Effects of Activity-Based Rehabilitation on Cancellous Bone Loss Following Contusion Spinal Cord Injury in Rats: 1171 Board #3 May 30 9:30 AM - 11:30 AM

Severe cancellous bone loss occurs after spinal cord injury (SCI), which increases fracture risk. Bodyweight-supported treadmill training (TM) and passive Cycle training are activity-based rehabilitation therapies that improve neuromuscular plasticity after SCI. However, the skeletal adaptations to these therapies remain unknown. PURPOSE: Determine whether TM or Cycle training alter the rate of cancellous bone loss in a rodent severe contusion SCI model. METHODS: 16-wk old male Sprague-Dawley rats received: 1) SHAM surgery (T9 laminectomy) (n=9), 2) T9 laminectomy plus severe contusion SCI (n=8), 3) SCI+TM (n=14), or 4) SCI+Cycle (n=7). TM and Cycle were initiated 1-wk post-SCI and consisted of two 20 min bouts/day for 3 wks. For TM, 40% bodyweight support was provided and the paralyzed hindlimbs were manually positioned into plantar stepping (3.5 m/min, increasing 0.1 m/min/day). For Cycle, the paralyzed hindlimbs were secured to pedals on a motor-driven bike and moved passively through a cycling motion that mimicked normal gait patterning (12 rotations/min). Distal femur cancellous bone was quantified before surgery (baseline), and at 2- and 4-wk post-surgery via in vivo microCT. Outcomes are reported as percent change from baseline. RESULTS: Across all groups, cancellous bone volume (cBV/TV) was reduced 52-75% at 2-wk and 54-84% at 4-wk, compared with baseline (p<0.01). cBV/TV loss was 22% greater in SCI at 2-wk and 29% greater at 4-wk vs SHAM (p<0.01), characterized by 28% lower trabecular number (Tb.N) and 90% higher trabecular separation (Tb.Sp) (p<0.01) and a higher trabecular pattern factor (Tb.Pf) (p<0.05) that indicates a less connected trabecular network. At 2-wk, neither TM nor Cycle prevented SCI-induced bone deficits. However, at 4-wk SCI+Cycle displayed 25-30% higher cBV/TV, 23-24% higher trabecular thickness (Tb.Th), 17-22% higher Tb.N, and lower Tb.Pf vs SCI and SCI+TM (p<0.01). Ultimately, no differences in cancellous bone outcomes were present between SCI+Cycle and SHAM at 4-wk, except for 16% higher Tb.Th in SCI+Cycle (p<0.01). CONCLUSION: Our data indicate Cycle better attenuated cancellous bone loss in rodents after severe SCI. The higher cBV/TV and Tb.Th in SCI+Cycle at 4-wk also suggests that this modality stimulated bone formation; although, further investigation is needed.