With the different dynamic properties of the brain and skull, plus different injury types and mechanisms, it is very difficult to produce a single injury criteria for the head.
These different dynamic characteristics coupled with the different types of impact object, flat or blunt, means that a number of different injury criteria are required to assess the injury severity. As the potential of skull fracture is linked to brain damage then injury criteria could be combined. In assessing injury mechanisms it has been proposed that in the case of skull fracture and brain injury there are different mechanisms for flat object (diffuse brain damage and remote linear fractures) and blunt object (localised depressed fractures and focal brain injuries) impacts. Therefore it is proposed to have different injury criteria based on the type of impact object.
Although the mechanism for brain axonal injury is internal shear and tensile forces, often produced by high angular or rotational accelerations, such accelerations would be very difficult to measure and interpret. However skull fractures are usually only produced by translational accelerations, therefore translational accelerations have been proposed for the basis of an injury criterion. The severity or level of brain injury or skull fracture is a combination of both the acceleration level and the duration of that acceleration. High accelerations can be tolerated for very short durations (200g for 2 msec) while lower accelerations for longer durations (80g for 200 msec) can not. Results from biomechanical tests have produced tolerance curves where the onset of brain injury / skull fracture is defined by acceleration plotted against loading duration. The equation best fitting the tolerance curve is acceleration to the power 2.5 multiplied by the time. In order to predict the onset of brain damage from the complex acceleration curve for the head, the Head Injury Criterion was produced, requiring computer analysis for calculation.
Where t1 and t2 are the initial and final times (expressed in seconds) of the interval during which the HIC attains a maximum value and a(t) is the resultant acceleration (expressed in G) measured at the head CG.
The time duration (t1-t2) used in the calculation should be taken as the contact time for the impact, however, this is often very difficult to ascertain in physical evaluations using crash test dummies or headform simulators. In using HIC for assessing the potential of concussion then a maximum time duration of 15 msec should be used, which was the maximum time duration for which the original tolerance curve was developed. Longer contact time durations can be used to predict skull fracture.
The highest acceleration, independent of location or direction, should be used in the Head Injury Criterion, which will therefore be the resultant acceleration measured at the heads centre of gravity.
The Head Injury Criterion (HIC) should be used for all impacts to the head, independent of impact type or location as it is the best predictor of brain concussion, which could significantly affect the occupant’s ability of egress. As well as being used for flat object impacts it should also be used for blunt objects to assess diffuse brain injuries. Also due to the varying strength of the skull, the Head Injury Criterion tolerance level would vary at different impact locations around the head, being potentially lower in the lateral direction to the frontal. Due to difficulties in assessing exactly the location and acceleration direction, it is proposed to use the same tolerance levels for any direction of impact.
Crash Test Dummies / Headform Simulators – The resultant accelerations at the centre of gravity (CoG) of the head or headform should be used with a either the head contact time duration or a maximum of 15 msec duration.
Computer Simulations – To compare with physical simulations and crash test results using crash test dummies use the resultant acceleration at the head centre of gravity.
A large number of biomechanical tests have been conducted to evaluate the fracture force of the skull for blunt object impactors using several different size small radius bars (20 – 50 mm radius) and circular impactors (less than 6.5 cm2). Force (N) being found to be a good indicator of the onset of skull fractures and therefore used as injury criteria for predicting injuries from blunt object impacts.
The skull fracture injury criteria should be used for all blunt object (<40 mm radius) impacts to the skull but not the facial area. The Head Injury Criteria should also be used to predict brain damage and concussion.
Crash test dummies / headform simulators – As installing loadcells to the dummies aluminium skull would be extremely difficult, particularly in maintaining the correct skull mass and inertia characteristics, the total force on the skull can be calculated from the resultant acceleration using F=ma. The head mass is approximately 4.5 Kg.
Computer Simulations – With complex finite element models of an actual human or crash test dummy head the actual forces at the impact point could be ascertained, but for consistency with the original biomechanics data and crash test dummies, the force calculated from the resultant head acceleration would be most applicable.
As the facial bones are covered in a relatively thick layer of skin and muscle a force intrusion based injury criterion would be most applicable for predicting the level of facial injury. The outer skin has a very low stiffness (0.25 KN at 12 mm) which increases on contact with the facial bones, 0.90 KN at 20 mm, to a maximum 3.3 KN at 40 mm. The facial injury criteria would be evaluated by the force or level of intrusion into a surface with the above force / intrusion characteristics.
To be used the alternative to the skull fracture injury criterion, when there is a known blunt object impact to the facial area (chin to eyebrows). The facial injury criteria is always used in conjunction with the Head Injury Criterion as brain damage or concussion can be associated with a facial impact.
Crash Test dummy / head form simulator – To assess the facial injury criterion a frangible facial insert with the correct facial stiffness would have to be developed and installed into the facial area. The force could then be calculated from the peak resultant acceleration.
Computer simulations – In both finite element and MADYMO computer models facial stiffness characteristics could be applied to the head models and force or intrusion used to evaluate the severity of the facial injury.
Dr. A.R. Payne
© MIRA 2001