Sensor Assessment of Gap Balance in Mobile-Bearing Unicompartmental Knee Arthroplasty

Summary

Here, we present a protocol to measure the gap contact force and gap balance in mobile-bearing unicompartmental knee arthroplasty (UKA). Along with the clinical and radiographic data, we hope to determine the normal range of the contact force and set up the threshold of the gap balance.

Abstract

The most important procedure of mobile-bearing unicompartmental knee arthroplasty (UKA) is to balance the knee flexion and extension gap. Conventionally, the balance was determined by the subjective assessment of plugging out the feeling gauge. Since it mainly depended on the surgeons' experience, the accuracy was always in doubt. In the past 10 years, pressure sensors have been introduced to guide the gap balance in total knee arthroplasty (TKA). However, the sensor technique was introduced to UKA very recently. Herein is our sensor assessment of the gap balance in 20 cases UKA by one experienced surgeon. The sensor was a custom-designed force sensor matrix according to the shape of the tibial trial of mobile-bearing UKA. The postoperative clinical outcomes and radiographic results were recorded for future comparison. We aim to use this method to assess more than 200 cases of UKA by various surgeons to ultimately standardize the gap-balance result.

Introduction

The mobile-bearing UKA is currently one of the most successful treating methods for anteromedial osteoarthritis (AMOA) of the knee¹. The balance of the flexion and extension gap during the operation is the key to a successful UKA^2,3. The gap overload might aggravate the wear of the mobile bearing. Moreover, the elevated gap contact force might lead to postoperative valgus deformity and degeneration of the lateral compartment⁴. Therefore, achieving an optimal gap tightness as well as an acceptable gap balance in UKA is an important part of the learning curve⁵. According to the mobile-bearing UKA manual of surgical technique⁶, the surgeon must use the feeling gauge to insert and plug out of the joint gap to "feel" the contact force. By evaluating the force required to insert and remove the insert, the surgeon could estimate whether the gap balance is acceptable. Therefore, the judgment depended mainly on the surgeon's experience.

In recent years, digital measurement of intraoperative gap balance of medial and lateral gap had been widely reported in total knee arthroplasty (TKA)^7,8,9. Recommendations for the threshold of the gap balance had also been set⁷. However, the sensor technique was introduced to UKA very recently without a well-recognized gap-balancing goal.

Last year, a force sensor specially designed to measure joint gap contact force during mobile-bearing UKA was introduced⁵. In the present research protocol, the sensor-guided gap force measurement method is demonstrated. In addition, a case series of 20 patients who had undertaken mobile-bearing UKA is included to assess the gap contact force and the gap balance. The final goal of this protocol is to determine the normal range of contact force and set up the threshold of gap balance in mobile-bearing UKA.

Protocol

This study was approved by the human ethics committee of the China-Japan Friendship Hospital (approval number 2020-50-k28).

1. Preparation and sterilization of the force sensor

1. Use abrasion-resistant adhesive tape to fix the force sensor on the upper surface of the tibial trial before sterilization.
2. Pack and sterilize the force sensor using low-temperature sterilization with hydrogen peroxide gas plasma (Figure 1).
   NOTE: The sensor should be fixed on the tibial trial to prevent the effect of the shear force.

2. Procedure of mobile-bearing UKA

1. Perform the operation procedure of mobile-bearing UKA according to the standard surgical instruction⁶ or the kinematic alignment technique introduced by Zhang et al.¹⁰.
2. Stop the procedure when all the bone cuttings are finished, and the gap balance is confirmed manually.

3. Installation of the force sensor

1. Install the force sensor along with the tibial trail first, and then install the femoral component.
2. Ensure the sensor, the USB line, and the laptop are connected properly. After that, insert the feeler gauge into the component gap, and place the knee joint at deep flexion of 120° as the start point of the measurement. (Figure 2).
   NOTE: Use a sterilized protractor to make sure the accuracy of the knee flexion angle.

4. Measurement and recording of raw data of the contact force

1. Record raw data of the force value using a computer program developed for this sensor.
2. First, pay attention to the right side of the operation interface (Figure 3), and set up the recording frequency at 10 Hz and the recording time at 5 s. Then, click on the Data Feedback button when the knee is placed at the flexion angle of 120°.
3. When the recording process is over, click on the Data Feedback button again when the knee flexion is 90°, then 60°, 45°, 20°, and 0° (Figure 3).
   ​NOTE: Raw data is saved into .txt files by the computer program, and further management is needed to acquire force value.

5. Management of raw data

1. Input the .txt file into a spreadsheet (digital table) for raw data conversion. Compute the average value of 50 records as the contact force.
   ​NOTE: The program can also show the distribution of the contact force.

6. Clinical and radiographic observations

1. Record the patient's demographic data such as age, gender, diagnosis, and the American Knee Society score (AKSS).
2. Take radiographs of anteroposterior, lateral, and full-length weight-bearing lower limb preoperatively and within 1 week postoperatively.
3. Measure the varus/valgus alignment of the femoral and tibial prosthesis (Figure 4-1), the flexion/extension alignment of the femoral prosthesis, and the tibial posterior slope (Figure 4-2).
4. Measure the hip-knee-ankle angle on the full-length lower limb radiographs both preoperatively and postoperatively. Measure the continuity of the prosthesis (Figure 4-3) and the convergence/divergence angle, which implies the axis of the femoral prosthesis relative to the surface of the tibial prosthesis (Figure 4-4).
5. Make sure these data are integrated and can be analyzed in the future.
   NOTE: The method of the radiographic measurement of the angles^6,11 are shown in Figure 4.

Results

Cohort demographics
The first 20 patients who undertook mobile-bearing UKA were enrolled in the China-Japan Friendship Hospital from March to June 2021. The surgeries were all done by a senior doctor with over 2,000 cases of UKA experience. The demographic along with prothesis data is shown in Table 1. The age ranged from 58-82 years, and the diagnoses were all AMOA.

Results of gap force and balance measurements
The pattern of gap forc...

Discussion

This study provided a detailed protocol of sensor technology in assessing the joint gap contact force and balance in mobile-bearing UKA. We hope to set up a goal of standard contact force as well as gap-balancing difference, which would allow the orthopedic surgeons to determine the bearing thickness and gap-balancing more easily in the future.

The overload of the joint gap may lead to postoperative valgus deformity of the limb, future degeneration of the lateral compartment, and even OA progr...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Oxford UKA	Zimmer/Biomet		For the catalog numbers refer to Oxford Partial Knee Microplasty Instrumentation (femoral component, tibial component, meniscus bearing)
Teflon Tape	3M		Abrasion resistant adhesive tape widely used in biomechanical experiments
Verasense	OrthoSensor	Verasense	TKA sensor
Excel	Microsoft		digital table software
STERRAD 100S sterilization system	Johnson&Johnson	STERRAD 100S	Low-temperature sterilizing with hydrogen peroxide gas plasma
UKA force sensor	Qingrui Boyuan	in house	Co-designed and produced by Qingrui Boyuan Technology
Computer program for recording raw data	Qingrui Boyuan	in house	Co-designed and produced by Qingrui Boyuan Technology
Protractor	Shanghai M&G Stationery Inc.	any	Sterilized in the sterilization system
USB line	Lenovo		any
Laptop	Lenovo		any basic configuration