Longitudinal Examination of Bone Loss in Male Rats After Moderate-Severe Contusion Spinal Cord Injury

To elucidate mechanisms of bone loss after spinal cord injury (SCI), we evaluated the time-course of cancellous and cortical bone microarchitectural deterioration via microcomputed tomography, measured histomorphometric and circulating bone turnover indices, and characterized the development of whole bone mechanical deficits in a clinically relevant experimental SCI model. 16-weeks-old male Sprague-Dawley rats received T-9 laminectomy (SHAM, n=50) or moderate-severe contusion SCI (n=52). Outcomes were assessed at 2-weeks, 1-month, 2-months, and 3-months post-surgery. SCI produced immediate sublesional paralysis and persistent hindlimb locomotor impairment. Higher circulating tartrate-resistant acid phosphatase 5b (bone resorption marker) and lower osteoblast bone surface and histomorphometric cancellous bone formation indices were present in SCI animals at 2-weeks post-surgery, suggesting uncoupled cancellous bone turnover. Distal femoral and proximal tibial cancellous bone volume, trabecular thickness, and trabecular number were markedly lower after SCI, with the residual cancellous network exhibiting less trabecular connectivity. Periosteal bone formation indices were lower at 2-weeks and 1-month post-SCI, preceding femoral cortical bone loss and the development of bone mechanical deficits at the distal femur and femoral diaphysis. SCI animals also exhibited lower serum testosterone than SHAM, until 2-months post-surgery, and lower serum leptin throughout. Our moderate-severe contusion SCI model displayed rapid cancellous bone deterioration and more gradual cortical bone loss and development of whole bone mechanical deficits, which likely resulted from a temporal uncoupling of bone turnover, similar to the sequalae observed in the motor-complete SCI population. Low testosterone and/or leptin may contribute to the molecular mechanisms underlying bone deterioration after SCI.