Evaluation Criteria for the Reduction of Minor Neck ...

Evaluation Criteria for the Reduction of Minor Neck Injuries during

Rear-end Impacts Based on Human Volunteer Experiments and

Accident Reconstruction Using Human FE Model Simulations

Koshiro Ono1)

, Susumu Ejima1)

, Kunio Yamazaki1)

, Fusako Sato1)

,

Jonas Aditya Pramudita2)

, Koji Kaneoka3)

, and Sadayuki Ujihashi 2)

1) Japan Automobile Research Institute

2) Tokyo Institute of Technology, Japan

3) Waseda University, Japan

ABSTRACT Minor neck injuries (whiplash associated disorder) during rear-end impacts have become the most frequent cause of permanent disabilities compared with rear-end impact injuries in other regions of the body. The social cost of these types of injuries have grown to be such an extensive burden that identification and clarification of the mechanism of neck injury, as well as the need for a method for evaluating the reduction of neck injuries through the use of an advanced seat and headrest, becomes imperative. Injury evaluation parameters were set using a human finite element model based on live volunteer and cadaver experiments, reinforced with data from 20 accidents. It is found that the displacements between the cervical vertebrae can cause neck injuries/persistent symptoms, and 2D strains of the inter-vertebral bodies as injury criteria are defined. The thresholds where WAD2+ injuries occur by making risk curves with NIC and the neck forces, and lead to a proposed neck injury evaluation method. Keywords: Volunteers, Cadavers, FE Model, Strain/strain rate, Cervical Vertebral Motion

WITH THE INCREASE in the number of rear-end impact accidents, the occurrence of minor neck injuries have likewise increased every year. Compared with rear-end impact injuries to other regions of the human body, neck injuries usually induce adverse effects, making it imperative to clearly establish the occurrence mechanism of neck injuries. It is also difficult to diagnose neck injuries resulting from low-speed rear-end impacts with either a CT or MRI (Davis, 1991), making the clarification of such occurrence mechanism particularly vital from a medical standpoint. Recently, using advanced X-ray cineradiographic imaging to capture cervical vertebral motions in human live volunteers during the impacts, possible causes of neck injuries due to whiplash motion have begun to be proposed (Ono et.al, 1997, 2006, Pramudita et. al., 2007). Moreover, for phenomena which are difficult to clarify through experiments, the use of human body finite element models for reconstructing cervical motion, made investigating the mechanisms of neck injuries possible. However, the criteria for minor neck injuries is yet to be established, and thus the use of human models and dummies for impacts, aimed at developing advanced seats with its corresponding method of evaluation are yet to be established as well. In this study, we have developed a technique enabling the evaluation of minor neck injuries due to impacts, and proposed a neck injury evaluation method which may contribute to developing safer advanced seats. With a human finite element model validated by human volunteer and cadaver experiments, we have also proposed injury evaluation parameters based on the results of the accident reconstruction simulations. In addition, we also established targeted criteria for neck injury evaluation using results from volunteer experiments and the accident reconstruction simulations by the human finite element models, and proposed an evaluation method with dummies for impacts. OUR RESEARCH FLOW CHART

The flow chart of our research for the proposed minor neck injury evaluation criteria is shown in Fig. 1. First, the rear-end accident data were supported by Folksam (upper right in Fig.

IRCOBI Conference - York (UK) - September 2009 381

1), and necessary information such as boundary conditions for the accident reconstruction simulation were selected. Next, based on cervical vertebral motions obtained from X-ray cineradiographic images in volunteer experiments (upper left of Fig. 1); we decided on the necessary physical quantities, such as, strains of the inter-vertebral bodies, as parameter studies for accident reconstruction simulations. In practice, we set strain thresholds where pain occurs in relation to the cervical vertebra motion of the subjects, who felt a sense of discomfort during the volunteer experiments, then examined physical quantities which correlate with the strains (lower left part of Fig. 1). Using these data, we carried out accident reconstruction simulations using JAMA human FE models (center of Fig. 1; Sugimoto and Yamazaki, 2005; Sato et al. 2009), and analyzed the strains of the relative motion of cervical vertebrae bodies that correspond to the occupant’s symptoms. At the same time, the head acceleration and neck forces were analyzed. Finally, we selected the physical parameters which correlate with the strains of the inter-vertebral bodies from the volunteer experiments and accident reconstruction simulations, proposed injury criteria, thresholds, and risk curves.

Fig. 1 Flow chart for the proposed neck injury criteria

APPLICATION OF VOLUNTEER EXPERIMENTS TO THE EVALUATION OF LOCALIZED CERVICAL VERTEBRAL MOTIONS To clarify localized motions of the cervical vertebrae which characteristically occur during impacts, simulated low-speed rear-end impacts with the use of volunteers were carried out, and strains of relative motion of inter-vertebral bodies were analyzed. In this study, we have provided: the method of the volunteer experiments, an outline of the experimental results, a summary of the cervical vertebral motion in the experiments, the conditions of the subjects during the experiments, and the method of analysis of the relative motion of cervical vertebrae. EXPERIMENTAL METHOD A simulated mini-sled was mounted on horizontal rails, on which a mass-production car seat was set (Fig. 2), and rear-end impact was simulated by pulling the sled forward. In order to obtain the head, neck and torso behavior of the volunteers during the rear impact, a high speed video camera (Redlake MotionXtra HG-100K) with a photographic capability of 500fps was used. Markers that were attached to the volunteers’ bodies were able to be tracked and analyzed. In order to obtain the cervical vertebral motion, a cineradiography system (Philips BH-5000) from the University of Tsukuba Hospital, with a photographic capability of 60fps was used. Accelerometers, angular velocity, strain gauges, electrodes for EMGs, and touch sensors were

382 IRCOBI Conference - York (UK) - September 2009

mounted to determine acceleration responses, rebound forces of the head restraint, EMG responses, and contact between the volunteer and the seat. Sled acceleration: 28m/s

2, 33m/s

2,

and 40m/s2, seat back angle: 20º and 25º, muscle condition: a combination of tensed and relaxed

states. The volunteers were six normal healthy adult males and three normal healthy adult females. The purpose, method, and risk of the experiments were explained to the volunteers, and the experiments were carried out with their agreement. Moreover, the experimental details were approved by the Special Committee of Ethics of Tsukuba University.

40 m/s 2 33 m/s 2 28 m/s 2

Sled Velocity

-1

0

1

2

3

0 100 200 300

Time [ms]

Vel

ocity

[m/s

]

Sled Acceleration

-40

-20

0

20

40

60

0 100 200 300

Time [ms]

Acc

eler

atio

n [m

/s2 ]

Fig. 3 Sled acceleration and velocity

ANALYSIS METHOD OF CERVICAL VERTEBRA STRAINS The 2D strain analysis has been proposed by Hoffman et al (1984), and we used the method, which was also applied to previous works

(Ianuzzi

et al, 2004). Assumption for relative motion

of cervical vertebrae is equivalent with the strain of facet joint capsule. At first, a region surrounded by straight lines joining the representative points in a cervical vertebra is assumed to be a 2D element. As shown in Fig. 4, a coordinate transformation is executed, and thereby the strain of the square center (0, 0) is obtained. This element is called the “isoparametric element,” which is frequently used in the finite element method.

Fig. 4 Coordinate transformation of 2D region of facet joint and/or inter-vertebral disk

Vertebra Ci

Vertebra Ci+1

representative point Front edge

x2

x1

Upper edge

Lower edge

Rear edge

Physical coordinate of facet joint Ci/Ci+1

r1

r2

① ②

③ ④

① ②

③ ④

(-1,-1) (1,-1)

(1,1) (-1,1)

(0,0)

Natural coordinate of facet joint Ci/Ci+1

transformation

Cam

Motor

Sled

Mass production seat with head rest

Shock absorber

Hydraulic cylinder

X

Z

Head accelerometer

T1 accelerometer

Sled accelerometer

Y

Fig.2 Schematic view of the mini-sled apparatus


In this method, the interpolation function of the coordinate transformation is as follows:

N(1)=1

4 1-r1 1-r2 1 N(2) =

1

4 1+r1 1-r2 (2)

N(3)=1

4 1+r1 1+r2 3 N(4)=

1

4 1-r1 1+r2 (4)

where the coordinate xi and the displacement ui are as follows:

xi = N j xi j 5 ui = N(j)ui

(j) (6)

From the above equations (1)-(6), the displacement and strain for calculating the strain of the square center are shown by the following Eq. (7).

(7)

This equation is a nonlinear function considering large deformations, where e11 is the vertical strain in the direction of x1 , e 22 the vertical strain in the direction of x2, and e 12 (e 21) is shear strain. In addition, Quin et al. (2007) have analyzed the rupture of soft tissues by principal strains. Thus, using vertical strains and shear strains obtained from Eq. (7), we analyzed the maximum/minimum principal strains and the maximum shear strains. The maximum/minimum principal strains and maximum shear strains are expressed by the following Eqs. (8) and (9).

(8)

(9)

Moreover, by differentiating Eqs. (8) and (9) with respect to time, we obtain the maximum/minimum principal strain rates and the maximum shear strain rates. RESULTS HUMAN VOLUNTEER TESTS: Typical characteristics of the appearance of the occupant and the cervical vertebral motions during rear-end impacts are as follows (Fig. 5). In Phase 1 (0-around 70 ms), before the head collides with head restraint, the torso (hip, back, and T1) interacted with the seat cushion and ramped up along the seat back straightening the curvature of the spinal column. Due to the straightening of the spine and the ramping-up of the torso, the head which stays due to inertia was knocked up by the torso, inducing buckling and an S-shape deformation of the neck. In phase 2 (around 70-130 ms), the head collides with the head restraint, and the upper torso (T1) moves towards the rear, and is extended by an impact force from the seat back. Thus, as the upper torso and the head touch and sink into the seat back and the head restraint, respectively, the head shows flexing relative to the upper torso. At this time, C0-C6 shows flexion with respect to C7. Furthermore, the head begins to turn to the direction of extension, and the rearward shift of the head and upper torso reaches maximum at around 130 ms. In phase 3 (around 130 ms) as the head and the torso rebounded from the seat, the head and the upper torso move forward. At this time, the upper torso turns into the direction of the flexing, while the head exhibits extension. After that, the head also begins to turn into the direction of the flexing. Furthermore, T1 departs from the seat back, and subsequently the head departs from the head restraint. At the time of contact with the head restraint, the upper cervical vertebra

j

j

i

j

j

i

i

i

i

j

j

i

jix

u

x

u

x

u

x

u

x

u

x

u

2

1

2

1

2

12

2

22112211minmax/

22

2

12

2

2211max_

2

shear


shows relative flexion and forward displacement. On the other hand, as the forward displacement of the upper cervical vertebra progresses with the forward movement of the torso, C5 and C6 on the other hand, show extension relative to C7 as it moves rearward, greatly inducing the tensile and forward shear on the upper cervical vertebra, and the compression and rearward shear on the C5/C6 and C6/C7. Digitizing errors (mean) in the vertebral motion analysis were 0.055 (+/-0.016SD) for principal strain and 0.071 (+/-0.031SD) for shear strain.

Fig.5 Typical characteristics of occupant motion and cervical vertebral motion during impact. The arrows of phase 1, phase 2, and phase 3 show the time flow just after the impact.

CONDITIONS OF SUBJECTS AFTER THE EXPERIMENTS: The conditions of the volunteers after experiments are shown in Table 1. Volunteers I, II and VI felt a sense of discomfort, such as stiffness in the shoulder and neck ache. The sense of discomfort occurred only on the day of the experiment, and the symptoms were comparatively mild.

Table 1 Neck discomfort after experiments

Volunteer

I

II

III

IV

V

VI Pain in neck while sleeping on test day

Neck discomfort

Stiff shoulder on test day

Stiff shoulder on test day

None

None

None

STRAIN-TIME HISTORY BASED ON THE SENSE OF DISCOMFORT FELT BY THE SUBJECTS: We have examined the thresholds of the strains through the cervical vertebral motions obtained from the volunteer experiments, and the sense of discomfort in the neck region that the subjects felt in the experiments. In the experiments, as the strains of the inter-vertebra joints cannot be directly measured, we estimated the strains, shear strains, and strain rates of the inter-vertebral discs using the analysis method shown in Fig. 4. The inter-vertebral discs are assumed to be a rigid body, and their strains and strain rates are assumed to be equivalent to the deformation strains of the vertebral facet joint capsules. The experiments for analysis were carried out at a seat back angle of 25º, a sled acceleration of 40 m/s

2, and with subjects in a relaxed condition (the most sever condition). For

maximum/minimum principal strains and the maximum shear strains of the inter-vertebra, the mean values of all the volunteers together with corridors are shown in Fig.6. The corridors were determined by the average value ± one standard deviation. Moreover, for maximum/minimum principal strain rates and the maximum shear strain rates of the inter-vertebra discs, the mean values of all the volunteers, together with corridors are shown in Fig. 7.


Fig.6 Mean values of maximum/minimum principal strains and the maximum shear strains of

inter-vertebra discs; 6 human volunteers


Fig.7 Mean values of maximum/minimum principal strain rates and the maximum shear strain

rates of inter-vertebra discs


STRAIN THRESHOLDS BY VOLUNTEER EXPERIMENTS: The occurrence of strains of each cervical vertebral segment differs. Also, the regions where the sense of discomfort felt by the volunteers occurs were not specified. Thus, as a method for establishing strain threshold, the following classification as shown in Fig.8 is adopted. Average strains and strain rates: For the cervical vertebrae, it may be assumed that the region where the sense of discomfort occurs can be represented by the average of the strain and strain rate occurring in each cervical vertebra. By determining the average of the strain and the strain rate in each segment of the subjects, and furthermore determining the average of the lower limit of hazard and the upper limit of safety and its standard deviation, the hazard zone, safety zone, and gray zone are defined. Maximum strains and strain rates: For the cervical vertebrae, it may be assumed that the region where there is a sense of discomfort is the region where the strain becomes the maximum in a specified segment. By this, the average of the maximum value of the strain occurring in C2/C3-C6/C7 and the strain rate at the time for each subject, the hazard zone, safety zone, and gray zone are defined. Maximum strain rates and strains: It may be assumed that, in the cervical vertebra, the region where the sense of discomfort occurs is the one where the strain rate in a specified segment becomes a maximum. Based on this, the maximum value of the strain rate occurring in C2/C3-C6/C7 and the strain in a case are obtained for each subject; using their mean values, the hazard zone, safety zone, and gray zone are defined. The schematic diagram for the method of establishing the strain threshold mentioned above is shown in Fig. 8. The thresholds of lower limit of hazard and the upper limit of safety, concerning the strains and strain rates in each method are summarized in Table 2.

Fig.8 Establishment method for strain threshold

Table 2 Strain thresholds in the sense of discomfort of subjects in volunteer experiments

Lower

limit of

Upper

limit of

Lower

limit of

Upper

limit of

Lower

limit of

Upper

limit of

Lower

limit of

Upper

limit of

Average

strain and

strain rate

0.019

(0.017)

0.055

(0.023)

0.015

(0.008)

0.047

(0.012)

0.863

(0.712)

2.679

(1.366)

0.818

(0.344)

1.814

(0.414)

Maximum

strain and

strain rate

0.104 0.129 0.1 0.102 0.146 0.644 0.101 1.218

Maximum

strain rate

and strain

0.06 0.066 0.061 0.074 2.728 5.021 2.05 2.243

Maximum principal Maximum shear Maximum principal Maximum shear

Strain Strain rate

Values in parentheses indicate the standard deviation.

-0.1

0

0.1

0.2

0.3

0 2 4 6 8

Str

ain

Max. Rate [1/s]

Ci/Ci+1 Max. Princ. Strain

Lo wer limit of hazard zoneSmallest strain rate of the

volunteer who feels a sense of discomfort

Lo wer limit of safety zoneLargest strain rate of the

volunteer who does not feels a sense of discomfort

Gray zone

S afety zone

H azard zone

Uppe r limit of safety zone

Lo wer limit of hazard zone

Largest strain of the volunteer

who does not feels a sense of discomfort

Smallest strain of the volunteer

who feels a sense of discomfort


SELECTED CASES FOR ACCIDENT RECONSTRUCTION SIMULATIONS: We have collected accident cases that involved the same type of vehicles equipped CPR (Crash Pulse Recorder) with the same seat as that of the human volunteer experiments. Under such conditions, 20 cases of the rear-end impact accidents were provided by Folksam, and we used 13 out of the 20 cases. Of the 20 occupants in the forward seat, 14 occupants suffered neck injury, and 6 had no injury. Crush pulses analyzed by CPR (ΔV, mean accelerations, and maximum accelerations), the occurrence or non-occurrence of neck injuries, the duration of symptoms, and WAD（whiplash-associated disorders）are summarized in Table 3. The WAD symptoms are based on the Quebec classification (Spitzer et.al., 1995) in Table 4, and the duration of neck injury symptoms is classified into four periods (no injury, shorter than one month, shorter than 6 months, and longer than 6 months) using medical records and interviews with the occupants. As incipient symptoms, the ache and stiffness on the neck are relatively frequent. On the other hand, as a long-term symptom, headaches also occur. RELATIONSHIP AMONG ΔV, MEAN, MAXIMUM ACCELERATIONS AND NECK INJURIES: Even at a low speed of 10.8 km/h, neck injuries with durations shorter than one month occur. As for gender difference, symptoms appeared in two out of five males and in six out of six females at ΔV larger than 20 km/h, and at mean accelerations larger than 5.0g. Moreover, for females, the neck injury symptoms appeared at mean accelerations larger than 4.0g and at ΔV larger than 20 km/h. ACCIDENT RECONSTRUCTION SIMULATIONS: Using human finite element models, with crash pulses recorded at the time of actual rear-end impacts as input conditions, we reconstructed the occupant’s motion during impacts. Using the head and neck motion obtained from these simulations, we have examined the relationship between the degree of injuries at the time of the actual impacts and the local displacement of cervical vertebra.

Human finite element models, seat models, and physical constitution scaling: The human finite element models correspond to the average physical type of American adult males, AM50, and skin, soft tissues, and visceral organs are modeled on the basis of skeleton data. The connection between bones is simulated by beam elements which imitate the ligaments, through which the motions of joints can be expressed. The structure of the vertebra, which influences the motion at the time of impacts, as well as the human vertebra structure, consists of the first (T1) to the twelfth (T12) thoracic vertebrae, and the first (L1) to the fifth (L5) lumbar vertebrae, and each vertebral body is modeled with a rigid body element. Each vertebral body is jointed through the ligament modeled with a bar element and the inter-vertebral disc modeled with a solid element. By rigidly jointing the upper and lower faces of each vertebral body and inter-vertebral disc, the motions of the inter-vertebral bodies are simulated. In this study, the human model (JAMA Rear-impact Human FE Model) mentioned above is validated for application by volunteer (Ono et.al, 2006, Pramudita et.al, 2007) and cadaver (White et.al, 2009) experiments. In particular, the validation of the human model was executed using a crash pulse of 40 m/s

2 (Fig. 3). In Fig. 11,

the maximum strains and maximum shear strains for the inter-vertebral disc of the volunteers are compared with those for the human model. With the same method used in the volunteer experiments, the maximum strains and maximum shear strains for the human model are determined by the estimation of the 2D vertebral strains of the human body model. The volunteer experimental data shown in Fig.11 are the mean values, and the mean values ±SD (standard deviation) of the data for six volunteers, which are shown by the corridor.

The seat model was made by measuring the external form of a seat that is the same as that of the accident car. The dynamical weight/displacement characteristics were estimated from static experiments for the seat alone. Moreover, the bending characteristics of the seat hinge and head restraint stay were obtained by analyzing the time hysteresis of the inclination angle of the seat back using the sled experiments with the BioRID-II dummy. Moreover, the height and weight of the occupant, which were indicated in the accident data, were used as the parameters to scale the physical constitution of the human FE model.


Table 3 Collected cases of accident

HR

No. D/P⊿v

[km/h]

Mean

Acc.[g]

Peak Acc.

[g]Neck/Spine Symptoms WAD Gender Age Height Weight Height

1 Driver 28.2 5.8 10.6 Injured 1-6 m 2 F 26 175 55 No.4

4 Driver 26.0 5.6 12.6 Injured >6 m 3 M 57 178 100 No.4

4 Passenger 26.0 5.6 12.6 Injured >6 m 3 F 57 168 80 No.3

2 Driver 23.3 6.7 14.7 Injured >6 m 2 F 59 156 60 No.1

8 Driver 20.4 5.2 12.8 Injured <1 m 1 F 22 171 63 No.4

8 Passenger 20.4 5.2 12.8 Injured <1 m 2 M 18 179 80 No.4

7 Driver 19.5 4.0 9.2 No injuries no 0 M 67 167 84 No.3

7 Passenger 19.5 4.0 9.2 Injured <1 m 1 F 72 165 63 No.3

10 Driver 17.6 5.0 12.4 Injured 1-6 m 1 M 74 175 62 No.4

10 Passenger 17.6 5.0 12.4 Injured 1-6 m 2 F 74 160 57 No.2

6 Driver 16.3 4.9 12.1 No injuries no 0 F 59 165 65 No.3

6 Passenger 16.3 4.9 12.1 Injured <1 m 1 M 88 170 70 No.3


11 Passenger 16.3 6.5 15.2 No injuries no 0 F 61 154 69 No.1


23 Driver 11.1 3.7 8.9 Injured <1 m 1 F 35 178 65 No.4

20 Driver 10.8 3.7 7.1 Injured <1 m 1 M 65 176 82 No.4

20 Passenger 10.8 3.7 7.1 No injuries no 0 M 68 176 77 No.4

24 Driver 8.8 3.5 7.5 Injured 1-6 m 1 F 35 165 55 No.3

3 Driver 14.7 5.2 7.5 Injured >6 m 2 F 35 165 55 No.3

CASE RECORDED CRASH PULSE REPORTED INJURY PASSENGER CHARACTERISTICS

Table 4 Quebec classification of WAD (Whiplash-associated disorders)

Fig.9 Relationship between ΔV and mean accelerations

Adjustment of head restraint height: There were no data concerning the head restraint height in the accident data. Thus, assuming that the occupant properly adjusted the head restraint height, we set the height so that the occupant’s occiput is proximate to the head restraint center. The relationship between the height of the occupant and head restraint is shown in Table 3 (Right-end column) and Fig.10.

Grade Clinical classification

0 The neck has no symptoms, and the physical finding is normal.

1 The neck has pain and stiffness, but the physical finding is normal.

2

In addition to neck symptoms, there is a limit of motion space of the

cervical vertebra and a localized tender point, suggesting neck

symptoms from the musculoskeletal system.

3In addition to neck symptoms, there are neurological findings such as

the tendon reflex disorder, adynamia, and perception disorder.

4 Dislocation and fracture of the cervical vertebra.

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35

Change of velocity [km/h]

No neck symptoms

Neck symptoms

mean acceleration [g]


(a) No.1 (b) No.2 (c) No.3 (d) No.4

Figure 10 Adjustment of head restraint height

Reconstruction simulations using accident data: In the accident reconstruction simulations, by using the analysis method of cervical vertebral strain for volunteer experiments, we estimated the 2D strains of the cervical vertebrate of the occupant models in all cases of accidents. The relationship between the strains/strain rates and WAD is shown in Fig. 12, where the


strains/strain rates are at maximum in C1/C2 to C6/C7 of each occupant. As seen from Fig. 12, the grade of WAD increases as the strains and strain rates of the cervical vertebrae increase. Fig. 13 shows the relationship between the strains or strain rates and ΔV, and Fig. 14 shows the relationship between the strains/strain rates and the mean accelerations. Moreover, Fig. 15 shows the relationship between the strains or strain rates and the maximum accelerations. In these figures, the linear approximation straight lines and multiple correlation coefficients are also shown.

The maximum principal strain and maximum shear strain are best correlated with ΔV among ΔV, mean acceleration, and maximum acceleration. On the other hand, the multiple correlation coefficients of the maximum principal strain rates and maximum shear strain rates with ΔV are a low value of approximately 0.13. The shear strain rates are highly correlated with the maximum accelerations, indicating that there is a difference between the strains and the strain rates. Thus, to analyze the relationship between neck injuries and crash pulses, not only ΔV but also the maximum acceleration should be considered.

Fig. 12 Relationship between the strains/strain rates and WAD

Fig.13 Relationship between the strains/strain rates and ΔV

R² = 0.3236

0

1

2

3

0 0.05 0.1 0.15 0.2

WA

D

Strain

(a) Max Strain

(C1-C7)

R² = 0.0757

0

1

2

3

0 2 4 6 8

WA

D

Strain Rate

(b) Max Strain Rate

(C1-C7)

R² = 0.341

0

1

2

3

0 0.05 0.1 0.15

WA

D

Strain

(c) Max Shear Strain

(C1-C7)

R² = 0.0906

0

1

2

3

0 1 2 3 4

WA

D

Strain Rate

(d) Max Shear Strain Rate

(C1-C7)

R² = 0.4792

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 0.05 0.1 0.15 0.2

⊿V

[k

m/h

]

Strain

(a) Max Strain

(C1-C7)

R² = 0.1491

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 2 4 6 8

⊿V

[k

m/h

]

Strain Rate

(b) Max Strain Rate

(C1-C7)

R² = 0.5126

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 0.05 0.1 0.15

⊿V

[k

m/h

]

Strain


(C1-C7)

R² = 0.1306

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 1 2 3 4

⊿V

[k

m/h

]

Strain Rate


(C1-C7)

Fig.14 Relationship between the strains/strain rates and mean acc.

Fig.15 Relationship between the strains/strain rates and peak acc.

R² = 0.3674

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

0 0.05 0.1 0.15 0.2

Mea

n A

cc.[

g]

Strain

(a) Max Strain

(C1-C7)

R² = 0.3426

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

0 2 4 6 8

Mea

n A

cc.[

g]

Strain Rate

(b) Max Strain Rate

(C1-C7)

R² = 0.3846

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

0 0.05 0.1 0.15

Mea

n A

cc.[

g]

Strain


(C1-C7)

R² = 0.3258

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

0 1 2 3 4

Mea

n A

cc.[

g]

Strain Rate


(C1-C7)

R² = 0.3268

0.0 2.0 4.0 6.0 8.0

10.0 12.0 14.0 16.0

0 0.05 0.1 0.15 0.2

Pea

k A

cc.[

g]

Strain

(a) Max Strain

(C1-C7)

R² = 0.4108

0.0 2.0 4.0 6.0 8.0

10.0 12.0 14.0 16.0

0 2 4 6 8

Pea

k A

cc.[

g]

Strain Rate

(b) Max Strain Rate

(C1-C7)

R² = 0.3334

0.0 2.0 4.0 6.0 8.0

10.0 12.0 14.0 16.0

0 0.05 0.1 0.15

Pea

k A

cc.[

g]

Strain


(C1-C7)

R² = 0.4002

0.0 2.0 4.0 6.0 8.0

10.0 12.0 14.0 16.0

0 1 2 3 4

Pea

k A

cc.[

g]

Strain Rate


(C1-C7)


DISCUSSION

THRESHOLD OF NECK INJURY OCCURRENCE BASED ON CERVICAL VERTEBRAL

MOTIONS: The present study attempted to investigate the threshold of neck injury occurrence

based on localized cervical vertebral motions. However, because none of the volunteers in the

experiment actually experienced neck injury, it was impossible to investigate the threshold for

actual neck injury occurrence. Therefore, the experiments were conducted not as an indicator of

actual neck injury, but rather as an indicator of the presence or absence of some sense of

discomfort. The corresponding relationship between the physical quantities and the strains of

the relative motion of cervical vertebral bodies for each subject were analyzed to investigate the

threshold of neck injury occurrence. Deng et.al. (2000),

Yoganandan et.al.(1998, 2001),

Winkelstein et.al.(1999), Sigmond et.al.(2001), and Stemper et. al. (2005) in their crash tests

that used cadavers, reported to have found that injuries of the inter-vertebral discs and the facet

joint capsule of the lower cervical vertebrae, and especially the tearing of the soft tissues,

occurred due to shearing and tension. Deng et.al. (2000) also reported that dynamic motion and

strain rates influenced the tearing of soft tissue in the cervical vertebra. Yoganandan et.al. (1998,

2001), Barnsley et.al. (1995) , Lord (1996), and Inami et.al. (2000) have reported that neck pain

was related to inter-vertebral discs and facet joint capsule injury resulting from treatment of the

disc joint block. Lee (2006), in his test using rats reported that strain of facet joint capsule was

related to pain. The volunteer low speed mini-sled test conducted by Anthors et al.(2006, 2007),

in which they hypothesized that the strains on the facet joint capsule and the motion of the

inter-vertebral disc were equivalent, measured the motion of the local transformation of the

inter-vertebral disk using sequential cineradiography of the vertebral motion during impact. The

maximum principal strain and principal strain rate, and the maximum shear strain and shear

strain rate, were determined from these motions. Injury thresholds were defined by the strain

value and strain rate at which volunteers felt some discomfort after the test.

CERVICAL VERTEBRAL STRAINS, PHYSICAL QUANTITIES MEASURABLE WITH DUMMIES, AND INJURY PARAMETERS: To extract parameters correlated with the strains and strain rates of the cervical vertebra, we summarize the relationship between physical quantities or injury parameters, measurable with dummies, and the strains in Table 5, where the physical quantities are neck forces (Upper Fx, Fz, My and Lower Fx, Fz, My) and currently proposed injury parameters are NIC, Nkm, LNL, TIG, Rebound V, OC-T1, head-T1 rotation angle. The classification method in Table 5 is as follows; In note 1 of Table 5, if each physical parameter in the volunteer experiments has a positive correlation with both of the strains and strain rates, then ○; if it has a positive correlation with the strains or strain rates, then △; if it does not have a positive correlation with either of the strains and strain rates, then ×. For the selection of physical parameters used in the accident reconstruction simulations, if the correlation coefficient of each physical parameter with the strain and strain rate is larger than 0.5, it is defined as that there is a positive correlation. In note 2 of Table 5, if both the strains and strain rates have a correlation coefficient larger than 0.5 (multiple correlation coefficient R

2 is

0.25), each physical parameter has ○; if the strains or strain rates have a correlation coefficient smaller than 0.5, it has △; if strains or strain rates does not have a positive correlation, it has ×.

Assuming that the physical parameters, which have a positive correlation with either principal strain or shear strain in the volunteer experiments and simulations, also have a positive correlation with the injury level, and that they are possible candidates for injury evaluation parameters, then we decide on such physical parameters. As shown Table 5, with four items, which are principal strains and shear strains both in the volunteer experiments and in the simulations; the comprehensive evaluation were done based on the above assumption. In this comprehensive evaluation, as NIC-Min do not have a positive correlation with both of the principal strains and shear strains in the volunteer experiments and the simulations, they were excluded. In addition, Rebound V，T1G，Nkm，LNL，OC-T1, and head-T1 rotation angle were also excluded from the candidate of the injury parameter because the following reasons:


Although Rebound V has a positive correlation with the strain, its maximum value appears in phase 4 (time frame after phase 3 shown in Fig.5 and Fig.16). However, the maximum value of the strain and strain rate appears in other phases (phases 2 and 3). The head and T1 accelerations can be taken into account in the calculation for NIC.; Nkm can be calculated by Upper Fx and My; OC-T1 and Head-T1 are the displacement evaluation.

Table 5 Physical quantities adopted as injury evaluation parameters and selection of injury evaluation criteria candidates.

SELECTION OF THE NIC AND NECK FORCES AS NECK INJURY EVALUATION PARAMETERS: By classifying the behavior of occupants and the cervical vertebral motion during interaction with the seat-head restraint in three phenomenon phases as shown in Fig.5, we found the following relationship between strains/strain rates and NIC/neck forces. Using this phase classification, we selected the NIC and neck forces as neck injury evaluation parameters. Fig.16 shows the interaction behavior of an occupant with the seat and the time dependence of NIC and Upper Neck Fz. As for Fig.5, the time-history is classified by phase 1 where the head of the occupant has not come in contact yet with the head restraint, phase 2 where the head makes contact with the head restraint, and phase 3 where the head rebounds away from the head restraint. Fig.16 demonstrates that strain rates (principal strains and shear strains) become maximal in phase 1, i.e., before (or immediately after) the contact with the head restraint, and in this time frame NIC becomes maximal. On the other hand, strains (principal strains and shear strains) become maximal in phase 2, i.e., during the contact with head restraint, and at this time

Principal

strainShear strain

Principal

strainShear strain

Forward △ × △ △ △

Backward ○ ○ × × ○

Tension △ ○ ○ ○ ○

Compression △ △ n/a n/a △

Extension × △ × × △

Flexion × × ○ ○ ○

Forward △ △ △ △ △

Backward ○ ○ ○ ○ ○

Tension ○ ○ ○ ○ ○

Compression △ △ n/a n/a △

Extension × △ × × △

Flexion × × ○ ○ ○

○ ○ △ △ ○

× × × × ×

× × ○ ○ ○

△ × × × △

× × ○ ○ ○

× △ ○ ○ ○

○ ○ △ △ ○

× ○ × × ○

Symbol

○

△

×

Symbol

○

△

×

Correlation coefficients with strain or strain rate larger than 0.5 (multiple correlation

coefficient R2 is 0.25)

Correlation coefficients with strain or strain rate smaller than 0.5 (multiple

correlation coefficient R2 is 0.25)

No positive correlation with either of strain and strain rate

Correlation with strain and strain rate

Positive correlation with both of strain and strain rate

Positive correlation with strain or strain rate

　No positive correlation with both of strain and strain rate

Note 2: Evaluation of physical parameters in simulations

Correlation with strain and strain rate

Lower Fz

Lower My

NIC Max

Note 1: Evaluation of physical parameters in volunteer experiments

Upper Fz

Upper My

Correlation statusInjury Evaluation

Parameters

Upper Fx

Lower Fx

Volunteer experiment Simulation

Neck-Torso Angle

Comprehensive

evaluation

NIC Min

T1G

Nkm

LNL

Rebound V

OC-T1


frame the neck forces and neck moments become maximal. Although we only show here the Upper Neck Fz, other neck forces such as Fx and moments exhibit similar tendency. Thus, physical parameters which synchronize with such a time-history of strains and strain rates are NIC and neck forces. For this reason, we have concluded that NIC and neck forces are appropriate neck injury evaluation parameters.

Fig.16 Interaction motion of an occupant with the seat, the time dependence of NIC & Upper Neck Fz and strain/strain rate: the arrows of phase 1, phase 2, and phase 3 show the time flow just after the impact.

Table 6 The 5% and 95% values in risk curves (WAD2+)

Injury Parameters

Before correction (WAD2+)

After correction (WAD2+)

Remarks

5% 95% 5% 95%

Principal strain (Max) 0.08 0.24 0.08 0.24

Shear strain (Max) 0.05 0.13 0.05 0.13

Principal strain rate (Max) - 10.8 2.68 10.8

Shear strain rate (Max) - 5.8 1.81 5.8

NIC Max. - 30 8 30 5%: refer volunteer data 95%: from Risk curve

Upper Fx Backward Shear - - 340 730 Substituted by Lower Fx

Upper Fz Tension 475 1130 475 1130 From risk curve

Upper My Extension - - 12 40 Substituted by Upper My

Flexion 12 40 12 40 From risk curve

Lower Fx Backward Shear 340 730 340 730 From risk curve

Lower Fz Tension - 1480 257 1480 5%: refer volunteer data

95%: from risk curve

Lower My Extension - - 12 40 Substituted by Upper My

Flexion - - 12 40 Substituted by Upper My

NECK INJURY EVALUATION CRITERIA AND THEIR RISK CURVES: The injury criteria were decided using physical quantities that are positively correlated with WAD2+, and their risk curves. However, the physical quantities which have no risk curves, that is, Upper Fx (backward, shear), Upper My extension, and Lower My extension, were selected as the injury evaluation parameter from the following reasons. Upper Fx has a positive correlation with both principal strains and shear strains in the volunteer experiments. For Upper My and Lower My extensions, injuries due to extension are also considered, and it was also necessary to evaluate suppression effects in the motions of the upper and lower neck due to the head restraint.

The parameters to be selected with no risk curves and the risk curves were decided according to the following assumptions. The shearing forces (x) of the upper and lower neck are assumed to be almost the similar, and thus Upper Fx was substituted by Lower Fx. The moment of upper and lower neck were assumed to be almost the same, and thus Upper My extension is

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 20 40 60 80 100 120 140 160Time [ms]

Stra

in

-6

-4

-2

0

2

4

6

Stra

in R

ate

[1/s

]

StrainStrain Rate

-20

-10

0

10

20

NIC

-1000-750-500-25002505007501000

For

ce [N

]

NICUpperNeckFz

NIC (Max)Tension (Max)

Strain Rate (Max)Strain (Max)

Phase3Phase2Phase1

HRContact

Head acceleration against HR:0m/s

HRDetouch


substituted by Lower My flexion. As for Lower My flexion, it was substituted by Upper My flexion.

According to the assumptions mentioned above, the risk curves of the physical parameters determined are shown in Fig. 17. A 95% confidence interval is also given in Fig. 17. Table 6 shows the 5% and 95% values of the risk curves. The 5% and 95% values are listed to extensively evaluate and distinguish the performance of the seats, while considering the difference of experimental data for the J-NCAP evaluation test methodology (Ikari et al., 2009). As the risk curve of Lower neck Fz tension and NIC do not start from the origin of coordinate, their 5% values were substituted by the volunteer experiment data, which are the mean values of the data of the volunteers who felt the sense of discomfort in the neck.

Fig.17 Risk curves of physical parameters. The dotted lines are a 95% confidence interval.

LIMITATION OF STUDY Volunteer experiments were carried out on 6 males and 3 females. There were 20 accident cases used for accident reconstruction simulations, and the seats used for the experiments are based on seats of only one type of mass production car. Thus, we think that more experiment and accident data are needed to generalize our results. Furthermore, we estimated strains, shear strains and strain rates of inter-vertebral discs by assuming that these strains and strain rates are equivalent to deformation strains of inter-vertebral joint capsules. The use of an Human Finite Element Model to calculate the strains and strain rates may also be a limitation concerning the accuracy of these calculations that are of course dependent on the quality of the validation of the model. CONCLUSIONS We have established the methodology that enables the measurement and evaluation of the minor neck injury that is induced by rear-end impacts for the development of safer, more advanced seats, and proposed a neck injury evaluation method. We have set injury evaluation parameters using a human finite element model based on live volunteer and cadaver experiments reinforced with data from 20 accidents. Moreover, we have obtained the threshold criteria for minor neck injury evaluation, and proposed an evaluation method with dummies for impacts. We believe that this study is one of the first in the world. The result is primarily concerned and focused with being a primary candidate for an injury evaluation method that would be scrutinized by WP29/GRSP/HR GTR, where the assessment and examination of minor neck injury reduction seats are deliberated in the world. Our results are as follows: 1) From the results of the volunteer experiments, it is found that the displacements between the

0%

20%

40%

60%

80%

100%

0 500 1000

pro

ba

bilit

y o

f W

AD

2+

Lower Neck Fx Backward (N)

Chi2=10.51

p=0.0012

R2 = 0.56

0%

20%

40%

60%

80%

100%

0 10 20 30 40 50

pro

ba

bilit

y o

f W

AD

2+

NIC_Max

Chi2=1.45p=0.23R2 = 0.097

0%

20%

40%

60%

80%

100%

0 20 40 60 80 100

pro

ba

bilit

y o

f W

AD

2+

Upper Neck My Flexion (Nm)

Chi2=9.44p=0.0021R2 = 0.52

0%

20%

40%

60%

80%

100%

0 500 1000 1500 2000

pro

ba

bilit

y o

f W

AD

2+

Upper Neck Fz Tension (N)

Chi2=5.85

p=0.016

R2 = 0.35

0%

20%

40%

60%

80%

100%

0 500 1000 1500 2000

pro

ba

bilit

y o

f W

AD

2+

Lower Neck Fz Tension (N)

Chi2=0.98p=0.32R2 = 0.066


cervical vertebrae can cause neck injuries, and defined 2D strains of the inter-vertebral bodies as injury criteria.

2) From accident reconstruction simulations using the data of 20 accidents, it is found that NIC and neck forces are physical values which are highly correlated with the strains of the relative cervical vertebral motion.

3) The thresholds (the upper and lower limit values) where WAD2+ injuries occur by making risk curves with NIC and the neck forces, and lead to a proposed neck injury evaluation method.

4) The proposed neck injury evaluation method can be applied to dummy experiments. 5) It is shown that the proposed neck injury evaluation method is usable for the J-NACP test. ACKNOWLEDGEMENT Many thanks to Dr. Anders Kullgren, Folksam for the consultation about the status of accidents in rear-end collisions, as well as for his invaluable contribution to the paper through the accident cases and the analysis he shared. REFERENCES Barnsley L., Lord S., and Bogduk N., Comparative local anesthetic blocks in the diagnosis of

cervical zygapophysial joint pain, Pain 55(1), 1993, 99-106 Davis J, Cervical Spine Hyperextension Injuries: MRI Findings, Radiology 180, pp.245-251,

1991 Deng B., Begeman P.C., Yang K.H., Tashman S. and King A.I., Kinematics of human cadaver

cervical spine during low speed rear-end impacts, Proceedings of the 44th Stapp Car Crash Conference, Paper No. 2000-01-SC13, 2000, 171-188

Hoffman A.H. and Grigg P., A method for measuring strains in soft tissue, J. Biomechanics 17(10), 1984, 795-800

Ianuzzi A., Little J.S., Chiu J.B., Baitner A., Kawchuk G.. and Khalsa P.S., Human lumbar facet joint capsule strains: I. During physiological motions, The Spine Journal 4, 2004, 141-152

Ikari T., Kaito K., Nakajima T, Yamazaki K., and Ono K., Japan New Car Assessment Program For

Minor Neck Injury Protection In Rear-End Collisions, Twenty First International Technical Conference on the Enhanced Safety Vehicles, Sindelfingen, Germany, (2009) (In press)

Inami S., Kaneoka K., Hayashi K., and Ochiai N., Types of Synovial Fold in the Cervical Facet Joint, J. Orthop Sci. 5, 2000, 475-480

Lee K.E., Franklin A.N., Davis M.B., and Winkelstein B.A., Tensile Cervical Facet Capsule Ligament Mechanics: Failure and Subfailure Responses in the Rat, Journal of Biomechanics 39, 2006, 1256-1264

Lord S.M., McDonald G.J., and Bogduk N., Percutaneous Radio Frequency Neurotomy of the Cervical Medial Branches: A Validated Treatment for Cervical Zygapophysial Joint Pain, Neurosurgery Quarterly 8(4), 1998, 288-308

Ono K., Kaneoka K., Wittek A., and Kajzer J., Cervical Injury Mechanism Based on the Analysis of Human Cervical Vertebral Motion and Head-Neck-Torso Kinematics During Low Speed Rear Impacts, Proceedings of the 41

st Stapp Car Crash Conference Proceedings,

SAE Paper 973340, 1997 Ono K, Ejima S., Suzuki Y., Kaneoka K., Fukushima M., and Ujihashi S., Prediction of Neck

Injury Risk Based on the Analysis of Localized Cervical Vertebral Motion of Human Volunteers during Low-Speed Rear Impacts, Proceedings of International IRCOBI Conference Biomechanics of Impacts, 2006, 103-114

Pramudita J.A., Ono K., Ejima S., Kaneoka K., Shiina I., Ujihashi S., Head/neck/torso behavior and cervical vertebral motion of human volunteers during low speed rear impact: mini-sled tests with mass production car seat, Proc. of International IRCOBI Conference, 2007, 201-217

Quinn K. P. and Winkelstein B. A., Cervical facet capsular ligament yield defines the threshold


for injury and persistent joint-mediated neck pain, Journal of Biomechanics 40(10), 2007, 2299-2306

Sato F., Antono J., Ejima S., Yamazaki K., Ono K., Pramudita J., Ujihashi S., and Kaneoka K., Evaluation Parameters and Criteria for the Reduction of Minor Neck Injuries during Rear-end Impacts - Human Volunteer Experiments and Accident Reconstruction Using Human FE Model Simulations -, JSAE Annual Congress (Autumn), 2009 (In press)

Siegmund G.P., Myers B.S., Davis M.B., Bohnet H.F., and Winkelstein B.A., Mechanical evidence of cervical facet capsule injury during whiplash: a cadaveric study using combined shear, compression and extension loading, Spine 26, 2001, 2095-2101

Spitzer W.O., Skovron M.L., Salmi J.D., Cassidy J.D., Duranceau J., Suissa S., and Zeiss E., Scientific Monograph of Quebec Task Force on Whiplash-Associated Disorder: Redefining “Whiplash” and Its Management, Spine, Vol.20, No.8S, April 15, 1995, pp.34s-39s

Stemper B.D., Yoganandan N., and Pintar F.A., Effects of Head Restraint Backset on Head-Neck Kinematics in Whiplash, Accident Analysis and Prevention Vol.38, 2006, 317-323

Sugimoto, T., Yamazaki, K., First Result from the JAMA Human Body Model Project, 19th International Technical Conference on the Enhanced Safety Vehicles, Washington 2005.

Yoganandan N., Kumaresan S. and Pintar F.A., Biomechanics of the cervical spine Part 2: Cervical spine soft tissue responses and biomechanical modeling, Clin. Biomech. 16, 2001, 1-27

Yoganandan N., Pintar F.A., Kumaresan S., and Elhagediab A., Biomechanical assessment of human cervical spine ligaments, Stapp Car Crash Journal 42, Paper No. 983159, 1998, 223-236

White A.N., Begeman C.P., Hardy N.W., Yang H.K., Ono. K.,Sato F., Kamiji K., and Yasuki T., Investigation of Upper Body and Cervical Spine Kinematics of Post Mortem Human Subjects (PMHS) during Low-Speed, Rear-End Impacts, Paper No. 09B-0440, 2009 SAE Congress (In press)

Winkelstein B.A., Nightingale R.W., Richardson W.J., and Myers B.S., Cervical facet joint mechanics: its application to whiplash injury, Stapp Car Crash Journal 43, Paper No. 99SC15, 1999, 243-252


Evaluation Criteria for the Reduction of Minor Neck ...

Documents

Transcript of Evaluation Criteria for the Reduction of Minor Neck ...