Astronauts show higher hip fracture rates after spaceflights longer than 90 days

New research has found that the rate of hip fractures in astronauts back on Earth was greater in the period after an astronaut had spent more than 90 days in space than in the period following shorter spaceflights, or in non-astronauts. The findings, reported in Mayo Clinic Proceedings, published by Elsevier, point to the need for expanded assessment of long-term skeletal health in astronauts.

NASA is planning multiple missions lasting longer than six months in space, including commercial low Earth orbit activity, Artemis IV's first human mission to the lunar south pole in 2028, and sustainable surface capabilities for the Moon in preparation for Mars. More human participation and more time spent in space exploration could harm the long-term skeletal health of astronauts, who are selected for their physical fitness and optimal health.

Muscle and bone loss caused by the microgravity environment of space is a common physiological phenomenon among astronauts, and suggests a risk of fragility arising from an adaptive response to disuse.

Jean D. Sibonga, PhD, of NASA's Johnson Space Center, lead investigator of the study "Increased Rates of Hip Fractures Associated With Longer Spaceflight Durations", noted that humans do not feel the effects of bone loss. After returning from space and feeling stable or recovered, astronauts may resume physically demanding activities despite underlying skeletal changes caused by prolonged disuse. A better understanding of bone health after spaceflight could help astronauts, patients and clinicians assess long-term health concerns, determine appropriate care and reduce the risk of injury.

Matthew T. Drake, MD, PhD, of the Endocrinology and Metabolic Bone Disease Service at the Hospital for Special Surgery in New York, co-author of the accompanying editorial "Bone Loss in Astronauts", said: "While data to date on bone loss associated with space flight are consistent, a major limitation is accuracy, as astronaut cohort sample sizes are small and spaceflights are rare."

To overcome that statistical limitation, the investigators surveyed a cohort of US-based astronauts for all fractures reported during annual clinical exams and analyzed the data using Bayesian probabilistic modeling. They did not find a higher incidence rate for all fractures, but the incidence rate for hip fractures in astronauts was increased after a long-duration spaceflight of more than 90 days.

The increased rate occurred at a younger age than would be expected in a non-astronaut population on Earth, and the hip fracture rate was greater than the rate before a long-duration mission or with no spaceflight exposure at all.

"Taken together, the finding of hip fracture occurrence at an age earlier than expected after long-duration spaceflight is clinical evidence that spaceflight-induced bone loss can lead to real long-term fracture consequences, which should be addressed accordingly," said Moshe Gertzulin, MD, of the Endocrinology and Metabolic Bone Disease Service at the Hospital for Special Surgery, New York, co-author of the accompanying editorial.

Sibonga emphasized the need to expand NASA's current bone surveillance program beyond dual-energy X-ray absorptiometry measurements of bone mineral density to better evaluate astronauts' long-term fracture risk. NASA has recently begun ordering quantitative computed tomography scans before and after spaceflight, which can provide more detailed information about different areas of the bone and whether astronauts have fully recovered. Without that insight, astronauts could risk placing too much strain on bones that have not yet regained their preflight condition.

To reduce these risks, the investigators advocate prophylactic countermeasures that would preserve an astronaut's baseline, pre-flight skeletal health. A fuller characterization of spaceflight-induced changes to bone mass, density, structure and microarchitecture will determine which interventions - whether drugs, exercise or dietary adjustments - are most beneficial for long-term skeletal health in astronauts.

CONTACT: Eileen Leahy, Elsevier, [email protected]