Crack propagation through sandwich bones due to low-velocity projectile impact

Projectile impact in sandwich bones typically results in formation of conoidal wounds exhibiting a larger region of damage on the inner cortical plate termed the bevel. To date, a number of hypotheses have been put forward to explain the formation of this wound type. The plug and spall hypothesis suggests that the conoidal morphology is produced by a two-phase mechanism of shear plug formation followed by internal bevel production during projectile exit. In contrast, the cone crack hypothesis suggests that such wounds are produced by cone crack propagation through the three laminae of the sandwich bone, resulting in the formation of bioceramic conoids consisting of all three bone laminae. In order to test these hypotheses, 28 non-human sandwich bones were impacted with 6-mm carbon steel spheres at velocities ranging from 26 to 96 metres per second (m/s). Impacts were filmed utilizing high-speed videography and fracture morphology analysed using micro-computerized tomography (μ-CT). Sequential increase in velocity successfully captured the genesis of conoidal wounds. Low-velocity impact produced circular depressed fractures in the outer cortex exhibiting angulated cortical fracture edges. An increase in velocity resulted in translaminar fracture and production of one intact and three fragmentary bioceramic conoids. At the highest velocities, conoids were fragmented and lost in the ejecta plume, with attached fragments undergoing dynamic movement during and after perforation. Significantly, projectile exit was not required for bevel production. The implications of these findings in wound interpretation are discussed.