Discussion
Post-mortem computed tomography (PMCT) is increasingly being adopted in the investigation of death, usually as an adjunct to autopsy, but increasingly in some countries to replace the internal examination [
1]. It has been reported to have an important role in the investigation of trauma due to its ability to investigate skeletal injury [
2]. In this context it has been shown to be useful in motorcycle fatalities [
3].
Motorcyclists remain some of the most at-risk road users worldwide. In the United Kingdom motorcyclists are 68 times more likely to suffer a fatality on the road per passenger mile than car occupants [
4]. Although helmet use has been shown to be effective in reducing injuries to the head, head injury is still the leading cause of death among both helmeted and non-helmeted riders [
5,
6]. As these are closed injuries the extent of the head injury may not be appreciated by those rendering aid at the roadside. Despite the clear benefit of wearing a helmet, there appear to be predictable patterns of skull fracture associated with certain types of helmet impacts. Cooter et al. reported a fracture of the middle cranial fossa traversing the clivus believing that an impact to the face-bar transmitted a force to the skull base via the mandibular condyles, a mechanism also considered by Harms [
7,
8]. Heavier helmets are more likely to result in a partial or complete ring fracture of the base of the skull when subjected to axial loading [
9].
The five cases presented here demonstrate the same fracture pattern: a circumferential fracture comprising of a linear fracture of the middle cranial fossa extending through the pituitary fossa, continuing to the temporal and occipital bones of the skull vault. They are presented in order of increasing severity. Case 1 was less severe without dissociation of the skull. In cases 2–5 the fracture was sufficiently severe to completely dissociate the skull into two parts. Case 3 had an additional associated defect more akin to a classic incomplete ring fracture of the skull base. Cases 2, 4 and 5 show the fracture-type largely alone. Of note all of the deceased had witness marks on their helmet indicating an impact to the face, except for the individual in case 2, who clearly had an impact to the chin from the injuries noted on examination. All died at the scene due to their head injuries, although several had other injuries to the chest or abdomen which contributed equally to death.
A ring fracture is classically caused by an impact either to the top of the head or a fall from a height landing feet first, and involves the petrous temporal bones bilaterally, the clivus and the posterior part of the foramen magnum [
10,
11]. However, the pattern described here also bears resemblance to a hinge (or ‘motorcyclist’s’) fracture in which a transverse crack extends the width of middle cranial fossa through the pituitary fossa [
10]. Krantz briefly described a fracture which may have similar features to our series, which resulted from ‘a force which lifts the upper part of the head from the base of the skull’ [
12]. The majority of these cases suffered an impact to the face. The lack of detail in these studies’ descriptions make it difficult however to confirm the similarity in the fracture pattern. Moskała et al. included an image of a ‘circular’ fracture in their study of PMCT in motorcyclists which shares the features in our cases; it is not further described, except to note that these types of fractures can be difficult to see on traditional autopsy [
3]. A review of basal skull fractures in relation to helmet-use also described several fractures with features similar to our cases [
13]. West et al. also demonstrated fractures extending from the skull base to the occipitomastoid suture (as in case 1), but these were not bilateral and did not involve full circumferential fracture of the occipital bone seen in cases 2–5 [
14]. The fracture pattern presented here has features of both a hinge and a ring fracture. Cases 2, 4 and 5 had clear evidence of an impact to the chin area, which could cause the fracture via force through the mandible, as described in the literature [
7,
8]. However although this may be the most frequent cause it is thought that skull base and vault fractures can result from impacts anywhere on the head [
13,
15].
We suggest that the fractures presented in the five cases here are distinctly associated with motorcycle incidents. Similar fractures may have been described briefly within larger studies, and they may be familiar to experienced autopsists, however to our knowledge, this is the first time the fracture has been described in detail and as a series illustrated using PMCT.
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