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13:00   Medical Instruments - Surgery II Chair: Joris Jaspers 13:00 15 mins THE ITERATIVE COMPUTATION OF DECOUPLED MODEL SYSTEMS Jörn Kretschmer, Knut Möller Abstract: Mathematical models are a major tool in providing insights in the physiological processes of the human body. They can be exploited to predict effects of therapeutic strategies in intensive care medicine. Medical Decision Support Systems (MDSS) might capitalize such predictions when searching for the optimal therapeutic setting. In critically ill patients that e.g. depend on mechanical ventilation these predictions should cover all main involved organ systems, such as pulmonary mechanics, gas exchange and cardiovascular dynamics. In a previously presented framework developed to support mechanical ventilation we combine elements of these three organ systems [1]. A complex interacting model system can be formed arbitrarily from submodels of these model families. Interaction is achieved through interfaces that enable the submodels to exchange parameters. Computing combinations of moderately complex submodels showed to be computationally costly. An MDSS needs to evaluate the model systems numerous times to find the optimal therapeutic setting. Thus, high computation costs would delay the treatment recommendation. Decoupling the interacting submodels allows for individual computation of submodels with different system dynamics. Unfortunately, direct model interaction as implemented in a coupled computing approach is not possible when submodels are decoupled. Therefore, interface signals need to be substituted by estimates. These estimates can be calculated by the decoupled models and can be improved by iterating the computation. Although it might seem that computing costs rise when iterating the simulation, the application of individual solvers should compensate the increased number of calculations needed. A test setup was created containing a respiratory mechanics model of third order, a tidal breathing 4-compartment gas exchange model [2, 3] and a 19-compartment cardiovascular model which is reactive to intrathoracic pressure [4]. Simulation error converged to a minimum after three iterations. Maximum simulation error after three iterations showed to be 1.1% compared to a coupled computing approach. Simulation error was found to be below measurement noise generally found in clinical data. Simulation time was reduced by factor 17 using three iterations. Applying the proposed calculation scheme, moderately complex model combinations can be made applicable for model based decision support. 13:15 15 mins HIERACHICAL MODEL SYSTEM OF RESPIRATORY MECHANICS FOR PARAMETER IDENTIFICATION Christoph Schranz, Knut Möller Abstract: The application of respiratory mechanics models combined with standardized ventilation manoeuvres enable investigations of patients’ lung mechanics directly at the bedside to optimize ventilation therapy [1]. Therefore, underlying effects of respiratory mechanics such as viscoelasticity, inhomogeneity and recruitment are uncovered by applying certain flow profiles. Subsequently, these effects can be captured by the corresponding model via parameter identification methods. Data sets of 13 patients with Acute Respiratory Distress Syndrome (ARDS) undergoing various ventilation maneuvers [2] are available along with a hierarchical model structure for simulation and parameter identification. The hierarchy is structured according to the model complexity. The first hierarchical layer contains the basic linear 1st order model (FOM) consisting of a serial arrangement of airway resistance and a compliant lung compartment. The second layer contains extensions of the FOM, linear 2nd order models (SOM) assuming viscoelastic tissue contributions (VEM – Viscoelastic Model) or ventilation inhomogeneities (IHM – Inhomogeneity Model). Additionally, the 2nd layer also includes model extensions of the FOM whith either a nonlineare resistance or compliant element. In the given hierarchy, a pressure dependent compliance model (PRM) is implemented [3]. The PRM was developed to simulate the sudden opening of alveolar regions (recruitment) by exceeding a certain opening pressure as observed in ARDS patients [4]. The third hierarchical layer contains a combination of the VEM and PRM, where viscoelasticity is assigned to the recruitable alveolar regions. All models in a particular layer are extensions of the simpler models in the layer above. Thus, the hierarchical structure can support gradient-based parameter identification processes by offering convenient initial values via a prior identification of simpler models in the hierarchy [5]. The applicability of a patient-specific FOM in various ventilation maneuvers proved to be critical since distinct time-depending effects are not considered. In contrast, a individualized SOM captured the observed dynamics and provided accurate simulations under static and dynamic conditions. Pressure dependent effects were successfully captured by the PRM. However, due to the lack of model-dynamics the patient-specific PRM was only applicable in the corresponding maneuver. Finally, only individualized PRVEMs could capture pressure depending recruitment effects together with the observed dynamics to provide accurate simulations in various scenarios. The proposed model hierarchy illustrates the applicability of different models under various circumstances and enables robust parameter identification of more complex models. Due to the combination of the models in the 2nd layer a first suggestion of a respiratory mechanics model could be proposed that is able to provide accurate simulations of a patient in various conditions. 13:30 15 mins ADVANCED STEERABLE AND REMOTE CONTROLLED INTERVENTIONAL INSTRUMENTS FOR IMAGE GUIDED PROCEDURES Helene Clogenson, John van den Dobbelsteen, Jenny Dankelman Abstract: Endovascular interventions have proven to be successful with millions of patients annually diagnosed and treated worldwide. However, this technique presents several weaknesses. The manoeuvrability of these instruments determines whether the target can be successfully reached. Yet, conventional endovascular instruments, catheters and guide-wires, are limited in shape and flexibility and therefore difficult to steer and control [1]. Furthermore endovascular interventions are performed under 2D fluoroscopy/angiography guidance, exposing patient and staff to accumulating ionizing radiations dozes. Consequently, any difficulties in navigating increase the exposure of patient and staff. Magnetic resonance imaging (MRI), on the other hand, has no known harmful effects, offers several advantages for both patients and interventionalist [2, 3]. The goal of this project is to develop an MRI-compatible and steerable endovascular instrument with improved manoeuvrability. In order to achieve more insight in the design requirements of steerable instruments for interventional procedures an experimental study was performed. While navigating the instruments, time-action analysis was used to investigate the relation between the geometry of bifurcations, the shape of catheters and the time taken to perform specific actions Based on the results of this study a 60cm long, 6Fr, polymer based, MRI-compatible catheter with a two-degree-of-freedom tip has been designed and is currently been assembled. Once realized the proposed instrument will provide the opportunity to steer, or adjust, the instrument tip to adapt to the geometry of the anatomy and bifurcation, thereby directly improving the distal control of the instruments. An evaluation of this steerable prototype will be performed. First, the main mechanicals properties of the prototype, such as stiffness and torque, will be measured. Then the instrument will be manipulated and evaluated in a vascular model with novices. Finally in-vitro and in-vivo experiments will be planned with experienced interventional radiologists. REFERENCES 1. Fu Y, L.H., Huang W, Wang S, Liang Z., Steerable catheters in minimally invasive vascular surgery. Int J Med Robot., 2009. 5(4): p. 381-91. 2. Saikus CE, L.R., Interventional cardiovascular magnetic resonance imaging: a new opportunity for image-guided interventions. JACC Cardiovasc Imaging., 2009. 2(11): p. 1321-31. 3. Krämer NA, K.S., Schmitz S, Linssen M, Schade H, Weiss S, Spüntrup E, Günther RW, Bücker A, Krombach GA., Preclinical evaluation of a novel fiber compound MR guidewire in vivo. Invest Radiol., 2009. 44(7): p. 390-7. 13:45 15 mins SHAPING PATIENT SPECIFIC TEMPLATES FOR ARTHROPLASTY TO OBTAIN HIGH DOCKING ROBUSTNESS Joost Mattheijer, Just Herder, Gabrielle Tuijthof, Rob Nelissen, Jenny Dankelman, Edward Valstar Abstract: In arthroplasty, worn out and painful joints are replaced with a prosthesis. Alignment instruments are used to find where bone cuts need to be made, in order to place the prosthesis with the correct alignment. This process can often be problematic and prone for errors, because only a small part of the involved bones – the joint - is exposed. Computer Assisted Surgery (CAS) techniques are being used in arthroplasty, in order to obtain accurate alignment of prosthetic components. Generally there are two CAS approaches used today: Camera-based CAS and Patient Specific Templates (PST’s). Camera-based CAS relies on – time consuming – registration of the actual bone surfaces as exposed during surgery and the virtual bone models resulting from CT or MRI. Bone, instruments and prosthesis using markers are subsequently tracked by a camera. For the PST method, the virtual bone models are used to fabricate plastic templates, representing the negative imprint of the joint surface [1]. During surgery, the templates are supposed to dock in the planned position only, taking away the need for a time-consuming registration process. Holes and slots that are incorporated in the templates are instantly aligned right and will be used to guide cutting instruments. Our research group is developing Configurable PST’s: templates that can be customized for every surgery to have a patient specific fit. As a first topic of research we are investigating the quality of the fit. The shape of the template – the location of bone-template contact and an application surface where the surgeon may push – determines the fit that may be perceived when the template is held in its planned position. The goal is to analyze the effect of the template’s shape onto the range of forces that may be applied, i.e. the docking robustness. The bony geometry is hereto used as an input to find suitable locations for bone-template contact and the application surface. The wrench space theory [2] widely used in the related research areas of robotic hands and workpart fixtures is employed to obtain an analytical method. With this method, templates can be fully shaped to obtain high docking robustness with minimal contact. REFERENCES [1] M. Hafez et al., “Computer-Assisted Total Knee Arthroplasty Using Patient-Specific Templates”, Navigation and MIS in Orthopedic Surgery, pp. 182-188, 2007. [2] B. Dizioğlu, K. Lakshiminarayana, “Mechanics of form closure”, Acta mechanica, Vol. 52, pp. 107-118, 1984. 14:00 15 mins IS SCOLIOSIS INDUCTION A GOOD MODEL FOR SCOLIOSIS CORRECTION? Gerdine Meijer, Martijn Wessels, Edsko Hekman, Jasper Homminga, Bart Verkerke Abstract: Scoliosis is a deformity of the spine and trunk, mainly characterized by a lateral deviation of the spinal column in combination with axial rotation of the vertebrae. In the current project, a new scoliosis correction implant was developed, which will apply small correction forces over a longer period of time [1]. Due to the visco-elastic properties and adaptation of the soft tissues of the spine, a complete correction is expected over time. Preclinical evaluation of new implants includes testing the efficacy in animal experiments. However, as scoliosis does not occur in animals, these tests study scoliosis induction rather than scoliosis correction. Methods for scoliosis correction in humans are thus tested by inducing scoliosis in animals. In this study, two previously developed finite element models were used [2], one of the healthy and one of the scoliotic spine (figure 1). The effects of a pure torsion moment (1.5 Nm) on the T8-vertebra of a healthy spine was compared to the effects of a correction moment at the same level (-1.5 Nm) on a scoliotic spine. The long-term correction and induction effects (including visco-elasticity and adaptation of soft tissues) on the 3D deformities were analyzed (table 1). Comparison of scoliosis correction and induction in our models showed that the mechanical effects of a torsion loading on a healthy spine differ from those on a scoliotic spine. Both showed effects in the axial rotation and lateral deformity, and no effects on the sagittal shape. However, the effects in the lateral plane were much higher in the scoliosis correction simulations than in the scoliosis induction simulations, which might be explained by the different functioning of the facet joints. Our study indicates that results from torsion-induced scoliosis in animals cannot be translated directly into scoliosis correction in humans or even in animals; from the results it is expected that the 3D correction in a real-world scoliosis procedure will be better than what was obtained in the scoliosis induction experiments in animals. 14:15 15 mins VISUALISATION TECHNIQUES FOR AIR FLOWS IN A LIVE OPERATING ROOM TO STUDY POTENTIAL SOURCES FOR INFECTION Ruud Verdaasdonk, Joost de Jong, Albert van der Veen, Keith Cover Abstract: Infections can be a serious complication after surgery. In the design of operating rooms (OR), a laminar flow of clean air is created above the field of surgery to prevent contaminations. In this study, various air flow visualization techniques have been developed and compared to optimize for high sensitivity, large field of view and practical to be applied in a live OR setting. The visualization techniques are based on contrast enhancement using either ‘analoge’ optical methods (Schlieren) or digital image processing methods enabling quantification of the images e.g. applying colour intensity projection. Small density gradients in air induced by e.g. temperature differences, will deflect light beams. By discriminating the deflected rays from background light using spatial filters or digital subtraction techniques, air flow can be visualized. An enormous contrast enhancement was obtained with a classical Schlieren (CS) setup using concave mirror and a spatial filter (knife edge). However, the field of view was limited to the size of the mirror (35 cm and 200 cm focal length). To enlarge the field of view, a background oriented Schlieren setup (BOS) was developed by placing a high contrast pattern (e.g. lines ~10/cm) in the background and subtracting images during the air flow either with a ‘analoge’ spatial filter or digitally. This proved less sensitive compared to CS. The Schlieren setup showed significant disturbance of the laminar flow of clean air above the operation table depending on the position of the operating lamp. Looking at the air flow around various instruments that are standard in the OR, air turbulence over 1 m distance was visible created by the cooling vent. When these vents are directed to the ground or into the operation field, the flow of clean air intended to prevent infections, could easily be disrupted. Schlieren techniques are useful to investigate potential sources for infection in a live operation room.