Augmented reality surgical guidance aids total knee arthroplasty
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Computer-assisted total joint arthroplasty has become more popular in the 2 decades since its introduction.
There are a range of options now available, including robotics and hand-held navigation devices, to assist with surgical planning and provide intraoperative guidance.
Source: Michael P. Nett, MD, FAAOS
Augmented reality-driven surgical guidance systems, such as the ARVIS (Augmented Reality Visualization and Information System) system (Enovis), which is a proprietary system used exclusively with Enovis EMPOWR hip and knee products, provide a new take on surgical guidance because these reduce the surgical guidance system to something that is worn and controlled by the surgeon that can seamlessly integrate into the surgical workflow.
The system has three components: an augmented reality (AR) headset that mounts to a surgical helmet and contains tracking cameras, headlight and AR display; a belt pack that powers the headset and runs the surgical guidance software; and one tray of reusable instruments that can be used for a total knee arthroplasty, unicondylar knee arthroplasty or total hip arthroplasty performed with the EMPOWR hip and knee implant systems. With the light, the headset weighs 260 grams. The only disposables with the system are two pins that are used to affix the reference trackers to the bone.
AR headset
The headset contains lenses that project the AR display into the user’s field of view, as well as infrared cameras that pick up the trackers on the anatomy and instruments. The headset mounts to a surgical helmet or an AR headband, and normal personal protective equipment is worn over the helmet (Figure 1).
Once the headset is on and the device is powered on, an AR display will appear a few feet in front of the surgeon’s view. The cursor on this display follows the surgeon’s head movement. Basic voice commands allow the surgeon to move forward, start navigation, stop navigation and place the system in sleep mode (Figure 2).
Source: Enovis Surgical
The first screen is used to select which procedure is taking place (hip or knee). Then the options screen can be used to set the operative side and preferences, such as for workflow and target alignment.
Surgical technique overview
For TKA, the reference trackers are placed inside the incision, which eliminates the need for puncture wounds outside of the incision. After registration of strategic landmarks and the guidance mode are entered, the ARVIS AR system will display numerical values along with anatomical reference images that allow the surgeon to precisely place femoral and tibial cut guides in optimal positions for that patient. Optimal positioning of the cutting guides is based on the goal of neutral mechanical alignment for most patients who undergo TKA. For a routine varus knee, that alignment is 3° flexion on the femur and neutral axial alignment. On the tibia, the tibial slope is set to 4° and the goal is for neutral mechanical alignment. The surgeon can set targets for any desired position based on surgeon preference and patient anatomy.
Throughout the procedure, instructions and clinically relevant values are displayed immediately in the surgeon’s field of view on the AR display. A tablet that mirrors what is shown on the AR display can be used optionally so the surgical team can see the surgeon’s vantage point.
The final screen for the procedure displays a dashboard with all the clinically relevant values for the surgeon to review. The displayed information can be recorded and placed in the patient’s medical record. The application can now be shut down or restarted if it was used for a bilateral TJA procedure.
Femoral registration
Once the femur is exposed, the surgeon attaches the reference tracker to the femur using double-headed threaded pins. The marking on this tracker should face the surgeon so it can be easily picked up by the cameras on the eyepiece that is mounted on the helmet.
After the tracker is attached, the surgeon uses the stylus to register the required anatomic landmarks to navigate the distal femoral resection. Once all required trackers are in view, the points are collected, and the location of that landmark is registered within the software (Figure 3).
Source: Enovis Surgical
The first step in every procedure is to register the checkpoint by placing the tip of the stylus at the tip of the reference tracker.
Once the checkpoint is complete, the surgeon registers the knee center by placing the tip of the registration stylus at the knee center. Then the anteroposterior (AP) axis is registered by aligning the axis of the stylus to the AP axis of the femur. Finally, the hip center is registered by rotating the leg in a circular motion. The center of rotation from this step is calculated and captured as the hip center.
In my patients with arthritic hips, the hip is not routinely radiographed, however these patients undergo a hip examination. If the exam shows the patient has a stiff hip that is arthritic, I recommend we proceed with total hip arthroplasty first, before proceeding with TKA. Replacing the knee below a stiff, arthritic hip may lead to a painful and stiff TKA from the referred pain and the inability of the patient to adequately rehabilitate following the knee surgery.
The company reports it is unaware of users who have had difficulty registering the hip center in a patient with an arthritic hip. However, the company provides the following contraindications for use of its AR system for TKA:
- hip ankylosis or other hip condition that limits range of motion of the hip to less than 10° abduction/adduction or flexion/extension; and
- severe hip arthritis or other disease of the hip that causes it to no longer approximate a ball joint.
The software has a check that requires a minimum range of motion of 20° for both abduction/adduction and flexion/extension.
Pin femoral cutting block
As the distal femoral cutting block is being positioned, the AR display shows values for varus/valgus and flexion/extension angles. Depth is mechanically set to 10 mm using paddles on the distal guide. When the block is in the correct location based on the plan for the patient’s anatomy, the surgeon pins the cutting block and makes the distal femoral resection.
If desired, the system can be used to check the cut. This is done by placing a flat tracker against the resected surface to confirm the varus/valgus and flexion/extension angles (Figure 4).
Source: Enovis Surgical
This optional step can be used to set rotation relative to the AP axis by default or relative to the transverse epicondylar axis if the option to register the medial and lateral epicondyle was selected at the beginning of the procedure. To do so, the surgeon places the tracker in the desired rotation, uses a marker or cautery to mark this angle on the bone and then uses this marking to guide placement of the AP sizer.
At this point, the navigated femoral steps are complete. The tracker can be removed, and the regular femur workflow can resume either now or after the proximal tibial resection has been made.
Tibial registration
The surgeon attaches the reference tracker to the medial tibial plateau using a single-headed threaded pin with the markings facing the surgeon, taking care not to cover the lowest part of the plateau because that point will be registered later. It is also important that tracker placement leaves sufficient room for the extramedullary (EM) guide.
When the cutting block is in the optimal position for the patient, the surgeon pins the block and places the tracker into the cutting slot of the proximal tibial cut guide, which is attached to the EM tibial guide. Once the camera on the eyepiece identifies the trackers, the screen will transition to the “set resection plane” view (Figure 5). Once the desired values are achieved, guidance is stopped. The tracker is removed from the cutting slot, the reference tracker is removed from the tibial plateau and the tibial resection is made. At this point, the guidance steps are complete.
Source: Enovis Surgical
The remainder of the procedure is completed with the surgeon’s preferred instrumentation and following the surgeon’s preferred workflow.
- Reference:
- Chytas D, et al. Front Surg. 2019;doi:10.3389/fsurg.2019.00038.
- For more information:
- Michael P. Nett, MD, FAAOS, chair of orthopedic surgery at South Shore University Hospital in Bay Shore, New York, can be reached at mnett@northwell.edu.
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