Bone fragility after spinal cord injury: reductions in stiffness and bone mineral at the distal femur and proximal tibia as a function of time

Abstract

Computed tomography and finite element modeling were used to assess bone structure at the knee as a function of time after spinal cord injury. Analyzed regions experienced degradation in stiffness, mineral density, and content. Changes were well described as an exponential decay over time, reaching a steady state 3.5 years after injury.

Introduction: Spinal cord injury (SCI) is associated with bone fragility and an increased risk of fracture around the knee. The purpose of this study was to investigate bone stiffness and mineral content at the distal femur and proximal tibia, using finite element (FE) and computed tomography (CT) measures. A cross-sectional design was used to compare differences between non-ambulatory individuals with SCI as a function of time after injury (0-50 years).

Methods: CT scans of the knee were obtained from 101 individuals who experienced an SCI 30 days to 50 years prior to participation. Subject-specific FE models were used to estimate stiffness under axial compression and torsional loading, and CT data was analyzed to assess volumetric bone mineral density (vBMD) and bone mineral content (BMC) for integral, cortical, and trabecular compartments of the epiphyseal, metaphyseal, and diaphyseal regions of the distal femur and proximal tibia.

Results: Bone degradation was well described as an exponential decay over time (R2 = 0.33-0.83), reaching steady-state levels within 3.6 years of SCI. Individuals at a steady state had 40 to 85% lower FE-derived bone stiffness and robust decreases in CT mineral measures, compared to individuals who were recently injured (t ≤ 47 days). Temporal and spatial patterns of bone loss were similar between the distal femur and proximal tibia.

Conclusions: After SCI, individuals experienced rapid and profound reductions in bone stiffness and bone mineral at the knee. FE models predicted similar reductions to axial and torsional stiffness, suggesting that both failure modes may be clinically relevant. Importantly, CT-derived measures of bone mineral alone underpredicted the impacts of SCI, compared to FE-derived measures of stiffness.

Trial registration: ClinicalTrials.gov (NCT01225055, NCT02325414).

Keywords: CT imaging; Finite element modeling; Osteoporosis; Spinal cord injury.

Conflict of interest statement

Ifaz T. Haider, Stacey M. Lobos, Narina Simonian, Thomas J. Schnitzer and W. Brent Edwards declare that they have no conflicts of interest related to this work.

Figures

Figure 1:

Model fits at the distal femur for select FE and CT parameters. Both models match the rapid bone loss observed in the acute phase, soon after injury. The single exponent model (Eq.3; LEFT) quickly reaches a steady-state value, while the double exponent model (Eq.6; RIGHT) predicts progressive bone loss up to 50 years after SCI. Both models explained a similar percentage of the total variation in measured data (R2 > 0.51).

Figure 2:

Model fits at the proximal tibia for select FE and CT parameters. As with the distal femur, shown in Figure 1, both models match the rapid bone loss observed in the acute phase, soon after injury. The single exponent model (Eq.3; LEFT) quickly reaches a steady-state value, while the double exponent model (Eq.6; RIGHT) predicts progressive bone loss up to 50 years after SCI. Both models explained a similar percentage of the total variation in measured data (R2 > 0.52).

Figure 3:

%Change in FE derived stiffness and select CT mineral measures, in patients who reached steady state (t > tss) compared to a recently injured reference group (t ≤ 47 days). The magnitude and spatial pattern of bone loss was similar between the proximal tibia (WHITE) and distal femur (GRAY). Losses were greatest at the epiphysis and progressively decreased, moving towards the diaphysis.