  In this paper it is shown that statistical mechanics in the form of
thermodynamic entropy can be used as a measure of the severity of individual
injuries (AIS), and that the correct way to account for multiple injuries is to
sum the entropies. It is further shown that summing entropies according to the
Planck-Boltzmann (P-B) definition of entropy is formally the same as ISS, which
is why ISS works. Approximate values of the probabilities of fatality are used
to calculate the Gibb's entropy, which is more accurate than the P-B entropy
far from equilibrium, and are shown to be again proportional to ISS. For the
categorisation of injury using entropies it is necessary to consider the
underlying entropy of the individuals morbidity to which is added the entropy
of trauma, which then may result in death. Adding in the underlying entropy and
summing entropies of all AIS3+ values gives a more extended scale than ISS, and
so entropy is considered the preferred measure. A small scale trial is
conducted of these concepts using the APROSYS In-Depth Pedestrian database, and
the differences between the measures are illustrated. It is shown that adopting
an entropy approach to categorising injury severity highlights the position of
the elderly, who have a reduced physiological reserve to resist further
traumatic onslaught. There are other informational entropy-like measures, here
called i-entropy, which can also be used to classify injury severity, which are
outlined. A large scale trial of these various entropy or i-entropy measures
needs to be conducted to assess the usefulness of the measures. In the
meantime, an age compensated ISS measure such as ASCOT or TRISS is recommended.
