The objective of this study was to track individual skeletal integrity (bone loss and recovery) particularly with regard to prolonged disuse, experienced by subjects participating in long duration bed rest studies.
Bone Densitometry was performed using a Hologic Discovery whole body densitometer located at the bed rest facility at the University of Texas Medical Branch (UTMB). This device employed the technique of Dual-energy X-Ray Absorptiometry (DXA). DXA was capable of distinguishing between hard tissue (bone) and soft tissue. Soft tissue was distinguished further as either lean tissue or fat. Subjects participated in scans of the following regions: whole body, lumbar spine, proximal femurs (hips), calcaneus (heel), and forarm (wrist). This protocol mirrors that of Medical Requirement 035L. The subjects were scanned in triplicate for all the sessions and total time (including setup) was about two hours. Scan analyses was performed using the analysis software provided by Hologic.
Due to the lack of load on the skeleton during long duration bed rest, bone mineral density changes may occur in the hips, spine, pelvis and heel. DXA results provided control data for longer duration bed rest studies, complement assessment of markers of bone and calcium metabolism, and allowed assessment of subject body composition.
The MR035G testing transitioned during the Campaign 11 (CFT70) Study Protocol and are utilizing the intelligent dual-energy x-ray absorptiometry (iDXA) instead of the Hologic DXA hardware. Bone Densitometry was performed using a General Electric Lunar whole-body densitometer located at John Sealy Hospital at UTMB, Galveston. This iDXA device can distinguish between hard tissue (bone) and soft tissue. Soft tissue can be distinguished further as either lean tissue or fat. Subject’s participated in scans of the following regions: whole body, lumbar spine, proximal femurs (hips), and forearm. The subjects were scanned in triplicate for the pre- and in-bed rest sessions; the follow-up tests were conducted at 6-month and 12-month as single scans.
This experiment has concluded in ground-based studies sponsored by NASAs Human Adaptation and Countermeasures Division. Bone mineral density changes were consistent with earlier bed rest and spaceflight studies, with statistically significant losses averaging 1% per month in the hip, pelvis, and heel. Recovery data were also consistent with data obtained after spaceflight. Bone biomarker data are described, and support the findings of previous studies. Specifically, the process of normal bone remodeling is uncoupled: increased bone resorption with no concomitant change in bone formation.
In conclusion, the Flight Analog Project (FAP) appears to be a valid test bed for skeletal disuse studies, and should provide a useful research platform for evaluating countermeasures to spaceflight-induced bone loss.