RESULTS
This investigation used noninvasive quantitative computed tomography (QCT) scans and finite-
element (FE) structural engineering modeling to compare the pre-flight and post-flight
geometry and strength of the proximal (segment joining the hip) femur of 13 astronauts who
had not taken any medical countermeasures to prevent bone loss during missions (this area of
the femur typically experienced the highest rate of bone loss while in space). FE calculations
determined up to a 2.6% per month decrease in proximal femoral strength while standing, and
up to a 2.0% decrease against falling for crew members exposed to microgravity for 4 to 6
months aboard the ISS.
Over long-duration
missions, the cumulative
effect represented a
significant reduction in
bone strength. The authors
considered estimates of
bone strength by this
method would be more
revealing since complex 3-D
and nonlinear changes in
Bone Mineral Density
(BMD) were taken into
account. Furthermore, QCT
scans showed that while
overall bone mass
appeared recoverable after
1 year back on Earth, the
regained bulk was from volumetric growth and the corticoid bone portion, but the reductions in
trabecular bone mineral density (tBMD) and strength, did not necessarily follow suit, and these
losses could potentially be permanent (Keyak 2009). An affiliated study augmented previous
investigations by Lang (2007) with the first long-term volumetric measurements of human bone
mineral status and bone geometry of the hip and spine of an 8-crew member subset for up to
4.5 years after long-duration spaceflights. Observed time points at landing, 1 year, and 2-4.5
years later showed bone recovery rate and characteristics for the spine and proximal femur in
line with previous findings (Sibonga 2007) in that the intergral bone mineral density (iBMD)
regained near preflight status over the following 2-4.5 years along with net gains in total
volume and trabecular bone for the hip region. However, volumetric density measurements by
QCT showed an overall trabecular bone density average decline to 88% of the preflight value
over the same period, and there were no indications that this quantity of tBMD loss would
eventually be recovered. Researchers also proposed that the persistent deficits in trabecular
bone and bone strength coupled with natural aging osteoporosis may significantly elevate the
risk for astronauts who underwent extended or repeat missions and predispose them to
premature osteoporosis later in life (Carpenter 2010).
Dual-energy X-ray absorptiometry (DEXA) assessment of bone mineral
density of the femoral neck (A) and the lumbar spine. Image courtesy of Dr
Caroline Lebreton, Raymond Poincaré Hospital, Garches, France.