Finite Element Analysis of Short- Versus Long-Segment Posterior Fixation for Thoracolumbar Burst Fracture

Relationship Between the Elastic Modulus of the Novel Pedicle Screw-Plate System and Biomechanical Properties Under Osteoporotic Condition: A Power-Law Regression Analysis Based on Parametric Finite Element Simulations

BACKGROUND AND OBJECTIVE

The novel pedicle screw-plate system (NPSPS) is a new internal fixation method for the thoracic spine that we proposed, which has demonstrated effectiveness through clinical practice and biomechanical testing. Nevertheless, the optimal elastic modulus of NPSPS (NPSPS-E) remains debated, particularly for osteoporosis patients. We propose a more efficient method to predict the biomechanical effects of NPSPS across varying elastic moduli in osteoporosis using parametric finite element (FE) analysis, establishing the regression relationship between NPSPS-E and biomechanical properties.

METHODS

An FE surgical model of NPSPS under osteoporotic conditions was developed. The NPSPS-E was linearly varied from 3.6 GPa (polyether ether ketone) to 110 GPa (titanium alloy). Using power-law regression analysis, a functional equation was established to correlate NPSPS-E with biomechanical properties under osteoporotic condition.

RESULTS

Power-law equations and regression models were successfully established between NPSPS-E and biomechanical prediction indices under osteoporotic condition (P<0.0001). As NPSPS-E increased, the range of motion (ROM) of the T8-T10 spinal segments decreased from 0.51°-4.06° to 0.24°-1.45°. The mean von Mises stress in the T8-T10 vertebrae declined from 1.36 MPa-2.03 MPa to 1.15 MPa-1.79 MPa. Concurrently, the stress shielding ratios and the total stress ratios of the NPSPS increased from 3.66%-48.07% and 13.96%-26.96% to 10.70%-56.20% and 52.62%-64.40%, respectively.

CONCLUSION

The functional equations derived from these models serve as a predictive tool to directly estimate the biomechanical effects of NPSPS across a range of elastic modulus under osteoporotic conditions, thereby facilitating the design and optimization of NPSPS materials.

Neural network surrogate and projected gradient descent for fast and reliable finite element model calibration: A case study on an intervertebral disc

Accurate calibration of finite element (FE) models is essential across various biomechanical applications, including human intervertebral discs (IVDs), to ensure their reliability and use in diagnosing and planning treatments. However, traditional calibration methods are computationally intensive, requiring iterative, derivative-free optimization algorithms that often take days to converge. This study addresses these challenges by introducing a novel, efficient, and effective calibration method demonstrated on a human L4-L5 IVD FE model as a case study using a neural network (NN) surrogate. The NN surrogate predicts simulation outcomes with high accuracy, outperforming other machine learning models, and significantly reduces the computational cost associated with traditional FE simulations. Next, a Projected Gradient Descent (PGD) approach guided by gradients of the NN surrogate is proposed to efficiently calibrate FE models. Our method explicitly enforces feasibility with a projection step, thus maintaining material bounds throughout the optimization process. The proposed method is evaluated against state-of-the-art Genetic Algorithm (GA) and inverse model baselines on synthetic and in vitro experimental datasets. Our approach demonstrates superior performance on synthetic data, achieving a Mean Absolute Error (MAE) of 0.06 compared to the baselines' MAE of 0.18 and 0.54, respectively. On experimental specimens, our method outperforms the baseline in 5 out of 6 cases. While our approach requires initial dataset generation and surrogate training, these steps are performed only once, and the actual calibration takes under three seconds. In contrast, traditional calibration time scales linearly with the number of specimens, taking up to 8 days in the worst-case. Such efficiency paves the way for applying more complex FE models, potentially extending beyond IVDs, and enabling accurate patient-specific simulations.

Comparison of Methods for Short-Segment Posterior Stabilization of Lumbar Spine Fractures and Thoracolumbar Junction

Background: Thoracolumbar and lumbar spine injuries account for 30-60% of spinal fractures, especially at the thoracolumbar junction. Conservative treatment is recommended for stable fractures without neurological symptoms, but studies suggest surgical intervention may offer better outcomes. However, there is no consensus on the best stabilization method. Methods: This non-randomized, prospective study was conducted on 114 patients divided into groups based on the surgical technique selected: pedicle stabilization using Schanz screw constructs (Group One, n = 37) stabilization above and below the fractured vertebra using pedicle screws (Group Two, n = 32), and intermediate fixation with a pedicle screw additionally inserted into the fractured vertebra (Group Three, n = 45). Outcomes were assessed using the Cobb angle, anterior and posterior vertebral wall height, and patient quality of life via the Visual Analog Scale (VAS) and Oswestry Disability Index (ODI). X-ray imaging was performed before, during, and after surgery in the control group. Results: This statistical study showed that the location and type of injury significantly influenced the choice of short-segment stabilization method. In the case of measuring the Cobb angle and the high anterior wall, the statistical analysis showed that the best result was observed in the Schanz Group. Patients from this study group had the lowest pain and the highest efficiency. Conclusions: Schanz screw stabilization may offer superior outcomes for thoracolumbar spine injuries, providing better clinical and quality of life results compared to other methods.

Influence of Screw Length and Cement Position Within the Injured Vertebra on Fixation Strength in Short-Segment Fixation of Osteoporotic Thoracolumbar Burst Fractures

OBJECTIVE

The prevalence of osteoporotic vertebral fractures has increased with aging populations, necessitating effective treatments such as percutaneous kyphoplasty combined with posterior screw fixation. However, biomechanical research on the effects of using short screws on fixation stability and bone stress or on the impact of bone cement bonding to screws on structural strength is lacking. This study aimed to optimize short-segment fixation strategies for osteoporotic thoracolumbar burst fractures by analyzing the biomechanical effects of pedicle screw length and bone-cement augmentation.

METHODS

Four models of the thoracolumbar spine were established using computed tomography data of a female volunteer: 1) short screws in the injured vertebra without contact with the bone cement, 2) long screws without contact with the bone cement, 3) long screws in contact with the bone cement, and 4) long screws without the bone cement. The 4 fixation models were simulated under physiological loads. The range of motion, implant stress, and segmental stability were assessed.

RESULTS

The 3 groups containing the bone cement exhibited similar performances in terms of stability and stress distribution, whereas the group without the bone cement exhibited a poorer biomechanical performance. Incorporation of the bone cement enhanced the biomechanical properties of the structure, and short screws in the injured vertebra without contact with the bone cement did not significantly compromise the biomechanical performance.

CONCLUSIONS

Short screws in injured vertebrae without contact with the bone cement can achieve satisfactory stability and stress distribution. It is feasible to implant short screws in the injured vertebrae, reduce the number of bilaterally injured vertebrae, and inject bone cement through the non-pedicle approach during the surgical procedure, which simplifies the surgical process.

Should the level of the posterior instrumentation combined with the intermediate screw be a short segment or a long segment in thoracolumbar fractures with fusion to the fractured segment?

PURPOSE

It was aimed to compare the results of long segment posterior instrumentation with intermediate pedicular screw + fusion at the level of the fractured segment including one vertebra above and one below the fractured vertebra (LSPI) and short segment posterior instrumentation with intermediate pedicular screw + fusion at the level of the fractured segment including one vertebra above and one below the fractured vertebra (SSPI) in the surgical treatment of thoracolumbar vertebral fractures.

METHODS

Ninety patients with thoracolumbar vertebral (T11-L2) fractures operated between March 2015 and February 2022 were included in this retrospective study. The patients were divided into two groups as those who underwent LSPI (n, 54; age, 40.3) and those who underwent SSPI (n, 36; age, 39.7). Radiological evaluations like vertebral compression angle (VCA), vertebral corpus heights (VCH), intraoperative parameters, and complications were compared between the groups.

RESULTS

Correction in early postoperative VCA was statistically significantly better in LSPI (p = 0.003). At 1-year follow-up, postoperative VCA correction was significantly more successful in LSPI (p = 0.001). There was no difference between the two groups in terms of correction loss in VCA measured at 1-year follow-up. There was no statistically significant difference between the two groups in terms of postoperative VCH, VCH at 1-year follow-up, and correction loss in VCH.

CONCLUSION

LSPI provides better postoperative kyphosis correction of the fractured vertebra than SSPI. Regarding the segment level of posterior instrumentation, there was no difference between the two groups in terms of the loss of achieved correction of VCA, ABH, and PBH at 1-year follow-up. Operating a thoracolumbar fracture with LSPI will lengthen the operation and increase the number of intraoperative fluoroscopies compared to SSPI.

Biomechanical Effect of Different Posterior Fixation Techniques on Stability and Adjacent Segment Degeneration in Treating Thoracolumbar Burst Fracture With Osteoporosis: A Finite Element Analysis

STUDY DESIGN

Finite element analysis.

OBJECTIVE

To investigate the biomechanical effect of four posterior fixation techniques on stability and adjacent segment degeneration in treating thoracolumbar burst fractures with osteoporosis.

SUMMARY OF BACKGROUND DATA

In terms of stability and adjacent segment degeneration, there remains no consensus or guidelines on the optimal technique for the treatment of thoracolumbar burst fractures in patients with osteoporosis.

MATERIALS AND METHODS

Images of CT scans were imported into MIMICS and further processed by Geomagic to build three-dimensional models of the T10-L5 region. A v-shaped osteotomy was performed on the L1 vertebral body to simulate a burst fracture in the setting of osteoporosis. Subsequently, four fixation techniques were designed using SolidWorks software. Range of motion (ROM) of the global spine, ROM distribution, ROM of adjacent segment, Von Mises stress on adjacent intervertebral disks, and facet joints were analyzed.

RESULTS

Among the four groups, the cortical bone screw fixation (CBT) showed the highest global ROM at 1.86°, while long-segmented pedicle screw fixation (LSPS) had the lowest global ROM at 1.25°. The LSPS had the smallest percentage of ROM of fractured vertebral body to fixed segment at 75.04%, suggesting the highest stability after fixation. The maximum ROM of the adjacent segment was observed in the CBT at 1.32°, while the LSPS exhibited the smallest at 0.89°. However, the LSPS group experienced larger maximum stress on the adjacent intervertebral disks (9.60 MPa) and facet joints (3.36 MPa), indicating an increasing risk of adjacent segment disease.

CONCLUSION

LSPS provided the greatest stability, while CBT provided the smallest amount of stability. However, the elevated stress on adjacent intervertebral disks and facet joints after LSPS fixation increased the possibility of adjacent segment degeneration. Cement-augmented pedicle screw fixation (CAPS) and combined cortical bone screw and pedicle screw fixation (CBT-PS) demonstrated significant biomechanical advantages in providing moderate fixation strength while reducing stress on the intervertebral disks and facet joints.

Biomechanical Evaluation of a Newly Developed Functional-Grade Composite Material for Pedicle Screws

OBJECTIVE

Pedicle screw and rod systems are widely employed in spine surgeries and loosening due to insufficient mechanical stimulation on the bone is frequently encountered in pedicle screws. This mechanical stimulation problem also arises due to the high rigidity of the implant material. This study aimed to develop new pedicle screws with composite material to solve the pedicle screw loosening problem.

METHODS

The vertebrae and vertebral disk were modeled in three dimension using computerized tomography images obtained from a patient. A commercially available pedicle screw was modeled using Fusion software, and all models were assembled in accordance with the surgical procedure. Pedicle screw models were also divided radially and longitudinally to resemble functionally graded materials, which are composite materials. The load was applied to the top of the T12 vertebra and the screw-vertebral system was fixed to the bottom of the L1 vertebra.

RESULTS

The strain results on the vertebrae were examined according to the mechanostat theorem. According to the results, functionally graded material (FGM) pedicle screw decreased the strain on the vertebral bones, and the positive effects on the bone were determined when using the radially functionally graded screws. The maximum stress values were also examined to determine the strengths of all the models.

CONCLUSION

In conclusion, FGM pedicle screw decreased the strain on the bone which is an important parameter for the loosening failure according to the study. The other important conclusion is that FGM pedicle screw can be the solution to the loosening of the screw but not in all vertebrae.

Comparative Outcomes of Percutaneous and Conventional Open Pedicle Screw Fixation for Single-level Thoracolumbar Spine Injury: Randomised Controlled Trial

Introduction

To compare post-operative outcomes of percutaneous pedicle screw fixation (PPSF) vs open pedicle screw fixation (OPSF) in patients with thoracolumbar spine fractures with no neurological deficits.

Materials and methods

In a randomised controlled trial, patients received short-segment fixation with intermediate screws. We assessed post-operative back pain (Visual Analog Scale or VAS), blood loss, operative/fluoroscopy times, radiographic parameters, and oswestry disability index (ODI) scores at 1, 2, 3, 6, 9, and 12 months.

Results

Between January 2018 and October 2019, 31 patients received PPSF and 30 OPSF. Mean intra-operative blood loss was 66.45 (±44.29) ml for PPSF vs 184.83 (±128.36) ml for OPSF (p<0.001). Fluoroscopy time averaged 2.36 (±0.76) minutes for PPSF vs 0.58 (±0.51) minutes for OPSF (p<0.001). No significant differences existed in operative time or post-operative VAS scores. Radiographic parameters (kyphosis angle and vertebral height ratios) didn't significantly differ post-operatively or at 12 months. However, ODI scores differed significantly at 6 months (p=0.025), with no difference at 12 months.

Conclusion

In this trial, PPSF was comparable to OPSF in improving ODI scores at 12 months but showed earlier improvement at 6 months and reduced blood loss. Radiographic outcomes remained similar between groups over 12 months.

Finite element analysis of different pedicle screw internal fixations for first lumbar vertebral fracture in different sports conditions

OBJECTIVE

Lumbar fractures are the most common spinal injuries, and surgery is required for severe fracture. This study aimed to investigate the variations in motion and stress in varying states of activity after minimally invasive and traditional open pedicle screw placement for L1 vertebral fracture stabilization.

METHODS

We studied a male volunteer (26 years old) with no history of chronic back pain or lumbar spine trauma. We used the finite element method for this investigation. Using finite element software, we created a three-dimensional model of L1 vertebral compression fracture. We also constructed models for four percutaneous pedicle screws spanning the fractured vertebra and four screws traversing the damaged vertebra with transverse fixation.

RESULTS

In all three-dimensional movement directions, the open pedicle fixation system experienced maximum stress higher than its percutaneous counterpart. With axial spinal rotation, von Mises stress on the traditional open pedicle screw was considerably lower than that with percutaneous pedicle fixation, but peak stress was elevated at the transverse connection. Traditional open pedicle fixation displayed less maximum displacement than percutaneous pedicle internal fixation.

CONCLUSIONS

During axial spinal movements, high peak stress is observed at the transverse connection. Patients should avoid excessive axial rotation of the spine during recovery.

Biomechanical Comparison of Different Treatment Strategies for Thoracolumbar Burst Fracture: A Finite Element Study

OBJECTIVE

The aim of this study was to compare the biomechanical performance of 6 pedicle screw internal fixation strategies for the treatment of burst fractures of the thoracolumbar spine using finite element (FE) analysis.

METHODS

A finite element model of the T11-L3 thoracolumbar segment was established to simulate L1 vertebral burst fractures, and 6 models were conducted under multidirectional loading conditions: P2-D2, P1-D1, P2-D1,P1-D, P1-BF-D1, and P1-UF-D1. The range of motion (ROM) in the T12-L2 region and the von Mises stresses of pedicle screws and rods under the 6 internal fixation models were mainly analyzed.

RESULTS

The maximum ROM and von Mises stress were obtained under flexion motion in all models. The P1-BF-D1 model had the least ROM and screw stress. However, when the injured vertebra was not nailed bilaterally, the P1-UF-D1 model had the smallest ROM; the maximum von Mises stress on the screw and rod was remarkably higher than that recorded in the other models. Moreover, the P2-D1 model had a ROM similar to that of the P1-D2 model, but with lower screw stress. The 2 models outperformed the P1-D1 model in all 6 conditions. The P2-D2 model had a similar ROM with the P2-D1 model; nevertheless, the maximum von Mises stress was not substantially reduced.

CONCLUSIONS

The P1-BF-D1 model exhibited better stability and less von Mises stress on the pedicle screws and rods, thereby reducing the risk of screw loosening and fracture. The P2-D1 internal fixation approach is recommended when the fractured vertebrae are not nailed bilaterally.

Long-segment versus short-segment fixation through a posterior approach for tuberculous spondylodiscitis of the mid-thoracic spine in adults: a study of mid- to long-term efficacy

BACKGROUND

This retrospective study aimed to perform a comparative evaluation of the mid- to long-term efficacy of long-segment and short-segment fixations via the posterior approach as a treatment for tuberculous spondylodiscitis in the mid-thoracic spine.

METHODS

A total of 95 patients with tuberculous spondylodiscitis in the mid-thoracic spine underwent surgery via the posterior approach including single-stage posterior debridement, interbody fusion, and pedicle screw fixation. Long-segment fixations were performed for 46 patients (group A), while short-segment fixations were performed for the other 49 patients (group B). Clinical and radiological outcomes were assessed during mid- to long-term follow-up.

RESULTS

The average follow-up periods for groups A and B were 75.5±11.8 and 76.8±11.6 months, respectively. The operative time and intraoperative blood loss were lower in group B than in group A (P<0.05). Both management approaches significantly corrected the kyphotic deformity detected either in the early postoperative period or at the final visit after long-term follow-up (P>0.05). Bony fusion was generated after average periods of 10.8±2.1 months and 11.0±2.0 months in groups A and B, respectively. Favorable outcomes were observed on assessment of neurological function and patients' well-being at the final follow-up.

CONCLUSIONS

No therapeutic differences were observed between long-segment and short-segment fixation as surgical treatment for mid-thoracic Pott's disease during mid- to long-term follow-up. Kyphotic deformity and neurological impairment were significantly relieved via both posterior fixation approaches, with patients' well-being reaching a favorable level. Moreover, short-segment fixation led to less blood loss and required a shorter operative time.

Biomechanical evaluation of different posterior fixation techniques for treating thoracolumbar burst fractures of osteoporosis old patients: a finite element analysis

Objective: To investigate the biomechanical characteristics of different posterior fixation techniques in treatment of osteoporotic thoracolumbar burst fractures by finite element analysis. Methods: The Dicom format images of T10-L5 segments were obtained from CT scanning of a volunteer, and transferred to the Geomagic Studio software, which was used to build digital models. L1 osteoporotic burst fracture and different posterior fixation techniques were simulated by SolidWorks software. The data of ROM, the maximum displacement of fixed segment, ROM of fractured L1 vertebrae, the stress on the screws and rods as well as on fractured L1 vertebrae under different movement conditions were collected and analysed by finite element analysis. Results: Among the four groups, the largest ROM of fixed segment, the maximum displacement of fixed segment and ROM of fractured vertebrae occurred in CBT, and the corresponding data was 1.3°, 2.57 mm and 1.37°, respectively. While the smallest ROM of fixed segment, the maximum displacement of fixed segment and ROM of fractured vertebrae was found in LSPS, and the corresponding data was 0.92°, 2.46 mm and 0.89°, respectively. The largest stress of screws was 390.97 Mpa, appeared in CBT, and the largest stress of rods was 84.68 MPa, appeared in LSPS. The stress concentrated at the junction area between the root screws and rods. The maximum stress on fractured vertebrae was 93.25 MPa, appeared in CBT and the minimum stress was 56.68 MPa, appeared in CAPS. And the stress of fractured vertebrae concentrated in the middle and posterior column of the fixed segment, especially in the posterior edge of the superior endplate. Conclusion: In this study, long-segment posterior fixation (LSPF) provided with the greatest stability of fixed segment after fixation, while cortical bone screw fixation (CBT) provided with the smallest stability. Cement-augmented pedicle screw-rod fixation (CAPS) and combined using cortical bone screw and pedicle screw fixation (CBT-PS) provided with the moderate stability. CBT-PS exhibited superiority in resistance of rotational torsion for using multiple connecting rods. CAPS and CBT-PS maybe biomechanically superior options for the surgical treatment of burst TL fractures in osteoporotic patients.

Clinic choice of long or short segment pedicle screw-rod fixation in the treatment of thoracolumbar burst fracture: From scan data to numerical study

Based on computerized tomography scanning images of human lumbar vertebrae, finite element (FE) analysis is performed to predict the stress of pedicle screws, rods, and fractured vertebra as well as the displacement of fractured vertebra after internal fixation treatment of thoracolumbar burst fracture. A three-dimensional FE model of L1-L5 lumbar vertebrae with L3 burst fracture has been established and four fixation methods, namely, short segment cross- and trans-injured vertebrae, long segment cross- and trans-injured vertebrae fixations, have been adopted to perform posterior pedicle fixation. The stress distributions of the screws, rods, and fractured vertebra and the total deformation of the fractured vertebra are investigated under six different physiological motions. From the view of the stress on the screw-rod system and the deformation of the fractured vertebral body, the long segment cross-injured vertebra fixation has the best mechanical performance, followed by the long segment trans-injured vertebra fixation, and then the short segment fixation trans-injured vertebra. The short segment fixation cross-injured vertebra performs the worst. Among the six motions, the forward flexion movement has the greatest impact on the screw-rod system and the fractured vertebra. However, the rotation motion greatly affects the stress of the screw in the long segment fixation. This indicates that the longer the fixed segment is, the more susceptible it is to human rotation. Thus, for patients with severe fracture, the long segment cross-injured vertebra is preferred. On the contrary, the short segment trans-injured vertebra fixation is optimal.

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.

Finite element analysis of minimally invasive nail placement and traditional nail placement in the treatment of lumbar 1 vertebral compression fracture

Using the finite element analysis method to help us better understand the biomechanical changes of the spine after surgery and the changes in the stress distribution around the screw implantation area. The finite element model of L1 vertebral compression fracture was constructed by using a large number of finite element programs. On the fracture model, 2 kinds of internal fixation devices are set up, namely: the first type of 4 screws across the injured vertebra through the adjacent upper and lower vertebrae + transverse connector; the second type of 4 screws crosses the injured vertebra through the adjacent upper and lower vertebrae + non-transverse connector. To study the distribution of the maximum displacement and von Mises stress of the intramedullary pedicle screws and rods of the 2 types of internal fixation devices after implantation in the spine under certain loading conditions. In traditional open pedicle screw fixation, the maximum stress in the pedicle screw fixation system in the direction of 3D movement is higher than in percutaneous pedicle screw fixation. There is no significant difference in the Von Mises stress of the pedicle screw between the 2 procedures when the spine performs flexion-extension and lateral flexion activities. When the spine is rotating axially, the Von Mises stress of the pedicle screw in conventional open surgery is significantly less than that of the screw in percutaneous pedicle screw fixation. Traditional open internal fixation produces stress peaks of 891.7 MPa and 886.34 MPa at the transverse joint during axial rotation. Only when the spine is rotating in the axial direction, the maximum displacement of traditional open pedicle screw fixation is smaller than that of percutaneous pedicle screw fixation. There is no significant difference in the maximum displacement between the 2 procedures when the spine is moving in other directions. Traditional open pedicle screw fixation can strengthen the stability of the spine in the direction of axial rotation, and can also be greater to reduce the maximum stress of the pedicle screw axial rotation, so the clinical treatment of unstable fractures of the thoracolumbar spine instability is of great significance.

Long segment versus short segment stabilization in thoracolumbar spine fracture: A retrospective clinical and radiological analysis

Introduction: Thoracolumbar spine fracture is one of the commonest spinal fractures. The treatment of choice of surgery is still controversial. This is a retrospective analysis of clinical and radiological outcomes of long and short segment stabilization for patients admitted with thoracolumbar fracture in our hospitals. Material and Method: Inclusion criteria included a single level of thoracolumbar fracture, excluding pre-existing deformity or spinal surgery, osteoporosis, and pathological fracture. Clinical parameters included visual analogue scale (VAS), operative time, intraoperative blood loss, and postoperative length of stay. Radiological parameters included wedge angle and anterior and posterior vertebral height ratio. Results: From June 2007 to May 2020, 56 patients (male = 31, female = 25) were recruited. There were 25 patients in the short segment group (open = 11, minimal invasive surgery (MIS) = 14) and 31 patients in the long segment group. Clinically, significantly better VAS at 6 months (1 vs. 1.96; p = 0.041), shorter post-op length of stay (16 days vs. 25 days; p = 0.01), and less blood loss (178 ml vs. 824 ml; p < 0.01) were found in the short segment group. Radiologically, the short segment group showed significantly better wedge angle at immediate (5° vs. 9.23°; p = 0.002) and long-term follow-up (7.41° vs. 11.43°; p = 0.01). Moreover, the short segment group has significantly better post-op anterior and posterior vertebral height ratios. Within the short segment group, the MIS group showed significantly less blood loss (82 ml vs. 303 ml; p < 0.01). However radiological parameters favour the open group. Discussion and Conclusion: For single-level thoracolumbar fracture, both long and short stabilizations were effective in reducing and stabilizing the fracture. However, the short segment group showed significantly superior results. Moreover, a minimally invasive technique in short segment stabilization showed even less blood loss but less optimal radiological results. Therefore, short segment stabilization could be the treatment of choice for traumatic thoracolumbar spine fractures.

Novel uniplanar pedicle screw systems applied to thoracolumbar fractures: a biomechanical study

Objective: In this study, the advantages of the internal fixation configuration composed of uniplanar pedicle screws in the treatment of thoracolumbar fractures were verified by biomechanical experimental methods, which provided the basis for subsequent clinical experiments and clinical applications. Methods: A total of 24 fresh cadaveric spine specimens (T12-L2) were utilized to conduct biomechanical experiments. Two different internal fixation configurations, namely, the 6-screw configuration and the 4-screw/2-NIS (new intermediate screws) configuration, were tested using fixed-axis pedicle screws (FAPS), uniplanar pedicle screws (UPPS), and polyaxial pedicle screws (PAPS) respectively. The spine specimens were uniformly loaded with 8NM pure force couples in the directions of anteflexion, extension, left bending, right bending, left rotation, and right rotation, and the range of motion (ROM) of the T12-L1 and L1-L2 segments of the spine was measured and recorded to access biomechanical stability. Results: No structural damage such as ligament rupture or fracture occurred during all experimental tests. In the 6-screw configuration, the ROM of the specimens in the UPPS group was significantly better than that of the PAPS group but weaker than those of the FAPS group (p < 0.01). In the 4-screw/2-NIS configuration, the results were identical to the biomechanical test results for the 6-screw configuration (p < 0.01). Conclusion: Biomechanical test results show that the internal fixation configuration with UPPS can maintain the stability of the spine well, and the results are better than that of PAPS. UPPS has both the biomechanical advantages of FAPS and the superiority of easy operation of PAPS. We believe it is an optional internal fixation device for minimally invasive treatment of thoracolumbar fractures.

Short-segment fixation and transpedicular bone grafting for the treatment of thoracolumbar spine fracture

PURPOSE

Thoracolumbar fracture is one of the most common fractures of spine. And short-segment posterior fixation including the fractured vertebra (SSPFI) is usually used for the surgical treatment of it. However, the outcomes of SSPFI for different types of thoracolumbar fractures are not clear, and whether it is necessary to perform transpedicular bone grafting is still controversial. This study was conducted to determine the clinical efficacy of SSPFI for the treatment of different types of single-level thoracolumbar fracture, and make clear what kind of fractures need transpedicular bone grafting during the surgery.

METHODS

Patients with single-level thoracolumbar fracture undergoing SSPFI surgery between January 2013 and June 2020 were included in this study. The operative duration, intraoperative blood loss, anterior vertebral height ratio (AVHR) and anterior vertebral height compressive ratio (AVHC) of the fractured vertebra, local kyphotic Cobb angle (LKA), vertebral wedge angle (VWA) and correction loss during follow up period were recorded. Outcomes between unilateral and bilateral pedicle screw fixation for fractured vertebra, between SSPFI with and without transpedicular bone grafting (TBG), and among different compressive degrees of fractured vertebrae were compared, respectively.

RESULTS

A total of 161 patients were included in this study. All the patients were followed up, and the mean follow-upped duration was 25.2 ± 3.1 months (6-52 months). At the final follow-up, the AVHR was greater, and the LKA and VWA were smaller in patients with bilateral fixation (6-screw fixation) than those with unilateral fixation (5-screw fixation) of AO type A3/A4 fractures (P < 0.001). The correction loss of AVHR, LKA and VWA in fractured vertebra were significantly great when preoperative AVHC was >50% (P < 0.05). For patients with AVHC >50%, the correction loss in patients with TBG were less than those without TBG at the final follow-up (P < 0.05).

CONCLUSIONS

SSPFI using bilateral fixation was more effective than unilateral fixation in maintaining the fractured vertebral height for AO type A3/A4 fractures. For patients with AVHC >50%, the loss of correction was more obvious and it can be decreased by transpedicular bone grafting.

Effect of endplate reduction on endplate healing morphology and intervertebral disc degeneration in patients with thoracolumbar vertebral fracture

OBJECTIVE

To determine the effect of endplate reduction on the final healing morphology and degenerative changes in intervertebral discs.

METHODS

Forty-eight patients with single-level thoracolumbar fractures with endplate injury were included. All patients underwent posterior reduction and pedicle screw fixation, and postoperative imaging was used to determine whether endplate reduction was successful. The healing morphology of the endplate was divided into three types: increased endplate curvature, irregular healing and traumatic Schmorl node. MRI was performed at baseline and at the last follow-up evaluation to observe changes in disc degeneration (disc height and nucleus pulposus signal) and Modic changes.

RESULTS

The reduction rate in the central area was significantly lower than that in the peripheral area (P = 0.017). In patients with successful reduction, 90.9% (20/22) of the endplates healed with increased curvature. In patients with an unsuccessful endplate reduction, 63.4% (26/41) of the endplates healed irregularly, and 34.1% (14/41) of the endplates formed traumatic Schmorl nodes. Endplate reduction was closely related to the final healing morphology of the endplate (P < 0.001), which had a significant protective effect on the degeneration of the intervertebral disc. At the last follow-up evaluation, there was no statistically significant correlation between different endplate healing morphologies and new Modic changes.

CONCLUSIONS

The reduction rate in the central area is significantly lower than that in the peripheral area. Although all of the intervertebral discs corresponding to fractured endplates had degenerated to different degrees, successful endplate fracture reduction can obviously delay the degeneration of intervertebral discs.

Does Instrumentation of the Fractured Level in Thoracolumbar Fixation Affect the Functional and Radiological Outcome?

STUDY DESIGN

Retrospective comparative study.

OBJECTIVES

To compare radiological and functional outcomes of patients with fixation constructs utilizing pedicle screw stabilization at the fracture level (FL group) versus patients with non-fracture level (NFL group) fixation in single level fractures of the thoracolumbar junction (T11-L1).

METHODS

53 patients of whom fracture level screw was used in 34 (FL group) were compared to 19 patients in NFL group. Radiological parameters analyzed were sagittal index, bi-segmental kyphosis (Cobb) angle and degree of vertebral height restoration. Prospectively collected patient reported functional outcomes and post-operative complications were also studied. Stepwise regression analysis adjusted by age, gender and functional scores was performed to account for the small numbers and unequal sizes of the groups.

RESULTS

Back pain score was significantly lower in the FL group (P < 0.025). Core Outcome Measures Index scores and leg pain scores, though low in the FL group, were not statistically significant. The regression analysis showed that the inclusion of the fracture-level screw was independently associated with a greater change in sagittal index and vertebral height restoration post-operatively. Sagittal index was maintained through to final follow up as well. The bi-segmental Cobb's angle correction was not associated with fracture-level screw construct. There was no significant difference between the groups for revision surgery, deep infection, implant failure or length of hospital stay.

CONCLUSION

The inclusion of the fracture-level pedicle screws in the fixation construct significantly improves the immediate and final measured radiological parameters, with improved functional scores in single level unstable vertebral fractures of the thoracolumbar junction.

Development and model form assessment of an automatic subject-specific vertebra reconstruction method

BACKGROUND

Current spine models for analog bench models, surgical navigation and training platforms are conventionally based on 3D models from anatomical human body polygon database or from time-consuming manual-labelled data. This work proposed a workflow of quick and accurate subject-specific vertebra reconstruction method and quantified the reconstructed model accuracy and model form errors.

METHODS

Four different neural networks were customized for vertebra segmentation. To validate the workflow in clinical applications, an excised human lumbar vertebra was scanned via CT and reconstructed into 3D CAD models using four refined networks. A reverse engineering solution was proposed to obtain the high-precision geometry of the excised vertebra as gold standard. The 3D model evaluation metrics and a finite element analysis (FEA) method were designed to reflect the model accuracy and model form errors.

RESULTS

The automatic segmentation networks achieved the best Dice score of 94.20% in validation datasets. The accuracy of reconstructed models was quantified with the best 3D Dice index of 92.80%, 3D IoU of 86.56%, Hausdorff distance of 1.60 mm, and the heatmaps and histograms were used for error visualization. The FEA results showed the impact of different geometries and reflected partial surface accuracy of the reconstructed vertebra under biomechanical loads with the closest percentage error of 4.2710% compared to the gold standard model.

CONCLUSIONS

In this work, a workflow of automatic subject-specific vertebra reconstruction method was proposed while the errors in geometry and FEA were quantified. Such errors should be considered when leveraging subject-specific modelling towards the development and improvement of treatments.

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.

Finite Element Analysis of a Novel Anterior Locking Plate for Thoracolumbar Burst Fracture

Purpose

The finite element analysis method was used to explore the biomechanical stability of a novel locking plate for thoracolumbar burst fracture fusion fixation.

Methods

The thoracolumbar CT imaging data from a normal volunteer was imported into finite software to build a normal model and three different simulated surgical models (the traditional double-segment fixation model A, the novel double-segment fixation model B, and the novel single-segment fixation model C). An axial pressure (500 N) and a torque (10 Nm) were exerted on the end plate of T12 to simulate activity of the spine. We recorded the range of motion (ROM) and the maximum stress value of the simulated cages and internal fixations.

Results

Model A has a larger ROM in all directions than model B (flexion 5.63%, extension 38.21%, left rotation 46.51%, right rotation 39.76%, left bending 9.45%, and right bending 11.45%). Model C also has a larger ROM in all directions than model B (flexion 555.63%, extension 51.42%, left rotation 56.98%, right rotation 55.42%, left bending 65.67%, and right bending 59.47%). The maximum stress of the cage in model A is smaller than that in model B except for the extension direction (flexion 96.81%, left rotation 175.96%, right rotation 265.73%, left bending 73.73%, and right bending 171.28%). The maximum stress value of the internal fixation in model A is greater than that in model B when models move in flexion (20.23%), extension (117.43%), and left rotation (21.34%).

Conclusion

The novel locking plate has a smaller structure and better performance in biomechanical stability, which may be more compatible with minimally invasive spinal tubular technology.

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.

A biomechanical study of proximal junctional kyphosis after posterior long segment fusion with vertebral body augmentation

Background Proximal junction kyphosis is a common clinical complication of posterior long-segment spinal fusion and vertebral body augmentation method is one of the effective approaches to prevent it. The purpose of this study was to explore the biomechanical effect of proximal junction kyphosis after posterior long-segment thoracolumbar fusion with different vertebral augmentation schemes using finite element analysis. Methods 3D nonlinear finite element models of T1-L5 spine posterior long-segment T8-L5 thoracolumbar fusion combined with T7, T8 and T7&T8 vertebral bone cement augmentation were constructed from human spine CT data and clinical surgical operation scheme to analyze the von Mises stress in the vertebrae, intervertebral discs pressure and pedicle screws system loads under the flexion, extension, lateral bending and axial rotation motion. Findings Compared with thoracolumbar posterior long-segment fusion model, T7 maximum stress in T7, T8 and T7&T8 vertebrae augmentation models were reduced by 8.64%, 7.17%, 8.51%;0.79%, -3.88%,1.67%;4.02%, 5.30%, 4.27% and 3.18%, 3.06%, -6.38% under the flexion, extension, lateral bending and axial rotation motion. T7/T8 intervertebral disc pressure in T7, T8, T7&T8 vertebral augmentation models were 36.71Mpa,29.78Mpa,36.47Mpa;22.25Mpa,18.35Mpa,22.06Mpa;84.27Mpa,68.17Mpa, 83.89Mpa and 52.23Mpa, 38.78Mpa,52.10Mpa under the same condition. The maximum stress 178.2Mpa of pedicle screws is mainly distributed at the root of screw. Interpretation Thoracolumbar posterior long-segment fusion with proximal double-segment vertebral augmentation should be recommended to prevent proximal junction kyphosis than single-segment augmentation. Simulation results can provide theoretical foundations and assist surgeons in selecting the appropriate operation scheme.

Comparison of Posterior Fixation Strategies for Thoracolumbar Burst Fracture: A Finite Element Study

The management of thoracolumbar (TL) burst fractures remained challenging. Due to the complex nature of the fractured vertebrae and the lack of clinical and biomechanical evidence, currently, there was still no guideline to select the optimal posterior fixation strategy for TL burst fracture. We utilized a T10-L3 TL finite element model to simulate L1 burst fracture and four surgical constructs with one- or two-level suprajacent and infrajacent instrumentation (U1L1, U1L2, U2L1, and U2L2). This study was aimed to compare the biomechanical properties and find an optimal fixation strategy for TL burst fracture in order to minimize motion in the fractured level without exerting significant burden in the construct. Our result showed that two-level infrajacent fixation (U1L2 and U2L2) resulted in greater global motion reduction ranging from 66.0 to 87.3% compared to 32.0 to 47.3% in one-level infrajacent fixation (U1L1 and U2L1). Flexion produced the largest pathological motion in the fractured level but the differences between the constructs were small, all within 0.26 deg. Comparisons in implant stress showed that U2L1 and U2L2 had an average 25.3 and 24.8% less von Mises stress in the pedicle screws compared to U1L1 and U1L2, respectively. The construct of U2L1 had better preservation of the physiological spinal motion while providing sufficient range of motion reduction at the fractured level. We suggested that U2L1 is a good alternative to the standard long-segment fixation with better preservation of physiological motion and without an increased risk of implant failure.

Different fixation pattern for thoracolumbar fracture of ankylosing spondylitis: A finite element analysis

The objective of this study is to establish an ankylosing spondylitis (AS) thoracolumbar fracture finite element (FE) model and provide a proper posterior fixation choice from the biomechanical perspective. The ankylosing spondylitis T9-L5 FE model was built and the range of motion (ROM) was compared to previous studies. The L1 transverse fracture was simulated and was separately fixed by five different patterns. The pull force and yielding force of the screws, the von Mises stress of the internal fixation, and the displacement of fracture site were analyzed to evaluate the proper fixation pattern for thoracolumbar fracture of AS. ROM of AS model was obviously restricted comparing to the normal vertebral experimental data. All the fixation patterns can stabilize the fracture. At least four levels of fixation can reduce the von Mises stress of the internal fixation. Four levels fixation has a higher pull force than the six levels fixation. Skipped level fixation did not reduce the stress, pull force and yielding force. The kyphosis correction did not change the biomechanical load. At least 4 levels fixation was needed for AS thoracolumbar fracture. The cemented screws should be chosen in 4 levels fixation to increase the holding of the screws. The skipped fixation has no advantage. The kyphosis correction can be chosen after weighing the pros and cons.

A Hybrid Uniplanar Pedicle Screw System with a New Intermediate Screw for Minimally Invasive Spinal Fixation: A Finite Element Analysis

Purpose

A hybrid pedicle screw system for minimally invasive spinal fixation was developed based on the uniplanar pedicle screw construct and a new intermediate screw. Its biomechanical performance was evaluated using finite element (FE) analysis.

Methods

A T12-L2 FE model was established to simulate the L1 vertebral compression fracture with Magerl classification A1.2. Six fixation models were developed to simulate the posterior pedicle screw fracture fixation, which were divided into two subgroups with different construct configurations: (1) six-monoaxial/uniplanar/polyaxial pedicle screw constructs and (2) four-monoaxial/uniplanar/polyaxial pedicle screw constructs with the new intermediate screw. After model validation, flexion, extension, lateral bending, and axial rotation with 7.5 Nm moments and preloading of 500 N vertical compression were applied to the FE models to compare the biomechanical performances of the six fixation models with maximum von Mises stress, range of motion, and maximum displacement of the vertebra.

Results

Under four loading scenarios, the maximum von Mises stresses were found to be at the roots of the upper or lower pedicle screws. In the cases of flexion, lateral bending, and axial rotation, the maximum von Mises stress of the uniplanar screw construct lay in between the monoaxial and polyaxial screw constructs in each subgroup. Considering lateral bending, the uniplanar screw construct enabled to lower the maximum von Mises stress than monoaxial and polyaxial pedicle screw constructs in each subgroup. Two subgroups showed comparable results of the maximum von Mises stress on the endplates, range of motion of T12-L1, and maximum displacement of T12 between the corresponding constructs with the new intermediate screw or not.

Conclusions

The observations shown in this study verified that the hybrid uniplanar pedicle screw system exhibited comparable biomechanical performance as compared with other posterior short-segment constructs. The potential advantage of this new fixation system may provide researchers and clinical practitioners an alternative for minimally invasive spinal fixation with vertebral augmentation.

Long versus Short Segment Instrumentation in Osteoporotic Thoracolumbar Vertebral Fracture

Study Design

Retrospective comparative study.

Purpose

This study aimed to compare clinical and radiological data and rate of mechanical complications in elderly patients treated with short segment (SSS) or long segment stabilization (LSS) for thoracolumbar junction osteoporotic vertebral fractures (OVFs).

Overview of Literature

A fervent debate is now focused on the treatment of OVF using SSS or LSS. High rate of complications is associated with pedicle screw fixation because of poor bone quality.

Methods

Patients over 65 years old with a T-score of <−2.5, affected by (T10–L2) vertebral fracture treated with LSS or SSS pedicle screw fixation, with at least 24 months of follow-up were evaluated. All patients were analyzed with conventional X-ray to evaluate bisegmental kyphotic angle (BKA) and clinically with Visual Analog Scale (VAS), Oswestry Disability Index (ODI), and rate of mechanical complications at 2, 6, 12, and 24 months. Data were expressed as mean±standard deviation. Student t-test was used to compare clinical scores between populations. Mann-Whitney U-test was used to analyze clinical and radiological variable, whereas Fisher’s exact test was used to identify differences in the rate of complications between groups.

Results

A total of 37 patients met the inclusion criteria. Mean follow-up was 33.97±9.26 months. For both groups, ODI and VAS significantly decreased over time with good results (p<0.00001). At the final follow-up, no significant differences were found in terms of ODI and VAS. There was no difference in correction of BKA between groups; however, a significant difference was found in LSS group between pre- and postoperative BKA (p=0.046), whereas no difference was found in SSS group. A significant difference in the rate of mechanical complications was found between groups (p=0.011).

Conclusions

Both treatments showed good clinical and radiological results; however, LSS group showed better BKA correction and lower mechanical complications than SSS group.

Traumatic Fractures of the Thoracic Spine

The majority of traumatic vertebral fractures occur at the thoracolumbar junction and the lumbar spine and less commonly at the mid-thoracic and upper thoracic spine. In accordance, a high number of articles are dealing with thoracolumbar fractures focusing on the thoracolumbar junction. Nonetheless, the biomechanics of the thoracic spine differ from the thoracolumbar junction and the lumbar vertebral spine. The aim of this review is to screen the literature dealing with acute traumatic thoracic vertebral fractures in patients with normal bone quality. Thereby, the diagnostic of thoracic vertebral body fractures should include a CT examination. Ideally, the CT should include the whole thoracic cage particularly in patients suffering high energy accidents or in those with clinical suspicion of concomitant thoracic injuries. Generally, concomitant thoracic injuries are frequently seen in patients with thoracic spine fractures. Particularly sternal fractures cause an increase in fracture instability. In case of doubt, long segment stabilization is recommended in patients with unstable mid- und upper thoracic fractures, particularly in those patients with a high grade of instability.