Poster P4 - Viscoelastic and Failure Properties of Spinal Ligaments in Yorkshire Piglets with Induced Scoliosis

Introduction: Both animal and computational models are used to assist in understanding growth modulation for the purpose of improving adolescent idiopathic scoliosis (AIS) treatment outcomes for developing growth modulation interventions. Creating accurate computational models of growth modulation in animal models then follows as a potentially useful puzzle piece in the eventual translation of knowledge about animal scoliosis to scoliosis in humans. While detailed level-specific adult human spinal ligament properties have been published, reported data on animal-specific spinal ligament mechanical properties are limited to non-existent. Moreover, the authors are not aware of any publication which has reported animal-specific spinal ligament properties which include comparisons by any categories such as age, level, sex, or curve-sidedness (in the case of scoliotic animal models). Granular data on animal spine ligaments can improve the accuracy of computational models in such cases. The current study obtained data from Yorkshire piglet spinal ligaments, harvested from piglets in which scoliosis was induced, so in this case, the addition of more accurate ligament properties in computational models can improve understanding of the internal stresses that exist during curve induction and correction, potentially aiding in translation of information gained in porcine models to human patients, given the similarity in human and porcine spinal ligaments.

Materials and

Methods: Nine inter-transverse ligaments (ITL) and seven inter-spinous ligament (ISL) samples were harvested ex vivo from 22-30 week-old female Yorkshire piglets.. Of these, two were from normative controls and three were from a scoliosis-induced treatment group. Upon harvesting, the samples were frozen. Prior to testing the samples were thawed in a refrigerator at 37℉ for 24 hrs. while wrapped in gauze soaked in 1x phosphate buffered saline. The biomechanical testing consisted of three phases: preconditioning, stress-relaxation, and failure. Preconditioning consisted of ten 2% strain cycles at a frequency of 0.5 Hz. Stress relaxation was performed by stretching the samples to 10% at a rate of 10%/s and holding at this strain for 300s. The sample was then returned to 0% displacement and held for 100s. Following the rest period, failure testing was performed by stretching each sample to 100% strain at a rate of 10% per second. A custom MATLAB script was used to quantify the time-dependent and failure properties of the ligament samples. Box and whiskers plots were used to identify ligament property differences between control and experimental groups, and, using SPSS, a related-samples Wilcoxon Signed Rank Test was used to assess level-matched sidedness differences in ITL properties of the experimental group.

Results, Conclusions, and Discussions: Table 1 lists the median first quartile (Q1), and third quartile (Q3) values of: percent decrease in stress at 300s of stress-relaxation, elastic modulus, failure stress, and strain at failure stress. An appreciable decrease in strain at failure stress was identified for scoliosis-induced ISLs compared to normal ones. Lastly, no significant (p ≥ 0.29) differences were found between concave and convex ITL properties of the experimental group.

This study provides an initial quantification of the material properties of spinal ligaments from normative and scoliosis-induced piglets. Such properties can serve as reference for validating high-fidelity computational animal models of scoliotic spine growth. Limited comparison in literature exists for the data obtained in this study. The data obtained on immature Yorkshire pig ISLs and ITLs is, at least, greater than the measured properties of the porcine denticulate ligament, which has a reported elastic modulus of 0.88 MPa. Additionally, no significant curve-sidedness differences in ITLs support the notion that not all spinal structures may be affected equally by lateral spinal curvature. Perhaps this may indicate a compensatory mechanism, allowing the spine some degree of stability despite spinal deformity. However, this result, along with the finding of decreased strain at failure stress for scoliosis-induced ISLs, require further investigation as this study is limited by a small sample size. Furthermore, additional studies are necessary to obtain more rigorous comparisons by age, level, and curve-sidedness, as well as to assist in the formation of hypotheses about any material property differences observed.

Growth modulation for human AIS patients can be better understood through insight gained from scoliotic spine animal models. Growth modulation in such animal models can be better understood with computational models informed by animal-specific spinal ligament properties, such as those reported herein.

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