Summary
This protocol presents a cadaveric study of a wireless sensor used in medial unicompartmental knee arthroplasty. The protocol includes the installation of an angle measuring device, standardized Oxford unicompartmental knee arthroplasty osteotomy, preliminary assessment of flexion-extension balance, and application of the sensor to measure flexion-extension gap pressure.
Abstract
Unicompartmental knee arthroplasty (UKA) is an effective treatment for end-stage anteromedial osteoarthritis (AMOA). The key to UKA is the flexion-extension gap balance, which is closely related to postoperative complications such as bearing dislocation, bearing wear, and arthritis progression. The traditional gap balance assessment is performed by indirectly sensing the tension of the medial collateral ligament by a gap gauge. It relies on the surgeon's feel and experience, which is imprecise and difficult for beginners. To accurately assess the flexion-extension gap balance of UKA, we developed a wireless sensor combination consisting of a metal base, a pressure sensor, and a cushion block. After osteotomy, the insertion of a wireless sensor combination allows the real-time measurement of intra-articular pressure. It accurately quantifies the flexion-extension gap balance parameters to guide further femur grinding and tibia osteotomy, to improve the accuracy of gap balance. We conducted an in vitro experiment with the wireless sensor combination. the results showed that there was a difference of 11.3 N after applying the traditional method of flexion-extension gap balance performed by an experienced expert.
Introduction
Knee osteoarthritis (KOA) is a global burden1, for which the stepwise treatment strategy is currently adopted. For end-stage unicompartmental KOA, unicompartmental knee arthroplasty (UKA) is an effective choice, with a 10-year survival rate of over 90%2. Medial UKA only replaces the severely worn medial compartment and preserves the natural lateral compartment, medial collateral ligament (MCL), and cruciate ligament3. The principle is to make the flexion gap and extension gap approximately the same by tibial osteotomy and femoral grinding, and to restore MCL tension after implantation of the prosthesis and bearing4. Compared with total knee arthroplasty, UKA has greater surgical difficulty and technical requirements. The main source is the proper balance of ligaments throughout the full range of motion of the knee3.
Traditionally, after preliminary osteotomy, the surgeon inserts a gap gauge in the joint space and indirectly determines whether the flexion and extension gaps are equal by feeling the tension of the MCL. However, the definition and sensation of balance are hardly the same, even for experienced surgeons. For beginners, it is more difficult to grasp the requirement of balance. The imbalance of the flexion-extension gap can lead to a series of complications5,6, resulting in an increased revision rate.
With the advancement of technology, some researchers have tried to apply tensors to UKA7,8. However, these researches are all on the fixed-bearing UKA, and the tensor may damage the MCL when used.
The emergence of sensors not only meets the demand for displaying the pressure in the knee joint gap, but various sensors often have less risk of MCL damage due to their small size9,10. In addition, the sensors currently used are all wired transmission, which may interfere with the aseptic operation and is not convenient enough to use.
In order to accurately measure the flexion-extension gap balance parameters, we developed a wireless sensor combination for UKA, which consists of a metal base, a wireless sensor with three pressure probes on the front, medial, and lateral sides, and a cushion block. The sensor combination measures and displays the pressure in the joint space in real time to help surgeons accurately assess whether the balance target has been achieved.
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Protocol
The protocol was approved by the Ethics Committee of Xuanwu Hospital (grant number: 2021-224) and was conducted in accordance with the Declaration of Helsinki. Informed consent was obtained from next of kin to use the cadavers.
1. Installation of angle measuring device
- Turn on the switch of the femur and tibia angle measuring device. Open the angle measurement software on the tablet computer, scan the QR codes of the two measuring devices, and click Bluetooth connection.
- Place the two angle measuring instruments on the horizontal table, click the Calibration button to calibrate, and tie them with straps 10 cm above and below the knee to measure the flexion angle of the knee in real time (Figure 1).
2. Standardized Oxford UKA osteotomy
- Place a cadaver in the supine position with the lower extremities draped in flexion and abduction over the outside of the operating table.
- Open the joint cavity by the medial parapatellar approach with a scalpel. Make a cut 3 cm distal to the joint line along the apex of the medial border of the patella, ending distally at 1 cm medial to the tibial tuberosity. Ensure the incision depth reaches the joint cavity.
- Remove osteophytes of the medial femoral condyle, intercondylar fossa, and anterior tibia using a rongeur.
- Insert different sizes of femoral sizing spoons to hook the posterior femoral condyle, and when the end of the spoon is about 1 mm away from the cartilage surface, the size of the femoral prosthesis corresponding to the spoon is suitable.
- Select a 3 mm G-clamp. Connect the G-clamp, tibial saw guide, and femoral sizing spoon together. Ensure that the guide's shaft is parallel with the long axis of the tibia in both the coronal and sagittal planes, and the ankle yoke points toward the ipsilateral anterior superior iliac spine.
- Make vertical and horizontal cuts on the tibia. Use the reciprocating saw to make a vertical tibial saw cut. Ensure the cut is just medial to the apex of the medial tibial spine. Advance the saw vertically down until it rests on the surface of the saw guide.
- Remove the shim from the tibial resection guide and insert the slotted 0 shim. Use the oscillating saw blade to excise the plateau. Remove the slotted shim, lever the plateau up with a broad osteotome, and remove it with the knee in extension.
- Make a hole in the distal femoral condyle. Ensure the hole is situated 1 cm anterior to the anterior edge of the intercondylar notch and in line with its medial wall.
- Insert the intramedullary rod in the hole. Connect the femoral drill guide with the intramedullary rod. Perform femoral drilling with the help of the femoral drill guide.
- Install the posterior resection guide and insert it into the drilled hole. Ensure the oscillating saw blade is guided by the underside of the posterior resection guide and perform the posterior femoral condyle osteotomy. Remove the guide and the bone fragment.
- Excise the medial meniscus. Leave a small cuff of the meniscus to protect the MCL. Completely remove the posterior horn.
- Insert a 0 femoral spigot. Attach the spherical mill onto the spigot and perform distal femoral milling.
3. Preliminary assessment of the flexion-extension gap
- Insert a femoral trial. Assess flexion-extension gap by gap gauge.
- Use angle measurement devices to monitor the flexion angle. Define the appropriate flexion-extension gap balance by inserting the gap gauge into the joint space with slight resistance, and the perceived tension as almost equal when the knee is in the flex position at 20° (extension gap) and 110° (flexion gap). If the gaps are not equal, grind the femur according to the difference value between the flexion and extension gaps until they are equal.
4. Application of sensor combination to measure flexion and extension gap pressure
- Remove the sensor magnetic induction power switch. Open the pressure measurement software on the tablet computer, scan the sensor QR code, and enter the measurement interface.
- Click the Connect Device button; the sensor will be automatically calibrated after successful connection.
- Select the appropriate thickness cushion block according to the gauge specification. Put the sensor on the metal base and install the cushion block on the sensor (Figure 2).
- Click Start working on the tablet computer. Insert the wireless sensor combination into the medial compartment and fit the metal base to the tibial osteotomy surface (Figure 3).
- Measure the flexion and extension gap pressure at 110° (Figure 4A,B) and 20° (Figure 4C, D) of knee flexion. Calculate the average values separately for three consecutive measurements.
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Representative Results
This in vitro study was performed on a 60-year-old female cadaver. With the S-size femoral prosthesis and 3 mm bearing the target, after performing femoral grinding and tibial osteotomy, the surgeon used the gap gauge to assess flexion-extension gap tension preliminarily and believed that balance was achieved.
After the femoral trial was installed, the wireless sensor was inserted into the medial joint space, and the intra-articular pressure was measured three times at 110° (flexion gap) and 20° (extension gap) of flexion. The flexion-extension gap pressure was 49.9 N-44.8 N, 47.1 N-25.9 N, and 42.0 N-34.2 N (Table 1). The pressure values for the flexion gap were quite consistent, while the pressure values for the extension gap were quite different. The average pressure in the flexion and extension gaps were 46.3 N and 35.0 N, respectively, with a mean difference of 11.3 N. Postoperative radiographs showed appropriate prosthesis positioning (Figure 5).
| Measurement times | Intra-articular pressure (N) | |
| Flexion 110° (flexion gap) | Flexion 20° (extension gap) | |
| 1 | 49.9 | 44.8 |
| 2 | 47.1 | 25.9 |
| 3 | 42.0 | 34.2 |
| Mean | 46.3 | 35.0 |
Table 1: Intra-articular pressure measured by the sensor.
Figure 1: The angle measurement devices. (A) The angle measurement devices were installed 10 cm above and below the center of the knee. (B) The measurement software can display the knee flexion angle in real time. Please click here to view a larger version of this figure.
Figure 2: The structure of the wireless sensor combination. The wireless sensor combination consists of (A) a metal base, (B) a wireless sensor with three pressure probes (yellow arrows), (C) and a cushion block. (D) The combination formed after nested assembly. Please click here to view a larger version of this figure.
Figure 3: Application of sensor combination. After the osteotomy and installation of the femoral trial, the wireless sensor combination is inserted in the medial compartment for measurement. Please click here to view a larger version of this figure.
Figure 4: Position to measure pressure. (A) The flexion gap pressure was measured at 110° of knee flexion; (B) the flexion gap pressure was 49.9 N. (C) The extension gap pressure was measured at 20° of knee flexion; (D) the extension gap pressure was 44.8 N. Please click here to view a larger version of this figure.
Figure 5: Postoperative imaging. Postoperative antero-posterior radiograph showed good tibial component positioning and coverage. A postoperative lateral radiograph showed good positioning and flexion angle of the femoral component. Abbreviations: AP = antero-posterior; LT = lateral). Please click here to view a larger version of this figure.
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Discussion
Mobile-bearing UKA is an effective treatment for anteromedial KOA. It has the advantages of less trauma, quick recovery, and maintaining normal knee proprioception11,12,13. The key to UKA is flexion-extension balance; that is, making the flexion gap and extension gap as equal as possible on the premise of restoring MCL tension14. The imbalance may lead to bearing dislocation, prosthesis wear, or progression in the lateral compartment15,16,17,18. Balance techniques are usually related to the surgeon's experience, which affects patient satisfaction and prosthesis survival.
Gap gauge is a widely used UKA gap balance tool now. The surgeon inserts the gap gauge into joint space and feels the gap tension to roughly determine whether the flexion and extension gap is balanced. This approach relies heavily on the surgeon's sensation and experience, so it is difficult for beginners to achieve a precise balance, which is one of the reasons for the steep learning curve of UKA and the development of prosthetic complications. In addition, this method does not meet the requirements of millimeter level femur grinding in UKA.
Subsequently, tensors were applied to UKA's gap balance assessment19. Tensors can apply a constant distraction force to joint space to restore the tension of the MCL. By measuring the distracting distance of the joint space, it can accurately measure the flexion and extension gap. However, because the tensor can exert different distraction forces, the distracting distance of the joint space changes when the MCL is not restored to normal tension or the MCL is over-distracted to injury. At present, an appropriate distraction force that can match different bearing thicknesses has not been agreed on7,8,19.
Different from the above two rough measurement tools, the wireless sensor we used is embedded with three integrated pressure probes, which can display the intra-articular pressure during the knee's full range of motion in real time. The wireless sensor converts the traditional rough feeling of MCL tension into accurate intra-articular pressure, and with the aid of an angle measuring device, surgeons can accurately assess flexion-extension balance. For surgeons, especially beginners, this can effectively assist in a precise osteotomy, shorten the learning curve, and improve the surgical effect.
In order to match different sizes of bearings and femoral prostheses, wireless sensors are also available in various sizes. In the process of use, the most important thing is to select an appropriate cushion block for the sensor according to the osteotomy plan; otherwise, it may lead to wear of the osteotomy surface and damage the flexion-extension gap balance.
Previous studies have reported sensors with fewer embedded pressure probes resulting in lower accuracy or wired transmission that does not meet the aseptic requirements during surgery20,21,22,23,24,25. The wireless sensor we used considers both measurement accuracy and aseptic requirements. In this in vitro study, we found that even an experienced surgeon could not achieve complete equivalence of flexion and extension gaps. To determine a reasonable pressure range for flexion-extension balance, further large-sample, multicenter in vivo studies are required.
However, this wireless sensor also has some limitations. First, the frequent insertion of the sensor combination with its metal base may wear the osteotomy surface of the tibial plateau, causing osteotomy errors, which is contrary to the original goal of precise flexion-extension balance. Secondly, the wireless sensor we used is a disposable device; the battery can only supply power for about 3 h. Currently, our team is improving the technology to enable the sensor to be reused and meet wireless charging requirements. Additionally, we are also designing a new cushion block that can completely simulate mobile bearings to display the contact point trajectory of the tibial and femoral prostheses in real time during knee flexion and extension movements.
The use of the wireless sensor can help to quantify intra-articular pressure and guide osteotomy to achieve accurate flexion-extension balance. This will compensate for the lack of gap balance experience in beginners and reduce the learning difficulty of UKA. The application of wireless sensors reflects the trend toward individualized and intelligent approaches to joint surgery and the technological innovation brought about by close interdisciplinary cooperation.
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Disclosures
The authors have nothing to disclose.
Acknowledgments
This work was supported by Beijing Hospitals Authority Clinical Medicine Development of Special Funding Support [grant numbers: XMLX202139]. We would like to express our gratitude to Diego Wang for valuable suggestions.
Materials
| Name | Company | Catalog Number | Comments |
| angle measuring device | AIQIAO(SHANGHAI) MEDICAL TECHNOLOGY CO., LTD. | 20203010141 | angle measuring device of femur,angle measuring device of tibia |
| Oxford Partial Knee System | Biomet UK LTD. | 20173130347 | Oxford UKA |
| Wireless sensor combination | AIQIAO(SHANGHAI) MEDICAL TECHNOLOGY CO., LTD. | 20212010325 | a metal base, a wireless sensor with three pressure probes, and a cushion block |
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