Ion Pappas

A reliable gait phase detection system

A new highly reliable gait phase detection system, which can be used in gait analysis applications and to control the gait cycle of a neuroprosthesis for walking, is described. The system was designed to detect in real-time the following gait phases: stance, heel-off, swing, and heel-strike. The gait phase detection system employed a gyroscope to measure the angular velocity of the foot and three force sensitive resistors to assess the forces exerted by the foot on the shoe sole during walking. A rule-based detection algorithm, which was running on a portable microprocessor board, processed the sensor signals. In the presented experimental study ten able bodied subjects and six subjects with impaired gait tested the device in both indoor and outdoor environments (0-25/spl deg/C). The subjects were asked to walk on flat and irregular surfaces, to step over small obstacles, to walk on inclined surfaces, and to ascend and descend stairs. Despite the significant variation in the individual walking styles the system achieved an overall detection reliability above 99% for both subject groups for the tasks involving walking on flat, irregular, and inclined surfaces. In the case of stair climbing and descending tasks the success rate of the system was above 99% for the able body subjects and above 96% for the subjects with impaired gait. The experiments also showed that the gait phase detection system, unlike other similar devices, was insensitive to perturbations caused by nonwalking activities such as weight shifting between legs during standing, feet sliding, sitting down, and standing up.

A reliable, gyroscope based gait phase detection sensor embedded in a shoe insole

This paper describes a new gait phase detection sensor (GPDS) and its application together with a functional electrical stimulation (FES) system for subjects with a drop-foot walking dysfunction. The gait phase detection sensor (sensors and processing unit) is entirely embedded in a shoe insole and detects in real time four gait phases (events) during the gait cycle: stance, heel-off, swing and heel-strike. The gait phase signal is used in a finite state control scheme to time the electrical stimulation sequences in order to generate a motion in the affected leg that is close to a physiological motion. The GPDS uses a miniature gyroscope that measures the rotational velocity of the foot and three force sensitive resistors that measure the force load on the shoe insole. Contrary to other systems using only force sensors, our system can easily differentiate between true walking and weight shifting from one leg to the other during standing. This is achieved through the combination of force sensors and a gyroscope sensor.

A New System for Computer-Aided Preoperative Planning and Intraoperative Navigation During Corrective Jaw Surgery

A new system for computer-aided corrective surgery of the jaws has been developed and introduced clinically. It combines three-dimensional (3-D) surgical planning with conventional dental occlusion planning. The developed software allows simulating the surgical correction on virtual 3-D models of the facial skeleton generated from computed tomography (CT) scans. Surgery planning and simulation include dynamic cephalometry, semi-automatic mirroring, interactive cutting of bone and segment repositioning. By coupling the software with a tracking system and with the help of a special registration procedure, we are able to acquire dental occlusion plans from plaster model mounts. Upon completion of the surgical plan, the setup is used to manufacture positioning splints for intraoperative guidance. The system provides further intraoperative assistance with the help of a display showing jaw positions and 3-D positioning guides updated in real time during the surgical procedure. The proposed approach offers the advantages of 3-D visualization and tracking technology without sacrificing long-proven cast-based techniques for dental occlusion evaluation. The system has been applied on one patient. Throughout this procedure, we have experienced improved assessment of pathology, increased precision, and augmented control

Stability criterion for controlling standing in able-bodied subjects

A new stability criterion that can be used to assess the standing condition of a subject from center of pressure (CoP) measurements is presented. This criterion can be applied, for example, to control a standing prosthesis, which should allow a paraplegic subject to stand up, sit down and stand safely without using hands for support. Experiments conducted with able-bodied subjects enabled us to establish a relationship between its stability and the subjects CoP position. Four CoP stability zones were identified: high preference, low preference, undesirable and unstable zones. The high preference zone is defined as the area where the CoP is found 99% of the time during quiet standing. The area where the CoP is found during the remaining 1% of the time is called the low preference zone. The undesirable zone is defined as the CoP area where the subject is forced to change posture in order to maintain balance, and the unstable zone is defined as the CoP area in which the subject is forced to step forward, backward or sideways to maintain stability. A general model of the proposed four stability zones was derived, which can be used to compute stability zones a priori for any subject and thus allows one to assess the subjects stability condition from the CoP measurements.

Development of a force amplitude- and location-sensing device designed to improve the ligament balancing procedure in TKA

To improve the ligament balancing procedure during total knee arthroplasty a force-sensing device to intraoperatively measure knee joint forces and moments has been developed. It consists of two sensitive plates, one for each condyle, a tibial base plate and a set of spaces to adapt the device thickness to the patient-specific tibiofemoral gap. Each sensitive plate is equipped with three deformable bridges instrumented with thick-film piezoresistive sensors, which allow accurate measurements of the amplitude and location of the tibiofemoral contact forces. The net varus-valgus moment is then computed to characterize the ligamentous imbalance. The developed device has a measurement range of 0-500 N and an intrinsic accuracy of 0.5% full scale. Experimental trials on a plastic knee joint model and on a cadaver specimen demonstrated the proper function of the device in situ. The results obtained indicated that the novel force-sensing device has an appropriate range of measurement and a strong potential to offer useful quantitative information and effective assistance during the ligament balancing procedure in total knee arthroplasty.

Visual control of a microrobot operating under a microscope

In this paper the vision-based control of a microrobot operating under a light microscope is presented. For high accuracy manipulation the parameters describing the relationship between the reference frame and the frame attached to the robot must be continuously and accurately updated These parameters are unknown a priori and are not directly measurable, but can be deduced by observing the scene change during the motion. The relative positioning accuracy is about 100 nm and trajectories can be precisely followed in 3 degrees of freedom. Experimental results are presented.

A task-oriented teleoperation system for assembly in the microworld

Operating a robot in the micro/nano-world presents challenges not found in the macro-world. Due to the small dimensions, the operator has no direct access to the objects and must assemble them by teleoperation. To allow a good manipulability, we propose a mixed teleoperation system which is composed of both direct and task-oriented teleoperation modes. The operator is provided with a set of visualization and manipulation tools on a workstation. This paper presents the micro-robot system, discusses some important issues in micro-teleoperation and finally presents some micro-manipulation tasks performed with the system.

Improved targeting device and computer navigation for accurate placement of brachytherapy needles.

Successful treatment of skull base tumors with interstitial brachytherapy requires high targeting accuracy for the brachytherapy needles to avoid harming vital anatomical structures. To enable safe placement of the needles in this area, we developed an image-based planning and navigation system for brachytherapy, which includes a custom-made mechanical positioning arm that allows rough and fine adjustment of the needle position. The fine-adjustment mechanism consists of an XYZ microstage at the base of the arm and a needle holder with two fine-adjustable inclinations. The rotation axes of the inclinations cross at the tip of the needle so that the inclinational adjustments do not interfere with the translational adjustments. A vacuum cushion and a noninvasive fixation frame are used for the head immobilization. To avoid mechanical bending of the needles due to the weight of attached tracking markers, which would be detrimental for targeting accuracy, only a single LED marker on the tail of the needle is used. An experimental phantom-based targeting study with this setup demonstrated that a positioning accuracy of 1.4 mm (rms) can be achieved. The study showed that the proposed setup allows brachytherapy needles to be easily aligned and inserted with high targeting accuracy according to a preliminary plan. The achievable accuracy is higher than if the needles are inserted manually. The proposed system can be linked to a standard afterloader and standard dosimetry planning module. The associated additional effort is reasonable for the clinical practice and therefore the proposed procedure provides a promising tool for the safe treatment of tumors in the skull base area.

3D surgical planning and navigation for CMF surgery

In this paper we describe a system for corrective and reconstructive CMF surgery that allows planning of bone segment relocations in 3D and transfer of the goal positions into an intra-operative navigation module, which provides guidance to realize the planned movement. In addition, the pre-operative planning module offers functions of mirroring and allows insertion of distraction devices. We present three clinical cases of CMF surgical procedures planned a posteriori with our application: bimaxillary realignment, involving subcondylar osteotomy of the mandible and LeFort I osteotomy, secondary orbital reconstruction and mandibular reconstruction.

New method to assess the registration of CT-MR images of the head

Due to their complementary information content, both X-ray computed tomography (CT) and magnetic resonance (MR) imaging are employed in certain clinical cases to improve the understanding of the pathology. To spatially relate the two datasets, image-registration and image fusion is employed. However, registration errors, either global or local, are common and are non-uniform within the image volume. In this paper, we propose a new algorithm, which assesses the quality of the registration locally within the CT-MR volume and provides visual, color-coded feedback to the user about the location and extent of good/bad correspondence between the two images. The proposed registration assessment algorithm is based on a correspondence analysis of bone structures in the CT and MR images. For that purpose, a custom segmentation algorithm for bone in MR images has been developed, which is based on a stochastic threshold computation method. This segmentation method for MR images and the CT-MR registration assessment algorithm were validated on simulated MR datasets and real CT-MR image pairs of the head. Some partial-volume effects occur at the borders of the bone structures and at the bone interfaces with air, which cannot be separated from bone in the MR image. The presented assessment method of CT-MR image registration offers the user a new tool to evaluate the overall and local quality of the registration. With this information, the user does not have to blindly trust the fused CT-MR datasets but can easily identify areas of inaccurate correspondence. The application of the algorithm is so far limited to T1-weighted MR and CT images of the head area.