Applied Assessment Method for Varus Thrust during Walking in Patients with Knee Osteoarthritis Using Acceleration Data Measured by an Inertial Measurement Unit

Abstract

We developed a novel quantitative method to assess varus thrust during walking using acceleration data obtained from an inertial measurement unit (IMU). This study aimed to examine the reliability of the developed index and to evaluate its ability to distinguish patients with knee osteoarthritis (OA) with varus thrust from healthy adults. Overall, 16 patients with knee OA and 16 healthy adults walked on a treadmill with IMUs attached to the tibial tuberosity and lateral femoral condyle. As an index of varus thrust, we used the root mean square (RMS) of acceleration in the mediolateral direction. This value was adjusted by dividing it by swing speed while walking (adjusted RMS, A-RMS) because the RMS of the acceleration was strongly coupled with the speed of motion. The intraclass correlation coefficients of A-RMS of the tibia and femur were 0.85 and 0.73, respectively. Significant differences were observed in the A-RMSs of the tibia and femur, with large effect sizes between the patients with knee OA and healthy adults (Cohen's d: 1.23 and 0.97, respectively). Our results indicate that A-RMS has good test-retest reproducibility and can differentiate patients with varus thrust from healthy adults.

Keywords: acceleration; gait; inertial measurement unit; knee; osteoarthritis; thrust.

Figure 1

Photograph of the inertial measurement units attached to the tibia and femur.

Figure 2

Typical raw acceleration data in the ML direction for the tibia and femur around the knee and the extracted data for the assessment of varus thrust. The upper and lower figures represent the raw acceleration data of the tibia (A) and femur (B) for several typical gait cycles during the walking state. Data for the sections colored light green were for each of the first halves of the stance phases, and they were extracted to calculate the index of varus thrust. ML, mediolateral.

Figure 3

Scatter plots between stRMS in the ML direction and the mean v-swRMS in the three planes. The left and right figures represent the plot of the data for the tibia (A) and femur (B), respectively. The cross marks indicated the values of patients with knee OA, and the circle marks indicate the values of healthy adults. Pearson’s correlation coefficient, r, is shown in the upper left corner of each plot. RMS, root mean square; stRMS, RMS of the acceleration data of half of the stance phase; v-swRMS, RMS of the angular velocity data of the swing phase; ML, mediolateral.

Figure 4

ROC curves of A-RMS for detecting patients with knee OA and varus thrust. The left and right figures represent the ROC curve of A-RMS in the ML direction of the (A) tibia (AUC: 0.81) and (B) femur (AUC: 0.75). ROC, receiver operating characteristic; A-RMS, adjusted root mean square of the acceleration data of half of the stance phase; OA, osteoarthritis; ML, mediolateral; AUC, area under the curve.