Hypogravitational Osteoporosis

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