Osteoporosis: a condition characterized by an absolute decrease in the amount of bone present to a level below which it is capable of maintaining the structural integrity of the skeleton. To state the obvious, Human beings have evolved under Earth’s gravity “1G”. Our musculoskeleton system have developed to help us navigate in this gravitational field, endowed with ability to adapt as needed under various stress, strains and available energy requirement.
The system consists of Bone a highly specialized and dynamic supporting tissue which provides the vertebrates its rigid infrastructure. It consists of specialized connective tissue cells called osteocytes and a matrix consisting of organic fibers held together by an organic cement which gives bone its tenacity, elasticity and its resilience. It also has an inorganic component located in the cement between the fibers consisting of calcium phosphate [85%]; Calcium carbonate [10%] ; others [5%] which give it the hardness and rigidity.
Other than providing the rigid infrastructure, it protects vital organs like the brain], serves as a complex lever system, acts as a storage area for calcium which is vital for human metabolism, houses the bone marrow within its mid cavity and to top it all it is capable of changing its architecture and mass in response to outside and inner stress. It is this dynamic remodeling of bone which is of primary interest in microgravity.
To feel the impact of this dynamicity it should be noted that a bone remodeling unit [a coupled phenomena of bone reabsorption and bone formation] is initiated and another finished about every ten seconds in a healthy adult. This dynamic system responds to mechanical stress or lack of it by increasing the bone mass/density or decreasing it as per the demand on the system. g; a person dealing with increased mechanical stress will respond with increased mass / density of the bone and a person who leads a sedentary life will have decreased mass/density of bone but the right amount to support his structure against the mechanical stresses she/she exists in. Hormones also play a major role as seen in postmenopausal females osteoporosis (lack of estrogens) in which the rate of bone reformation is usually normal with the rate of bone re-absorption increased.
In Skeletal system whose mass represent a dynamic homeostasis in 1g weight- bearing,when placed in microgravity for any extended period of time requiring practically no weight bearing, the regulatory system of bone/calcium reacts by decreasing its mass. After all, why carry all that extra mass and use all that energy to maintain what is not needed? Logically the greatest loss -demineralization- occurs in the weight bearing bones of the leg [Os Calcis] and spine. Bone loss has been estimated by calcium-balance studies and excretion studies.
An increased urinary excretion of calcium , hydroxyproline & phosphorus has been noted in the first 8 to 10 days of microgravity suggestive of increased bone re-absorption. Rapid increase of urinary calcium has been noted after takeoff with a plateau reached by day 30. In contrast, there was a steady increase off mean fecal calcium throughout the stay in microgravity and was not reduced until day 20 of return to 1 G while urinary calcium content usually returned to preflight level by day 10 of return to 1G.
There is also significant evidence derived primarily from rodent studies that seem to suggest decreased bone formation as a factor in hypogravitational osteoporosis. Boy Frame,M. D a member of NASA’s LifeScience Advisory Committee [LSAC] postulated that “the initial pathologic event after the astronauts enter zero gravity occurs in the bone itself, and that changes in mineral homeostasis and the calcitropic hormones are secondary to this. It appears that zero gravity in some ways stimulate bone re-absorption, possibly through altered bioelectrical fields or altered distribution of tension and pressure on bone cells themselves.
It is possible that gravitational and muscular strains on the skeletal system cause friction between bone crystals which creates bioelectrical fields. This bioelectrical effect in some way may stimulate bone cells and affect bone remodeling. ” In the early missions, X-ray densitometry was used to measure the weight-bearing bones pre & post flight. In the later Apollo, Skylab and Spacelab missions Photon absorptiometry (a more sensitive indicator of bone mineral content) was utilized.
The results of these studies indicated that bone mass [mineral content] was in the range of 3. to 8% on flight longer than two weeks and varying directly with the length of the stay in microgravity. The accuracy of these measurements have been questioned since the margin of error for these measurements is 3 to 7% a range being close to the estimated bone loss. Whatever the mechanism of Hypogravitational Osteoporosis, it is one of the more serious biomedical hazard of prolonged stay in microgravity. Many forms of weight loading exercises have been tried by the astronauts & cosmonauts to reduce the space related osteoporosis.
Although isometric exercises have not been effective, use of Bungee space suit have shown some results. However use of Bungee space suit [made in such a way that everybody motion is resisted by springs and elastic bands inducing stress and strain on muscles and skeletal system] for 6 to 8 hrs a day necessary to achieve the desired effect are cumbersome and require significant workload and reduces efficiency thereby impractical for long term use other than proving a theoretical principle in preventing hypogravitational osteoporosis.
Skylab experience has shown us that in spite of space related osteoporosis humans can function in microgravity for six to nine months and return to earth’s gravity. However since adults may rebuild only two-third of the skeletal mass lost, even 0. 3 % of calcium loss per month though small in relation to the total skeletal mass becomes significant when Mars mission of 18 months is contemplated. Since adults may rebuild only two-thirds of the skeletal mass lost in microgravity, even short durations can cause additive effects.
This problem becomes even greater in females who are already prone to hormonal osteoporosis on Earth. So far several studies are under way with no significant results. Much study has yet to be done and multiple experiments were scheduled on the Spacelab Life Science [SLS] shuttle missions prior to the Challenger tragedy. Members of LSAC had recommended that bone biopsies need to be performed for essential studies of bone histomorphometric changes to understand hypogravitational osteoporosis.
In the past, astronauts with the Right Stuff had been resistant and distrustful of medical experiments but with scientific personnel with life science training we should be able to obtain valid hard data. [It is of interest that in the SLS mission, two of the mission specialists were to have been physicians, one physiologist and one veterinarian. ] After all is said, the problem is easily resolved by creation of artificial gravity in rotating structures. However if the structure is not large enough the problem of Coriolis effect must be faced.
To put the problem of space related osteoporosis in perspective we should review our definition of Osteoporosis: a condition characterized by an absolute decrease in the amount of bone present to a level below which it is capable of maintaining the structural integrity of the skeleton. In microgravity where locomotion consists mostly of swimming actions with stress being exerted on upper extremities than lower limbs resulting in reduction of weight bearing bones of lower extremities and spine which are NOT needed for maintaining the structural integrity of the skeleton.
So in microgravity the skeletal system adapts in a marvelous manner and problem arises only when this microgravity adapted person need to return to higher gravitational field. So the problem is really a problem of re-adaptation to Earth’s gravity. To the groups wanting to justify space related research: Medical expense due to osteoporosis in elderly women is close to 4 billion dollars a year and significant work in this field alone could justify all space life science work.
It is the opinion of many the problem of osteoporosis on earth and hypogravity will be solved or contained, and once large rotating structures are built the problem will become academic. For completeness sake: Dr. Graveline, at the School of Aerospace Medicine, raised a litter of mice on a animal centrifuge simulating 2G and compared them with a litter mates raised in 1G. “They were Herculean in their build, and unusually strong…. ” reported Dr. Graveline. Also X-ray studies showed the 2G mice to have a skeletal density to be far greater than their 1G litter mates.