The Journal of Trauma: Injury, Infection, and Critical Care

Volume 38(3), March 1995, pp 459-462

[Letter To The Editor]

ISSN: 0022-5282

Zugriffsnummer: 00005373-199503000-00032

[Letter To The Editor]

Letter to the Editor

Treviranus, G. R. S. ; trainee in Psychiatry, MD

Neurobiology and Mechanical Safety

Munich, Germany

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To the Editor:

This letter is in response to the very interesting article by Siegel et al. [1]

I feel that the data discussed herein is relevant, and that the exchange of

information between the trauma and impact research communities should be

encouraged.

Side impact is not side impact is not frontal impact

Lateral crash injury description is intricate enough. [2] It becomes nearly senseless in biomechanical terms without making the difference between struck-side and offside occupants, as in the paper by Spiegel et al. that finds no difference in the incidence of thoracic injuries between FB and LB patients. Although there remains the value for car design investments, which have to consider the overall effect, it is not clear why this ``biomechanical'' information has not been provided (with little effort) to surgeons and biomechanics alike.

An even more selective approach considers directly struck struckside passengers. A TRRL study of towed cars [3]showed a difference of MAIS 3 to 6, cases of 28% for the directly struck compared with 13% for the directly struck compared with 13% for the remaining.Belt effectiveness decreased rapidly with intrusion: 0.33 (25 cm), 0.14 (50 cm),and 0.02 (75 cm).

Whereas off-side lateral and frontal crash victims usually have to deal with the

secondary collision (i.e., into the belt)--in which speed is never higher than

the collision speed [4] and corresponds to the change in velocity--nearside

occupants are subjected to the ``punch,'' [5] a force-pulse that initially is

practically independent of the path, wherein only the door and the occupant are

encountered, and only secondarily is decelerated by interaction with the entire

mass of the car hit. This pulse initially--at the moment of contact--causes the

door to intrude, with approximately 60% of the plain relative collision speed,

[6,7] that therefore represents a viable parameter. The second collision is very

early; any measure to delay contact to the time of the descending limb of the

pulse is welcome. This principle in side impact directly underlies the

discussion of thoracic, pelvic, and contact cranial intrusion-caused injuries,

and indirectly underlies that of noncontact cranial or cervical injuries.

Fortunately “delta-v” in side impact usually represents the collision speed of

a reference car of constant mass, [8] and despite the name should not be

calculated from the conservation of momentum principle, as in the frontal

collision case between vehicles. But often, there remains a doubt when this is

not made explicit, especially in acute angle collisions. Consider that a frontal

collision with a Dv from 120 to 90 km/hour is not comparable in severity

(deformation, etc.) with one with a Dv from 30 to 0 km/hour. The

equivalent test speed actually denotes the difference of the squares of the

velocities as encountered in the kinetic energy expression, with the difference

between the energetic delta-v's being about sevenfold between 6300 and 900. Now,

some of the severest lateral collisions stem from highspeed longitudinal acute

angle collisions, which only secondarily evolve into side impacts. Herein, a

true delta-v can be an appropriate description. Often, the victim is ejected or

crushed by his or her own overturning car, and the impact energy has to be

estimated individually. In this we are not told about the laterofrontal status

of the 27% ejected, nor about their degree of ejection.

The basis for comparison of belt effectiveness between frontal and lateral

crashes may be either simple occurrence or crash severity, with the former being

closer to economic aspects and the latter closer to impact biomechanics. By

choosing the former, the question of exposure becomes very intricate. [9]

Exposure representing all of the parameters affecting the outcome variable

except the influencing factor studied at the moment is one aspect. If one tries

to move in the direction of impact biomechanics, by choosing the latter,

questions of exposure disappear, but one is stuck with the technical details. In

both cases, ratios do help a lot. The precondition as always is that one

compares apples with apples. As previously shown, “delta-v” does not represent

the same for frontal and lateral crashes when considering injuries. In crash

severity, plain relative collision speed is more closely related to intrusion

velocity of the door, which is the main factor in injury causation to near-side

occupants struck in the side. The static damage (delta-v) is not sufficiently

correlated with the early door intrusion speed. Only the victim's precrash speed

is mentioned in the considered study, which is without any major consequence for

injury causation in blunt-angle side crashes. [10]

Laterofrontal Indexes

Being interested in car design decisions, we systematically introduced a ratio between data referring to lateral impacts to the corresponding data for frontal impacts, [11] because this LFI provides comparability between studies referring to different populations (e.g., with different amounts of rear collisions considered in the same 100% or certain different kinds of exposure).

A further advantage consists of eliminating effects of the mean progress in

contact mitigation. Many design decisions trade lateral for frontal crashworthiness.[12] (The vehicle designer needs to know, for example, the relative importance and severity of side and frontal impacts before producing a vehicle with optimum and economic crash protection under real-life conditions.) Some examples from Siegel et al. [1] illustrate this: overall LFI risk for brain injuries = 1.30;LFI for lung injuries = 1.47; LFI for LE fx = 0.66; belted LFI for brain injuries = 2.10; LFI for facial lacerations = 0.53; b-LFI for facial fx = 1.0; unbelted LFI risk for brain injuries = 1.10; LFI for facial lacerations = 0.56; and ub-LFI for facial fx = 0.35. These data are calculated from relative figures, if one is interested in risk (from absolute figures, if one isinterested in cost).

Retention effectiveness

Side impacts from a slightly forward direction are 50% more harmful in causing injuries [13]; safety belts are effective in mitigating this forward component, but retention of the torso decreases [14] after 02:30 or 11:30 forward. Belt effectivities [15] are best expressed as effectiveness indices. [Effectivity, which typically varies with collision severity denotes the relative decrease of (or the relative occurrence of) events after introduction of a device (before minus after)/before.] To calculate these indices (e.g., for belts), relative figures have to be used. Belt effectivities as obtainable from the data of Siegel et al. [1] are nil for facial lacerations and all pelvic fractures, but were 0.36 for frontal and 0.17 for lateral crash cerebral injuries; and for LE fractures, 0.16 and 0.63, respectively. The LFI belt effectivities, therefore, were 0.47 for brain and 3.9 for LE fx, respectively.

A TRRL study [16] from the United Kingdom of towed cars established data that

allowed calculation of belt effectiveness for “struck-side” and “off-side”

passengers in respect to overall trauma severity consecutively restricting the

analysis to more severe cases (MAIS 1+, MAIS 2+, and MAIS 3+ and stratifying for

delta-v. Belts were effective at 16.1 km/hour for MAIS 1+ (0.50) for both

positions, for MAIS 2+ (0.50) for struck-side only, at 32.2 and 48.3 km/hour only for struck-side MAIS 2+ (0.41), and ineffective at 64.4 km/hour. They even

seemed to be counterproductive for offside passengers at 32.2 km/hour,

considering the Abbreviated Injury Score (AIS) 1+. Numbers were insufficient for

more severe crashes, but Mackay et al. [17] (analyzing a stratified sample,

excluding side-swipes and rollovers, but including noncar objects) showed that

off-side occupants profit greatly--and more than near-side occupants--from

retention. They egress the shoulder belt in 35% of the cases, sustaining mild to

moderate head injury. In the 20% AIS 3+ head injuries, the intrusion crossed the

bisection in 70%, and nearly so in the rest, whereby belt retention was made

superfluous. With increasing crash severity, belt effectiveness decreases in

frontal, and more rapidly so, in side impacts, [18] but it probably increases

transiently for off-side occupants, which constitute one in three. General

frontseat effectiveness has been estimated by Evans in 1986 to be 0.43. Viano et

al. [19,20] pointed out that crashworthiness improvements have become asymptotic

with present technology, although frontal airbags save a further 28% occupant

deaths in frontal [21] and 19% in all crashes [22] together. Moderate to

severely injured were decreased 25% to 29% in 1990 models with airbags, compared

with automatic belts. [22] Side airbags await validation.

Contact intrusions

The Rigid Belt-Light Vehicle, [23,24] built by the Swiss Light Vehicle Project of the Swiss Federal Institute of Technology and by the University of Zurich (I. R. Kaeser), has proven to some extent (as several atypical details occurred) that a less than 700-kg vehicle can resist a frontal or lateral 50 km/hour collision, with no relevant intrusion and dummy decelerations within the new ECE standards. No high-technology materials were used. Although this resistance is related to low mass, in large cars parts of the compartment (e.g., seat-and-pedal shell) can be built as a ``car-in-the-car.'' The conclusion of Siegel et al.--that cars resisting a delta-v of 50.7 km/hour (30mph) can and should be built and will be rewarding--is corroborated by this achievement.

A further decrease could be expected from a new generation of rigid light-weight automotive seats [25] that move transversely under the impact while

they keep the occupant upright sideways by sustaining him or her at the shoulder

in any upright or forwardly flexed position.[26] (According to the spatial

concept as developed by General Motors Research Laboratories: “Since a majority

of side impacts have principal directions of force contained in a 30degrees

range from pure lateral to forward, the body zones were enlarged to include the

projection of the body regions along an axis pure laterally through 30degrees

forward of lateral.” The same banana-shaped shoulder support will have to carry

the lateral head (and abdominal) airbag, because lateral rotations of the head

have to be avoided. The latter event constitutes the most effectively harmful

rotational mechanism, causing the spectrum [27] slight commotion to diffuse

axonal injury.

Strother et al. [28] on the other hand have shown that contact occurs long

before intrusion is complete and that intrusion is a factor via collision

severity only when predicting injury severity. The problem is often more the

spatial concentration of impact forces than intrusion itself, which may even

lower impact forces by decreasing free acceleration paths. Obviously, without

intrusion, contact often would not occur (at least when belts are worn and

sufficient anterior space is available, as in the Swiss Rigid Belt-Light

Vehicle), or would occur at a lower speed despite equal collision severity. On

the other hand it has become clear [29] that rigidifying the compartment of

conventional cars is useful only to a certain degree, whereafter occupant loads

increase as the shock-mitigating properties of the shell are replaced by

repellent properties, causing a narrow pulse.

For the surgeon, immediate knowledge of the accident mechanism and its probable

consequences are invaluable, and therefore the surgeon seems the one to profit

most from these studies until the necessary adjustments in design are made. This

is a relevant area of research. Many trauma centers are still far from

establishing a complete trauma mechanism hypothesis at an early stage. These

data are especially valuable in suspecting and treating retroperitoneal

bleedings. Most case reports even fail to give the most basic circumstances

(e.g., direction of impact).

Not all the cases — “the biggest flaw?”

In contrast to the general opinion,heralded by Evans,[30] the leading specialist in the field, it does make sense to compare only fractions of occupants defined by injury only between car-to-car crashes, considering just the ratio. The mathematical reason is that, if you remove subgroups of lesser injured from a given subpopulation, the nominal percentages of the remaining groups (e.g., severely injured, killed) change in a proportional, not necessarily known, way. The ratio between nominal percentages remains constant, somewhat less so between corresponding percentages across populations, thus providing very valuable information. [For example, the ratio between percentages of injured at equal severity degrees side-wise across to frontally impacted cars. The factor changing the values for the frontal crash cases will be slightly different, but still in a further determinable way.] One

becomes, therefore, far more independent of the cut-off point. If the proportion

is known - which is easily extracted from larger statistics - a direct comparison can be established, if one agrees to a population of reference. Evans' further argument [28] - that comparing percentages of the remaining groups yields tautological constant ratios - does not hold true, because the percentual distributions of degrees of severity in a spectrum of crash severities are S-shaped and not horizontal. This would be necessary for constant, tautological ratios that are empirically known from background investigations. [31] Where the ratio is constant, as it happens, over a range of interest in crash severity,this is the result of the “coarseness” of the MAIS on the AIS injury scale. One can resort to adding the number of fractured ribs [32] or to using pairs of the two uppermost AIS scores. [33]

The only ”flaw” in the article by Siegel et al., in my opinion, is the (until

today) insufficient delta-v frontolateral comparability and maybe the inattention to heterogeneous side-impact populations, not that it merely concentrates on the multiply injured. These patients decrease in number when the belt rate goes up, but in a certain percentage continue not to wear belts. Although in the small Zurich series of mostly inner-city fatal lateral-impact victims, only 2 of 8 would have survived if they had worn belts.

REFERENCES

1. Siegel JH, Mason-Gonzalex S, Dischinger P: Safety belt restraints and

compartment intrusions in frontal and lateral motor vehicle crashes: Mechanisms

of injuries, complications, and acute care costs. J Trauma 5:736, 1993

2. Huelke DF, Sherman HW, Stigmeyer JL: Side Impacts of the Passenger Compartment—Clinical Studies from Field Accident Investigations. SAE International Congress, Detroit, SAE 890379, 1989, pp 21-23