The impact velocity and bone fracture pattern: Forensic perspective
Introduction
Bone trauma is an important source of information regarding the circumstances that “led” to the death of the victim [1]. Proper fracture interpretation may assist in identifying the location and number of impact sites, establishing the sequence of blows, and determining the characteristics of the object that inflicted the injuries [2], [3]. Studies on fracture pattern in the forensic setting are of importance in cases such as homicidal assault, suicide, falls, child abuse, and road traffic accidents. Knowledge on fracture associated with specific modes of trauma can be used to predict the nature of the injury. For example, in cases where homicidal victims and child abuse are suspected, identifying the fracture type may assist in determining the direction of the force and whether the bone was twisted or angulated. In the case of suspected fall, analysis of the fracture pattern may assist in determining the type of fall (simple fall or fall from a height), the surface of the impact, and in some cases, the landing orientation of the victim [4]. Analysis of fracture pattern could also be a useful tool for accident reconstruction purposes [5]. There are a limited number of loading modes to which bone can be subjected, and these results in predictable fracture patterns [6]. These patterns are usually classified into 6 classic types: transverse, oblique or butterfly, spiral, segmental and comminuted. This classification of fracture patterns is derived largely from the medical literature where determination of the stability of the injury, probable extent of associated soft tissue damage, and the prognosis for recovery are the primary motivations [7]. Transverse fracture runs at approximately right angles to the long axis of the long bone [8]. This fracture type can be the result of force producing bending [9] or severe angulations, but not necessarily under compression from the normal weight-bearing functions [7], or the result of force producing tension [10]. Transverse fractures become increasingly more comminuted as a result of direct trauma with progressively greater force. Completed and displaced transverse fractures often result from mechanisms of high energy, such as injuries involving encounters with cars or falls from significant heights [9]. Oblique fractures run diagonally across the diaphysis with short blunt fractures usually ending at a 45° angle with no vertical segment [8], [10]. An oblique fracture usually results from the combination of angulation and axial compressive forces of moderate intensity [7] or a combination of torsion and bending (when bending is the dominant loading) [11]. The fracture morphology reflects the predominant loading type (a long oblique is common when torsion is the predominant force or short oblique when the predominant force is bending or compression) [9]. The more common patterns are oblique transverse and butterfly fractures (or indirect wedge-shaped fractures) where the initial fracture is perpendicular to the long axis (representing the failure in tension), while the latter portion is oblique (representing the compression failure). Oblique-transverse and butterfly fractures are commonly seen in the lower extremities when the thigh or calf receives a lateral blow involving weight, as in the case of pedestrians injured by automobiles [10]. Butterfly fractures usually occur at low speed of impact. At higher speeds of impact, “dynamic” noncharacteristic transverse or multifragment fractures are usually observed [4]. Few words about Butterfly fracture are due here as this type of fracture is commonly used in forensic cases for establishing the position of a pedestrian in relation to a motor vehicle [15]. The mechanism of this fracture was the subject matter of Messerer's study in the late 19th century [16]. The rules put forward by him concerning the location of the base of the wedge (from the impact side) and its apex (according to the force direction) has become, from the early 60th of the previous century, a ‘standards’ in forensic medicine and is treated almost dogmatically both in the literature (especially textbooks) and in practice [15], [22], [23], [24], [25]. This is surprising considering the growing evidence to suggest the presence of a reversed phenomenon, i.e., the apex and not the base is directed toward the impact site (“false butterfly fracture”), in some of the cases. For example: Spitz and Russell [12] in their study on pedestrian leg impact found that in some cases, even a “false” wedge-shaped fragment may be seen. This observation was repeated by other studies [4], [13], [14], [20]. Rich [4] claimed that a typical bending fracture (Messerer's wedge) can indicates the direction of impact only when the bone was bent at the moment of impact [4]. Already in 1963 Patscheider proved experimentally the possibility of the occurrence of “false” indirect wedge-shaped tibial and femoral fractures by hitting rigidly fixed human and animal bones with weighted pendulum [18]. On 1999 Teresinski and Mydro examined 14 femurs with wedge-shaped fractures following pedestrians’ car accidents to evaluate the evidential value of wedge-shaped fractures: in only 50% of the cases a “true” wedge fractures (“Messerer's fractures”) were found, 21% of the cases manifested “false” wedge fractures and the rest had the wedge at the impact side [15]. Spiral fractures are caused by rotational forces on the bone [7] or a combination of torsion and bending (when torsion is the dominant loading) [4]. These fractures tend to be the result of low-velocity forces [7] and were produced only from torsional loading in experimental testing of human cadaver long bones [5]. Torsion creates a state of pure shear between parallel transverse planes. In other planes (at other angles with respect to the longitudinal axis), tensile and compressive stresses are present, and they become maximum at a 45° angle to the longitudinal axis [17]. The fracture has long, sharp, pointed ends and vertical segment, which is the last component to form [10], in contrast to the ends of bones in oblique fractures, which are short, blunt and rounded [4]. The direction of the spiral indicates the direction of the torsional forces [10] and can be used to reconstruct the events that produced the fracture [7]. When multiple fractures leave diaphyseal portions separated from the proximal or the distal ends, the intervening segment is called a segmental fracture. This defect may result from multiple simultaneous fractures as would occur when a bone is hit at two points or by a large surface [7]. A comminuted fracture is one in which more than two fragments are generated [8] and usually results from relatively high levels of force [10]. Such fractures are most common in the lower extremity, which is often weight-bearing at the time of impact by an extraneous object. They are commonly seen in the legs of pedestrians hit by motor vehicles. The ability to distinguish between fracture patterns in bones following an impact at different velocities is extremely important. The need for traffic accident reconstruction is of major importance, since road traffic injuries are the leading cause of death worldwide among young people aged 10–24 years (World Health Organization) [27]. Analyses of the lower-extremity fracture patterns are particularly relevant in car-to-pedestrian impacts, since they reflect the actual location of the pedestrian relative to the vehicle and can shed light on the speed of the car when this is in question. It is well known that the greater the magnitude of the force, the higher its energy content, and hence, the more bone destruction. Conversely, the more complex the fracture pattern the greater the energy needed to produce the fracture [10]. A high-energy direct blow to an adult bone will cause a markedly comminuted fracture [13], [29] typically associated with extensive soft tissue injury and indicating a large amount of energy dissipation in conjunction with a rapid loading rate [9], [30]. Tissues surrounding bone, including muscle, tendons, ligaments, fat, and skin, can affect the fracture pattern by absorbing some of the load energy and also by creating additional load [6]. Studies on bone-energy interaction revealed only a general correlation between the fracture pattern and the mechanism of the insult [31] (cited from Rich [4]). Attempts to determine the crash speed on the basis of the severity of injuries were reported in a text book however this report has yielded no reliable methods of crash speed determination [32]. Text books on bone trauma mostly describe and define fracture types in relation to the direction of loading and loading type applied [7], [8], [10], [33], [34]. Studies on the association between the mechanical properties of the bone and physical injury concentrated mainly on microcracking behavior, their location, initiation and propagation [35], [36], [37], [38], [39]. These studies however did not explain the correlation between the macrocracking behavior on a whole bone in relation to different types of force applied. Any attempt to predict the behavior of a skeletal region under loading must reflect both the material properties of the bone in that region and its structure [40]. Although there is a consensus regarding the mechanism that produces certain fracture pattern, there are clearly competing theories in the medical literature in relation to others. In addition, most studies analyzing fracture patterns in 3-point bending did not include additional axial compression loading as when the thigh receives a lateral blow during weight bearing, as in the case of pedestrians injured by vehicles. In addition, the information regarding whether the bones were complete or partial during testing, and the precise site of impact is uncertain. Fracture pattern analysis can be a complicated process especially since in most cases the fracture pattern is far from being classic. The determination of the mechanism that results in particular patterns is better approached through experimentation rather than theorizing. Unfortunately, many unsubstantiated theories have been repeated and referenced for decades. Most studies that looked into the micro-morphology of fractures were performed by engineers who were interested in exploring the mechanical properties of bones. Physicians, on the other hand, were more interested in the macro-morphology of fractures (classification of fracture patterns), where prognosis for recovery and fixation are of major concern. Currently, there is no a single study that has examined, in a comprehensive way, the morphological and metrical characteristics of fractures in regard to the impact energy of the force applied in a forensic perspective, simulating a situation occurring in pedestrian road traffic accidents.
Section snippets
Objective of study
This study has two objectives, to establish a usable fracture analysis method and to reveal the association between the energy of the force and the fracture pattern.
Material and methods
Experimental set-up: the Dynatup Model POE 2000 (Instron Co.) low energy pendulum impact machine, shown in Fig. 1, was utilized to apply impact loading on pig femoral bones. Custom-made supports for holding each bone in place during loading were fabricated. The clamp shaft was designed to support the bone while the impact load was oriented in perpendicular to the longitudinal axis of the bone (Fig. 2). The bone clamp shaft was adjusted to position the bone for three-point bending with
Results
This study describes and analyzes fracture pattern in two modes: qualitative and quantitative. In the quantitative mode the fracture measurements are summarized. In the qualitative mode, schematic illustrations of the fracture patterns are presented for each test. Mean values (±SD) of bone and fracture characteristics relating to different impact energy are presented in Table 2. No significant differences in cross-sectional moment of inertia were found between the femora in the three tests
Summary
A bone fracture following a low-velocity impact can be identified mainly as a simple transverse fracture. The impacted and the contralateral sides are mostly characterized by a short longitudinal line that crosses the transverse fracture toward the epiphyses (on the impacted aspect, the meeting point of the transverse line and longitudinal line is the point of impact). However, a fracture following a moderate- or high-velocity impact results in a comminuted or multifragment double butterfly
Discussion
In the current study, a complete transverse fracture is observed in bones subjected to a low-velocity impact, but although expected, the butterfly pattern is absent (Fig. 4) [4]. In bones exposed to a moderate- or high-velocity impact, the fracture pattern becomes significantly more comminuted as was found previously [4] and is characterized by a greater number of longer fracture lines, in particular, on the impacted (lateral) aspect (Fig. 5, Fig. 6). A larger chip fragment is also present in
Conclusions
This study, to the best of our knowledge, is the first to examine experimentally variant fracture features in relation to the energy of the force applied, from a forensic perspective. Reconstructing an injury scenario can be complicated especially in cases of comminuted and non-characteristic fractures. In this study, new and significant correlations between fracture pattern and impact characteristics were found. This can serve as a core principle for future studies and forensic case analysis.
Limitations of the study
This study is sample-specific and further validation (independent laboratory and sample) is required in order to develop an appropriate prediction model for bones in general and human bones in particular. For the case of simplicity, bones were impacted at mid-shaft. Other sites of impact may yield different results. Finally, the setting itself bears on the results, i.e., the direction of impact simulated by the Instron apparatus was applied perpendicular to the bone diaphysis hence, different
Acknowledgments
Grant sponsors: Dan David Foundation, Tassia and Dr. Joseph Meychan Chair for the History and Philosophy of Medicine.
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Department of Evolutionary Genetics, Max Planck Institute for Evolutionary Anthropology, 04103 Leipzig, Germany.