EXPERIMENTAL STUDY OF DYNAMICS OF THE HUMAN THORAX WITH NUSS IMPLANT

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1 Jan AWREJCEWICZ, Bartosz ŁUCZAK, Department of Automatics and Biomechanics, Technical University of Łódź EXPERIMENTAL STUDY OF DYNAMICS OF THE HUMAN THORAX WITH NUSS IMPLANT Summary. The experimental rig was developed in order to perform more detailed investigation of the human rib cage responses and injuries subjected to impact loads and to validate numerical result published recently by the authors. Thorax model was established with impact velocities ranging between 2-4 m/s and impactor masses of kg. From these impacts, force-deflection, force-time and deflection-time curves were generated. The curves define the response of the human thorax to the given impact conditions. In an effort to understand response to blunt impacts of human thorax with implant, the same techniques were employed. 1. INTRODUCTION Developed in the mid 1970s the Anthropometric Test Device (ATD) has become the important tool for investigate restrain systems. Biofidelic response to thorax impact loading has been significant performance criterion for ATD s. The thoracic loading response criteria for ATD s reflected the need for accurate evaluation of crash conditions involving anterior chest impact with the steering wheel. In the mid 1960s the extensive research of thorax dynamics respond was made by group of General Motors scientists managed by Kroell [4]. The laboratory tests involved impact of pendulum to the seated subject s central sternum. Flat disk pendulum with 152 mm diameter was similar in profile to a steering wheel hub (Fig. 1). Chest deflection and impactor force were recorded. Figure 1. Kroell experimental rig [4]

2 All types of crash test dummies ribcage element were developed to math the force deflection response based on the Kroell tests involving impactor velocities of 4,3 and 6,7 m/s [4]. From the 1980s use of restraint belt increases [3], and in 1990s passive restrain systems (airbags or three point belt) were a standard in all new cars. Nowadays, researchers believe that higher loading rates will become less important and lower loading rates, resulting from interaction with shoulder belt and airbags, will become increasingly important. In the new thorax, designing to achieve humanlike biofidelity in response to low loading rates, and even quasi-static loading conditions, may be of equal or greater importance than designing to achieve biofidelity at a higher loading rates [5]. The main goal of our research was to design and build a new thorax model which has a biofidelity with low loading rates patterns. The autors also postulate using such type of thorax model to investigate the new chest implants [1,2]. In minimally invasive pectus excavatum repair procedure the implants are used to elevate and hold deformed sternum in desired position. Nuss implant left in a human organism for two or even more years. During such a long period of time a patient may participate in a road accident or another traumatic incident. Therefore, an investigation of a ribcage with implant responses to impact loads should be carried out. 2. METHOD 2.1. Construction of new ribcage model Constuction of a new ribcage model was preceded by wide analysis of constructions of various types of ATD s ribcages (Fig. 2). Figure 2. Frontal impact dummy torsos. In constructions presented above the spring steel ribs with visco-elastic damping material are used to simulate the highly rate dependent response recorded for cadavers during Kroell tests. The damping material itself was insufficient to achieve the required stiffening under high rate loading. Thus, increasing the elastic stiffness of the ribcages was required, which however, compromises response at low loading rates, such as those generated by shoulder belts [6]. In new construction the authors gave up using steel ribs in favor of using ribs made from polyurethane. Each of rib was milling in PVC sheet as a negative and polyurethane was poured into moulds. Since the dynamical validation of ribcage was performed by comparison with whole body cadavers tests, the thorax construction must take into account the inertia of

3 head, legs, arms, etc. To obtain proper weight and inertia distribution of ribcage the additional masses were added (see Fig. 3). Head mass Arm mass Legs mass Figure 3. New ribcage model In previous models of frontal impact dummies the spine segments was stiff and undeformable. In what follows the proposed by the authors a new spine assembly is full flexible and it is believed that it is more biofidelic Construction of experimental rig The construction of experimental rig was based on Hybrid III thorax calibration test stand (Fig. 4). To realize validation process of a new ribcage model the protocol of National Highway Traffic Safety Administration (NHTSA) and European New Car Assessment Programme (EuroNCAP) in part thorax calibration has been applied. Figure 4. Hybrid III thorax test stand

4 In the experimental rig a rigid disk initiated by energy of spring was used as an impactor. In construction of experimental rig the authors gave up of using a pendulum, because main assumption of design was to construct a portable rig (Fig. 5). Impactor Guide rails Figure 5. Experimental rig The time deflection curves are obtained from deflection sensor, which has been constructed and validated by the authors (see Fig. 6). The sensor has been calibrated and standardized by comparative measurements. Dynamical changes of voltage has been recorded in oscilloscope, and after that the data have been transformed to the deflection - time form. Affixed to sternum Affixed to thoracic vertebrae Potentiometer Figure 6. Deflection sensor and sensor calibration

5 To measure dynamical force magnitude an accelerometer sensor affixed to the impactor has been applied (mass of impactor is known). 3. PRELIMINARY RESULTS In this report only preliminary results are presented. The results are obtained for the following impact conditions: the impactor mass is 23,4 kg and impactor speed is 4,3 m/s. The obtained data is compared with the results of Kroell test [4] and also with numerical results published recently by the authors [1, 2] for the same impact conditions (Fig. 7). Figure 7. Thorax model frontal impact results (continuous line), results of Kroell cadavers test (grey region) and FEM analysis (dashed line) [1, 2]. 4. CONCLUSIONS Analysis of preliminary results (see Fig. 7) allowed to recognize that stiffness of the new ribcage model is not sufficient for that impact conditions. The authors proposed some modification of ribcage construction, introducing the stiffening steel elements in posterior ends of each rib. The next problem noted in the final phase of movement are vibrations of the ribcage elements. The authors believe that mentioned stiffening elements will reduce these harmful effects. It should be emphasized that the presented so far results do not yield a full characteristic of dynamical response of thorax model to impact loads. The authors, in the next phases of validation, are going to make more precise analysis of ribcage biofidelity. However, the experimental rig should be still slightly modified to obtain more humanlike results. The proposed future modification of the experimental rig will enable to measure quasistatic properties of a ribcage model, applying the Cavanaugh tests [3]. 5. ACKNOWLEDGEMENT The research has been financially supported by the Ministry of Science and Education under grant No. 4T07A01627 for years

6 6. REFERENCES [1] J. Awrejcewicz, B. Łuczak, Numerical Analysis of the Lorenz Pectus Implant, Proceedings of the SMC Conference, Zakopane, Poland, [2] J. Awrejcewicz, B. Łuczak, Minimaly invasive Pectus excavatum Repair procedure numerical study Proceedings of the 8th Conference on Dynamical Systems - Theory and Applications, Eds: J. Awrejcewicz, D. Sendkowski, J. Mrozowski, Łódź, Poland, December 12-15, [3] J. Cavanaugh, K. Jespen, A. King, Quasi static frontal loading on the thorax of cadavers and Hybrid III dummy. Proceedings on the 16 th International Workshop on Human Subject for Biomechanical Research, Atlanta, pp. 3-18, [4] C. Kroell, Thoracic response to blunt frontal loading in The Human Thorax Anatomy, Injury, and Biomechanics, Society of Automotive Engineers publication P-67, pp 49-77, [5] G Shaw, J. Crandall, J. Butcher, Biofidelity Evaluation of the THOR Advanced Frontal Crash Test Dummy. IRCOBI Conference on the Biomechanics of Impact 2000.