Assessment of hip fracture risk in astronauts exposed to long-term weightlessness

Background. A human exploration mission to Mars could take place within 10 years. During the 6 to 12 month journey astronauts would likely lose bone mineral density (BMD) at a mean rate of 1–2 percent per month in weight-bearing areas, approximately 10 times the rate associated with normal ageing. There exists an important need to quantify the fracture risk associated with this loss. Methods. Using computational modeling, the factor of risk for hip fracture (applied load divided by failure load) was assessed following 0, 6, and 12 months of weightlessness for: (1) the mid-stance phase of gait, and (2) a fall to the side impacting the greater trochanter. Peak applied loading was calculated for Earth and Mars gravity levels using the equations of motion for three-segment models representing locomotion and falls. Mars simulations included extravehicular activity (EVA, with spacesuit) and intravehicular activity (IVA). The structural properties of the femur were analyzed using a three-dimensional finite element model derived from quantitative computed tomography scans of a representative cadaveric femur. Space flight associated changes in density, geometry, and muscle strength were incorporated. Results. Peak applied joint contact force ranges for mid-stance were: 1.2–2.5 kN (Earth), 0.9–1.8 kN (Mars IVA), and 1.5–2.4 kN (Mars EVA). Peak applied joint contact forces for fall impact were: 4.2–8.0 kN (Earth), 2.7–5.1 kN (Mars IVA), and 3.1–5.0 kN (Mars EVA). Femoral strength in mid-stance decreased from 5.9–6.1 kN (0 months) to 5.1–5.4 kN (12 months), while femoral strength in fall impact decreased from 4.2–4.4 kN (0 months) to 3.8–4.0 kN (12 months). Typically, the factor of risk for hip fracture was highest for falls in Earth gravity following 12 months of weightlessness (1.12–2.08), and lowest for IVA locomotion in Mars gravity (0.26–0.49). All fall conditions yielded a high likelihood of fracture. Astronauts are advised to take precautions against falling following long duration space flight and could benefit from the temporary use of hip pads. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)