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

Abstract

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.