Abstract
The present paper presents a scanning electron microscope (SEM) analysis of the genesis and microarchitecture of experimentally induced cortical entry fractures in porcine scapulae impacted at velocities ranging from 54 to 897 m/s. SEM observation was conducted on polyurethane replicas cast from negative silicone moulds. Analysis of the sequence of fracture processes operative during projectile impact revealed the presence of ring cracks at the site of impact, confirming that penetration in sandwich bones is achieved by cone crack propagation. Despite impulsive loading, two forms of plastic deformation were identified in the cortical bone surrounding the entry fracture up to a maximum velocity of 871 m/s. Microscopic radial and concentric cracks were associated with projectile impact, and the role of pores and pits as stress concentrators was captured. Possible underlying mechanisms for the observed plastic deformation are described, and the diagnostic utility of SEM analysis is presented.
Highlights
The typical fracture produced by perpendicular projectile impact in tri-layered sandwich bones exhibits a circular hole in the outer cortical layer and a bevel flaring in the direction of projectile travel on the exiting side [1, 2]
In order to examine the micromorphology of cortical entry fractures and to identify features that might be of potential diagnostic utility, the present paper presents the findings of an scanning electron microscope (SEM) analysis of projectile fractures induced in the cortical bone of porcine sandwich bones at impact velocities ranging from 54 m per second (m/s) to 897 m/s
With some displaying few pores and some presenting with an abundance of them
Summary
The typical fracture produced by perpendicular projectile impact in tri-layered sandwich bones exhibits a circular hole in the outer cortical layer (the cortical entry) and a bevel flaring in the direction of projectile travel on the exiting side [1, 2]. Recent work indicates that conoidal fractures are produced by propagation of a Hertzian cone crack through the three laminae of the sandwich bone [3, 4] This translaminar fracture mechanism results in the production of tri-layered conoids of bone that typically undergo comminution during high-velocity impact, traces may remain in the form of cortical roof and floor fragments (Fig. 1). The correlation between this distinct conoidal fracture morphology and bullet involvement led to it being considered diagnostic of ballistic impact quite early in skeletal trauma analyses [2], and it is largely still considered so today [5]. In skeletonised and/ or fragmentary material, multiple factors can add considerable complexity to the differential diagnoses of perforating holes in skeletal elements [6]
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