Effects of long-term microgravity exposure on cancellous and cortical weight-bearing bones of cosmonauts

Summary

Background

Microgravity has been thought to induce osteoporosis because of reduced weight-bearing. However, up to now, few data have been available about its precise nature and timecourse.

Methods

We measured bone mineral density (BMD) at the distal radius and tibia in 15 cosmonauts of the Russian MIR space station who sojourned in space either 1 (n=two), 2 (two), or 6 months (11). After recovery periods of similar duration to the space missions, BMD was measured for the 2-month and 6-month crews.

Findings

Neither cancellous nor cortical bone of the radius was significantly changed at any of the timepoints. On the contrary, in the weight-bearing tibial site, cancellous BMD loss was already present after the first month and deteriorated with mission duration. In tibial cortices, bone loss was noted after a 2-month flight. In the 6-month group, cortical bone loss was less pronounced than that for cancellous bone. In some individuals, tibial deterioration was great. Actual BMD did not depend on preceding cumulative periods spent in space. During recovery, tibial bone loss persisted, suggesting that the time needed to recover is longer than the mission duration.

Interpretation

In space, despite physical training, bone loss is an adaptive process that can become pathological after recovery on Earth. Striking interindividual variations in bone responses seem to suggest a need for adequate crew preselection. Targeted treatment or prevention strategies would be useful, not only for space purposes, but also for the increasing number of osteoporotic patients on Earth.

Introduction

Various models of physical exercise overloading have been shown to preserve or increase skeletal mass. By contrast, reduction in mechanical use—as in sedentary people or in diseases associated with paralysis and unloading, such as prolonged bed rest—is associated with bone loss and is likely to be a contributory factor in age-related osteopenia. Because gravity seems to be a major constraint, spaceflight has been judged as the ultimate model to determine the role of gravity on the human skeleton.

Reduced mechanical use, because of hypodynamia (decreased forces) and hypokinesia (fewer movements), is thought to be the main factor leading to bone loss in space. Microgravity-induced bone loss has been suggested to be similar to disuse-osteoporosis on Earth, which constitutes a challenging public health problem.

Despite the many cosmonauts who have occupied the Russian space station MIR and its predecessor, Salyut, few data on bone mass are available. Suspected microgravity-induced osteoporosis was noted as early as the first human space flights in the 1970s. Bone loss was detected in the calcaneum, and not in the arm in the three Skylab mission crew members studied,¹ although major interindividual variations were recorded. Subsequent data about bone loss were derived from long-term Soviet/Russian missions.2, 3 Bone loss in the calcaneum was also recorded after missions lasting 75–184 days, and the degree of bone loss was roughly correlated with flight duration. Vertebral bone mineral density (BMD) decreased in four cosmonauts after missions lasting up to 7 months.⁴

Adequate research programmes were set up only 10 years ago to acquire a better knowledge of physiological changes, and to develop appropriate space medicine. Collaborative studies between US and Russian space agencies allowed assessment of cosmonaut bone mass. Vertebral bone loss was initially measured by computed tomography.⁵ In subsequent research work, a dual photon densitometer (DXA, HOLOGIC, Waltham, MA, USA) was used to measure BMD. Greater BMD loss was recorded in the lower part of the body than the upper part, associated with 0·3-0·4% whole body bone mass per month.6, 7 However, a fall in BMD values in the pelvis, femoral neck, and trochanter were reported, with no significant whole body bone loss in cosmonauts who spent 131-312 days on the MIR space station.⁷

Altogether, the data up to now suggest that bone loss is site-specific, and that there is no complete recovery after return to Earth. French and Russian space agencies have collaborated to measure bone mass of cosmonauts, both in cancellous and cortical bone of the radius, a nonweight-bearing bone, and of the tibia, a weight-bearing bone, by means of peripheral quantitative computed tomodensitometry (pQCT). We provide a kinetic analysis of the changes in various bone sites in human beings exposed to microgravity and recovery.