Predicting patient-specific rates of bone loss at fracture-prone sites after spinal cord injury

Purpose: People with spinal cord injury (SCI) experience bone loss and have an elevated rate of fracture in the paralysed limbs. The literature suggests an exponential time course of bone loss after SCI, but true rates may vary between patients. We propose systematic evaluation of bone status in the early stages of SCI to identify fast bone losers. Method: A case series of six patients with complete SCI were scanned using peripheral quantitative computed tomography within 5 weeks and at 4, 8 and 12 months post-injury. Bone mineral density (BMD) and bone mineral content (BMC) were measured at fracture-prone sites in the tibia and femur. Patient-specific-predictions (PSP) of expected rates of bone loss were produced by individualising published model equations according to each patient's measured values at baseline. Wilcoxon Signed-Rank tests were used to identify changes between time-points; chi-squared tests for differences between measured and PSP values. Results: In the lower limbs, mean values decreased significantly between baseline and 8 months post-injury, by 19-31% for trabecular BMD, 21-32% for total BMD, and 9-29% for BMC. Most subjects showed no significant differences between PSP and measured values, but individuals with significantly faster rates of bone loss than predicted should be investigated further. Conclusions: There was considerable intersubject variability in rates of bone loss after SCI. Patients showing the fastest bone loss could benefit from continued follow-up and possibly treatment.