New Findings Confirm Regional Internal Disc Strain Changes During Simulation of Repetitive Lifting Motions

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.