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New Delhi: The ambiguity of the science underlying the bone density test being proposed to establish the age of one of the six accused in the rape and murder of a 23-year-old woman in Delhi last month implies the findings could be legally challenged.

In other words, establishing the age of the accused, who is currently identified as a 17-year-old and hence a minor, is not an open-and-shut case as implied.

Dr S.K.S. Marya, an orthopaedic surgeon at the Max Super Speciality Hospital in Delhi, said only radiologists and forensic experts performed such tests. “It’s accurate, but it too has error limits. You can with a great level of confidence tell someone a few years apart, but a window of two and six months is harder to pinpoint."

Earlier this week, the police charged five of those accused of raping and torturing the woman in a moving bus. She eventually succumbed to her injuries in a Singapore hospital.

A bone density test isn’t a forensic test. It is a routine examination conducted to measure the calcium content in bones and evaluate the risk of osteoporosis. It is generally useful only in those above 50, especially women, whose calcium levels correlate well with the risk of osteoporosis.

That said, the bones do offer significant insight into one’s age with the requisite caveats. Forensic orthopaedics is a field that’s different from that of orthopaedicians who use bone density measurements. It’s used in criminal investigations as well as in paleoanthropological research to estimate the age of human or human-like fossils from excavated or exhumed remains.

Taking X-rays of specific regions of the skeleton—the only medical equipment that is needed—and matching them with sample profiles can say whether teenagers, on the cusp of adulthood, show bone development commensurate with their age.

The Smithsonian National Museum of Natural History explains on its website that the collarbone is the last bone to complete its growth, usually around 25, and the tibia—one of the strongest bones and below the knee—completes growth at about age 16 or 17 in girls and 18 or 19 in boys. In fact, the tibia completely fuses with the rest of the skeletal structure only around the age of 20 in males.

Investigators determining age also look at osteons—small tubes that carry blood into the bones—to determine age. Microscopic exams show these changes, which can indicate adult age to within five-10 years. Younger adults have fewer and larger osteons. Older adults have smaller osteons and more osteon fragments, as new ones form and disrupt older ones.

However, like most such data, these findings rely on statistical correlations between bone development and age, and it’s quite possible that some children may develop faster or slower than what the tests and charts expect them to. Worse, it is not possible to establish the age precisely to a month, leaving it open for legal challenge.

Legally, however, only anyone above 18 can be considered for harsh punishment such as life imprisonment or death. “The law is explicit on that, and any tests that have inbuilt errors of accuracy will always be interpreted in favour of the accused," said Shyam Nandan, a Delhi-based lawyer. However, “depending on the crime, those below 16 and between 16 and 18 are also dealt with differently".

Meanwhile, public outrage over the crime has prompted calls across the political class in various states for an amendment in the definition of what it means to be a juvenile.

“There was a general consensus among state director generals of police and state chief secretaries to lower the juvenile age from 18 to 16," a senior home ministry official said on condition of anonymity. The home ministry has called a special meeting of top law enforcement officials of the country to discuss ways to strengthen safety of women in the country as well as prevent crime against scheduled castes and scheduled tribes.

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