Quantitative ultrasound imaging monitoring progressive disuse osteopenia and mechanical stimulation mitigation in calcaneus region through a 90-day bed rest human study

Abstract

Background: Osteoporosis parallels aging and functional mechanical unloading (e.g., space flight and bed rest), jeopardizing mineral density, microstructure, and integrity of bone and leading to an increased risk of fracture. A way to combat this deterioration is to harness the sensitivity of bone to mechanical signals.

Objective: This study evaluates the longitudinal effect of a dynamic mechanical loading through the heel on human bone in vivo during 90-day bed rest, monitored by quantitative ultrasound (QUS) imaging and dual-energy X-ray absorptiometry (DXA) in localized regions of interests, i.e., calcaneus.

Methods: A total of 29 bed rest individuals were evaluated (11 control and 18 treatment) with a brief (10-minute) daily low-intensity (0.3g), high-frequency (30Hz) dynamic mechanical stimulation countermeasure through vibrational inhibition bone erosion (VIBE). Both QUS and DXA detected longitudinal bone density and quality changes.

Results: Ultrasound velocity (UV) decreased in the control group and increased in the group treated with low-intensity loading. The UV increased by 1.9% and 1.6% at 60- and 90-day bed rest (p=0.01) in VIBE over control groups. A trend was found in broadband ultrasound attenuation (BUA), with a VIBE benefit of 1.8% at day 60 and 0.5% at day 90 in comparison with control (p=0.5). Bone mineral density (BMD) assessed by DXA decreased -4.50% for control individuals and -2.18% for VIBE individuals, showing a moderate effect of the mechanical intervention (p=0.19). Significant correlations between QUS and DXA were observed, with a combined BUA and UV vs. BMD: r²=0.70.

Conclusion: These results indicated that low-intensity, high-frequency loading has the potential to mitigate regional bone loss induced by long-term bed rest and that QUS imaging may be able to assess the subtle changes in bone alteration.

Translational potential of this article: Quantitative ultrasound has shown the efficacy of noninvasively assessing bone mass and structural properties in cadaver and isolated trabecular bone samples. While its ability in measuring in vivo bone quality and density is still unclear, a scanning confocal ultrasound imaging is developed and can perform an instant assessment for the subtle changes of such bone loss. This ultrasound imaging modality can potentially be used in the clinical assessment of bone mass. Moreover, physical stimulation has shown the ability to prevent bone loss induced by functional disuse and estrogen deficiency in animal models. However, its treatment capability is unclear. This study has shown that low-magnitude mechanical signals, introduced using low-intensity vibration (LIV), can mitigate regional bone loss caused by functional disuse. Thus localized mechanical treatment, and the quantitative ultrasound imaging have shown translational potential to noninvasively attenuate bone loss, and assess bone mass in the clinic, e.g., in an extreme condition such as long-term space mission, and long-term bedrest such as in case of spinal cord injury.

Keywords: Bed rest; Bone remodelling; Confocal acoustic navigation; Osteoporosis and osteopenia; Ultrasound imaging; Whole-body vibration.

Figure 1

Short-term (<6 months) space mission and bed rest induced the loss of bone mineral density, averaged per month loss. Data adapted from reference LeBlanc et al., 2000, 2007 , . BMD = bone mineral density.

Figure 2

Age-related BMD loss in both cortical and trabecular bones in women and men. Longitudinal data were assessed using DXA. Data are adapted from Khosla A, and Riggs BL, 2005 . BMD = bone mineral density; DXA = dual-energy X-ray absorptiometry.

Figure 3

Daily mechanical loading was applied to the bed rest individuals with a 6-degree head-down tilt for 90 days during which no physical exercise was allowed. The low-magnitude, high-frequency stimulation was applied to the individuals while in a supine position using a vest and a bungee spring system, which loaded the individual horizontally to the vibration platform on mobile support positioned at the foot of the bed. The mechanical vibration was set at 0.3 g, 30 Hz and 10 min per day.

Figure 4

QUS attenuation images and obtained irregular ROI by automatic procedures at baseline, day 60 and day 90 for bed rest individuals. QUS = quantitative ultrasound; ROI = region of interest.

Figure 5

SCAN system shows the change in UV (mean ± Standard Error) and BUA (mean ± standard error) from baseline to 60 days and 90 days, for 11 control individuals and 18 VIBE individuals. BUA = broadband ultrasound attenuation; SCAN = scanning confocal acoustic diagnostic navigation; UV = ultrasound velocity; VIBE = vibrational inhibition of bone erosion.

Figure 6

SCAN system–determined BMD showed a high correlation (r² = 0.70) with DXA-measured BMD. BMD = bone mineral density; DXA = dual-energy X-ray absorptiometry; SCAN = scanning confocal acoustic diagnostic navigation.

Figure 7

SCAN system–determined BMD change (mean ± standard error) from 90-day bed rest showed a similar pattern to DXA-determined BMD change. BMD = bone mineral density; DXA = dual-energy X-ray absorptiometry; SCAN = scanning confocal acoustic diagnostic navigation; VIBE = vibrational inhibition of bone erosion.

Figure 8

SCAN system shows the BUA images measured at baseline, 60 days and 90 days (from left to right) for one control individual. Arrow indicates regional bone property changes due to bed rest. BUA = broadband ultrasound attenuation; SCAN = scanning confocal acoustic diagnostic navigation.