In Situ Lumbar Facet Capsular Ligament Strains Due to Joint Pressure and Residual Strain

Abstract

Abstract The lumbar facet capsular ligament, which surrounds and limits the motion of each facet joint, is mechanically significant and has been the subject of multiple previous mechanical characterization studies. Those studies, however, were performed on isolated tissue samples and could not assess the ligament's mechanical state in vivo, where attachment to rigid bone and joint pressure forces lead to non-zero strain even when the spine is unloaded. In this work, we quantified these two effects using cadaveric lumbar spines (5 spines, 20 total facet joints harvested from L2-L5). The effect of joint pressure was measured by injecting saline into the joint space and tracking 3-D capsule surface motion, and prestrain due to attachment was measured by dissecting a large tissue section from the bone and tracking motion between the on-bone and free states. We measured joint pressures of roughly 15-40kPa and local 1st principal strains of up to 25-50% for 0.3mL saline injected into the joint space. The largest stretches were in the bone-to-bone direction in regions spanning the joint space. When the ligament was released from the vertebrae, it shrank by 4-5% on average, with local maximum (negative) principal strains of up to 30%. Based on these measurements and previous tests on isolated lumbar facet capsular ligaments, we conclude that the facet capsular ligament's normal in vivo state is in tension, and that the collagen in the ligament is likely uncrimped even when the spine is unloaded.