Stress-relaxation and Failure Properties of Spinal Ligaments in Pigs with and without Scoliosis-like Deformity

This study addresses the need for accurate tissue material properties in computational models of the spine. Fundamental to functional spinal unit (FSU) biomechanics are the stress-relaxation and failure properties of the interspinous and intertransverse ligaments (ISL and ITL, respectively). These were measured and compared in normative and scoliotic spines using a Yorkshire pig model. Skeletally immature female pig spines were posterolaterally tethered to induce scoliosis. 23 ISLs and 17 ITLs excised from normative and scoliotic spine segments underwent stress-relaxation and failure tests. Group comparisons tested the effects of scoliosis (magnitude, convexity versus concavity), spinal level (thoracic, lumbar), and pig age. Scoliosis altered ligament geometry and material behavior. The cross-sectional area of concave ITLs was greater than that of normative ITLs, which was larger than that of convex ITLs. ISL and ITL elastic moduli varied across groups. Scoliotic ISL failure strain was higher than that of the normative ISL, and convex ITL failure strain was higher than that of the normative ITL. The long-term relaxation time of ISLs from scoliotic FSUs was longer than that of normative ISLs. These findings demonstrate differences in dimensions and stress-relaxation, and failure material properties of the ISL and ITL associated with scoliosis. Obtained values were comparable to reported human and porcine ligament material properties and were incorporated into an FSU finite element model and shown to affect range of motion. Such property differences may inform quantification of ligament properties with higher sample sizes, related surgical techniques, and further computational modeling.