Milwaukee Journal Sentinel, WI
08-05-07
Making an impression
Bite-mark study could bolster use as evidence
By BRANDY BENEDICT
bbenedict@journalsentinel.com
It's commonly believed that no two sets of human teeth are alike. Chips, jagged
edges, crooked teeth and gaps all contribute to a person's unique smile.
It seems reasonable, then, that bite patterns made by those teeth should be as
unique as the teeth themselves, though this has not been scientifically proven.
Now, a team of researchers at Marquette University has gotten one step closer.
By measuring the position and rotation of individual teeth from hundreds of bite
samples collected from volunteers, the researchers can statistically estimate how
often the characteristics of a particular bite pattern occur in the population and
determine the probability of someone else having the same characteristics.
The researchers ultimately hope that the scientific analysis of the patterns will
provide a more objective basis for analysis and comparison of bite-mark
evidence in criminal trials.
"This project will put science behind the impression that a person's pattern of
bites is unique," said Thomas Radmer, assistant professor and director of oral
and maxillofacial surgery at Marquette.
L. Thomas Johnson, adjunct professor at Marquette University's School of
Dentistry, explained that while intuition may say that each set of teeth would
make a distinct bite pattern, there is no scientific database proving it.
So Johnson and Radmer decided to create one.
During a two-year period they collected more than 400 bite patterns from male
volunteers ages 18 to 44, the demographic most likely to be the aggressors in
violent crimes involving biting.
Each subject bit down on a copper-coated wax wafer, simultaneously capturing
the indentations of his top and bottom teeth. The copper coating on the wafer
ensured that the bite marks would show up clearly when scanned into a
computer.
For each sample they measured six characteristics: tooth width; degree of
rotation of a tooth; inward or outward displacement; arch width; spaces between
teeth; and any missing or extra teeth.
Measurements of each bite mark pattern initially were taken by hand and had to
be checked for consistency.
"This is not an exact science; it's a bit subjective," Johnson said. "There has to be
a human mind and eye to compare."
Now, a computer can do all the hard work and get it right every time.
Thomas Wirtz, director of the dental informatics program at Marquette,
developed a computer program to measure the bite pattern characteristics
automatically, eliminating any errors in measurement.
His program looks at every pixel in the image to locate the positions of all the
teeth, and then uses formulas to calculate distances and angles.
It takes 15 to 20 minutes to measure a sample by hand, but Wirtz's computer
program can do it in about 10 seconds, a huge improvement considering the
researchers ultimately would like to build a database of tens of thousands of
samples.
"Without the automated program it would be a monumental task," Radmer said.
Controversial as evidence
Bite marks have a controversial history as evidence in criminal trials. First
brought up as forensic evidence in 1870, bite marks did not formally gain legal
credibility in the U.S. courts until 1954. Since then, bite-mark evidence has been
used in a number of cases, most famously to convict serial killer Ted Bundy.
But occasionally forensic dentists have been known to make some costly
mistakes.
In April 2002, DNA evidence freed Ray Krone, dubbed the "snaggletooth killer,"
who spent 10 years in an Arizona prison, including two on death row, based
solely on incorrectly analyzed bite marks.
Bite-mark patterns can vary wildly in clarity, and like a bruise, they can change
over time. The quality of a bite mark also depends on the substance a person
has bitten, as bite marks on gum or Styrofoam cups can be more defined than
bite marks on skin. The force of the bite, the angle of the bite and the area bitten
can all affect the appearance of a bite mark.
Forensic dentists apply their expertise to formulate an opinion on whether the
accused could have made the particular bite mark patterns.
But the subjective nature of bite-mark analysis and interpretation has been the
focus of harsh criticism, and forensic dentistry has been called a "junk science."
"False convictions leave bite marks with a black eye," said Donald Simley, a
certified forensic dentist based in Madison. The study "should help the whole
science of bite-mark evidence."
Radmer agreed: "The defense attorneys have been taking aim at this type of
evidence in jury trials as being subjective, not objective."
'Worthwhile' study
With statistics to back up professional opinion, the researchers hope to make
bite-mark forensics more scientifically sound.
"What the Marquette study is trying to do is provide an objective, reliable basis to
answer the critics," said Daniel Blinka, a professor in the Marquette Law School
and legal consultant for the study. "It has a broader significance in providing a
template for other areas of forensic expertise, like fingerprint and handwriting
analysis, to look to and follow."
Wirtz's computer program could be adapted to measure the width of tire treads,
or width of ridges in a fingerprint. "There are multiple applications throughout the
field of forensics," Radmer said.
The study was funded with a nearly $110,000 grant from the National Institute of
Justice through the Midwest Forensic Resource Center, part of the Ames
Laboratory Research Center at Iowa State University. The researchers
expect to complete their study by Sept. 30 and will present results in February at
the annual meeting of the American Academy of Forensic Science in
Washington, D.C.
The researchers agreed that the pilot study shows there is some predictability
and provides a basis for further studies.
"We've shown with a very high confidence level that the (bite) pattern is unique,"
Radmer said.
Johnson, too, is pleased with the results: "I think it was worthwhile."