(D-143) Effects of Long-Duration Spaceflight on Lumbar Vertebral Strength

Introduction::

Exposure to microgravity throughout long-duration spaceflight induces musculoskeletal changes including a loss of bone mineral density (BMD). Computed tomography (CT) images of astronauts on long-duration spaceflights can quantify changes in vertebral morphology and BMD pre- to post-flight. Together, these spinal changes can lead to an increased risk of injury (e.g., vertebral fracture, disc herniation), especially during dynamic load events such as launching and landing. Compared to terrestrial controls, there is a 36 times higher risk of disk herniation among astronauts, motivating the interest in researching the changes in vertebral geometry as a partial explanation for the observed increase in spinal injury risk. Finite element (FE) models, such as the average male Global Human Body Models Consortium (GHBMC) model have been developed and the spinal response has been validated with post mortem human subject data. To understand the effects of the localized changes in vertebral geometry and BMD, subject-specific FE models at pre-flight and post-flight timepoints are needed. The goal of this study is to develop subject-specific FE models of astronaut lumbar vertebrae to quantify pre- to post-flight changes in vertebral compressive strength.

Materials and Methods:: The base FE vertebral models used for the morphing were sourced from the GHBMC 50^th percentile male detailed occupant model M50-O (v6.0). The target geometries of pre- and post-flight L1 vertebrae were manually segmented from pre-flight and post-flight quantitative CT images obtained from nine astronauts who underwent long-duration (6+ months) spaceflight missions. Volumetric BMD (mg/cm^3) was measured with in-house method which calculates the L1 trabecular BMD values using the pixel intensities from the astronauts’ quantitative CT and a bone phantom with rods of known Hounsfield Unit values imaged in the CT for calibration. The GHBMC vertebra was morphed to each target subject-specific geometry using the open-source Advanced Normalization Tools to create the subject-specific L1 FE model at the pre-flight and post-flight timepoints (Figure 1). Bone strength of each subject-specific (pre- and post-flight) L1 vertebra was estimated through a quasi-static uniaxial compression test in the LS-Dyna FE modeling solver. A simulated impactor compressed the superior endplate of each vertebral body at a rate of 0.15mm/ms for 30ms shown in Figure 2. For each vertebra, the evaluated compressive strength was calculated as the total reaction force generated at 0.61% effective plastic strain.

Results, Conclusions, and Discussions:: Model morphing results indicated the surface deviation among developed subject-specific vertebrae had a root mean square (RMS) of 0.248mm with a standard deviation of 0.028mm compared to the ground truth segmentation from the CT. From the simulations, the average pre-flight resultant force measured to 9.98kN with a standard deviation of 1.69kN. The mean postflight vertebral strength was 9.63kN with a standard deviation of 1.39kN. A paired, one tailed t test between flight state and vertebral strength derived the t stat 0.87 and p value 0.20. Seven out of the nine subjects experienced a decrease in vertebral strength when under the 0.61% effective plastic strain. Two thirds of the subjects experienced an increased BMD in the lumbar vertebrae with an average increase of 5.25%. The percent change in vertebral strength along with the percent change in BMD from pre- to post-flight is shown in Figure 3. However, the increase in BMD showed no effect on the decrease in vertebrae strength. The RMS of the morphed subject-specific vertebrae yields minimal variance in vertebral geometry allowing for an accurate measurement of vertebral strength The decreased resultant force indicates a possible increase in risk of fracture and injury following dynamic spaceflight events (e.g., landings upon return to Earth). We saw a loss of 2.75% in vertebral strength with long-duration spaceflight with a variability of -25.68-15.38% across the nine astronauts studied. The correlation between flight state and decreased vertebral strength can be explained by the extended lack of gravitational force leading the body to perceive it does not require as much strength. The outcome of this study demonstrates the differences in bone strength within subject-specific models. The importance of developing FE models with the time-specific geometry of the subject before and after long-duration spaceflight to predict injury risk is also highlighted. Future studies will create and run simulations on cervical and thoracic vertebrae and full body models with the entire subject-specific spines, which will highlight differences in injury risk for astronauts pre- and post-flight.

Acknowledgements (Optional): : This study was funded by a NASA Human Research Program grant (NNX16AP89G) and NSF REU Site award #1950281.

References (Optional): : Sibonga, J., et al. National Aeronautics and Space Administration. 2017.  Johnston SL, et al. Aviat Space Environ med 81:566-574. 2010.