On-joint Mechanics of the Lumbar Facet Capsular Ligament

Abstract

Chronic pain presents a substantial socioeconomic burden and public health challenge, with low back pain (LBP) being the primary reason for activity restriction in individuals under 45 years of age in the United States. Between 70 to 85% of the entire adult population in the US will experience LBP, often idiopathic, at some point in their lives. While numerous factors can contribute to LBP, the facet joint and the surrounding facet capsular ligament (FCL) are recognized as potential sources. Given the FCL’s possible role in LBP and its innervation with nociceptive (pain) and proprioceptive (position) nerve endings, there is considerable interest in its mechanical behavior. Unfortunately, the unloaded in situ configuration and the internal loading state of the FCL is often unknown, presenting challenges when simulating realistic mechanical response. The goal of this dissertation was to determine the on-joint in vivo behavior of the ligament. Here we show how the variation in collagen and elastin fibers within the microstructure affects bending mechanics, and why pressurization of the inner joint space and ligament-bone attachment is mechanically important. We found that collagen content, distribution, and crimp length are vital to the ligament’s through-thickness mechanics. Furthermore, we found that the lumbar facet capsular ligament is under constant tensile strain in vivo due to its attachment to the bone and inner joint pressure. Both have large effects on overall on-joint ligament stresses and strains. Our novel enhanced multiscale finite element model of on-joint mechanics of the lumbar facet capsular ligament demonstrates the complex behavior of the ligament and how it remains in tension even when it is unstretched or compressed relative to the neutral joint position. Multiscale model facilitates further insights on in vivo ligament mechanics and influence therapeutic approaches to low back pain.