Skeletal trauma analysis typically relies on post-hoc interpretation of fracture features such as crack configuration and fragment shape. Understanding how cracks initiate and propagate is foundational for explaining how such features form, especially in complex trauma where fragments may not be recovered or cannot be fully reconstructed. This study used high-speed video to document crack initiation and propagation in 30 whole human long bones (femora, tibiae, and humeri) from ten cadaveric donors following projectile impact with 9 mm full metal jacket bullets. Two cameras recording at 55,000 frames per second were positioned to capture crack formation at the entrance and exit sites. Fracture sequences were typically complete within 5–8 frames (91–145 μs), with the longest lasting 10 frames (182 μs). A consistent entrance site sequence emerged across all bones and impact locations: initial chipping at the impact site, followed by radial crack propagation, then formation of circumferential cracks between adjacent radial cracks. The entrance sequence mirrors patterns reported in cranial projectile impacts. Crack propagation opposite the entrance was more complex and diverged from cranial patterns. Longitudinal cracks often formed before exit defects and sometimes curved back toward the entrance, intersecting entrance radial cracks. Transverse cracks occasionally developed between longitudinal cracks. The results suggest greater interaction between entrance and exit cracks in long bones than in crania, likely due to their smaller diameter. The fracture sequences captured in these experiments can inform more accurate interpretations of long bone projectile trauma.
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