frontal impact pmhs sled tests for fe torso ...ircobi conference - york (uk) - september 2009 341...

FRONTAL IMPACT PMHS SLED TESTS FOR FE TORSO MODEL DEVELOPMENT

Greg Shaw, Dan Parent, Sergey Purtsezov, David Lessley, Jason Kerrigan, Jaeho Shin, Jeff Crandall University of Virginia Center for Applied Biomechanics

Yoshio Zama, Susumu Ejima Japan Automobile Research Institute

Koichi Kamiji, Tsuyoshi Yasuki Japan Automobile Manufacturers Association, Inc.

ABSTRACT This study evaluated the response of PMHS in 40 km/h frontal sled tests. Three male PMHS were restrained on a rigid planar seat by a custom 3-point shoulder and lap belt. Ribcage deformation was comprehensively quantified using three-dimensional trajectories of multiple skeletal sites on the torso provided using a motion tracking system. The motion of the lower ribcage up and away from the spine may have contributed to sternal fractures early in the event and suggests that fracture risk may not be fully described solely by anterior ribcage displacement toward the spine. The tests produced valuable kinematic data that will be useful in developing a more biofidelic human model. Keywords: Frontal impact, cadavers, thorax, deformations INJURY TO THE CHEST is the principle cause of death in approximately 30 percent of traffic fatalities Mulligan et al. 1994). A more recent study of crash data bases (National Automotive Sampling System (NASS) (1993 to 2001) and Crash Injury Research and Engineering Network (CIREN) (1996 to 2004)) found that chest injuries in motor vehicle crashes continues to be a major cause of morbidity and mortality (Nirula and Pintar 2008).

Since the early 1960s, the goal of reducing chest injuries has motivated numerous studies of human thoracic response to loading (Backaitis, 1994). In frontal crashes, thoracic deformation due to anterior chest loading is generally accepted as the parameter that best correlates to rib and sternal fractures, the most frequently observed thoracic injury for occupants in vehicles equipped with contemporary restraint systems (Kent et al. 2003a).

Improved biofidelity of the chest subjected to restraint loading remains a priority for improving frontal impact dummies and computational models. A simulated impact (sled test) provides the most realistic conditions for defining human response (as approximated by post mortem human subjects (PMHS), the best available human surrogate).

The objective of this study is a comprehensive analysis of the response of three belted PMHS subjected to frontal sled tests in support of the development and validation of a human occupant finite element model. Emphasis was placed on providing comprehensive documentation of all initial occupant and sensor positions, boundary conditions, and response measurements. In addition to the sensor information, detailed three-dimensional motion of skeletal and surface structures was recorded. METHOD

Three male PMHS with approximately 50th percentile stature and mass (Table 1) were subjected to the same simulated 40 km/h frontal crash on deceleration test sled. The subjects, preserved by freezing and confirmed free of infectious diseases including HIV and Hepatitis B and C, were positioned on a rigid planar seat with their torso and head supported to approximate the seated posture of a right front passenger by an adjustable matrix of cables (Figure 1). The primary goal was to position the subject torso so that the shoulder belt loaded the shoulder and anterior ribcage in manner representative of a typical mid size passenger car.

Restraint consisted of a custom 3-point shoulder and lap belt. Each section was separately adjustable for length and joined near the subject’s left hip, a location approximating that of a stalk–mounted buckle. Neither belt segment included a retractor. The restraint webbing, replaced for each test, was manufactured by Narricut (International twill pattern 13195, 6-8% elongation, 6000 lbf minimum tensile strength).

IRCOBI Conference - York (UK) - September 2009 341

Table 1. Subject Characteristics Test: 1358 1359 1360

PMHS ID No. 425 426 428 Age at Time of Death 54 49 57

Cause of Death CVA and Atrial fibulation Lung cancer Neoplasm of brain Body Mass (kg) 78.5 76.2 63.5

Stature (cm) 177 184 175 Pelvis and lower extremity movements were restricted by a rigid knee bolster (adjusted to be in

contact with the knees at the time of impact) and by a footrest with ankle straps (Figure 1). The combination of a snug lap belt, the rigid, channeled knee bolster and footrest was designed to allow little pelvic or lower extremity movement during the event. Bilateral posterior pelvic blocks helped to maintain lower body positioning.

Subject anthropometry and subject crash responses were recorded in order to facilitate finite element modeling. Anatomical landmarks and body segment cross sections were digitized using a 3D measurement device (FaroArm). Subject seating position was held constant for the tests in order to reduce response variability. The location of the subject H-point relative to the seat plate and restraint belt anchors, as estimated by palpating the greater trochanter, varied within a range of 4 mm across all subjects. Pretest torso angle was 64 to 66 degrees which was defined by the angle (relative to horizontal) of a line through the center of the greater trochanter and the 1st thoracic vertebral spinous process projected onto the sagittal plane (Figure 1). Adjustments to the upper shoulder belt anchor location produced an angle from the anchor to the top of the shoulder of 26 to 27 degrees. Belt angle in the frontal plane varied from 51 to 55 degrees.

A Buck Restraint geometry similar to 1998 Ford Taurus front passenger position. B Seat Rigid horizontal aluminum plate. C Knee bolster Adjustable bilateral non-padded knee channels atop right and left 5-axis load cells. D Thigh support Helps maintain pelvic and lower extremity position. E Pelvic block Adjustable bilateral blocks to prevent pelvic posterior migration pre-test. F Footrest Adjustable bilateral channels with ankle straps to immobilize feet and lower legs. G Seat load cell 6-axis load cell supporting seat. H Footrest load cell 6-axis load cell supporting right and left foot plates. I Right lap belt anchor Center of bolt head securing the right belt to the anchor mount defines the buck coordinate

system for 3D movement data. Lap belt length adjuster tongue mounts here. J Upper shoulder belt anchor Center of bolt head securing upper shoulder belt segment. Adjustable in Z and Y. K Back Adjustable back and head support provided by horizontal wires attached to vertical cables. L Anterior head support Plastic tape held head in position pre-test. M T8 marker plate Marker plate screwed to the 8th thoracic vertebra. N H-point Approximated by palpating the greater trochanter. O Torso angle Line between H-point and T1. P Belt angle to shoulder From upper belt anchor to top of right shoulder. Q Belt angle across chest Angle of belt at sternum centerline.

Figure 1. Test hardware and subject positioning.

342 IRCOBI Conference - York (UK) - September 2009

Accelerometers were installed at the head, spine, and sternum. Angular rate sensors were paired with the triaxial accelerometers mounted on the head and the first thoracic vertebrae (T1). Load cells recorded belt tension and subject seat, knee bolster, and footrest loads. Immediately prior to testing, the PMHS’s lungs were inflated with 2.5 liters of air. The tracheal tube through which the air was delivered was left open. Subject kinematic data including torso deflection was provided using a 16-camera 1000Hz Vicon MX™ three-dimensional (3D) motion capture system to track to motion of retroreflective spherical targets through a calibrated 3D space lying within the cameras’ collective field of view (Figure 2). The calibration procedure, performed prior to the testing of each subject, established the position and orientation of cameras with respect to one another which was used to reconstruct the 3D target locations from multiple 2D camera images via a triangulation algorithm. Four-target clusters were secured to selected anatomical locations including the head, spine, pelvis and anterior ribcage to facilitate the use of rigid body mechanics to determine the actual motion of the corresponding bone at each time step using a mathematical coordinate transformation (Figure 3). Target cluster location details are illustrated in Table 2. Strain gages were glued to the sternum, right clavicle, and to ribs 3, 5, and 7 as shown in the Table 2 figure.

Figure 2. Subject in Vicon capture volume.

Figure 3. Subject motion capture measurement hardware.

A – 5/16 Vicon MX13 cameras. B – Reflective markers on subject. C – Vicon camera positions from above. Note that there are 4 cameras directly in front of the subject.


Table 2. Subject Kinematic Measurement and Strain Gage Locations Measurement Location Abbreviation

Right Acromion (RA) Left Acromion (LA) Sternum C (ST) Upper Left Rib (4th) (UL) Upper Right Rib (4th) (UR) Lower Left Rib (7-8 junction) (LL) Lower Right Rib (7-8 junction) (LR) Head A (HEAD) T1 A (T1) T8 B (LA) L2 B (L2) L4 B (L4) Pelvis B (PELVIS)

LA

LL

UL

ST

RA

UR

LR

LA

HEAD

RA

T8

T1

L2

PELVISL4

A – Triaxial acceleration and angular rate, B - Triaxial acceleration, C – Uniaxial acceleration (local x-axis)

VIDEO DATA PROCESSING: Video data, captured in 2D at 1000 Hz by the 16 cameras, was reconstructed using Vicon IQ software to yield 3D trajectories of each tracked marker with respect to a global coordinate system. From this global data set, a local bone-based coordinate system was developed for each marker cluster (Figure 4). All marker hardware (Figure 3) was rigidly fixed to the underlying bone surface at each measurement location, thus satisfying a rigid body assumption. Six degree of freedom (6DOF) (translational and rotational) motion of the bone at each measurement location was obtained from its corresponding marker cluster using a local coordinate transformation accomplished using hardware digitization and CT data where applicable. While analysis of the rotational data was performed, these rotational results are beyond the scope of this paper and are not reported, however this rotational information is an integral part of the coordinate transformation process used to obtain the presented translational data at all 6DOF measurement locations (Table 2). The trajectory of each marker cluster was smoothed through a rigidity constraint using the least squares pose (LSP) estimator as performed by (Cappozzo et al. 1997).

Anterior chest displacement was measured relative to an orthogonal spine-based coordinate system created using the motions from the T1, T8, L2, and L4 vertebral bodies. This spine-based coordinate system will be referred to as the “spine” coordinate system and was created as described in this section. First, a local coordinate system was created on each vertebral body using CT data and a process similar to that recommended by (Wu et al. 2005). Then, at each time step during the test, a cubic spline interpolation based on the position of the vertebral body centers was used to represent the shape of the PMHS spine segment between the T1 and L4 vertebrae. The origin of the spine coordinate system coincided with the origin of the T8 vertebral body coordinate system. The vector tangent to the spline at the T8 vertebra and pointing inferiorly was defined as the spine Z-axis. The spine x-axis was defined as the unit vector normal to the spine Z-axis and the T8 vertebral y-axis. The spine Y-axis was normal to both the spine X and Z axes. Position (trajectory) data are presented in this spine coordinate system (X (positive anterior), Z (positive inferior), and Y (positive to the subject’s right)).

Figure 4. Coordinate transformation to obtain bone position and orientation relative to the

global coordinate system. Marker (M), Bone (B), Global Coordinate System (G), and Transformation (T).

Measurement site Strain gage site


SENSOR DATA PROCESSING AND PRESENTATION: The data was not scaled to account for differences in subject mass or size. Instrument data was collected with TDAS, an onboard data acquisition system (Diversified Technical Systems Inc.) that acquires electronic data at 10,000 samples/sec, was hardware-filtered to 3000 Hz, debiased, filtered to SAE J211-prescribed filter classes, and truncated. The data is reported in accordance with the SAE coordinate system (positive axes: X forward, Y to the right, Z down (Figure 1). SUBJECT INJURY ASSESSMENT: Post test subject injury assessment included identifying ribcage fractures during autopsy. The torso was denuded and organs removed so that the inner surface of ribs could be examined and palpated in order to identify subtle fractures including incomplete (monocortical) or cartilage fractures. Soft tissue, spinal and lower extremity injuries were also assessed. RESULTANT DISPLACEMENT MAGNITUDE AND DIRECTION: Equations 1 – 4 were employed to facilitate presentation of the thoracic displacement data by quantifying 1) the distribution of displacement magnitudes across the anterior thorax 2) the apportioning of each resultant magnitude along the X, Y, and Z axes (i.e., the resultant directionality), and 3) the variation in these measures across tests. For a selected time, tj, during a test, j, at a particular measurement location, i, the resultant

displacement, )(t Res jji, , was calculated from the corresponding displacement components, )(t X jji, , )(t Y jji, , and )(t Z jji, using Equation 1. The normalized displacement components, ji,NormX , ji,NormY

, and ji,NormZ were calculated using Equation 2. At a given measurement location, i, the mean normalized

displacement components, iNormX , iNormY , and iNormZ , corresponding to the three (N=3) tests, were

obtained from Equation 3. The standard deviations, iNormXσ , iNormYσ , and iNormZσ in the mean normalized displacement components across tests for a particular measurement location, i, is given by Equation 4. These equations will be referenced in the presented results.

)(t Z)(t Y)(t X)(t Res jji,22

jji,2

jji,2

jji, ++= [1]

)(t Res)(t X

Xjji,

jji,Norm ji,

= , )(t Res

)(t YY

jji,

jji,Norm ji,

= , )(t Res

)(t ZZ

jji,

jji,ji,Norm = [2]

∑=

=N

1jNormNorm

ji,i XN1X , ∑

=

=N

1jNorm ji,

YN1Y iNorm ∑

=

=N

1jNormNorm

ji,i ZN1Z [3]

2

Norm

N

1jNormX

)X(XN1σ iji,iNorm

−= ∑=

, 2Norm

N

1jNormY

)Y(YN1σ iji,

−= ∑=

iNorm

, 2Norm

N

1jNormZ

)Z(ZN1σ iji,

−= ∑=

iNorm

[4]

RESULTS The forward movement of the subjects’ right clavicle was arrested by the shoulder belt at approximately 60ms. Appendix A quantifies subject movement. Peak chest deflection (anterior ribcage movement toward the spine) coincided with peak shoulder belt tension (6900N ave.) at approximately 90 to 110 ms (Figures 5, 6, 7). Time (ms): 20 60 100 140

Figure 5. High speed images for test 1358. Images are similar for tests 1359 and 1360.

Two cross-sections of the ribcage are shown in Figure 6. The plane of the upper site cross-section at each time step (0, 58, and 105ms) is defined by 3 points: the origin of the T8 vertebral body coordinate system (located at the midpoint of the centers of the upper and lower endplates of the


vertebral body) and the origins of the left and right upper rib site coordinate systems (located at the center of the rib cross-section at the rib mount). The lines, shown overlaid with a drawing of the ribcage at 0ms, connect the projections of the points lying on the y-axes of the vertebra and ribs coordinate systems. The plane of the lower site cross-section is defined by 3 points: the origin of the L2 vertebral body coordinate system and the origins of the left and right lower rib site coordinate systems. Appendix B plots the trajectories of the anterior ribcage relative to the T8 coordinate system.

LL

UL

ST

UR

LR

A - Upper Site Cross Section B - Lower Site Cross Section Figure 6. Ribcage deformation plotted at 0, 58, and 105 ms.

There were similar patterns of anterior ribcage movement with respect to the spine. There was an initial phase of movement from 0 to 60ms when the two upper and left lower rib sites moved away from the spine (+X axis) 2-24mm. These sites then moved towards the spine 20-69mm (86-116ms). The right lower rib site, unrestrained by the shoulder belt, moved away from the spine 37-43mm (95-120 ms). This site also moved up (-Z axis) 56-87mm (97-120 ms) (Figure 8).

Although the general pattern of chest deflection was similar for the tests, there were differences recorded for test 1358 including X-axis deflection. Upper right deflection for 1358, - 20mm, was a factor of 3 lower than the average of the values for tests 1359 (-60mm) and 1360 (-63mm) (Figure 7). However, upper left chest deflection for 1358 was a factor of 2 higher than it was for the other two tests. This test-to-test variability is evident in the standard deviations for the displacement component magnitudes plotted in Figure 9.

This difference may have been due to variation in shoulder belt position. The position of the shoulder belt for 1358 was closer to the left side than for tests 1359 and 1360 (Figure 10). There was little lateral (Y-axis) movement of the belt from 0 to 120 ms. FRACTURES: All three subjects sustained fractures (Table 3). Sternal fractures occurred in all three tests. Fractures to the body of the sternum occurred in tests 1358 and 1360. In test 1359, the cartilage separated from the body of the sternum at the rib 5/6 sterno-costal joint. The fractures were near the lower border of the shoulder belt (Figure 11). In test 1358 only, the sternal facture was near the right border of the aluminum plate used to mount the sternal marker plate. No soft tissue injuries were observed.

Table 3. Subject Fractures. Test Subject AIS90A Rib and Cartilage

Fractures Sternal Fractures Clavicle

Fractures

# Comment # Comment #

1358 425 450240.4 B

15 7 right, 8 left

1 Complete fracture along right border of mount (Figure 11).

1

1359 426 450240.4 B

9 5 right, 4 left

2 Minor fractures. One at the rib 5/6 sterno-costal joint resulting in the cartilage separating from the body of the sternum.

0

1360 428 450230.3 5 All 5 left 1 Diagonal complete fracture. 0

Notes: A – Abbreviated Injury Scale AIS 90, updated 1998. Association for the Advancement of Automotive Medicine, Des Plaines IL, 60018, USA. B – AIS 4 because “> 3 ribs on each of two sides with stable chest or NSF (not further specified). Although the chest may be unstable, which would result in an AIS 5 coding, it not possible to make this determination for PMHS subjects.

0ms

58ms 105ms

mmmm

mm mm


0

5

10

15Acceleration Pulse (g)

0

2500

5000

7500Shoulder Belt Tension (N)

-2500

0

2500

5000

Sternum Strain (μstrain)

-60

-40

-20

0Sternum Deflection, X-axis (mm)

-60

-40

-20

0Upper Left Chest Deflection, X-axis (mm)

-60

-40

-20

0Upper Right Chest Deflection, X-axis (mm)

-40

-20

0

20Lower Left Chest Deflection, X-axis (mm)

0

20

40

60 Lower Right Chest Deflection, X-axis (mm)

0 20 40 60 80 100 120 140

0.5 Test 1358 Test 1359 Test 1360 Figure 7. Sensor time-history plots. Breaks in the marker trajectory traces indicate that the

marker(s) were visible to an insufficient number of cameras. A- One of three axes recorded on the sternum. See Appendix C.

A

Time (ms)


Figure 8. Normalized resultant and component displacement magnitudes (Refer to Equations 1 -

4) at 58 ms and the time of C-max (peak X-axis deflection toward the spine).

Test 1359 @ Time of Cmax

0.0

1.0

2.0

3.0

4.0

5.0


0.0

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l Res

. / S

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um R

es.

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UR LL LR

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pone

nt D

isp,

/ L

ocal

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ulta

nt D

isp.

Local X Disp. / Local Res.Local Y Disp. / Local Res.Local Z Disp. / Local Res.

-1.0

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Test 1358 @ 58 ms

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es.

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isp.


Test 1359 @ 58 ms

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Test 1360 @ 58 ms

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Figure 9. Average ± 1 S.D. of normalized resultant and component displacement magnitudes at

58 ms and the time of C-max for each subject.

1358 1359 1360

-200-150-100-50050100150200

-850

-800

-750

-700

-650

-600

-550

-500

-450

Y-axis Displacement [mm]

Z-ax

is D

ispl

acem

ent [

mm

]

SternumURULLRLLNotchXiphoidBelt

-200-150-100-50050100150200Y-axis Displacement [mm]

SternumURULLRLLNotchXiphoidBelt

-200-150-100-50050100150200


SternumURULLRLLBeltNotchXiphoid

Figure 10. Shoulder belt position relative to chest deflection measurement sites. Note that the

measurement locations on the bone are plotted rather than the marker plate positions.

Average @ 58 ms

0.0

1.0

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5.0Lo

cal R

es.

/ Ste

rnum

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.

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p.


Average @ Time of Cmax

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S

X 0

Y = 0

1358 1359 1360

Figure 11. Approximate fracture (red slashes) locations. 1358 sternal mount plate (A) and strain gage (B).

DISCUSSION

The tests were conducted without incident. Achieving the target subject posture was facilitated by the available adjustable positioning components. The test series yielded a substantial amount of information. 3D motion information was recorded for 13 marker arrays mounted to the skeleton providing both translation and rotation data. Additional single markers provided translation data for the shoulder belt and the extremities. Subject accelerations and interface loads including seat, knee bolster, and footrest loads also were recorded but are not reported in this paper. This discussion is limited to the response of the anterior ribcage. BELT POSITION: All the subjects were prepared and positioned according to the same procedures. The 1358 shoulder belt position, approximately 50 mm more leftward than for the other tests relative to the ribcage marker mount sites (Figure 10), was unexpected.. Although spinal curvature (scoliosis) did not appear to be a dominant factor, torso anatomical variations that included a prominent abdomen, could explain the deviation in shoulder belt path. Qualitatively, the belt path for all three subjects appeared reasonable and we expect that such variation is common in passenger vehicles. ANTERIOR RIBCAGE MOVEMENT PATTERNS: As expected, the dominant response to the shoulder belt restraint acting on the torso was motion of the anterior ribcage was toward the spine (-X-axis displacement). However, motion away from the spine also was observed for all sites between 40 and 70 ms (Figure 6 and 7). The lower right site continued moving away from the spine throughout the event with an average peak X-axis displacement of 37mm.

A review of the 3D data confirmed that the bulge-out behavior, consistently observed in all three tests, was physical and not an artifact caused by insufficient or corrupt optical trajectory information. In prior PMHS sled tests for which chestbands were used to record the external contours of the torso, a modest bulge-out was observed for the unloaded lower quadrant for some subjects restrained by a non-force limiting 3-point belt without an airbag (Figure 12) (Forman et al. 2006 ). Rouhana et al. (2003) reported that the anterior ribcage moved away from the spine in frontal sled tests involving subjects restrained by 4-pt belts.

Figure 12 . Chestband contours showing lower torso cross sections relative to the spine for three

PMHS subjected to a 29 (A) and 38 (B) km/h frontal impacts.

The observed pattern of ribcage deformation may explain the sternal fractures that occurred at a low level of displacement toward the spine. The first fracture signature for the sternal strain gage (in

Spine Spine Spine

S

Y = 0

Belt Path R L

Dimensions in mm A A B

Pretest contour

Contour at max. chest deflection


tests 1358 and 1360) occurred at approximately 58 ms (Figure 7). Abrupt discontinuities also were recorded by the sternal local x-axis accelerometer trace at the same time. However, at this time, the sternum had deflected toward the spine only 11mm (1358) and 6mm (1360), magnitudes not usually associated with sternum fractures and more commonly observed (Howell et al. 2005) rib fractures. In sled tests of belted PMHS, Crandall et al. (1997) reported an average ratio of 12 rib fractures for subjects who also sustained a single sternal fracture. Subjects who did not have a sternal fracture had an average of 7 rib fractures.

Age-matched rib fracture risk curves developed using PMHS subjected to quasi-static and dynamic loading of the sternum (Kent et al. 2003b) suggests that the chance of sustaining at least one rib fracture (at the time of the observed sternal fracture, 58 ms) was 23 percent for both of the test 1358 and 1360 subjects and that the chance of multiple (>6) fractures was less than one percent.

In quasi-static thorax indentor loading tests, Cavanaugh found that 25 mm of skeletal deflection did not result in ribcage fractures but that 50 mm did (Schneider et al. 1989, 1992 and Cavanaugh et al. 1988). Results from a similar test series confirmed that approximately 25 mm of deflection was non-injurious (Shaw et al. 2007).

A possible reason for the sternal fractures at such a low level of deflection may be the relative movement of the lower ribcage which was moving away from the spine at the time the fractures occurred (Figure 6). The lower right for 1358 was 26 mm farther away from the spine for a differential of 37 mm. The lower right for 1360 was 33 mm farther away from the spine for a differential of 39 mm. In terms of deflection towards the spine, the Kent et al. (2003b) risk curves suggest a 52 and 64 percent chance of a single rib fracture and a 5 and 10 percent chance for more than 6 rib fractures for the test subjects at the 58 ms deflection level. Assuming that sternal fractures are associated with multiple rib fractures, it is reasonable to assume that the risk of sternal fracture for the test subjects was higher than indicated by the recorded 6 – 11 mm deflection toward the spine. Two mechanisms acting in combination appear to have caused the bulge out behavior: asymmetrical loading of the ribcage by a diagonal belt and inertial loading of the ribcage and underlying organs (Figure 13). Diagonal belt loading alone did not result in bulge out on the unloaded lower side in bench top tests (Lessley et al. 2008). Using these tests as a baseline, an H-model simulation was used to conduct a study in which the baseline case, belt loading only, was compared to ribcage loading that involved both belt and inertial loading (Haug et al. 2004). The addition of inertial loading produced a modest bulge out. Although the magnitude of the bulge out was not as great as recorded in the sled tests, apparently because the model’s ribcage is stiffer than that of the PMHS (Shin et al. 2009), the trend was similar.

The observed motion of the lower right rib site relative to the sternum is consistent with a hinging effect along the line of the lower shoulder belt edge (Figure 6) This hinging movement resulted in triaxial motion including substantial upward movement (- Z axis) (Figure 8). At the time of the sternal fractures in tests 1358 and 1360, this site had moved up 11mm (1358) and 15mm (1360) relative to the sternum. Lateral movement (Y axis) relative to the sternum at this time was 13mm (1358) and 21mm (1360). At 58 ms the largest resultant magnitude was observed consistently at the lower right (Figure 9).

Further investigation with additional subjects is required to determine if the observed ribcage deformation described by the recorded values is consistent with fracture. If differential movement is confirmed to be the cause of the sternal fractures, this finding would suggest that inertial loading of the thorax contents contributes to ribcage fractures and, furthermore, that restraint systems such as force-limiting belt and air bag systems, reduce fracture risk (Knobloch et al. 2006, Foret-Bruno et al. 2001, Crandall et al. 1997) in part by reducing differential ribcage movement. If further research confirms that anterior ribcage deformation is a risk factor for rib cage fractures, parameters in addition to C-max, maximum X-axis anterior cage-to-spine displacement (Kent et al. 2003a), are required to fully characterize fracture risk. Figure 13 illustrates how C-max may underestimate fracture risk if a bulge out occurs. In the case of distributed loading produced, for example, by an airbag, the the anterior ribcage moves toward the spine decreasing lateral rib radius of curvature. In this case, rib radius decreases as the sternum-to-spine distance decreases and C-max adequately characterizes ribcage deformation. Assymetrical anterior loading by the shoulder belt, however, allows the contralateral side of the anterior ribcage to move away from the spine, creating a reduced radius of curvature on the


anterior ribcage. In this condition, there is increased fracture risk despite little movement of the anterior ribcage toward the spine and C-max is not sufficient to quantify ribcage deformation.

A - Distributed loading B – Decreased lateral rib radius of curvature C - Assymetric anterior loading by the shoulder belt D - Contralateral side of the anterior ribcage movement away from the spine E - Hinging motion resulting in a reduced radius of curvature on the anterior ribcage F - Little movement of the sternum toward the spine relative to the distributed loading condition.

Figure 13. Ribcage deflection conditions.

RELEVANCE OF FINDINGS ON MODEL DEVELOPMENT: The study provides consistent evidence for anterior ribcage movement away from the spine, especially for the ribcage portion unrestrained by the shoulder belt. This suggests that models should be able to accurately represent inertial loads acting on the ribcage and organs. The observed differences in loading pattern produced by the lateral shift in the belt path, if confirmed by additional study, should become a requirement for computational-model torso response.

The chest deflection response (-X-axis displacement) was not measurably affected by rib and sternal fractures. The results in Table 4, which includes the average X-axis deflection for sites that moved toward the spine and the total number of rib and sternal fractures, indicate that there was no relationship between the two parameters. These results were consistent with observations made in other studies (Kent et al. 2004, Duma et al. 2005 Shaw et al. 2007) and suggest that the structural integrity of the ribcage and ribcage response is relatively insensitive to the extent of fracturing observed in this study.

Table 4. Relationship Between Ribcage Fractures and X-axis Displacement

Test Displacement (mm) # Rib+sternal Fractures 1358 -50 16 1359 -40 11 1360 -54 6

LIMITATIONS AND RECOMMENDATIONS FOR FURTHER STUDY: Only three subjects were tested. Because PMHS vary considerably, data from additional subjects tested under similar conditions is required to evaluate the validity of the reported observations.

A preliminary error analysis of the 3D movement data indicates that the measured location of a single marker is no more than 3mm from its actual position. However, a more thorough study is needed in order to better define the composite error associated with many steps required to generate the reported trajectories (See Method above).

Defining chest deflection proved to be difficult because establishing a spine coordinate system required combining position data for multiple sites, T1, T8, L2, and L4. Translation data for anterior rib sites measured relative to coordinate systems attached to any one vertebral site magnified the effect of minor vertebral rotations. Although the T1, T8, L2, and L4 spine definition reasonably characterized the spine in terms of chest deflection, variations on how it was constructed were found to have a substantial effect on anterior rib translation values. Further study is required to ensure that the new spinal and ribcage kinematic data produced by these tests is used to develop metrics that better characterize injury-correlated ribcage deformation.

Distributed Loading Assymetrical Loading


SUMMARY AND CONCLUSIONS This study evaluated the response of three male PMHS in 40 km/h frontal sled tests. The subjects

were restrained on a rigid planar seat by bilateral rigid knee bolsters, pelvic blocks, and a custom 3-point shoulder and lap belt that approximated the restraint geometry of the right front passenger position of a standard mid-size sedan. A system of 16 cameras provided 3D torso movement including chest deflection.

Variation in shoulder belt placement was reflected in anterior ribcage deflection patterns. The motion of the unrestrained lower right ribcage up and away from the spine may have contributed to sternal fractures early in the event. The wealth of kinematic data* produced by this test series, when combined with data from subjects tested in similar conditions, will facilitate the development of improved human models. *Contact Greg Shaw [email protected] to request additional study information.

ACKNOWLEDGEMENTS General hardware, data acquisition, and procedure development was supported by NHSTA. JARI provided support for these tests and was intimately involved in the design, planning, and execution of this study. Key JARI members attended the tests and post test subject injury assessments. This high level of involvement resulted in productive communication and a clear understanding of test objectives, results, and limitations. Narricut Industries Inc. supplied the restraint webbing. REFERENCES

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Crandall, JR, Bass, CR, Pilkey, WD, Miller, HJ, Sikorski, J, Wilkins, M. (1997) Thoracic Response and Injury with Belt, Driver Side Airbag, and Force Limited Belt Restraint Systems. International Journal of Crashworthiness, 2(1): 119-132.

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Forman, J, Lessley, DJ, Shaw, CG, Evans, J, Kent, RW, Rouhana, S, Prasad, P. (2006) Thoracic Response of Belted PMHS, the Hybrid III, and the THOR-NT Mid-Sized Male Surrogates in Low Speed, Frontal Crashes. Stapp Car Crash Journal, 50: 191-215.

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Kent, RW, Patrie, J, Poteau, F, Matsuoka, F, Mullen, C. (2003b) Development of an age-dependent thoracic injury criterion for frontal impact restraint loading. Paper 72, Proceedings of the 18th International Technical Conference on the Enhanced Safety of Vehicles (ESV).

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Knobloch, K, Wagner, S, Haasper, C, Probst, C, Krettek, C, Otte, D, Richter, M. (2006) Sternal Fractures Occur Most Often in Old Cars to Seat-Belted Drivers Without Any Airbag Often With Concomitant Spinal Injuries: Clinical Findings and Technical Collision Variables Among 42,055 Crash Victims. The Annals of Thoracic Surgery, 82(2): 444-450.

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Appendix A Position With Respect to the Buck Coordinate System

-500501001502002503003504004500

50

100

150

200

-50050100150200250300350400450

0

50

100

-50050100150200250300350400450

-50

0

50

5075100125150175200

-25

0

5075100125150175200

-25

0

5075100125150175200

-25

0

25

0100200300400

-1000

-900

-800

-700

-600

-500

-400

-300

00.5 Test 1358

Test 1359

Test 1360

- Axis values represent position in millimeters - Markers denote 25-millisecond increments

Position in XZ Plane Position in XY Plane


Appendix B Position With Respect to the T8 Coordinate System

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

70

80

90

100

110

120

130

140

150

160

170

180


X-a

xis

Dis

plac

emen

t [m

m]

RL

-100 -80 -60 -40 -20 0 20 40 60 80 100 120 140

60

80

100

120

140

160

180

200

220


L R

Upper Left, Sternum, Upper Right Lower Left, Lower Right

60 80 100 120 140 160 180 200 220

-20

0

20

40

60

80

100

120

140

160

X-axis Displacement [mm]

Z-ax

is D

ispl

acem

ent [

mm

]

60 80 100 120 140 160 180 200 220 240

-60

-40

-20

0

20

40

60

80

100

120

140


Z-ax

is D

ispl

acem

ent [

mm

]

Upper, Lower Left Sternum Upper, Lower Right

Appendix C Strain Gage Time-Histories (0-130 ms)

-8000-6000-4000-2000

0

R3 Med - 71 [μS]

-4000-2000

020004000

R5 Lat - 73 [μS]

-8000-6000-4000-2000

02000

R5 Med - 72 [μS]

-20000

200040006000

R7 Lat - 75 [μS]

-2000

0

2000

4000

6000 R7 Med - 74 [μS]

-20000

2000400060008000

Stern Mid - 82 [μS]

-4000-2000

0200040006000

Stern Right - 83 [μS]

0

2000

4000

6000

Clav - 76 [μS]

10000-8000-6000-4000-2000

02000

L5 Lat - 85 [μS]

-15000

-10000

-5000

0

L5 Med - 84 [μS]

-10000-8000-6000-4000-2000

02000

L7 Lat - 91 [μS]

-10000-8000-6000-4000-2000

02000

L7 Med - 86 [μS]

0 20 40 60 80 100 120 1 0 20 40 60 80 100 120 1 0 20 40 60 80 100 120 1 0 20 40 60 80 100 120 1

05 Test 1358

Test 1359

Test 1360

- Axis values represent position in millimeters - Markers denote 25-millisecond increments

60 80 100 120 140 160 180 200 220 240

-60

-40

-20

0

20


Z-ax

is D

isp.

[mm

]

ms


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