A team of NASA-funded researchers from University of California-San Francisco and Baylor College of Medicine has used a new method to measure and characterize bone loss caused by prolonged spaceflight.

Study results appear in the online version of the Journal of Bone and Mineral Research, which can be viewed at http://www.jbmr-online.org.

The researchers used three-dimensional x-ray computed tomography [CT] to study the effect of prolonged weightlessness on the bone mineral density and structure of the hip in a group of 14 American and Russian crew members making flights of four-to six-month duration on the International Space Station (ISS).

This study is the first to use CT imaging to three-dimensionally quantify spaceflight-related bone loss in the hip and to estimate changes in hip bone strength. Previous studies have used a two-dimensional imaging technology called dual x-ray absorptiometry (DXA).

"Our study demonstrates that bone loss, especially from the central skeleton, occurs in the ISS crewmembers at a rate comparable to that observed almost a decade before in the crew of the Russian MIR spacecraft," said Thomas Lang, PhD, UCSF associate professor of radiology and principal investigator on the study.

"The lack of clear progress in the interval between the MIR and ISS missions indicates a need for continued efforts to improve musculoskeletal conditioning regimens during longer space missions, such as those proposed for the moon and Mars."

He added that pharmacological interventions, such as drug therapies that suppress the accelerated bone resorption which is associated with weightlessness, are being evaluated by other NASA-funded researchers as aids to prevent bone loss during long-duration space travel.

The researchers also analyzed loss of density in vertebrae, or back bones, which, along with the hip, are the skeletal sites associated most with serious osteoporotic fractures in the elderly. The study found that on average, the ISS crew lost vertebral bone at a rate of 0.8-0.9 percent per month, which was consistent with data from earlier long-duration missions.

However, in contrast to a previous study of a small group of Soviet cosmonauts, the researchers did not observe pronounced bone loss in the spiny posterior elements of the vertebrae which protect the spinal cord and provide attachment points for the lower back muscles. The researchers speculate that this may be due to a new resistive exercise device that exercises lower back muscles, thus maintaining mechanical load on these structures.

The CT measurements in the hip were performed pre- and post-flight to measure bone loss in the porous bone in the interior of the hip (the trabecular bone) and in the dense outer shell of the hip bone (the cortex). On average, the ISS crew lost trabecular bone at a rate of 2.2 to 2.7 percent for each month in space and cortical bone at a rate of 1.6 to 1.7 percent per month.

The investigators used information from the CT images to estimate changes in the strength of the hip bone. They found that on average the hip bone strength declined by 2.5 percent for each month of flight. Because the amount of bone loss increases with mission length, said Lang, astronauts on multi-year explorations may face increased risk of fracture upon return to gravity. In addition, those who do not recover the lost bone may be at increased risk of fracture as they age, he added.

Other research has noted that it may be possible to use the measurements of the heel bone (measured by DXA and ultrasound technology) as indicators for how much central skeletal bone astronauts are losing in space. The heel is an appealing measurement site because it is easily accessible, explained Lang. In this study, the research team compared heel measurements to spine and hip measurements in ISS crew members to see if this was the case. Results did not support the use of heel measurements as indicators for what is occurring in the hip and spine, said Lang.

While the study provides new information regarding how bone is lost in space flight, there were several limitations. First, there was a small sample size, a problem inherent to almost all human spaceflight studies. Another limitation was the duration of the missions, which lasted 4-6 months. While this time period is considered a long space flight, it is much shorter than the length of potential missions to Mars. Since it is not known whether bone loss continues at the same rate or reaches a plateau, these data should not be used to predict bone changes over much longer missions.

Co-researchers on the study include: Adrian LeBlanc, PhD, professor of medicine, Baylor College of Medicine; Harlan Evans, PhD, senior scientist, Wyle Laboratories; Ying Lu, PhD, UCSF associate professor of radiology; Harry Genant, MD, UCSF professor emeritus of radiology; and Alice Yu, BS, staff research associate in the UCSF department of radiology. The research was funded by NASA.