Spaceflight-induced bone loss alters failure mode and reduces bending strength in murine spinal segments

Abstract

Intervertebral disc herniation rates are quadrupled in astronauts following spaceflight. While bending motions are main contributors to herniation, the effects of microgravity on the bending properties of spinal discs are unknown. Consequently, the goal of this study was to quantify the bending properties of tail discs from mice with or without microgravity exposure. Caudal motion segments from six mice returned from a 30-day Bion M1 mission and eight vivarium controls were loaded to failure in four-point bending. After testing, specimens were processed using histology to determine the location of failure, and adjacent motion segments were scanned with micro-computed tomography (μCT) to quantify bone properties. We observed that spaceflight significantly shortened the nonlinear toe region of the force-displacement curve by 32% and reduced the bending strength by 17%. Flight mouse spinal segments tended to fail within the growth plate and epiphyseal bone, while controls tended to fail at the disc-vertebra junction. Spaceflight significantly reduced vertebral bone volume fraction, bone mineral density, and trabecular thickness, which may explain the tendency of flight specimens to fail within the epiphyseal bone. Together, these results indicate that vertebral bone loss during spaceflight may degrade spine bending properties and contribute to increased disc herniation risk in astronauts.

Keywords: four-point bending; herniation; intervertebral disc; murine; spaceflight.

Figure 1

T2-weighted MR images of an astronaut’s L4–L5 disc (A) immediately post-flight and (B) 30 days after landing, with a visible disc herniation (white arrow). Image courtesy of Dr. Alan Hargens.

Figure 2

Astronaut disc herniation photographed during surgery showing a tear at the endplate junction (B). The annulus fibrosus (A) has been avulsed from the vertebra and the nucleus pulposus herniated through the space. (C) is a retractor retracting the ligamentum flavum, and (D) is fat tissue. Image kindly provided by the authors of ref. under the Creative Commons Attribution License.

Figure 3

Mechanical testing 4-point bending setup (left) and spacing of bending supports (right).

Figure 4

Representative load-displacement curves for flight and control mice. Toe region displacement and force at failure are decreased for flight mice, while stiffness (slope of linear region) remains relatively constant.

Figure 5

Representative μCT scan showing the volume of interest used for analysis (light gray) highlighted within the entire vertebra (dark gray). Note the volume of interest only includes the trabecular core.

Figure 6

Bending strength, toe region, disc height and displacement at failure are all reduced for flight mice (FLT, n = 6) compared to controls (CTL, n = 8). ^*p <0.05, +0.05 <p <0.1, unpaired t-tests. Error bars represent standard deviation.

Figure 7

Representative histology images showing (A, B) failure at the endplate junction in control spinal segments and (C, D) failure in the growth plate and epiphyseal bone in post-flight spinal segments. (A and C) show the failure locations in the regions bounded by the dashed boxes and (B and D) show the failure locations indicated by arrows. Image parameters: (A, C) 4 × magnification (scale bars: 500 μm); (B, D) 10 × magnification (scale bars: 200 μm). These sections were stained with a tri-chrome Mallory–Heidenhain stain.

Figure 8

Bone parameters for trabecular bone VOI adjacent to growth plate in control (CTL, n = 5) and flight (FLT, n = 3) specimens. (A) Trabecular bone volume fraction, (B) trabecular thickness, and (C) bone mineral density all decreased post-spaceflight. ^*p <0.05, ^**p <0.005, unpaired t-tests. Error bars represent standard deviation.

Figure 9

Representative μCT scans of trabecular volume of interest for control and flight mice. Flight specimens show significant bone loss compared to pre-flight controls.

Figure 10

Regression between mechanical properties and bone properties for trabecular bone volume of interest. White squares represent flight specimens and dark circles represent controls. Displacement at failure increased with (A) trabecular bone volume fraction and (B) bone mineral density. (C) Toe region displacement increased with trabecular thickness and (D) bending strength increased with bone mineral density as trends. Bone mineral density significantly increased with (E) trabecular thickness and (F) trabecular number.

Figure 11

(A) Trabecular thickness for entire vertebra μCT analysis, showing large decrease post-flight (^**p <0.001, unpaired t-test); (B and C) Regression between (B) toe displacement and trabecular thickness, and (C) BMD and trabecular number for entire vertebra analysis, showing significant positive correlations. White squares represent flight specimens and dark circles represent controls.