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

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Abstract

Summary

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)

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Acknowledgements

We thank our colleagues Dr. Alan S. Anschel, Dr. David Chen, and Dr. Ki H. Kim at the Shirley Ryan AbilityLab (formerly Rehabilitation Institute of Chicago) and Dr. Michelle Gittler and Dr. Ray Lee at Schwab Rehabilitation Hospital for their support in our recruitment efforts. We would also like to thank Matthew Giffhorn, Dr. Elaine Gregory, Kendra Harmon, Amy Lange, Julia Marks, Dr. Amanpreet Saini, Bernard Stephens, and Renita Yeasted for their help with recruitment and data collection.

Funding

This research was supported by Department of Defense U.S. Army Medical Research and Materiel Command (Grant Numbers SC090010 and SC130125). REDCap is supported at FSM by the Northwestern University Clinical and Translational Science (NUCATS) Institute. Research reported in this publication was also supported, in part, by the National Institutes of Health’s National Center for Advancing Translational Sciences (Grant Numbers UL1TR001422 and UL1TR000150). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

Author information

Authors and Affiliations

  1. Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, AB, T2N 1N4, USA

    I.T. Haider, S.M. Lobos & W.B. Edwards

  2. McCaig Institute for Bone and Joint Health, Cumming School of Medicine, University of Calgary, Calgary, AB, T2N 1N4, USA

    I.T. Haider, S.M. Lobos & W.B. Edwards

  3. Department of Physical Medicine and Rehabilitation, Northwestern University Feinberg School of Medicine, Chicago, IL, 60611, USA

    N. Simonian & T.J. Schnitzer

  4. Northwestern University Clinical and Translational Sciences Institute, Northwestern University Feinberg School of Medicine, Chicago, IL, 60611, USA

    N. Simonian

  5. Department of Physical Medicine and Rehabilitation, Rehabilitation Institute of Chicago (d.b.a. Shirley Ryan AbilityLab), Chicago, IL, 60611, USA

    T.J. Schnitzer

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  1. I.T. Haider
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  2. S.M. Lobos
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  3. N. Simonian
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  4. T.J. Schnitzer
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  5. W.B. Edwards
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Corresponding author

Correspondence to I.T. Haider.

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Electronic supplementary material

Figure A1

Segmentation of representative slices from a distal femur. Simple thresholding (GREEN) was used to identify the periosteal boundary. Some manual clean-up (PURPLE) was required to fill any gaps. Image slices at the epiphysis (TOP) required more manual effort compared to slices at the diaphysis (BOTTOM). (JPG 71 kb)

Figure A2

Representative FE model of the distal femur (TOP) and proximal tibia (BOTTOM) generated from one participant. Each bone was separated into three models, representing the epiphysis, metaphysis and diaphysis. The undeformed shape, and boundary conditions are shown on the LEFT. One end was full fixed (RED) while loads were applied to a reference point (WHITE CIRCLE). The model was then patient to torsion (MIDDLE) and axial compression (RIGHT). For clarity, the torsional and axial deformations shown are exaggerated by a factor of 15 and 10, respectively. (JPG 389 kb)

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Haider, I., Lobos, S., Simonian, N. et al. Bone fragility after spinal cord injury: reductions in stiffness and bone mineral at the distal femur and proximal tibia as a function of time. Osteoporos Int 29, 2703–2715 (2018). https://doi.org/10.1007/s00198-018-4733-0

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