Ondrej Cakrt

Volume of confidence ellipsoid: a technique for quantifying trunk sway during stance

Abstract The position of the trunk can be negatively affected by many diseases. This work focuses on a noninvasive method of quantifying human postural stability and identifying defects in balance and coordination as a result of the nervous system pathology. We used a three-degree-of-freedom orientation tracker (Xsens MTx unit) placed on a patient’s trunk and measured three-dimensional (3-D) data (pitch, roll, and yaw) during quiet stance. The principal component analysis was used to analyze the data and to determine the volume of 3-D 95% confidence ellipsoid. Using this method, we were able to model the distribution of the measured 3-D data (pitch, roll, and yaw). Eight patients with degenerative cerebellar disease and eight healthy subjects in this study were measured during stance, with eyes open and eyes closed, and statistical analysis was performed. The results of the new method based on the 3-D confidence ellipsoid show that the volumes related to the patients are significantly larger than the volumes related to the healthy subjects. The concept of confidence ellipsoid volume, although known to the biomechanics community, has not been used before to study the postural balance problems. The method can also be used to study, for example, head and pelvis movements or alignments during stance.

ASSESSMENT OF POSTURAL INSTABILITY IN PATIENTS WITH A NEUROLOGICAL DISORDER USING A TRI-AXIAL ACCELEROMETER

Current techniques for quantifying human postural stability during quiet standing have several limitations. The main problem is that only two movement variables are evaluated, though a better description of complex three-dimensional (3-D) movements can be provided with the use of three variables. A single tri-axial accelerometer placed on the trunk was used to measure 3-D data. We are able to evaluate 3-D movements using a method based on the volume of confidence ellipsoid (VE) of the set of points obtained by plotting three accelerations against each other. Our method was used to identify and evaluate pathological balance control. In this study, measurements were made of patients with progressive cerebellar ataxia, and also control measurements of healthy subjects, and a statistical analysis was performed. The results show that the VEs of the neurological disorder patients are significantly larger than the VEs of the healthy subjects. It can be seen that the quantitative method based on VE is very sensitive for identifying changes in stability, and that it is able to distinguish between neurological disorder patients and healthy subjects.

VOLUME OF CONVEX HULL: A TECHNIQUE FOR QUANTIFYING HUMAN POSTURAL STABILITY

Many disorders, such as nervous system disorders, can affect orientation of the body segments in 3D space negatively. Patients with these disorders often show body segments instability during stance tasks. Nowadays, 3-axis gyroscopes are about to be used to measure postural stability. The main objective of the paper is to describe a method which would be suitable for quantifying postural stability and 3D movement as a whole using a cheap 3-axis gyroscope. New method based on the volume of a 3D convex hull (CH) obtained by plotting pitch, roll and yaw angles versus each other was proposed for quantitative evaluation of 3D trunk sway. The sway was measured while patients with degenerative cerebellar disorder (Pts) and eleven healthy subjects (HSs) performed quiet stance on a firm surface (FiS) and foam surface (FoS) with open eyes (OE) and closed eyes (CE). The CH was used to identify differences in balance control, and there were significant differences found between the two groups. The median (Mdn) of the volume of Pts with OE on FiS is four times larger than the Mdn of the volume of HS with OE on FiS. The Mdn of the volume of Pts with CE on FoS is 80 times larger than the Mdn of the volume of HS with CE on FoS. It was therefore found that the volume of CH is suitable for quantifying postural sway and identifying differences in balance control.

COMPARATIVE MEASUREMENTS OF HEAD ANGULAR MOVEMENTS USING A CAMERA SYSTEM AND A GYROSCOPE SYSTEM

Assessments of body-segment angular movements are very important in the rehabilitation process. Head angular movements are measured and analyzed for use in studies of stability and posture. However, there is no methodology for assessing angular movements of the head, and it has not been verified whether data measured by fundamentally different MoCap systems will lead to the same results. In this study, we used a camera system and a 3DOF orientation tracker placed on the subject’s head, and measured inclination (roll) and flexion (pitch) during quiet stance. The total length and the mean velocity of the traces of the pitch versus roll plots were used to measure and analyze head orientation. Using these methods, we are able to model the distribution of the measured 2D data, and to evaluate stability and posture. The results show that the total lengths and the mean velocities related to the 3DOF orientation tracker do not differ significantly from the total lengths and the mean velocities of traces related to the IR medical camera. We also found that the systems are not interchangeable, and that the same type of system must be used each time. The designed methods can be used for studies not only of head movements but also of movements of other segments of the human body, and can be used to compare other types of MoCap systems, depending on the requirements for a specific rehabilitation examination.

Quantifying postural stability of patients with cerebellar disorder during quiet stance using three-axis accelerometer

Abstract This work focuses on the novelty of applying a 3-D postural analysis on the cerebellar disorders diagnosis and on introduction of an alternative to recent methods of quantifying the human postural stability during quiet stance, which uses a three-axis accelerometer. It introduces an advantage in the form of an ability to evaluate a complex three-dimensional (3-D) movement, as opposed to a major limitation of todaýs alternatives that evaluate only two coordinates in space. 3-D data (anterior-posterior, superior-inferior, and medio-lateral accelerations of a patients trunk) were obtained using a three-axis accelerometer (Xsens Mtx), enabling us to evaluate 3-D translational body movements on the basis of the average velocity (AV) of the point and the total length (TL) of the 3-dimensional trajectory. Then a pathological balance control has been identified from the 3-D plot of the three trajectories. The data were obtained from patients suffering from the progressive cerebellar ataxia as well as from healthy participants and were analyzed statistically. The analysis revealed several significant differences between the AVs and TLs of the patients versus the healthy participants.

Postural Stability Evaluation of Patients Undergoing Vestibular Schwannoma Microsurgery Employing the Inertial Measurement Unit

The article focuses on a noninvasive method and system of quantifying postural stability of patients undergoing vestibular schwannoma microsurgery. Recent alternatives quantifying human postural stability are rather limited. The major drawback is that the posturography system can evaluate only two physical quantities of body movement and can be measured only on a transverse plane. A complex movement pattern can be, however, described more precisely while using three physical quantities of 3-D movement. This is the reason why an inertial measurement unit (Xsens MTx unit), through which we obtained 3-D data (three Euler angles or three orthogonal accelerations), was placed on the patients trunk. Having employed this novel method based on the volume of irregular polyhedron of 3-D body movement during quiet standing, it was possible to evaluate postural stability. To identify and evaluate pathological balance control of patients undergoing vestibular schwannoma microsurgery, it was necessary to calculate the volume polyhedron using the 3-D Leibniz method and to plot three variables against each other. For the needs of this study, measurements and statistical analysis were made on nine patients. The results obtained by the inertial measurement unit showed no evidence of improvement in postural stability shortly after surgery (4 days). The results were consistent with the results obtained by the posturography system. The evaluated translation variables (acceleration) and rotary variables (angles) measured by the inertial measurement unit correlate strongly with the results of the posturography system. The proposed method and application of the inertial measurement unit for the purpose of measuring patients with vestibular schwannoma appear to be suitable for medical practice. Moreover, the inertial measurement unit is portable and, when compared to other traditional posturography systems, economically affordable. Inertial measurement units can alternatively be implemented in mobile phones or watches.

Comparative Measurement of the Head Orientation Using Camera System and Gyroscope System

The aim of article is to compare two fundamentally different MoCap systems used to measure head orientation. We used IR medical camera and one 3DOF orientation tracker placed on subject’s head and measured 3-D data - inclination (roll), flexion (pitch) and rotation (yaw). The volumes of 3D confidence ellipsoids were used to compare the two systems. Using the volumes of confidence ellipsoids, we were able to model the distribution of the measured 3-D data. Ten healthy subjects in the study were measured and statistical analysis was performed. The results of the comparative method based on the confidence ellipsoid show that the volumes related to the 3DOF orientation tracker are significantly larger than the volumes related to the IR medical camera. Although the results are different when using different MoCap systems, the MoCap systems identified the same subject’s behavior and deterioration or improvement of stability. The comparative measurement based on concept of confidence ellipsoid volume has not been used before to study the MoCap systems used to measure head orientation. The methods can be used to compare other types of MoCap systems and can also be used to study other segments of human body.

Vestibular Compensation following Retrosigmoid Vestibular Schwannoma Surgery

Objectives: Balance problems following vestibular schwannoma (VS) surgery are most apparent in the acute stage and usually improve over time. Persisting vestibular symptoms are related to significant morbidity and decreased quality of life. Methods: Eighty-nine patients undergoing unilateral VS resection via a retrosigmoid approach during 2008-2010 were prospectively analyzed. Definitive assessment of vestibular compensation was performed 24 months postoperatively. Persistent vertigo or spontaneous nystagmus and deviation of subjective visual vertical were classified as non-compensated vestibular pathology. We performed multivariate statistic analysis (Anova) of patient (age, gender), tumor (size – Koos and international classification), preoperative vestibular pathology (ipsilateral/contralateral peripheral, central), and intraoperative (conventional or minimally-invasive approach, tumor adherence, consistency, bleeding, cystic, origo, cerebellar injury) factors. To assess impact of vestibular symptoms on quality of life, we employed Dizziness Handicap Inventory (DHI) and SF36 questionnaires. Results: Twelve of 89 patients undergoing VS surgery suffered from disordered vestibular compensation. Among the factors analyzed, the advanced age seems to be a crucial negative prospective factor of vestibular compensation (P =0.006) (55 ± 10 years in noncompensated vs. 44 ± 13 years in compensated patients). Disbalance was more prevalent in the group of uncompensated patients (P = 4 x 10-7). Surprisingly, poor compensation did not correlate with worse DHI and SF36 scoring. Conclusions: Identification of factors that may contribute to a poor vestibular compensation is crucial for both VS management planning and proper postoperative vestibular rehabilitation. Employment of biofeedback and vestibular prehabituation represent promising methods to improve and speed up postoperative vestibular compensation.