ObjectiveTo ascertain the biomechanical effects of a degenerated L4–L5 segment on the lower lumbar spine through a comprehensive simulation of disc degeneration.MethodsA three‐dimensional nonlinear finite element model of a normal L3–S1 lumbar spine was constructed and validated. This normal model was then modified such that three degenerated models with different degrees of degeneration (mild, moderate, or severe) at the L4–L5 level were constructed. While experiencing a follower compressive load (500 N), hybrid moment loads were applied to all models to determine range of motion (ROM), intradiscal pressure (IDP), maximum von Mises stress in the annulus, maximum shear stress in the annulus, and facet joint force.ResultsAs the degree of disc degeneration increased, the ROM of the L4–L5 degenerated segment declined dramatically in all postures (flexion: 5.79°–1.91°; extension: 5.53°–2.62°; right lateral bending: 4.47°–1.46°; left lateral bending: 4.86°–1.61°; right axial rotation: 2.69°–0.74°; left axial rotation: 2.69°–0.74°), while the ROM in adjacent segments increased (1.88°–8.19°). The largest percent decrease in motion of the L4–L5 segment due to disc degeneration was in right axial rotation (75%), left axial rotation (69%), flexion (67%), right lateral bending (67%), left lateral bending right (67%), and extension (53%). The change in the trend of the IDP was the same as that of the ROM. Specifically, the IDP decreased (flexion: 0.592–0.09 MPa; extension: 0.678–0.334 MPa; right lateral bending: 0.498–0.205 MPa; left lateral bending: 0.523–0.272 MPa; right axial rotation: 0.535–0.246 MPa; left axial rotation: 0.53–0.266 MPa) in the L4–L5 segment, while the IDP in adjacent segments increased (0.511–0.789 MPa). The maximum von Mises stress and maximum shear stress of the annulus in whole lumbar spine segments increased (L4–L5 segment: 0.413–2.626 MPa and 0.412–2.783 MPa, respectively; adjacent segment of L4–L5: 0.356–1.493 MPa and 0.359–1.718 MPa, respectively) as degeneration of the disc progressively increased. There was no apparent regularity in facet joint force in the degenerated segment as the degree of disc degeneration increased. Nevertheless, facet joint forces in adjacent healthy segments increased as the degree of disc degeneration increased (extension: 49.7–295.3 N; lateral bending: 3.5–171.2 N; axial rotation: 140.2–258.8 N).ConclusionDegenerated discs caused changes in the motion and loading pattern of the degenerated segments and adjacent normal segments. The abnormal load and motion in the degenerated models risked accelerating degeneration in the adjacent normal segments. In addition, accurate simulation of degenerated facet joints is essential for predicting changes in facet joint loads following disc degeneration.

Objective: To evaluate the strain-rate-dependent viscoelastic properties of the intervertebral disc by in vitro experiments. Method: The biomechanical experiments were conducted from September 2019 to December 2019. The lumbar spines of sheep were purchased within 4-6 hours from the local slaughterhouse, and the intervertebral disc samples were divided into three groups. In rupture group, the samples were used to test the mechanical behavior of the intervertebral disc rupture at different strain rates. In fatigue injury group, the samples were used to test the mechanical behavior of fatigue injury on the intervertebral disc under different strain rates. In internal displacement group, the samples were used to test the internal displacement distribution of the intervertebral disc at different strain rates by applying an optimized digital image correlation (DIC) technique. Results: Both the yielding and cracking phenomenon occurs at fast and medium loading rates, while only the yielding phenomenon occurs at a slow loading rate. The yield stress, compressive strength, and elastic modulus all increase with the increase of the strain rate, while the yield strain decreases with the increase of the strain rate. The logarithm of the elastic modulus in the intervertebral disc is approximately linear with the logarithm of the strain rate under different strain rates. Both before and after fatigue loading, the stiffness in the loading and unloading curves of the intervertebral disc is inconsistent, forming a hysteresis loop, which is caused by the viscoelastic effect. The strain rate has no significant effect on the internal displacement distribution of the intervertebral disc. Based on the experimental data, the constitutive relationship of the intervertebral disc at different strain rates is obtained. The fitting curves are well coupled with the experimental data, while the fitting parameters are approximately linear with the logarithm of the strain rate. Conclusions: These experiments indicate that the strain rate has a significant effect on the mechanical behavior of the intervertebral disc rupture and fatigue injury, while the constitutive equation can predict the rate-dependent mechanical behavior of lumbar intervertebral disc under flexion very well. These results have important theoretical guiding significance for preventing lumbar disc herniation in daily life.

In this study, we developed and validated a refined three-dimensional finite element model of middle femoral comminuted fracture to compare the biomechanical stability after two kinds of plate fixation: a newly designed assembly locking compression plate (NALCP) and a locking compression plate (LCP). CT data of a male volunteer was converted to middle femoral comminuted fracture finite element analysis model. The fracture was fixated by NALCP and LCP. Stress distributions were observed. Under slow walking load and torsion load, the stress distribution tendency of the two plates was roughly uniform. The anterolateral femur was the tension stress area, and the bone block shifted toward the anterolateral femur. Maximum stress was found on the lateral border of the number 5 countersink of the plate. Under a slow walking load, the NALCP maximum stress was 2.160e+03 MPa and the LCP was 8.561e+02 MPa. Under torsion load, the NALCP maximum stress was 2.260e+03 MPa and the LCP was 6.813e+02 MPa. Based on those results of finite element analysis, the NALCP can provide adequate mechanical stability for comminuted fractures, which would help fixate the bone block and promote bone healing.

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Neck injury is one of the most frequent spine injuries due to the complex structure of the cervical spine. The high incidence of neck injuries in collision accidents can bring a heavy economic burden to the society. Therefore, knowing the potential mechanisms of cervical spine injury and dysfunction is significant for improving its prevention and treatment. The research on cervical spine dynamics mainly concerns the fields of automobile safety, aeronautics, and astronautics. Numerical simulation methods are beneficial to better understand the stresses and strains developed in soft tissues with investigators and have been roundly used in cervical biomechanics. In this article, the simulation methods for the development and application of cervical spine dynamic problems in the recent years have been reviewed. The study focused mainly on multibody and finite element models. The structure, material properties, and application fields, especially the whiplash injury, were analyzed in detail. It has been shown that simulation methods have made remarkable progress in the research of cervical dynamic injury mechanisms, and some suggestions on the research of cervical dynamics in the future have been proposed.