Biomechanical Response of Human Cervical Spine to Direct Loading of the Head

Currently, dummies are used to evaluate the potential for neck injuries under both inertial loading and direct contact loading, such as in the case of air bag loading. However, there is currently little known data on the biomechanical response of the human neck under direct head contact loading conditions. The objective of this preliminary research is to develop an experimental approach and analysis method that can characterize the response of human neck under low severity head impact loading conditions and compare the results with those for a dummy. The tests include three healthy adult male volunteers with average age of 22, as well as a Hybrid III (HY-III) test dummy. Low-level impact loads to the head and face of each subject were applied via a strap at one of three locations in various directions: an upward load applied to the chin, a rearward load applied to the chin, and a rearward load applied to the forehead. The resultant forces and moments at the occipital condyle were calculated using the measured applied loads, inertial properties of the head, and translational and angular accelerations of the head. For the volunteers, activation of the neck musculature was determined using EMG. Although two initial muscle activation conditions, relaxed and tensed, were investigated, only the relaxed results are presented in this study. For each of the test conditions, the three relaxed volunteers showed similar head rotation patterns, with the highest head rotational velocities observed for the chin-upwards loading condition. The head rotation for the relaxed volunteers was also substantial in the chin-upward and forehead rearward cases, up to 25 degrees. By contrast the dummy’s head rotation was small for all loading modes, less than 5 degrees. It is expected that as the intensity of the applied load increases and/or the volunteers tense the muscles in the cervical spine, these differences will decrease. The calculated forces at the occipital condyle were comparable for the volunteers and dummy, although the calculated moments in the dummy were somewhat higher than those for the relaxed volunteers. Although preliminary testing at low impact severity demonstrates that there may be differences in the moment-angle response between relaxed human volunteers and the HY-III dummy, it remains to be demonstrated if these differences will occur for higher-severity impacts associated with air bags or if these differences will occur for tensed human volunteers. However, once the overall moment-angle response of the head and neck to impact loading is characterized for both low and high levels, the next important step is to understand how the forces and moments are partitioned between the ligamentous cervical spine and the surrounding musculature. Thus, if differences exist, this data should contribute to the improvement of the physical dummy head/neck and its instrumentation to achieve better biofidelic interactions with the environment and to have a greater ability to accurately evaluate the potential for injury to the cervical spine and spinal cord. Key word: Dummies, Neck, Biofidelity, Human Body THE HYBRID III NECK HAS been used for many years to evaluate the potential for injury in vehicle crash testing. However, more recently, there has been debate 1)-4) about the performance of the HY-III neck in testing where the head and neck of the dummy are directly loaded by the deploying airbag. It has been observed that sometimes in this scenario, large shear forces and bending moments are measured at the upper neck load cell without significant neck bending deformation. This phenomenon has not been observed in existing biomechanical corridors, which were developed based upon inertial rather than direct contact head loading. Some have suggested this phenomenon is caused by the differences in performance among different types of vehicle occupant restraint systems, such that well-designed systems do not interact with the dummy in a manner that generates this loading pattern, while others 3) have hypothesized that the design of the HY-III neck, either because of the stiffness of the OC joint or the design of the underside of the chin, allows non-biofidelic interactions with the deploying airbag, resulting in these high shear forces and moments in the absence of significant neck bending deformations. The human neck is an extremely complex structure, with various ligaments and muscle groups controlling the articulation between the head and the top of the cervical spine as well as controlling the articulations between the cervical spine vertebrae. The head/neck articulation is provided by the occipital condyle joint, but it is controlled by the ligaments and muscles connecting the head to both the top of the cervical spine and structures below. Thus, the overall moment experienced by the head is the sum of the moments produced by the OC and the moment produced by the surrounding musculature. In isolation, the human ligamentous occipital condyle joint exhibits a very low stiffness over the first 10 to 20 degrees of flexion or extension (Figure 1a). Depending on the level of muscular activation, the human muscular neck is capable of generating a range of bending stiffnesses. For instance, the human muscular neck would have a fairly low bending stiffness when the cervical musculature is partially activated to hold the head upright. However in a real world crash, the human muscular neck may have a much higher bending stiffness when occupants brace themselves for an impending collision. For air bag loading to out-of-position occupants, there may not be sufficient time to activate cervical muscles or the occupants may not be aware of the impending crash. Although the stiffness of the muscular spine is important in determining the kinematics and kinetics of the head and neck due to both direct and inertial loads, severe injuries of the ligamentous spine and spinal cord are most likely a function of only the loads transmitted through the ligamentous spine. Thus, it would be ideal for a dummy neck to accurately replicate and measure the loads in the ligamentous spine directly. The HY-III dummy neck simplifies the structure of the human neck by representing the neck as an elastic column and the articulation between the head and top of the cervical spine as a revolute joint. The HY-III uses one mechanical element, the nodding block, to transmit moments between the head and neck. Consequently, the stiffness of the nodding blocks represents the combined stiffness of the human ligamentous occipital condyle joint and the cervical muscles attached to the head. The moment-angle behavior of the HY-III neck under quasi-static loading is shown in Figure 1b, where the angle is measured between the head and the top of the cervical spine. This level of stiffness is needed in part to keep the dummy’s head in a stable, upright position. Current neck injury criteria attempt to account for the single load path in the HY-III neck design by establishing neck tolerance values that are the sum of allowable forces in the ligamentous and muscular spine. For instance, neck tension tolerance values for in-position occupants are typically higher than that for out-of-position occupants, because in-position occupants are assumed to have activated cervical musculature which will carry more load than relaxed musculature 5) . Fig 1a,b: Static Flexibility of the Upper Neck Joint of the Hybrid III and Human Occipital Condyle Joint. The angle is measured between between the occiput and C2 for the 15 female ligamentous OC-C2 spinal motion segments (Used with permission of Nightingale) 6) and between the head and top of the neck for the Hybrid III. To better understand the partitioning of loads between the ligamentous and muscular neck, three simplified loading conditions were selected for investigation based on analysis of the interaction of the deploying air bag with the head and neck of crash test dummies in rigid barrier and offset frontal crash tests. These loading conditions were: upward load applied to the chin, rearward load applied to the chin, and rearward load applied to the forehead. The objectives of this study are: 1) To develop an experimental approach and analysis method to characterize the response of the human head and neck to direct loading, and 2) To compare the head and neck responses of relaxed human volunteers and HY-III dummy for low-intensity direct head loading. Then, this work together with additional follow-on efforts, will attempt to determine whether the current approach of adjusting injury criteria limits for particular crash conditions will remain the best overall injury evaluation strategy or if it will be necessary to modify either the dummy neck’s physical design and/or the type and location of its instrumentation. EXPERIMENTAL METHODS SUBJECTS (VOLUNTEERS) AND INFORMED CONSENT: Test subjects for this study included the three volunteers and the Hybrid III crash test dummy. Three healthy adult male volunteers with average age of 22, without any history of neck injury were selected as the subjects. X-ray photographs confirmed that the volunteers did not show signs of cervical spine degeneration. The experimental protocol was reviewed and approved by the Ethics Committee of Tsukuba University. All volunteers submitted their informed consent in writing according to the Helsinki Declaration 7) prior to the implementation of the tests. The height, weight and sitting height of each subject are shown in Table 1. Table 1 Subjects Age Sex Height Sitting Weight Estimated Head Estimated Inertia (cm) Height (cm) (kg) Weight (kg) of Head (10*5 g-cm*2) 1 KK 25 M 170 85 78 4.68 2.43 2 KY 22 M 169 86 74 4.52 2.35 3 HT 21 M 173 85 80 4.76 2.48 4 HY-III 91 78 4.48* 2.11* *: actual value For the volunteer tests, two initial conditions were investigated, relaxed and tensed muscles. Electromyographs (EMG) were used to determine qualitatively the level of activation of the neck muscles. Skin electrodes were placed over the left sternocleidomastoid muscles (SCM), left paravertebral muscles (PVM), left trapezius muscles (TZM), and right side SCM. Dipole electrodes, which were approximately 5 mm in diameter, spaced at 2 cm intervals for the EMG measurements. For the relaxed muscle condition, the subjects were asked to relax and the EMGs were used to confirm low levels of muscle activation. For the tensed muscle condition, the subjects were asked to statically contract their neck muscles. For both initial conditions, the subjects were not aware of the exact timing of when the tests were initiated.