Melvin and Evans (SAE #710871) The tolerance of the frontal bone to force can be characterized based on the impactor area.

a. Small impactor areas produce penetration fractures,

b. Intermediate areas comminuted depressed and local linear fractures and

c. Large impactor areas, remote linear fractures fitting shell theory.

SAE J885 page 9 summarizes the fracture forces to the frontal, parietal and occipital bones based on various cadaveric studies.

Two general trends:

Temporal and parietal bones are weaker than frontal and occipital bones.

The force to fracture is smaller for a smaller impactor surface.

Wayne State Tolerance Curve (WSTC): This is the foundation for most currently accepted head injury indices. It shows the relationship between acceleration level and impulse duration necessary to produce skull fracture.

Short pulses of high acceleration can produce injury and lower accelerations require longer pulses to produce injury.

Lissner et al (1960) first suggested the curve after impacting embalmed cadavers on the forehead. An accelerometer was positioned on the center rear of the skull.

Gadd (1961) proposed plotting the injury curve on a log-log scale to achieve a straight tolerance line.

The WSTC was changed to effective or average acceleration on the x-axis to conform to the more irregular and trapezoidal waves summarized by Eiband (1959) in volunteer studies. The asymptote of the curve for very long durations was increased from 42 to 80 g’s based on further study. (For non-rectangular pulses peak acceleration can be higher than 80 g’s).

Ono et al (1980) used primate and cadaver skulls impacts that further reinforced the WSTC. Cerebral concussion was produced by pure translational acceleration of the skull. A tolerance curve for cerebral concussion to humans was developed by scaling the monkey data. This was called the Japan Head Tolerance Curve (JHTC). The difference in the WSTC and the JHTC between 1 and 10 ms is negligible. There are minor differences for longer durations. Ono et al showed that the threshold for skull fracture is slightly higher than for cerebral concussion.

Gadd Severity Index (SI): The slope of Gadd’s log-log plot was –2.5 and 2.5 became his power weighing factor. The severity index formula was the integral of a^2.5dt, and it was suggested that if SI exceeded 1000, there was a threat to life.

Head Injury Criterion (HIC): Versace (1971, 15^th Stapp) suggested an alternative to SI in which effecitve acceleration was:

1/t aⁿdt

and was raised to the 2.5 power and multiplied by time duration.

This formula could handle long duration low acceleration impacts in which there was a high tolerance to injury and more irregular impacts of multiple pulses. The HIC was adopted by the NHTSA as a rule in June of 1972.

HIC = < 1000

The time duration, t₂ – t₁ has been controversial. Initially it was restricted to head contact other then to the belt system. Later, it was changed to include any pulse duration, whether there was head contact or not. Field data shows little evidence of brain injury in the absence of head contact. Current FMVSS 208 limits the search for maximum HIC to 36 ms.

Backaitis (1981) and Eppinger (1981) said HIC could be interpreted as a measure of the rate of change of specific kinetic energy imparted to the head.

Prasad and Mertz (SAE #851246) recommended HIC duration be limited to 15 ms or less. Based on their analysis of cadaver data, and using the analysis technique of Mertz and Weber (1982), a HIC of 1000 represented a 16% risk of AIS 4 or greater brain injury.