Fibula response to disuse: a longitudinal analysis in people with spinal cord injury

SUMMARY: Fibular response to disuse has been described in cross-sectional but not longitudinal studies. This study assessed fibular bone changes in people with spinal cord injury. Fibular bone loss was less than in the tibia and was not correlated together. This might explain low fibular fracture incidents in these patients. PURPOSE: Cross-sectional studies suggest that the fibula responds differently to loading and disuse compared to the tibia. Whilst tibial bone changes following spinal cord injury (SCI) have been established in longitudinal studies, fibular changes remain unexplored. METHODS: Fibular and tibial bone parameters were assessed in 13 individuals with SCI (aged 16–76 years). Peripheral quantitative computed tomography scans were acquired at 4%, 38% and 66% distal–proximal tibia length at 5 weeks and 12 months post-injury. Changes in 4% site total bone mineral content (BMC), total cross-sectional area (CSA) and bone mineral density (BMD), and 38% and 66% sites total BMC, total CSA, cortical BMD and cortical CSA were assessed using paired T-tests. Relationships between bone loss in the two bones at equivalent sites were assessed using paired T-tests and correlation. RESULTS: At the 4% site, fibular total BMC and BMD losses were less than tibial losses (− 6.9 ± 5.1% and − 6.6 ± 6.0% vs − 14.8 ± 12.4% and − 14.4 ± 12.4%, p = 0.02 and p = 0.03, respectively). Similarly, at the 66% site, fibular BMC losses were less than those in the tibia (− 2.0 ± 2.6% vs − 4.3 ± 3.6%, p = 0.03), but there was no difference at 38% (− 1.8 ± 3.5% vs − 3.8 ± 2.1%, p = 0.1). No correlation was observed for BMC changes between the two bones (all p > 0.25). CONCLUSION: These results support cross-sectional evidence of smaller disuse-related bone loss in the fibula compared to the tibia. These results may in part explain lower incidence of fibula fractures in individuals with chronic SCI. The lack of association between losses in the two bones might point to different underlying mechanisms.