The effect of screw tunnels on the biomechanical stability of vertebral body after pedicle screws removal: a finite element analysis

The effect of different fixation systems on oblique lumbar interbody fusion under vibration conditions

Despite the fact that lower back pain caused by degenerative lumbar spine pathologies seriously affects the quality of life, however, there is a paucity of research on the biomechanical properties of different auxiliary fixation systems for its primary treatment (oblique lumbar interbody fusion) under vibratory environments. In order to study the effects of different fixation systems of OLIF surgery on the vibration characteristics of the human lumbar spine under whole-body vibration (WBV), a finite element (FE) model of OLIF surgery with five different fixation systems was established by modifying a previously established model of the normal lumbar spine (L1-S1). In this study, a compressive follower load of 500 N and a sinusoidal axial vertical load of ±40 N at the frequency of 5 Hz with a duration of 0.6 s was applied. The results showed that the bilateral pedicle screw fixation model had the highest resistance to cage subsidence and maintenance of disc height under WBV. In contrast, the lateral plate fixation model exerted very high stresses on important tissues, which would be detrimental to the patient's late recovery and reduction of complications. Therefore, this study suggests that drivers and related practitioners who are often in vibrating environments should have bilateral pedicle screws for OLIF surgery, and side plates are not recommended to be used as a separate immobilization system. Additionally, the lateral plate is not recommended to be used as a separate fixation system.

Comparative efficacy of robotic-assisted and freehand techniques for pedicle screw placement in spinal disorders: a meta-analysis and systematic review

The efficacy and safety of robotic-assisted pedicle screw placement compared to traditional fluoroscopy-guided techniques are of great interest in the field of spinal surgery. This systematic review and meta-analysis aimed to compare the outcomes of these two methods in patients with spinal diseases. Following the PRISMA guidelines, we conducted a systematic search across PubMed, Embase, Web of Science, and Cochrane Library. We included randomized controlled trials comparing robotic-assisted and fluoroscopy-guided pedicle screw placement in patients with spinal diseases. Outcome measures included the accuracy of pedicle screw placement, postoperative complication rates, intraoperative radiation exposure time, and duration of surgery. Data were analyzed using Stata software. Our analysis included 12 studies. It revealed significantly higher accuracy in pedicle screw placement with robotic assistance (odds ratio [OR] = 2.83, 95% confidence interval [CI] = 2.20-3.64, P < 0.01). Postoperative complication rates, intraoperative radiation exposure time, and duration of surgery were similar between the two techniques (OR = 0.72, 95% CI = 0.31 to 1.68, P = 0.56 for complication rates; weighted mean difference [WMD] = - 0.13, 95% CI = - 0.93 to 0.68, P = 0.86 for radiation exposure time; WMD = 0.30, 95% CI = - 0.06 to 0.66, P = 0.06 for duration of surgery). Robotic-assisted pedicle screw placement offers superior placement accuracy compared to fluoroscopy-guided techniques. Postoperative complication rates, intraoperative radiation exposure time, and duration of surgery were comparable for both methods. Future studies should explore the potential for fewer complications with the robotic-assisted approach as suggested by the lower point estimate.

The cranial vertebral body suffers a higher risk of adjacent vertebral fracture due to the poor biomechanical environment in patients with percutaneous vertebralplasty

BACKGROUND CONTEXT

Adjacent vertebral fracture (AVF), a frequent complication of PVP, is influenced by factors such as osteoporosis progression, increased intervertebral cement leakage (ICL), and biomechanical deterioration. Notably, the risk of AVF is notably elevated in the cranial vertebral body compared with the caudal counterpart. Despite this knowledge, the underlying pathological mechanism remains elusive.

PURPOSE

This study delves into the role of biomechanical deterioration as a pivotal factor in the heightened risk of AVF in the cranial vertebral body following PVP. By isolating this variable, we aim to unravel its prominence relative to other potential risk factors.

STUDY DESIGN

A retrospective study and corresponding numerical mechanical simulations.

PATIENT SAMPLE

Clinical data from 101 patients treated by PVP were reviewed in this study.

OUTCOME MEASURES

Clinical assessments involved measuring Hounsfield unit (HU) values of adjacent vertebral bodies as a representation of patients' bone mineral density (BMD). Additionally, the rates of ICL were compared among these patients. Numerical simulations were conducted to compute stress values in the cranial and caudal vertebral bodies under various body positions.

METHODS

In a retrospective analysis of PVP patients spanning July 2016 to August 2019, we scrutinized the HU values of adjacent vertebral bodies to discern disparities in BMD between cranial and caudal regions. Additionally, we compared ICL rates on both cranial and caudal sides. To augment our investigation, well-validated numerical models simulated the PVP procedure, enabling the computation of maximum stress values in cranial and caudal vertebral bodies across varying body positions.

RESULTS

The incidence rate of cranial AVF was significantly higher than the caudal side. No notable distinctions in HU values or ICL rates were observed between the cranial and caudal sides. The incidence of AVF showed no significant elevation in patients with ICL in either region. However, numerical simulations unveiled heightened stress values in the cranial vertebral body.

CONCLUSIONS

In patients postPVP, the cranial vertebral body faces a heightened risk of AVF, primarily attributed to biomechanical deterioration rather than lower BMD or an elevated ICL rate.

A finite element study and mathematical modeling of lumbar pedicle screw along with various design parameters

BACKGROUND

Lumbar pedicle screw is one of the most common and important elements in the field of lumbar surgery. It plays a great role in rectifying the spinal alignment and stabilization providing strength and stability to the affected area of spine. In spinal surgery, minimally invasive techniques and minor incisions are made which makes it less painful for the patients than the traditional methods. Moreover, the screws are not needed to be removed after the surgery which is yet another great advantage of the pedicle screw.

METHOD

In this study, 3D Finite Element (FE) model of human L4 vertebrae is taken for analysis using image processing tool. Pedicle screw design with varying mechanical and geometrical properties has been carried out at different applied loads on it along with considering the effect of frictional forces between all contact surfaces.

RESULT

Mathematical relationship among stress, strain, pitch of the screw and diameter have been developed for different thread profiles which will be beneficial for researchers for further development of pedicle screw implants.

CONCLUSION

Results from the different analysis shows that bending stress on the screw for different loads at triangular pitch is higher than the trapezoidal. Hence, trapezoidal thread is efficacious than triangular thread. In case of vertebral bone, the magnitude of stress is less for trapezoidal screw than triangular and stress has a linear relationship with pitch length. In term of strain, triangular thread develops more strain than trapezoidal thread. A set of mathematical relation has been developed for different thread profile based on pitch length, stress and strain which gives the idea about von Mises stress and strain.

Effect of screw tunnels on proximal femur strength after screw removal: A finite element analysis

BACKGROUND

The presence of screw tunnels in the femoral neck is a problem for patients with proximal femoral fractures after removal of internal fixation. The question of how much does the existence of the screw tunnels affect the strength of the femur and whether the patient needs to be protected with an adjunctive device has been controversial. The objective of this finite element analysis was to determine (1) whether the screw tunnels affects normal weight bearing after removal of internal fixation of a proximal femur fracture, (2) which screw tunnels parameters affect the weight bearing capacity of the entire femur.

HYPOTHESIS

The presence of the screw tunnels reduces the load-bearing capacity of the femur, and the arrangement, diameter and wall thickness of the screw tunnels affect the load-bearing capacity of the femur.

MATERIALS AND METHODS

Twenty patients who underwent surgical treatment for proximal femur fracture at our hospital were included in the study. Computed tomography (CT) values of the screw tunnel wall in the femur after removal of internal fixations were analysed. Mimics v16.0 and Hypermesh v13.0 software programs were used to generate 3-dimensional (3D) tetrahedral finite element models of the proximal femur with different screw tunnel numbers, diameters, thicknesses, and arrangements. An acetabulum exerting a vertical pressure load of 600N on the femoral head was simulated and the force on various parts of the femur in each model was calculated.

RESULTS

There was no difference in the Hounsfield Units of the tunnel walls and cortical bone of the proximal femur (893.48±61.28 vs. 926.34±58.43; p=0.091). In each of the 3D models, the cancellous bone was the first structure to reach maximal stress. The compressive strength of the femur decreased with increasing thickness of the screw tunnel wall and decreased with increasing tunnel diameter. The femoral neck model with the inverted triangle screw tunnel arrangement had the highest compressive strength.

DISCUSSION

The femoral neck with screw tunnels can withstand day-to-day stress without special intervention. For femoral neck fractures fixed with cannulated screws, inverted triangle screws are recommended; For a single screw tunnel in the femoral neck, the larger the diameter of the femoral neck internal screw channel, the weaker the load-bearing capacity of the femur.

LEVEL OF EVIDENCE

III; well-designed computational non-experimental study.

Biomechanical characteristics of 2 different posterior fixation methods of bilateral pedicle screws: A finite element analysis

BACKGROUND

To explore the biomechanical characteristics of 2 posterior bilateral pedicle screw fixation methods using finite element analysis.

METHODS

A normal L3-5 finite element model was established. Based on the verification of its effectiveness, 2 different posterior internal fixation methods were simulated: bilateral pedicle screws (model A) were placed in the L3 and L5 vertebral bodies, and bilateral pedicle screws (model B) were placed in the L3, L4, and L5 vertebral bodies. The stability and stress differences of intervertebral discs, endplates, screws, and rods between models were compared.

RESULTS

Compared with the normal model, the maximum stress of the range of motion, intervertebral disc, and endplate of the 2 models decreased significantly. Under the 6 working conditions, the 2 internal fixation methods have similar effects on the stress of the endplate and intervertebral disc, but the maximum stress of the screws and rods of model B is smaller than that of model A.

CONCLUSIONS

Based on these results, it was found that bilateral pedicle screw fixation in 2 vertebrae L3 and L5 can achieve similar stability as bilateral pedicle screw fixation in 3 vertebrae L3, L4, and L5. However, the maximum stress of the screw and rod in model B is less than that in model A, so this internal fixation method can effectively reduce the risk of fracture. The 3-dimensional finite element model established in this study is in line with the biomechanical characteristics of the spine and can be used for further studies on spinal column biomechanics. This information can serve as a reference for clinicians for surgical selection.

Poor bone mineral density aggravates adjacent segment's motility compensation in patients with oblique lumbar interbody fusion with and without pedicle screw fixation: An in silico study

Objective

Motility compensation increases the risk of adjacent segment diseases (ASDs). Previous studies have demonstrated that patients with ASD have a poor bone mineral density (BMD), and changes in BMD affect the biomechanical environment of bones and tissues, possibly leading to an increase in ASD incidence. However, whether poor BMD increases the risk of ASD by aggravating the motility compensation of the adjacent segment remains unclear. The present study aimed to clarify this relationship in oblique lumbar interbody fusion (OLIF) models with different BMDs and additional fixation methods.

Methods

Stand-alone (S-A) OLIF and OLIF fixed with bilateral pedicle screws (BPS) were simulated in the L4–L5 segment of our well-validated lumbosacral model. Range of motions (ROMs) and stiffness in the surgical segment and at the cranial and caudal sides’ adjacent segments were computed under flexion, extension, and unilateral bending and axial rotation loading conditions.

Results

Under most loading conditions, the motility compensation of both cranial and caudal segments adjacent to the OLIF segment steeply aggravated with BMD reduction in S-A and BPS OLIF models. More severe motility compensation of the adjacent segment was observed in BPS models than in S-A models. Correspondingly, the surgical segment's stiffness of S-A models was apparently lower than that of BPS models (S-A models showed higher ROMs and lower stiffness in the surgical segment).

Conclusion

Poor BMD aggravates the motility compensation of adjacent segments after both S-A OLIF and OLIF with BPS fixation. This variation may cause a higher risk of ASD in OLIF patients with poor BMD. S-A OLIF cannot provide instant postoperative stability; therefore, the daily motions of patients with S-A OLIF should be restricted before ideal interbody fusion to avoid surgical segment complications.

Biomechanical study of oblique lumbar interbody fusion (OLIF) augmented with different types of instrumentation: a finite element analysis

Background

To explore the biomechanical differences in oblique lumbar interbody fusion (OLIF) augmented by different types of instrumentation.

Methods

A three-dimensional nonlinear finite element (FE) model of an intact L3-S1 lumbar spine was built and validated. The intact model was modified to develop five OLIF surgery models (Stand-alone OLIF; OLIF with lateral plate fixation [OLIF + LPF]; OLIF with unilateral pedicle screws fixation [OLIF + UPSF]; OLIF with bilateral pedicle screws fixation [OLIF + BPSF]; OLIF with translaminar facet joint fixation + unilateral pedicle screws fixation [OLIF + TFJF + UPSF]) in which the surgical segment was L4–L5. Under a follower load of 500 N, a 7.5-Nm moment was applied to all lumbar spine models to calculate the range of motion (ROM), equivalent stress peak of fixation instruments (ESPFI), equivalent stress peak of cage (ESPC), equivalent stress peak of cortical endplate (ESPCE), and equivalent stress average value of cancellous bone (ESAVCB).

Results

Compared with the intact model, the ROM of the L4–L5 segment in each OLIF surgery model decreased by > 80%. The ROM values of adjacent segments were not significantly different. The ESPFI, ESPC, and ESPCE values of the OLIF + BPSF model were smaller than those of the other OLIF surgery models. The ESAVCB value of the normal lumbar model was less than the ESAVCB values of all OLIF surgical models. In most postures, the ESPFI, ESPCE, and ESAVCB values of the OLIF + LPF model were the largest. The ESPC was higher in the Stand-alone OLIF model than in the other OLIF models. The stresses of several important components of the OLIF + UPSF and OLIF + TFJF + UPSF models were between those of the OLIF + LPF and OLIF + BPSF models.

Conclusions

Our biomechanical FE analysis indicated the greater ability of OLIF + BPSF to retain lumbar stability, resist cage subsidence, and maintain disc height. Therefore, in the augmentation of OLIF, bilateral pedicle screws fixation may be the best approach.

Biomechanical Effects of Pedicle Screw Positioning on the Surgical Segment in Models After Oblique Lumbar Interbody Fusion: An in-silico Study

Background

Bilateral pedicle screw (BPS) is the “gold standard” of fixation methods for patients with lumbar interbody fusion. Biomechanical deterioration initially triggers complications in the surgical segment. Studies proved that BPS positions and trajectory changes affect the local biomechanical environment. However, no study illustrates the biomechanical effect of insertional screw positions’ change on the surgical segment.

Methods

Oblique lumbar interbody fusion (OLIF) with different BPS insertional positions has been simulated in a well-validated lumbo-sacral model. Fixation stability and stress responses on the surgical segment were evaluated under identical loading conditions.

Results

There is no clear variation tendency for the risk of BPS failure and the change of strain energy density of the grafted bone. However, shifting the insertional screw position close to the surgical segment will increase the range of motions (ROM) in the surgical segment and lead to stress concentration of bony structures, especially in the caudal side of the surgical segment.

Conclusion

Adjusting the insertional position of BPS close to the surgical segment in OLIF models will lead to stress concentration of bony structures and surgical segmental instability. Therefore, reducing BPS’s fixation length was not recommended, which may increase the risk of segmental instability, non-union, and cage subsidence.

Thoracic vertebra fixation with a novel screw-plate system based on computed tomography imaging and finite element method

BACKGROUND AND OBJECTIVE

The traditional pedicle screw-rod internal fixation system has been widely used for thoracic diseases in clinical practice, but its high profile increases the damage to soft tissue, leading to long-term intractable back stiffness. The purpose of this study is to compare biomechanical advantages between the new spine pedicle screw-plate internal fixation system and traditional pedicle screw-rod internal fixation system using finite element analysis.

METHODS

Based on computed tomography (CT), four three-dimensional finite element models of T7-T9 were constructed. The downward concentrated force of 150 N and the moment of 5 Nm was applied to the models to simulate six physiological activities, including flexion, extension, left and right lateral bending, left and right axial torsion. The maximum displacement, range of motion (ROM) and maximum stress of the two models in six physiological activities, was measured to evaluate the biomechanical advantages of the novel pedicle screw-plate internal fixation system.

RESULTS

The novel pedicle screw-plate internal fixation system has a lower profile than the traditional pedicle screw-rod internal fixation system. With regards to the stability, the maximum displacement of the models of two internal fixation systems decreased by 56.2%-91.4% under the six motion status when comparing with the unstable model. Meanwhile, the ROM remained unchanged between the two models of internal fixation systems besides the left lateral bending. However, there is no significant difference in the ROM between the models of the two internal fixation systems in left lateral bending motion (P = 0.203). In terms of the strength, the maximum stress in the model with the new pedicle screw-plate internal fixation system was higher than that of model with the traditional pedicle screw-rod internal fixation system in every motion status but left and right lateral bending motion.

CONCLUSIONS

The novel pedicle screw-plate internal fixation system has lower profile in orthopedics and higher strength, However, it has no disadvantage when comparing with the traditional pedicle screw-rod internal fixation system in terms of the stability. In summary, we suggest that the novel spine pedicle screw-plate system can be used as a new internal fixation and provide better comfort for patients.

[A finite element analysis of petal-shaped poly-axial locking plate fixation in treatment of Y-shaped patellar fracture]

Objective

To establish the finite element model of Y-shaped patellar fracture fixed with titanium-alloy petal-shaped poly-axial locking plate and to implement the finite element mechanical analysis.

Methods

The three-dimensional model was created by software Mimics 19.0, Rhino 5.0, and 3-Matic 11.0. The finite element analysis was implemented by ANSYS Workbench 16.0 to calculate the Von-Mises stress and displacement. Before calculated, the upper and lower poles of the patella were constrained. The 2.0, 3.5, and 4.4 MPa compressive stresses were applied to the 1/3 patellofemoral joint surface of the lower, middle, and upper part of the patella respectively, and to simulated the force upon patella when knee flexion of 20, 45, and 90°.

Results

The number of nodes and elements of the finite element model obtained was 456 839 and 245 449, respectively. The max value of Von-Mises stress of all the three conditions simulated was 151.48 MPa under condition simulating the knee flexion of 90°, which was lower than the yield strength value of the titanium-alloy and patella. The max total displacement value was 0.092 8 mm under condition simulating knee flexion of 45°, which was acceptable according to clinical criterion. The stress concentrated around the non-vertical fracture line and near the area where the screws were sparse.

Conclusion

The titanium-alloy petal-shaped poly-axial locking plate have enough biomechanical stiffness to fix the Y-shaped patellar fracture, but the result need to be proved in future.