Gap Osteo 1: A new acceptable bone health standard using an expanded surrogate for bone health needs to be defined for the flight environment.
Last Published:  07/30/21 01:05:31 PM (Central)
Responsible Element: Human Health Countermeasures (HHC)
Status: Open
Description
Initial State:
 

The current NASA-STD-3001 Volume I standards for Bone are based on clinical guidelines for diagnosing osteoporosis in perimenopausal and postmenopausal women and in men older than 50 years of age, as developed by World Health Organization [WHO] and International Society of Clinical Densitometry [ISCD]. These guidelines are based upon the measurement of areal Bone Mineral Density (aBMD) by the Dual-energy X-ray absorptiometry (DXA) modality. These medical standards for bone health are used to qualify applicants to astronaut candidacy, to certify an astronaut for spaceflight, and to establish a non-permissible outcome for missions; it is this last application of standards that drives the required efficacy of countermeasures and could disqualify an astronaut from a specific spaceflight mission. In June 2010, an expert panel composed of policy-makers in the ISCD and the WHO and practitioners in the field of osteoporosis was convened (“Bone Summit”) to review NASA’s assessment of skeletal integrity in astronauts. This panel stated that the aBMD-based guidelines were not the appropriate standards for decision-making at NASA because the astronaut cohort is not a patient population at risk for primary osteoporosis and fragility fractures.  Because of the novel adaptation of bones to spaceflight, the panel also suggested that Finite Element (FE) Modeling be explored.  FE modeling is being investigated in terrestrial clinical studies for its accurate estimation of bone strength which might serve as an additional index for bone health in astronauts especially after spaceflight missions. In addition, finite element (FE) estimates of hip strength (for given loading orientations) supports a biomechanical approach to assessing the probability of overloading the hip – as a surrogate for fracture outcomes.
Recommendations from Bone Summit:

  • Standards should focus on bone failure loads, i.e., an estimation of bone strength rather than a measured surrogate for bone strength.
  • Individualized assessments of bone strength should be pursued to support clinical decisions because of the atypical constraints in the space program (e.g., limited knowledge base, low subject numbers, aggressive time schedules). 
  • NASA should explore the evidence from multiple population studies that use FE Modeling to estimate bone strength of the hip. (Note: A hip fracture is rated by the Integrated Medical Model to have the greatest consequence – i.e., possible evacuation and loss of mission objectives. The hip is also a common site for age-related, osteoporotic fractures.)   
  • FE bone strength data from population studies with fracture outcome should be investigated to generate evidence-based cut-points to set boundaries for acceptable ranges of bone strength required for astronaut applicant screening, astronaut flight certification and a permissible outcome following spaceflight.
  • Similar approaches may be pursued for the vertebral bodies.  
  • If current aBMD-based standards are not modified, then NASA runs the risk of underestimating the impact of spaceflight as a risk factor for fractures.


Interim Stages/Metrics (Sequential):

1)  Examine Finite Element [FE] modeling data from the Mr. Os, Rochester Bone Health Study, AGES, and ISS astronauts> and propose method to derive candidate bone medical standards for astronauts (10%). FE Task Group I

2a)  Develop a dataset of FE estimated hip strengths from quantitative computed tomography [QCT] scans of aging populations from which to determine FE strength cut-points for clinical and operational decision-making tool (65%). Dataset shall to be produced with one consistent FE modeling method and QCT scanners shall be cross-calibrated.

2b) The FE Task Group II produces a recommended set of FE strength cut-points (20%).

3) Obtain approval for recommendations to update NASA-STD-3001 Volume 1 (5%) following review by Bone Research & Clinical Advisory panel.

Approach: 

 

Note:  The variability of aBMD data from the spine (both astronaut and general population data) precluded the Bone Summit Panel from making any recommendation for a surveillance method outside of using the DXA modality. Still, evaluating the changes to spinal/vertebral integrity is of great clinical concern and innovative capabilities to improve the assessment of spinal/vertebral integrity should be investigated (Gap Osteo3).


Target for Closure

Identified cut-points of hip bone strength, estimated by Finite Element Modeling [FEM], that represent operating bands for skeletal integrity: a) FEM-based bone strength  that denotes a minimum permissible outcome limit for hip bone integrity (kilo-Newton bone strength) after all spaceflight exposures and b) FEM-based bone strengths following 12- and 36-month spaceflights as determined by a physiologically-relevant FE trend model for skeletal unloading.

Mappings
Risk Risk of Bone Fracture due to Spaceflight-induced Changes to Bone
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Multi-Disciplinary Research Plans

Documentation:
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