Experimental fracture investigations concerning the hyoid bone fracture

Extensive literature exists concerning the etiology and morphology of fractures of the hyoid bone (h.b.) in cases of fatal strangulation. There is an agreement to the effect that fractures are mostly located within the distal third of the cornua majora (c.m.). Although several predictors have been described very accurately, the fracture analysis has neither been based on the osseous construction nor on the stress distribution of the c.m. under strangulation resulting from the configuration and its details. This especially applies to the apex of the distal ends of the big horns, the bulbi. The objective of the experimental break tests that were performed was to contribute to elucidating the biomechanics of the horizontal and vertical fractures and to fractures of the bulbi.

Break tests in the a.p. direction line were carried out on 28 unfixed h.b. of adults on a specially constructed test bench by continuously increasing the tension until a fracture/dislocation occurred. The test arrangement followed the constellation of typical-symmetrical hanging. The selection criteria were the symmetry of the h.b. and the gender. Before and after the experiments, a radiological depiction (DIMA system 20 kV, 10 sec; Institute for Diagnostic Radiology, University-Hospital Goettingen) was carried out, followed by a preparative depiction under magnifying glass control. The h.b. configuration was classified according to the following types: hyperbole-, parabola-, and horseshoe-type. Following this classification, the results were related to the findings achieved by the photo-elastic model experiments. By this, the results of the experimental fracture tests could be specifically compared to the tension distribution within the model.

A total of 70 % of the experimentally-produced fractures were located within the distal third of the c.m. This matches with the frequency distribution in real typical symmetrical hanging. Following the radiological and preparative investigations that were carried out, the c.m. have to be viewed as tubular bones. The transition regions of different osseous strength/elasticity are thus to be considered as areas of increased vulnerability. For the distal third of the c.m., it is the level at which the dense spongiosa/compacta of the shaft part turn into wide-meshed spongiosa and tender compacta of the bulbi. Additionally, the bulbi themselves represent a locus of reduced strength in which the fractures were located basally and/or apically in the transition. It was not only in the whole c.m. that the direction in which the fragment was snapped off or fractured was not random, as all fractures were located on the broad side of the horn, following the applied force.

The experimental fracture tests explained the known accumulation of fractures in the distal third of the c.m. in cases of hanging with the knot of the rope located against the neck. It could be demonstrated radiologically and preparatively that, anatomically, the big horn of the h.b. is a tubular bone. From this, a new approach to the forensic reconstruction of trauma can be derived. The transitional area from the shaft into the bulbus represents a locus minoris resistentiae. In case of pressing the h.b. towards the cervical spine under ventral application of force, one could expect a point load of the bulbi. Two different types of bulbus fractures showed that this load is diagnostically relevant. The fracture direction is also of diagnostic value. It depends on the angle that is formed by the c.m. with their broad side towards the horizontal. This angle may even change for about 90° for the c.m. in the course from proximal to distal movement. By adjustment of the alignment of the broad side with the stress distribution within the different types of the h.b., a mechanically justified answer can be given to the question of why a horizontal fracture appears in the one case and a vertical fracture in the other.