A biomechanical investigation of thoracolumbar burst fracture under vertical impact loads using finite element method

Factors Associated with Intradiscal Vacuum Phenomenon after Traumatic Thoracolumbar Fracture

Introduction  Posttraumatic kyphosis of the thoracolumbar spine is a possible cause of deterioration of activities of daily living. Thus, postoperative kyphosis is an important issue in treating traumatic thoracolumbar fractures. The intradiscal vacuum phenomenon (IVP) after a traumatic thoracolumbar fracture is considered an important predictor of severe kyphosis after implant removal. However, the associated factors are not yet clear. Methods  The study included data from 94 intervertebral discs on the cephalocaudal side of 47 fractured vertebrae of 45 patients for traumatic thoracolumbar fracture due to high-energy trauma. We assessed the demographics of patients (age, sex, cause of injury, location of injured vertebra, fracture type, cephalocaudal side), imaging finding (kyphosis angle of fractured vertebra at the injury, endplate fracture on computed tomography [CT], intervertebral injury on magnetic resonance image [MRI]), and IVP on CT conducted more than 6 months after surgery. We divided the intervertebral discs into an IVP group and a non-IVP group. To identify factors associated with an IVP, univariate analysis and multivariate logistic regression analysis were conducted. Results  IVP was observed in 27 (29%) of 94 intervertebral discs on CTs conducted at an average of 14.0 months postoperatively. In univariate analysis, the IVP group ( n  = 27) had a significantly more cephalic side of the injured vertebra, endplate fracture on CT, and disc injury on MRI compared with the non-IVP group ( n  = 67). A multivariate logistic regression analysis was conducted to identify factors associated with IVP. The cephalic side (odds ratio [OR] = 4.183, 95% confidence interval [CI] = 1.269-13.785) and endplate fracture on CT (OR = 9.564, 95% CI = 1.940-47.143) were identified as independent factors associated with IVP. Conclusions  IVP was observed in 27 (29%) of 94 intervertebral discs. The cephalic side and endplate fracture on CT were identified as independent factors associated with IVP.

Assessing the risk of re-fracture related to the percentage of partial union in scaphoid waist fractures

Aims

There is ambiguity surrounding the degree of scaphoid union required to safely allow mobilization following scaphoid waist fracture. Premature mobilization could lead to refracture, but late mobilization may cause stiffness and delay return to normal function. This study aims to explore the risk of refracture at different stages of scaphoid waist fracture union in three common fracture patterns, using a novel finite element method.

Methods

The most common anatomical variant of the scaphoid was modelled from a CT scan of a healthy hand and wrist using 3D Slicer freeware. This model was uploaded into COMSOL Multiphysics software to enable the application of physiological enhancements. Three common waist fracture patterns were produced following the Russe classification. Each fracture had differing stages of healing, ranging from 10% to 90% partial union, with increments of 10% union assessed. A physiological force of 100 N acting on the distal pole was applied, with the risk of refracture assessed using the Von Mises stress.

Results

Overall, 90% to 30% fracture unions demonstrated a small, gradual increase in the Von Mises stress of all fracture patterns (16.0 MPa to 240.5 MPa). All fracture patterns showed a greater increase in Von Mises stress from 30% to 10% partial union (680.8 MPa to 6,288.6 MPa).

Conclusion

Previous studies have suggested 25%, 50%, and 75% partial union as sufficient for resuming hand and wrist mobilization. This study shows that 30% union is sufficient to return to normal hand and wrist function in all three fracture patterns. Both 50% and 75% union are unnecessary and increase the risk of post-fracture stiffness. This study has also demonstrated the feasibility of finite element analysis (FEA) in scaphoid waist fracture research. FEA is a sustainable method which does not require the use of finite scaphoid cadavers, hence increasing accessibility into future scaphoid waist fracture-related research.

Finite element study of sagittal fracture location on thoracolumbar fracture treatment

Background: Posterior internal fixation is the main method used for the treatment of thoracolumbar fractures. Fractures often occur in the upper 1/3 of the vertebral body. However, they can also occur in the middle or lower 1/3 of the vertebral body. At present, there is no report discussing the potential effects of sagittal location on instrument biomechanics or surgical strategy. The object of this study was to investigate the effect of the sagittal location of the fracture region of the vertebral body on the biomechanics of the internal fixation system and surgical strategy. Methods: A finite element model of the T11-L3 thoracolumbar segment was established based on a healthy person's CT scan. Different sagittal fracture location finite element models were created by resection of the upper 1/3, middle 1/3, and lower 1/3 of the L1 vertebral body. Three surgical strategies were utilized in this study, namely, proximal 1 level and distal 1 level (P1-D1), proximal 2 level and distal 1 level (P2-D1), and proximal 1 level and distal 2 levels (P1-D2). Nine fixation finite element models were created by combining fracture location and fixation strategies. Range of motion, von Mises stress, and stress distribution were analyzed to evaluate the effects on the instrument biomechanics and the selection of surgical strategy. Results: In all three different fixation strategies, the maximum von Mises stress location on the screw did not change with the sagittal location of the fracture site; nevertheless, the maximum von Mises stress differed. The maximum rod stress was located at the fracture site, with its value and location changed slightly. In the same fixation strategy, a limited effect of sagittal location on the range of motion was observed. P2D1 resulted in a shorter range of motion and lower screw stress for all sagittal locations of the fracture compared with the other strategies; however, rod stress was similar between strategies. Conclusion: The sagittal location of a fracture may affect the intensity and distribution of stress on the fixation system but does not influence the selection of surgical strategy.

Development and Validation of a Whole Human Body Finite Element Model with Detailed Lumbar Spine

OBJECTIVE

Investigations showed that low back pain of occupational drivers might be closely related to the whole-body vibration. Restricted by ethical concerns, the finite element method had become a viable alternative to invasive human experiments. Many mechanical behaviors of the human spine inside of the human body were unclear; therefore, a human whole-body finite element model might be required to better understand the lumbar behavior under whole-body vibration.

METHODS

In this study, a human whole-body finite element model with a detailed lumbar spine segment was developed. Several validations were performed to ensure the correctness of this model.

RESULTS

The results of anthropometry and geometry validation, static validation, and dynamic validation were presented in this study. The validation results showed that the whole human body model was reasonable and valid by comparing with published data.

CONCLUSIONS

The model developed in this study could reflect the biomechanical response of the human lumbar spine under vibration and could be used in further vibration analysis and offer proposals for protecting human body under whole-body vibration environment.

Movement posture and injury pattern of pelvis-lumbar spine of seated human impacted by the vertical high loads: a finite element analysis

The injury conditions of the sitting position occupant inside the military equipment are highly related to the vertical impact environments. In this study, a detailed three-dimensional finite element (FE) model of pelvis-lumbar spine of seated human with nonlinear material property and strain failure criterion was developed and validated. A series of sinusoidal accelerations with a constant peak speed of 8 m/s and frequencies ranging from 10 Hz to 90 Hz were loaded on the FE model to investigate the injury conditions under different high loading rates. The results indicated that the injury patterns mainly include wedge fracture of the junction between lumbar spine and pelvis, and comminuted fracture of ischial tuberosity. The bending moment caused by the large angle deflection of pelvis under 10 Hz case (low rate) and the acting force caused by the excessive curvature of lumbar spine under 30 Hz-70 Hz cases (medium and high rate) cause the junction wedge fractured, while the high impact force under 30 Hz-50 Hz cases (medium rate) leads to comminuted fracture of the ischial tuberosity. The associated mechanism that the shorter the time interval between the peak of seat loading speed and the peak of hip muscle compression, the more serious dynamic responses of pelvis-lumbar spine is revealed for the first time.

Finite element study on whether posterior upper wall fracture is a risk factor for the failure of short-segment pedicle screw fixation in the treatment of L1 burst fracture

PURPOSE

To establish the finite element model of T12 and L2 (T12-L2) pedicle screw fixation for severe L1 burst fracture, and quantitatively simulate and analyze the screw stress and vertebral displacement in different degrees of L1 posterior upper wall fracture (PUWF), and evaluate whether PUWF degree is a risk factor for fixation failure.

METHODS

The data of 6 healthy volunteers were used to establish a finite element model of T12-L2 pedicle screw fixation for type A severe burst fractures. The stress and displacement of the conventional and Schanz pedicle screws for the different degrees of PUWF (including the anterior upper wall of the vertebral canal and the bipedicle) were evaluated.

RESULTS

The maximum stress and L1 displacement of conventional and Schanz pedicle screws were positively correlated with the severity of the PUWF (P<0.05). During anterior flexion, the maximum stress of conventional pedicle screws for 70% type I were 538.3±59.75MPa and the maximum stress of Schanz pedicle screws for 90% type Ⅱ, 90% type Ⅲ and 70% type IV fractures were close to the fatigue threshold. The maximum stress during anterior flexion were significantly higher than those during posterior extension, bending and rotation (P<0.05).

CONCLUSION

The posterior upper wall fracture of vertebral body (VB) of type A burst fracture is not an independent risk factor for the failure of short-segment pedicle screw fixation (SSPSF). Anterior flexion of type A fractures combined with severe PUWF of VB was a risk factor for the failure of SSPSF.

Finite Element Method for the Evaluation of the Human Spine: A Literature Overview

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 Effect of Thoracolumbar Injury Classification in the Clinical Outcome of Operative and Non-Operative Treatments

This review assesses the validity of a biomechanical approach using finite element analysis in the Thoracolumbar Injury Classification and Severity Score System (TLICS) by addressing the “gray zone” decision discrepancy of thoracolumbar spinal injuries. A systematic review was performed using the keywords “Thoracolumbar Injury Classification” AND “finite element analysis of the spinal column” to evaluate the validity of the TLICS and finite element analysis of the thoracolumbar spinal column. Results were classified according to the main conclusions and level of evidence. Thirteen articles are included. Four of the articles evaluated the TLICS in comparison to other classification systems of thoracolumbar spinal injuries. A notable finding is that the TLICS had inconsistencies with other classification systems in the treatment of burst fractures without neurological deficits. One article evaluated the TLICS with the inclusion of magnetic resonance imaging (MRI) in the evaluation, which decreased the agreement between the suggested and actual treatment. Among the three finite element analysis studies, limited data have been published on the posterior ligamentous complex (PLC) status when an injury is suspected or indeterminate. The TLICS has been a reliable classification system in the management of single-column fractures and three-column injuries treated with surgical stabilization. Special attention to enhancing the TLICS classification system by eliminating the “gray zone” of a TLICS score of 4 is essential. Biomedical computational modeling evaluating the PLC status of indeterminate or injury suspected is needed to enhance the current TLICS system and to clarify the decision discrepancy in the “gray zone.”

Biomechanical modelling of the facet joints: a review of methods and validation processes in finite element analysis

There is an increased interest in studying the biomechanics of the facet joints. For in silico studies, it is therefore important to understand the level of reliability of models for outputs of interest related to the facet joints. In this work, a systematic review of finite element models of multi-level spinal section with facet joints output of interest was performed. The review focused on the methodology used to model the facet joints and its associated validation. From the 110 papers analysed, 18 presented some validation of the facet joints outputs. Validation was done by comparing outputs to literature data, either computational or experimental values; with the major drawback that, when comparing to computational values, the baseline data was rarely validated. Analysis of the modelling methodology showed that there seems to be a compromise made between accuracy of the geometry and nonlinearity of the cartilage behaviour in compression. Most models either used a soft contact representation of the cartilage layer at the joint or included a cartilage layer which was linear elastic. Most concerning, soft contact models usually did not contain much information on the pressure-overclosure law. This review shows that to increase the reliability of in silico model of the spine for facet joints outputs, more needs to be done regarding the description of the methods used to model the facet joints, and the validation for specific outputs of interest needs to be more thorough, with recommendation to systematically share input and output data of validation studies.

Distribution of Young's modulus at various sampling points in a human lumbar spine vertebral body

BACKGROUND CONTEXT

Mathematical modeling for creating computer spine models is one of the basic methods underlying many scientific publications. The accuracy of strength parameters of tissues introduced into such models translates directly into the reliability of obtained results. Experimental determination of Young's modulus (E) in various areas of spongy bone tissue seems to be crucial for creating a reliable spine model without excessive simplifications in the form of a single E value for the whole vertebral body.

PURPOSE

The aim of the study was to determine Young's modulus in different parts of the lumbar vertebral column for samples subjected to compression and bending.

STUDY DESIGN

Cylindrical spongy bone tissue samples were subjected to bending and compression strength tests.

METHODS

The study included 975 pathologically unchanged samples of spongy bone tissue harvested from the lumbar vertebrae of 15 male donors. The samples were subjected to compression or bending strength tests and then Young's modulus was determined for each sample depending on its location in the vertebral body. The samples were tested differently between given locations within one vertebra as well as between vertebrae.

RESULTS

Compressed specimens are characterized by highly significantly different Young's modulus values depending on the location in the vertebral body. Samples No. 7 and No. 9 in the anterior part of the vertebral body have highly significantly higher Young's modulus values compared to those in the posterior part of the vertebral body for all lumbar vertebrae. Samples subjected to bending showed significant differences (p<.05) between samples located closer to the vertebral canal (No.16, No.17) and samples located further away (No.14, No.15) with higher values for the samples located in the posterior part of the vertebral body.

CONCLUSIONS

Accommodating the anisotropic structure of spongy bone in computer models and the application of different Young's module values for areas within one vertebral body will allow one to obtain realistic results of computer simulations used.

CLINICAL SIGNIFICANCE

Determining the exact strength parameters of spongy bone tissue within one vertebra and changes in these properties in subsequent vertebrae will allow to create more accurate computer models of the lumbar spine and the whole spine. This, in turn, will translate into more reliable computer simulations used, among others, to determine the risk of fractures or osteoporotic changes, or simulation of the procedure of spinal fusion.