New Findings Confirm Regional Internal Disc Strain Changes During Simulation of Repetitive Lifting Motions
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Annals of Biomedical Engineering (Ó 2019) https://doi.org/10.1007/s10439-019-02250-z
New Findings Confirm Regional Internal Disc Strain Changes During Simulation of Repetitive Lifting Motions D. B. AMIN, C. M. MOAWAD, and J. J. COSTI Biomechanics and Implants Research Group, Medical Device Research Institute, College of Science and Engineering, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia (Received 24 October 2018; accepted 20 March 2019) Associate Editor Jane Grande-Allen oversaw the review of this article.
Abstract—To understand the mechanisms of disc injuries that result from repetitive loading, it is important to measure disc deformations and use MRI to quantify disc damage. The aim of this study was to measure internal disc strains during simulated repetitive lifting and their relation to disc injury. Eight cadaveric lumbar segments underwent a pre-test MRI and 20,000 cycles of loading under combined compression (1.0 MPa), hyperflexion, and right axial rotation (2°), which simulated bending and twisting while lifting a 20 kg box. The remaining eight segments had a grid of tantalum wires inserted and used stereoradiography to calculate maximum shear strain (MSS) at increasing cycles. Post-test MRI revealed that 73% of specimens were injured after repetitive loading (annular protrusion, endplate failure, or lumbar disc herniation). MSS at cycle 20,000 was significantly larger than all earlier cycles (p < 0.003). MSS in the anterior, left posterolateral, and left lateral regions was significantly greater than the nucleus region (p < 0.006). Large strains, annular protrusion and herniation in the posterolateral regions were found in this study, which is consistent with clinical observations. In vitro strains can be used to develop more-robust computational models for understanding of the specimen-specific effects of repetitive lifting on disc tissue. Keywords—Maximum shear strain, In-vitro strain, Repetitive loading, Disc herniation, Annular protrusion, Biomechanics.
INTRODUCTION Lumbar disc herniation (LDH) is commonly associated with work related injuries that involve low back stressing through repetitive lifting or movement seen in individuals aged 25–55 years.16 Epidemiological and in vitro studies have shown that LDH can occur during compression combined with flexion, axial rotation Address correspondence to J. J. Costi, Biomechanics and Implants Research Group, Medical Device Research Institute, College of Science and Engineering, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia. Electronic mail: john.costi@flinders.edu.au
and/or lateral bending, resulting in posterior–lateral or posterior herniation.1,13,17–19 Furthermore, studies have looked at propagation of the injury, where nuclear extrusion or protrusion occurred via endplate junction failure29,38 or annular tears.13 However, successfully reproducing LDH during repetitive loading has proven challenging, which limits a full understanding of the injury. Few in vitro studies have examined herniation under repetitive loading
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