In Vitro Application of a Wireless Sensor in Flexion-Extension Gap Balance of Unicompartmental Knee Arthroplasty

Our wireless sensor quantifies the pressure parameters of the flexion-extension gap balance in real-time, which is the key to unicompartmental knee arthroplasty. The sensor can display the compartment pressure in real-time, helping the surgeon to assess the flexion-extension balance more accurately and enables wireless data transmission to meet intraoperative sterility requirements. UKA is an important treatment for osteoarthritis of the knee, and the sensor used in this protocol can help with the dynamic intraoperative assessment of the UKA, which is critical to its success.

This sensor is specially designed for the mobile-bearing UKA and it can perfectly fit UKA's prosthetic trial. In addition, we have designed matching sensors for different prosthetic sizes. This sensor is specially designed for UKA beginners to shorten their learning curve.

The sensor is very easy to operate and there are no specific techniques. Begin by switching on the femur and tibia angle measurement device. Open the angle measurement software on the computer.

Scan the QR codes of the two measuring devices and click Bluetooth connection. Place the two angle measurement instruments on a horizontal table and click the Calibration button to calibrate. Tie the instruments with straps on the knee to measure knee flexion angle in real-time.

Place a cadaver in the supine position with the lower extremities draped in flexion and abduction over the outside of the operating table. Using the medial parapatellar approach, open the joint cavity with a scalpel. Now, make an incision about three centimeters distal to the joint line along the apex of the medial patella border, ending it distally about one centimeter medial to the tibial tuberosity.

Using a rongeur, remove the osteophytes of the medial femoral condyle, intercondylar fossa, and the anterior tibia. Insert a one millimeter femoral sizing spoon into the medial compartment to determine the size of the joint space with cartilage. Insert different sizes of femoral sizing spoons to hook the posterior femoral condyle.

When the end of the spoon is about one millimeter away from the cartilage surface, the size of the femoral prosthesis corresponding to the spoon is chosen. Next, connect a three millimeter G-clamp to a tibial saw guide and a femoral sizing spoon. Ensure the guide's shaft is parallel to the tibial long axis in both coronal and sagittal planes and that the ankle yoke points to the ipsilateral anterior superior iliac spine.

Use the reciprocating saw to make a vertical tibial saw cut. 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 zero shim, then use the oscillating saw blade to excise the plateau.

Now, 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. Insert the intramedullary rod in the hole.

Next, connect the femoral drill guide to the intramedullary rod and perform femoral drilling. Install the posterior resection guide into the drilled holes of the femur. Using the oscillating saw, perform the posterior femoral condyle osteotomy and remove the guide and the bone fragment.

Next, excise the medial meniscus, leaving a small cuff of the meniscus to protect the MCL. Completely remove the posterior horn. Insert a zero femoral spigot and attach the spherical mill onto the spigot to perform distal femoral milling, Insert a femoral trial and assess the flexion-extension gap by the gap gauge.

Use angle measurement devices to monitor the flexion angle. Insert the gap gauge into the joint space with slight resistance to define the flexion-extension gap balance. If the gaps are not equal, grind the femur according to the difference values until equal.

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. 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.

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. Measure the flexion and extension gap pressure at 110 degrees and at 20 degrees of knee flexion.

Calculate the average values separately for three consecutive measurements. The pressure values for the flexion gap were quite consistent, while the pressure values for the extension gap were quite different with an average pressure of 46.3 N in inflection and 35.0 N in extension gaps. Postoperative radiographs showed appropriate prosthesis positioning with good tibial and femoral positioning and flexion angle.

When performing pressure measurement, select the sensor corresponding to the size of the prosthesis to avoid damage to the osteotomy surface and affect the flexion-extension gap balance. After gap balancing, install femoral and tibial components in balance. In the future, we need large sample multicenter status to ascertain the acceptable range of the flexion-extension gap pressure difference mirrored by the sensor and its improvement on the long-term effect of UKA.