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Astronomy And Space Science: Your Bones in Space
SIG
Hypogravitational Osteoporosis: A review of literature. By Lambert Titus
Parker. May 19 1987. (GEnie Spaceport)
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. -eg; 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 3.2% 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.
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