The bilateral lower extremities of four adult cadaveric specimens (8 knees) with intact hip, knee, and ankle joints were used in this study. The cadavers were ethically sourced and free of knee-related diseases. The mean age at the time of death was 65.5 ± 3.35 years with a mean body mass index of 23.8 kg/m2. Two of the four cadavers were females. The knee implants used were sourced from Johnson & Johnson and ICON (two common prostheses used in TKA in China). The osteotomy plan was created according to the preoperative software of the ROPA TKA system, and the implant prosthesis was installed after the robot-assisted osteotomy procedure was completed. The four surgeons who participated in this study were knee arthroplasty specialists and who were familiar with the basic principles and operating procedures of the ROPA TKA system. This training consisted of a theoretical training on the functions of the ROPA TKA system, the practice of what was learnt during 3 total knee replacements performed on sawbones and 2 TKAs performed on cadavers before the beginning of the study. To standardize the protocol, the target HKA angle was 180°, with 90° for both the tibial and femoral coronal angles. The femoral valgus angle (FVA) was preoperatively set to 6° and the PTS was set to 3°. Then, the results were compared with those of the bone cuts performed using the robotic system. For each bone cut, the resection thickness was measured with a caliper 3 times by 2 different observers and compared with the planned resection value [18].

The ROPA TKA system consists of three parts: the navigator, the robotic arm vehicle and the main control trolley. Figure 1 shows the ROPA TKA robot system and the schematic diagram of its placement in the operating room. In the experimental process, the navigator recognizes and tracks the optical tracer and provides real-time feedback regarding the positions of the power tool and the surgical area to the main control trolley. The main control trolley is embedded in the software of the ROPA TKA system, which can complete the preoperative planning and merge the real-time data of the robotic arm trolley and navigator to execute intraoperative navigation algorithms. The end of the robotic arm trolley is connected to a power tool, and based on the navigational information, it recognizes the safe area for bone cutting. Based on the navigational information, the safe zone for osteotomy is identified, and the power and activity range of the power tool are restricted to prevent excessive osteotomy or accidental damage to the ligaments and other soft tissues around the knee joint, thus assisting the operator in completing precise and safe osteotomy operations.

(A) The illustration of the ROPA TKA robot system. (B) The schematic diagram of the placement of the ROPA TKA robot system in the operating room

A 3D CT scan of each experimental cadaver was performed before the beginning of the experiment, and the obtained data were imported into the preoperative planning software of the ROPA TKA system in DICOM format. Then, CT segmentation and 3D reconstruction were performed to obtain a 3D model of the femur and tibia of the cadaver. Subsequently, osteotomy program planning was performed, including the angle and thickness of the osteotomies of the distal femur, anterior and posterior condyles and tibia, as well as the types of femoral and tibial prosthetic implants.

Experimental procedure: The surgeon performed preoperative planning with AI three-dimensional preoperative planning software to automatically determine the ideal resection thickness and angle for a balanced and well-aligned TKA procedure. Figure 2 shows the preoperative planning for a cadaveric trial of the ROPA TKA system, which shows the planned femoral and tibial angles as well as the balanced gap. All cadaveric knee replacements were performed using the medial approach. The cadaveric specimen was fixed on the experimental table, and the skin and subcutaneous tissues were incised sequentially to fully reveal the distal femur, the anterior condyle and the tibial plateau. Two rigid body trackers were placed on each cadaveric knee, one on the femur and one on the tibia, to align the robot after the robot was calibrated. Figure 3 shows the installation of navigation on the femur and tibia of a cadaver in preparation for bone resection after the completion of calibration using the ROPA TKA system. The ROPA TKA system can display the relative positions of the femur and tibia as well as the osteotomy flexion and extension gaps in real time, at which point the surgeon can confirm the adjustment of the osteotomy parameters according to the soft tissue condition of the specimen. All planned angle and resection thickness values were recorded. The osteotomy robot was then used to perform the distal femoral cut first, followed by the tibial cut. Figure 4 shows the measurement of intraoperative gap and osteotomy thickness.

The operator performs preoperative planning using specialised software to determine the ideal resection thickness and angle to obtain a balanced and well-aligned TKA.

The schematic diagram of intraoperative calibration and osteotomy. (A) and (B): The calibration of femoral and tibial on the ROPA TKA system. (C) The robotic arm of the ROPA TKA robotic system is fixed in the desired position determined by the surgical plan based on the operator’s intraoperative planning. Once the cutting jig is set and fixed in the correct position, the surgeon performs the cuts

(A) The illustration of intraoperative adjustment of osteotomy parameters according to the soft tissue condition of the specimen. (B) The chematic diagram of intraoperative tibial osteotomy on the ROPA TKA system

The ROPA assists doctors in performing intraoperative osteotomy operations through its osteotomy function. The position and angle information of the osteotomy surface are consistent with the preoperative plan, and the preoperative prosthesis planning osteotomy surface is accurately implemented through the positioning of the robotic arm. The software interface displays a total of 6 osteotomy surfaces for the femur and tibia (femoral anterior condyle, femoral anterior oblique, distal femur, femoral posterior oblique, femoral posterior condyle, and tibial plateau). Among them, the femoral anterior condyle, femoral posterior condyle, and tibial plateau provide safe boundary protection (to prevent damage to ligaments during the osteotomy process). During the osteotomy process, the software provides real-time feedback on the position and angle of the robotic arm’s movement, ensuring the visualization and accuracy of the intraoperative operation data. After completing femoral and tibial osteotomy. The prosthesis was fitted after the accuracy was confirmed, and the femoral and tibial tracers and fixation nails were then removed. Finally, the incision was closed.

The observers were trained in the method for measuring bone block thickness before the experiment and participated in intraoperative resection thickness measurements after stabilizing the measurements. The thickness of intraoperative resections was measured using a calibrated Vernier caliper (Mitutoyo, Japan). After each incision with the ROPA TKA system, the thickness of the resected bone was measured. Each cut was measured 3 times by 2 different independent observers. For each cadaver, the thickness of the cuts of the distal femur, anterior and posterior femoral condyles and proximal tibia was measured.

To verify the accuracy of the prosthetic position, each specimen was examined by X-ray. The cadaveric specimen was placed in the lying position, with both lower limbs straightened, internally rotated by 15°, and the patella facing anteriorly. The joint position of the cadaveric specimen was fixed with sponge pads. Orthopantomographs of the hip, knee, and ankle joints were taken, and the DICOM files of the three radiographs were exported and merged to form a full-length radiograph of the lower limbs. Then, the image was imported into Image-Pro software, which was used to measure the postoperative HKA, FFC, FTC, and PTS. HKA is formed by lines connecting the centers of the femoral head, the knee and the talus. The FFC is the lateral angle between the femur mechanical axis and the line across the bottom of the femoral condyles. The FTC is the medial angle between the tibial mechanical axis and the line across the bottom of the tibial plateau. The PTS is the angle between the articular surface of the tibial plateau and the horizontal line on lateral X-ray of the lower limb. The detailed measurement schematic is shown in Fig. 5. To ensure measurement accuracy and reduce measurement error, two nonparticipating orthopedic surgeons with rich measurement experience obtained the measurements, and if the difference between two measurements was too -large (≥ 0.5°), a third nonparticipating orthopedic surgeon obtained the measurement, and the final result was taken as the mean of the two similar measurements.

Measurement of HKA, FFC, FTC, FVA. (A) Line a was the femoral mechanical axis, and line b was the tibial mechanical axis; the medial angle formed between them was recorded as the HKA. (B) Line c was the line across the bottom of the femoral condyles, line d was the line across the bottom of the tibial plateau on the anteroposterior radiograph; the lateral angle between line a and line c was recorded as FFC, and the medial angle between line b and line d was recorded as FTC. (C) Line e was the anatomical axis of the femur, the acute angle between line a and line e was recorded as FVC.

The data were analyzed using SPSS version 26 (IBM, Armonk, NY, USA). After the normality of the data was checked, descriptive statistics (mean, standard deviation and extreme deviation) were calculated. Continuous data are expressed as the mean and standard deviation. The proportion of differences within ± 1° and ± 2° was calculated for alignment values. Similarly, the proportions of differences within ± 1 mm and ± 2 mm were calculated for resection thicknesses. Hypotheses were proposed, and the difference between the ROPA robotic system navigation group and preoperative planning group was calculated to test whether the difference was normally distributed. When there were missing data or obvious outliers that had a large impact on the results, the obvious outliers in the data were removed, and replaced with the mean or median values. Paired t tests were selected for data with a normal distribution, and the signed rank sum test was selected for data with a skewed distribution. Conclusions were drawn based on the test P value. Confidence intervals of 95% were set a priori for both the t test and the 99% prediction interval. P < 0.05 was considered to indicate statistical significance.