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Comparison of HIC and BRIC Head Injury Risk in IIHS Frontal Crash Tests to Real-World Head Injuries

The Insurance Institute for Highway Safety (IIHS) has been measuring head injury criterion (HIC), a measure based on linear impact skull fracture data, to assess head injury risk in its front crash tests since 1995. In 2012, IIHS added instrumentation to measure brain injury criterion (BrIC), a rotationally based injury measure derived from animal data correlated to humans through computational modeling. BrIC is intended to complement HIC rather than replace it. Head injury risk associated with HIC and BrIC values measured with a Hybrid III dummy in 138 front crash tests was compared with real-world injury rates in similar frontal crash configurations calculated from the National Automotive Sampling System Crashworthiness Data System (NASS CDS) database. NASS CDS identified 1.3-5 percent AIS3+ head injury rates in crashes similar to the test configurations. The mechanisms of injury represented by HIC and BrIC are a subset of all head injuries; therefore, the NASS-indicated head injury rates inherently may be an overprediction of injuries directly applicable to these formulas. In crash tests, HIC AIS3+ head injury risk ranged 0-22 percent and BrIC AIS3+ head injury risk ranged 3-85 percent. BrIC AIS3+ head injury risk greater than 50 percent was associated with a variety of head kinematic events including front airbag loading, head contact with instrument panel, and non-contact forward excursion. The published injury risk curve for BrIC indicates that crash tests represent significantly higher serious head injury risk than observed in real-world crashes of similar configurations. Hybrid III may produce exaggerated measures of BrIC or, if accurate, the BrIC formula may need to be reexamined against the underlying animal test data to determine the limitations of BrIC, and the proposed injury risk curves need to be re-evaluated against real human injury risk. Despite its origins as an indicator of skull fracture risk, the range of HIC-based head injury risk observed in crash tests more closely reflects the real-world head injury rates than the range of BrIC-based head injury risk.