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Title: An electrogoniometer to measure spinal curvature
Author: Smit, Philip C.
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2014
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Biomechanical motion capture is the process of recording the movements of people or animals. As an analysis tool it offers valuable insight into human motion and is useful to monitor treatment during rehabilitation. The spine in particular receives a significant amount of attention by biomechanical researchers, as spinal health is directly related to the quality-of-life of an individual. Spinal motion capture improves the understanding of the function and vulnerabilities of the spine as a mechanical structure and the analysis of spinal kinematics, in conjunction with spinal loading, offers a method for analysing therapeutic interventions. Numerous motion capture systems and devices are currently available, each with its own strengths and weaknesses. The systems or devices selected by researchers are usually determined by study objectives. For example, a video motion capture system would clearly not be appropriate in a study designed to monitor lower back movements of factory workers. Instead, unobtrusive accelerometry based devices would be more suitable to measure kinematics in a free-living environment. Accelerometry has its drawbacks however. It is limited to only global pitch and roll measurements and requires a subject to make relatively slow movements (i.e. the acceleration component of movement measured by the accelerometer must be significantly smaller than one g). In general, trade-offs exist between accuracy, obtrusiveness, ease-of-use, cost, mobility (degrees-of-freedom) and clinical versus free-living measurements. This thesis proposes an electrogoniometer, which meets many of the above mentioned criteria. The electrogoniometer aims to be accurate yet unobtrusive, low cost (perhaps less than £5 000, compared to the £100 000 price tag of high-end marker-based video motion capture system) and measures high mobility movement (typically the rotation components of a spinal motion segment) and do so within a free-living setting. The electrogoniometer is composed of discrete goniometers, referred to as goniometer-nodes. The goniometer-nodes are chained serially together to construct a multi degree-of-freedom electrogoniometer. The goniometer-nodes consist of mechanical structures embedded with optical sensors, each capable of measuring four degrees-of-freedom (three rotations and one translation). The mechanical structure's articulation is determined by processing the optical sensor data using the principles of triangulation and trilateration. The articulation is measured relative to a local reference frame (i.e. relative to the proximal-end of the node). Since local reference frame measurements are involved, accuracy and precision are important. Poor accuracy and precision will result in measurement errors propagating through the chain. The rotation accuracy is estimated to be better than 2° per axis (which is much less than the typical 5° accuracy of commercial goniometers) and a displacement (translation) accuracy of less than 0.2 mm. Precision is estimated better than 0.5° degrees per axis and 0.1 mm. The device is particularly suitable to measure spinal movement. It is attached to the backof a subject, similar to commercial electrogoniometers. It monitors the spinal kinematics on a continuous basis and transmits the data to a computer via a wireless adaptor. The kinematic data is then available for further analysis. This thesis initially investigates the mechanical and sensor design of the goniometer-nodes. A mechanical composite structure consisting of an universal (two rotations) and cylindrical (one rotation and translation) joint was utilised. Optical emitter-detector pairs were embedded within the structure, and a mathematical model was derived to predict the response of the detectors based upon the kinematic input. A custom instrument was developed to calibrate the nodes. Five nodes were assembled and calibrated, and then chained together to produce the electrogoniometer. The second part of the thesis evaluates the device. Reflective triads were attached to the base of each node within the chain. The device was then manipulated manually and compared against a video motion capture system for accuracy and precision. Analysis of the results showed a local reference frame accuracy and precision of 1.9 ± 1.0° per axis for rotation and 3.5 ± 1.8 mm for translation. The video captured measurements were also compared to the calibration results and proved to be worse than predicted. The cause was traced to the calibration instrument and the measurement method. Although the accuracy and precision specification were not met, it was concluded that the proof-of-concept electrogoniometer demonstrated here has merit as a low-cost motion capture device. The optical measurement method from which the electrogoniometer kinematics are determined, shows promise as a novel kinematic sensing method. It was concluded that with further refinement and improvements of the custom-build calibration instrument, the accuracy and precision requirements can be met. Nonetheless, the concepts and principles have been shown to be valid, and with additional resources, this electrogoniometer can be a viable biomechanical research device.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available