10:30   Sensors and Man-machine interfacing
Evaluation of mono and stereo marker tracking in see-through head mounted displays for surgical interventions
Abdullah Thabit, Wiro Niessen, Eppo Wolvius, Theo van Walsum
Abstract: Introduction: Optical tracking systems are widely used in surgical interventions. However, despite their great utility and accuracy, they are expensive and require time and effort to setup for surgeries. Head mounted displays such as the Microsoft HoloLens may provide an attractive alternative for surgical navigation that also permits augmented reality visualization. The HoloLens is equipped with multiple sensors for tracking and scene understanding. Mono and stereo-vision in the HoloLens have been reported to be used for marker tracking, but no extensive evaluation on accuracy has been done to compare the two approaches. The objective of our work is to investigate the tracking performance of various camera setups in the HoloLens, and to study the effect of the marker size, marker distance from camera, and camera resolution on marker localization accuracy. We also investigate the speed and stability of marker pose for each camera setup. Methods: The tracking approaches were evaluated using ArUco markers. For mono-vision, the marker pose was determined by utilizing the camera intrinsics, while for stereo-vision, triangulation of the marker corners was used to find the marker pose in 3D. An NDI optical tracking system served as a ground truth and the tracking accuracy of both approaches were compared. A combined mono-stereo tracking setup was then proposed and evaluated. Results: The results show that mono-vision is more accurate in marker localization than stereo-vision when high resolution is used. However, this comes at the expense of higher frame processing time. Alternatively, our proposed low-resolution mono-stereo tracking setup outperformed each tracking approach individually and was comparable to high resolution mono tracking, with an overall translational error of 1.8 ± 0.6mm for a marker size of 10cm at 50cm distance. Conclusion: The tracking accuracy of mono and stereo vision in the HoloLens is still relatively high for surgical navigation, but it has the potential to allow the use of the HoloLens for applications where perpendicular view can be maintained. Moreover, our proposed combined mono-stereo tracking setup improved the tracking accuracy while maintaining a reasonable frame processing time, but other ways of integrating the two approaches should be investigated for further improvement.
Virtual extensions for augmented reality surgical instruments alignment
Mohamed Benmahdjoub, Wiro Niessen, Eppo Wolvius, Theo van Walsum
Abstract: Background: Augmented reality (AR) has been used in surgery for navigation purposes. It solves the handeye coordination issue, and the recurrent switch of focus between the screen and the patient. These challenges are apparent during the use of conventional navigation system. However, AR solves, only partially, the depth perception issue. Therefore, drilling surgical operations that require surgeons to perform instrument alignment with pre-planned trajectories, can be hard to perform using AR. The purpose of this study is to investigate ways to perform alignment tasks in AR. Methods: We conducted a user evaluation study. Participants were asked to perform alignment tasks under three conditions: no instrument augmentation (N), realistic instrument augmentation (R), and virtual extensions instrument augmentation (VE). Objective measurements such as instrument positional error with respect to the planning, orientation error, head and instrument velocity were collected. Additionally, subjective measures were collected by means of the forms NASA-TLX and SUS to measure the usability, and the frustration levels in each condition. A customised form, asking the participants to rank their best performance according to their perception was also filled. Results: 18 participants have participated in the study resulting in 324 performed task (3 conditions and 6 repetitions). Virtual extensions metaphor allowed the participants to perform better in terms of positional and orientation error achieving a median of (1.9 mm with IQR=1 mm) and (1.8°, IQR = 1.3°) respectively. Moreover, participants experienced less frustration and felt more confident when aligning the instrument under the VE condition. Conclusion: This study presents a new visualization metaphor (VE) in AR setups for instrument alignment. Using VE, participants reached a median of 2 mm and 2° of positional and orientation error and experienced lower frustration levels. This method could facilitate alignment tasks under medical AR environments.
Urea monitoring in plasma, sweat and saliva of patients during hemodialysis
Sophie Adelaars, Daan van de Kerkhof, Arthur Bouwman, Eduard Pelssers, Massimo Mischi
Abstract: In the Netherlands, approximately 5000 patients are being treated by hemodialysis to support renal function. The procedure is time-consuming with cycles lasting around 4 hours with three to five sessions per week. As patients are admitted to the hospital for half a day every dialysis session, the high frequency of hemodialysis procedures results in a high burden for the patients’ quality of life. Hemodialysis at home is considered a promising development to improve quality of life. In addition, the adequacy of the hemodialysis is only monitored once every 3-4 months by measuring urea concentrations in blood and fluctuations in adequacy are potentially missed. Consequently, there is a clinical need to perform continuous and non-invasive monitoring of hemodialysis not using patients’ blood. In this pilot study sweat and saliva creatinine and urea will be analyzed and compared to plasma analysis in a group of 40 hemodialysis patients. Sweat analysis is currently rarely used for laboratory diagnostics. However, the development of microfluidic sensors able to handle low volume bio-fluids makes sweat sensing an emerging technology for non-invasive and continuous analyte monitoring. Although urea analysis in sweat has been investigated already since 1914, little is known about the correlation between the sweat urea concentration and the plasma urea concentration. For hemodialysis patients, a (semi-)continuous sweat sensor that can accurately measure the urea concentration at distance would ideally mean a non-invasive and continuous method to closely monitor the treatment adequacy and the course of urea over the day. This innovation potentially opens up adequate home-monitoring and patient specific treatment and as a consequence, improvement of the patients’ quality of life. The main aim of this first study is to clarify the relation of urea concentrations between sweat and saliva versus serum. This improves the current fundamental knowledge that is needed for the use of alternative monitoring techniques that could, if feasible, be applied in home monitoring using microfluidic sensor technology. Data collection for this study is planned to start October 2021.
The hundred dollar hand, Using 3D-printing to make prostheses accessible worldwide
Gerwin Smit, Jan van Frankenhuyzen
Abstract: Background A major part of the amputees lives in developing countries and does not have access to prosthetic healthcare. New technology, like 3D-printing, offers a potential solution to provide these amputees with affordable prostheses. Many recently developed 3D-printed hands are fragile and it remains unknown on how well they perform on functionality and durability [1]. Aim Our aim was to develop a 3D-printed hand prosthesis that is low-cost, lightweight, durable, functional, requires a low actuation force and has a hand-like appearance. Method The design of the hand prosthesis should be low-cost and allow for local production. We used 3D-printed PLA wherever possible and steel (laser-cutting) where necessary (e.g. loaded parts and axes). The hand is Voluntary Closing and can be controlled with a shoulder strap, using an efficient force transmission. The hand has 1 DOF, similar to conventional prosthetic hands and can be fitted with a cosmetic glove. It was tested in a mechanical evaluation, a duration test (>200.000 loading cycles) and in functional tests (Box and Block Test or BBT) with able-bodied subjects (n=20) using a prosthesis-simulator. Results We designed a low-cost hand that can be produced and sold for a price below $100, hence its name: ‘Hundred Dollar Hand’. The hand can be locally produced, using a standard 3D-printer and laser cutter. The hand, without glove, weighs 220 grams. The mechanical evaluation showed the hand needs a relatively low actuation force (~75 N to pinch 20 N). The hand withstood over 200.000 loading cycles in the duration test, needing only minor repair. The BBT functional test showed a score of 31.9±4.8 blocks/min. When covered with a cosmetic glove, the hand has a hand-like appearance, comparable to standard prosthetic hands. The hand is currently being marketed in India. Discussion and Conclusion We present a low-cost 3D-printed hand prosthesis, the ‘Hundred Dollar Hand’ that is durable, functional, lightweight and requires a low actuation force. When covered with a cosmetic glove, it has a hand-like appearance. Unlike many other studies on 3D-printed hands, our study provides data on durability and functionality. References 1. Ten Kate, 2017, Disab. and Rehab. Assis. Tech.
Patterns of physical activity over time in older patients rehabilitating after hip fracture surgery
Dieuwke van Dartel, Ying Wang, Han Hegeman, Marloes Vermeer, Miriam Vollenbroek-Hutten
Abstract: Objective: This study aimed to investigate the patterns of physical activity over time in older patients rehabilitating after hip fracture surgery by continuously monitoring physical activity. Furthermore, it was assessed whether the patterns of physical activity were associated with patient characteristics. Method: Physical activity of patients aged 70 years, who were surgically treated for a hip fracture and in rehabilitation at a skilled nursing home, was continuously measured using a MOX accelerometer. The intensity of physical activity per day was calculated from the accelerometer signals to describe a patient’s activity level and plotted over time. With the help of two experts in the geriatric rehabilitation field we identified 6 unique physical activity patterns based on visual inspection. Subsequently, 18 raters (all healthcare professionals) classified independently for each patient which physical activity pattern is most characterizing their recovery process. Results: Data from 66 patients were used in this study. Most common patterns of overall physical activity over time were a S-shape pattern of physical activity (n=23, 34.8%) and an upward linear pattern (n=15, 22.7%). The S-shape pattern showed a slow increase in overall physical activity at the beginning of the rehabilitation, followed by a steep increase, and a plateau phase at the end of rehabilitation. The upward linear pattern showed a linear increase in overall physical activity during the whole rehabilitation. Results showed a significant difference between the duration of rehabilitation stay and the different patterns. Patients with an upward linear pattern had the shortest duration of stay. In addition, patients with an upward linear pattern of overall physical activity also had better functionality and mobility scores at admission to the rehabilitation compared with the other patterns. Conclusion: Different patterns of overall physical activity over time in older patients rehabilitating after hip fracture surgery were found in this study. Results showed that the mobility and functionality at admission to the rehabilitation seemed to be associated with the pattern of overall physical activity. Furthermore, it is shown that the duration of rehabilitation stay depends on the patterns of physical activity.
Evaluation of tracking relative feet and CoM movement using only three iMUS during variable overground gait in chronic stroke patients
Roelien Russcher, Mohamed Irfan Mohamed Refai, Bert-Jan van Beijnum, Jaap Buurke
Abstract: Stroke patients often experience balance and stability problems, due to hemiparesis. Clinical therapy is therefore focused on improving mobility and functional capacity. There is however a lack of objective information about the underlying mechanisms for improvement during rehabilitation, both in the clinic and at home. Information about the centre of mass (CoM) position relative to the feet can be used to objectively measure balance and stability. A system using only three Inertial Measurement Units (IMUs) for tracking relative feet and CoM positions has been proposed as a portable alternative to optical motion capture [1]. The system employs a set of biomechanical constraints including the Centroidal Moment Pivot theory and has been validated in six healthy participants. Step detection is an important part of the algorithm and was based on a generalized likelihood ratio test [2]. The goal of the current study was to evaluate this system in one chronic stroke patient with functional ambulation category 5 against reference systems such as VICON© and ForceShoesTM. The algorithm was re-optimized before application for impaired gait. The participant was asked to perform tasks based on clinical tests (e.g. straight walking, Figure-8 test). The study was approved by the local ethical committee, and all participants consented to the study. Measurements were executed in the gait lab at Roessingh Research and Development, Enschede. The mean absolute error (±SD) in estimating step length and step width were 4.6 ± 2.4 cm and 4.9 ± 3.3 cm for straight walking, and 6.8 ± 3.4 cm and 13.7 ± 5.4 cm for the Figure-8 test. The results for straight line walking are comparable to the results obtained in healthy subjects [1], although the results in that study were an average of different walking tasks. The algorithm was not optimized for turning, as can be seen in the errors of the Figure-8 test. Nevertheless, the approach shows that after optimization of the algorithm the system is able to track relative feet and CoM movements in one chronic stroke patient. Alternative step detection methods should be investigated, after which further validation is recommended. [1] M.I. Mohamed Refai, B.J.F. van Beijnum, J.H. Buurke, and P.H. Veltink, “Portable gait lab: Tracking relative distances of feet and com using three imus,” IEEE Transactions on Neural Systems and Rehabilitation Engineering, vol. 28, no. 10, pp. 2255-2264, 2020. [2] I. Skog, P. Handel, J.O. Nilsson, and J. Rantakokko, “Zero-velocity detection-an algorithm evaluation,” IEEE transactions on biomedical engineering, vol. 57, no. 11, pp. 2657-2666, 2010.

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