Biomechanical effects of vertebroplasty on thoracolumbar burst fracture with transpedicular fixation: a finite element model analysis

Finite element analysis of biomechanical effects of percutaneous cement discoplasty in scoliosis

OBJECTIVE

To investigate the effect of bone cement on the vertebral body and biomechanical properties in percutaneous cement discoplasty (PCD) for degenerative lumbar disc disease.

METHODS

Three-dimensional reconstruction of L2 ~ L3 vertebral bodies was performed in a healthy volunteer, and the corresponding finite element model of the spine was established. Biomechanical analysis was performed on the changes in stress distribution in different groups of models by applying quantitative loads.

RESULTS

Models with percutaneous discoplasty (PCD) showed improved stability under various stress conditions, and intervertebral foraminal heights were superior to models without discoplasty.

CONCLUSION

Cement discoplasty can improve the stability of the vertebral body to a certain extent and restore a certain height of the intervertebral foramen, which has a good development prospect and potential.

Finite element analysis of biomechanical effects of mineralized collagen modified bone cement on adjacent vertebral body after vertebroplasty

Objective: To investigate whether mineralized collagen modified polymethyl methacrylate (MC-PMMA) bone cement impacts the implanted vertebral body and adjacent segments and the feasibility of biomechanical properties compared with common bone cement in the treatment of osteoporotic vertebral compression fractures (OVCF). Methods: A healthy volunteer was selected to perform a three-dimensional reconstruction of the T11-L1 vertebral body to establish the corresponding finite element model of the spine, and the changes in the stress distribution of different types of cement were biomechanically analyzed in groups by applying quantitative loads. Results: The stress distribution of the T11-L1 vertebral body was similar between the two bone types of cement under various stress conditions. Conclusion: Mineralized collagen modified bone cement had the advantages of promoting bone regeneration, good biocompatibility, good transformability, and coupling, and had support strength not inferior to common PMMA bone cement, indicating it has good development prospects and potential.

A 20-Year Review of Biomechanical Experimental Studies on Spine Implants Used for Percutaneous Surgical Repair of Vertebral Compression Fractures

A vertebral compression fracture (VCF) is an injury to a vertebra of the spine affecting the cortical walls and/or middle cancellous section. The most common risk factor for a VCF is osteoporosis, thus predisposing the elderly and postmenopausal women to this injury. Clinical consequences include loss of vertebral height, kyphotic deformity, altered stance, back pain, reduced mobility, reduced abdominal space, and reduced thoracic space, as well as early mortality. To restore vertebral mechanical stability, overall spine function, and patient quality of life, the original percutaneous surgical intervention has been vertebroplasty, whereby bone cement is injected into the affected vertebra. Because vertebroplasty cannot fully restore vertebral height, newer surgical techniques have been developed, such as kyphoplasty, stents, jacks, coils, and cubes. But, relatively few studies have experimentally assessed the biomechanical performance of these newer procedures. This article reviews over 20 years of scientific literature that has experimentally evaluated the biomechanics of percutaneous VCF repair methods. Specifically, this article describes the basic operating principles of the repair methods, the study protocols used to experimentally assess their biomechanical performance, and the actual biomechanical data measured, as well as giving a number of recommendations for future research directions.

Biomechanical Analysis of the External Fixation in a Lumbar Fracture Model: A Finite Element Study

Purpose

This study aimed to investigate the biomechanical characteristics of the external spinal fixation for treating lumbar fracture through finite element analysis (FEA) and provide a theoretical basis for its further application.

Methods

Two different models of L3 fracture fixed with the external spinal fixation and the internal fixation system respectively were constructed. The ROM, maximum stresses at L3, and the screws of the two models were measured under load control. Subsequently, the applied torque, the maximum stressed at L3, L1/2, L2/3, L3/4, L4/5 discs and the screws were analyzed under displacement control.

Results

Under load control, the external fixation model reserved more ROM than the internal fixation model (40.4–48.0% vs 30.5–41.0%). Compared to the internal fixation model, the maximum stresses at L3 and screws in the external fixation model were increased. Under displacement control, the external fixation model required fewer moments (N·mm) than the internal fixation model (flexion: 7500 vs 12,294; extension: 7500 vs 9027). Further, the maximum stresses at L3 and the screws in the external fixation model were greater than those of the internal fixation model, while the maximum stresses at the upper and lower adjacent discs of fixed segments were less than the internal fixation model.

Conclusion

Compared to the internal fixation system, the external fixation has a better stress distribution with the greater overall mobility. It theoretically reduces the stress concentration of the adjacent discs and the stress shielding of the fractured vertebral body.

Finite Element Analysis of a New Type of Spinal Protection Device for the Prevention and Treatment of Osteoporotic Vertebral Compression Fractures

Objective

To study the effectiveness of a new spinal protection device for preventing and treating osteoporotic vertebral compression fractures (OVCFs) by finite element analysis (FEA).

Methods

One healthy volunteer and one patient with 1‐segment lumbar vertebral compression fractures were included in this experimental study. The DICOM files of two different lumbar spiral computed tomography (CT) scans were converted into STL files, and 3D finite element models of the lumbar spine were generated for normal and L1 vertebral fracture spines. A new type of spinal protection device was applied to reduce the stress on the anterior vertebral edge and direct the center of gravity posteriorly. The stress distribution characteristics of different finite element models of the lumbar spine were analyzed, revealing the characteristics of the stress distributed along the spine under the action of the new spinal protection device.

Results

Under normal conditions, the stress was mainly distributed in the middle and posterior columns of the spine. When the anterior border of the L1 vertebral body was fractured and collapsed, the stress distribution shifted toward the anterior column due to the center of gravity being directed forward. According to finite element analysis of the spine with the new protection device, the stress in the middle and posterior columns tended to increase, and that in the anterior column decreased. After the new type of spinal fixation device was applied, the stress at the L1 and L2 vertebral endplates decreased to a certain extent, especially that at the L1 vertebral body. The maximum stress on the L1 vertebral body decreased by 20% after the auxiliary device was applied.

Conclusions

According to the FEA results, the new spinal protection device can effectively prevent and treat osteoporotic vertebral compression fractures (OVCFs), and can alter the stress distribution in the spine and reduce the stress in the anterior column of the vertebral body, especially in vertebral compression fractures.

Biomechanical comparative evaluation of percutaneous fixations with vertebral expansion for vertebral compression fractures: an experimental and finite element study

This study uses in vitro experiments and validated finite element models (FEM) to analyze the effect of posterior fixation, alone or associated with expandable device (ED) and/or cement. 3-dimensional FEMs of intact, fractured and instrumented spine were built and compared with experimental load-displacement curves. FEM ranges of motion were within the experimental corridors. Stresses appeared sensitive to both implant configuration and fracture severity with a stress reduction up to 84%. The FEM highlighted that for a same instrumental strategy, different biomechanical performances were observed according to fracture severity. When bone continuity is altered, both ED and cement may be needed.

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.

Unilateral Biportal Endoscopic-Guided Transcorporeal Vertebroplasty with Neural Decompression for Treating a Traumatic Lumbar Fracture of L5

BACKGROUND

Lumbar burst fractures (complete or incomplete) of L5 have a low incidence, accounting for 1.2% of all spinal burst fractures. Treatment for these fractures remains controversial. Decompression of neural elements and stabilization of the spine to preserve lordosis and avoid kyphotic deformity are recommended when a patient has a neurological deficit and an unstable fracture. Otherwise, the fracture could be managed conservatively.

METHODS

We report a detailed step-by-step unilateral biportal endoscopy technique used in a patient with an L5 incomplete burst fracture and neurological deficit.

RESULTS

The patient had an acceptable immediate postoperative course; lower back pain and radicular symptoms improved significantly after surgery.

CONCLUSIONS

Our unilateral biportal endoscopy technique for L5 incomplete burst fractures offers the capability to perform enough decompression of neural elements and assist other procedure-related maneuvers under direct endoscopic visualization. This technique could be considered another minimally invasive spine surgery option for treating selected patients with L5 incomplete burst fractures.

Biomechanical study of space frame structure based on bone cement screw

Stability of space frame structures with bone cement screw reinforcement by biomechanical testing was analyzed. Seven complete human spine specimens with osteoporosis were selected. Three specimens were separated into 18 vertebral bodies. Nine vertebral bodies were randomly selected and bone cement screws were implanted on both sides. Bone cement was used to form a bridge at the front end of the two screws (single vertebral group A). The other nine vertebral bodies were implanted with cement screws on both sides, but the front ends of the two screws were not bridged (single vertebral group B). The remaining spine specimens were used for biomechanical testing of the overall stability of the three-dimensional frame. The four specimens were osteotomized, and then two specimens were randomly selected. Bone cement screws were implanted on both sides of the vertebral body, and a bone cement bridge was formed at the front end of the two screws to establish a three-dimensional frame structure (multi-vertebral group A). The other two spine specimens were implanted with cement screws on both sides of the vertebral body, but the front ends of the two screws were not bridged (multi-vertebral group B). A statistical difference was found between the extractive force of bridged and non-bridged specimens. Group B showed some loosening of screws after the test. The stability of the triangle structure screw, which was formed after the bridge was established at the front end of the single-vertebral bone cement screw, was significantly enhanced. Moreover, the stability was significantly improved after the three-dimensional frame structure was established in the multi-vertebral body group, providing a new method for clinical improvement of the stability and reliability of internal fixation in patients with severe osteoporosis and spinal disease.

Comparative finite element analysis of three implants fixing stable and unstable subtrochanteric femoral fractures: Proximal Femoral Nail Antirotation (PFNA), Proximal Femoral Locking Plate (PFLP), and Reverse Less Invasive Stabilization System (LISS)

BACKGROUND

The optimal type of fixation implant for managing subtrochanteric fractures (STFs) is debated, as uncertainty continues to surround the comparative biomechanical performance of the proximal femoral nail antirotation (PFNA), proximal femoral locking plate (PFLP), and reverse less invasive stabilisation system (LISS). No studies have used finite element analysis (FEA) to compare these three devices. The objective of this study was to use FEA to compare the PFNA, PFLP, and LISS used to treat STFs based on the following criteria: (1) stress distribution on the femur and implant, (2) peak stress and stress on the medial side of the femur near the fracture site, and (3) smallest axial displacement of the femoral head.

HYPOTHESIS

Of the three implants, the PFNA has the best biomechanical performance when used for STF fixation.

METHODS

FEA was used to assess synthetic bone responses to the three implants used to fix three STF types, namely, Seinsheimer I, III, and IV, characterised by increasing bone loss and/or comminution with subsequent instability. Loading was with 1400N axial compression force.

RESULTS

The LISS and PFLP exhibited similar biomechanical properties in all three fracture types. However, with the Seinsheimer IV fracture, the triangular configuration of the PFLP resulted in stress concentration at the medial and lateral sides of the implant junction. With the Seinsheimer I and III fractures, the PFNA resulted in higher peak stress (183.85MPa and 364.58MPa, respectively) compared to the PFLP (102.90MPa and 177.52MPa) and LISS (116.55MPa and 227.97MPa). With the Seinsheimer IV fracture, peak stress was highest with LISS (2310.40MPa) and was higher with PFLP (2054.90MPa) than with PFNA (1313.30MPa). With the Seinsheimer I and III fractures, the axial femoral head displacement was greater with the PFNA (0.74mm and 1.13mm, respectively) than with the PFLP (0.48mm and 1.02mm) and LISS (0.52mm and 0.92mm). With the Seinsheimer IV fracture, in contrast, the PFNA produced less axial femoral head displacement (4.1mm) compared to the PFLP (12.03mm) and LISS (16.56mm).

DISCUSSION

With unstable (Seinsheimer IV) STFs, fixation stability was better with the PFNA compared to the PFLP and LISS. In contrast, with stable STFs (Seinsheimer I and III), the PFLP and LISS offered greater stability, with similar biomechanical effects. However, with Seinsheimer III fractures, the stress on the implant-femur interface was greater with the LISS.

LEVEL OF EVIDENCE

IV, basic science study.

Biomechanical effects of USS fixation with different screw insertion depths on the vertebrae stiffness and screw stress for the treatment of the L1 fracture

OBJECTIVE

To evaluate the biomechanical effects of internal fixation with different screw insertion depths on vertebrae stiffness and screw stress for L1 fracture.

METHODS

The established L1 fracture was fixed with 10 different depths of screw insertion: 10-100% screw-path length (SPL). Loading on the T12 endplate was simulated.

RESULTS

Screws inserted to 60-100% depths has a higher axial displacement of screw against injured vertebrae and maximum stress of screws compared to those of screws inserted to 30-50% depths and 10-20% (P< 0.05). No significant difference was noted among 60-100% SPL groups. Under single loading condition, the incidence rate of maximum stress of each screw ranged from 16.7-50.0%. Chi-square test showed superior screw has a higher incidence rate of maximum stress than inferior screw (P< 0.05).

CONCLUSIONS

Screws inserted to 60% depth or more can achieve effective strength to withstand the postoperative height correction loss of the L1 vertebrae fracture. However, continuous prolonged depth of screw insertion did not significantly increase the effective strength of the screw against injured vertebrae and maximum equivalent stress of screws. The incidence rate of the maximum stress of each screw in correlated with position of screw insertion but not associated with the screw insertion depth.

Biomechanical effects of different vertebral heights after augmentation of osteoporotic vertebral compression fracture: a three-dimensional finite element analysis

Background

Clinical results have shown that different vertebral heights have been restored post-augmentation of osteoporotic vertebral compression fractures (OVCFs) and the treatment results are consistent. However, no significant results regarding biomechanical effects post-augmentation have been found with different types of vertebral deformity or vertebral heights by biomechanical analysis. Therefore, the present study aimed to investigate the biomechanical effects between different vertebral heights of OVCFs before and after augmentation using three-dimensional finite element analysis.

Methods

Four patients with OVCFs of T12 underwent computed tomography (CT) of the T11-L1 levels. The CT images were reconstructed as simulated three-dimensional finite-element models of the T11-L1 levels (before and after the T12 vertebra was augmented with cement). Four different kinds of vertebral height models included Genant semi-quantitative grades 0, 1, 2, and 3, which simulated unilateral augmentation. These models were assumed to represent vertical compression and flexion, left flexion, and right flexion loads, and the von Mises stresses of the T12 vertebral body were assessed under different vertebral heights before and after bone cement augmentation.

Results

Data showed that the von Mises stresses significantly increased under four loads of OVCFs of the T12 vertebral body before the operation from grade 0 to grade 3 vertebral heights. The maximum stress of grade 3 vertebral height pre-augmentation was produced at approximately 200%, and at more than 200% for grade 0. The von Mises stresses were significantly different between different vertebral heights preoperatively. The von Mises stresses of the T12 vertebral body significantly decreased in four different loads and at different vertebral body heights (grades 0–3) after augmentation. There was no significant difference between the von Mises stresses of grade 0, 1, and 3 vertebral heights postoperatively. The von Mises stress significantly decreased between pre-augmentation and post-augmentation in T12 OVCF models of grade 0–3 vertebral heights.

Conclusion

Vertebral augmentation can sufficiently reduce von Mises stresses at different heights of OVCFs of the vertebral body, although this technique does not completely restore vertebral height to the anatomical criteria.

A REVIEW ON BIOMECHANICAL ASPECTS OF VERTEBROPLASTY AND KYPHOPLASTY USING FINITE ELEMENT MODELING-BASED METHODS

The vertebroplasty (VP) and kyphoplasty (KP) are two minimally invasive surgeries using cement augmentation to treat the osteoporotic vertebrae in elderlies in order to relieve pain and prevent the continuation of microfractures. Biomechanists have always tried to assess the mechanical behavior of vertebrae after cement augmentation by using both the experimental and theoretical methods such as finite element modeling (FEM).

In this study, 31 related articles using FEM in analyzing the VP and KP have been reviewed. This study included two main categories of spinal load distribution and tension in vertebrae after the VP and KP operations. This could be obtained by conducting FEM on the whole spine or other sectors of it such as intervertebral disc (IVD) or end plates (EPs). This study also referred to articles predicting the probability of adjacent fractures following VP and KP. The most common software employed in FEM was ABAQUS, applied for static and dynamic loads’ analyses.

It was found that most of the reviewed articles adopted reverse engineering techniques by converting 2D computed tomography (CT) scan images into 3D reconstructed models. The material properties were generally taken from the literature. In more than 80% of studies, the model geometry was based on CT data of the spine. Almost 45% of the studies have attempted to compare the simulated vertebra after augmentation with experimental results taken from the literature (5% of the reviewed articles) or their own experimental tests (40% of the reviewed articles).

Biomechanical assessment of new surgical method instead of kyphoplasty to improve the mechanical behavior of the vertebra: Micro finite element study

AIM

To reduce post treatments of kyphoplasty, as a common treatment for osteoporotic vertebrae.

METHODS

This study suggests a new method for treating vertebrae by setting the hexagonal porous structure instead of the rigid bone cement mass in the kyphoplasty (KP). The KP procedure was performed on the fresh ovine vertebra of the level L1. Micro finite element modeling was performed based on micro computed tomography of ovine trabecular cube. The hexagonal porous structure was set on one cube instead of the bone cement mass. For the implant designing, two geometrical parameters were considered: Spacing diameter and thickness.

RESULTS

The results of micro finite element analyses indicated the improvement in the mechanical behavior of the vertebra treated by the hexagonal porous structures, as compared to those treated by vertebroplasty (VP) and KP under static loading. The improvement in the mechanical behavior of the vertebra, was observed as 54% decrease in the amount of maximum Von Misses stress (improvement of stress distribution), in trabecular cube with embedded hexagonal structure, as compared to VP and KP. This is comparable to the results of the experimental study already performed; it was shown that the improvement of mechanical behavior of the vertebra was observed as: 83% increase in the range of displacements before getting to the ultimate strength (increasing the toughness) after setting hexagonal pearls inside vertebrae. Both the material and geometry of implant influenced the amount of Von Mises stress in the structure.

CONCLUSION

The new proposed method can be offered as a substitute for the KP. The implant geometry had a more obvious effect on the amount of Von Mises stress, as compared to the implant material.

Treatment of thoracolumbar burst fractures by short-segment pedicle screw fixation using a combination of two additional pedicle screws and vertebroplasty at the level of the fracture: a finite element analysis

Background

Traditional one-above and one-below four-screw posterior short-segment instrumentation is used for unstable thoracolumbar burst fractures. However, this method has a high rate of implant failure and early loss of reduction. The purpose of this study was to use finite element (FE) analysis to determine the effect of treating thoracolumbar burst fractures by short-segment pedicle screw fixation using a combination of two additional pedicle screws and vertebroplasty at the level of the fracture.

Methods

An intact T11-L1 spine FE model was created from the computed tomography images of a male subject. Four fixation models with posterior fusion devices (pedicle screws, rods, cross-link) were established to simulate an unstable thoracolumbar fracture with different fusion surgeries: short-segment fixation with: 1) a link (S-L); 2) intermediate bilateral screws (S-I); 3) a link and calcium sulfate cement (S-L-C); 4) intermediate bilateral screws and calcium sulfate cement (S-I-C). Different loading conditions (flexion, extension, lateral bending, and axial rotation) were applied on the models and analyzed with a FE package. The range of motion (ROM), and the maximum value and distribution of the implant stress, and the stress in the facet joint, were compared between the intact and fixation models.

Results

The ROM in flexion, extension, axial rotation, and lateral bending was the smallest in the S-I-C model, followed by the S-I, S-L-C, and S-L models. Maximum von Mises stress values were larger under lateral bending and axial rotation loadings than under flexion and extension loading. High stress was concentrated at the crosslink and rod junctions. Maximal von Mises stress on the superior vertebral body for all loading conditions was larger than that on the inferior vertebral body. The maximal von Mises stress of the pedicle screws during all states of motion were 265.3 MPa in S-L fixation, 192.9 MPa in S-I fixation, 258.4 MPa in S-L-C fixation, and 162.3 MPa in S-I-C fixation.

Conclusions

Short-segment fixation with two intermediate pedicle screws together with calcium sulfate cement at the fractured vertebrae may provide a stiffer construct and less von Mises stress of the pedicle screws and rods as compared to other types of short-segment fixation.

In silico investigation of vertebroplasty as a stand-alone treatment for vertebral burst fractures

BACKGROUND

The use of percutaneous vertebroplasty as a stand-alone treatment for stable vertebral burst fractures has been investigated in vitro and in clinical studies. These studies present inconsistent results on the mechanical response of vertebroplasty-treated burst fractures. In addition, observations of the loss of sagittal alignment after vertebroplasty raise questions on the applicability of vertebroplasty for burst fractures. Therefore, the aim of this study was to investigate the mechanical stability of burst fractures after stand-alone treatment by vertebroplasty.

METHODS

Finite element simulations were performed with models generated from two laboratory-induced burst fractures in human thoracolumbar specimens. The burst fracture models were virtually injected with various cement volumes using a unipedicular or bipedicular approach. The models were subjected to four individual loads (compression, lateral bending, extension and torsion) and a multi-axial load case in the physiological range.

FINDINGS

All treated burst fractures showed improvements in stiffness and a reduction in inter-fragmentary displacements, thus potentially providing a suitable mechanical environment for fracture healing. However, large volumes of the trabecular bone (<43%), cement (<53%) and bone-cement composite (<58%) were predicted to experience strain levels exceeding the yield point. While damage was not specifically modeled, this implies a potential collapse of the treated vertebra due to local failure.

INTERPRETATION

To improve the primary stability and to prevent the collapse of treated burst fractures, the use of posterior instrumentation is suggested as an adjunct to vertebroplasty.

The biomechanics of pedicle screw augmentation with cement

BACKGROUND CONTEXT

A persistent challenge in spine surgery is improving screw fixation in patients with poor bone quality. Augmenting pedicle screw fixation with cement appears to be a promising approach.

PURPOSE

The purpose of this study was to survey the literature and assess the previous biomechanical studies on pedicle screw augmentation with cement to provide in-depth discussions of the biomechanical benefits of multiple parameters in screw augmentation.

STUDY DESIGN/SETTING

This is a systematic literature review.

METHODS

A search of Medline was performed, combining search terms of pedicle screw, augmentation, vertebroplasty, kyphoplasty, polymethylmethacrylate, calcium phosphate, or calcium sulfate. The retrieved articles and their references were reviewed, and articles dealing with biomechanical testing were included in this article.

RESULTS

Polymethylmethacrylate is an effective material for enhancing pedicle screw fixation in both osteoporosis and revision spine surgery models. Several other calcium ceramics also appear promising, although further work is needed in material development. Although fenestrated screw delivery appears to have some benefits, it results in similar screw fixation to prefilling the cement with a solid screw. Some differences in screw biomechanics were noted with varying cement volume and curing time, and some benefits from a kyphoplasty approach over a vertebroplasty approach have been noted. Additionally, in cadaveric models, cemented-augmented screws were able to be removed, albeit at higher extraction torques, without catastrophic damage to the vertebral body. However, there is a risk of cement extravasation leading to potentially neurological or cardiovascular complications with cement use. A major limitation of these reviewed studies is that biomechanical tests were generally performed at screw implantation or after a limited cyclic loading cycle; thus, the results may not be entirely clinically applicable. This is particularly true in the case of the bioactive calcium ceramics, as these biomechanical studies would not have measured the effects of osseointegration.

CONCLUSIONS

Polymethylmethacrylate and various calcium ceramics appear promising for the augmentation of pedicle screw fixation biomechanically in both osteoporosis and revision spine surgery models. Further translational studies should be performed, and the results summarized in this review will need to be correlated with the clinical outcomes.

Biomechanical effects of cement distribution in the fractured area on osteoporotic vertebral compression fractures: a three-dimensional finite element analysis

BACKGROUND

According to some clinical studies, insufficient cement distribution (ID) in the fractured area and asymmetrical cement distribution around the fractured area were thought to be the reasons for unrelieved pain and recollapse after percutaneous vertebral augmentation (PVA) in the treatment of symptomatic osteoporotic vertebral compression fractures.

METHODS

Finite element methods were used to investigate the biomechanical variance among three patterns of cement distribution (ID and sufficient cement distribution in the fractured area and asymmetrical cement distribution around the fractured area including upward [BU] and downward [BD] cement distribution).

RESULTS

Compared with fractured vertebra before PVA, distribution of von Mises stress in the cancellous bone was transferred to be concentrated at the cancellous bone surrounding cement after PVA, whereas it was not changed in the cortical bone. Compared with sufficient cement distribution group, maximum von Mises stress in the cancellous bone and cortical bone and maximum displacement of augmented vertebra increased significantly in the ID group, whereas asymmetrical cement distribution around the fractured area in BU and BD groups mainly increased maximum von Mises stress in the cancellous bone significantly. Similar results could be seen in all loading conditions.

CONCLUSIONS

ID in the fractured area may lead to unrelieved pain after PVA in the treatment of symptomatic osteoporotic vertebral compression fractures as maximum displacement of augmented vertebral body increased significantly. Both ID in the fractured area and asymmetrical cement distribution around the fractured area are more likely to induce recollapse of augmented vertebra because they increased maximum von Mises stress in the cancellous bone and cortical bone of augmented vertebra significantly.