18th IAA Humans in Space Symposium (2011)
2082.pdf
Different time courses of bone losses in the cortical and trabecular compartments in
different experimental immobilization models
Jörn Rittweger
Institute of Aerospace Medicine, German Aerospace Center (DLR), Cologne, Germany
Introduction: Past research has shown that different parts of the skeleton respond differently to
microgravity exposure and to unloading. This is true for different anatomical regions (Vico, et al. 2000),
as well as for different tissue compartments (Lang, et al. 2006). In particular, it has been suggested
recently that bone losses in response to experimental bed rest (Rittweger, et al. 2009) as well due to spinal
cord injury (Rittweger, et al. 2010) are more pronounced from the cortical than from the trabecular
compartment. The present paper therefore compares the time course of trabecular and cortical bone losses
from the distal tibia epiphysis in four past studies.
Materials and Methods: Previously published data were re-analyzed from the control groups of a) the
long term bed rest (LTBR) study in Toulouse (90 days of -6° head-down-tilt, N = 9), b) the 1st Berlin Bed
Rest (BBR) study (56 days horizontal bed rest, N = 10), c) the Valdoltra 2007 (Valdo07) study (35 days
of horizontal bed rest, N = 10), and d) the unilateral lower limb suspension (ULLS) study (24 days of onelegged limb suspension, N = 8). Across all four studies, peripheral quantitative computed tomography
(pQCT) was used in the same way to study bone losses. Scans were obtained at 4% of the tibia’s length
from its distal end, and subsequent image analysis identified the trabecular region as the inner 45% of the
total bone area. The cortical compartment was defined as all pixels with apparent volumetric bone mineral
density (vBMC) > 480 mg/cm³. Deviations from baseline were computed for all measurement days during
the immobilization period and during follow-up, and one-sample t-tests were performed to assess
statistical significance of these deviations.
Results: Bone losses from the cortical compartment were almost linear over time and tended to start from
the first day of the immobilization phase. They were comparable across studies and became significant
from the 14th day of immobilization onwards (exception: day 21 of ULLS, P = 0,088). Trabecular bone
losses, by contrast, were not significant before the 31st day of immobilization, and in the ULLS study there
was an increase rather than a decrease in trabecular vBMC between the 7th day of immobilization and the
4th day of recovery. Recovery of bone losses during follow-up measurements tended to be balanced
between compartments and across studies.
Discussion: This systematic data analysis clearly demonstrates that immobilization-induced bone losses
precede trabecular bone losses by approximately 14 days, irrespective of whether the immobilization is
performed in -6° HDT, horizontal or vertical position. The high bone remodelling rate on the endocortical
envelope offers itself as a likely explanation for the readiness of cortical losses. The most striking finding
of this paper, however, is the trabecular vBMC gain during the immobilization phase of the ULLS study.
Whether this finding is due to a) the vertical immobilization model, b) the greater measurement frequency
as compared to the other studies, or c) the fact that only one leg was immobilized in the ULLS study
should be addressed by future studies.
References
Lang TF, Leblanc AD, Evans HJ and Lu Y. Adaptation of the proximal femur to skeletal reloading after long-duration spaceflight.
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Rittweger J, Goosey-Tolfrey VL, Cointry G and Ferretti JL. Structural analysis of the human tibia in men with spinal cord injury
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