The purpose of this experiment was to determine the effect of microgravity on the skeletal system during space flight. Studies conducted during the Gemini 4, 5, and 7, and Apollo 7 and 8 missions used x-ray densitometry to measure the amount of bone mineral present in the os calcis, radius, ulna, and phalanges. The studies revealed significant bone mineral losses in all areas measured. The degree of loss appeared excessive for such short periods in microgravity. Further evaluation of the data determined that bone mineral loses were lower than what was first measured. The overestimates were primarily due to problems inherent in the measurement techniques used. Scientists concluded that x-ray densitometry was unreliable for calculating bone mineral losses. It was not until a much more stable method of measuring bone mineral content, called photon absorptiometry, was developed that bone mineral studies continued during Apollo 14, 15, and 16.
Early reports of significant bone mineral losses in the 5- to 14-day Gemini and Apollo missions emphasized the need for correlating mineral losses observed in ground-based studies with those observed during microgravity. Ground-based studies designed to mimic the microgravity condition were used to estimate when changes were likely to occur during a mission. Bed rest served as an experimental model since the loss of bone mineral in bed rested and immobilized patients had been observed during periods of up to 36 weeks.
It was determined during these studies that the amount of initial mineral content in the os calcis could influence the rate of mineral loss. Bed rested subjects who exhibited a high mineral content at the beginning of bed rest showed the least mineral loss both in percentage and in quantity. Conversely, subjects who exhibited a low mineral content at the onset of bed rest showed a greater rate of mineral loss. The bed rest model was also used to determine what remedial measures might be used to both reduce bone mineral loss and predict bone mineral losses in the astronauts.
Thus, another goal of this study was to establish time courses for both the bed rest and flight situations so that better estimates could be obtained on the risk of prolonged space flight as translated from the ground-based bed rest studies.
A photon scanning device, called a rectilinear bone mineral scanner, was used during the experiment. The extremity to be measured was placed in tissue equivalent material (TEM) to compensate for the irregular thickness of tissue cover that surrounded the bone. The extremity, encased in the TEM, was placed between the photon source, Iodine-125 (125I), and a photon detector. The photon source and detector were aligned in direct opposition to each other so that a 3-millimeter source beam was directed into a 3-millimeter entrance on the detector. The absorptiometer scanned the section of bone in a series of sixteen straight, parallel lines covering the entire area. The scanner could be configured to accommodate either the upper or lower extremity. As the transmitted beam passed through the segment of bone, the detector on the opposite side measured the intensity of the radiation after it had passed through the TEM and the bone. Researchers were then able to calculate the amount of mineral content in the bone segment. The entire system was calibrated before and after each scan by making four passes over a Witt-Cameron standard.
Because of space restrictions in the quarantine facility and isolation restrictions, only a single scanner device could be used during the postflight period of Apollo 14. For this reason, arm and heel scans were performed separately using the same scanner in each of the two configurations. On the two subsequent missions, arm and heel studies were performed simultaneously both preflight and postflight, because quarantine was no longer required.
Mineral losses were not observed in the os calcis, radius, or ulna in the Apollo 14 or Apollo 16 crewmembers. Only two Apollo 15 crewmembers lost mineral from the os calcis. Both crewmembers regained the bone mineral losses and were near baseline values by the end of two weeks. The magnitude of loss, when compared to the control subjects, was 5 to 6 percent. Mineral losses were not observed in radius or ulna mineral during the Apollo 15 flight.
A significant increase in fatty tissue was observed on the plantar side of the os calcis in one subject of Apollo 14 immediately postflight. This subject showed a 34 percent increase in fat equivalence when compared to immediate preflight measurements. This increase would have resulted in a 4.3 percent overestimation of bone mineral if the soft tissue contribution had not been measured. In contrast, the other two crewmembers showed between a 8.1 and 8.4 percent increase in fat equivalence with a potential 2.5 and 2.2 percent overestimation in bone mineral. No significant changes in the soft tissue composition were observed in the Apollo 15 or 16 crewmembers.
Researchers established a predictive model for bone mineral loss for the bed rested subjects. In this model, the ratio of initial bone mineral content to the initial 24-hour urinary hydroxyproline excretion was compared to observed losses. The greater the ratio, the slower and smaller the mineral losses and, conversely, the smaller the ratio, the faster and greater the mineral losses. The accurate measurement of baseline 24-hour urinary hydroxyproline excretion was therefore an essential requirement for these predictions. Unfortunately, no time course could be established for the microgravity state because of limited data. Researchers hoped that additional data collected during the Skylab missions would determine a time course of bone mineral losses that could be compared with the bed rest data. Therefore, bed rest studies could be established as a good model for the effects of weight unloading. Secondly, bed rest studies could serve as a predictive measure for longer space missions.
It was concluded that mineral losses from bone incident to periods of microgravity were comparable to that observed in bed rested subjects and that the magnitude was not severe. If these losses were allowed to continue unabated for a prolonged period of time, the consequences might be more serious because the losses were probably not confined to the bones described.