3. Injury criteria for thoracic trauma

Side pole impact concentrates energy into a narrow contact patch on the lateral thorax, so the resulting injury pattern is dominated by localized rib loading and rapid chest wall compression rather than by whole-body deceleration. To evaluate occupant protection, engineers translate measured physical responses — displacement, velocity, and force — into standardized injury criteria that can be compared against regulatory and consumer-test thresholds. This section explains the four criteria most relevant to lateral thoracic trauma: chest deflection, the Viscous Criterion (VC), rib fracture risk, and the AIS/ISS injury scoring scales, and shows how they are computed and interpreted in practice.

Sign convention and measurement reference

Before any criterion is computed, the reference frame must be fixed. In side impact testing, chest deflection is defined as the inward displacement of the rib cage relative to the spine, measured along the lateral axis of the dummy or human body model. Compression is positive when the chest wall moves toward the spine (inward). Deflection rate is the first time derivative of that displacement, dD/dt, positive during inward compression. All criteria below use this convention consistently; a sign flip will invert the interpretation of VC and must be avoided.

Chest deflection

Chest deflection D(t) is the peak lateral displacement of the chest, usually reported in millimeters at the upper, middle, and lower rib levels. It is the most direct geometric measure of how far the thorax is crushed and correlates strongly with rib fracture count. In the Euro NCAP side pole test, for example, the WorldSID 50th percentile male dummy records rib deflections at multiple locations, and the assessment uses the peak value across those channels. A typical engineering workflow extracts the peak deflection from each rib channel, compares it against the criterion curve, and reports the worst-case channel.

Deflection alone, however, does not capture how fast the compression occurs. A chest compressed slowly to 40 mm may sustain fewer fractures than one compressed to 30 mm in a few milliseconds, because biological tissue is rate-sensitive. This limitation motivates the Viscous Criterion.

Viscous Criterion (VC)

The Viscous Criterion, introduced by Viano and Lau, combines compression and compression rate into a single soft-tissue injury metric. It is defined as the product of the instantaneous chest compression C(t) (deflection normalized by the initial chest depth, so it is dimensionless) and the compression velocity V(t) = dC/dt:

VC(t) = C(t) * dC(t)/dt      [units: 1/s]

where  C(t)  = D(t) / D0        (D0 = initial chest depth)
       dC/dt = (1/D0) * dD/dt

VC is evaluated continuously over the impact and the reported value is the maximum of VC(t), denoted VCmax. Because it multiplies normalized compression by rate, VC peaks during the rapid loading phase rather than at maximum deflection. The commonly cited injury threshold for the 50th percentile adult is approximately VCmax = 1.0 m/s for a 25% risk of AIS 3+ thoracic injury, with risk rising steeply above that value. In practice, engineers compute VC from the same deflection channel used for peak deflection, so both metrics come from one measurement.

Rib fracture risk

Rib fracture risk is often expressed as a probability curve derived from cadaver and dummy correlation studies. A widely used form relates the number of rib fractures to chest deflection and occupant age, since bone strength declines with age. A representative logistic model has the form:

P(fracture) = 1 / (1 + exp(-(a + b*D + c*age)))

Example coefficients (illustrative, not a specific published set):
  a = -6.0, b = 0.08 per mm, c = 0.05 per year
  D = 35 mm, age = 45 years
  P = 1 / (1 + exp(-(-6.0 + 2.8 + 2.25))) = 1 / (1 + exp(0.95)) ≈ 0.28

The example above is a teaching illustration of the calculation structure, not a validated prediction. In real assessments, the coefficients come from the specific dummy-to-human correlation study being applied, and the output is interpreted as the probability of a given fracture severity, typically AIS 2 or AIS 3 rib fractures. The key engineering insight is that the same deflection produces higher risk in older occupants, so a design that passes for a 45-year-old may fail for a 65-year-old.

AIS and ISS scoring

The Abbreviated Injury Scale (AIS) grades individual injuries on a 1–6 severity scale, where 1 is minor and 6 is currently untreatable/fatal. For the thorax, AIS 2 includes simple rib fractures, AIS 3 includes multiple rib fractures or a flail chest, and AIS 4 includes major thoracic organ injury. The Injury Severity Score (ISS) aggregates multiple injuries across body regions:

ISS = A1² + A2² + A3²

where A1, A2, A3 are the three highest AIS scores
from three different body regions (0 if fewer regions injured).
ISS ranges 0–75; ISS ≥ 16 is conventionally "major trauma".

For a side pole impact producing three rib fractures (AIS 3) and a minor lung contusion (AIS 2), the ISS would be 3² + 2² = 13. This aggregation matters because a single AIS 3 thoracic injury plus a moderate head injury can push ISS into the major-trauma range even when neither injury alone is catastrophic.

Putting the criteria to work: a worked assessment

Consider a side pole impact simulation or sled test with a 50th percentile male dummy. Suppose the filtered mid-rib deflection channel shows a peak of Dmax = 38 mm at t = 42 ms, with an initial chest depth of D0 = 230 mm. The normalized compression at peak is C = 38/230 ≈ 0.165. If the compression rate at that instant is dC/dt ≈ 4.5 s⁻¹, then VC = 0.165 × 4.5 ≈ 0.74 m/s, below the 1.0 m/s reference threshold. However, VC often peaks earlier: if at t = 30 ms the compression is 0.10 and the rate is 11 s⁻¹, then VC = 1.1 m/s, which exceeds the threshold. This illustrates why VCmax must be found over the full time history, not at peak deflection.

Applying the illustrative fracture model with D = 38 mm and age 45 gives a probability near 0.30 for the example coefficients above. The engineer would then check whether the design target (for example, VCmax below 1.0 m/s and fracture probability below a program-specific limit) is met. If not, the iteration typically targets the door trim, armrest stiffness, or side airbag coverage, since these control the local loading rate on the chest.

Interpreting results and common pitfalls

Three practical issues recur in side impact assessments. First, channel selection: the worst rib channel is not always the mid-rib, and reporting only one channel can understate risk. Second, time alignment: deflection and rate must be sampled on the same time base, and any resampling must preserve the peak. Third, the distinction between dummy-measured deflection and human injury: criteria curves are statistical correlations, so a passing value reduces but does not eliminate injury risk, particularly for older or smaller occupants. Reporting Dmax, VCmax, fracture probability, and the resulting AIS/ISS together gives a complete picture that a single number cannot.

The criteria form a layered assessment: deflection describes how far the chest is crushed, VC describes how violently, fracture risk translates that into a probability for a given occupant, and AIS/ISS place the outcome in a clinical severity framework. Used together, they let an engineer judge not only whether a design passes a threshold but how much margin exists and which loading mechanism dominates.