Bone steady-state is established at reduced bone strength after spinal cord injury: a longitudinal study using peripheral quantitative computed tomography (pQCT)

Spinal cord injury (SCI) is associated with a marked and rapid sublesional bone loss. So far, reports about the time course of adaptive changes in bone mass and structure in people with chronic and complete SCI are conflicting. Both, a continuous decline of bone parameters throughout the chronic phase of immobilisation as well as stabilisation of bone status on a low level have been documented. In our recently published cross-sectional study we suggested that subjects with a complete SCI reach a new bone steady-state in the paralysed limbs after extensive bone loss was complete. In addition, we described a time loss curve for each measured bone mineral density and geometry parameter and calculated its individual time to reach steady-state (tsteady-state). The aim of the present study was to test the findings of our cross-sectional study in a longitudinal design. Thirty-nine male subjects of the original cross-sectional study with complete SCI and paralysis duration between 0.9 and 34 years were included. Two follow-up pQCT measurements at 15 and 30 months after baseline measurement were performed at the distal epiphyses and mid shafts of the femur, tibia and radius. From the epiphyseal scans, bone mass, trabecular and total BMD were calculated. From the shaft scans, bone mass and cortical BMD, total and cortical cross-sectional areas and cortical thickness were determined. Repeated measures ANOVAs were performed with bone data at baseline, after 15 months and 30 months. Analyses were performed including only subjects with a lesion duration > or =t(steady-state) for each particular bone parameter. Bone parameters of tibial and femoral epi- and diaphyses were found to show no statistically significant differences between the three time points. Relative changes in bone parameters were small and ranged from -1.72% to +0.51% in the femur and from -1.67% to +0.42% in the tibia within 30 months of monitoring. Our data confirm the temporal limitation of the bone loss after complete SCI with stabilisation of BMD and geometric properties on a lower level-a finding of clinical importance considering the treatment strategies of bone loss after SCI with respect to lesion duration.