Hypogravitational Osteoporosis

Osteoporosis is a condition characterized by a decreased
bone mass, involving loss of both mineral and protein
matrix components to a level below the amount which is
necessary to maintain the structural integrity of the
skeleton. To state the obvious, Human beings have evolved
under Earth's gravity "1G". Our musculoskeleton system has
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. 

In addition to 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 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 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. For example; 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.
Both types will have the right amount to support his/her
structure against the mechanical stresses in which he/she
exists. Hormones also play a major role as seen in
postmenopausal females osteoporosis (lack of estrogen) in
which the rate of bone reformation is usually normal with
the rate of bone re-absorption increased. 

In Skeletal systems 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 Life Science 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 and
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's
motion is resisted by springs and elastic bands inducing
stress and strain on muscles and skeletal system] for 6 to
8 hours a day, necessary to achieve the desired effect, are
cumbersome and require significant workload. This reduces
efficiency, thereby making their wear 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-thirds 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 had been resistant and distrustful
of medical experiments but with scientific personnel with
life science training, we should be able to obtain
significant 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 locomotion consists mostly of swimming
actions with stress being exerted on upper extremities
rather than lower limbs. This results in a reduction of
weight bearing bones of lower extremities and spine as they
are NOT needed for maintaining the structural integrity of
the skeleton. Therefore, in microgravity, the skeletal
system adapts in a marvelous manner and a problem arises
only when this microgravity adapted person needs to return
to a higher gravitational field. Therefore, 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 that 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 an animal
centrifuge simulating 2G and compared it with a litter
raised in 1G. "The 2G group was Herculean in their build,
and unusually strong...." reported Dr. Graveline. Also
X-ray studies showed that the 2G mice had a far greater
skeletal density than their 1G litter mates.