Estimation of the spinal twisting angle using inertial measurement units during a rod derotation surgery in idiopathic scoliosis patients

• We measured the rod twisting angle during rod-derotation surgery using two IMUs.
• Without an alignment procedure, the Euler angle method was adopted in this study.
• We obtained the results with the correlation coefficient of 0.904 and RMSE of 0.680°.
• This result provides the basis method assisting the spinal correction surgery.

This study proposes a method to measure the twisting angle of a rod during a spinal correction surgery in real-time without performing an alignment procedure by using the six-axis data (tri-axis acceleration, tri-axis rate gyro) of the IMU and a Tait-Bryan Euler angle algorithm. To determine whether the twisting angle calculation algorithm offered an improvement, typical procedures described in existing studies were implemented using the same experimental data and were then validated using a three-dimensional motion analysis system (Vicon 460 motion analysis system). The correlation coefficients and the RMSE of the proposed method were 0.904 and 0.680° in the servo-motor experiment, and 0.988 and 0.691° in the mock surgery experiment, respectively, and these values were not significantly different from those calculated through other methods in previous studies. Therefore, if the proposed method is used during surgery, an alignment procedure and the assumptions following the morphology of body, which are limitations of prior research, are not necessary. Also the twisting angle of the rod can be observed without using magnetic data of IMU in real time during surgery. It is expected that the correction loss, which is a serious problem that can occur in patients after spinal correction, could be prevented.