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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.
