In vivo relationships between lumbar facet joint and intervertebral disc composition and diurnal deformation
Introduction
Motion of the spine is facilitated at each spinal level by a “three joint complex”, an intervertebral disc in the anterior spinal column and two facet joints in the posterior spinal column (Gellhorn et al., 2013a). The intervertebral discs are fibrocartilaginous structures that dissipate compressive loads and provide spinal flexibility in forward/backward bending, side-to-side bending, and rotation. The facet joints are bilateral synovial joints that stabilize the spine by constraining rotation and bending during these motions (Gellhorn et al., 2013a). Similar to other diarthroidal joints, the facets have a synovial capsule, articulating cartilage surfaces, and a meniscoid structure (Koes et al., 2006). Natural, age-related arthritis of the lumbar facets is universal, affecting >90% of individuals by age 50 (Eubanks et al., 2007a) and may trigger instability in the spinal column (Cohen et al., 2013; Kong et al., 2009). Similarly, the discs are subject age-related degeneration and mechanical decline (Kettler et al., 2011; O'Connell et al., 2011) which may trigger spinal instability (Mimura et al., 1994). While incident disc degeneration and facet arthritis are linked (Jarraya et al., 2018), it is not clear how.
Some authors provide evidence that disc degeneration precedes facet arthritis (Fujiwara et al., 1999; Suri et al., 2011), while others demonstrate that facet arthritis precedes disc degeneration (Eubanks et al., 2007b). Disc and facet degeneration may be governed by distinct pathological mechanisms, as serum biomarkers associated with disc degeneration differ from those associated with facet osteoarthritis (Goode et al., 2017). Or, the pathologies could be interrelated, as ex vivo cadaver studies and finite element analyses demonstrate load-sharing between the anterior and posterior spinal column (Jaumard et al., 2011). Still, the in vivo relationship between disc biomechanics and facet biomechanics has not yet been not determined.
Our lab has developed non-invasive magnetic resonance imaging (MRI) tools to measure the functional and compositional properties of cartilaginous tissues and improve clinical musculoskeletal imaging standards. In previous work, we applied MRI and three-dimensional (3D) modeling to quantify diurnal changes in lumbar disc geometry with sub-pixel measurement precision (Martin et al., 2018a). This process involves developing 3D models of the discs before and after daily activity to measure site-specific differences in disc compression. To evaluate the relationships between soft tissue composition and function, our lab has additionally measured quantitative MRI biomarkers of soft tissue composition like T1rho relaxation time (Gullbrand et al., 2016; Martin et al., 2015); these methods allow for the detection and quantification of degenerative changes in the intervertebral discs (Blumenkrantz et al., 2006) and knee (Heckelman et al., 2020) and facet cartilage (Zhang et al., 2017). Towards determining facet function, other researchers have measured facet motion in vivo in asymptomatic, symptomatic, and unstable/symptomatic volunteers (Jaumard et al., 2014; Kozanek et al., 2009; Li et al., 2011; Yao et al., 2013). These studies provide foundational data on in vivo facet biomechanics. Using MRI and 3D modeling techniques developed in our lab, we can build on this work and evaluate the relationships between in vivo disc and facet composition and function.
Our global hypothesis for this work is that aberrant mechanical cross-talk links disc and facet disease. Thus, the objective of this study was to determine the relationships between facet and disc composition (quantitative T1ρ MRI) and facet and disc function (diurnal changes in facet width, disc height) at the L4-L5 and L5-S1 spinal levels. We used MRI and 3D modeling to develop morning and evening 3D models of the facet joint space to measure diurnal changes in joint width as a measure of facet functional properties. We also measured facet joint T1rho relaxation time a quantitative MRI biomarker of composition to evaluate the relationship between facet joint composition and function. In parallel, we evaluated the intervertebral disc composition (T1ρ MRI, degeneration grading) and function (diurnal deformations) to define the relationships between disc and facet joint composition and function. We hypothesized (1) that the decrease in facet space width from AM to PM is related to facet space T1ρ relaxation time, (2) that diurnal changes in disc height and facet joint width are correlated, and, (3) that there are relationships between disc degeneration grade and the diurnal change in facet joint composition and function.
Section snippets
Participants
Ten healthy participants (age: mean 25, range 21–30 years; BMI: mean 23.1, range 19.1–29.0 kg/m2; 5 M/5F) without history of spinal deformity, spine surgery, back pain, sciatica, diabetes, smoking, or arthritis in the knee, hip, hand, or neck were recruited to measure the composition and diurnal changes in geometry of the discs and facets at the L4-L5 and L5-S1 spinal levels (Table 1). We confirmed each participant had a healthy spine and was free of back pain using the National Institutes of
Results
Facet space width varied significantly by level, while facet T1rho and the diurnal change in facet space width did not. Similarly, disc height varied by level, while NP T1rho and the diurnal change in disc height did not. Manual segmentation was a precise method for measuring facet space width and disc height. In both cases the joint geometry could be measured with subpixel precision (facet space width: COV, 1.2% or 20 μm; disc height: COV, 3.0% or 450 μm). Using this technique, we identified
Discussion
In this study, we evaluated a group of young, asymptomatic volunteers to determine the relationships between geometry, composition, and mechanical function of the intervertebral disc and its adjacent facet joints, i.e. the ‘three joint complex’. We found that baseline facet space width was positively associated with facet space T1rho (fluid content) and negatively associated with disc T1rho. When we grouped facet joints by the degeneration grade of their adjacent discs, we found that facets
Conclusion
In this study, we evaluated the relationships between composition and function in the ‘three joint complex’, the intervertebral discs and its two adjacent facet joints. We found an interdependence between disc and facet composition, but not between disc and facet mechanical function. Because we cannot confirm the mechanical interdependence of the disc and its adjacent facets, we point to other factors, like genetics, to link the parallel degeneration processes of these spine components.
Declaration of Competing Interest
None.
Acknowledgement
The authors thank Jean Shaffer and Raven Boykin from the Duke Center for Advanced Magnetic Resonance Development and Dr. Claude T. Moorman. JTM was funded by a grant from the National Institutes of Health (F32 AR071223).
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