- Email: [email protected]
P065 Computer analysis of pressure mat data
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INTRA-SUBJECT L. Chiari’, ‘DEIS - Universita
REPEATABILITY OF POSTUROGRAPHIC PARAMETERS U. Della Croci?, A. Cappozzo’, A. Cappello’ di Bologna; ‘Dip. Scienze Biomediche - Universiti Sassari, Italy
di
INTRODUCTION: Upright posture is investigated to provide information about motor ability in patients and in the elderly. The clinical effectiveness of relevant tests is limited by the difficulty of extracting meaningful and reliable parameters. Posture is generally analysed by means of a dynamometric platform which provides the time-course of the centre of pressure (COP). Several methods have been proposed for the calculation of posturographic parameters (PPs) from COP trajectories. These are global (GL) parameters: maximal antero-posterior (AP) and medio-lateral (ML) displacement; mean sway amplitude (MA); length of the sway path (SP); mean frequency (MF); sway area (SA) [l]. In addition, statistical mechanics (SM) parameters have been proposed in the form of diffusion coefficients (D) and scaling exponents (H) [2]. In this paper new SM (NSM) PPs are illustrated which were obtained using a modified version of Stabilogram Diffusion Analysis [2] and intraindividual repeatability of all above mentioned parameters is compared. METHODS: Experiments were carried out on 10 subjects while assuming a standardised upright posture, with open or closed eyes. For each experimen-tal condition five trials were made using a three component force plate. Acquisitions lasted 30 s and the sampling frequency was 100 samples per second (sps). The time series were then downsampled to a rate of 50 sps. The NSM we-re obtained from a single model of bidimensio-nal fractional brownian motion. The relation-ship between mean square displacement, <&*>, and time lag, At, (&>=2DAtzH) plot-ted in a logarithmic plane allows for the estima-tion of both K=log(2D) and H through linear regression in the two quasi-linear regions of the curve (short, s, and long term, 1, respectively). An optimised logarithmic spacing was chosen to avoid clustering of points in the I region. To describe intrasubject relative to inter-subject repeatability, the index VARI was defined as the standard deviation of the ensemble deviations from the means divided by the standard deviation of the means (computed for each subject and operating condition) %.
RESULTS AND DISCUSSION: The figure shows the values of VAR% relative to all PPs analysed. The NSM PPs showed to be more repeatable than the corresponding SM and, more so, with respect to the GL PPs. In addition, both SM and NSM PPs are more promising in terms of physiological interpretation [2]. CONCLUSIONS: Future work includes the use of the NSM PPs for the classification of different populations of subjects and their consequent validation. REFERENCES: 1. Hufschmidt A. et al., Arch. Psychiat. Nervenkr., 228, 135-150, 1980. 2. Collins J.J. et al., Exp. Brain Res., 103, 151-163, 1995. ACKNOWLEDGEMENTS: Supported by lstituto Superiore di Saniti, Roma, Italy. CORRESPONDENCE: Lorenzo Chiari, DEIS, Universita di Bologna, V.le Risorgimento 2, I-40136 Bologna, Italy. tel:+39 51 6443067, fax:+39 51 644354O,lchiari@deis,uniboit
1 l’h Conference
,
COMPUTER ANALYSIS OF PRESSURE MAT DATA M. Neal’, H. Holstein’, S. Khodadadeh’ ‘Department of Computer Science, University of Wales, Aberystwyth, UK. INTRODUCTION: Measurement of foot pressure is an essential diagnostic procedure in the management of common medical conditions, such as diabetes’. Standard foot pressure measurement equipment, the pedobarograph, is too expensive to be routinely available in centres of primary health care, yet would provide valuable screening data. We have modified the procedure of obtaining foot pressure data form the Harris and Beath mat*.‘, making it easier to use while retaining the essential features of low cost, flush floor mounting, portability and high spatial resolution. METHODS: Having obtained a foot print in the usual manner, we depart from traditional procedure by recording the mat data with a scanner, for digital storage in a patient database. The original paper print can then be discarded. The scanned data is normalised to allow for changes in inking levels over different applications of the mat. The mat is also periodically calibrated by applying a known range of loads by means of a roller. Comparison with the actual foot print then allows absolute pressures to be calculated. RESULTS: We obtain digitised pressures with excellent spatial resolution, as shown below. We are currently applying image analysis on mat data from 35 diabetic patients, in a left-right asymmetry and high pressure region classification study, for which the analysis is to be presented.
DISCUSSION: Reduction of mat data to absolute pressures encounters limitations of hysteresis. While this effect can be studied by applying loads at different speeds, the load time exposure is in general not known during use, and to this extent the pressure interpretation must remain uncertain. However, the mat does record the ground reaction to foot application, and therefore gives a faithful representation of regions of maximum wear, such as is relevant in indicating likely sites of skin deterioration. CONCLUSION: Our proposed procedure takes advantage of the positive qualities of the Harris and Beath method of foot pressure measurement, while greatly enhancing the usefulness of the resulting data through digital storage. REFERENCES: 1. Boulton, AGM. The importance of abnormal foot pressures and gait in the causation of foot ulcers. In: Brenner MA (Ed.), “Management of the diabetic foot”, 11-21, 1987. 2. Harris RJ, Beath T: Army Foot Survey - An investigation of foot ailments in Canadian soldiers. Ottawa: National Research Council of Canada, 1947. 3. Rose GK, Welton EA and Marshall T: The diagnosis of flat foot in the child. 3 Bone Surg 67B:71, 1985. CORRESPONDENCE: H. Holstein, [email protected]
of the ES& July 8-I 1 98, Toulouse,
France
87
&& I<
INTRA-SUBJECT L. Chiari’, ‘DEIS - Universita
REPEATABILITY OF POSTUROGRAPHIC PARAMETERS U. Della Croci?, A. Cappozzo’, A. Cappello’ di Bologna; ‘Dip. Scienze Biomediche - Universiti Sassari, Italy
di
INTRODUCTION: Upright posture is investigated to provide information about motor ability in patients and in the elderly. The clinical effectiveness of relevant tests is limited by the difficulty of extracting meaningful and reliable parameters. Posture is generally analysed by means of a dynamometric platform which provides the time-course of the centre of pressure (COP). Several methods have been proposed for the calculation of posturographic parameters (PPs) from COP trajectories. These are global (GL) parameters: maximal antero-posterior (AP) and medio-lateral (ML) displacement; mean sway amplitude (MA); length of the sway path (SP); mean frequency (MF); sway area (SA) [l]. In addition, statistical mechanics (SM) parameters have been proposed in the form of diffusion coefficients (D) and scaling exponents (H) [2]. In this paper new SM (NSM) PPs are illustrated which were obtained using a modified version of Stabilogram Diffusion Analysis [2] and intraindividual repeatability of all above mentioned parameters is compared. METHODS: Experiments were carried out on 10 subjects while assuming a standardised upright posture, with open or closed eyes. For each experimen-tal condition five trials were made using a three component force plate. Acquisitions lasted 30 s and the sampling frequency was 100 samples per second (sps). The time series were then downsampled to a rate of 50 sps. The NSM we-re obtained from a single model of bidimensio-nal fractional brownian motion. The relation-ship between mean square displacement, <&*>, and time lag, At, (&>=2DAtzH) plot-ted in a logarithmic plane allows for the estima-tion of both K=log(2D) and H through linear regression in the two quasi-linear regions of the curve (short, s, and long term, 1, respectively). An optimised logarithmic spacing was chosen to avoid clustering of points in the I region. To describe intrasubject relative to inter-subject repeatability, the index VARI was defined as the standard deviation of the ensemble deviations from the means divided by the standard deviation of the means (computed for each subject and operating condition) %.
RESULTS AND DISCUSSION: The figure shows the values of VAR% relative to all PPs analysed. The NSM PPs showed to be more repeatable than the corresponding SM and, more so, with respect to the GL PPs. In addition, both SM and NSM PPs are more promising in terms of physiological interpretation [2]. CONCLUSIONS: Future work includes the use of the NSM PPs for the classification of different populations of subjects and their consequent validation. REFERENCES: 1. Hufschmidt A. et al., Arch. Psychiat. Nervenkr., 228, 135-150, 1980. 2. Collins J.J. et al., Exp. Brain Res., 103, 151-163, 1995. ACKNOWLEDGEMENTS: Supported by lstituto Superiore di Saniti, Roma, Italy. CORRESPONDENCE: Lorenzo Chiari, DEIS, Universita di Bologna, V.le Risorgimento 2, I-40136 Bologna, Italy. tel:+39 51 6443067, fax:+39 51 644354O,lchiari@deis,uniboit
1 l’h Conference
,
COMPUTER ANALYSIS OF PRESSURE MAT DATA M. Neal’, H. Holstein’, S. Khodadadeh’ ‘Department of Computer Science, University of Wales, Aberystwyth, UK. INTRODUCTION: Measurement of foot pressure is an essential diagnostic procedure in the management of common medical conditions, such as diabetes’. Standard foot pressure measurement equipment, the pedobarograph, is too expensive to be routinely available in centres of primary health care, yet would provide valuable screening data. We have modified the procedure of obtaining foot pressure data form the Harris and Beath mat*.‘, making it easier to use while retaining the essential features of low cost, flush floor mounting, portability and high spatial resolution. METHODS: Having obtained a foot print in the usual manner, we depart from traditional procedure by recording the mat data with a scanner, for digital storage in a patient database. The original paper print can then be discarded. The scanned data is normalised to allow for changes in inking levels over different applications of the mat. The mat is also periodically calibrated by applying a known range of loads by means of a roller. Comparison with the actual foot print then allows absolute pressures to be calculated. RESULTS: We obtain digitised pressures with excellent spatial resolution, as shown below. We are currently applying image analysis on mat data from 35 diabetic patients, in a left-right asymmetry and high pressure region classification study, for which the analysis is to be presented.
DISCUSSION: Reduction of mat data to absolute pressures encounters limitations of hysteresis. While this effect can be studied by applying loads at different speeds, the load time exposure is in general not known during use, and to this extent the pressure interpretation must remain uncertain. However, the mat does record the ground reaction to foot application, and therefore gives a faithful representation of regions of maximum wear, such as is relevant in indicating likely sites of skin deterioration. CONCLUSION: Our proposed procedure takes advantage of the positive qualities of the Harris and Beath method of foot pressure measurement, while greatly enhancing the usefulness of the resulting data through digital storage. REFERENCES: 1. Boulton, AGM. The importance of abnormal foot pressures and gait in the causation of foot ulcers. In: Brenner MA (Ed.), “Management of the diabetic foot”, 11-21, 1987. 2. Harris RJ, Beath T: Army Foot Survey - An investigation of foot ailments in Canadian soldiers. Ottawa: National Research Council of Canada, 1947. 3. Rose GK, Welton EA and Marshall T: The diagnosis of flat foot in the child. 3 Bone Surg 67B:71, 1985. CORRESPONDENCE: H. Holstein, [email protected]
of the ES& July 8-I 1 98, Toulouse,
France
87