Observaciones generales:

With respect to neck injury, the Neck Injury Criteria (NIC) (Bostrom et al., 1996), Nij

(Klinch et al., 1996; Kleinberger et al., 1998), and Nkm (Schmitt et al., 2001) are often used.

Other proposed criteria are Intervetebral Neck Injury Criterion (IV-NIC) (Panjabi et al., 1999), Neck Displacement Criterion (NDC) (Viano and Davidsson, 2001), Lower Neck Load

Index (LNL) (Heitplatz et al., 2003), and Peak Virtual Power (PVP) (Sturgess, 2001).

A general limit of such injury criteria is the fact that they can be determined under controlled conditions, i.e. in experiments. Real world crashes canot be assessed retrospectively through those criteria, because there is no possibility to measure the neck loads in the real world. With respect to soft tissue neck injuries, this poses a problem as those cases often result in legal procedures requiring an assessment by an expert witness to clarify the likeliness whether the injury claimed is causally linked to an accident. Therefore special schemes were developed to biomechanically assess this causality.

In this Section only NIC, Nij and PVP are explained with respect to the main purposes of

the present study. More details about other criteria can be found in the literature.

2.2.1 Neck Injury Criterion (NIC)

The Neck Injury Criterion (NIC) was developed by Bostrom et al. (1996). The definition of the NIC as a function of time was validated based on animal experiments. The NIC expression is given in Eq. (2.1) as

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Where NIC stands for Neck Injury Criterion, arel is the relative acceleration difference

between C1-C7 in gs (g = 9.81 m/ s2, ℓ is the length of the pig’s cervical spine (0.2 m or 7.8 in) and vrel is the relative horizontal velocity of neck C1-C7 in m/s in sled or car crash. The

threshold value above which a significant risk of sustaining minor (AIS 1) neck injury is assumed to be inherent was set to be 15 m2 / s2 (Schmitt et. al. 2004).

2.2.2 Normalized Neck Injury Criterion (Nij)

Klinch et al. (1996) and Kleinberger et al. (1998) proposed this US-NHTSA criterion to assess neck injury. Recently, the Nij criterion was included as part of FMVSS 208. The Nij

criterion developed implies a linear combination of the axial forces and the flexion/extension bending moment, both normalized by critical intercept values:

(2.2)

Where Nij is the Normalized Neck Injury Criteria, FY and MOCZ are the axial force and

the sagittal bending moment at the occipital condyle, respectively. FYC indicates the critical

axial load values of neck tension and compression, and MZC indicates the critical values of

neck flexion moment at the occipital condyle. The intercept values of MZC and FYC in new

FMVSS 208 are as shown in Table 2.4. ZC OCZ YC Y ij M M F F N = +

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Table 2.4: The force and moment intercept values used in Nij (Lee et al., 2003)

Dummy MZC (Flexion/Extension) FYC (Compression/Tension) [Nm] [N] H-III 50% 310/ 135 6160/ 6806 H-III 5% 155/ 67 3880/ 4287 H-III 5%* 155/ 61 3880/ 3880 H-III 6 year 93/ 37 2800/ 2800 H-III 3 year 68/ 27 2120/ 2120 *Out of position

Currently there is very little correlation between neck injuries received by occupants in real accidents and this calculated injury criteria. The dummy neck loads (from the Hybrid III) obtained from New Car Assessment Program (NCAP) testing were compared with NASS accident, and data incidence of neck injury in the filed accident data. Kleinberger et al. (1998) proposed an AIS 5+ neck injury risk curves for human in frontal impacts, Figure 2.2.

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2.2.3 Peak Virtual Power (PVP)

The concept of Peak Virtual Power (PVP) or the maximum rate of Entropy production was first introduced by Sturgess (2001) as a universal injury criterion in the following form:

F V FV t U PVP = . = . ∂ ∂ = ∧ ∧ (2.3) Where t U ∂ ∂ ∧

is the specific virtual power per unit mass, F is the force, and V is the

impact velocity. For a particular impact, V is also the change in the velocity (∆V).

In the research of Sturgess (2002a), it was assumed that injuries in Impact Trauma can be modeled as mechanical dissipative processes, and the formalism of Continuum Damage

Mechanics (CDM) based on Irreversible Thermodynamics was applied to Impact Trauma. An

objective, self-consistent injury criterion of “Peak Virtual Power” was derived.

In the research of Sturgess et al. (2001), it has shown that the PVP can model the severity of injury at the micro, and macro scales, and can model neck impact injuries as well, if not better than, NIC. Other scientifically valid injury criteria such as the “Viscous Criterion” (Viano and Lau, 1986), and “Margulies and Thibault” criterion (Margulies and Thibault, 1989) were also shown to be derivable from PVP, and taken as further confirmation of the concept of PVP as a universal injury criterion.

It was shown that PVP predicted the severity of injury, measured on AIS scale, in around 90% of cases for all types of injury to all body regions (brain, skull, thorax, spine, upper and lower extremities) for car occupants from Co-operative Crash Injury Study in the UK (CCIS – Phase 7) and NASS-CDC Databases. Values of the Correlation Coefficient (R2) which are greater than 0.98 were routinely observed in that research (Sturgess, 2002a).

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It was shown that in general the Lower Bound of severity of injury is proportional to ΔV3 or (ETS3), for restrained vehicle occupants according to the form in Eq. (2.4):

Severity of Injury α AIS α PI% α PVP (2.4) α aV|max α a^ α a2 Δt α V3 α ΔV3 or (ETS3)

For unrestrained occupants, the Upper Bound is proportional to ΔV2 or (ETS2), Eq.(2.5): AIS α PVP α aV|max α a^ α a2 Δt α V2 α ΔV2 or (ETS2) (2.5)

Where AIS is the Abbreviated Injury Scale, a is the acceleration, ETS is the Equivalent

Test Speed and PI% is the probability of injury.

Sturgess et al. (2002b) have shown that the concept of Peak Virtual Power (PVP) as an injury criterion can predict injuries to pedestrians, and that the injury severity is proportional to V2, for slight injuries, and V3 for serious and fatal injuries. Simulations using MADYMO for head and chest injuries showed that, to a first degree of approximation, the influence of vehicle contact stiffness on the acceleration of the body is approximately linear.

In a recent PhD work, Jiang and Sturgess (2008) developed a reconstruction simulation for head injury in rollover real world accidents. The MAIS results achieved from the “Master PVP Curve” indicates the head injury severity well which shows the PVP is a good indicator of head injury from both macro and micro viewpoints. It was concluded that PVP could be a suitable candidate for an objective universal injury criterion.

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