Schlaff, Cody D.

The future of space exploration will include a prolonged presence on the Moon, commercial low-orbit spaceflight, and long-range missions to Deep Space, including a prolonged Martian presence. Understanding the effects the space environment will have on astronauts’ musculoskeletal system is mission critical and include both microgravity and space radiation. In spaceflight, crewmembers are exposed to a vast mixture of radiation species and energies including cosmic rays (CR) from galactic cosmic radiation (GCR), solar ejections, and neutrons. Any trip beyond the protection of the Earth’s electromagnetic field will expose astronauts to the near-maximum levels of lifetime allowable radiation exposure. We have previously reviewed how microgravity induces pathophysiological adaptations in the spine and how countermeasure strategies can play a role in minimizing astronaut morbidity. Now, through the National Aeronautics and Space Administration’s (NASA) Human Research Roadmap (HRR), there is a renewed interest in characterizing and mitigating the effects of radiation as astronauts prepare for the Artemis missions and beyond. Thus, our aim in this critical narrative review is to focus on how the second greatest challenge to crewmembers’ health, radiation, and identify how potential countermeasures will affect the spine.

Study Design: Controlled Laboratory Study Objective: To compare multilevel posterior cervical fusion (PCF) constructs stopping at C7, T1, and T2 under cyclic load to determine the range of motion (ROM) between the lowest instrumented level and lowest instrumented-adjacent level (LIV-1). Summary of Background Data: PCF is a mainstay of treatment for various cervical spine conditions. The transition between the flexible cervical spine and rigid thoracic spine can lead to construct failure at the cervicothoracic junction. There is little evidence to determine the most appropriate level at which to stop a multilevel PCF. Methods: Fifteen human cadaveric cervicothoracic spines were randomly assigned to 1 of 3 treatment groups: PCF stopping at C7, T1, or T2. Specimens were tested in their native state, following a simulated PCF, and after cyclic loading. Specimens were loaded in flexion-extension), lateral bending, and axial rotation. Three-dimensional kinematics were recorded to evaluate ROM. Results: The C7 group had greater flexion-extension motion than the T1 and T2 groups following instrumentation (10.17±0.83 degree vs. 2.77±1.66 degree and 1.06±0.55 degree, P <0.001), and after cyclic loading (10.42±2.30 degree vs. 2.47±0.64 degree and 1.99±1.23 degree, P <0.001). There was no significant difference between the T1 and T2 groups. The C7 group had greater lateral bending ROM than both thoracic groups after instrumentation (8.81±3.44 degree vs. 3.51±2.52 degree, P =0.013 and 1.99±1.99 degree, P =0.003) and after cyclic loading. The C7 group had greater axial rotation motion than the thoracic groups (4.46±2.27 degree vs. 1.26±0.69 degree, P =0.010; and 0.73±0.74 degree, P =0.003) following cyclic loading. Conclusion: Motion at the cervicothoracic junction is significantly greater when a multilevel PCF stops at C7 rather than T1 or T2. This is likely attributable to the transition from a flexible cervical spine to a rigid thoracic spine. Although this does not account for in vivo fusion, surgeons should consider extending multilevel PCF constructs to T1 when feasible. Level of Evidence: Not applicable.