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Registrar
The Australian Craniofacial Unit
Adelaide Children's Hospital and Royal Adelaide Hospital
Australia
DAVID J. DAVID AC, MBBS (Adel), FRCS, FRCSE,
FRACS
Head of Unit
The Australian Craniofacial Unit
Adelaide Children's Hospital and Royal Adelaide Hospital
Australia
ABSTRACT
Craniofacial fracture patterns were investigated in hospitalized patients and fatally injured subjects to determine the influence of helmets, spectacles, and dentures. Standardized clinical and radiographic assessment techniques were developed and fracture patterns were recorded at operation or necropsy. All fracture data were encoded into an alpha-numerical system for analysis.
From the hospitalized group interesting associations emerged of naso-ethmoidal fracturing with spectacle wearing and maxillary fracturing with denture wearing. Hospitalized motorcyclists who had worn open-face helmets, or full-face helmets with flexible face-bars, had sustained facial fracturing but minimal brain injury. In contrast, motorcyclists killed from anterior craniofacial impact whilst wearing full-face helmets with rigid face-bars had sustained fatal skull base fracturing in the absence of significant facial trauma.
It was therefore postulated that impacts to the face-bar of a full-face helmet may be transmitted through the mandibular rami and mandibular condyles to the skull base with subsequent fracturing of the middle cranial fossa, the integrity of the mid-facial skeleton being preserved.
Helmet deformation patterns, as delineated by computed tomography, provided support for the proposed mechanism of energy transfer to the skull base. Independent neuropathological examinations of the brains of the fatally injured motorcyclists revealed a high incidence of ponto-medullary disruption. This further strengthened the postulate because separation of the robust pons from the slender medulla may result from axial traction imparted to the brainstem by an upward fracturing middle cranial fossa shelf. Furthermore the skull base fractures traversed the mid-line through, or near to, the spheno-occipital synchondrosis which has an heterogenous morphology and is part of a potential 'fault line' across the skull base. That such a skull base fracture can occur from upward impaction of the skull base by column loading of the mandibular ramus has been noted in reports of judicial hangings in which the knot was placed beneath the mandibular angle.
From this study it was concluded that objects worn at impact on the craniofacial region may influence significantly the final craniofacial fracture pattern and this may be detrimental for some motorcyclists wearing full-face helmets.
INTRODUCTION
The labyrinthine architecture of human facial bones provides a propensity for their collapse at impact and they may thereby act as an effective energy absorber by preventing injury to the brain.(1) This concept is frequently reinforced by clinical observations of patients who have severe facial fracturing but only minimal overt brain injury. The cushioning potential of facial fracturing was recorded by Le Fort in 1901(2) whilst reporting on his experiments to delineate the lines of mechanical weakness in facial bones. In a similar way, in the 1940's, Cairns and Holbourn(3) noted the sparing of brain injury in helmeted motorcyclists who had sustained facial fractures. In contrast to the present-day surgical enthusiasm to devise new reparative techniques for facial fractures, only scant attention has been given to an analysis of observed fracture patterns and their potential mechanisms of production. One area that has remained void of investigation has been the influence on subsequent craniofacial fracture patterns of objects fitted to the craniofacial region ; in particular, objects such as helmets, spectacles, and dentures. This is surprising because these are commonly worn by people sustaining facial trauma. Whilst considering that facial fractures may actually play a protective role for the brain by absorbing impact energy, it is interesting to note that motorcycle helmet manufacturers have determined to produce helmets that incorporate a face-bar which is designed to prevent facial trauma. So popular have full-face helmets become that over 80% of motorcyclists wear them.(4) However, disturbing international reports are now emerging of inexplicable skull base fracturing in fatally injured motorcyclists wearing full-face helmets. (5-9)
OBJECTIVE
With the foregoing in mind, one objective of the present study was to compare and contrast the patterns of fracturing within the craniofacial region of surviving motorcyclists with those of fatally injured motorcyclists in an endeavour to elucidate a mechanism responsible for the fatal skull base fracturing that has been reported in full- face helmet wearers.
METHODOLOGY
To effect an analysis of fracture patterns with due regard for the complex anatomy of the craniofacial region required the formulation of an accurate method to code numerically the bony disruption. An alpha-numerical coding system was devised(10) and this was employed to analyse fracture data obtained from a variety of sources including clinical examination, radiographs, and operative, as well as post-mortem examinations.
A systematic method was developed for the clinical examination of the traumatized craniofacial. region of 50 hospitalized patients. Furthermore an optimal combination of radiographic imaging was ascertained and computed tomography (CT) was found to be the single most informative mode of imaging. Using the clinicoradiographic methods so devised, the fracture information of clinical patients was then coded into the alpha- numerical system. Road accident details were also collected and the influence assessed of objects fitted to the craniofacial region. Interesting associations emerged. High numerical scores of fracturing in the naso-ethmoidal region were associated with the wearing of heavy framed spectacles by four patients who had sustained lateral or oblique impacts. High numerical scores of fracturing in the maxillary region were associated with the wearing of upper dentures by six patients who had received impacts to the upper lip region. These preliminary studies demonstrated that the alphanumerical coding system was an effective method for the analysis of a wide range of fracture variants and also that the chosen clinical and radiographic techniques were providing accurate data efficiently, and that road traffic details could be incorporated with clinical details to generate meaningful associations about simple objects fitted to the craniofacial region.
Equipped with these techniques it was then considered appropriate to examine a series of 24 fatally-injured subjects, with particular reference to motorcyclists. With permission from the State Coroner, road traffic accident victims were examined with the palpation sequence of the systematic clinical examination method developed for clinical patients. If the clinical examination suggested the presence of craniofacial fracturing and other selection criteria were met, then the cadaver was transferred to a CT scanning facility and scanned according to the protocol devised for clinical patients. The cadavers then underwent a routine postmortem examination after which a formal facial dissection was performed using standard surgical approaches. Anthropomorphic landmarks were measured on the skull-base and the physical distances to fracture lines ascertained. In addition, the helmets worn by motorcyclists were uniquely examined with a CT scanning sequence that was developed for this project(11).
FINDINGS
When compared with the hospitalized group of motorcyclists the alpha-numerical coding of the fatally injured group revealed a statistically significant difference between their craniofacial injury patterns. The hospitalized group had high scores of facial fracturing but low scores of cranial fracturing. In contrast, fatally injured motorcyclists who had received impacts to the facial region whilst wearing full face helmets had sustained unsurvivable skull base fracturing (Figure 1), but low scores of facial fracturing were recorded. Indeed their observed skull base fractures traversed the middle cranial fossa just posterior to the underlying temporo-mandibular joints (Figure 2). Post-mortem, anthropomorphic data and dissection information of the cranial base and computed tomographic reformats of the craniofacial skeleton distraction patterns provided a foundation for a postulated mechanism to explain the fatal skull base fracturing.
It was proposed that an impact to the face bar of a motorcyclists' helmet may load the chinstrap, which is mounted at the rear of the facebar, and that this would transmit the force to the mandibular condyles and such force - loading would be sufficient to cause fracturing of the skull base(l 2) -(Figure 3).Helmet deformation patterns observed using computed tomography provided further support for the proposed mechanism.
These findings stimulated further investigations to test the validity of the proposed mechanism of injury production. Routine neuropathological reports were retrieved of examinations of the brains of the fatally injured. In the sample studied, a high incidence of tearing at the junction of the pons and medulla further strengthened the proposed sequence of face bar - chin strap - mandibular condyle - skull base force transmission because it was anticipated that fracturing of the clivus with upward distraction of fracture edges should cause brain stem damage. This became the catalyst for a thorough independent search (co-ordinated by Professor Simpson - Neurosurgeon, The University of Adelaide Road Accident Research Unit) for ponto-medullary trauma in fatally injured road accident victims(13).
From a consecutive series of 988 brains from autopsies on road accident victims there were 36 cases of unequivocal gross brain stem tearing. The proportion of motorcyclists was double the expected figure and of the 15 motorcyclists, 13 were known to have been wearing helmets at impact and 11 of these wore full face helmets. Furthermore, the principal impact point was the face or helmet face bar in nine of the motorcyclists.
The possibility remained that these injuries could still have been the result of severe cervical hyperextension rather than an upward impacting force through the mandibular condyles to the skull base. From a review of the craniofacial fracture patterns of people who had sustained an upward impacting force to their mandible it was interesting to note that the victims of judicial hangings in which the knot was placed beneath the angle of the mandible (thus imparting to the skull base an upward impacting force) could sustain fatal skull base fracturing of a similar ipsilateral pattern to that observed in the motorcyclist series. This contrasted significantly with the judicial hanging injury pattern observed in those victims who were hanged with the knot placed beneath the chin (thus providing a severe cervical hyperextension force) as they sustained fractures of their axis(14).
Another potential source of support for the postulated mechanism of skull base fracturing relates to the spheno-occipital synchondrosis as it was through, or near to, this zone that the fractures traversed the clivus. The motorcyclist group (and most victims of judicial hangings) are usually males in the age range of 17 to 25 years. In this age group the spheno-occipital synchondrosis may be a weak zone because it is undergoing ossification and often has heterogeneous morphology. Interestingly two children who were investigated in this study after receiving fatal injuries from facial impacts at the mandibular level had both disrupted their spheno-occipital synchondrosis.
CONCLUSIONS AND FUTURE IMPLICATIONS
So from this investigation a mechanism has been postulated for the generation of fatal skull base fracturing in motorcyclists who sustain impacts at face level whilst wearing full-face helmets. To arrive at this postulate a new method has been developed for the coding of fractures in the craniofacial region, a systematic clinical examination process has been formulated, and an optimal sequence was derived for the radiographic imaging of craniofacial trauma. These techniques have been applied to a sample of clinical patients with fracturing and by incorporating road accident information, associations were then made between the wearing of simple objects (such as dentures and spectacles) and the final fracture patterns. From the application of these methods to a group of fatally injured motorcyclists a postulated mechanism has been advanced to explain the disturbingly high incidence of skull base fractures observed internationally in motorcyclists killed whilst wearing full-face helmets. Support for the postulate has been drawn from the CT scanning of helmets and also from an independent study of ponto-medullary injuries as well as reported observations of the injuries sustained by judicial hanging victims.
From this study it was concluded that objects worn. at impact on the craniofacial region may influence significantly the final craniofacial fracture pattern and this may be detrimental for some motorcyclists wearing full-face helmets. The positive correlation of full-face helmet wearing with fatal middle cranial fossa
fractures and ponto-medullary injuries, in the absence of facial fractures, raised questions about the efficacy of rigid face-bars. Conversely, the lack of significant neurotrauma in cases suffering facial fractures provides support for the beneficial energy absorbing role of facial fracturing. It is therefore recommended that energy absorption properties be incorporated into the face-bar Of a full-face helmet to overcome the paradox of facial injury prevention at the expense of fatal skull base injury propagation.
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| Figure 3a - Impact to a helmet face bar may be transmitted to the chinstrap and then to the mandibular condyles. |
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| Figure 3b - Subsequent impaction of the skull base by the mandibular condyles causes fracturing across the middle cranial fossa. |
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| Figure 3c - The middle cranial fossa fractures traversed the sphenoid in the region of the spheno-occipital synchondrosis. |
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| Figure 3d - Upward distraction of fracture edges imparts a longitudinal fraction on the brainstem. |