Augmented reality-aided unicompartmental knee arthroplasty – Journal of Experimental Orthopaedics

This retrospective single-arm cohort study was approved by the institutional ethics board. We reviewed 11 consecutive UKAs performed using the AR-based portable navigation system to proximal tibial resection between January 2020 and November 2021. All patients provided written informed consent.

All surgeries were performed by one surgeon (ST) using a Persona Partial Knee System (Zimmer-Biomet, Warsaw, IN). Tibial bone resection was performed using the AR-based portable navigation system. The target angle of tibial resection was determined for each patient using the original varus angle and posterior slope of the proximal tibia as references. In the coronal plane, we aimed to resect the proximal tibia perpendicular to the mechanical axis of the femur except in patients with preoperative varus angle of the proximal tibia ≥ 6°, in whom we set the target varus angle to 2° or 3° because varus alignment exceeding 4° was reported to be associated with translation in the mediolateral direction [16]. In the sagittal plane, we aimed to resect the proximal tibia with the same angle of the preoperative posterior slope of the medial compartment in each patient. However, we reduced the target angle in patients with a preoperative posterior slope ≥ 6° because excessive posterior slope has been shown to be associated with greater tension of the anterior cruciate ligament and excessive translation in the mediolateral direction [16]. Following tibial bone resection, the distal femur was resected according to the spacer block technique. All prostheses were implanted using Simplex P bone cement (Stryker, Mahwah, NJ).

The tibial resection angle was measured using a standing long-leg radiograph with ImageJ software (US National Institutes of Health, Bethesda, MD). The coronal alignment was measured with reference to the perpendicular line connecting the midpoint of the tibial plateau and the midpoint of the tibial plafond [17]. The sagittal alignment was measured with reference to the perpendicular line connecting the anterior one third of the medial tibial plateau and midpoint of the tibial plafond [17]. Angles were recorded to two decimal places and rounded off to one decimal place. The accuracy of the AR-based navigation system was assessed by calculating the difference between the preoperative target angle and postoperative measured angle. Any complications were recorded with special attention to periprosthetic fracture.

Surgical technique of tibial bone resection using AR-based portable navigation system

AR technology projects digital information onto the real world [11, 12]. The AR-based navigation system enables the surgeon to see the tibial mechanical axis on the patient’s leg and the tibial cutting angle in real-time.

The extramedullary tibial cutting guide of the AR-based navigation system carries two markers with Quick Response (QR) codes (Figs. 1 and 2). The extramedullary tibial cutting guide of the AR-based navigation system was set on the patient’s lower leg in a similar manner to the standard cutting guide. First, the ankle clamp was placed proximal to the malleolar. Second, the surgeon inserted one pin to fix the cutting guide parallel to the anteroposterior axis of the tibia because the AR-based navigation system was programmed to recognize the direction of the pin as the anteroposterior axis (Fig. 2).

AR-based portable navigation system enables the surgeon to see the tibial mechanical axis in the surgical field through the smartphone (green line indicated by red arrow). On the smartphone display, the color of the marker turns blue after the smartphone camera recognized the QR code. The extramedullary tibial cutting guide carries two markers with QR codes

The surgeon aligns the cutting block of the proximal tibia while viewing the varus/valgus alignment, posterior slope, and medial resection depth on the smartphone display. Based on preoperative planning, the surgeon set the cutting block on the varus angle of 0.1°, posterior slope of 6.6°, and medial depth of 5 mm (red arrow). First, one fixation pin (white arrow) is inserted parallel to the anteroposterior axis to fix the extramedullary guide. Second, another pin (white arrowhead) is inserted to fix the cutting block. Note that no extra pins are required to attach the sensor of the AR-based navigation system compared with standard extramedullary guide and cutting block

The sensor of the AR-based navigation system is the camera of the smartphone. The smartphone camera recognizes the QR code of the extramedullary tibial cutting guide. Using a pointer marked with a QR code, the surgeon registers three bony landmarks: the most prominent point of the medial malleolus, the most prominent point of the lateral malleolus, and the tibial center on the tibial plateau (Fig. 3). Visualization of registration points is a major advantage of the AR-based navigation system, which enables surgeon to easily recognize whether the registered point is wrong or not (Fig. 4). The navigation system creates a 3-dimensional tibial coordinate system to express the position of the tibial cutting guide. The three lines constituting the 3-dimensional coordinate system of AR-based navigation are: (1) the tibial mechanical axis; (2) the tibial anteroposterior axis; and (3) the cross product of these two tibial axes. The tibial mechanical axis is defined as the line connecting the center of the ankle and the tibial center on the tibial plateau. The registration of medial compartments of the tibial plateau is used for the resection level of the tibia, allowing the surgeon to view the proximal tibia resection level.

Registration of the medial malleolus using a pointer marked with a QR code (red arrow). Note that the AR-based portable navigation system works if the smartphone recognizes only one of the two QR codes

Awareness of inappropriate registration. As the AR-based navigation system can visualize the registration point, the surgeon can easily recognize which point is inappropriate. The registration point of medial malleolus floats on the patient’s leg in this patient (red arrow)

After completing registration, the AR-based navigation system enables the surgeon to view the reference lines superimposed on the tibia on their smartphone display (Fig. 1). The surgeon can also view the angles of varus/valgus and posterior slope on the display (Fig. 2). The surgeon fixes the tibial resection block while viewing these angles. As the guide pinhole below the tibial prosthesis can be a stress point increasing the risk of tibial stress fracture [1], the cutting block of the AR-based navigation system has only one pinhole that is located at the cross point of the vertical and horizontal bone cut lines (Fig. 2). Following fixation of the tibial resection block, the surgeon cuts the proximal tibia in the standard manner.

After tibial bone resection, the AR-based navigation system allows the surgeon to verify the actual cutting angle and depth of resection. The display of the smartphone shows varus/valgus angle, tibial slope angle, rotation angle, and the thickness of bone resection (Fig. 5).

Verification of tibial bone cutting. The AR-based navigation system allows the surgeon to confirm the actual cutting angle and depth of resection