Bone mineral measurement: Skylab experiment M-078

Area of Science:

• Space medicine
• Bone physiology
• Biomedical engineering

Background:

• Immobilization and bed rest are known to cause bone mineral loss.
• Early space missions (Gemini) suggested significant bone mineral loss during short weightless periods.
• Concerns about bone loss in astronauts necessitated more precise measurement techniques.

Purpose of the Study:

• To assess bone mineral changes during prolonged weightlessness.
• To compare bone mineral loss in bed-rest studies with actual spaceflight data.
• To develop predictive models for bone mineral loss in astronauts.

Main Methods:

• Utilized prolonged bed-rest studies (30-36 weeks) with calcium balance and bone mineral absorptiometry.
• Employed a monoenergetic photon source (125I) and scintillation detector for precise bone mineral measurement in radius, ulna, and os calcis.
• Applied the same absorptiometric technique to Apollo 14, 15, and 16 astronauts and Skylab missions.

Main Results:

• Bed-rest studies showed variable bone mineral losses, with significant loss in the os calcis after 9 months but minimal loss in radius and ulna.
• Spaceflight data (Apollo, Skylab) generally aligned with bed-rest findings, showing minimal or no significant bone mineral loss in most astronauts.
• Bone loss variability was correlated with initial urinary hydroxyproline excretion and os calcis mineral content, enabling prediction.

Conclusions:

• Bone mineral loss during weightlessness is variable and can be predicted based on individual physiological markers.
• Current data suggest less severe bone mineral loss in spaceflight than initially feared from early Gemini missions.
• Predictive models derived from bed-rest studies are relevant for assessing bone health in astronauts on longer missions.