Regional cortical and trabecular bone loss after spinal cord injury

Abstract

Spinal cord injury (SCI) triggers rapid loss of trabecular bone mineral density (BMD) in bone epiphyses and a loss of cortical cross-sectional area (CSA) in bone diaphyses, increasing fracture risk for people with SCI. The purpose of this study was to measure trabecular BMD and cortical CSA loss at several previously unexamined lower-limb sites (4% fibula, 12% femur, 86% tibia, cortical) in individuals with SCI. Using peripheral quantitative computed tomography, we scanned 13 participants with SCI longitudinally and 16 on one occasion; 21 participants without SCI served as controls. In the first year post-SCI, 15% to 35% of BMD was lost at the distal femur, proximal tibia, and distal fibula. Bone loss at the distal fibula accelerated between 1 and 2 years post-SCI. BMD at these sites reached a steady state value of ~50% of the non-SCI value 4 years post-SCI. At the tibia diaphysis, cortical CSA decline was slower, eventually reaching 65% of the non-SCI value. Because of the extensive loss of bone observed at these sites, careful consideration needs to be given to the dose of musculoskeletal stress delivered during rehabilitation interventions like standing, muscle electrical stimulation, and aggressive stretching of spastic muscles.

Figure 1

Representative peripheral quantitative computed tomography images for participant without spinal cord injury (SCI) and participant with SCI (3.67 yr).

Figure 2

Mean ± standard deviation trabecular bone mineral density (BMD) or cortical cross-sectional area (CSA) for each sampled site across time. * = Significantly different from bin 5, ** = significantly different from bins 4 and 5, and # = significantly different from bins 3–5. All p < 0.05. SCI = spinal cord injury.

Figure 3

Mean ± standard deviation trabecular bone mineral density (BMD) or cortical cross-sectional area (CSA), normalized to non-spinal cord injury (SCI) values. * = Significantly different from distal tibia site, # = significantly different from proximal tibia and distal femur sites, and ^ = significantly different from distal femur site. All p < 0.05. Gray = trabecular site, hatched = cortical site.

Figure 4

Rate of bone loss as function of time after spinal cord injury (SCI). (a) Bone mineral density (BMD) loss for trabecular sites. (b) Cortical bone cross-sectional area (CSA) loss for tibia diaphysis. Data for time bin 5 were subdivided into bins 5a and 5b to explore whether bone loss reached steady state (0% loss per year) in participants with very long duration SCI (>10 yr).