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Gunshot induced indirect femoral fracture: mechanism of injury and fracture morphology
  1. David C Kieser1,
  2. D J Carr2,
  3. S C J Leclair3,
  4. I Horsfall2,
  5. J C Theis1,
  6. M V Swain4 and
  7. J A Kieser4
  1. 1Surgical Sciences, Orthopaedic Surgery, Health Sciences, Dunedin School of Medicine, University of Otago, Dunedin, Otago, New Zealand and the New Zealand Defence Force
  2. 2Impact and Armour Group, Department of Engineering and Applied Science, Cranfield Defence and Security, Defence Academy of the United Kingdom, Shrivenham, Wiltshire, UK
  3. 3University of Angers, Angers, France
  4. 4Sir John Walsh Research Institute, University of Otago, Dunedin, Otago, New Zealand
  1. Correspondence to Capt Dr D C Kieser, Surgical Sciences, Orthopaedic Surgery, Health Sciences, Dunedin School of Medicine, University of Otago, PO Box 6458, Dunedin, Otago 9016, New Zealand; kieserdavid{at}gmail.com

Abstract

Introduction Indirect ballistic fractures occur when a projectile passes close to, but not contacting, the bone. The mechanism of how these fractures occur is not yet proven, but recently the acoustic shockwave has been excluded as a cause. The objective of this study is to determine whether the expanding temporary cavity, the collapse of this cavity or its oscillation causes these fractures. In addition, we describe the fracture morphology and biomechanical causes of this injury.

Method 40 fresh deer femora were strain gauged and embedded in ballistic gelatin before being shot with four different projectiles with varying distances off the bone. Pressure recordings, chronographs and radar allowed assessment of local pressures and energy transfer. High-speed video allowed the temporal relationship between the temporary cavity and fracture formation to be analysed, while sample dissection allowed the fracture morphology to be described.

Results The fractures produced were consistently wedge-shaped and caused by the expansion of the temporary cavity, flexing the bone beyond its yield point, causing tension failure on the cortex opposite the expanding temporary cavity and a compression wedge on the side of the cavity. Local pressure was not predictive of fracture formation but the energy transfer to the gelatin block was predictive.

Conclusions Indirect fractures are caused by the expansion of the temporary cavity and relate to the proximity of this cavity to the bone. Fractures occur from flexion of the bone and classically display wedge-shaped fracture patterns with the apex of the wedge pointing away from the expanding cavity.

  • Orthopaedic & Trauma Surgery
  • Radiology & Imaging
  • Accident & Emergency Medicine
  • Received March 27, 2013.
  • Accepted April 29, 2013.

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