Hypogravitational Osteoporosis: A review of literature.
By Lambert Titus Parker. May 19 1987.
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
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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
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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
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& 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
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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.
