The average age of population increases worldwide, so does the number of total hip replacement surgeries. Total hip replacement, however, often involves a risk of dislocation and prosthetic impingement. To minimize the risk after surgery, we propose an instrumented hip prosthesis that estimates the relative pose between prostheses intraoperatively and ensures the placement of prostheses within a safe zone. We create a model of the hip prosthesis as a ball and socket joint, which has four degrees of freedom (DOFs), including 3-DOF rotation and 1-DOF translation. We mount a camera and an inertial measurement unit (IMU) inside the hollow ball, or "femoral head prosthesis," while printing customized patterns on the internal surface of the socket, or "acetabular cup." Since the sensors were rigidly fixed to the femoral head prosthesis, measuring its motions poses a sensor ego-motion estimation problem. By matching feature points in images of the reference patterns, we propose a monocular vision based method with a relative error of less than 7% in the 3-DOF rotation and 8% in the 1-DOF translation. Further, to reduce system power consumption, we apply the IMU with its data fused by an extended Kalman filter to replace the camera in the 3-DOF rotation estimation, which yields a less than 4.8% relative error and a 21.6% decrease in power consumption. Experimental results show that the best approach to prosthesis pose estimation is a combination of monocular vision-based translation estimation and IMU-based rotation estimation, and we have verified the feasibility and validity of this system in prosthesis pose estimation.
To improve the positioning accuracy of implants in Total Hip Replacement (THR) surgeries, a visual-aided wireless monitoring system for THR surgery is proposed in this paper. This system aims to measure and display the contact distribution and relative pose between femoral head and acetabulum prosthesis during the surgery to help surgeons obtain accurate position of implants. The system consists of two parts: the Sensors Array Measuring System (SAMS) and the display part. The SAMS is composed of a sensors array (including contact sensors and an image sensor), signal conditioning circuits, a low power microcontroller (MCU), and a low-power transceiver. The SAMS is designed to estimate the relative pose of femoral head component to acetabular component. The display part processes the data from sensors and demonstrates the contact distribution and the pose of the prothesis during the surgery in 3-D graphics. The two parts of the system communicate with each other on an RF link at the band of 400 MHz. The signal conditioning circuits have been designed and fabricated in 0.18 μm CMOS process. Testing results show that the resolution of the signal conditioning circuits is 60.1 μ Vpp (1.35 g) with ±100 mVpp input. The chip can operate under 1.2-to-3.6 V supply voltage for single battery applications with 116-160 μ A current consumption. The system has been verified by the simulation with rotation quaternion and translation vector. The experimental results show that the contact distribution and relative pose of the two components could be measured and demonstrated in real time. The relative error of rotation is less than 8% and the actual relative error of translation is less than 10%.
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