Introduction: Disruption of annular fibers occurs through traumatic or physiologic mechanisms and can involve tearing of inner fibers while superficial fibers remain intact. Injuries change segmental biomechanics by altering load sharing between anatomical components.
Methods: This study incorporated 16 cadaveric lumbar spine motion segments (mean age: 54±10yr) to quantify biomechanical changes following two patterns of annular disruption, categorized using MRI. Concentrated disruptions were focused near the disc center and involved annulus and nucleus. Linear disruptions were elongated in and focused toward the outer third of the annulus. Specimens were tested in tension/compression, anterior/posterior shear, and R/L lateral shear before and after annular disruption.
Results: . Annular disruption produced notable changes in compressive and anterior-posterior shear response, whereas tension and lateral shear were unaffected. Compressive stiffness increased by 36% for concentrated disruptions and decreased by 26% for linear disruptions. Anterior-to-posterior shear stiffness was unaffected by concentrated disruptions, but increased by 37% for linear disruptions. Posterior-anterior shear stiffness increased by 30% for linear disruptions (p<0.05).
Conclusions: Findings demonstrate complex load-sharing relationship between annular fibers, nucleus, and posterior elements of the intervertebral segment. Disruption of one component did not automatically result in decreased segmental stiffness. In many cases, disruption of the annulus/nucleus affected the integrity of the disc and contributed to a stiffer response. Concentrated disruption of the annulus and nucleus led to increased compressive stiffness that resulted from the lost integrity of the nucleus contributing to a decreased ability to distribute compressive loads to the annular fibers. Increased anterior-posterior stiffness for linear disruptions resulted from increased facet joint contribution due to lost integrity of annular fibers. This study provides a baseline for continued study of changes in lumbar mechanics due to annular disruption and their contribution to accelerated segmental degenerative changes due to altered load sharing.
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Learning Objectives: discuss load sharing on lumbar discs and its influence on disc degeneration