Hypogravitational Osteoporosis Essay, Research Paper

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.