2nd World Congress for Chinese Biomedical Engineers (WCCBME2004) Beijing 27-29 Sept
FE ANALYSIS OF HEAD-NECK RESPONSES DURING CAR IMPACT Ee-Chon Teo, Qing-Hang Zhang, *Peter Vee-Sin Lee, *Kian-Wee Tan, Hong-Wan Ng School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798 *Defence Medical and Environmental Research Institute @ DSO National Laboratories, Singapore 117510 Email:
[email protected] Fax: (65) 67954632 1. INTRODUCTION Whiplash is the common occurrence during which the cervical spine is frequently injured. The whiplash injuries as a result of the cervical acceleration/deceleration syndrome within this span of short duration are often dangerous and often associated with spinal cord injuries. It is therefore important to understand the underlying mechanisms of this kind of injury and dysfunction, leading to improved prevention, diagnosis, and treatment. Accordingly, in current study, a detailed threedimensional FE model of the whole head-neck complex developed and validated previously was used to investigate the biomechanical responses of the head and cervical spine under various impact conditions. 2. METHODS A 3-d FE model of the C0-C7 reported previously was used for this study [1]. Briefly, the 3-d FE model of the head and cervical vertebrae were developed with geometrical data based on the actual geometry of a 68 year-old male cadaver specimen. A flexible digitizer was used to capture the coordinates of the surface profile of the bony structures (head, C1C7) continuously and these data were subsequently processed for the FE mesh generation. For the modeling of the intervertebral discs (IVDs), the basic geometries taken from the average values reported in literature [2] were used. Furthermore, the major ligaments and muscle groups associated with the cervical spine were also incorporated in the model, for which the attachment points were determined from anatomic text. Figure 1 shows the final C0-C7 FE model consists of 21,765 elements and 29,066 nodes and the global XYZ coordinate system. The material properties of the elements representing the skull and vertebrae were assumed to be elastic and perfectly plastic, homogenous and isotropic. The ligaments were modeled using nonlinear link elements, which only permit tensile axial force transmission to simulate the actual characteristic of human ligaments. For the muscles, only passive effects of muscle materials were considered. The material properties of various components were derived from literature[3]-[5]. For the impact simulation, a trapezoidal acceleration history shown in Figure 2, which was adapted in experimental study conducted by Ewing et al[6], was applied on the inferior surface of C7 vertebral body. 8
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Figure 1. FE model of the C0-C7 (lateral and posterior views)
Figure 2. The simulated impact conditions. (a) Impact direction; (b) Input acceleration on C7 inferior surface.
3. RESULTS Figure 3 shows the predicted variation of rotational angles of C0-C1 and C6-C7 during front and rear-end impact. For the rear-end impact, after the first 10ms after impact, the lower segments (between C3 to C6) turned to extension motion with different angulations while the upper segments (between C0 to C3) maintained the flexion motion for much longer duration before turning to extension motion. During the 20-70ms period, the whole C0-C7 structure formed a S-shaped curvature with flexion at the upper levels and extension at the lower levels. After that, all the motion segments went into extension and the entire cervical spine formed a C-shaped curvature. For each motion segment, the variations of the rotational angles against time were nearly axisymmetric under front and rear-end impact (Figure 3a). Although the time of the upper cervical segments change from one side (extension\flexion) to the opposite one (flexion\extension) were a little different (Figure 3a), the magnitude of the peak rotational angles were nearly the same, which cause the axisymmetric curvature of the C0 rotation with respect to C7 in sagittal plane (Figure 4a). In this period (the 150ms duration), the C6-C7 segment experienced much higher extension/flexion angles (about 21o) as compared against its physiological normal range[7] of 8 o, implying a higher potential of injury at this segment. Compared with those in sagittal plane, the lateral rotational angles of the motion segments under side impact were more synchronous (Figure 3b). After much shorter time (less than 30ms), all the motion segments bent into one side, and then turn back simultaneously. The period the motion segments in one side and the relevant rotational magnitudes were all much
2nd World Congress for Chinese Biomedical Engineers (WCCBME2004) Beijing 27-29 Sept
lower than those in sagittal plane. As shown in Figure 4, at 125ms, the C0 has turned from one side to the opposite one under side impact while it just went through the peak angle in one side under front or rear-end impact. It is obvious that the response of the neck were greatly affected by the impact direction and the impact acceleration history. This point was further testified when compare the response of the motion segments under rear-side and front-side impact. Under these two conditions, the rotational angles of the motion segments in sagittal plane were still nearly axisymmetric while those in lateral place were much different. The peak lateral bending angle of C0 with respect to C7 under rear-side impact was higher than that under front-side impact. It was also noticed that although the peak rotational angles of each motion segment in sagittal and lateral plane were not greater than normal range under these two condition, the coupling torsion of C2-C3 and C3-C4, however, were close to their normal limit, which imply a potential torsional injury of these two segments under oblique impact. 30
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(a) In sagtittal plane under rear and front impact (b) Lateral bending under side impact Figure 3. Predicted rotational angles of C0-C1 and C6-C7 under different impact conditions.
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Figure 4. Predicted rotational angles of C0 respect to C7 under various conditions. (a) In saggittal plane; (b) Under lateral bending
4. CONCLUSIONS We have successfully used the previously developed C0-C7 FE model to characterize the post responses of the headneck simulating the response of them under various impact conditions. It was found that the rotational responses of the head-neck were greatly affected by the impact direction and the input acceleration. During whiplash, the lower levels especially C6-C7 may experience hyperextension in the early phase due to high acceleration of C7. These levels could be a potential area to be in high risk during whiplash. Under the same acceleration history, the potential injury under front or rear-end impact were greater than that under side impact. For an oblique impact direction, the possibility of the coupling torsional injury in the middle cervical segments should be concerned. REFERENCES [1] [2] [3] [4] [5] [6] [7]
Teo EC, Zhang QH, Lee VS, Ng HW, Tan KW. Finite element analysis of head-neck kinematics during whiplash. 13TH International Conference on Mechanics in Medicine and Biology. 12-15 November 2003. Tainan, Taiwan. Gilad I, Nissan M. A study of vertebra and disc geometric relations of the human cervical and lumbar spine. Spine 1986 Mar;11(2):154-157. Gibson LJ, Ashby MF: Cellular Solids: Structures & Properties. Pergamon Press, Oxford, 1997. Deng Y-C, Goldsmith W. Response of a human head/neck/upper-torso replica to dynamic loading-II: Analytical/numerical model. Journal of biomechanics 1987, 20(5): 487-97. Yamada H. Strength of biological materials. The Williams & Wilkins Company, Baltimore, 1970. Ewing CL, Thomas DJ, Lustick L : Multiaxis dynamic response of the human head and neck to impact acceleration. AGARD Conference Proceedings 1978 Nov 253: A5-1. Panjabi MM, Cholewicki J, Nibu K, Grauer JN, Babat LB, Dvorak J. Mechanism of whiplash injury. Clinical Biomechanics. 1998 Jun;13(4-5):239-249.
