HISTORY
A 27-year-old U.S. Army soldier with a gunshot wound to the head was flown from a forward operating base to the 332nd Expeditionary Medical Group, an air evacuation hospital in Balad, Iraq.
FINDINGS
Initial head CT (Figures 1-3) showed a keyhole defect high on the parietal bone on the left, impacted bone fragments in the parafalcine region with some pneumocephalus, and a 3 x 3 x 2-cm high parietal subarachnoid hemorrhage and intraparenchymal hemorrhage. No effacement of the basal cisterns, intraventricular bleeding, or midline shift was noted.
DIAGNOSIS
Keyhole fracture of the skull.
DISCUSSION
A gunshot wound that strikes the cranium tangentially with enough force to penetrate or perforate the skull will produce a characteristic keyhole fracture pattern.
In gunshot wounds with a tangential trajectory to the skull, horizontal and vertical force vectors (Figure 1) govern the resultant trauma. The vertical force vector of the bullet striking the outer table of bone creates a circular defect, and secondary fractures created by bone or bullet fragments propagate from the initial point of impact. This force can dislodge a segment of bone that is pressed upward as the bullet travels underneath and is the means of the characteristic exit-type feature keyhole fractures.
This type of injury may lead the examiner to believe that the intracranium was spared, since it may appear like a superficial grazing injury. While the bullet and a portion of any bone fragments may indeed exit along the initial path of the bullet, inward-driven bone or bullet fragments from the vertical force component can create a fracture of the inner table of bone and, possibly, fragments can travel into the intracranium. The inward-driven bone fragments can then travel in the calvaria in a characteristic pattern radiating out from the initial point of impact, as in our case.
Keyhole fractures, although rare, represent an entity deserving of contemplation when triaging gunshot wounds to the skull. The discrepancy between observed and actual craniocerebral trauma in keyhole fractures stresses the importance of radiographic imaging of gunshot wounds to the skull in determining the extent of damage as well as the lie of any inward-driven bone fragments or debris.
Submitted by 2nd Lt. Aaron M. Jackson, MSC; 2nd Lt. Brett K. Searcey, MSC, USAF; Col. Les Folio, MC, USAF; Dr. James G. Smirniotopoulos. All contributors practice in the radiology and radiological sciences department at the Uniformed Services University of the Health Sciences in Bethesda, MD.
BIBLIOGRAPHY
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