Hip load capacity cut-points for Astronaut Skeletal Health NASA Finite Element Strength Task Group Recommendations

Abstract

Concerns raised at a 2010 Bone Summit held for National Aeronautics and Space Administration Johnson Space Center led experts in finite element (FE) modeling for hip fracture prediction to propose including hip load capacity in the standards for astronaut skeletal health. The current standards for bone are based upon areal bone mineral density (aBMD) measurements by dual X-ray absorptiometry (DXA) and an adaptation of aBMD cut-points for fragility fractures. Task Group members recommended (i) a minimum permissible outcome limit (POL) for post-mission hip bone load capacity, (ii) use of FE hip load capacity to further screen applicants to astronaut corps, (iii) a minimum pre-flight standard for a second long-duration mission, and (iv) a method for assessing which post-mission physical activities might increase an astronaut's risk for fracture after return. QCT-FE models of eight astronaut were analyzed using nonlinear single-limb stance (NLS) and posterolateral fall (NLF) loading configurations. QCT data from the Age Gene/Environment Susceptibility (AGES) Reykjavik cohort and the Rochester Epidemiology Project were analyzed using identical modeling procedures. The 75^th percentile of NLS hip load capacity for fractured elderly males of the AGES cohort (9537N) was selected as a post-mission POL. The NLF model, in combination with a Probabilistic Risk Assessment tool, was used to assess the likelihood of exceeding the hip load capacity during post-flight activities. There was no recommendation to replace the current DXA-based standards. However, FE estimation of hip load capacity appeared more meaningful for younger, physically active astronauts and was recommended to supplement aBMD cut-points.

Fig. 1

a Current pre-flight standard operating procedure for skeletal assessment. b Current post-flight standard operating procedure for skeletal assessment. R refers to time point for return from mission

Fig. 2

a Nonlinear stance subject data with age and FE load capacity, the dashed line represents the 75^th percentile FE load capacity cut-point at 9537N. b Nonlinear fall population data with age and FE load capacity, the dashed line represents the 75^th percentile FE load capacity cut-point at 3664N. The 75^th percentile is defined from the AGES male fracture cohort. Astronaut data are represented at a single time point to maintain non-identifiable data

Fig. 3

a Correlation data for the Mayo cohort between DXA T-score and NLS FE outcomes. R² = 0.77. b Correlation data for the astronaut cohort between DXA T-score and NLS FE outcomes. Pre- and post-flight data are analyzed as a combined set. R² = 0.13. c Correlation data for the Mayo cohort between DXA T-score and NLF FE outcomes. R² = 0.63. d Correlation data for the astronaut cohort between DXA T-score and NLF FE outcomes. Pre- and post-flight data are analyzed as a combined set. R² = 0.43

Fig. 4

a Nonlinear stance clinical chart with T-score and FE load capacity. The vertical dashed line represents the DXA clinical standard at a T-score of −1.0 and the horizontal dashed line represents the 75^th percentile FE load capacity cut-point at 9537N. b Nonlinear fall clinical chart with T-score and FE load capacity. The vertical dashed line represents the DXA clinical standard at a T-score of −1.0 and the horizontal dashed line represents the 75^th percentile FE load capacity cut-point at 3664N

Fig. 5

Application of the PRA model to determine fracture risk. Each line reflects the distribution of loads for a specific mission with a different exposure to gravity (e.g., ISS, Moon, Mars, and return to Earth). Probabilities are based on an assessed NLF bone load capacity equivalent to the NLF cut-point of 3664N. a The distribution of loads expected to be applied to the hip for different missions for low-energy events (loads between 1–4 kN). The probability of experiencing an overloading event after return to Earth is 0.13 (e.g., tripping and falling). b Assessment of probability of fracture using the NLF load capacity cut-point after a long-duration spaceflight mission and return to Earth for high-energy events (e.g., collision while playing football). The probability of experiencing an overloading event is 0.67

Fig. 6

a Recommended standard operating procedure to integrate QCT-FE into current pre-flight monitoring standard operating procedure. QCT-FE is to be used for a biomechanical assessment of hip integrity. b Recommended standard operating procedure to integrate QCT-FE into current post-flight monitoring standard operating procedure. QCT-FE is to be used as a comprehensive index for monitoring post-flight skeletal health recovery