The facet capsular ligaments encapsulate the bilateral spinal facet joints and are common sources of painful injury due to afferent innervation. These ligaments exhibit architectural complexity, which is suspected to contribute to the experimentally observed lack of co-localization between macroscopic strain and microstructural tissue damage. The heterogeneous and multiscale nature of this ligament, combined with challenges in experimentally measuring its microscale mechanics, hinders the ability to understand sensory mechanisms under normal or injurious loading. Therefore, image-based, subject-specific, multiscale finite-element models were constructed to predict the mechanical responses of the human cervical facet capsular ligament under uniaxial tensile stretch. The models precisely simulated the force–displacement responses for all samples (R2 = 0.99 ± 0.01) and showed promise in predicting the magnitude and location of peak regional strains at two different displacements. Yet, there was a loss of agreement between the model and experiment in terms of fiber organization at large tissue stretch, possibly due to a lack of accounting for tissue failure. The mean fiber stretch ratio predicted by the models was found to be significantly higher in regions that exhibited anomalous fiber realignment experimentally than in regions with normal realignment (p < 0.002). The devel- opment of microstructural abnormalities was associated with the predicted fiber-level stretch (p < 0.009), but not with the elemental maximum principal stress or maximum principal strain by logistic regression. The multiscale models elucidate a potential mechanical basis for predicting injury-prone tissue domains and for defining the relationships between macroscopic ligament stretch and microscale pathophysiology in the subfailure regime.
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Acknowledgements This work was supported by a Grant from the NIH (U01EB016638), an AMTI Force and Motion Scholarship, and the Catherine Sharpe Foundation. The authors are grateful for Dr. Kyle Quinn for generating the polarized light images, Dr. Edward Sander for early work with establishing image-based multiscale models and Jared Zitnay for assistance with implementing those models. The authors thank the Minnesota Supercomputing Institute at the University of Minnesota for providing resources.
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- Cervical spine
- Facet capsular ligament
- Fiber-level mechanics
- Microstructural injury
- Multiscale model
- Polarized light imaging