For years, bone mineral density (BMD) -- a measure of how much actual bone mineral a bone contains -- has been used to determine which patients are candidates for spinal surgery and which are not. It was assumed that a low BMD, found in patients with osteoporosis, was a risk factor for vertebral fractures following spinal procedures.

However, a study presented today at the American Academy of Orthopaedic Surgeons (AAOS) 2018 Annual Meeting suggests that, at least in one type of spinal surgery, this may not be the case.

The authors report that in patients undergoing spinal fusions in their lower spine – known as the sacral region – a lower BMD alone may not increase the risk of sacral vertebral fractures. The findings suggest that surgeons should evaluate patients across a whole range of bone fracture risk factors, and not necessarily be reassured when a patient has a normal BMD.

"We better understand that there are many factors that impact bone 'quality'," says Hospital for Special Surgery (HSS) spinal surgeon Alexander P. Hughes, MD, who served as senior author on the new study. "It turns out that the mineral density measurement of bone alone may be less important than other factors. BMD is a measure of bone quantity, not quality." Among other factors that could increase sacral fracture risk are how the body maintains collagen, a connective tissue that helps maintain bone elasticity.

Dr. Hughes points out that while sacral fractures are rare, impacting less than 1 percent of fusion patients, when they do occur they can have serious consequences.

"Sacral fractures not only mean patients will have to undergo revision surgeries, they also may develop neurologic complications," he explains.

Dr. Hughes explains that previous data supporting a link between spinal surgeries and sacral fractures was purely anecdotal, and that some prior reports suggested that only a small percentage of patients sustaining sacral fractures after fusion surgery had osteoporosis. To test whether or not this is case, he and a team of his HSS colleagues reviewed BMD scores in patients who had experienced sacral fractures following a spinal fusion surgery and those who had not. They also employed an experimental application of qCT that allows an assessment of the BMD of the sacrum itself.  Neither decreased global BMD or sacral BMD was found to differ significantly between the two groups.

The new study is part of a larger research initiative at HSS aimed at better understanding and treating disorders of the bone and other connective tissues. Dr. Hughes feels the findings could have implications for other spinal surgeries and orthopedic procedures as well.

"This could have direct implications on surgical decision making,” he says. “We’re realizing that bone mineral density has its limitations."