A real-time gyroscopic system for three-dimensional measurement of lumbar spine motion

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Abstract

This paper describes a new method of measuring the three-dimensional movements of the lumbar spine in real time. The measurement system consisted of solid-state gyroscopes which were attached to the trunk. They measured the angular rates of rotations in three dimensions, which were then integrated to obtain the orientation. The sensors contained gravitometers and magnetometers which provided additional information for eliminating any drift of the gyroscopes. The reliability of the data provided by the gyroscopic system was examined in a group of 19 young healthy subjects. The similarity of the movement–time curves obtained in three repeated measurements was assessed by the coefficient of multiple correlation. The coefficients were found to be high, ranging from 0.972 to 0.991. The reliability of the data was slightly lower for measuring axial rotation. The device did not only quantify the kinematic patterns in the primary plane of movements, but also the accompanying movements in the other planes. Flexion and extension was found to be mainly confined to the sagittal plane, whereas lateral bending and axial rotation always accompanied each other. It was concluded that the inertial tracking device would be a useful tool for clinical measurement as well as biomechanical investigations.

Introduction

Measurement of lumbar spine motion is routinely employed in clinical assessment and diagnosis of low back pain. Most of the clinical techniques, such as the fingertips-to-floor, skin distension and inclinometers methods [1], [2], [3], involve measurements in one or two dimensions only, but the lumbar spine exhibits complex three-dimensional motions.

Various techniques have been previously employed to measure the three-dimensional movements of the lumbar spine. Opto-electronic devices [4], stereoradiography [5], [6], and electromagnetic tracking systems [7], [8], [9], [10], [11], [12] appear to be able to provide accurate data, but they are generally less than ideal. The optical methods are too complex, time consuming and expensive, and these disadvantages make them unsuitable for routine clinical use. Radiographic techniques are complicated, and have the inherent health risk of repeated X-ray exposure. They are unable to provide information about the kinematic patterns of movements, only measuring the end points of motion. Electromagnetic devices, which are highly accurate and inexpensive, have been shown to be a promising technique for clinical evaluation of joint motion [7], [8], [9], [10], [11], [12]. However, they may be adversely affected by the presence of metals [9], [13], and correction for metallic distortion is time consuming and complicated.

The literature review shows that there is a need to develop a new method of measuring lumbar spine motion that can be used routinely in clinical assessment. In the aerospace and robotic industries, gyroscopes are widely used to provide information on position and orientation of rigid bodies [14], [15]. It is highly possible that similar techniques can be developed for measuring lumbar spine motion. Gyroscope is an angular velocity sensor which is based on the measurement of the Coriolis force of a vibrating device [14]. The angular orientation can then be obtained from integration of the gyroscopic signals. The modern solid-state gyroscope uses ceramic vibrating unit [16], [17], [18], [19]. They are ultra-small, lightweight and easy to apply and have a quick response, making them ideal for joint motion measurement.

One limitation of solid-state gyroscope is the zero frequency offset, or bias, when the sensor is stationary. The bias will lead to an angular drift after integration of the gyroscopic signals, but there are many methods of correcting such error. Previous authors performed high pass filtering of the signals to eliminate the bias [20], [21]. Luinge et al. [17] proposed predicting the drift error using a Kalman filter and signals from accelerometers, and compensated for the error using a feedback loop. Alternatively, Williamson and Andrews [18] employed a mathematical technique that performed automatic nulling and resetting of the gyroscopes.

The feasibility of using gyroscopes to measure motions of lower limb joints had been examined in previous research [17], [18], [19], [22]. Motion data derived from these gyroscopes was shown to be highly correlated with those determined by a video-based motion analysis system. However, the feasibility of using a three-dimensional system for measuring lumbar spine motion has not been explored.

The purpose of this study was to examine the reliability of a new method of measuring lumbar spine motion using a three-dimensional gyroscopic system.

Section snippets

Subjects

Nineteen subjects (15 men and 4 women, mean age=22±5, mean height=1.65±0.31 m, mean weight=66.2±11.3 kg) agreed to participate in this study. They were in good health with no history of back pain or leg pain that may be attributed to the back within the last 12 months. They were excluded if they had undergone previous back surgery, had a fracture, dislocation or any structural defects of vertebral structures.

The study was approved by the Ethics Committee of the Department of Rehabilitation

Results

The mean maximum ranges of the movements of the lumbar spine are presented in Fig. 1. The mean differences in the maximum ranges determined in the three trials were found to be small, ranging from 1.02° to 1.55° (Fig. 1). The movement patterns of all subjects were also highly similar among the three trials. Fig. 2 shows the consistency of the normalised movement–time curves in repeated measurements in one of the subjects. The mean CMC was found to be high in all anatomical movements, ranging

Discussion

The results of this study indicate that the inertial tracking system is a reliable device for measuring movements of the lumbar spine. The range of movement determined was highly consistent among the various trials. The system has a number of attractive features that will make it an ideal clinical tool. It is simple to use, highly portable and inexpensive when compared to conventional motion analysis systems such as opto-electronic devices. It does not have the problem of metallic interference

Conclusion

In conclusion, the inertial tracking device described in this study was capable of producing reliable data. It has several attractive features which will make it a useful tool for clinical measurement as well as biomechanical investigations. It is simple to use, highly portable and inexpensive. It does not have the problem of metallic interference which is commonly observed in electromagnetic tracking. Another unique feature is that it provides real-time, three-dimensional kinematic information

Acknowledgements

This study was supported by the Internal Competitive Research Grant of the Hong Kong Polytechnic University. The authors are grateful to C.W. Tsang, Roy Wong, Maggie Wong, Carol Yip, and Colin Yip for their assistance in collecting the data.

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