Two per cent of the African-American population is affected. But up to 60 per cent of Alaskan natives have this condition. It could be linked to genetic or lifestyle factors, but exactly what those are remains unknown.

It appears that separation of the bone occurs with repetitive mechanical load or stress. But many people experiencing similar loads don't develop spondylolysis. So what's the difference between someone who has spondylolysis and someone who doesn't? That remains a mystery for now.

Scientists are actively studying this problem. Recent studies of the tissue that forms around the fracture might offer us some clues. It could be that abnormal tissue in the area can't handle the load across the low back.

Or perhaps fractures occur that heal with callus (bone tissue) in some people and they never develop back pain. Others have poor fracture healing because the defect fills in with tissue that's more like tendons or ligaments. It's strong but not strong enough to withstand the mechanical forces on the spine. They develop painful low back symptoms and have an X-ray showing the fracture.

More study is needed to answer the many questions about this condition, including why such a high proportion of Alaskan natives are affected.