Finite element analysis of the influence of three-joint spinal complex on the change of the intervertebral disc bulge and height

Prediction of the biomechanical behaviour of the lumbar spine under multi-axis whole-body vibration using a whole-body finite element model

Low back pain has been reported to have a high prevalence among occupational drivers. Whole-body vibration during the driving environment has been found to be a possible factor leading to low back pain. Vibration loads might lead to degeneration and herniation of the intervertebral disc, which would increase incidence of low back problems among drivers. Some previous studies have reported the effects of whole-body vibration on the human body, but studies on the internal dynamic responses of the lumbar spine under multi-axis vibration are limited. In this study, the internal biomechanical response of the intervertebral disc was extracted to investigate the biomechanical behaviour of the lumbar spine under a multi-axial vibration in a whole-body environment. A whole-body finite element model, including skin, soft tissues, the bone skeleton, internal organs and a detailed ligamentous lumbar spine, was used to provide a whole-body condition for analyses. The results showed that both vibrations close to vertical and fore-and-aft resonance frequencies would increase the transmission of vibrations in the intervertebral disc, and vertical vibration might have a greater effect on the lumbar spine than fore-and-aft vibration. The larger deformation of the posterior region of the intervertebral disc in a multi-axis vibration environment might contribute to the higher susceptibility of the posterior region of the intervertebral disc to injury. The findings of this study revealed the dynamic behaviours of the lumbar spine in multi-axis vehicle vibration conditions, and suggested that both vertical and fore-and-aft vibration should be considered for protecting the lumbar health of occupational drivers.

Will the adjustment of insertional pedicle screw positions affect the risk of adjacent segment diseases biomechanically? An in-silico study

Background

The fixation-induced biomechanical deterioration will increase the risk of adjacent segment diseases (ASD) after lumbar interbody fusion with Bilateral pedicle screw (BPS) fixation. The accurate adjustment of insertional pedicle screw positions is possible, and published studies have reported its mechanical effects. However, no studies clarified that adjusting insertional screw positions would affect the postoperative biomechanical environment and the risk of ASD. The objective of this study was to identify this issue and provide theoretical references for the optimization of insertional pedicle screw position selections.

Methods

The oblique lumbar interbody fusion fixed by BPS with different insertional positions has been simulated in the L4-L5 segment of our previously constructed and validated lumbosacral model. Biomechanical indicators related to ASD have been computed and recorded under flexion, extension, bending, and axial rotation loading conditions.

Results

The change of screw insertional positions has more apparent biomechanical effects on the cranial than the caudal segment. Positive collections can be observed between the reduction of the fixation length and the alleviation of motility compensation and stress concentration on facet cartilages. By contrast, no pronounced tendency of stress distribution on the intervertebral discs can be observed with the change of screw positions.

Conclusions

Reducing the fixation stiffness by adjusting the insertional screw positions could alleviate the biomechanical deterioration and be an effective method to reduce the risk of ASD caused by BPS.

Effect of overload on changes in mechanical and structural properties of the annulus fibrosus of the intervertebral disc

The research focussed on analysing structural and mechanical properties in the intervertebral disc (IVD), caused by long-term cyclic loading. Spinal motion segments were divided into two groups: the control (C), and the group in which it was analysed the impact of posterior column in the load-bearing system of the spine-specimens with intact posterior column (IPC) and without posterior column (WPC). To evaluate the structural and mechanical changes, the specimens were tested with simulation of 100,000 compression-flexion load cycles after which it was performed macroscopic analysis. Mechanical properties of the annulus fibrosis (AF) from the anterior and posterior regions of the IVD were tested at the uniaxial tension test. The stiffness coefficient values were statistically 32% higher in the WPC group (110 N/mm) than in the IPC (79 N/mm). The dynamics of increase in this parameter does not correspond with the course of decrease in height loss. WPC segments revealed clear structural changes that mainly involve the posterior regions of the IVD (bulging and delamination with the effect of separation of collagen fibre bundles). Pathological changes also caused decreases in the value of stress in the AF. The greatest changes in the stress value about group C (7.43 ± 4.49 MPa) were observed in the front part of the fibrous ring, where this value was for IPC 4.49 ± 4.78 MPa and WPC 2.56 ± 1.01 MPa. The research indicates that the applied load model allows simulating damage that occurs in pathological IVD. And the posterior column's presence affects this change's dynamics, structural and mechanical properties of AF.

TELD with limited foraminoplasty has potential biomechanical advantages over TELD with large annuloplasty: an in-silico study

Background

Facetectomy, an important procedure in the in–out and out–in techniques of transforaminal endoscopic lumbar discectomy (TELD), is related to the deterioration of the postoperative biomechanical environment and poor prognosis. Facetectomy may be avoided in TELD with large annuloplasty, but iatrogenic injury of the annulus and a high grade of nucleotomy have been reported as risk factors influencing poor prognosis. These risk factors may be alleviated in TELD with limited foraminoplasty, and the grade of facetectomy in this surgery can be reduced by using an endoscopic dynamic drill.

Methods

An intact lumbo-sacral finite element (FE) model and the corresponding model with adjacent segment degeneration were constructed and validated to evaluate the risk of biomechanical deterioration and related postoperative complications of TELD with large annuloplasty and TELD with limited foraminoplasty. Changes in various biomechanical indicators were then computed to evaluate the risk of postoperative complications in the surgical segment.

Results

Compared with the intact FE models, the model of TELD with limited foraminoplasty demonstrated slight biomechanical deterioration, whereas the model of TELD with large annuloplasty revealed obvious biomechanical deterioration. Degenerative changes in adjacent segments magnified, rather than altered, the overall trends of biomechanical change.

Conclusions

TELD with limited foraminoplasty presents potential biomechanical advantages over TELD with large annuloplasty. Iatrogenic injury of the annulus and a high grade of nucleotomy are risk factors for postoperative biomechanical deterioration and complications of the surgical segment.

Biomechanical role of osteoporosis in the vibration characteristics of human spine after lumbar interbody fusion

Lumbar vertebrae osteoporosis is the most common challenge for lumbar interbody fusion, and this challenge has been widely concerned by scholars for many years. However, under whole-body vibration, osteoporosis how to affect the vibration characteristics of the fusion lumbar spine, complications, and fusion outcomes is urgent to know. The L1-L5 finite element model of lumbar spine was modified to simulate the transforaminal lumbar interbody fusion model with the bilateral pedicle screw fixator at L4-L5 level. A 5 Hz, 40 N sinusoidal vertical load supplemented with a 400 N preload was used to simulate the vertical vibration of human body. The results showed that under whole-body vibration, osteoporosis of fused vertebrae may cause the adjacent segments more unstable and increase the risk of adjacent segment diseases, subsidence, cage failure, rod failure, and lumbar instability. Osteoporosis of the fused vertebrae may cause the vertebral cells an unstable, inhibited growth and lead to poorer fusion outcomes. The findings may assist us in understanding the effect of osteoporosis on the vibration characteristics of lumbar spine fusion and provide references to clinical treatments for lumbar interbody fusion and lumbar vertebrae osteoporosis.

A Planar Model of an Ankle Joint with Optimized Material Parameters and Hertzian Contact Pairs

The ankle is one of the most complicated joints in the human body. Its features a plethora of elements with complex behavior. Their functions could be better understood using a planar model of the joint with low parameter count and low numerical complexity. In this study, an accurate planar model of the ankle with optimized material parameters was presented. In order to obtain the model, we proposed an optimizational approach, which fine-tuned the material parameters of two-dimensional links substituting three-dimensional ligaments of the ankle. Furthermore, the cartilage in the model was replaced with Hertzian contact pairs. The model was solved in statics under moment loads up to 5 Nm. The obtained results showed that the structure exhibited angular displacements in the range of the ankle joint and that their range was higher in dorsiflexion than plantarflexion. The structure also displayed a characteristic ramp up of the angular stiffness. The results obtained from the optimized model were in accordance with the experimental results for the ankle. Therefore, the proposed method for fine-tuning the material parameters of its links could be considered viable.

Structural and Material Optimization for Automatic Synthesis of Spine-Segment Mechanisms for Humanoid Robots with Custom Stiffness Profiles

Typical artificial joints for humanoid robots use actual human body joints only as an inspiration. The load responses of these structures rarely match those of the corresponding joints, which is important when applying the robots in environments tailored to humans. In this study, we proposed a novel, automated method for designing substitutes for a human intervertebral joint. The substitutes were considered as two platforms, connected by a set of flexible links. Their structural and material parameters were obtained through optimization with a structured Genetic Algorithm, based on the reference angular stiffnesses. The proposed approach was tested in three numerical scenarios. In the first test, a mechanism with angular stiffnesses corresponded to the actual L4–L5 intervertebral joint. Scenarios 2 and 3 featured mechanisms with geometry and structure comparable to the joint, but with custom stiffness profiles. The obtained results proved the effectiveness of the proposed method. It could be employed in the design of artificial joints for humanoid robots and orthotic structures for the human spine. As the approach is general, it could also be extended to different body joints.