==============================================================================
 Your Bones in Space                            ASTRONOMY AND SPACE SCIENCE
 ------------------------------------------------------------------------------
 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
 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.
 
 
 