PARALYZED PATIENTS HAVE WEAKER BONES, HIGHER RISK OF FRACTURES THAN EXPECTED
People paralyzed by
spinal cord injuries lose mechanical strength in their leg bones faster, and
more significantly, than previously believed, putting them at greater risk for
fractures from minor stresses, according to a new study by a research team at
Worcester Polytechnic Institute (WPI).
The results suggest
that physicians need to begin therapies for spinal cord injury patients sooner
to maintain bone mass and strength. The data also serve as a warning to
physicians treating patients with osteoporosis to think beyond the standard
bone density test when assessing risks of hip and other fractures. Details of
the study are reported in the paper "Reduction in Proximal Femoral
Strength in Patients With Acute Spinal Cord Injury" published by the Journal
of Bone and Mineral Research.
"It's not just a
question of how much bone mass is lost, but where that loss is occurring,"
said Karen Troy, PhD, assistant professor of biomedical engineering at WPI and
senior author of the paper. "We found that bone loss occurred sooner in
mechanically important areas and significantly increased the risk of
fracture."
Estimates of the
number of Americans living with spinal cord injuries range from 300,000,
according to the National Spinal Cord Injury Statistical Center, to 1.2 million
in a study funded by the Christopher and Dana Reeve Foundation. Both sources
agree that the leading causes of these injuries are auto accidents, workplace
accidents, falls, sports injuries, and violent crime.
Healthy bones adapt to
the mechanical forces they encounter, with new bone formation and existing bone
resorption constantly occurring to meet the body's needs. When bones stop
carrying loads, however, they begin to lose mass and weaken. For patients with
spinal cord injuries, this dramatic decline in bone strength often causes
broken legs or knees from otherwise minor impact or stresses. "Their bones
are so fragile, that just the act of rolling over in bed can snap their knee or
leg," Troy said.
In the current study,
Troy and her co-authors, W. Brent Edwards at the University of Calgary and
Thomas Schnitzer at Northwestern University, captured and analyzed data from 13
spinal cord injury patients treated at the Rehabilitation Institute of Chicago.
Each patient received two standard scans (a DXA bone mineral density scan and a
CT scan) of their leg bones at specified time intervals for nearly four months
after the original injury. The scans documented the change in bone mass over
time. The team then used sophisticated computer modeling systems to process the
scan data and simulate how the amount and distribution of bone loss would
affect the ability to sustain mechanical loads and movements.
That analysis showed
that patients in the study lost 2 percent of their leg bone mass each month,
yet that correlated with a 6.9 percent loss in leg bone strength. "In just
3.5 months, reductions in strength for some patients were on the order of that
predicted for lifetime declines owing to aging," the authors wrote.
Starting therapies
early to maintain bone mass and strength is important for these patients, Troy
noted, not only to prevent injuries but also to keep patients eligible for
treatments and technologies now in development. "In ten or fifteen years,
with advances in tissue regeneration to repair the spinal cord, and exoskeleton
assist devices, many of these people will have the opportunity to get back on
their feet, if their bones are strong enough to carry the load," Troy
said. "It's very difficult to restore bone mass once it's lost, so the
better approach is to prevent the loss in the first place."
Bone loss and the risk
of fracture is also a serious concern for the 54 million people in the United
States who have low bone density or osteoporosis.1 The three-fold difference
observed in bone mechanical strength versus density in the current study should
prompt additional review of how physicians assess risk and treat patients with
this condition. "Bone mineral density is important, but it doesn't tell
the whole story," Troy said.
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