The mechanical consequence of actual bone loss and simulated bone recovery in acute spinal cord injury

Introduction: Spinal cord injury (SCI) is characterized by rapid bone loss and an increased risk of fragility fracture around regions of the knee. Our purpose was to quantify changes in torsional stiffness K and strength T_ult at the proximal tibia due to actual bone loss and simulated bone recovery in acute SCI. Methods: Computed tomography scans were acquired on ten subjects with acute SCI at serial time points separated by a mean of 3.9months (range 3.0 to 4.8months). Reductions in bone mineral were quantified and a validated subject-specific finite element modeling procedure was used to predict changes in K and T_ult. The modeling procedure was subsequently used to examine the effect of simulated hypothetical treatments, in which bone mineral of the proximal tibiae were restored to baseline levels, while all other parameters were held constant. Results: During the acute period of SCI, subjects lost 8.3±4.9% (p<0.001) of their bone mineral density (BMD). Reductions in K (-9.9±6.5%; p=0.002) were similar in magnitude to reductions in BMD, however reductions in T_ult (-15.8±13.8%; p=0.005) were some 2 times greater than the reductions in BMD. Owing to structural changes in geometry and mineral distribution, T_ult was not necessarily recovered when bone mineral was restored to baseline, but was dependent upon the degree of bone loss prior to hypothetical treatments (r≥0.719; p≤0.019). Conclusions: Therapeutic interventions to halt or attenuate bone loss associated with SCI should be implemented soon after injury in an attempt to preserve mechanical integrity and prevent fracture.