Accuracy of an electromagnetic measurement device and application to the measurement and description of knee joint motion

The aims of this study were to analyse the accuracy and sensitivity to metals of an electromagnetic motion measurement device and to use it to provide a description of motion of the human knee joint. A calibrated grid and sine bar were used to measure translational and rotational accuracy, which were found to be +/-0.23 per cent of the step size (translation) and +/-2.0 per cent of the step size (rotation) within an optimal operational zone of minimal positional error for which the receiver to transmitter separation was between 271 and 723 mm. The presence of multiple receivers was found to have a significant effect on the accuracy only when positioned within 30 mm of one another. Mild steel was found to have a significant detrimental effect when within 150 mm of the transmitter or receiver. A stainless steel bone screw had no effect on the accuracy of the device. A mathematical description of knee joint motion is presented which describes the motion in terms of clinical rotations. The device is a useful tool for measuring joint motion in the operating theatre and laboratory, owing to its accuracy and insensitivity to surgical alloys.