Bone Density Study Gives Forensic Investigators New Tool
September 22, 2016
All good forensics stories are stories of science. Whether the CSI folks are testing for matching fiber, Dexter is charting the physics of a blood spatter analysis or Sherlock Holmes is deducing where the killer is lurking based on the type of dirt found in the footprints at the crime scene, deductions from the smallest crumbs of evidence are the driving force behind these stories.
Among the scientific tools forensic investigators use is ballistics, or the study of projectiles, especially bullets, and the marks they make. While some ballistics stories are a little more fantasy than science fiction—see Batman digitally rebuilding a shattered bullet to get the Joker’s fingerprints in the Dark Knight—the size and shape of a bullet hole can make the difference between exonerating an innocent person and proving the guilt of a criminal.
There are times, however, when gathering ballistic evidence becomes difficult. A bullet’s entry into the human body causes a much different effect than entry into a brick wall, and now new research from NC State University is helping to make that science clearer.
NC State professor of biological sciences Ann Ross found a relationship between bone density and the size hole a given caliber of bullet will make in that bone. Ross says this work could potentially give investigators another tool to gather ballistics data in a place where it can be tricky to find.
“There are so many factors involved in a medical or legal investigation that it's essential to get as much data as we can when these cases come in,” Ross said. The study was published in the International Journal of Legal Medicine.
Many times, when a bullet hits a hard, solid surface like wood or brick, the bullet will lodge itself in the surface where it can be recovered. Ross says the picture is rarely as simple with human gunshot wounds.
“When you get a gunshot wound, bullets can go through and through and sometimes they get left at the scene,” Ross said.
This means that often the gunshot wound itself is the only place investigators can go to get ballistics data. Ross began working on the relationship between ballistics and the solid state makeup of bone as a graduate student, when she studied the relationship between bone thickness and the size of a gunshot wound.
For this study, Ross used data from the same bones as in her master’s work: 18 skulls with gunshot wounds for which the caliber of bullet was previously known, mostly .22, .32 and .38 caliber rounds. For a given caliber, fluctuations in bone mineral density reflect those in bullet caliber and wound size. The greater the caliber of the bullet, the larger the minimum diameter of the hole.
Given those two relationships, investigators can begin to use the size of a bullet hole and a victim’s bone mineral density, which can be measured using X-Ray technology, to determine the caliber of a bullet.
This study is what is called a “proof-of-concept,” which means that Ross and her colleagues used the 18 skulls to prove that relationships between bullet caliber, minimum hole diameter and bone mineral density exist. More studies using more skulls will be necessary to nail down the exact mathematics of that relationship.
Further, this study used .22, .32 and .38 caliber, low-velocity wounds. Ross says that different relationships will describe wounds from other caliber bullets. High velocity rounds, such as .357 magnum rounds, Ross says, cause much more damage and different studies are needed to determine the math of those wounds.
Still, Ross says, these results could begin to help medical examiners and other criminal investigators more reliably gather ballistics data in cases where very little is available, potentially filling in important gaps.
“If you have missing pieces of info,” Ross said, “it makes it more difficult to try in a court and you don’t want an innocent person to go to court.”
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.