Bone mineral content was measured in the central left os calcis and the right distal radius and ulna using a photon absorptiometer. The device used a monoenergetic photon source, the 27.5 keV X-ray of iodine-125 (¹²⁵I), and a sodium iodine crystal scintillation detector. The photon source and the scintillation detector were mounted on a scanner yoke in direct opposition to each other. The source and detector were collimated so that a 3-millimeter beam was similarly viewed by a 3-millimeter entrance collimator on the detector. The yoke, mounted on the scanner, was able to scan a limb placed between the source and detector in a rectilinear raster pattern. The scanner could be configured to accommodate either the upper or lower extremity. The limb to be scanned was placed in tissue equivalent material to compensate for the irregular thickness of tissue cover that surrounds the bone. The foot was placed in a Plexiglas box filled with water and the arm was encased in Superstuff when placed on a platform between uprights.

The most distal 2.0-centimeter portion of the radius and ulna was measured and reported as mean mineral content in grams of ash per centimeter of bone length. The mineral content of the central 2.50-centimeter section of the os calcis was reported in mg/cm² of hydroxyapatite. Mineral content was calculated using the following method. The count rate of the transmitted beam through the tissue and tissue equivalent was designated as Ko*. Transmittance through bone was designated as K. The absorbance through this segment was given as the log of the ratio (Ko*/K). The sum of the values across the bone was proportional to the mineral content in this segment. Sixteen rows were measured.

The entire system was calibrated before and after each subject scan using a standardization method called a Witt-Cameron standard. The reference standard was used so researchers could calibrate scanning systems from different laboratories and more easily compare the results.

Simulation studies conducted in 15 healthy male volunteers showed that prolonged bed rest could result in significant mineral loss in the central os calcis (up to 40 percent). However, little or no os calcis mineral loss was observed during the first month of bed rest. In contrast, the radius and ulna failed to exhibit mineral losses during periods of up to 30 weeks of bed rest. Using the data collected, researchers were able to calculate a prediction term for each subject that could be used to estimate potential mineral loss for any subject whose prediction term had been determined. When the prediction term for each subject was plotted against the mineral losses observed, a series of regression lines was derived. The regression lines were used to estimate potential mineral losses for any subject whose prediction term had been determined. It could be seen that a high prediction term was associated with little os calcis mineral loss and a low prediction term was associated with larger losses.

Having established bone mineral loss profiles for simulated weightlessness on Earth, the photon absorptiometric technique and associated prediction term were applied to subjects A, B, and C of Skylab 2 (28-day mission), subjects D, E, and F of Skylab 3 (59 days), and G, H, and I of Skylab 4 (84-days). Measurements for the prime crews were carried out preflight at about 30, 15, and 5 days before launch. The data consisted of one x-ray and three bone scans. The scans were averaged to provide a baseline value for each astronaut. Identical tests were conducted on recovery day and days 1 and 7, and at variable times thereafter. The crewmembers of Skylab 2 and Skylab 3 were studied until each had returned to baseline. The Skylab 4 crew study was terminated before two of the crewmembers had returned to baseline levels.

A series of control subject measurements was also made in parallel with the crew. Seven subjects were studied during Skylab 2 and Skylab 4, and six subjects during Skylab 3. Data collection consisted of preflight x-ray and bone scans.

RESULTS:
No bone mineral losses were seen in the radius and ulna on any crewmember for any of the missions. No significant bone mineral loss from the os calcis was found in the crewmembers of Skylab 2. Subject F of Skylab 3 showed significant mineral loss in the left central os calcis, but it appeared to be within the predicted limits previously determined by bed rest studies. This subject regained his os calcis mineral by day 87 postflight. Some loss from the os calcis was noted in subject I of Skylab 4 on the first day after recovery. Subject H of Skylab 4 also showed some loss from the os calcis on both the first and fourth day after recovery. The two subjects of Skylab 4 had not returned to preflight levels by day 95 postflight. Bone mineral loss was not noted in the os calcis of subject G.

Of the nine crewmembers, only the prediction terms for four of the Skylab 2 and Skylab 3 crewmembers fell within the limits observed in the bed rested subjects for the os calcis. Bone mineral loss was predicted preflight in subject I of Skylab 3 and observed postflight. The two crewmembers of Skylab 3 who had higher prediction terms did not lose mineral. The Skylab 4 crew had high prediction terms outside the limits set by the bed rest study. Therefore, predictions for this crew based upon the bed rested data was not possible.

The postflight bone mineral findings of the three Skylab crews suggested those bone mineral losses resulting from microgravity mirrored the patterns established from bed rest studies. Researchers concluded that the bed rest study was a closer approximation of the flight situation with regard to bone mineral changes than previously suspected. Furthermore, bone mineral losses observed in the Skylab crews were of no clinical concern and they did not represent a significant hazard for missions of the Skylab visit durations. A prediction term was proposed in an attempt to translate bed rest data into the weightless condition. In general, this was applicable to microgravity in all crewmembers whose prediction term fell within the limits set by the bed rest study.

The os calcis mineral content was measured in the nine Skylab crewmembers and eight control subjects five years after the conclusion of the Skylab Program. The follow-up study utilized a photon absorptiometric technique that used a functionally identical scanner and radiation source to that used during the original experiment. The measurements were compared with preflight measurements in an attempt to discover any long-term effects of space flight on the skeletal system. The mineral content of all but one of the Skylab astronauts was reduced since flight, and the total difference was significant. Even so, the mineral losses observed in the later study were not clinically significant. They did, however, lend support to the hypothesis that space flight had a direct lasting effect on the human skeletal system. Researchers concluded that additional studies would be necessary to substantiate the findings.