A New Neck Injury Criterion in Combined Vertical/Frontal Crashes with Head Supported Mass

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1 A New Neck Injury Criterion in Combined Vertical/Frontal Crashes with Head Supported Mass Cameron R. Dale Bass, Lucy Donnellan, Robert Salzar, Scott Lucas, Benny Folk, Martin Davis, Karin Rafaels, Chris Planchak, Kevin Meyerhoff and Adam Ziemba University of Virginia Center for Applied Biomechanics Nabih Alem U.S. Army Aeromedical Research Laboratory ABSTRACT This study developed a new neck injury risk function suitable for use in frontal crashes with occupant orientation ranging from predominantly horizontal to predominantly vertical in the occupant anterior-posterior (A/P) direction. In this study, 36 cadaveric head/neck complexes and 6 whole cadavers were tested under impact scenarios with varying total head mounted mass and center of gravity locations. Matched Hybrid III and THOR dummy tests were also performed. The resulting injury criterion is based on a beam model of the lower cervical spine and is expressed as FZ M Y BC = + FZC M YC where F z is the axial compression/tension force, and M y is the A/P flexion moment, both at the C7/T1 intervertebral disc. The derived constant values of F ZC are 566 N in axial tension and 543 N in axial compression and M YC is 141 N-m in A/P flexion. These values are comparable to single axis injury tolerance values described in the literature and in existing motor vehicle injury standards for the upper cervical spine (FMVSS-28, 23). Beam criterion values (BC) of 1. correspond to a 5% risk of an AIS 2 injury in the human cervical spine. The THOR dummy was found to reproduce the kinematics of the cadaver better than did the Hybrid III dummy. However, response curves showed differences between the THOR and the cadaver that were due to anatomy. Keywords: NECK, FRONTAL IMPACTS, INJURY CRITERIA, DUMMIES The introduction of the hard helmet to military aviators in the 195s helped to protect the head from impacts (McEntire and Shanahan, 1997). However, the mass of the hard helmet, along with the additional mass added to the helmet in the form of night vision goggles (NVGs), oxygen masks, communication devices, and other devices in the 197s and 198s, led to neck injuries that were not seen previously. The cervical spine has to support this additional weight (up to 3.5 kg) along with the weight of the head, which is approximately 3.5 kg. The most important injury mechanisms due to head supported mass (HSM) are neck axial tension and flexion. In general, compression and extension injuries are not seen due to the recent aircraft seat design requirements that include headrests and load limiting vertical energy absorption capabilities (McEntire and Shanahan, 1998). Shanahan and Shanahan (1989) found that the level of the C7 vertebra was the most common injury location in helicopter crashes from 1979 to However, several of these injuries are believed to be from compression under high vertical impacts in seats not containing energy absorption capabilities. Further, a series of preliminary sled impact tests performed by Bass (22) with Hybrid III dummies in a predominantly vertical orientation showed relatively large lower neck forces and moments in a common helicopter impact scenario. Based on these studies, it is expected that the lower cervical spine will be the site of injury during this test series. Six sled and thirty-six cadaver head-neck component tests were performed for this study. This study addresses whether Nij, a current neck injury criterion used with crash test dummies, is applicable to aviation crashes when head supported mass is worn. In addition, a new injury criterion, the Beam Criterion, was developed specifically for neck injury with added mass based on numerous cadaveric experiments. Matched Hybrid III and THOR dummy tests were performed to allow kinematic and dynamic comparison between cadaver and dummy. Finally, full-body cadaver, Hybrid III, and THOR sled tests were performed to confirm the suitability of component test injury patterns. IRCOBI Conference - Madrid (Spain) - September 26 75

2 NECK INJURY CRITERIA: Through the years, several injury criteria have been developed to predict neck injuries. These injury criteria are defined for a number of different conditions such as injury mechanism (e.g., compression, flexion), acceleration environment (e.g., high acceleration, long-term vibration), and impact condition (e.g., frontal collision, rear collision). Neck injury criteria have been developed for both minor and serious neck injury. Simple Loading: Early studies investigated simple loading conditions such as uniaxial force or single plane bending. Mertz et al. (Mertz and Patrick, 1971, Mertz et al., 1978) defined the limits for axial compression, flexion, and extension moments using Hybrid III dummy reconstructions of serious injuries to football players. Two axial compression injury reference values were derived, one in which the football player is charging a tackling block (6.75 kn) and another in which the player is stationary (4. kn). In addition, Mertz and Patrick performed 9 static and 178 dynamic sled tests on volunteers and cadavers to determine appropriate flexion and extension injury threshold moment values. The maximum dynamic moment that the volunteer endured was 88 N-m, which caused a sharp pain that lasted for several days. When similar tests were run using cadavers under flexion, a value of 19 N-m was determined to be the maximum moment without ligamentous or bony damage, but with possible muscle damage. During flexion tests, no cadavers were found to have ligamentous or bone damage. When cadavers underwent extension tests, a 47 N-m torque about the occipital condyles was non-injurious, while a 57 N-m torque caused ligamentous damage. The flexion and extension critical values were scaled to a 5 th percentile adult male, and are defined as being 19 N-m and 57 N-m, respectively. The tolerance levels for simple tension and shear loads were developed by Nyquist et al. (198) using reconstructions of Swedish field accident data with a Hybrid III dummy in frontal impact tests. These reconstructions produced a tolerance level of 33 N in simple tension and 3 N in simple shear loading in the neck. These simple loading values are the basis for the simple loading criteria in current Federal Motor Vehicle Safety Standards (FMVSS 28, 23). Combined Loading: Since simple loading rarely occurs in automotive and military crashes, combined loading must be considered. Several injury criteria have been developed for combined loading that result in either minor injuries or serious injuries. Prasad and Daniels (1984) suggested that combined forces and moments may cause neck injuries. Cheng et al. (1982) performed experiments under combined loading by restraining the cadaver s chest and applying a uniform frontal restraining load to the chest using an airbag. Four of the six cadavers tested were injured in the upper neck. After performing cadaver sled tests, Hybrid III tests were performed to determine bending loads, as well as shear and axial loads. Cheng found a resultant neck force (shear and axial) of 62 N for injury. As the test data is left censored, it is likely this represents an upper bound on the force for fracture occurring at the base of the skull. Kleinberger et al. (1998) developed that an injury criterion that includes the combined effects of axial loading and bending moments was necessary and so expanded on the simple loading criteria defined above. Nij is defined as a linear combination for loads and moments, so that FZ M Y Nij = + (Equation 1) F M ZC where F Z is the axial load in the upper neck, either tension or compression, F ZC is the critical axial load, M Y is the moment in the sagittal plane, either flexion or extension, and M YC is the critical moment. When little or no moment is present, the simple peak tension and compression values serve as the critical values (Eppinger et al., 2). Values from the 2 final rule are shown in Table 1. These values provide a demarcation for which Nij greater than 1.; this line corresponds to a 22% risk of AIS 3+ neck injury. Dummy Size Tension (N) Compression (N) Flexion (N-m) Extension (N-m) 5 th Percentile Male Table 1. Nij intercepts for tension, compression, flexion, and extension (FMVSS 23). Other published injury criteria (c.f. Bostrom et al., 2, Svensson et al., 1993, Yoganandan et al., 22, and Schmitt et al., 21) are for minor injuries or different injury mechanisms than are expected in these tests. Nij is potentially the most applicable criterion as it assesses severe injuries based on the axial force and A/P bending moment in the neck, the expected injury mechanism. YC 76 IRCOBI Conference - Madrid (Spain) - September 26

3 METHODS Two types of tests were performed in this study. First, sled tests were performed with whole body cadaveric specimens, a Hybrid III dummy and a THOR dummy. Second, component tests were performed using head/neck complexes of the three surrogates. Full cadaver tests were performed to assess injury locations compared with locations found in Shanahan and Shanahan (1989) and to provide supporting information for component tests. Component tests were performed to assess a breadth of experimental conditions with more instrumentation than is possible in full sled tests. A general overview of the test methods is given below. HEAD SUPPORTED MASS: For all tests, a device based on a cranial fixation halo (PMT, Inc., Model ) was used to allow changes in the center of gravity of the head supported mass in the coronal plane and a rigid connection to cadaveric specimens. A pair of graphite composite halo rings, each mounted at 15 from the vertical, was connected by a bridging fixture as shown in Figure 1. This fixture was positioned on the head of the specimen using sharpened mounting screws to grip the skull of the specimen. The halo was mounted so that the original center of gravity of the device was located at the intersection of the Frankfort plane and the mid-coronal plane. The nominal mass of the halo device was.894 kg and the nominal moments of inertia for the halo device alone are I xx = 1,75 kgmm 2, I yy = 693 kg-mm 2, and I zz = 12,4 kg-mm 2. Mass was added to the halo device using cylindrical weights. The center of gravity locations of the head supported mass ranged from mm to 118 mm above the head center of gravity, and the total head supported mass ranged from 2 kg to 4 kg. Moments of inertia ranged from 13,1 kg-mm 2 to 11, kg-mm 2. Variation in the moment of inertia of the head supported mass has been shown in calculations to have a limited effect on head dynamics, while changing the head center of gravity location has a large effect (Bass, 22). CADAVERIC SPECIMENS: Cadaveric specimens were procured in accordance with state and federal regulations and are subject to the oversight of the University of Virginia Cadaver Use Committee. All cadaveric specimens were fresh frozen with no evidence of wasting disease, Hepatitis B, Hepatitis C or HIV. The bone quality of each specimen was assessed using a histogram technique (QBMAP, The IRIS) on pretest CT scans; no specimen had evidence of osteoporosis (T-Score > 2.5 for UCSF 25 year old dataset), preexisting fractures of significant cervical spinal disease that might compromise skeletal strength. The specimen average mass for component tests was 79±19 kg compared with a 5 th percentile male value of approximately 79 kg, and the average height was 1761±79 mm compared with a 5 th percentile male value of approximately 178 mm. The average age of the specimens in all component tests was 59 years, and no cadavers tested were older than 74 years. The specimen average mass for sled tests was 69±26 kg and the average height was 1655±15 mm. The average age of the specimens was 61 years, and no cadavers tested were older than 7 years. Detailed specimen anthropometry is shown in Appendix A cm 3.8 cm Center of Gravity z 11.1 cm 11.4 cm y x Note: Drawing Not To Scale Figure 1. Halo head mounted mass device, front and side view, schematic and coordinate system for halo head supported mass device. DUMMIES (HYBRID III AND THOR): Both Hybrid III and THOR-alpha dummies were used for the component tests and for the sled tests. Hybrid III has a relatively stiff neck that has limited ability to enter a shearing motion. THOR is an advanced crash test dummy developed by NHTSA and its collaborators. It has a more compliant neck, which allows for a more biofidelic shearing motion. Both dummies, however, were designed to simulate human dynamics with musculature for frontal crashes in an automobile test environment. So, the flexion-tension or flexion-compression loading condition with head supported mass and a substantial vertical component may be outside the usual range of application of these dummies. IRCOBI Conference - Madrid (Spain) - September 26 77

4 SLED TEST SETUP: For the full-body validation tests, an acceleration sled system (Via Systems 713) was used with a triangular deceleration pulse based on helicopter crash deceleration time histories (Alem, 22) pulse is used with the peak deceleration varying depending on the severity of injury desired. The carriage test fixture is comprised of several major components, including the seat, the head support, the universal test fixture (carriage), and the angle support. The seat and head support fixtures were used in previous small female neck testing (Bolton, 22), and the geometric angles, planes, and anchor points were designed to be representative of an aviator seat. Test instrumentation included six axis head and T6 accelerometer/angular rate sensor packages, sled accelerometers, acoustic fracture sensors, and seat base load cells as shown in Figure 2. The lumbar region of the seat back was covered with two inches of foam with a slot cut along the centerline to provide clearance for instrumentation installed on the spine of the cadaveric subjects. The seatback support was fixed at 3 from the impact vector. The specimen was initially centered and aligned in the seat. Three one-inch wide nylon straps were located in the upper torso across the chest, and one strap passed over each shoulder and between the legs; the restraints were firmly tightened to limit the mobility of the upper torso. For cadaveric tests, the head was propped to a position similar to that of a Hybrid III dummy using soft viscoelastic foam. The position of the six axis angular rate sensor/accelerometer package mount was then measured. For the sled tests, the deceleration profile was triangular in shape, similar to helicopter crash seat test deceleration profiles. This pulse was selected in consultation with US Army Aeromedical Research Laboratory (USAARL) personnel (Alem, 22) and was obtained using a programmable hydraulic decelerator. The pulse shown is a 3 g deceleration profile. The entering speed and peak deceleration generally varied throughout the dummy and cadaver sled test series, but the generic shape of the profiles was similar. Figure 2. Schematic drawings showing sled cadaver instrumentation. Six cadaver sled tests were performed to investigate the injuries and kinematics of a specimen in realistic crash conditions as shown in Table 2. The primary experimental variables in the test series included the head supported mass at, 1.7, and 2 kg, and the impact peak acceleration. Each fullscale cadaver test held the location of the added mass constant at the head CG, and the seat angle constant at 3. Average Test Specimen Added Mass (kg) Peak Sled Decel. (g) Sled Decel (g) Δ V (km/hr) HM3_cad1.95 FRM HM3_cad2.98 FRF HM3_cad3.974 FRF HM3_cad4.975 FRF HM3_cad5.976 WMA HM3_cad6.977 FRM Table 2. Cadaver sled test matrix COMPONENT TEST SETUP: The specimen head/neck complexes were disarticulated below the T4 vertebral body. Skin and outer layer musculature were removed from the T1-T4 vertebral bodies while maintaining ligamentous structure to facilitate potting of the neck and mounting of the sensors (Figure 3). Screws were implanted into the T4 and T3 vertebral bodies to ensure immobilization of the T3-T4 spinal segment in the potting fixture, and the spine was oriented in a potting fixture with 78 IRCOBI Conference - Madrid (Spain) - September 26

5 normal T4 kyphosis to allow positioning of the specimen in a natural upright posture. The spine was potted using an epoxy compound (Buehler Inc., Epoxide). Sufficient muscle and other flesh were left on the T2 vertebral body to enable some flexibility in the T2/T3 spinal segment as the T2 vertebral body was partially embedded in the epoxy. The component tests were performed using a pneumatic impactor (Via Systems 928-2). The headneck complex was suspended on the underside of the component sled and the lower neck load cell is placed between the head-neck and the sled.. Depending on test condition, impacts were moderated using viscoelastic padding or a hydraulic decelerator. To prevent injuries owing to rebound, a cushioned backstop and hydraulic decelerator were used to decelerate the head. To position the specimen, the head-neck complex was mounted on the inferior side of the mini-sled. The initial neck angle measured from the local horizontal (9 from the impact direction) was selected using wedge fixtures of selected angles as shown in Figure 3b. To position the neck in a natural lordotic posture, the spines of the cadavers were compressed using frangible tape that held the head in compression until the time of the test. Once the final position is achieved, the specimen is allowed to relax under compression for 15 minutes to obtain normal ligamentous response before the test (Lucas et al., 25). Impact 45 Load Cell Potting Cup a) b) Figure 3. a) Schematic drawing of specimen instrumentation, b) Orientation of test setup 3 positive A baseline head mounted mass of 2. kg was selected for the impactor dummy and cadaver tests. The deceleration pulse for the first two series (cad1-cad17, cad36) was designed to be very short to maximize the shear force in the neck. The remaining tests were performed using a longer duration and a smaller magnitude pulse. In addition to a variation in acceleration, other variables (initial head-neck angle, added mass location, and total mass) were tested (Table 3, Table 4). All data was sampled at 1, Hz with hardware anti-aliasing at 33 Hz using an eight-pole Butterworth filter. Series # Cad. Tests Variable Constants Series I 9 Acceleration Head-Neck Angle, Total Mass, Mass Location (CG) Series II 9 Head-Neck Angle Total Mass, Mass Location (CG) Series III 9 Mass Location Head-Neck Angle, Total Mass Series IV 9 Misc (Total Mass, Angle, Acceleration) Table 3. Generic test matrix for cadaver component tests. RESULTS INJURY RESULTS: The sled injuries from post-test CT scans and post-test necropsy are shown in Table 5. Approximately 7% of the component injuries were posterior ligaments, of which 83% were located between C5 and T2. The predominant injuries were supraspinous ligament and ligamentum flavum tears and transactions. The component injuries from CT and necropsy for series I, series II, series III, and series IV are included in Table 6, The injuries from the sled tests have a very similar pattern. Approximately 67% of the sled injuries were posterior ligaments, of which 79% were located between C5 and T2. Approximate maximum AIS (MAIS) values (AAAM, 1998) were assessed for each injury as shown in Table 6; however, most of the current AIS codes for the cervical spine are related to the condition of the spinal cord. If the cord is compromised above C3, the threat-to-life is extremely high IRCOBI Conference - Madrid (Spain) - September 26 79

6 (AIS 6); if the cord is compromised below C3, there is a better chance of survival (AIS 4 or 5). An injury in which the vertebral body is fractured and the ALL ruptured is classified as an AIS 2 or 3 depending on the severity of the fracture. In all cases, the spinal cord was severely autolyzed, so the spinal cord injury was not easily determined. The spinal cord is especially vulnerable to injury when the posterior ligaments (e.g., supraspinous ligament, intraspinous ligament, ligamentum flavum) were torn or transected. If the posterior ligaments were injured in conjunction with the posterior longitudinal ligament, it was assumed that the spinal cord would be compromised and the corresponding AIS score was increased. Added Mass Mass Position (Relative to Head cg) Neck Angle Maximum Sled Test Body Number (kg) X (cm) Z (cm) (deg) Velocity (m/s) Series I HM2_cad1 FRM HM2_cad2 FRM HM2_cad3 FRM HM2_cad4 FRM HM2_cad5 FRM HM2_cad6 FRM HM2_cad7 FRM HM2_cad8 FRF HM2_cad9 WFA Series II HM2_cad1 PMA HM2_cad11 PMA HM2_cad12 PMA HM2_cad13 PMA HM2_cad14 PMA HM2_cad15 PMA HM2_cad16 PMA HM2_cad17 PFA Series III HM2_cad18 PMA HM2_cad19 PFA HM2_cad2 PFA HM2_cad21 PMA HM2_cad22 PMA HM2_cad23 23-FRM HM2_cad24 PMA HM2_cad25 PMA HM2_cad26 PMA Series IV HM2_cad27 PMA HM2_cad28 PFA HM2_cad29 PFA HM2_cad3 PMA HM2_cad31 WFA HM2_cad32 PMA HM2_cad33 PMA HM2_cad34 PMA HM2_cad35 PFA Series II HM2_cad36 PMA Table 4. Cadaver component test matrix. Test Injuries MAIS Hm3_cad1.95 Ligament tear (ALL) (C3/C4), ligament tears (LF, SSP) (C7/T1), endplate crushing at C7 and T1 (1 cm in width) 3 Hm3_cad2.98 Ligament transection (7-8%) ALL (C2/C3), Disc crushing C2/C3, Osteophyte crushed at C2/C3 anterior, C7/T1 LF 9% transected, C4/C5 PLL 3% transected 4 Hm3_cad3.974 LF 2% transected C6/C7, LF completely transected C7/T1, ISP transected C7/T1 3 Hm3_cad4.975 SSP and ISP tear C6/C7 2 Hm3_cad5.976 SSP transection C1/C2, ISP tear C1/C2, SSP, ISP, LF transection C5/C6, disc crushed C5/C6 3 Hm3_cad6.977 Minor damage to intervertebral disc at C6/C7 1 Table 5. Sled cadaver CT and necropsy results. 8 IRCOBI Conference - Madrid (Spain) - September 26

7 Test Injuries MAIS Hm2_cad1 Anterior vertebral body crushing C4-C5, Increased laxity (C4-C5) 2 Hm2_cad2 Lgament transactions (T2-T3) 6 Hm2_cad3 Ligament transactions (C7-T1) 6 Hm2_cad4 Partial ALL ligament tear (C5-C6), minor C5 crushing, C5-C6 IVD damage 3 Hm2_cad5 Ligament tear (C7-T1, ALL), SSP, PLL severely distended, IVD damage (C7-T1) 4 Hm2_cad6 None Hm2_cad7 None Hm2_cad8 Ligament tears (C7-T1) (LF, SSP, ISP), Dens Fracture 4 Hm2_cad9 Ligament tears (C5-C6) (LF, PLL, capsular ligaments) 5 Hm2_cad1 None Hm2_cad11 Ligament tears (C6-C7) (LF, SSP), other ligaments disrupted (C6-C7) 5 Hm2_cad12 Ligament tear (C5-C6) (LF), ALL, PLL permanent deformation (C5-C6), intervertebral disc damage (C5-C6) 4 Hm2_cad13 None Hm2_cad14 ALL permanent deformation (C6-C7), SSP, ISP excess laxity (C6-C7), LF tear (C5- C6, C6-C7, C7-T1) 3 Hm2_cad15 ALL tear (C5-C6), associated with osteophyte 2 Hm2_cad16 Fracture of anterior, inferior end plate of C5 Hm2_cad17 None Hm2_cad18 Disc tear on posterior side (C3-C4), Capsular ligaments torn (C4-T1) 3 Hm2_cad19 Ligament tear (C4-C6) (LF) 2 Hm2_cad2 None Hm2_cad21 LF and PLL permanent deformation (C4-C6) 1 Hm2_cad22 Ligament tear (C3-C4) (capsular) 2 Hm2_cad23 LF permanent deformation (C5-C7) 1 Hm2_cad24 None Hm2_cad25 LF and PLL permanent deformation (C2-C4) 1 Hm2_cad26 LF and PLL permanent deformation (C4-C5) 1 Hm2_cad27 None Hm2_cad28 None Hm2_cad29 C1 fracture on the posterior arch 3 Hm2_cad3 None Hm2_cad31 ALL and PLL permanent deformation (C5/C6) 2 Hm2_cad32 None Hm2_cad33 None Hm2_cad34 None Hm2_cad35 None Hm2_cad36 None Table 6. Component cadaver CT and necropsy results. DUMMY VS. CADAVER KINEMATICS SLED TESTS: The kinematic responses of cadavers, the Hybrid III dummy, and the THOR dummy in similar test conditions are remarkably different. This difference can be compared using high speed video motion analysis. Under shearing motion characteristic of tension/flexion injuries under inertial loading, the video frame comparison in Figure 4 shows the THOR head performing a motion that appears qualitatively similar to a human-like frontal S-shaped bending motion before a transition to simple bending shown in Figure 5. In contrast, the Hybrid III neck rapidly converts shearing motions to simple C-shaped bending modes. Typically, THOR head center of mass translated about 4 cm in the sled centered frame before reaching joint stops and flipping into simple flexion, the cadaver translated ~35 cm, and Hybrid III only translated ~1 cm under the conditions tested in this study. Figure 4. Each subject shown undergoing shearing motions with head supported mass (from left to right): Hybrid III, cadaver, THOR. IRCOBI Conference - Madrid (Spain) - September 26 81

8 Figure 5. Hybrid III, cadaver, and THOR shown in peak flexion. Typical head acceleration time histories from similar THOR, Hybrid III and cadaver sled tests are compared in Figure 6 and Figure 7. Owing to the structural differences between the cadaver and THOR dummy, the inertial frame head anterior/posterior (A/P) accelerations differ. The THOR achieves globally similar A/P shearing motion in the head using 1) an extended rotational range of motion in the OC compared with the Hybrid III, and 2) a local c-shaped mode in the neck. So, the resulting dynamics of the head center of gravity are different for the THOR and the cadaver. For the Hybrid III, the impact produces an early acceleration in Z and X demonstrating the lack of a head lag phenomenon seen in cadavers and volunteers (c.f. Thunnissen and Wismans, 1995) For the cadaver, the neck shearing motion may engage the facet joints, locally stiffening the neck. There is increased local head A/P acceleration in the cadaver that is not seen in the dummy followed by a steeper local A/P deceleration. Further, there is more complex dynamics in local A/P head acceleration in the THOR acceleration. Similar behavior in local head superior/inferior acceleration is seen in Figure 7. Deceleration in the vertical direction (superiorly) is seen in the cadaver under shearing motions. From these dynamics, we infer that the neck reaches facet joint stops and acts as a stiff beam under this loading. Using this inference, when the head reaches limits of rotation about the C/C1 joint, head acceleration occurs in the local superior/inferior axis. This behavior is less pronounced in the THOR dummy and absent in the Hybrid III owing to the lack of free shearing capability and posterior facet joint support. Mid torso accelerations (T6) for the THOR and cadaver tests are shown in Figure 8 and Figure 9. The corresponding Hybrid III tests were not instrumented with the accelerometer package. There is some evidence that the cadaver package contacted the back support during rotation. However, both the THOR and the cadaver show local accelerations that are commensurate with the underlying 25 g sled deceleration time history indicating the desired limitation of motion in the torso from the restraints. SENSOR DATA: All data was filtered to SAE J211 standards and was mass compensated, where appropriate, to account for the mass of the potting cup and tissue between the load cell and the region of interest (C7-T1). The forces and moments were then transferred from the location of the load cell to the location of interest (C7-T1) using the dynamics data. Dummy forces and moments were translated to the OC using standard techniques (e.g. as specified in SAE J1733 (1994) for the Hybrid III and by GESAC, Inc (Shams, 24) for the THOR). As the dynamics is constrained to the midsagittal plane, FY, MX, and MZ are small compared to FX, FZ, and MY. For the cadaver component tests, the peak shear force (FX) peak tensile force (FZ), and peak flexion moment (MY) are shown in Table 7. By design, the ratio of peak compression force and tension force (FZ) and A/P bending moment (MY) are altered using impactor force and velocity, initial head neck angle, position and value of the head supported mass. As shown in Figure 1, Series I concentrated on large initial shear resulting in low ratios of peak axial force to bending moment. However, Series II changed neck angle which increased peak axial tension relative to peak flexion moment. Series III changed the angle and location changing the ratio of compression and tension relative to the peak flexion moment, and Series IV investigated cross conditions resulting in differing ratios of peak axial force to peak flexion moments. DISCUSSION COMPONENT NIJ CALCULATIONS: To calculate the Nij criterion for neck injury, the axial force and A/P bending moment measured in the upper neck load cell of the Hybrid III dummy are used. The intercept values used for the Hybrid III are for the 5 th percentile male dummy (Eppinger, et al.2). Nij values for Hybrid III tests that are matched tests to cadaver series I (constant neck 82 IRCOBI Conference - Madrid (Spain) - September 26

9 angle and added mass with varying acceleration) are presented in Figure 11. It is clear from these results that the Nij values from these matched Hybrid III tests suggest a lower risk of injury than is seen in the component tests. Nij is not calculated for the cadaver tests because an upper neck load cell can not be installed in the upper neck without disrupting the anatomy. However, differences in kinematics with head supported mass in predominantly frontal/vertical impacts prevent the Hybrid III from being a suitable assessment tool Cadaver - HM3_cad2.98 Thor - HM5_atd3.962 Hybrid III - HM5_cad Head X Acceleration (g) Time (ms) Figure 6. Head local (head fixed system) anterioposterior (X) acceleration from cadaver sled test HM3_cad2.98, THOR test HM5_atd3.962, and Hybrid III test HM3_atd49.14 for similar test conditions Head Z Acceleration (g) Cadaver - HM3_cad2.98 Thor - HM5_atd3.962 Hybrid III - HM5_atd Time (ms) Figure 7. Head local (head fixed system) superioinferior (Z) acceleration from cadaver sled test HM3_cad2.98, THOR test HM5_atd3.962, and Hybrid III test HM3_atd for similar test conditions Cadaver - HM3_cad2.98 Thor - HM5_atd3.962 Torso X Acceleration (g) Time (ms) Figure 8. Torso local (head fixed system) anterioposterior (X) acceleration from cadaver sled test HM3_cad2.98, THOR Test HM5_atd3.962 for similar test conditions. IRCOBI Conference - Madrid (Spain) - September 26 83

10 15 1 Cadaver - HM3_cad2.98 Thor - HM5_atd3.962 Torso Z Acceleration (g) Time (ms) Figure 9. Torso local (head fixed system) superioinferior (Z) acceleration from cadaver sled test HM3_cad2.98, THOR test HM5_atd3.962 for similar test conditions. Test Number Peak Sled Velocity (m/s) Vertical Distance of Added Mass to Head CG (mm) Added Mass (kg) Initial Head- Neck Angle (deg) Peak Forward Shear Force (N) Peak Tensile Force (N) Peak Flexion Moment (N-m) HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad HM2_cad Table 7. Peak A/P shear, axial force, and flexion moment for cadaver component tests. 84 IRCOBI Conference - Madrid (Spain) - September 26

11 Peak Axial Force/ Peak Flexion Moment Compression/Flexion Tension/Flexion HM2_cad1 HM2_cad2 HM2_cad3 HM2_cad4 HM2_cad5 HM2_cad6 HM2_cad7 HM2_cad8 HM2_cad9 HM2_cad1 HM2_cad11 HM2_cad12 HM2_cad13 HM2_cad14 HM2_cad15 HM2_cad16 HM2_cad17 HM2_cad36 HM2_cad18 HM2_cad19 HM2_cad2 HM2_cad21 HM2_cad22 HM2_cad23 HM2_cad24 HM2_cad25 HM2_cad26 HM2_cad27 HM2_cad28 HM2_cad29 HM2_cad3 HM2_cad31 HM2_cad32 HM2_cad33 HM2_cad34 HM2_cad35 Series I Series II Series III Series IV Figure 1. Ratio of peak axial force to peak moment in compression/flexion and tension/flexion Nij Headmass4_18-4. m/s Headmass4_2-4. m/s Headmass4_ m/s Headmass4_ m/s Headmass4_ m/s Headmass4_ m/s Figure 11. Hybrid III Series I Nij values. All tests were performed at a zero degree initial head-neck angle, 2. kg HSM mounted at the head CG, and varying peak velocities shown above. As there are no intercept coefficients currently available for use with the THOR dummy, THOR Nij calculations used the Hybrid III 5 th percentile male values. Nij values for THOR, series I are presented in Figure 12. As no injury reference values or intercepts are available for THOR, the values are provided for comparison. However, they are far lower than those for the Hybrid III under matched conditions for Series I. This suggests that unmodified Nij may not be used with THOR for impacts with head supported mass. BEAM CRITERION: Nij is generally seen as a predictor of upper neck injury. In contrast, the current study saw injuries at the lower neck (C5-T1) in the cadaveric component testing. These injuries likely arise from the increased moment to the lower neck caused by inertial loading under head supported mass. This increased moment would not be as pronounced at the upper neck which is the likely explanation of low injury risk Nij values. To predict injury in the lower neck, a different approach must be taken. Here, a simple beam criterion based on the stress in a beam (similar to the concepts behind Nij) is proposed as follows: IRCOBI Conference - Madrid (Spain) - September 26 85

12 FZ M Y Beam = + + G( F X ) Equation 2 F M ZC YC where F Z is the axial load in the neck, either tension or compression, F ZC is the critical axial load, M Y is the flexion moment in the sagittal plane, M YC is the critical moment, and G(F X ) is some undetermined function of shear. The Beam Criterion should be evaluated at the intervertebral disk of C7-T1 to predict injuries occurring about C7-T1 under inertial loading. The criterion is intended to be evaluated as the maximum of series of instantaneous Beam criteria calculated from the force and moment time histories. It is likely that the philosophical basis for this criterion in the lower neck is better than that for the upper neck since the spinal flexion/extension freedom of motion of the lower neck spinal segments is less than that of the OC-C1 segment. Further, the addition of a shear force contribution (as yet undetermined) is based on the observation that simple beam theory does not account for finite deformation mechanics in the spine. For finite deformations, a portion of the shear force converts to a normal stress. However, for the neck, this conversion is unknown. When the neck undergoes a shear force, it forces the ligaments into tension, so some of the shear force must convert to axial force, which affects the normal stress Nij Headmass4_61-5. m/s Headmass4_ m/s Headmass4_ m/s Headmass4_ m/s Headmass4_ m/s Headmass4_ m/s Headmass4_ m/s Headmass4_ m/s Headmass4_7-5. m/s Headmass4_ m/s Headmass4_72-7. m/s Headmass4_ m/s Headmass4_74-6. m/s Headmass4_ m/s Headmass4_ m/s Figure 12. THOR dummy component series I Nij values (All tests at initial head-neck angle, 2. kg HSM mounted at the head CG, and varying peak velocities). Initially, the Beam Criterion is evaluated without the addition of the shear function, since the shear conversion to normal stress in the neck is unknown. The critical values chosen as a starting point are the FMVSS-28 simple bending values. These critical values used are shown in Table 8. Tension 417 N Compression 4 N Flexion 19 N-m Table 8. Initial critical values used for calculating Beam Criterion. INJURY RISK: A survival analysis (Hosmer, 23) using the FMVSS-28 critical values was performed using Minitab version 14 (Minitab, Inc, State College, PA) for injuries of MAIS 2. A parametric analysis was performed with arbitrarily censored data using a logistic curve (right censored for non-injury tests and left censored for injury tests). Maximum likelihood estimates were used in the calculation of the survival function. The injury risk for a logistic regression is given as Equation 3 1 Risk( BC) = a BC 1+ exp b 86 IRCOBI Conference - Madrid (Spain) - September 26

13 where BC is the Beam Criterion value and a and b are the coefficients of the logistic distribution. Figure 13 shows the injury risk curve generated for BC calculated using FMVSS critical values with a 5% risk of injury at BC =.9 and a standard deviation of % Injury Risk = Beam Criteria of.928 ±.3883 Risk of Injury Injury Risk Curve 95 % Confidence Interval Injury No Injury Beam Criterion Figure 13. Beam Criterion injury risk curve using FMVSS critical values. The failure curve for the Beam Criterion may be optimized to minimize the standard deviation at the 5% injury risk while constraining the mean BC to be 1.. The latter satisfies the usual assumptions for beam failure criteria. To determine these optimal critical values, the ratio between the tension and flexion critical values was varied. Tension and flexion were chosen because they are the prominent force/moment mechanisms of injury during this test series. The ratio between compression and tension forces was chosen to be constant. By changing the ratio of tension force to flexion force, new injury risk curves may be created with new means and standard deviation values. The standard deviation values may be minimized using the ratio subject to the constraint that the mean BC is 1.. The results of this optimization process for axial force and A/P moment are shown in Table 9. These values compare well with previous FMVSS-28 intercepts for Nij and single mode injury criteria and values reported in the literature discussed above. The resulting injury risk curve shown in Figure 14 shows a mean of 1. with a standard deviation of.38. This satisfies both of the above requirements of having a low standard deviation and a mean of 1.. In addition to the case with no shear presented above, an injury risk curve was developed for BC with shear in the form: 2 2.5* FX + FZ Beam = + M Y Equation 4 FZC M YC where F X is the shear force, F Z is the axial load in the neck, either tension or compression, F ZC is critical axial load, M Y is the flexion moment in the sagittal plane, and M YC is the critical moment. Fifty percent of the shear force is added to the axial force. The addition of shear was not found to significantly impact the predictive nature of the curves. Scaling techniques were also performed using body mass, vertebral height, vertebral width, and tensile strength (based on age), but were not found to increase the predictive nature of the curves. Tension 566 N Compression 543 N Flexion 141 N-m Table 9. Optimized critical values for Beam Criterion. IRCOBI Conference - Madrid (Spain) - September 26 87

14 % Injury Risk = Beam Criteria of ±.3761 Risk of Injury Injury Risk Curve 95 % Confidence Interval Injury No Injury Beam Criterion Figure 14. BC injury risk curve calculated for optimized critical values. CONCLUSIONS This study performed a series of tests, including sled tests and head/neck component tests on both dummy and cadaveric subjects, to assess the risk of neck injury from increased head mounted mass. Various parameters were investigated including value of head supported mass, the location of the center of gravity of the head supported mass relative to the head center of mass, the location of the head supported mass, the severity of impact, and the initial angle of impact. In testing, injuries were seen that are similar to those seen in impact events with military rotary wing aircraft with head supported mass under inertial loading. These injuries were located mostly in the lower cervical spine and were predominantly ligamentous and disk injuries for both sled and component cadaveric tests. An injury criterion was developed using cadaveric component head/neck complexes that is based on a survival analysis with an assumed underlying logistic distribution for dynamic variables measurable in a dummy and calculable using computational programs. This injury criterion uses a lower neck beam criterion for the failure stress in a beam (similar to the concepts behind Nij) as follows: FZ M Y Beam = + FZC M YC where F Z is the axial load in the neck, either tension or compression, F ZC is critical axial load, M Y is the flexion moment in the sagittal plane, and M YC is the critical A/P moment. The Beam Criterion should be evaluated at the intervertebral disk of C7/T7 to predict injuries occurring about C7/T1 under inertial loading. Optimized critical values for this criterion derived from minimizing mean error in the survival analysis are presented in Table 9. The effect of lower neck anterioposterior shear was found to be low for the test conditions considered, including high shear conditions. Levels of shear conversion to normal loads to 5% do not substantially change the survival function above. This may, however, change under conditions in which lower neck inertial loading is combined with upper neck impact loading. Computational investigations should be performed to investigate more complex impact conditions. However, forward A/P shear contributes to a stiffening of the cadaveric necks substantially affecting dynamics. The existing Nij criteria evaluated at the upper neck were not found to be adequate for use for the neck injury with head supported mass under inertial loading. There are two reasons for this. 88 IRCOBI Conference - Madrid (Spain) - September 26

15 1. The Nij criteria are based on the Hybrid III dummy. However, the Hybrid III dummy is not generally adequate for simulating human motions under purely inertial flexion/tension loading. In such loading, the dummy shows no humanlike lag of head rotation with neck rotation. The effect of this is exacerbated by head supported mass under inertial loading. The testing emphasized the limited biofidelity of the Hybrid III neck, both from a joint torque basis and for structural biofidelity. The structure of the neck, with a heavy central cable and high bending stiffness, produced lower neck loads and moments that are substantially higher than those from the upper neck used in the current injury criterion. The increased compliance of the human neck would likely not produce such loads and moment in realistic circumstances with realistic phasing. Further, there is a potential for increasing lower neck loads through muscular interactions. However, that will not produce a pure moment about the joint center, but will produce enhanced neck tension and shear forces. The effect of neck tensioning in pilots during impacts is unknown, but should be investigated. 2. Upper neck flexion moment is generally lower than lower neck flexion moment under the conditions investigated. This emphasizes the importance of the moment arm of the head and head supported mass system in the dummy response. The effect of this moment arm is exacerbated by head supported mass. The differences between the dynamic behavior of cadavers and the THOR dummy in sled testing emphasize differences in moment/angle properties and anatomy between humans and THOR. Though THOR kinematics is improved over the Hybrid III dummy, there is a limited representation of the inferred effect of facet joint stops in forward shear under inertial loading. In the human response, this likely contributes to a substantial stiffening of the neck under A/P shear with the head forward of the lower neck, engaging the cervical facets. The gross kinematics of the Hybrid III was found to be different from that of the cadavers in sled testing, while the THOR dummy was found to have similar kinematics to the cadavers in sled testing. THOR was found to have different dynamic behavior than the cadaveric specimens, however, in complex s-shaped bending as the THOR-alpha neck does not represent shear motion joint stop (facet joints) behavior inferred from cadaveric specimen dynamics. Owing to similarities in structural response, this may also be true for the THOR-NT and THOR-FT necks. There are several limitations of this study. Cadaveric response may be different than human response under active and passive muscle control; the cadaver is neither a tensed nor a relaxed human. However, during the impact event (~8 ms), there is not sufficient time for voluntary muscular action. So, active pretensioning, passive musculature, and reflex responses remain. However, there are limits to the influence of muscle response beyond pre-positioning during an impact, and to a certain extent, the positioning effects of passive and active musculature may be simulated as done in these experiments. In addition, the dummies used in this study were designed for automobile impacts in a seated posture. Further, the intent of the dummies is to include the muscle response of humans which is not reproduced in cadaveric specimens. ACKNOWLEDGEMENTS The authors gratefully acknowledge support from the U.S. Army Aeromedical Research Laboratory, the U.S. Army Office of Scientific Research, the Battelle Memorial Foundation and the UVa School of Engineering and Applied Sciences for this study. REFERENCES AAAM (1998)The Abbreviated Injury Scale, 1998 Update. Association for the Advancement of Automotive Medicine. Alem, N. (22) personal communication. Bass, C.R. (22) Impact and Head Supported Mass. Report Headmass1. University of Virginia, Center for Applied Biomechanics, Charlottesville, VA. Bolton, J. (22) Horizontal Decelerator Ejection Seat Testing Using a Hybrid III Test Dummy and a Human Surrogate. Final Report to Naval Air Warfare Center. University of Virginia, Automobile Safety Laboratory, Charlottesville, VA. Bostrom, O., Bohman, K., Haland, Y., and Kullgren, A. (2) New AIS1 Long-Term Neck Injury Criteria Candidates Based on Real Frontal Crash Analysis, IRCOBI. IRCOBI Conference - Madrid (Spain) - September 26 89

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