the method comprising determining one or more size parameters of a bone resection surface viewable via a visualization device; selecting, based on the one or more size parameters of the bone resection surface and from a plurality of implants, an implant; and outputting for display, via the visualization device, a graphical representation of the selected implant relative to the bone resection surface. 
     Example 13: The method of example 12, wherein selecting the implant comprises: determining, based on the one or more size parameters, a diameter of the implant; identifying a center of a taper connection of an implant stem within the bone resection surface; and determining, based on the identified center, an offset value for the selected implant. 
     Example 14: The method of example 13, wherein the offset value comprises an offset distance and an offset orientation. 
     Example 15: The method of example 13 or example 14, wherein the offset value comprises an offset adaptor size. 
     Example 16: The method of any of examples 12-15, further comprising outputting for display, via the visualization device, a graphical representation of a native resected bone relative to the graphical representation of the selected implant. 
     Example 17: The method of any of examples 12-16, further comprising determining a change in position of the visualization device relative to the bone resection surface; and updating the display of the graphical representation of the selected implant in response to determining the change in position so as to maintain a position of the graphical representation relative to the bone resection surface. 
     Example 18: The method of any of examples 12-17, wherein the bone resection surface comprises a humeral resection surface; and the plurality of implants comprise prosthetic humeral heads. 
     Example 19: The method of any of examples 12-18, further comprising outputting for display additional surgical guidance information. 
     Example 20: The method of example 19, wherein the additional surgical guidance information comprises a graphical element indicating a correct offset orientation of an offset adaptor. 
     Example 21: The method of example 20, wherein the additional surgical guidance information comprises an arrow having a color indicative of the correct offset orientation. 
     Example 22: A system for guiding a joint replacement surgery, the system comprising: means for determining one or more size parameters of a bone resection surface viewable via a visualization device; means for selecting, based on the one or more size parameters of the bone resection surface and from a plurality of implants, an implant; and 
     means for outputting for displaying a graphical representation of the selected implant relative to the bone resection surface. 
     Example 23: The system of example 22, further comprising means for performing the method of any of examples 12-21. 
     Example 24: A computer-readable storage medium storing instructions that when executed cause one or more processors to determine one or more size parameters of a bone resection surface viewable via a visualization device; select, based on the one or more size parameters of the bone resection surface and from a plurality of implants, an implant; and output for display a graphical representation of the selected implant relative to the bone resection surface. 
     Example 25: The computer-readable storage medium of example 24, further comprising instructions that cause the one or more processors to perform the method of any of examples 12-21. 
     While the techniques 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 circuity,” 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.