DESIGN OF AN INSTRUMENTED PROSTHESIS FOR MEASUREMENT OF JOINT FORCES INTRAOPERATIVELY

Introduction Dislocation continues to be a common complication of total hip arthroplasty (THA) [1]. Although many factors affect the prevalence of dislocation, achieving proper intraoperative soft tissue tension is one of the main surgical goals to reduce this risk. However, a sensor to measure the soft tissue of ball joints i.e. hip and shoulder has not yet been developed. The sensor enables surgeons to adjust the size or position of the implants depending on soft tissue tension. Hence, we have developed a sensor-instrumented modular femoral head for THA to measure soft-tissue tension intraoperatively [2]. This study demonstrates the possibility of a soft tissue tension and joint angle data connection using a wireless system. Materials and Methods The sensor-instrumented modular femoral head that we developed was made of epoxy resin with linear strain gauges (BTM-1C, Tokyo Sokki, Japan) inside the head and a triple-axis gyroscope (MPU-6500). Strain outputs and angle data from the gyroscope were transferred to a computer via a 2.4 GHz wireless link (RN42, Bluetooth Module). Data logging was performed by a custom program using C++ (Microsoft Visual Studio 2012) via both wired and wireless link. The strain gauges were embedded inside the head. For the calibration study, the sensor was fixed in a clamping block of an angle vice to permit changes in the direction of force. The calibration jig with the angle vice was placed on top of a low-friction two-dimensional translation table that eliminated horizontal constraints. A constant vertical force was applied using a vertical die set. The experimental setup is shown in Fig. 1. Instead of a portable battery, a DC electric power supply is used (bottom left). A picture of the Gyroscope and the radio module is inserted (bottom right). The force values and applied angles were changed recording strain gauge and angle outputs. Results When the loads were applied on the z axis (at θ = 0), the output strains of ez showed increases with increases of the force values. When the cone angle (θ) was decreased from 0˚, angle changes were measured by the gyro scope. Output angles of both wired and wireless data are plotted to be compared (Fig. 2). Although data from a wireless system have a worse signal to noise ratio, it could capture the angle changes. Strain outputs of ez were plotted changing the cone angles and force values. ez showed decreases with increasing of cone angles (Fig.3). Discussion This device was connected to a data logger with a wireless system in order to diminish the risk of infection. Although the amount and quality of the data connection were not good enough, the possibility and the concept of intraoperative wireless measurement have been shown. With a more sophisticated sensor system, this will be useful to permit optimized intraoperative soft tissue tension. Acknowledgement This study is supported by the Grant-in-Aid for Scientific Research (C) 26350511. To view tables/figures, please contact authors directly.