Abstract
Introduction
Dislocation continues to be a common complication of total hip arthroplasty (THA). Many factors affect the prevalence of dislocation after THA, including soft tissue laxity, surgical approach, component position, patient factors, and component design [1]. Achieving proper intraoperative soft tissue tension is one of the surgical goals to reduce the risk of the dislocation. However, reports of the intraoperative soft tissue tension measurements have not been enough yet. One way to quantify the intraoperative soft tissue tension is to measure joint forces using an instrumented prosthesis. Hence, we have developed a sensor-instrumented modular femoral head of THA to measure the soft-tissue tension intraoperatively. The goal of this study was to design and calibrate the sensor.
Materials and Methods
The sensor-instrumented modular femoral head that we developed was made of polycarbonate with four linear strain gauges (BTM-1C, Tokyo Sokki Kenkyujo Co., Ltd., JP). To fabricate the sensor, four penetrant holes (1.6 millimeter in diameter), parallel to the coordinate axes were produced (Fig1). The strain gauges were embedded on inside wall of these holes. Finally, the holes were filled by epoxy resin (A-2 adhesive, Tokyo Sokki Kenkyujo Co., Ltd., JP). For calibration study, the sensor was fixed in a clamping block of an angle vice to permit change of force directions. The calibration jig with the angle vice was placed on top of a low-friction x-y translation table that eliminated horizontal constrains. Known forces (Fi) were applied by a standard material testing machine (Instron4204, INSTRON, Norwood, MA) through a polyethylene insert (Fig. 2). Two different series of forces were applied. One is that force values were increased from zero to 600 N on the z axis. And the other force pattern is 600 N forces were applied by changing force angles. The external force vector (Fi) can be expressed in terms of the strain gauge outputs as follows:
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Fi = T Si
where T is a calibration matrix and Si corresponds to the outputs of the strain gauges. Calibration errors were calculated according to well-established methods [2].
Results
When the loads were applied on the z axis, the output strains of ε1 showed increases with increase of the force values (Fig.3). A coefficient of determination of least square linear regression between εz and the force values was 0.86. When the cone angle was decreased from 90˚ in the x-z plane, εz decreased and εx increased concurrently (Fig.4). The average absolute error of the force was 23.9%.
Discussion
This device was connected to a data logger with wires. In order to remove these wires to diminish the risk of infection, we will use wireless system (nRF24LE1, Nordic Semiconductor, Inc., Norway). Although the calibration matrix and the errors were acquired, the error value was not good enough to calculate the applied forces yet. With more calibration results and wireless system, this system will be useful to permit optimized intraoperative soft tissue tension.
