Regional and temporal variation in bone loss during the first year following spinal cord injury

Osteoporosis is a consequence of spinal cord injury (SCI) that leads to fragility fractures. Visual assessment of bone scans suggests regional variation in bone loss, but this has not been objectively characterised. In addition, substantial inter-individual variation in bone loss following SCI has been reported but it is unclear how to identify fast bone losers. Therefore, to examine regional bone loss, tibial bone parameters were assessed in 13 individuals with SCI (aged 16-76 years). Peripheral quantitative computed tomography scans at 4 % and 66 % tibia length were acquired within 5 weeks, 4 months and 12 months postinjury. Changes in total bone mineral content (BMC), and bone mineral density (BMD) were assessed in ten concentric sectors at the 4 % site. Regional changes in BMC and cortical BMD were analysed in thirty-six polar sectors at the 66 % site using linear mixed effects models. Relationships between regional and total loss at 4 months and 12 months timepoints were assessed using Pearson correlation. At the 4 % site, total BMC (P = 0.001) decreased with time. Relative losses were equal across the sectors (all P > 0.1). At the 66 % site, BMC and cortical BMD absolute losses were similar (all P > 0.3 and P > 0.05, respectively) across polar sectors, but relative loss was greatest in the posterior region (all P < 0.01). At both sites, total BMC loss at 4 months was strongly positively associated with the total loss at 12 months (r = 0.84 and r = 0.82 respectively, both P < 0.001). This correlation was stronger than those observed with 4-month BMD loss in several radial and polar sectors (r = 0.56-0.77, P < 0.05). These results confirm that SCI-induced bone loss varies regionally in the tibial diaphysis. Moreover, bone loss at 4 months is a strong predictor of total loss 12 months postinjury. More studies on larger populations are required to confirm these findings.