279 Sensitivity of selfdynamisable internal fixator to change of bar length and clamp distance Marko Simenonov *,** , Nikola Korunović * , Miroslav Trajanović * , Manfred Zehn ** , Milorad Mitković *** * University of Niš, Faculty of Mechanical Engineering, Niš, Serbia ** Technische Universität Berlin, Berlin, Germany *** University of Niš, Faculty of medicine, Niš, Serbia mare.simeonov@gmail.com nikola.korunovic@masfak.ni.ac.rs miroslav.trajanovic@masfak.ni.ac.rs manfred.zehn@tu-berlin.de mitkovic@gmail.com Abstract—Selfdynamisable internal fixator (SIF) is a type of medical device used in internal fixation of long bones. Occasional SIF failure requires an extra surgery, which causes an additional trauma for the patient. To minimize the risk of failure, a methodology was established for optimization of SIF structure and position. It is based on sensitivity studies or structural optimization procedures, performed using finite element method (FEM). Automatic creation of FEM models, for any combination of values of dimensional and positional parameters, is enabled trough creation of a robust and flexible CAD model and establishment of bi-directional associativity between CAD and FEM models. This paper focuses on the results of sensitivity study, performed using the defined methodology. I. INTRODUCTION Fractures of long bones typically occur as a consequence of traffic accidents or extreme sports injuries and usually require stabilization by surgery. Selfdynamisable internal fixator (SIF), invented by prof. Mitković, is a type of medical device used in internal fixation of long bones [1]. The structure of SIF is modular (Fig. 1). One of the modules, trochanteric unit with bar, is produced in four different sizes. The number and mutual position of clamps can also be changed, depending on application. Figure 1. SIF, configured for subtrochanteric femoral fracture treatment Selfdynamisable internal fixator is designed to allow for the change of axial position of fixated bone parts, which means that the bone parts can get closer over time. In this way, there exists a greater probability of healing process success, thus an extra surgery, which represents an additional discomfort to the patient, is usually avoided [2]. Occasionally a failure occurs in SIF components during the healing process. In those cases an extra surgery is still required. It is highly desirable to minimize the number of such occurrences. Failure of SIF components is related to its stress state during the healing process. For each specific patient and type of femoral fracture there exists an optimal configuration and position of SIF components that minimizes component stresses and thus the possibility of failure. In that manner, a research goal was set which aimed to establish a methodology for optimization of SIF configuration and position considering any subject- specific case of femoral fracture. At the same time, the whole optimization process was foreseen to be as robust and fast as possible so it may be used in everyday medical practice. II. METHODOLOGY The defined methodology for optimization of SIF configuration and position consist of four steps, explained below. 1. In the first step, the subject-specific CAD model of femur is created. The creation process consists of two stages: the creation of a polygonal model of the femur and the creation of a CAD model of the femur. Polygonal model is obtained as a result of a pre- defined procedure, which starts with CT scanning of patient’s lower extremities. The CAD model is than created from medical images, in a process that includes the extraction of the zone of interest, point cloud creation, triangulation, surface fitting and creation of solid features. In order to obtain the CAD model of femur, it is necessary to remove the segments of the polygonal model that actually don’t belong to the femoral bone (parts of the vascular system visible due to insertion of contrast media or surrounding bones). Than the interior of the model is cleaned, whereby the excessive geometry that mainly originates from trabecular bone structure is removed. After the polygonal model is healed and, if required, its smoothness is increased, a closed NURBS surface that represents the outer envelope of the model can be created using surface fitting procedures. By filling the surface with solid geometry, CAD model of femur is created [3], [4]. 7th International Conference on Information Society and Technology ICIST 2017