Marker-Free Tracking of Facet Capsule Motion Using Polarization-Sensitive Optical Coherence Tomography

Abstract

We proposed and tested a method by which surface strains of biological tissues can be captured without the use of fiducial markers by instead, utilizing the inherent structure of the tissue. We used polarization-sensitive optical coherence tomography (PS OCT) to obtain volumetric data through the thickness and across a partial surface of the lumbar facet capsular ligament (FCL) during three cases of static bending. Reflectivity and phase retardance were calculated from two polarization channels, and a power spectrum analysis was performed on each a-line to extract the dominant banding frequency (a measure of degree of fiber alignment) through the maximum value of the power spectrum (maximum power). Maximum powers of all a-lines for each case were used to create 2D visualizations, which were subsequently tracked via digital image correlation. In-plane strains were calculated from measured 2D deformations and converted to 3D surface strains by including out-of-plane motion obtained from the PS OCT image. In-plane strains correlated with 3D strains (R2 ≥ 0.95). Using PS OCT for marker-free motion tracking of biological tissues is a promising new technique because it relies on the structural characteristics of the tissue to monitor displacement instead of external fiducial markers.

Image-based multiscale mechanical modeling shows the importance of structural heterogeneity in the human lumbar facet capsular ligament

“…Polarization-sensitive OCT (PS-OCT) (De Boer et al 1997), however, can provide retardance and optic axis orientation contrasts that originate from tissue anisotropy (birefringence), in a noninvasive reflectance-based image. PS-OCT has been used previously to study the structure of various birefringent biological tissues, including bone (De Boer et al 1999), brain (Wang et al 2011, 2014; Liu et al 2017), and lumbar FCL (Claeson et al 2015). The collagen fiber organization in the ligament can be assessed by retardance and optic axis orientation contrast from PS-OCT.…”

Computer simulation of lumbar flexion shows shear of the facet capsular ligament

“…One principal strain was calculated to represent the entire FCL surface at four displacements within a series, but the regional variations in strain across the FCL were not reported. A previous study from our group [5] aimed to quantify lumbar FCL deformations by optically tracking structural characteristics inherent to the FCL over the course of static bending. Although our previous work extracted 3D FCL deformations, we were only able to characterize the deformation of a portion of the capsule.…”

Planar biaxial extension of the lumbar facet capsular ligament reveals significant in-plane shear forces

“…The FCLs were prepared in a planar orientation for biaxial testing, however, a slight curvature, although greatly reduced from the in situ curvature, still existed, and the experiment was replicated with a finite element model that assumed a planar surface. Therefore, in-plane surface strains output by the model would underestimate the experimental strains, which may account for some of the error between the experimental and model strains (3-D motion of the lumbar FCL is discussed further in Claeson et al (2015)). …”

“…The lumbar FCL, however, is opaque and too thick (approximately 2.5mm) to allow imaging via this transmission-based method. Other modalities, such as polarization-sensitive optical coherence tomography (Wang et al, 2011, Claeson et al, 2015) or scanning electron microscopy (Iatridis and ap Gwynn, 2004; Provenzano and Vanderby, 2006) could be considered in the future. Finally, it is also emphasized that we focused on healthy tissue in the current study, but low back pain can occur with lumbar FCL degradation (Cohen, 2007; Lewinnek and Warfield, 1986), therefore, a comparison between healthy and degenerated samples would be in order.…”

“…To our knowledge, the in-plane shear forces and strains have never been quantified in testing of the lumbar FCL, leaving a significant gap in our knowledge of the tissue’s mechanical behavior. Digital image correlation (Boyle et al, 2014; Claeson et al, 2015; Kelly et al, 2007; Raghupathy et al, 2011), however, can now provide tracking of the full displacement field, and six-degree-of-freedom (6-DOF) load cells can measure the shear forces, enabling a much fuller examination of the mechanical response of a complex tissue such as the lumbar FCL. Thus, the objective of this work was to perform biaxial experiments on human cadaveric lumbar FCL tissue and incorporate shear forces into the analysis.…”