Biomechanical assessment of a PEEK rod system for semi-rigid fixation of lumbar fusion constructs

Comparison of biomechanical effects of polyetheretherketone (PEEK) rods and titanium rods in lumbar long-segment instrumentation: a finite element study

Introduction

Polyetheretherketone (PEEK) lumbar fusion rods have been successfully used in short-segment posterior instrumentation to prevent adjacent segment degeneration. However, limited studies have reported their application in lumbar long-segment instrumentation. This study aimed to compare the biomechanical performances of PEEK rods and titanium rods in lumbar long-segment instrumentation using finite element (FE) models, with the expectation of providing clinical guidance.

Methods

A lumbar FE model (A) and four lumbar fixation FE models (BI, CI, BII, CII) of the L1-S1 vertebral body were developed using CT image segmentation (A: intact model; BI: intact model with L2-S1 PEEK rod internal fixation; CI: intact model with L2-S1 titanium rod internal fixation; BII: intact model with L3-S1 PEEK rod internal fixation; CII: intact model with L3-S1 titanium rod internal fixation). A 150-N preload was applied to the top surface of L1, similar to the intact model. The stresses on the lumbar intervertebral disc, facet joint, pedicle screws, and rods were calculated to evaluate the biomechanical effect of the different fixation procedures in lumbar long-segment instrumented surgery.

Results

Under the four physiological motion states, the average stresses on the adjacent segment intervertebral disc and facet joint in all fixation models were greater than those in the intact model. Furthermore, the average stresses on the adjacent segment intervertebral disc and facet joint were greater in models CI and CII than in models BI and BII, respectively. The average stresses on the pedicle screws and rods were decreased in models BI and BII compared with models CI and CII under the four physiological motion states, respectively.

Discussion

The PEEK rod internal fixation system may have better biomechanical properties than the titanium rod internal fixation system in delaying adjacent segment degeneration, improving the lumbar function of postoperative patients, and reducing the risk of screw loosening and breakage in lumbar long-segment instrumentation.

Retrieval analysis of PEEK rods pedicle screw system: three cases analysis

PURPOSE

To analyze the characteristics of PEEK rods retrieved in vivo, specifically their wear and deformation, biodegradability, histocompatibility, and mechanical properties.

METHOD

Six PEEK rods were retrieved from revision surgeries along with periprosthetic tissue. The retrieved PEEK rods were evaluated for surface damage and internal changes using Micro-CT, while light and electron microscopy were utilized to determine any histological changes in periprosthetic tissues. Patient history was gathered from medical records. Two intact and retrieved PEEK rods were used for fatigue testing analysis by sinusoidal load to the spinal construct.

RESULTS

All implants showed evidence of plastic deformation around the screw-rod interface, while the inner structure of PEEK rods appeared unchanged with no visible voids or cracks. Examining images captured through light and electron microscopy indicated that phagocytosis of macrophages around PEEK rods was less severe in comparison to the screw-rod interface. The results of an energy spectrum analysis suggested that the distribution of tissue elements around PEEK rods did not differ significantly from normal tissue. During fatigue testing, it was found that the retrieved PEEK rods cracked after 1.36 million tests, whereas the intact PEEK rods completed 5 million fatigue tests without any failure.

CONCLUSION

PEEK rods demonstrate satisfactory biocompatibility, corrosion resistance, chemical stability, and mechanical properties. Nevertheless, it is observed that the indentation at the junction between the nut and the rod exhibits relatively weak strength, making it susceptible to breakage. As a precautionary measure, it is recommended to secure the nut with a counter wrench, applying the preset torque to prevent overtightening.

Biomechanical comparison of polyetheretherketone rods and titanium alloy rods in transforaminal lumbar interbody fusion: a finite element analysis

BACKGROUND

Whether polyetheretherketone (PEEK) rods have potential as an alternative to titanium alloy (Ti) rods in transforaminal lumbar interbody fusion (TLIF) remains unclear, especially in cases with insufficient anterior support due to the absence of a cage. The purpose of this study was to investigate biomechanical differences between PEEK rods and Ti rods in TLIF with and without a cage.

METHODS

An intact L1-L5 lumbar finite element model was constructed and validated. Accordingly, four TLIF models were developed: (1) Ti rods with a cage; (2) PEEK rods with a cage; (3) Ti rods without a cage; and (4) PEEK rods without a cage. The biomechanical properties were then compared among the four TLIF constructs.

RESULTS

With or without a cage, no obvious differences were found in the effect of PEEK rods and Ti rods on the range of motion, adjacent disc stress, and adjacent facet joint force. Compared to Ti rods, PEEK rods increase the average bone graft strain (270.8-6055.2 µE vs. 319.0-8751.6 µE). Moreover, PEEK rods reduced the stresses on the screw-rod system (23.1-96.0 MPa vs. 7.2-48.4 MPa) but increased the stresses on the cage (4.6-35.2 MPa vs. 5.6-40.9 MPa) and endplates (5.7-32.5 MPa vs. 6.6-37.6 MPa).

CONCLUSIONS

Regardless of whether a cage was used for TLIF, PEEK rods theoretically have the potential to serve as an alternative to Ti rods because they may provide certain stability, increase the bone graft strain, and reduce the posterior instrumentation stress, which might promote bony fusion and decrease instrumentation failure.

Biomechanical effects of screws of different materials on vertebra-pediculoplasty: a finite element study

Background: The effects of cannulated screws made of polyetheretherketone (PEEK) on the biomechanical properties of the vertebral body during vertebra-pediculoplasty remain unclear. This study aimed to investigate whether PEEK screws have the potential to replace titanium alloy screws. Methods: The surgical model of two different materials of screws was constructed using the finite element method. The biomechanical effects of the two models on the vertebral body under different working conditions were compared. Results: ① The peak von Mises stress of PEEK screws was significantly lower than that of titanium screws, with a reduction ranging from 52% to 80%. ② The von Mises stress values for the injured T12 spine were similar for both materials. Additionally, the segmental range of motion and intervertebral disc pressure showed no significant difference between the two materials. Conclusion: PEEK screws demonstrated advantages over titanium screws and may serve as a viable alternative for screw materials in vertebra-pediculoplasty.

Design and 3D printing of novel titanium spine rods with lower flexural modulus and stiffness profile with optimised imaging compatibility

PURPOSE

To manufacture and test 3D printed novel design titanium spine rods with lower flexural modulus and stiffness compared to standard solid titanium rods for use in metastatic spine tumour surgery (MSTS) and osteoporosis.

METHODS

Novel design titanium spine rods were designed and 3D printed. Three-point bending test was performed to assess mechanical performance of rods, while a French bender was used to assess intraoperative rod contourability. Furthermore, 3D printed spine rods were tested for CT & MR imaging compatibility using phantom setup.

RESULTS

Different spine rod designs generated includes shell, voronoi, gyroid, diamond, weaire-phelan, kelvin, and star. Tests showed 3D printed rods had lower flexural modulus with reduction ranging from 2 to 25% versus standard rod. Shell rods exhibited highest reduction in flexural modulus of 25% (~ 77.4 GPa) and star rod exhibited lowest reduction in flexural modulus of 2% (100.8GPa). 3D printed rod showed reduction in stiffness ranging from 40 to 59%. Shell rod displayed highest reduction in stiffness of 59% (179.9 N/mm) and gyroid had least reduction in stiffness of 40% (~ 259.2 N/mm). Rod bending test showed that except gyroid, other rod designs demonstrated lesser bending difficulty versus standard rod. All 3D printed rods demonstrated improved CT/MR imaging compatibility with reduced artefacts versus standard rod.

CONCLUSION

By utilising novel design approach, we successfully generated a spine rod design portfolio with lower flexural modulus/stiffness profile and better CT/MR imaging compatibility for potential use in MSTS/other conditions such as osteoporosis. Thus, exploration of new rod designs in surgical application could enhance treatment outcome and improve quality of life for patients.

Polyetheretherketone (PEEK) rods versus titanium rods for posterior lumbar fusion surgery: a systematic review and meta-analysis

BACKGROUND

Rigid fixation, represented by titanium rods, is a widely used fixation technique for lumbar fusion. However, this technique carries the risk of degeneration of adjacent segments. In recent years, the semi-rigid fixation technique represented by PEEK rods has gradually matured, and its effectiveness has been verified by numerous studies. The aim of this study was to systematically evaluate the effectiveness of these two fixation modalities in posterior lumbar fusion surgery.

METHODS

Studies meeting the inclusion criteria were searched in PubMed, Cochrane Library, ScienceDirect, Embase, CNKI, and Wanfang databases. After data extraction and quality assessment of included studies, meta-analysis was performed using STATA 15.1 software. The protocol for this systematic review was registered on INPLASY (2021110049) and is available in full on the inplasy.com ( https://inplasy.com/inplasy-2021-11-0049/ ).

RESULTS

Fifteen relevant studies were finally included, including eight prospective studies and seven retrospective studies. The results of meta-analysis showed that in ODI (P = 0.000), JOA score (P = 0.017), VAS score for lower limb pain (P = 0.027), fusion rate of bone graft at week 12 (P = 0.001), fusion rate of bone graft at last follow-up (P = 0.028), there was a statistical difference between the two groups. The PEEK rod group was superior to the titanium rod group in the above aspects. While in VAS score for LBP (P = 0.396), there was no statistical difference between the two groups.

CONCLUSION

Both PEEK rods and titanium rods are effective fixation materials in lumbar fusion surgery. PEEK rods may be superior to titanium rods in improving postoperative function and improving bone graft fusion rates. However, given the limitations of this study, whether these conclusions are applicable needs further research.

Radiological outcomes of PEEK rods in patients with lumbar degenerative diseases: A minimum 5-year follow-up

Purpose

To determine the long-term radiological outcomes of PEEK rods in patients with lumbar degenerative diseases.

Methods

Radiological outcomes of cohort cases with lumbar degenerative diseases following PEEK rods were retrospectively studied. Disc height index (DHI) and range of motion (ROM) were measured by x-rays. The CT scans and reconstruction were used to determine screw breakage, rods fracture, screw loosening and intervertebral bony fusion status. The MRI scans were used to evaluate the changes of intervertebral discs at the non-fusion segments and adjacent segments in terms of Pfirrmann Classification.

Results

A total of 40 patients completed the mean of 74.8 ± 9.6 months follow-up, with 32 patients undergoing hybrid surgery and 8 patients undergoing non-fusion surgery. The mean DHI changed from preoperative 0.34 to 0.36 at the final follow-up and the ROM declined from 8.8° preoperatively to 3.2° at the final visit, however, both had no statistical differences. Of the 40 levels underwent non-fusion procedure, 9 levels showed disc rehydration with 7 patients from Grade 4 to Grade 3 and 2 patients from Grade 3 to Grade 2. The other 30 cases did not show distinctive change. No screw loosening or rods breakage were detected during the follow-up periods.

Conclusion

PEEK rods have obvious protective effects on degenerated intervertebral disc of non-fusion segments and the incidence of complications related to internal fixation is low. PEEK rods pedicle screw system is safe and effective in the treatment of lumbar degenerative diseases.

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.

Comparison of novel stabilisation device with various stabilisation approaches: A finite element based biomechanical analysis

The treatment of spinal failure requires suitable instrumentation, which is based on numerous concepts such as rigid fixation, semi-rigid and dynamic stabilisation. In the present work, the biomechanical investigation of various fixation systems on the lumbar segment L2-L3 was performed employing finite element analysis. Different devices were considered: novel stabilisation device (NSD), rigid implant (RI) and existing dynamic stabilisation device (EDSD). All instrumented models were loaded with a condition of 400 N compressive force with a moment of 10Nm during flexion, extension, lateral bending and axial rotation. The results of range of motion change (RMC), von-Mises stress and strain were compared. The spinal biomechanics post instrumentation resulted significantly sensitive to the geometrical feature of the implant. The obtained results showed that NSD has intermediate motion characteristics in between dynamic stabilisation and rigid fixation. However, the optimum features of a novel stabilisation device for the treatment of spinal failure still need to be verified employing in-vivo, in-vitro studies.

Biomechanical feasibility of semi-rigid stabilization and semi-rigid lumbar interbody fusion: a finite element study

BACKGROUND

Semi-rigid lumbar fusion offers a compromise between pedicle screw-based rigid fixation and non-instrumented lumbar fusion. However, the use of semi-rigid interspinous stabilization (SIS) with interspinous spacer and ligamentoplasty and semi-rigid posterior instrumentation (SPI) to assist interbody cage as fusion constructs remained controversial. The purpose of this study is to investigate the biomechanical properties of semi-rigidly stabilized lumbar fusion using SIS or SPI and their effect on adjacent levels using finite element (FE) method.

METHOD

Eight FE models were constructed to simulate the lumbosacral spine. In the non-fusion constructs, semi-rigid stabilization with (i) semi-rigid interspinous spacer and artificial ligaments (PD-SIS), and (ii) PI with semi-rigid rods were simulated (PD + SPI). For fusion constructs, the spinal models were implanted with (iii) PEEK cage only (Cage), (iv) PEEK cage and SIS (Cage+SIS), (v) PEEK cage and SPI (Cage+SPI), (vi) PEEK cage and rigid PI (Cage+PI).

RESULT

The comparison of flexion-extension range of motion (ROM) in the operated level showed the difference between Cage+SIS, Cage+SPI, and Cage+PI was less than 0.05 degree. In axial rotation, ROM of Cage+SIS were greater than Cage+PI by 0.81 degree. In the infrajacent level, while Cage+PI increased the ROM by 24.1, 27,7, 25.9, and 10.3% and Cage+SPI increased the ROM by 26.1, 30.0, 27.1, and 10.8% in flexion, extension, lateral bending and axial rotation respectively, Cage+SIS only increased the ROM by 3.6, 2.8, and 11.2% in flexion, extension, and lateral bending and reduced the ROM by 1.5% in axial rotation. The comparison of the von Mises stress showed that SIS reduced the adjacent IVD stress by 9.0%. The simulation of the strain energy showed a difference between constructs less than 7.9%, but all constructs increased the strain energy in the infradjacent level.

CONCLUSION

FE simulation showed semi-rigid fusion constructs including Cage+SIS and Cage+SPI can provide sufficient stabilization and flexion-extension ROM reduction at the fusion level. In addition, SIS-assisted fusion resulted in less hypermobility and less von Mises stress in the adjacent levels. However, SIS-assisted fusion had a disadvantage of less ROM reduction in lateral bending and axial rotation. Further clinical studies are warranted to investigate the clinical efficacy and safety of semi-rigid fusions.

Comparison of clinical and radiological results of dynamic and rigid instrumentation in degenerative lumbar spinal stenosis

Objective

Lumbar spinal stenosis is defined as a clinical syndrome characterized by neurogenic claudication or radicular pain due to the narrowing of the spinal canal or neural foramen and the compression of its neural elements. Surgical treatment is applied to decompress the neural structures. In some cases, transpedicular instrumentation and fusion may also be applied. In this study, we aimed to investigate and compare the preoperative and postoperative, clinical and radiological aspects of patients with lumbar spinal stenosis who underwent lumbar instrumentation using a polyetheretherketone (PEEK) rod or a titanium rod.

Materials and Methods:

In this study, the files of 293 patients who underwent posterior lumbar transpedicular stabilization between January 2015 and February 2018 in the Neurosurgery Clinic of Ümraniye Training and Research Hospital were reviewed retrospectively. Patients who did not meet the study criteria were excluded, and 127 patients who met the criteria and underwent posterior lumbar transpedicular stabilization due to lumbar spinal stenosis and/or lumbar degenerative disc disease were retrospectively reviewed. The patients were divided into two groups, dynamic and rigid, according to the rod types used. The two groups were compared using various postoperative clinical and radiological parameters.

Results:

The demographic data, surgical data, Visual Analog Scale-Oswestry Disability Index (VAS-ODI) data, and radiological data of both groups were carefully examined. There were 63 patients in the rigid group and 64 patients in the dynamic group. The age range in both groups was from 30 to 78 years, with a mean age of 56.44 years; 99 of the cases were female and 28 were male. The analysis of the participants' demographic data showed no significant differences between the two groups. Compared with the preoperative data, the postoperative evaluations revealed a significant decrease in VAS and ODI, but no significant difference was observed between the two groups. There was no difference between the two groups in terms of duration of surgery, follow-up time, operating distances, hospitalization duration, pseudoarthrosis, or fusion. Regarding the total and segmental range of motion, the affection was less in the dynamic group, which allowed for more movement. While there was no difference in disc height index between the two preoperative groups, it was observed that it was better maintained in the rigid group in the postoperative long term. Regarding foraminal height (FH), there was no difference between the two groups in the preoperative and early postoperative periods, but in the long term, FH was better maintained in the dynamic group. The long-term follow-ups revealed that adjacent segment disease (ASD) had developed in 19 patients in the rigid group, whereas ASD developed in only nine patients in the dynamic group. Based on these results, the probability of developing significant ASD in the rigid group was higher.

Conclusion:

Previous experience with PEEK rod systems has demonstrated physiological spine movement, increased fusion rates, minimal complications, reduction in adjacent segment degeneration, and biomechanical compatibility. Although further long-term studies are needed and the cost of PEEK systems is likely to be a barrier, the results of the present study support the use of PEEK rods and other dynamic systems in spinal surgery.

Evolution of materials for implants in metastatic spine disease till date - Have we found an ideal material?

"Metastatic Spine Disease" (MSD) often requires surgical intervention and instrumentation with spinal implants. Ti6Al4V is widely used in metastatic spine tumor surgery (MSTS) and is the current implant material of choice due to improved biocompatibility, mechanical properties, and compatibility with imaging modalities compared to stainless steel. However, it is still not the ideal implant material due to the following issues. Ti6Al4V implants cause stress-shielding as their Young's modulus (110 gigapascal [GPa]) is higher than cortical bone (17-21 GPa). Ti6Al4V also generates artifacts on CT and MRI, which interfere with the process of postoperative radiotherapy (RT), including treatment planning and delivery. Similarly, charged particle therapy is hindered in the presence of Ti6Al4V. In addition, artifacts on CT and MRI may result in delayed recognition of tumor recurrence and postoperative complications. In comparison, polyether-ether-ketone (PEEK) is a promising alternative. PEEK has a low Young's modulus (3.6 GPa), which results in optimal load-sharing and produces minimal artifacts on imaging with less hinderance on postoperative RT. However, PEEK is bioinert and unable to provide sufficient stability in the immediate postoperative period. This issue may possibly be mitigated by combining PEEK with other materials to form composites or through surface modification, although further research is required in these areas. With the increasing incidence of MSD, it is an opportune time for the development of spinal implants that possess all the ideal material properties for use in MSTS. Our review will explore whether there is a current ideal implant material, available alternatives and whether these require further investigation.

Biomechanical analysis of lumbar interbody fusion supplemented with various posterior stabilization systems

PURPOSE

Biomechanical comparison between rigid and non-rigid posterior stabilization systems following lumbar interbody fusion has been conducted in several studies. However, most of these previous studies mainly focused on investigating biomechanics of adjacent spinal segments or spine stability. The objective of the present study was to compare biomechanical responses of the fusion devices when using different posterior instrumentations.

METHODS

Finite-element model of the intact human lumbar spine (L1-sacrum) was modified to simulate implantation of the fusion cage at L4-L5 level supplemented with different posterior stabilization systems including (i) pedicle screw-based fixation using rigid connecting rods (titanium rods), (ii) pedicle screw-based fixation using flexible connecting rods (PEEK rods) and (iii) dynamic interspinous spacer (DIAM). Stress responses were compared among these various models under bending moments.

RESULTS

The highest and lowest stresses in endplate, fusion cage and bone graft were found at the fused L4-L5 level with DIAM and titanium rod stabilization systems, respectively. When using PEEK rod for the pedicle screw fixation, peak stress in the pedicle screw was lower but the ratio of peak stress in the rods to yield stress of the rod material was higher than using titanium rod.

CONCLUSIONS

Compared with conventional rigid posterior stabilization system, the use of non-rigid stabilization system (i.e., the PEEK rod system and DIAM system) following lumbar interbody fusion might increase the risks of cage subsidence and cage damage, but promote bony fusion due to higher stress in the bone graft. For the pedicle screw-based rod stabilization system, using PEEK rod might reduce the risk of screw breakage but increased breakage risk of the rod itself.

Lumbar Fusion With Polyetheretherketone Rods Use for Patients With Degenerative Disease

Introduction

Polyetheretherketone (PEEK) rods for lumbar fusion have been available since 2007. However, literature about their utility is sparse and of mixed outcomes.

Methods

A retrospective review of PEEK rod lumbar fusion cases was performed. Data were analyzed from 108 patients of the senior author Donald Ross who underwent PEEK lumbar fusion.

Results

There were 97 single and 11 2-level fusions. Rates of tobacco use, diabetes mellitus, low bone density, depression, and immunosuppression were 23.1%, 24.1%, 14.8%, 32.4%, and 6.5%, respectively. In the study population, the mean age was 60.2 years, body mass index was 30.1, and there was a mean 31.3 months for follow-up. There were no wound infections or new neurologic deficits. Of 81 patients with > 11 months of follow-up, 70 (86.4%) had an arthrodesis, 8 (9.9%) had no arthrodesis, and 3 (3.7%) were indeterminate. No patients had revision fusion surgery and 2 patients had adjacent level fusions at 27 and 60 months. One patient had an adjacent segment laminectomy at 18 months and one a foraminotomy at 89 months, resulting in a 3.7% adjacent segment surgery rate. Mean preoperative Short Form-36 (SF-36) physical functioning (PF) score and Oswestry Disability Index (ODI) score were 28.9 and 24.8, respectively. Mean SF-36 PF postoperative score at 1 and 2 years were 59.3 and 65, respectively. Mean ODI postoperative score at 1 year was 14.5.

Conclusions

In a large patient cohort lumbar fusion with PEEK rods can be undertaken with low complication rates, satisfactory clinical improvements, low rates of hardware failure or need for revision surgery. Longer follow-up is needed to confirm findings.

Do PEEK Rods for Posterior Instrumented Fusion in the Lumbar Spine Reduce the Risk of Adjacent Segment Disease?

BACKGROUND

Polyetheretherketone (PEEK) rods were clinically introduced in the mid-2000s as an alternative to titanium (Ti) rods for posterior instrumented lumbar spine fusion, theorized to reduce the risk of adjacent segment disease (ASD). However, few studies have follow-up beyond 2 years. Consequently, we conducted a matched cohort study using data from Kaiser Permanente's spine registry to compare the 2 rod systems and risk for outcomes.

METHODS

Patients aged ≥18 undergoing first posterior lumbar fusion for a degenerative diagnosis from 2009 to 2018 using either a PEEK or a Ti rod were identified. Fusions using Ti rods were 2:1 propensity score matched to PEEK rods on the following factors: patient age, body mass index, smoking, American Society of Anesthesiologists classification, diagnosis, interbody use, bone morphogenic protein use, number of levels fused, fusion levels, and operative year. The matched sample included 154 PEEK and 308 Ti fusions. We used Cox regression to evaluate ASD and nonunion, and logistic regression to evaluate 90-day emergency department (ED) visit, readmission, and complication.

RESULTS

We did not observe a difference in risk for ASD (hazard ratio = 1.02, 95% confidence interval [CI] = 0.66-1.59) or ED visit (odds ratio [OR] = 0.88, 95% CI = 0.48-1.59). A lower likelihood of readmission (OR = 0.34, 95% CI = 0.13-0.94) was observed following PEEK fusion compared with Ti. No nonunions or 90-day complications were observed for the PEEK group; 5 (2-year cumulative incidence = 0.7%) nonunions and 4 (1.3%) complications were observed for the Ti group.

CONCLUSIONS

Our multicenter study did not support the hypothesis that PEEK rods are associated with a lower ASD risk. Reasons for readmission need to be identified to better understand the differences observed here. Further study of patients with TLIF using Ti and PEEK rods and posterolateral fusion with Ti and PEEK rods is needed.

CLINICAL RELEVANCE

The present study adds to the literature supporting their midterm effectiveness of PEEK rods compared with Ti rods for both their safety and their effectiveness at the 5-7-year follow-up.

LEVEL OF EVIDENCE

3.

Biomechanical Investigation Between Rigid and Semirigid Posterolateral Fixation During Daily Activities: Geometrically Parametric Poroelastic Finite Element Analyses

While spinal fusion using rigid rods remains the gold standard treatment modality for various lumbar degenerative conditions, its adverse effects, including accelerated adjacent segment disease (ASD), are well known. In order to better understand the performance of semirigid constructs using polyetheretherketone (PEEK) in fixation surgeries, the objective of this study was to analyze the biomechanical performance of PEEK versus Ti rods using a geometrically patient-specific poroelastic finite element (FE) analyses. Ten subject-specific preoperative models were developed, and the validity of the models was evaluated with previous studies. Furthermore, FE models of those lumbar spines were regenerated based on postoperation images for posterolateral fixation at the L4–L5 level. Biomechanical responses for instrumented and adjacent intervertebral discs (IVDs) were analyzed and compared subjected to static and cyclic loading. The preoperative model results were well comparable with previous FE studies. The PEEK construct demonstrated a slightly increased range of motion (ROM) at the instrumented level, but decreased ROM at adjacent levels, as compared with the Ti. However, no significant changes were detected during axial rotation. During cyclic loading, disc height loss, fluid loss, axial stress, and collagen fiber strain in the adjacent IVDs were higher for the Ti construct when compared with the intact and PEEK models. Increased ROM, experienced stress in AF, and fiber strain at adjacent levels were observed for the Ti rod group compared with the intact and PEEK rod group, which can indicate the risk of ASD for rigid fixation. Similar to the aforementioned pattern, disc height loss and fluid loss were significantly higher at adjacent levels in the Ti rod group after cycling loading which alter the fluid–solid interaction of the adjacent IVDs. This phenomenon debilitates the damping quality, which results in disc disability in absorbing stress. Such finding may suggest the advantage of using a semirigid fixation system to decrease the chance of ASD.

Can Polyether Ether Ketone Dethrone Titanium as the Choice Implant Material for Metastatic Spine Tumor Surgery?

Instrumentation during metastatic spine tumor surgery (MSTS) provides stability to the spinal column in patients with pathologic fracture or iatrogenic instability produced while undergoing extensive decompression. Titanium is the current implant material of choice in MSTS. However, it hinders radiotherapy planning and generates artifacts, with magnetic resonance imaging and computed tomography scans used for postoperative evaluation of tumor recurrence and/or complications. The high modulus of elasticity of titanium (110 GPa) results in stress shielding, which may lead to construct failure at the bone-implant interface. Polyether ether ketone (PEEK), a thermoplastic polymer, is an emerging alternative to titanium for use in MSTS. The modulus of elasticity of PEEK (3.6 GPa) is close to that of cortical bone (17-21 GPa), resulting in minimal stress shielding. Its radiolucent and nonmetallic properties cause minimal interference with magnetic resonance imaging and computed tomography scans. PEEK also causes low-dose perturbation for radiotherapy planning. However, PEEK has reduced bioactivity with bone and lacks sufficient rigidity to be used as rods in MSTS. The reduced bioactivity of PEEK may be addressed by 1) surface modification (introducing porosity or bioactive coating with hydroxyapatite [HA] or titanium) and 2) forming composites with HA/titanium. The mechanical properties of PEEK may be improved by forming composites with HA or carbon fiber. Despite these modifications, all PEEK and PEEK-based implants are difficult to handle and contour intraoperatively. Our review provides a comprehensive overview of PEEK and modified PEEK implants, with a description of their properties and limitations, potentially serving as a basis for their future development and use in MSTS.

Biomechanical design using in-vitro finite element modeling of distal femur fracture plates made from semi-rigid materials versus traditional metals for post-operative toe-touch weight-bearing

This proof-of-concept study designs distal femur fracture plates from semi-rigid materials vs. traditional metals for toe-touch weight-bearing recommended to patients immediately after surgery. The two-fold goal was to (a) reduce stress shielding (SS) by increasing cortical bone stress thereby reducing the risk of bone absorption and plate loosening, and (b) reduce delayed healing (DH) via early callus formation by optimizing axial interfragmentary motion (AIM). Finite element analysis was used to design semi-rigid plates whose elastic moduli E ensured plates permitted AIM of 0.2 - 1 mm for early callus formation. A low hip joint force of 700 N (i.e. 100% x body weight) was applied, which corresponds to a typical 140 N toe-touch foot-to-ground force (i.e. 20% x body weight) recommended to patients after surgery. Analysis was done using 2 screw materials (steel or titanium) and types (locked or non-locked). Steel and titanium plates were also analyzed. Semi-rigid plates (vs. metal plates) had lower overall femur/plate construct stiffnesses of 508 - 1482 N/mm, higher cortical bone stresses under the plate by 2.02x - 3.27x thereby reducing SS, and lower E values of 414 - 2302 MPa to permit AIM of 0.2 - 1 mm thereby reducing DH.

Clinical and radiological outcomes after correction of degenerative lumbar scoliosis with dynamic stabilization (with the help of a rigid rod); and describing an alternative technique

Sharing clinical and radiological results in patients with degenerative lumbar scoliosis (DLS) treated surgically with dynamic system and describing an alternative technique for scoliosis correction. Between 2013 and 2018, 48 patients with flexible degenerative lumbar scoliosis (DLS) were operated with dynamic stabilization with Polyetheretherketone Rod (PEEK rod) after rigid rod application. Preoperative and postoperative scoliosis angles (standing and supine) were statistically compared. Preoperative and postoperative low back pain (LBP) Visual Analogue Scale (VAS) and Oswestry Disability Index (ODI) scores were compared. In addition, preoperative C7 Sagittal Vertical Axis (SVA) values and lumbar lordosis angles were compared with postoperative values. The mean follow-up period of the patients was 48.3 months (range 30-76), the mean age was 67.08 (range 49-84). While the average Cobb angle of all patients was 9.65 in preoperative supine position, the average Cobb angle with standing position was 19.73. The mean standing Cobb angle of the patients after surgery was 3.52. The mean Cobb angle in the supine position after surgery was 3.02. The difference between the preoperative and postoperative patients' Cobb angles in standing and supine position were statistically significant (p:0,000, p:0,000, respectively). The differences of VAS and ODI scores between preoperative and postoperative period were statistically significantly (p:0,000, p:0,000; respectively). Especially in patients with flexible LDS, the technique we have defined and dynamic stabilization with PEEK rod provides significant correction. There was no loss of correction in our patients during postoperative follow-up period. PEEK rod is insufficient for lordosis increase and correction of SVA values.

Carbon fiber-reinforced PEEK versus titanium implants: an in vitro comparison of susceptibility artifacts in CT and MR imaging

Artifacts in computed tomography (CT) and magnetic resonance imaging (MRI) due to titanium implants in spine surgery are known to cause difficulties in follow-up imaging, radiation planning, and precise dose delivery in patients with spinal tumors. Carbon fiber–reinforced polyetheretherketon (CFRP) implants aim to reduce these artifacts. Our aim was to analyze susceptibility artifacts of these implants using a standardized in vitro model. Titanium and CFRP screw-rod phantoms were embedded in 3% agarose gel. Phantoms were scanned with Siemens Somatom AS Open and 3.0-T Siemens Skyra scanners. Regions of interest (ROIs) were plotted and analyzed for CT and MRI at clinically relevant localizations. CT voxel–based imaging analysis showed a significant difference of artifact intensity and central overlay between titanium and CFRP phantoms. For the virtual regions of the spinal canal, titanium implants (ti) presented − 30.7 HU vs. 33.4 HU mean for CFRP (p < 0.001), at the posterior margin of the vertebral body 68.9 HU (ti) vs. 59.8 HU (CFRP) (p < 0.001) and at the anterior part of the vertebral body 201.2 HU (ti) vs. 70.4 HU (CFRP) (p < 0.001), respectively. MRI data was only visually interpreted due to the low sample size and lack of an objective measuring system as Hounsfield units in CT. CT imaging of the phantom with typical implant configuration for thoracic stabilization could demonstrate a significant artifact reduction in CFRP implants compared with titanium implants for evaluation of index structures. Radiolucency with less artifacts provides a better interpretation of follow-up imaging, radiation planning, and more precise dose delivery.

A new concept of motion preservation surgery of the cervical spine: PEEK rods for the posterior cervical region

BACKGROUND

Laminectomy may cause kyphotic postoperative deformity in the cervical region leading to segmental instability over time. Laminoplasty may be an alternative procedure to laminectomy, as it protects the spine against post-laminectomy kyphosis; however, similar to laminectomy, laminoplasty may cause sagittal plane deformities by destructing or weakening the dorsal tension band.

OBJECTIVE

Using finite element analysis (FE), we attempted to determine whether a posterior motion preservation system (PEEK posterior rod system concept) could overcome the postoperative complications of laminectomy and laminoplasty and eliminate the side effects of rigid posterior stabilization in the cervical region.

METHODS

We compared PEEK rods in four different diameters with a titanium rod for posterior cervical fixation. The present study may lead to motion preservation systems of the cervical vertebra.

RESULTS

When PEEK rod is compared with titanium rod, considerable increase in range of motion is observed.

CONCLUSIONS

PEEK rod-lateral mass screw instrumentation systems may be useful in motion preservation surgery of the posterior cervical region.

A Novel Spine Fixation System Made Entirely of Carbon-Fiber-Reinforced PEEK Composite: An In Vitro Mechanical Evaluation

BACKGROUND

Semirigid spine fixation systems utilizing nonmetallic materials have emerged as a promising innovation to overcome the inherent disadvantages of metal instrumentation in spine surgery. This study tests the mechanical properties of a novel spine fixation system made entirely of carbon-fiber-reinforced PEEK (CFR-PEEK) composite material (CarboClear System, CarboFix Orthopedics Ltd., Israel).

METHODS

An in vitro mechanical evaluation of the CFR-PEEK CarboClear system was conducted in compliance with the American Society for Testing and Materials (ASTM) F1717, F2193, and F543 standards.

RESULTS

The mean bending yield load, bending ultimate load, and bending stiffness of the construct were 322 N, 363 N, and 45 N/mm, respectively. All tested samples completed 5 × 10⁶ dynamic cycles successfully, with no evidence of fatigue failure at increasing load levels, up to 83% of ultimate bending load. The mean torsional stiffness was 1.0 Nm/deg and the mean screw axial pull-out strength was 2,037 N.

CONCLUSION

The CarboClear Pedicle Screw System has mechanical properties comparable to those of other commonly used titanium-made systems, with superior fatigue properties. The fatigue resistance, modulus of elasticity which is very similar to that of bone, radiolucency, and CT/MRI artifact-free feature of this spine fixation system made entirely of CFR-PEEK may offer advantages over traditional spine fixation systems made of metal alloys.

Topping-off technique for stabilization of lumbar degenerative instabilities in 322 patients

OBJECTIVE

Semi-rigid instrumentation (SRI) was introduced to take advantage of the concept of load sharing in surgery for spinal stabilization. The authors investigated a topping-off technique in which interbody fusion is not performed in the uppermost motion segment, thus creating a smooth transition from stabilized to free motion segments. SRI using the topping-off technique also reduces the motion of the adjacent segments, which may reduce the risk of adjacent segment disease (ASD), a frequently observed sequela of instrumentation and fusion, but this technique may also increase the possibility of screw loosening (SL). In the present study the authors aimed to systematically evaluate reoperation rates, clinical outcomes, and potential risk factors and incidences of ASD and SL for this novel approach.

METHODS

The authors collected data for the first 322 patients enrolled at their institution from 2009 to 2015 who underwent surgery performed using the topping-off technique. Reoperation rates, patient satisfaction, and other outcome measures were evaluated. All patients underwent pedicle screw-based semi-rigid stabilization of the lumbar spine with a polyetheretherketone (PEEK) rod system.

RESULTS

Implantation of PEEK rods during revision surgery was performed in 59.9% of patients. A median of 3 motion segments (range 1-5 segments) were included and a median of 2 motion segments (range 0-4 segments) were fused. A total of 89.4% of patients underwent fusion, 73.3% by transforaminal lumbar interbody fusion (TLIF), 18.4% by anterior lumbar interbody fusion (ALIF), 3.1% by extreme lateral interbody fusion (XLIF), 0.3% by oblique lumbar interbody fusion (OLIF), and 4.9% by combined approaches in the same surgery. Combined radicular and lumbar pain according to a visual analog scale was reduced from 7.9 ± 1.0 to 4.0 ± 3.1, with 56.2% of patients indicating benefit from surgery. After maximum follow-up (4.3 ± 1.8 years), the reoperation rate was 16.4%.

CONCLUSIONS

The PEEK rod concept including the topping-off principle seems safe, with at least average patient satisfaction in this patient group. Considering the low rate of first-tier surgeries, the presented results seem at least comparable to those of most other series. Follow-up studies are needed to determine long-term outcomes, particularly with respect to ASD, which might be reduced by the presented approach.

Biomechanical analysis of single-level interbody fusion with different internal fixation rod materials: a finite element analysis

Background

Lumbar spinal fusion with rigid spinal fixators as one of the high risk factors related to adjacent-segment failure. The purpose of this study is to investigate how the material properties of spinal fixation rods influence the biomechanical behavior at the instrumented and adjacent levels through the use of the finite element method.

Methods

Five finite element models were constructed in our study to simulate the human spine pre- and post-surgery. For the four post-surgical models, the spines were implanted with rods made of three different materials: (i) titanium rod, (ii) PEEK rod with interbody PEEK cage, (iii) Biodegradable rod with interbody PEEK cage, and (iv) PEEK cage without pedicle screw fixation (no rods).

Results

Fusion of the lumbar spine using PEEK or biodegradable rods allowed a similar ROM at both the fusion and adjacent levels under all conditions. The models with PEEK and biodegradable rods also showed a similar increase in contact forces at adjacent facet joints, but both were less than the model with a titanium rod.

Conclusions

Flexible rods or cages with non-instrumented fusion can mitigate the increased contact forces on adjacent facet joints typically found following spinal fixation, and could also reduce the level of stress shielding at the bone graft.

Material failure in dynamic spine implants: are the standardized implant tests before market launch sufficient?

PURPOSE

International Standards Organization (ISO) 12189 and American Society for Testing and Materials F2624 are two standard material specification and test methods for spinal implant devices. The aim of this study was to assess whether the existing and required tests before market launch are sufficient.

METHODS

In three prospective studies, patients were treated due to degenerative disease of the lumbar spine or spondylolisthesis with lumbar interbody fusion and dynamic stabilization of the cranial adjacent level. The CD HORIZON BalanC rod and S⁴ Dynamic rod were implanted in 45 and 11 patients, respectively.

RESULTS

A fatigue fracture of the material of the topping off system has been found in five cases (11%) for the group fitted with the CD HORIZON BalanC rod. In the group using the S⁴ Dynamic rod group, a material failure of the dynamic part was demonstrated in seven patients (64%). All three studies were interrupted due to these results, and a report to the Federal Institute for Drugs and Medical Devices was generated.

CONCLUSION

Spinal implants have to be checked by a notified body before market launch. The notified body verifies whether the implants fulfil the requirements of the current standards. These declared studies suggest that the current standards for the testing of load bearing capacity and stand ability of dynamic spine implants might be insufficient. Revised standards depicting sufficient deformation and load pattern have to be developed and counted as a requirement for the market launch of an implant. These slides can be retrieved under Electronic Supplementary Material.

Investigation into the biomechanics of lumbar spine micro-dynamic pedicle screw

Background

Numerous reports have shown that rigid spinal fixation contributes to a series of unwanted complications in lumbar fusion procedure. This innovative micro-dynamic pedicle screw study was designed to investigate the biomechanical performance of lumbar implants using numerical simulation technique and biomechanical experiment.

Methods

Instrumented finite element models of three configurations (dynamic fixation, rigid fixation and hybrid fixation) using a functional L3-L4 lumbar unit were developed, to compare the range of motion of the lumbar spine and stress values on the endplate and implants. An in vitro experiment was simultaneously conducted using 18 intact porcine lumbar spines and segmental motion analyses were performed as well.

Results

Simulation results indicated that the dynamic fixation and the hybrid fixation models respectively increased the range of motion of the lumbar spine by 95 and 60% in flexion and by 83 and 55% in extension, compared with the rigid fixation model. The use of micro-dynamic pedicle screw led to higher stress on endplates and lower stress on pedicle screws. The outcome of the in vitro experiment demonstrated that the micro-dynamic pedicle screw could provide better range of motion at the instrumented segments than a rigid fixation.

Conclusion

The micro-dynamic pedicle screw has the advantage of providing better range of motion than conventional pedicle screw in flexion-extension, without compromising stabilization, and has the potential of bringing the load transfer behavior of fusional segment closer to normal and also lowers the stress values of pedicle screws.

Comparison of rigid and semi-rigid instrumentation under acute load on vertebrae treated with posterior lumbar interbody fusion/transforaminal lumbar interbody fusion procedures: An experimental study

Rigid and semi-rigid fixations are investigated several times in order to compare their biomechanical stability. Interbody fusion techniques are also preferable for maintaining the sagittal balance by protecting the disk height. In this study, the biomechanical comparison of semi-rigid and rigid fixations with posterior lumbar interbody fusion or transforaminal lumbar interbody fusion procedures is conducted under trauma. There were four different test groups to analyze the effect of acute load on treated ovine vertebrae. First and second groups were fixed with polyetheretherketone rods and transforaminal lumbar interbody fusion and posterior lumbar interbody fusion cages, respectively. Third and fourth groups were fixed with titanium rods and posterior lumbar interbody fusion and transforaminal lumbar interbody fusion cages, respectively. The drop tests were conducted with 7 kg weight. There were six samples in each group so the drop test repeated 24 times in total. The test samples were photographed and X-rayed (laterally and anteroposteriorly) before and after drop test. Two fractures were observed on group 1. Conversely, there were no fractures observed for group 2. There were no anterior element fractures for both groups 1 and 2. However, one fracture seen on group 3 was anterior element fracture, whereas the other three were posterior element fractures. All three fractures were anterior element fractures for group 4. Treated vertebrae with polyetheretherketone rods and posterior lumbar interbody fusion cages showed the best durability to the drop tests among the groups. Semi-rigid fixation gave better results than rigid fixation according to failed segments. Posterior lumbar interbody fusion cages seem to be better option for semi-rigid fixation, however mentioned surgical disadvantages must be considered.

Semirigid Waved Rod System for the Treatment of the Degenerative Lumbar Diseases

BACKGROUND

The semirigid pedicle screw instrumentation has gained wide popularity in recent decennium in lumbar fusion surgery. However, few data were documented to compare the clinical efficacy between semirigid and traditional rigid pedicle screw systems.

MATERIALS AND METHODS

A total of 96 patients with degenerative lumbar diseases were selected to perform operations between 2008 and 2013. The patients were prospectively randomized into 2 groups: 50 patients were managed by semirigid waved rod systems and 46 patients were intervened by traditional rigid straight stiff rod systems. X-rays and computed tomography were utilized to examine the interbody fusion status in the follow-up in detail. Surgical parameters such as operative time, blood loss, and total hospital stay were calculated and compared. Visual Analog Scale and Oswestry Disability Index were used to assess clinical efficacy postoperatively.

RESULTS

No significant differences were found about demographic data between groups. There were no significant differences regarding the surgical parameters including operative time, blood loss, and total hospital stay. Visual Analog Scale and Oswestry Disability Index postoperatively were also similar between the 2 instrumentations (P>0.05). The fusion rate was higher in the semirigid group (45/50) than in the traditional group (34/46) (P=0.039) at the final follow-up.

CONCLUSIONS

Waved rod may be better in facilitating interbody fusion compared with traditional straight rod, although waved rod and straight stiff rod can both get similar clinical efficacy. Meanwhile, waved rod is likely superior in alleviating adjacent degeneration segments.

Impact of spinal rod stiffness on porcine lumbar biomechanics: Finite element model validation and parametric study

A three-dimensional finite element model of the porcine lumbar spine (L1-L6) was used to assess the effect of spinal rod stiffness on lumbar biomechanics. The model was validated through a comparison with in vitro measurements performed on six porcine spine specimens. The validation metrics employed included intervertebral rotations and the nucleus pressure in the first instrumented intervertebral disc. The numerical results obtained suggest that rod stiffness values as low as 0.1 GPa are required to reduce the mobility gradient between the adjacent and instrumented segments and the nucleus pressures across the porcine lumbar spine significantly. Stiffness variations above this threshold value have no significant effect on spine biomechanics. For such low-stiffness rods, intervertebral rotations in the instrumented zone must be monitored closely in order to guarantee solid fusion. Looking ahead, the proposed model will serve to examine the transverse process hooks and variable stiffness rods in order to further smooth the transition between the adjacent and instrumented segments, while preserving the stability of the instrumented zone, which is needed for fusion.

Limiting interpedicular screw displacement increases shear forces in screws: A finite element study

BACKGROUND CONTEXT

Screw loosening has been reported for non-fusion devices. Forces on pedicle screws could be related to kinematic parameters as the interpedicular displacement (ID), which consists of the displacement between superior and inferior screw heads from full extension to full flexion.

PURPOSE

To investigate the relationship between ID and screw loosening for different designs of posterior implants using a finite element model.

METHODS

An L3-sacrum previously validated spine FE model was used. Three-rod designs were considered in L4-L5 segment: a rigid screw-rod implant, a flexible one and a specific design with a sliding rod providing limited restrain in ID. In order to simulate intermediate configurations, the friction coefficient between the sliding rods and connectors were varied. The sacrum was rigidly fixed. Rotations (flexion-extension, lateral bending and axial rotation) were applied to L3, for each modeled configuration: intact, injured, injured with different implants. Model consistency was checked with existing experimental in vitro data on intact and instrumented segments. Screw loads were computed as well as ID.

RESULTS

In flexion-extension, the ID was less than 2mm for rigid (R) and flexible (F) constructs and 5.5mm for intact spine and the sliding implant (S3). Screw's shear forces were 272N, 153N, 43N respectively for R, F and S3 constructs.

CONCLUSIONS

Implants that allow ID presented lower screws loads. A compromise between the ability of the implant to withstand compressive forces, which requires longitudinal stiffness, and its ability to allow ID could be important for future implant designs in order to prevent screw loosening.

Reduction of intradiscal pressure by the use of polycarbonate-urethane rods as compared to titanium rods in posterior thoracolumbar spinal fixation

Loss of sagittal alignment and balance in adult spinal deformity can cause severe pain, disability and progressive neurological deficit. When conservative treatment has failed, spinal fusion using rigid instrumentation is currently the salvage treatment to stop further curve progression. However, fusion surgery is associated with high revision rates due to instrumentation failure and proximal junctional failure, especially if patients also suffer from osteoporosis. To address these drawbacks, a less rigid rod construct is proposed, which is hypothesized to provide a more gradual transition of force and load distribution over spinal segments in comparison to stiff titanium rods. In this study, the effect of variation in rod stiffness on the intradiscal pressure (IDP) of fixed spinal segments during flexion-compression loading was assessed. An ex vivo multisegment (porcine) flexion-compression spine test comparing rigid titanium rods with more flexible polycarbonate-urethane (PCU) rods was used. An increase in peak IDP was found for both the titanium and PCU instrumentation groups as compared to the uninstrumented controls. The peak IDP for the spines instrumented with the PCU rods was significantly lower in comparison to the titanium instrumentation group. These results demonstrated the differences in mechanical load transfer characteristics between PCU and titanium rod constructs when subjected to flexion-compression loading. The concept of stabilization with a less rigid rod may be an alternative to fusion with rigid instrumentation, with the aim of decreasing mechanical stress on the instrumented segments and the possible benefit of a decrease in the incidence of screw pullout.

Biomechanical testing of a PEEK-based dynamic instrumentation device in a lumbar spine model

BACKGROUND

The purpose of this study was to investigate the range-of-motion after posterior polyetheretherketone-based rod stabilisation combined with a dynamic silicone hinge in order to compare it with titanium rigid stabilisation.

METHODS

Five human cadaveric lumbar spines with four vertebra each (L2 to L5) were tested in a temperature adjustable spine-testing set-up in four trials: (1) native measurement; (2) kinematics after rigid monosegmental titanium rod instrumentation with anterior intervertebral bracing of the segment L4/5; (3) kinematics after hybrid posterior polyetheretherketone rod instrumentation combined with a silicone hinge within the adjacent level (L3/4) and (4) kinematics after additional decompression with laminectomy of L4 and bilateral resection of the inferior articular processes (L3). During all steps, the specimens were loaded quasi-statically with 1°/s with pure moment up to 7.5Nm in flexion/extension, lateral bending and axial rotation.

FINDINGS

In comparison to the native cadaveric spine, both the titanium device and polyetheretherketone-based device reduce the range-of-motion within the level L4/5 significantly (flexion/extension: reduction of 77%, p<0.001; lateral bending: reduction of 62%, p<0.001; axial rotation: reduction of 71%, p<0.001). There was a clear stabilisation effect after hybrid-instrumentation within the level L3/4, especially in flexion/extension (64%, p<0.001) and lateral bending (62%, p<0.001) but without any effect on the axial rotation. Any temperature dependency has not been observed.

INTERPRETATION

Surprisingly, the hybrid device compensates for laminectomy L4 and destabilising procedure within the level L3/4 in comparison to other implants. Further studies must be performed to show its effectiveness regarding the adjacent segment instability.

Hybrid Rigid-Deformable Model for Prediction of Neighboring Intervertebral Disk Loads During Flexion Movement After Lumbar Interbody Fusion at L3-4 Level

Knowledge of spinal loads in neighboring disks after interbody fusion plays an important role in the clinical decision of this treatment as well as in the elucidation of its effect. However, controversial findings are still noted in the literature. Moreover, there are no existing models for efficient prediction of intervertebral disk stresses within annulus fibrosus (AF) and nucleus pulposus (NP) regions. In this present study, a new hybrid rigid-deformable modeling workflow was established to quantify the mechanical stress behaviors within AF and NP regions of the L1-2, L2-3, and L4-5 disks after interbody fusion at L3-4 level. The changes in spinal loads were compared with results of the intact model without interbody fusion. The fusion outcomes revealed maximal stress changes (10%) in AF region of L1-2 disk and in NP region of L2-3 disk. The minimal stress change (1%) is noted at the NP region of the L1-2 disk. The validation of simulation outcomes of fused and intact lumbar spine models against those of other computational models and in vivo measurements showed good agreements. Thus, this present study may be used as a novel design guideline for a specific implant and surgical scenario of the lumbar spine disorders.

Kinematics and load-sharing of an anterior thoracolumbar spinal reconstruction construct with PEEK rods: An in vitro biomechanical study

BACKGROUND

Polyetheretherketone rod constructs provide adequate spinal stability. Kinematics and load sharing of anterior thoracolumbar reconstruction with polyetheretherketone rods under preload remains unknown.

METHODS

Eight human cadaveric specimens (T11-L3) were subjected to a pure moment of 5.0Nm in flexion-extension, lateral bending and axial rotation, and flexion-extension with a compressive preload of 300N. An anterior reconstruction of L1 corpectomy was conducted with a surrogate bone graft and anterior rod constructs (polyetheretherketone or titanium rods). An axial load-cell was built in the surrogate bone graft to measure the compressive force in the graft. Range of motion, neutral zone and compressive force in the graft were compared between constructs.

FINDINGS

The polyetheretherketone rod construct resulted in more motion than the titanium rod construct, particularly in extension (P=0.011) and axial rotation (P=0.001), but less motion than the intact in all directions except in axial rotation. There was no difference in range of motion or neutral zone between constructs in flexion-extension under preload. The polyetheretherketone rod construct kept the graft compressed 52N which was similar to the titanium rod construct (63N), but allowed the graft compressed more under the preload (203N vs. 123N, P=0.003). The compressive forces fluctuated in flexion-extension without preload, but increased in flexion and decreased in extension under preload.

INTERPRETATION

The polyetheretherketone rod construct allowed more motion compared to the titanium rod construct, but provided stability in flexion and lateral bending without preload, and flexion and extension under preload. The anterior graft shared higher load under preload, particularly for the polyetheretherketone rod construct. The results of this study suggest that rigidity of rods in the anterior reconstruction affects kinematic behavior and load sharing.

Clinical and biomechanical researches of polyetheretherketone (PEEK) rods for semi-rigid lumbar fusion: a systematic review

Lumbar spinal fusion using rigid rods is a common surgical technique. However, adjacent segment disease and other adverse effects can occur. Dynamic stabilization devices preserve physiologic motion and reduce painful stress but have a high rate of construct failure and reoperation. Polyetheretherketone (PEEK) rods for semi-rigid fusions have a similar stiffness and adequate stabilization power compared with titanium rods, but with improved load sharing and reduced mechanical failure. The purpose of this paper is to review and evaluate the clinical and biomechanical performance of PEEK rods. A systematic review of clinical and biomechanical studies was conducted. A literature search using the PubMed, EMBASE, and Cochrane Library databases identified studies that met the eligibility criteria. Eight clinical studies and 15 biomechanical studies were included in this systematic review. The visual analog scale and the Oswestry disability index improved significantly in most studies, with satisfactory fusion rates. The occurrence of adjacent segment disease was low. In biomechanical studies, PEEK rods demonstrated a superior load-sharing distribution, a larger adjacent segment range of motion, and reduced stress at the rod-screw/screw-bone interfaces compared with titanium rods. The PEEK rod construct was simple to assemble and had a reliable in vivo performance compared with dynamic devices. The quality of clinical studies was low with confounding results, although results from mechanical studies were encouraging. There is no evidence strong enough to confirm better outcomes with PEEK rods than titanium rods. More studies with better protocols, a larger sample size, and a longer follow-up time are needed.

Clinical and Radiological Comparison of Semirigid (WavefleX) and Rigid System for the Lumbar Spine

OBJECTIVE

Spinal fusion operation is an effective treatment in the spinal pathology, but it could change the physiological distribution of load at the instrumented and adjacent segments. This retrospective study compared the radiological and clinical outcomes of patients undergoing lumbar fusion with semirigid rods versus rigid rods system.

METHODS

Using transpedicular fixation and posterior lumbar interbody fusion at the level of L4/L5, 20 patients were treated with semirigid rods (WavefleX, SR group), and 20 patients with rigid rods (titanium, RR group). Clinical and radiological outcomes were evaluated, including visual analog score for lower back pain and leg pain, Prolo functional and economic scores, statues of implanted instruments, fusion rate, and complications during 24-month follow-up.

RESULTS

Clinical scores were significantly improved until postoperative 24-month follow-up as compared with preoperative scores in both groups (p<0.05), with similar levels of improvement observed at the same time points postoperatively between the 2 groups. Prolo economic scores were significantly improved in SR group compared to RR until 12 months, but this improvement became similar after 18 months. The overall fusion rate was 94.1% until the 24-month follow-up for both groups. No significant complication was observed in both groups.

CONCLUSION

The results of the present study indicate that semirigid rods system with posterior lumbar interbody fusion showed similar clinical and radiological result with rigid rods system until 2 years after instrumentation. The WavefleX rods system, as a semirigid rods with unique characteristics, may be an effective alternative treatment for patients in lumbar fusion.

Adjacent-Level Hypermobility and Instrumented-Level Fatigue Loosening With Titanium and PEEK Rods for a Pedicle Screw System: An In Vitro Study

Adjacent-level disease is a common iatrogenic complication seen among patients undergoing spinal fusion for low back pain. This is attributed to the postsurgical differences in stiffness between the spinal levels, which result in abnormal forces, stress shielding, and hypermobility at the adjacent levels. In addition, as most patients undergoing these surgeries are osteoporotic, screw loosening at the index level is a complication that commonly accompanies adjacent-level disease. Recent studies indicate that a rod with lower rigidity than that of titanium may help to overcome these detrimental effects at the adjacent level. The present study was conducted in vitro using 12 L1-S1 specimens divided into groups of six, with each group instrumented with either titanium rods or PEEK (polyetheretherketone) rods. The test protocol included subjecting intact specimens to pure moments of 10 Nm in extension and flexion using an FS20 Biomechanical Spine Test System (Applied Test Systems) followed by hybrid moments on the instrumented specimens to achieve the same L1-S1 motion as that of the intact specimens. During the protocol's later phase, the L4-L5 units from each specimen were segmented for cyclic loading followed by postfatigue kinematic analysis to highlight the differences in motion pre- and postfatigue. The objectives included the in vitro comparison of (1) the adjacent-level motion before and after instrumentation with PEEK and titanium rods and (2) the pre- and postfatigue motion at the instrumented level with PEEK and titanium rods. The results showed that the adjacent levels above the instrumentation caused increased flexion and extension with both PEEK and titanium rods. The postfatigue kinematic data showed that the motion at the instrumented level (L4-L5) increased significantly in both flexion and extension compared to prefatigue motion in titanium groups. However, there was no significant difference in motion between the pre- and postfatigue data in the PEEK group.

Flexible Stabilisation of the Degenerative Lumbar Spine Using PEEK Rods

Posterior lumbar interbody fusion using cages, titanium rods, and pedicle screws is considered today as the gold standard of surgical treatment of lumbar degenerative disease and has produced satisfying long-term fusion rates. However this rigid material could change the physiological distribution of load at the instrumental and adjacent segments, a main cause of implant failure and adjacent segment disease, responsible for a high rate of further surgery in the following years. More recently, semirigid instrumentation systems using rods made of polyetheretherketone (PEEK) have been introduced. This clinical study of 21 patients focuses on the clinical and radiological outcomes of patients with lumbar degenerative disease treated with Initial VEOS PEEK^®-Optima system (Innov'Spine, France) composed of rods made from PEEK-OPTIMA^® polymer (Invibio Biomaterial Solutions, UK) without arthrodesis. With an average follow-up of 2 years and half, the chances of reoperation were significantly reduced (4.8%), quality of life was improved (ODI = 16%), and the adjacent disc was preserved in more than 70% of cases. Based on these results, combined with the biomechanical and clinical data already published, PEEK rods systems can be considered as a safe and effective alternative solution to rigid ones.

Non-fusion procedure using PEEK rod systems for lumbar degenerative diseases: clinical experience with a 2-year follow-up

Background

Polyetheretherketone (PEEK) rod system is a novel pedicle-based dynamic stabilization system. This study evaluated clinical and radiographic outcomes of non-fusion surgery by PEEK rod systems for treatment of degenerative lumbar diseases with a 2-year follow-up.

Methods

From February 2012 to October 2012, 38 patients who underwent non-fusion surgery using PEEK rod systems were included in the study. Data on Oswestry Disability Index (ODI) score and Japanese Orthopaedics Association (JOA) score were collected and radiographs were obtained to evaluate disc height index (DHI) and range of motion (ROM) at each interval.

Results

Both JOA and ODI scores significantly improved postoperatively. DHI showed a slight increase immediately after the surgery but gradually dropped below preoperative levels. Mean ROM values changed from 8.8° preoperatively to 1.8° at the 2-year follow-up point. Screw loosening occurred in one case at the 2-year follow-up.

Conclusions

The preliminary results indicated a significant improvement in clinical outcomes and advantageous implant safety. The non-fusion procedure using PEEK rod systems might be a viable alternative for treatment of lumbar degenerative diseases. The distraction technique needs to be improved for better postoperative DHI.

ISO 12189 standard for the preclinical evaluation of posterior spinal stabilization devices--II: A parametric comparative study

The International Standardization Organization (ISO) 12189 standard was recently introduced to preclinically evaluate and compare the mechanical properties of posterior stabilization devices. This scenario presents some new significant steps ahead over the vertebrectomy model recommended by American Society for Testing and Materials (ASTM) F1717 standard: the modular anterior support allows for describing a closer scenario to the effective clinical use as well as to test very flexible and dynamic posterior stabilization devices. Despite these significant advantages, ISO 12189 received little attention in the literature. Anatomical parameters depending on the spinal level were compared to the published data or original measurements on biplanar stereoradiography on 13 patients. Other mechanical variables, describing the test set-up design, were considered and all parameters were investigated using a numerical parametric finite element model. Stress values were calculated by also considering their worst-case combination. The standard set-up represents quite well the anatomy of an instrumented average thoracolumbar segment. The parametric comparative analysis demonstrates a significant (even beyond +350%) maximum increase in the stress on the device, compared to the standard currently in use. The anterior support stiffness plays the most detrimental effect (maximum stress increases up to 396%). The initial precompression step has an important role in determining the final stress values achieved at peak load (up to +76%). Moreover, when combining these two contributions, an even higher stress increase may be achieved (up to 473%). Despite the other anatomical parameters playing a secondary role, their worst-case combination demonstrates that a device could potentially undergo higher stresses than those reached according to standard suggestions (maximum increase of 22.4% at L1). Any user/designer should be aware of these effects when using ISO 12189 standard for the preclinical evaluation of posterior spinal stabilization devices.

ISO 12189 standard for the preclinical evaluation of posterior spinal stabilization devices--I: Assembly procedure and validation

The International Standardization Organization introduced standard 12189 for the preclinical evaluation of the mechanical reliability of posterior stabilization devices. The well-known vertebrectomy model formalized in standard F1717 by the American Society for Testing and Materials was modified with the introduction of a modular anterior support made up of three calibrated springs, which allows to describe a more realistic scenario, closer to the effective clinical use, as well to test even very flexible and dynamic posterior stabilization implants. Despite these important improvements, ISO 12189 received very little attention in the literature. The aim of the work is to provide a systematic procedure for the assembly and validation of a finite element model capable of describing the experimental test according to ISO 12189. The validated finite element model is able to catch very well the effective stiffness of the unassembled and assembled constructs (percentage differences <2% and <10%, respectively). As concern the assembled construct, the experimentally measured and predicted strains were found in a good agreement (R2 > 0.75, root mean square error < 30%), but the procedure without precompression lead to much better results (R2 > 0.96, root mean square error < 10%). Given the prediction errors of the assembled construct fall within the experimental range of repeatability, the finite element model can be systematically implemented to support the mechanical design of a variety of spinal implants, to quantitatively investigate the load-sharing mechanism, as well as to investigate the loading conditions set by ISO 12189 standard.

Biomechanical assessment of the stabilization capacity of monolithic spinal rods with different flexural stiffness and anchoring arrangement

BACKGROUND

Spinal disorders can be treated by several means including fusion surgery. Rigid posterior instrumentations are used to obtain the stability needed for fusion. However, the abrupt stiffness variation between the stabilized and intact segments leads to proximal junctional kyphosis. The concept of spinal rods with variable flexural stiffness is proposed to create a more gradual transition at the end of the instrumentation.

METHOD

Biomechanical tests were conducted on porcine spine segments (L1-L6) to assess the stabilization capacity of spinal rods with different flexural stiffness. Dual-rod fusion constructs containing three kinds of rods (Ti, Ti-Ni superelastic, and Ti-Ni half stiff-half superelastic) were implanted using two anchor arrangements: pedicle screws at all levels or pedicle screws at all levels except for upper instrumented vertebra in which case pedicle screws were replaced with transverse process hooks. Specimens were loaded in forward flexion, extension, and lateral bending before and after implantation of the fusion constructs. The effects of different rods on specimen stiffness, vertebra mobility, intradiscal pressures, and anchor forces were evaluated.

FINDING

The differences in rod properties had a moderate impact on the biomechanics of the instrumented spine when only pedicle screws were used. However, this effect was amplified when transverse process hooks were used as proximal anchors.

INTERPRETATION

Combining transverse hooks and softer (Ti-Ni superelastic and Ti-Ni half stiff-half superelastic) rods provided more motion at the upper instrumented level and applied less force on the anchors, potentially improving the load sharing capacity of the instrumentation.

Biomechanical analysis between PEEK and titanium screw-rods spinal construct subjected to fatigue loading

STUDY DESIGN

An in vitro fatigue loading test with porcine specimens.

OBJECTIVES

To comparatively analyze the in vitro biomechanical performance of Polyetheretherketone (PEEK) and Titanium rods construct subjected to a battery of fatigue loading testing.

SUMMARY OF BACKGROUND DATA

PEEK rods construct has been proposed to allow better load sharing among spinal components than the more traditional Titanium rods constructs. However, such proposal has largely derived from single-load in vitro testing and the biomechanical differences when subjected to fatigue loading remain unknown.

METHODS

Twenty-four fresh 4-level motion segment were harvested from porcine. Specimens were randomly assigned into 3 groups: (1) intact, (2) destabilized group with Titanium alloy rods, and (3) destabilized group with PEEK rods. All specimens were subjected to a fatigue loading procedure with the disk height and intradiscal pressure (IDP) of the instrumented and adjacent levels were recorded and used for analysis. The stress levels on the rods and bone stress near the screw-bone interface were also collected to investigate the likely failure rates of the 2 constructs.

RESULTS

Titanium rods construct demonstrated a minimum amount of loss of disk height and IDP at the instrumented level; however, a significant loss of the disk height and IDP at adjacent levels compared with the intact spine were identified. In contrast, the disk height and IDP of the PEEK rods were found to be comparable with those of the intact spine for all levels. The PEEK rods group also showed significantly less bone stress near the screw-bone interface compared with the Titanium rods group.

CONCLUSIONS

The current study has demonstrated the differences in biomechanical characteristics of PEEK and Titanium rods construct when subjected to fatigue loading. More specifically, the result is indicative of the potential benefits of the PEEK rods construct in reducing the risks of adjacent segment disease and implant failure rates.

The effect of pedicle screw implantation depth and angle on the loading and stiffness of a spinal fusion assembly

BACKGROUND

The increasing prevalence of spine disorders in industrialized environments has impaired the quality of life in the elder population. In an effort to relieve pain, physicians strive to improve treatment through the consideration of patient specific characteristics during preoperative planning of procedures such as spinal fusion.

OBJECTIVE

This study aims at quantifying aspects of spondylodesis to the loading and mobility of the utilized instrumentation, as the use of rigid vs. motion sparing materials as well as implantation angle and depth of the pedicle screws are still subject to controversy among surgeons.

METHODS

A fixation assembly was reverse engineered based on µCT measurements of the involved instrumentation. Two pedicle screws were connected with a rod, thus representing a mono-segmental fixation device. The pedicle screws were embedded in hexahedral structures simulated by bone properties. Upon validation and verification, the response of the model to a compressive and a torsional load was simulated in ANSYS 14, while altering the implantation depth and insertion angle of the pedicle screws along with the rod material.

RESULTS

The mobility of the instrumentation was drastically increased (by up to 390%) when PEEK rods were used in place of traditional Ti ones, a tendency observed at varying extent for all simulated scenarios. Shallow implantation induced a slight stress increase (∼21%) on the implant and a notable distressing of the bony tissue (∼44%), whereas inclined screw positioning was overall beneficial to the developing stress fields in both, with bone profiting a max. stress release of ∼15% during the application of torsion.

CONCLUSIONS

The investigation presented refined insight into the biomechanical response of a spinal fusion device. As expected, rigid fixation seems preferable in fusion oriented instrumentation whereas semi rigid devices should be employed for non-fusion applications. Shallow implantation resulted in a slight posterior offset of the stabilization device, which could be beneficial in the treatment of osteoporotic patients.

In Vitro Comparison of Dynesys, PEEK, and Titanium Constructs in the Lumbar Spine

Introduction. Pedicle based posterior dynamic stabilization systems aim to stabilize the pathologic spine while also allowing sufficient motion to mitigate adjacent level effects. Two flexible constructs that have been proposed to act in such a manner, the Dynesys Dynamic Stabilization System and PEEK rod, have yet to be directly compared in vitro to a rigid Titanium rod. Methods. Human lumbar specimens were tested in flexion extension, lateral bending, and axial torsion to evaluate the following conditions at L4-L5: Intact, Dynesys, PEEK rod, Titanium rod, and Destabilized. Intervertebral range of motion, interpedicular travel, and interpedicular displacement metrics were evaluated from 3rd-cycle data using an optoelectric tracking system. Results. Statistically significant decreases in ROM compared to Intact and Destabilized conditions were detected for the instrumented conditions during flexion extension and lateral bending. AT ROM was significantly less than Destabilized but not the Intact condition. Similar trends were found for interpedicular displacement in all modes of loading; however, interpedicular travel trends were less consistent. More importantly, no metrics under any mode of loading revealed significant differences between Dynesys, PEEK, and Titanium. Conclusion. The results of this study support previous findings that Dynesys and PEEK constructs behave similarly to a Titanium rod in vitro.

Polyetheretherketone (PEEK) intervertebral cage as a cause of chronic systemic allergy: a case report

BACKGROUND CONTEXT

Polyetheretherketone (PEEK) is an organic polymer thermoplastic with strong mechanical and chemical resistance properties. It has been used in industry to fabricate items for demanding applications such as bearings, piston parts, compressor plate valves, and cable insulation. Since the early 1980s, polyetheretherketone polymers have been increasingly used in orthopedic and spinal surgery applications. Numerous studies and years of clinical experience have confirmed the biocompatibility of this material.

PURPOSE

The purpose of the study was to report a case of chronic systemic allergy after anterior cervical decompression and fusion (ACDF) and implantation of an intervertebral PEEK cage, with resolution of symptoms after removal of PEEK cage.

STUDY DESIGN/SETTING

This study is a case report with clinical evidence for allergy to PEEK.

METHODS

The methods involve clinical findings and review of current literature.

RESULTS

After ACDF and implantation of an intervertebral PEEK cage, the patient had developed an angioedema-like picture marked by severe redness, itching, swelling of his tongue, and skin thickening. A skin patch test was positive for PEEK. Removal of the implant resulted in the resolution of his allergy symptoms shortly after surgery.

CONCLUSIONS

Tissue reactions to PEEK are extremely rare. Herein, we present the first report of a chronic allergic response to interbody PEEK material.