Bone loss following spinal cord injury in a rat model

Abstract

The current study was undertaken to follow the time course of bone loss in the proximal tibia of rats over several weeks following thoracic contusion spinal cord injury (SCI) of varying severity. It was hypothesized that bone loss would be more pronounced in the more severely injured animals, and that hindlimb weight bearing would help prevent bone loss. Twenty-six female Sprague-Dawley rats (200-225 g, 6-7 weeks old) received standard thoracic (T9) injuries at energies of 6.25, 12.5, 25, or 50 g-cm. The rats were scored weekly for hindlimb function during locomotion. At 0, 2 or 3, and 8 weeks, high-resolution micro-CT images of each right tibia were obtained. Mechanical indentation testing was done to measure the compressive strength of the cancellous bone structure. The 6.25 g-cm group showed near normal locomotion, the 12.5 and 25 g-cm groups showed the ability to frequently or occasionally generate weight-supported plantar steps, respectively, and the 50 g-cm group showed only movement without weight-supported plantar stepping. The 6.25, 12.5 and 25 g-cm groups remained at the same level of bone volume fraction (cancBV/TV=0.24±0.07), while the 50 g-cm group experienced severe bone loss (67%), resulting in significantly lower (p<0.05) bone volume fraction (cancBV/TV=0.11±0.05) at 8 weeks. Proximal tibia cancellous bone strength was reduced by approximately 50% in these severely injured rats. Instead of a linear proportionality between injury severity and bone loss, there appears to be a distinct functional threshold, marked by occasional weight-supported stepping, above which bone loss does not occur.

FIG. 1.

The custom in vivo micro-CT scanner was used to scan the proximal tibial metaphysis at a nominal resolution of 28 μm.

FIG. 2.

The 3-mm section of the proximal metaphysis was reconstructed in three dimensions (A), and then segmented to isolate cancellous bone (B) and cortical bone (C).

FIG. 3.

Graph of the Basso-Beattie-Bresnahan (BBB) locomotor score for each rat group shows the recovery of each group over time, with continuing lack of plantar weight support in the severely-injured (50 g-cm) group, as indicated by a BBB score below 10 (dashed line at 9.5).

FIG. 4.

(A) The cancellous bone volume fraction in the proximal tibia metaphysis decreased approximately 25% in each injury severity group over the first few weeks (p<0.01) relative to baseline. The severely-injured group lost a significant amount of bone over 3 weeks (*p<0.01), and lost 67% of its cancellous bone over 8 weeks (**p<0.01) relative to baseline. The 8-week bone volume fraction in the severely-injured group was also significantly different than the less-severely-injured groups (p<0.01). (B) The cortical bone volume fraction in the proximal tibia metaphysis decreased approximately 11% in each injury severity group over the first few weeks (p<0.01) relative to baseline. The severely-injured group lost a significant amount of bone over 3 weeks (*p<0.01), and lost 32% of its cortical bone over 8 weeks (**p<0.01) relative to baseline. The 8-week cortical bone volume in the severely-injured group was also significantly different than the less-severely-injured groups (p<0.01). (C) The mechanical indentation strength of the proximal tibia metaphyseal cancellous bone decreased significantly (p<0.05), by approximately 50% in the severely-injured group (50 g-cm) compared to the other groups.

FIG. 5.

The indentation strength and the cancellous bone volume fraction are correlated (r²=0.594) at 8 weeks post-spinal cord injury, and the correlation is significant (p<0.001; BV/TV, bone volume/total volume).