The protocol follows guidelines of the Human Research Ethics Committee of the Medical University of Warsaw. The patient gave informed consent to the procedure and acknowledged the fact that it will be recorded. The patient agreed to that prior to the procedure.

NOTE: The basic criterion for including the patient in the surgery project was the necessity to intervene because of the anatomical dysfunction, which made it impossible to use a standard implant. Mixed reality was aimed at better placement of the prosthesis, increasing the chances of successful surgery.

1. Preparation

1. Prepare a custom-made acetabular implant and plan the surgical procedure before the patient's hospitalization.
   NOTE: In this case study, persons skilled in the art of medical image diagnostics prepared the custom-made acetabular implant.
   1. Before the planned admission to the hospital, perform an x-ray in the diagnostic imaging unit.
   2. Perform a pelvic X-ray in an anterior-posterior projection.
   3. Assess the current condition of the patient's pelvis based on the X-ray.
   4. Compare the image obtained with the earlier X-ray images.
2. Take a pelvic CT scan and acquire DICOM (Digital Imaging and Communications in Medicine) files according to the protocol.
   1. Place the patient on the moveable CT scan platform.
   2. Click on the thickness button and select 512 x 512 px thickness for the scans.
   3. Click on the parameter that determines the thickness of the 1 mm layer.
   4. Start the procedure by clicking, wait for the test result.
3. Ask an engineer to design an implant proposal that can be sent digitally as a technical scheme, or a 3D printed model prototype (Figure 1).
   1. Visualize the computed tomography result in the DICOM viewer.
   2. Determine the needs for the implementation of the implant, taking into account the current anatomical conditions of the patient, biomechanics, and the function of the joint.
   3. Consult the engineer for suggestions on implants, including fixation.
   4. Approve the project and wait for shipment.
      NOTE: The final shape of the implant involves combining 3D data from a patient's CT scan with the input of a design engineer and a surgeon.
4. Print the custom-made 3D implant from the titanium alloy powder (TiAl6V4) using electron beam technology^26,27. Inside a chamber containing small amounts of TiAl6V4 powder, each time the electron beam is fired, there is selective melting and accumulation of material (plasma coating).
5. Check whether the implant was sterilized. Sterilization of implant trials and the final implant was guaranteed by the manufacturer.

2. Pre-surgery checkups

1. Perform a standard of care laboratory tests and specialist consultations.
   1. Exclude patients with potential periprosthetic joint infection (no radiological features, normal c-reactive protein (<10 mg/L), and erythrocyte sedimentation rate of 1-10 mm/h for women, 3-15 mm/h for men).
2. Check for the clinical signs of infection such as fever (systemic), pain, swelling, redness (local), and reduced joint function²⁸.
   1. Exclude patients having signs of local inflammation during the clinical examinations (redness, temperature increase, pain, swelling and loss of function indicate local inflammation). The patient gave complete informed consent to the operation.

3. Mixed reality model

NOTE: The process is performed to achieve proper implant and pelvis visualization, which will be used intraoperatively.

1. Process the pelvic CT DICOM file into a holographic representation using dedicated application.
   1. Load the CT image into mixed reality headset from acquired CT DICOM files.
      1. Open holographic DICOM Viewer.
      2. Select the folder containing CT DICOM files.
      3. Check the IP that is displayed when the headset is switched on and enter it in a designated place in the holographic DICOM Viewer.
      4. Click on the Connect button to be able to see the visualization in the mixed reality headset.
   2. Segment the pelvis bone tissue structures. This is performed manually by using the Scissors option. When the option is enabled, the user clicks the left mouse button and moves the mouse around to remove the structures that are selected with this tool.
      1. End the selection with another click of the left mouse button, which creates a pop-up for the user to confirm that he/she wants to cut out the structures that are selected.
         NOTE: The user can choose areas to be cut out from the visualization in both 3D and 2D views. It is possible to remove the structures from within or outside the selection. This is repeated until only the necessary parts of the CT image are visible.
   3. Choose a predefined transfer function (color visualization parameters) dedicated to orthopedic procedures from the list of available functions by clicking on its name: CT Bone Endoprosthesis. If needed, adjust the visualization by changing the window and level using the right mouse button connected with mouse movement in the 3D visualization window.
   4. Connect to the headset to see the prepared visualization in the 3D holographic space. Adjust the image using voice commands: Rotate, Zoom, Cut Smart, and Hand Gestures.
   5. Use the Cut Smart command to use and adjust a cutting plane that is perpendicular to user's line of sight. The closer the user moves the head into the hologram, the deeper the plane goes.
   6. Perform these movements to see the inner parts of the visualization because structures that are located anteriorly to the plane are not visualized.
      NOTE: This view is important to assess the geometrical relations between structures (pelvis, femur, and implant) (Figure 2 and Figure 3).

4. Surgery

1. Perform the surgical procedure of revision hip arthroplasty due to aseptic loosening of the acetabular component with the use of a custom-made acetabular implant and mixed reality device^14,16,29. Use a scalpel, an electrosurgical knife with a coagulator, a Luer tool and cutters for the operation.
   1. Give 1.5 g of ceftriaxone intravenously 30 min prior to the skin incision, and two subsequent doses are to be given on the day of the surgery to prevent infection. Initiate thromboprophylaxis on the day before the surgery with low molecular weight heparin (LMWH). Continue the single daily dose of 40 mg enoxaparin for 30 days after the procedure.
   2. Place and secure the patient under general anesthesia, lying down on the operating table.
   3. Release the connective tissue adhesions using the Hardinge access to the hip joint and remove the loose acetabular implant.
   4. Perform the operation in the same way as other revision procedures of the hip joint, but use a wider access.
   5. Remove all soft tissues from the surface of the acetabulum so that the shape is exactly the same as in the model provided. The implant model must perfectly adhere to the surface of the acetabular bone.
   6. Fix the new uncemented implant using specially designed screws that stabilize the implant.
   7. Perform a femoral nerve block after surgery.
2. Intraoperative holographic visualization of processed images
   1. Load the visualization of the DICOM CT scan prepared in the pre-procedural planning to the mixed reality application.
   2. Connect the mixed reality headset to the mixed reality application to see the prepared visualization in the 3D holographic space.
   3. Use intraoperative holographic visualization of the processed images to achieve adequate and precise pelvic bone surface preparation as well as for removal of the excess of connective tissue that developed as a response to loosening of the acetabular component.
   4. Ensure that the operator looks at the holographic visualization as a reference image.
   5. Use a scalpel, an electrosurgical knife with a coagulator, a luer tool, and cutters for the operation. Visualization of the 3D pelvis model should minimize the risk of damaging neurovascular structures and mistakes in implant placement.
   6. Ensure that the head-mounted display is connected to the workstation through a WiFi network. The processing of the images and the rendering is performed on the workstation and the results are displayed on the headset as holograms. Use gestures and voice commands. If necessary, get help from an engineer with POV preview.

5. Postoperative care

1. Make the patient undergo a standard rehabilitation and recovery protocol, including rehabilitation and mobilization on the first day after surgery^30,31,32.
   NOTE: Rehabilitation was implemented by a dedicated team experienced in the hip and knee arthroplasty.
2. Implement pharmacological thromboprophylaxis. Thromboprophylaxis was initiated on the day before the surgery with low molecular weight heparin (LMWH). The single daily dose of 40 mg enoxaparin was continued for 30 days after the procedure.