Comparative analysis of the biomechanics of the adjacent segments after minimally invasive cervical surgeries versus anterior cervical discectomy and fusion: A finite element study
“…A contradiction was observed during the construction of the FEA models; specifically, macroscopic irregular structures in the models constructed with the ‘reconstructive method’ inevitably reduce the repeatability of FEA, and irregular surfaces in these models increase the incidence of mesh deviation, the related computational burden and the risk of computational divergence [ 5 , 11 , 15 , 16 ]. Oversimplified FEA models constructed with the standard geometry method eliminate almost all the structural details and possibly reduce the credibility of FEA [ 4 , 17 – 19 ]. By contrast, the models constructed with the presented method (i.e., using the fitted curves to depict the external contour of model structures) could result in the balance between the repeatability and structure details of FEA models.…”
Section: Discussionmentioning
confidence: 99%
“…However, the resulting structural oversimplification is a visible limitation of FEA and may reduce the credibility of FEA for this model; as such, oversimplification cannot simulate the details of a model’s outline, especially in the posterior column (Fig. 1 ) [ 4 , 18 , 19 ]. Fig.…”
Backgrounds
Finite element analysis (FEA) is an important tool during the spinal biomechanical study. Irregular surfaces in FEA models directly reconstructed based on imaging data may increase the computational burden and decrease the computational credibility. Definitions of the relative nucleus position and its cross-sectional area ratio do not conform to a uniform standard in FEA.
Methods
To increase the accuracy and efficiency of FEA, nucleus position and cross-sectional area ratio were measured from imaging data. A FEA model with smoothened surfaces was constructed using measured values. Nucleus position was calibrated by estimating the differences in the range of motion (RoM) between the FEA model and that of an in-vitro study. Then, the differences were re-estimated by comparing the RoM, the intradiscal pressure, the facet contact force, and the disc compression to validate the measured and calibrated indicators. The computational time in different models was also recorded to evaluate the efficiency.
Results
Computational results indicated that 99% of accuracy was attained when measured and calibrated indicators were set in the FEA model, with a model validation of greater than 90% attained under almost all of the loading conditions. Computational time decreased by around 70% in the fitted model with smoothened surfaces compared with that of the reconstructed model.
Conclusions
The computational accuracy and efficiency of in-silico study can be improved in the lumbar FEA model constructed using smoothened surfaces with measured and calibrated relative nucleus position and its cross-sectional area ratio.
“…A contradiction was observed during the construction of the FEA models; specifically, macroscopic irregular structures in the models constructed with the ‘reconstructive method’ inevitably reduce the repeatability of FEA, and irregular surfaces in these models increase the incidence of mesh deviation, the related computational burden and the risk of computational divergence [ 5 , 11 , 15 , 16 ]. Oversimplified FEA models constructed with the standard geometry method eliminate almost all the structural details and possibly reduce the credibility of FEA [ 4 , 17 – 19 ]. By contrast, the models constructed with the presented method (i.e., using the fitted curves to depict the external contour of model structures) could result in the balance between the repeatability and structure details of FEA models.…”
Section: Discussionmentioning
confidence: 99%
“…However, the resulting structural oversimplification is a visible limitation of FEA and may reduce the credibility of FEA for this model; as such, oversimplification cannot simulate the details of a model’s outline, especially in the posterior column (Fig. 1 ) [ 4 , 18 , 19 ]. Fig.…”
Backgrounds
Finite element analysis (FEA) is an important tool during the spinal biomechanical study. Irregular surfaces in FEA models directly reconstructed based on imaging data may increase the computational burden and decrease the computational credibility. Definitions of the relative nucleus position and its cross-sectional area ratio do not conform to a uniform standard in FEA.
Methods
To increase the accuracy and efficiency of FEA, nucleus position and cross-sectional area ratio were measured from imaging data. A FEA model with smoothened surfaces was constructed using measured values. Nucleus position was calibrated by estimating the differences in the range of motion (RoM) between the FEA model and that of an in-vitro study. Then, the differences were re-estimated by comparing the RoM, the intradiscal pressure, the facet contact force, and the disc compression to validate the measured and calibrated indicators. The computational time in different models was also recorded to evaluate the efficiency.
Results
Computational results indicated that 99% of accuracy was attained when measured and calibrated indicators were set in the FEA model, with a model validation of greater than 90% attained under almost all of the loading conditions. Computational time decreased by around 70% in the fitted model with smoothened surfaces compared with that of the reconstructed model.
Conclusions
The computational accuracy and efficiency of in-silico study can be improved in the lumbar FEA model constructed using smoothened surfaces with measured and calibrated relative nucleus position and its cross-sectional area ratio.
“…FEM can be used to examine the following issues relating to degenerative disc disease: (a) the etiology of intervertebral disc degeneration (IVDD), including biological and biomechanical pathogenesis, (b) the biology, biochemistry and biomechanics of IVDD, and (c) the examination of biological and surgical management for IVDD [37][38][39][40][41][42][43][44]. In particular, in a recent FEM study that ascertained the biomechanical consequences of a degenerated L4-L5 segment, it was found that abnormal loading and motion in the degenerated models enhanced degeneration in the neighboring normal segments [45,46].…”
The finite element method (FEM) represents a computer simulation method, originally used in civil engineering, which dates back to the early 1940s. Applications of FEM have also been used in numerous medical areas and in orthopedic surgery. Computing technology has improved over the years and as a result, more complex problems, such as those involving the spine, can be analyzed. The spine is a complex anatomical structure that maintains the erect posture and supports considerable loads. Applications of FEM in the spine have contributed to the understanding of bone biomechanics, both in healthy and abnormal conditions, such as scoliosis, fractures (trauma), degenerative disc disease and osteoporosis. However, since FEM is only a digital simulation of the real condition, it will never exactly simulate in vivo results. In particular, when it concerns biomechanics, there are many features that are difficult to represent in a FEM. More FEM studies and spine research are required in order to examine interpersonal spine stiffness, young spine biomechanics and model accuracy. In the future, patient-specific models will be used for better patient evaluations as well as for better pre- and inter-operative planning.
“…The annulus was assumed to be hypoelastic [31,33], and the nucleus was set as an incompressible 'semi-uid pad' [28,29]. Ligaments structures and capsules of ZJ were de ned as cable elements in the pre-processing step of FEA [23,34]. Contact elements de ned facet cartilages of ZJ, and its frictional coe cient was set as zero [35,36].…”
Section: Boundary and Loading Conditionsmentioning
Background: The posterior fixation system (PFS) is extensively used in the treatment of spinal diseases. However, the fixation-induced pathological stress distribution pattern (e.g., stress concentration and motility compensation) will increase the risk of adjacent segment diseases (ASD). The accurate adjustment of insertional screw positions is possible under the guidance of the C-arm during percutaneous pedicle screw insertion. Although studies reported that the adjustment of insertional screw positions would affect the stiffness of the fusion segment, there were still no studies that clarified the issue that if the adjustment of insertional screw positions would affect the postoperative biomechanical environment and the risk of ASD. Methods: To investigate if the change of screw positions and related moment arm in adjacent segments will affect mechanical indicators related to ASD, an intact lumbosacral model has been constructed and validated based on our anteriorly constructed model, the oblique lumbar interbody fusion fixed by bilateral PFS with different insertional positions has been simulated in the L4-L5 segment. Models were computed under completely identical loading conditions, including flexion, extension, bending, and rotation. Motility parameters, stress distribution in the intervertebral disc (IVD), and zygapophyseal joints (ZJ) in both cranial and caudal sides of functional units were recorded. Results: Consist with anterior published studies, the change of fixation lengths has a more apparent biomechanical effect on the cranial than the caudal segment. Positive collections can be observed between the reduction of the moment arm in adjacent segments and the aggravation of motility compensation. Furthermore, which can also lead to stress concentration on ZJ facet cartilages. By contrast, no pronounced tendency of stress distribution on IVD can be observed with the change of moment arm. Conclusions: during LIF operations fixed by percutaneous PFS, reducing the fixation stiffness by adjusting the insertional screw positions on the sagittal plane could alleviate the biomechanical deterioration and may be an effective method to reduce the risk of ASD (adjacent segmental instability in the short term and spinal stenosis in the long term).
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