Finite element model predicts the biomechanical performance of transforaminal lumbar interbody fusion with various porous additive manufactured cages

The effect of polymethylmethacrylate augmentation on the primary stability of stand-alone implant construct versus posterior stabilization in oblique lumbar interbody fusion with osteoporotic bone quality- a finite element study

BACKGROUND CONTEXT

Oblique lumbar interbody fusion (OLIF) can provide an ideal minimally invasive solution for achieving spinal fusion in an older, more frail population where decreased bone quality can be a limiting factor. Stabilization can be achieved with bilateral pedicle screws (BPS), which require additional incisions and longer operative time. Alternatively, a novel self-anchoring stand-alone lateral plate system (SSA) can be used, where no additional incisions are required. Based on the relevant literature, BPS constructs provide greater primary biomechanical stability compared to lateral plate constructs, including SSA. This difference is further increased by osteoporosis. Screw augmentation in spinal fusion surgeries is commonly used; however, in the case of OLIF, it is a fairly new concept, lacking a consensus-based guideline.

PURPOSE

This comparative finite element (FE) study aimed to investigate the effect of PMMA screw augmentation on the primary stability of a stand-alone implant construct versus posterior stabilization in OLIF with osteoporotic bone quality.

STUDY DESIGN

The biomechanical effect of screw augmentation was studied inside an in-silico environment using computer-aided FE analysis.

METHODS

A previously validated and published L2-L4 FE model with normal and osteoporotic bone material properties was used. Geometries based on the OLIF implants (BPS, SSA) were created and placed inside the L3-L4 motion segment with increasing volumes (1-6 cm3) of PMMA augmentation. A follower load of 400 N and 10 Nm bending moment (in the three anatomical planes) were applied to the surgical FE models with different bone material properties. The operated L3-L4 segmental range of motion (ROM), the inserted cage's maximal caudal displacements, and L4 cranial bony endplate principal stress values were measured.

RESULTS

The nonaugmented values for the BPS construct were generally lower compared to SSA, and the difference was increased by osteoporosis. In osteoporotic bone, PMMA augmentation gradually decreased the investigated parameters and the difference between the two constructs as well. Between 3 cm3 and 4 cm3 of injected PMMA volume per screw, the difference between augmented SSA and standard BPS became comparable.

CONCLUSIONS

Based on this study, augmentation can enhance the primary stability of the constructs and decrease the difference between them. Considering leakage as a possible complication, between 3 cm3 and 4 cm3 of injected PMMA per screw can be an adequate amount for SSA augmentation. However, further in silico, and possibly in vitro and clinical testing is required to thoroughly understand the investigated biomechanical aspects.

CLINICAL SIGNIFICANCE

This study sheds light on the possible biomechanical advantage offered by augmented OLIF implants and provides a theoretical augmentation amount for the SSA construct. Based on the findings, the concept of an SSA device with PMMA augmentation capability is desirable.

Finite element analysis of implanted lumbar spine: Effects of open laminectomy plus PLF and open laminectomy plus TLIF surgical approaches on L3-L4 FSU

Several finite element (FE) studies reported performances of various lumbar fusion surgical approaches. However, comparative studies on the performance of Open Laminectomy plus Posterolateral Fusion (OL-PLF) and Open Laminectomy plus Transforaminal Interbody Fusion (OL-TLIF) surgical approaches are rare. In the current FE study, the variation in ranges of motions (ROM), stress-strain distributions in an implanted functional spinal unit (FSU) and caudal adjacent soft structures between OL-PLF and OL-TLIF virtual models were investigated. The implanted lumbar spine FE models were developed from subject-specific computed tomography images of an intact spine and solved for physiological loadings such as compression, flexion, extension and lateral bending. Reductions in the ROMs of L1-L5 (49 % to 59 %) and L3-L4 implanted FSUs (91 % to 96 %) were observed for both models. Under all the loading cases, the maximum von Mises strain observed in the implanted segment of both models exceeds the mean compressive yield strain for the vertebra. The maximum von Mises stress and strain observed on the caudal adjacent soft structures of both the implanted models are at least 22 % higher than the natural spine model. The findings indicate the risk of failure in the implanted FSUs and higher chances of adjacent segment degeneration for both models.

Dislodgment Effects of Different Cage Arrangements in Posterior Lumbar Interbody Fusion: A Finite Element Study

The vertebral cage has been widely used in posterior lumbar interbody fusion. The risk of cage dislodgment is high for patients undergoing lumbar fusion surgery. Therefore, the main objective of this study was to use a lumbar fusion model to investigate the effects of cage dislodgment on different cage arrangements after PLIF. Finite element analysis was used to compare three PEEK cage placements, together with the fibula-type cage, with respect to the four kinds of lumbar movements. The results revealed that a horizontal cage arrangement could provide a better ability to resist cage dislodgment. Overall lumbar flexion movements were confirmed to produce a greater amount of cage slip than the other three lumbar movements. The lower part of the lumbar fusion segment could create a greater amount of cage dislodgment for all of the lumbar movements. Using an autograft with a fibula as a vertebral cage cannot effectively reduce cage dislodgment. Considering the maximum movement type in lumbar flexion, we suggest that a horizontal arrangement of the PEEK cage might be considered when a single PEEK cage is placed in the fusion segment, as doing so can effectively reduce the extent of cage dislodgment.

Biomechanical Comparison of Different Surgical Approaches for the Treatment of Adjacent Segment Diseases after Primary Transforaminal Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND AND OBJECTIVE

Adjacent segment disease (ASD) is a well-known complication after interbody fusion. Revision surgery is necessary for symptomatic ASD to further decompress and fix the affected segment. However, no optimal construct is accepted as a standard in treating ASD. The purpose of this study was to compare the biomechanical effects of different surgical approaches for the treatment of ASD after primary transforaminal lumbar interbody fusion (TLIF).

METHODS

A finite element model of the L1-S1 was conducted based on computed tomography scan images. The primary surgery model was developed with a single-level TLIF at L4-L5 segment. The revision surgical models were developed with anterior lumbar interbody fusion (ALIF), lateral lumbar interbody fusion (LLIF), or TLIF at L3-L4 segment. The range of motion (ROM), intradiscal pressure (IDP), and the stress in cages were compared to investigate the biomechanical influences of different surgical approaches.

RESULTS

The results indicated that all the three surgical approaches can stabilize the spinal segment by reducing the ROM at revision level. The ROM and IDP at adjacent segments of revision model of TLIF was greater than those of other revision models. While revision surgery with ALIF and LLIF had similar effects on the ROM and IDP of adjacent segments. Compared among all the surgical models, cage stress in revision model of TLIF was the maximum in extension and axial rotation.

CONCLUSION

The IDP at adjacent segments and stress in cages of revision model of TLIF was greater than those of ALIF and LLIF. This may be that direct extension of the surgical segment in the same direction results in stress concentration.

Biomechanical analysis of functionally graded porous interbody cage for lumbar spinal fusion

Functionally graded porous (FGP) interbody cage might offer a trade-off between porosity-based reduction of stiffness and mechanical properties. Using finite element models of intact and implanted lumbar functional spinal unit (FSU), the study investigated the quantitative deviations in load transfer and adaptive changes in bone density distributions around FGP interbody cages. The cage had three graded porosities: FGP-A, -B, and -C corresponded to a maximum porosity levels of 48%, 65% and 78%, respectively. Efficacy of the FGP cages were evaluated by comparing the numerically predicted results of solid-Ti and uniformly porous 78% porosity (P78) cage. Variations in stiffness and interface condition affected the strain distribution and bone remodelling around the cages. Peak strains of 0.5-1% were observed in less number of peri-prosthetic bone elements for the FGP cages as compared to the solid-Ti cage. Strains and bone apposition were considerably higher for the bonded implant-bone interface condition than the debonded case. For the FGP-C with bonded interface condition, bone apposition of 11-20% was predicted in the L4 and L5 regions of interest (ROIs); whereas the debonded model exhibited 6-10% increase in bone density. The deviations in bone density change between FGP-C and P78 model were 3-8% for L4 and L5 ROIs. FGP resulted in a reduced average micromotion (∼70-106 μm) as compared to solid-Ti (116 μm), for all physiologic movements. Compared to solid-Ti and uniformly porous cages, the FGP cage seems to be a viable alternative considering the conflicting nature of strength and porosity.

An overview of 3D printed metal implants in orthopedic applications: Present and future perspectives

With the ability to produce components with complex and precise structures, additive manufacturing or 3D printing techniques are now widely applied in both industry and consumer markets. The emergence of tissue engineering has facilitated the application of 3D printing in the field of biomedical implants. 3D printed implants with proper structural design can not only eliminate the stress shielding effect but also improve in vivo biocompatibility and functionality. By combining medical images derived from technologies such as X-ray scanning, CT, MRI, or ultrasonic scanning, 3D printing can be used to create patient-specific implants with almost the same anatomical structures as the injured tissues. Numerous clinical trials have already been conducted with customized implants. However, the limited availability of raw materials for printing and a lack of guidance from related regulations or laws may impede the development of 3D printing in medical implants. This review provides information on the current state of 3D printing techniques in orthopedic implant applications. The current challenges and future perspectives are also included.

Biomechanical Analysis of Lumbar Interbody Fusion Cages With Various Elastic Moduli in Osteoporotic and Non-osteoporotic Lumbar Spine: A Finite Element Analysis

STUDY DESIGN

Finite element analysis (FEA).

OBJECTIVE

This study aimed to explore the effects of cage elastic modulus (Cage-E) on the endplate stress in different bone conditions: osteoporosis (OP) and non-osteoporosis (non-OP). We also explored the correlation between endplate thickness and endplate stress.

METHOD

The FEA models of L4-L5 with lumbar interbody fusion were designed to access the effects of Cage-E on the endplate stress in different bone conditions. Two groups of the Young's moduli of bony structure were assigned to simulate the conditions of OP and non-OP, and the bony endplates were analyzed in 2 kinds of thicknesses: .5 mm and 1.0 mm, with the insertion of cages with different Young's moduli including .5, 1.5, 3, 5, 10, and 20 GPa. After model validation, an axial compressive load of 400 N and a flexion/extension moment of 7.5Nm was performed on the superior surface of L4 vertebral body in order to analyze the distribution of stress.

RESULT

The maximum Von Mises stress in the endplates increased by up to 100% in the OP model compared with non-OP model under the same condition of cage-E and endplate thickness. In both OP and non-OP models, the maximum endplate stress decreased as the cage-E decreased, but the maximum stress in the lumbar posterior fixation increased as the cage-E decreased. Thinner endplate thickness was associated with increased endplate stress.

CONCLUSION

The endplate stress is higher in osteoporotic bone than non-osteoporotic bone, which explains part of the mechanism of OP-related cage subsidence. It is reasonable to reduce the endplate stress by reducing the cage-E, but we should balance the risk of fixation failure. Endplate thickness is also important when evaluating the cage subsidence risk.

Subsidence of a partially porous titanium lumbar cage produced by electron beam melting technology

The lumbar intervertebral devices are widely used in the surgical treatment of lumbar diseases. The subsidence represents a serious clinical issue during the healing process, mainly when the interfaces between the implant and the vertebral bodies are not well designed. The aim of this study is the evaluation of subsidence risk for two different devices. The devices have the same shape, but one of them includes a filling micro lattice structure. The effect of the micro lattice structure on the subsidence behavior of the implant was evaluated by means of both experimental tests and finite element analyses. Compressive tests were carried out by using blocks made of grade 15 polyurethane, which simulate the vertebral bone. Non-linear, quasi-static finite element analyses were performed to simulate experimental and physiologic conditions. The experimental tests and the FE analyses showed that the subsidence risk is higher for the device without micro lattice structure, due to the smaller contact surface. Moreover, an overload in the central zone of the contact surface was detected in the same device and it could cause the implant failure. Thus, the micro lattice structure allows a homogenous pressure distribution at the implant-bone interface.

Biomechanical and clinical studies on lumbar spine fusion surgery: a review

Low back pain is associated with degenerative disc diseases of the spine. Surgical treatment includes fusion and non-fusion types. The gold standard is fusion surgery, wherein the affected vertebral segment is fused. The common complication of fusion surgery is adjacent segment degeneration (ASD). The ASD often leads to revision surgery, calling for a further fusion of adjacent segments. The existing designs of nonfusion type implants are associated with clinical problems such as subsidence, difficulty in implantation, and the requirement of revision surgeries. Various surgical approaches have been adopted by the surgeons to insert the spinal implants into the affected segment. Over the years, extensive biomechanical investigations have been reported on various surgical approaches and prostheses to predict the outcomes of lumbar spine implantations. Computer models have been proven to be very effective in identifying the best prosthesis and surgical procedure. The objective of the study was to review the literature on biomechanical studies for the treatment of lumbar spinal degenerative diseases. A critical review of the clinical and biomechanical studies on fusion spine surgeries was undertaken. The important modeling parameters, challenges, and limitations of the current studies were identified, showing the future research directions.

Effect of Interbody Implants on the Biomechanical Behavior of Lateral Lumbar Interbody Fusion: A Finite Element Study

Porous titanium interbody scaffolds are growing in popularity due to their appealing advantages for bone ingrowth. This study aimed to investigate the biomechanical effects of scaffold materials in both normal and osteoporotic lumbar spines using a finite element (FE) model. Four scaffold materials were compared: Ti6Al4V (Ti), PEEK, porous titanium of 65% porosity (P65), and porous titanium of 80% porosity (P80). In addition, the range of motion (ROM), endplate stress, scaffold stress, and pedicle screw stress were calculated and compared. The results showed that the ROM decreased by more than 96% after surgery, and the solid Ti scaffold provided the lowest ROM (1.2-3.4% of the intact case) at the surgical segment among all models. Compared to solid Ti, PEEK decreased the scaffold stress by 53-66 and the endplate stress by 0-33%, while porous Ti decreased the scaffold stress by 20-32% and the endplate stress by 0-32%. Further, compared with P65, P80 slightly increased the ROM (<0.03°) and pedicle screw stress (<4%) and decreased the endplate stress by 0-13% and scaffold stress by approximately 18%. Moreover, the osteoporotic lumbar spine provided higher ROMs, endplate stresses, scaffold stresses, and pedicle screw stresses in all motion modes. The porous Ti scaffolds may offer an alternative for lateral lumbar interbody fusion.

Impact of surface roughness and bulk porosity on spinal interbody implants

Spinal fusion surgeries are performed to treat a multitude of cervical and lumbar diseases that lead to pain and disability. Spinal interbody fusion involves inserting a cage between the spinal vertebrae, and is often utilized for indirect neurologic decompression, correction of spinal alignment, anterior column stability, and increased fusion rate. The long-term success of interbody fusion relies on complete osseointegration between the implant surface and vertebral end plates. Titanium (Ti)-based alloys and polyetheretherketone (PEEK) interbody cages represent the most commonly utilized materials and provide sufficient mechanics and biocompatibility to assist in fusion. However, modification to the surface and bulk characteristics of these materials has been shown to maximize osseointegration and long-term stability. Specifically, the introduction of intrinsic porosity and surface roughness has been shown to affect spinal interbody mechanics, vascularization, osteoblast attachment, and ingrowth potential. This narrative review synthesizes the mechanical, in vitro, in vivo, and clinical effects on fusion efficacy associated with introduction of porosity in Ti (neat and alloy) and PEEK intervertebral implants.

Recent advancement in finite element analysis of spinal interbody cages: A review

Finite element analysis (FEA) is a widely used tool in a variety of industries and research endeavors. With its application to spine biomechanics, FEA has contributed to a better understanding of the spine, its components, and its behavior in physiological and pathological conditions, as well as assisting in the design and application of spinal instrumentation, particularly spinal interbody cages (ICs). IC is a highly effective instrumentation for achieving spinal fusion that has been used to treat a variety of spinal disorders, including degenerative disc disease, trauma, tumor reconstruction, and scoliosis. The application of FEA lets new designs be thoroughly "tested" before a cage is even manufactured, allowing bio-mechanical responses and spinal fusion processes that cannot easily be experimented upon in vivo to be examined and "diagnosis" to be performed, which is an important addition to clinical and in vitro experimental studies. This paper reviews the recent progress of FEA in spinal ICs over the last six years. It demonstrates how modeling can aid in evaluating the biomechanical response of cage materials, cage design, and fixation devices, understanding bone formation mechanisms, comparing the benefits of various fusion techniques, and investigating the impact of pathological structures. It also summarizes the various limitations brought about by modeling simplification and looks forward to the significant advancement of spine FEA research as computing efficiency and software capabilities increase. In conclusion, in such a fast-paced field, the FEA is critical for spinal IC studies. It helps in quantitatively and visually demonstrating the cage characteristics after implanting, lowering surgeons' learning costs for new cage products, and probably assisting them in determining the best IC for patients.

Biomechanical studies of different numbers and positions of cage implantation on minimally invasive transforaminal interbody fusion: A finite element analysis

BACKGROUND

The position and number of cages in minimally invasive transforaminal interbody fusion (MIS-TLIF) are mainly determined by surgeons based on their individual experience. Therefore, it is important to investigate the optimal number and position of cages in MIS-TLIF.

METHODS

The lumbar model was created based on a 24-year-old volunteer's computed tomography data and then tested using three different cage implantation methods: single transverse cage implantation (model A), single oblique 45° cage implantation (model B), and double vertical cage implantation (model C). A preload of 500 N and a moment of 10 Nm were applied to the models to simulate lumbar motion, and the models' range of motion (ROM), ROM ratio, peak stress of the internal fixation system, and cage were assessed.

RESULTS

The ROM ratios of models A, B, and C were significantly reduced by >71% compared with the intact model under all motions. Although there were subtle differences in the ROM ratio for models A, B, and C, the trends were similar. The peak stress of the internal fixation system appeared in model B of 136.05 MPa (right lateral bending), which was 2.07 times that of model A and 1.62 times that of model C under the same condition. Model C had the lowest cage stress, which was superior to that of the single-cage model.

CONCLUSION

In MIS-TLIF, single long-cage transversal implantation is a promising standard implantation method, and double short-cage implantation is recommended for patients with severe osteoporosis.

[Application of three-dimensional printed porous titanium alloy cage and poly-ether-ether-ketone cage in posterior lumbar interbody fusion]

OBJECTIVE

To compare the effectiveness between three-dimensional (3D) printed porous titanium alloy cage (3D Cage) and poly-ether-ether-ketone cage (PEEK Cage) in the posterior lumbar interbody fusion (PLIF).

METHODS

A total of 66 patients who were scheduled to undergo PLIF between January 2018 and June 2019 were selected as the research subjects, and were divided into the trial group (implantation of 3D Cage, n=33) and the control group (implantation of PEEK Cage, n=33) according to the random number table method. Among them, 1 case in the trial group did not complete the follow-up exclusion study, and finally 32 cases in the trial group and 33 cases in the control group were included in the statistical analysis. There was no significant difference in gender, age, etiology, disease duration, surgical segment, and preoperative Japanese Orthopaedic Association (JOA) score between the two groups (P>0.05). The operation time, intraoperative blood loss, complications, JOA score, intervertebral height loss, and interbody fusion were recorded and compared between the two groups.

RESULTS

The operations of two groups were completed successfully. There was 1 case of dural rupture complicated with cerebrospinal fluid leakage during operation in the trial group, and no complication occurred in the other patients of the two groups. All incisions healed by first intention. There was no significant difference in operation time and intraoperative blood loss between groups (P>0.05). All patients were followed up 12-24 months (mean, 16.7 months). The JOA scores at 1 year after operation in both groups significantly improved when compared with those before operation (P<0.05); there was no significant difference between groups (P>0.05) in the difference between pre- and post-operation and the improvement rate of JOA score at 1 year after operation. X-ray film reexamination showed that there was no screw loosening, screw rod fracture, Cage collapse, or immune rejection in the two groups during follow-up. At 3 months and 1 year after operation, the rate of intervertebral height loss was significantly lower in the trial group than in the control group (P<0.05). At 3 and 6 months after operation, the interbody fusion rating of trial group was significantly better in the trial group than in the control group (P<0.05); and at 1 year after operation, there was no significant difference between groups (P>0.05).

CONCLUSION

There is no significant difference between 3D Cage and PEEK Cage in PLIF, in terms of operation time, intraoperative blood loss, complications, postoperative neurological recovery, and final intervertebral fusion. But the former can effectively reduce vertebral body subsidence and accelerate intervertebral fusion.

Biomechanical Evaluation of Transforaminal Lumbar Interbody Fusion with Coflex-F and Pedicle Screw Fixation: Finite Element Analysis of Static and Vibration Conditions

OBJECTIVE

To investigate the biomechanics of transforaminal lumbar interbody fusion (TLIF) with interspinous process device (IPD) or pedicle screw fixation under both static and vibration conditions by the finite element (FE) method.

METHOD

A validated FE model of the L1-5 lumbar spine was used in this study. This FE model derived from computed tomography images of a healthy female adult volunteer of appropriate age. Then the model was modified to simulate L3-4 TLIF. Four conditions were compared: (i) intact; (ii) TLIF combined with bilateral pedicle screw fixation (BPSF); (iii) TLIF combined with U-shaped IPD Coflex-F (CF); and (iv) TLIF combined with unilateral pedicle screw fixation (UPSF). The intact and surgical FE models were analyzed under static and vibration loading conditions respectively. For static loading conditions, four motion modes (flexion, extension, lateral bending, and axial rotation) were simulated. For vibration loading conditions, the dynamic responses of lumbar spine under sinusoidal vertical load were simulated.

RESULT

Under static loading conditions, compared with intact case, BPSF decreased range of motion (ROM) by 92%, 95%, 89% and 92% in flexion, extension, lateral bending and axial rotation, respectively. While CF decreased ROM by 87%, 90%, 69% and 80%, and UPSF decreased ROM by 84%, 89%, 66% and 82%, respectively. Compared with CF, UPSF increased the endplate stress by 5%-8% in flexion, 7%-10% in extension, 2%-4% in lateral bending, and decreased the endplate stress by 16%-19% in axial rotation. Compared with CF, UPSF increased the cage stress by 9% in flexion, 10% in extension, and decreased the cage stress by 3% in lateral bending, and 13% in axial rotation. BPSF decreased the stress responses of endplates and cage compared with CF and UPSF. Compared BPSF, CF decreased the facet joint force (FJF) by 6%-13%, and UPSF decreased the FJF by 4%-12%. During vibration loading conditions, compared with BPSF, CF reduced maximum values of the FJF by 16%-32%, and vibration amplitudes by 22%-35%, while UPSF reduced maximum values by 20%-40%, and vibration amplitudes by 31%-45%.

CONCLUSION

Compared with other surgical models, BPSF increased the stability of lumbar spine, and also showed advantages in cage stress and endplate stress. CF showed advantages in IDP and FJF especially during vertical vibration, which may lead to lower risk of adjacent segment degeneration. CF may be an effective alternative to pedicle screw fixation in TLIF procedures.

Subsidence and fusion performance of a 3D-printed porous interbody cage with stress-optimized body lattice and microporous endplates - a comprehensive mechanical and biological analysis

BACKGROUND CONTEXT

Cage subsidence remains a serious complication after spinal fusion surgery. Novel porous designs in the cage body or endplate offer attractive options to improve subsidence and osseointegration performance.

PURPOSE

To elucidate the relative contribution of a porous design in each of the two major domains (body and endplates) to cage stiffness and subsidence performance, using standardized mechanical testing methods, and to analyze the fusion progression via an established ovine interbody fusion model to support the mechanical testing findings.

STUDY DESIGN/SETTING

A comparative preclinical study using standardized mechanical testing and established animal model.

METHODS

To isolate the subsidence performance contributed by each porous cage design feature, namely the stress-optimized body lattice (vs. a solid body) and microporous endplates (vs. smooth endplates), four groups of cages (two-by-two combination of these two features) were tested in: (1) static axial compression of the cage (per ASTM F2077) and (2) static subsidence (per ASTM F2267). To evaluate the progression of fusion, titanium cages were created with a microporous endplate and internal lattice architecture analogous to commercial implants used in subsidence testing and implanted in an endplate-sparing, ovine intervertebral body fusion model.

RESULTS

The cage stiffness was reduced by 16.7% by the porous body lattice, and by 16.6% by the microporous endplates. The porous titanium cage with both porous features showed the lowest stiffness with a value of 40.4±0.3 kN/mm (Mean±SEM) and a block stiffness of 1976.8±27.4 N/mm for subsidence. The body lattice showed no significant impact on the block stiffness (1.4% reduction), while the microporous endplates decreased the block stiffness significantly by 24.9% (p<.0001). All segments implanted with porous titanium cages were deemed rigidly fused by manual palpation, except one at 12 weeks, consistent with robotic ROM testing and radiographic and histologic observations. A reduction in ROM was noted from 12 to 26 weeks (4.1±1.6° to 2.2±1.4° in lateral bending, p<.05; 2.1±0.6° to 1.5±0.3° in axial rotation, p<.05); and 3.3±1.6° to 1.9±1.2° in flexion extension, p=.07). Bone in the available void improved with time in the central aperture (54±35% to 83±13%, p<.05) and porous cage structure (19±26% to 37±21%, p=.15).

CONCLUSIONS

Body lattice and microporous endplates features can effectively reduce the cage stiffness, therefore reducing the risk of stress shielding and promoting early fusion. While body lattice showed no impact on block stiffness and the microporous endplates reduced the block stiffness, a titanium cage with microporous endplates and internal lattice supported bone ingrowth and segmental mechanical stability as early as 12 weeks in ovine interbody fusion.

CLINICAL SIGNIFICANCE

Porous titanium cage architecture can offer an attractive solution to increase the available space for bone ingrowth and bridging to support successful spinal fusion while mitigating risks of increased subsidence.

Biomaterials for Interbody Fusion in Bone Tissue Engineering

In recent years, interbody fusion cages have played an important role in interbody fusion surgery for treating diseases like disc protrusion and spondylolisthesis. However, traditional cages cannot achieve satisfactory results due to their unreasonable design, poor material biocompatibility, and induced osteogenesis ability, limiting their application. There are currently 3 ways to improve the fusion effect, as follows. First, the interbody fusion cage is designed to facilitate bone ingrowth through the preliminary design. Second, choose interbody fusion cages made of different materials to meet the variable needs of interbody fusion. Finally, complete post-processing steps, such as coating the designed cage, to achieve a suitable osseointegration microstructure, and add other bioactive materials to achieve the most suitable biological microenvironment of bone tissue and improve the fusion effect. The focus of this review is on the design methods of interbody fusion cages, a comparison of the advantages and disadvantages of various materials, the influence of post-processing techniques and additional materials on interbody fusion, and the prospects for the future development of interbody fusion cages.

Finite Element Analysis of a Novel Fusion Strategy in Minimally Invasive Transforaminal Lumbar Interbody Fusion

Purpose

To evaluate the biomechanics of a novel fusion strategy (hybrid internal fixation+horizontal cage position) in minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF).

Methods

MIS-TLIF finite element models for three fusion strategies were created based on computed tomography images, namely, Model-A, hybrid internal fixation (ipsilateral pedicle screw and contralateral translaminar facet screw fixation)+horizontal cage position; Model-B, bilateral pedicle screw (BPS) fixation+horizontal cage position; and Model-C, BPS fixation+oblique 45° cage position. A preload of 500 N and a moment of 10 Nm were applied to the models to simulate lumbar motion, and the models' range of motion (ROM), peak stress of the internal fixation system, and cage were assessed.

Results

The ROM for Models A, B, and C were not different (P > 0.05) but were significantly lower than the ROM of Model-INT (P < 0.0001). Although there were subtle differences in the ROM ratio for Models A, B, and C, the trend was similar. The peak stress of the internal fixation system was significantly higher in Model-A than that of Models B and C, but only the difference between Models A and B was significant (P < 0.05). The peak stress of the cage in Model-A was significantly lower than that of Models B and C (P < 0.01).

Conclusion

Hybrid internal fixation with horizontal single cage implantation can provide the same biomechanical stability as traditional fixation while reducing peak stress on the cage and vertebral endplate.

Bone Remodelling Around Solid and Porous Interbody Cages in the Lumbar Spine

Spinal fusion is an effective surgical treatment for intervertebral disc degeneration. However, the consequences of implantation with interbody cages on load transfer and bone remodelling in the vertebral bodies has scarcely been investigated. Using detailed 3D models of an intact and implanted lumbar spine and the strain energy density based bone remodelling algorithm, this study investigated the evolutionary changes in bone density distributions around porous and solid interbody cages. Follower load technique and submodelling approach were employed to simulate applied loading conditions on the lumbar spine models. The study determined the relationship between mechanical properties and parametrical characteristics of porous Body-centered-cubic (BCC) models, which corroborated well with Gibson-Ashby and exponential regression models. Variations in porosity affected the peri-prosthetic stress distributions and bone remodelling around the cages. In comparison to the solid cage, stresses and strains in the cancellous bone decreased with an increase in cage porosity; whereas the range of motion increased. For the solid cage, increase in bone density of 20-28% was predicted in the L4 inferior and L5 superior regions; whereas the model with 78% porosity exhibited a small 3-5% change in bone density. An overall increase of 9-14% bone density was predicted in the L4 and L5 vertebrae after remodelling for solid interbody cages, which may influence disc degeneration in the adjacent segment. In comparison to the solid cage, an interbody cage with 65-78% porosity could be a viable and promising alternative, provided sufficient mechanical strength is offered.

Biomechanical changes at the adjacent segments induced by a lordotic porous interbody fusion cage

Biomechanical changes at the adjacent segments after interbody fusion are common instigators of adjacent segment degeneration (ASD). This study aims to investigate how the presence of a lordotic porous cage affects the biomechanical performance of the adjacent segments. A finite element model (FEM) of a lumbar spine implanted with a lordotic cage at L3-L4 was validated by in-vitro testing. The stress distribution on the cage and range of motion (ROM) of L3-L4 were used to assess the stability of the implant. Three angles of cage (0° = non-restoration, 7° = normal restoration and 11° = over-restoration) were modelled with different porosities (0%, 30% and 60%) and evaluated in the motions of flexion, extension, lateral bending and rotation. The ROM, intervertebral disc pressure (IDP) and facet joint force (FJF) were used to evaluate biomechanical changes at the adjacent segments in each model. The results indicated that porous cages produced more uniform stress distribution, but cage porosity did not influence the ROM, IDP and FJF at L2-L3 and L4-L5. Increasing the cage lordotic angle acted to decrease the ROM and IDP, and increase the FJF of L4-L5, but did not alter the ROM of L2-L3. In conclusion, changes in ROM, IDP and FJF at the adjacent segments were mainly influenced by the lordotic angle of the cage and not by the porosity. A larger angle of lordotic cage was shown to reduce the ROM and IDP, and increase the FJF of the lower segment (L4-L5), but had little effect on the ROM of the upper segment (L2-L3).

Relationship between the elastic modulus of the cage material and the biomechanical properties of transforaminal lumbar interbody fusion: A logarithmic regression analysis based on parametric finite element simulations

BACKGROUND AND OBJECTIVE

Conventional method for evaluating the biomechanical effects of a specific elastic modulus of cage (cage-E) on spinal fusions requires establishing a "one-on-one" biomechanical model, which seems laborious and inefficient when dealing with the emergence of numerous cage materials with various cage-Es. We aim to offer a much convenient method to instantly predicting the biomechanical effects of any targeted cage-E on transforaminal lumbar interbody fusion (TLIF) by using a parametric finite element (FE) analysis to determining the regression relationship between cage-E and biomechanical properties of TLIF.

MATERIALS AND METHODS

A L4/5 FE TLIF construct was modeled. Cage-E was linearly increased from 0.1 GPa (cancellous bone) to 110 GPa (titanium alloy). The function equations for assessing the influence of cage-E on the biomechanical indexes of TLIF were established using a logarithmic regression analysis.

EXPERIMENTAL RESULTS

As cage-E increased from 0.1 GPa to 110 GPa, all the biomechanical indexes initially increased or decayed rapidly, and then slowed over time. Logarithmic regression models and functional equations were successfully established between cage-E and these indexes (P<0.0001). Their determination coefficients ranged from 0.72 to 0.99. The range of motions decreased from 0.37-1.10° to 0.20-1.07°. The mean stresses of the central and peripheral grafts reduced from 0.10-0.41 and 0.25-0.42 MPa to 0.03-0.04 and 0.19-0.27 MPa, respectively. In addition, the maximum stresses of the screw-bone interface and posterior instrumentation reduced from 11.76-25.04 and 8.91-84.68 MPa to 9.71-18.92 and 6.99-70.59 MPa, respectively. Finally, the maximum stresses of the cage and endplate increased from 0.28-1.35 MPa and 3.90-8.63 MPa to 14.86-36.16 MPa and 11.01-36.55 MPa, respectively.

CONCLUSIONS

The decrease of cage-E reduces the risks of cage subsidence, cage breakage, and pseudarthrosis, while increasing the risk of instrumentation failure. The logarithmic regression models optimally demonstrate the relationship between cage-E and biomechanical properties of TLIF. The functional equations based on these models can be adopted to predict the biomechanical effects of any targeted cage-Es on TLIF, which effectively simplifies the procedures for the biomechanical assessments of cage materials.

Computational comparison of three different cage porosities in posterior lumbar interbody fusion with porous cage

Porous interbody cages, manufactured using additive laser melting technology, have recently been used in lumbar fusion surgery. The major advantage of a porous cage is the presence of space inside the cage for bone ingrowth. However, the biomechanical effects of different porosities on the lumbar segment with and without bone fusion (ingrowth) are still unclear. Hence, the present study aimed to compare the biomechanical responses, including the stress and range of motion (ROM) of the lumbar L3-L4 segments with three different types of porous cages along with a posterior instrument (PI) with and without bone fusion using computer simulation. A lumbar L3-L4 segment model with a PI and porous cages was used in this study. Three different porosities, namely 12.5, 41.2, and 80.84% were used. The diameter of the pores of the porous cage was uniformly set to 0.5 mm. In addition, a traditional PEEK cage was used in this study. Two different bone statuses, with and without bone fusion (ingrowth into the pores of the porous cage and the inner space of the PEEK cage), were considered. The results indicated that although the contact pressure on the bone surface reduced, the cage stress increased with increasing cage porosity. Furthermore, cage stress and contact pressure also increased in cases with bone fusion compared with those without bone fusion. The contact pressure on the bone surface with a cage porosity of 80.8% decreased by 40% (from 943.1 to 575.5 MPa), 37.7% (from 133 to 82.9 MPa), 40.4% (from 690.8 to 412 MPa), and 34.2% (from 533 to 351.1 MPa), respectively, for flexion, extension, lateral bending, and rotation, respectively, compared with that with a cage porosity of 12.5%. The rotational ROM of the PEEK cage with bone fusion was clearly larger than those of the porous cages. Porous cages have recently become popular owing to improved manufacturing technology. This study provides scientific data on the strength and weakness of porous cages with different porosities for clinical use.

Biomechanical comparison of four types of instrumentation constructs for revision surgery in lumbar adjacent segment disease: A finite element study

BACKGROUND

Different constructs are applied in revision surgery (RS) for adjacent segment disease (ASD) aiming to further decompress and fixate the affected segment(s) in two ways: replacing or preserving the primary implants. This study aimed to compare the biomechanical properties of four constructs with different configurations.

METHODS

An T12-L5 finite element (FE) model was constructed and validated. Primary surgery was performed at L4-L5 and instrumented from L3 to L5. Thereafter, RS was undertook by decompressing L2-L3 and fixated with implant-replacing construct A, or implant-preserving construct B, C or D. Range of motion (ROM) and intervertebral disc pressure (IDP) were compared. Maximum von Mises stress on the rods between Construct A and B was evaluated.

RESULTS

An obvious reduction of ROM was observed when the FE model was instrumented with four constructs respectively. The overall changing characteristics of ROM were approximately identical among four constructs. The changing characteristic of IDP among four constructs was similar. The degree of IDP reduction of Construct B was comparable to Construct A, while that of Construct C was comparable to Construct D. Maximum von Mises stress on the rods between Construct A and B indicated that no stress concentration was recorded at the locking part of the connector rod.

CONCLUSIONS

The biomechanics of implant-preserving constructs were comparable to the traditional implant-replacing construct. The location of side-by-side connector could not affect the stability of Construct C and D. Construct B might be an optimal choice in RS for less dissection, less complication and more convenience in manipulation.

Numerical analysis of the mechanical behaviour of intact and implanted lumbar functional spinal units: Effects of loading and boundary conditions

The objective of this study was to develop an improved finite element (FE) model of a lumbar functional spinal unit (FSU) and to subsequently analyse the deviations in load transfer owing to implantation. The effects of loading and boundary conditions on load transfer in intact and implanted FSUs and its relationship with the potential risk of vertebral fracture were investigated. The FE models of L1-L5 and L3-L4 FSUs, intact and implanted, were developed using patient-specific CT-scan dataset and segmentation of cortical and cancellous bone regions. The effect of submodelling technique, as compared to artificial boundary conditions, on the elastic behaviour of lumbar spine was examined. Applied forces and moments, corresponding to physiologic movements, were used as loading conditions. Results indicated that the loading and boundary conditions considerably affect stress-strain distributions within a FSU. This study, based on an improved FE model of a vertebra, highlights the importance of using the submodelling technique to adequately evaluate the mechanical behaviour of a FSU. In the intact FSU, strains of 200-400 µε were observed in the cancellous bone of vertebral body and pedicles. High equivalent stresses of 10-25 MPa and 1-5 MPa were generated around the pars interarticularis for cortical and cancellous regions, respectively. Implantation caused reductions of 85%-92% in the range of motion for all movements. Insertion of the intervertebral cage resulted in major deviations in load transfer across a FSU for all movements. The cancellous bone around cage experienced pronounced increase in stresses of 10-15 MPa, which indicated potential risk of failure initiation in the vertebra.

Vertebral Endplate Cyst Formation in Relation to Properties of Interbody Cages

Objective

This retrospective study aimed to compare vertebral endplate cyst formation (VECF), an early predictor for pseudoarthrosis, in different types of interbody cages.

Methods

We reviewed 84 cases treated with single-level posterior/transforaminal lumbar interbody fusion. We utilized a polyetheretherketone cage in 20 cases (group P), a titanium cage in 16 cases (group Ti), a titanium-coating polyetheretherketone cage in 13 cases (group TiP) and a porous tantalum cage in 35 cases (group Tn). VECF was evaluated comparing the computed tomography scans taken at day 0 and 6-month postoperation. We defined VECF (+) as enlargement of a pre-existing cyst or de novo formation of a cyst with the diameter over 2 mm. We calculated the adjusted odds ratio (OR) and 95% confidence intervals (CIs) as an indicator of association between different types of cages and VECF using a logistic regression model.

Results

VECF was observed in 13 (65%), 7 (44%), 9 (69%), and 8 (23%) cases in groups P, Ti, TiP and Tn, respectively. VECF correlated with the type of cage (p = 0.04). In comparison with group P, the proportion of VECF (+) cases was lower in group Tn (OR, 0.16; 95% CI, 0.04–0.60) but not different in group Ti (OR, 0.47; 95% CI, 0.10–2.20) and group TiP (OR, 1.06; 95% CI, 0.21–5.28). No patient underwent additional surgery for the fused spinal level during the follow-up periods (average, 37.9 months; range, 6–76 months).

Conclusion

VECF was the least in the porous Tn cage, suggesting its potential superiority for initial stability.

The anterior and traverse cage can provide optimal biomechanical performance for both traditional and percutaneous endoscopic transforaminal lumbar interbody fusion

BACKGROUND

Transforaminal lumbar interbody fusion (TLIF) is a well-established surgical treatment for patients with lumbar degenerative disc disease; however, the optimal position for the interbody fusion cage in TLIF procedures for reducing cage-related complications remains uncertain. The present study aims to compare the biomechanical effects between different cage positions in TLIF and percutaneous endoscopic-TLIF (PE-TLIF).

METHOD

An intact finite element model of L3-L5 from computed tomography images of a 25-year-old healthy male without any lumbar disease was reconstructed and validated. TLIF and PE-TLIF were performed on L4-L5 with bilateral pedicle screws fixation. Two surgical finite element models were subjected to loads with six degrees of freedom. The range of motion (ROM) and von Mises stress of the implantations and endplates were measured for the anterior, middle, and posterior district and the traverse or oblique direction of the cage respectively.

RESULTS

As the cage was implanted forward, the ROMs in the fused L4-L5 segments and the von Mises stress of the cage and endplates decreased while the von Mises stress of the screws increased; this was also shown in the traverse cage when compared with the oblique cage (A-90-compared with A-45- had a 31.3%, 1.7%, 12.6%, and 5.7% decrease in FL, EX, LB and AR). The ROMs (TLIF A-45 increase of 80.8%, 23.8%, and 12.2% in FL, EX, and LB when compared with PE-TLIF), cage stress, and endplate stress of PE-TLIF were lower than those of TLIF.

CONCLUSIONS

Considering the ROM of the fusion segments, implanting the cage in the anterior district in the traverse direction can effectively enhance the fusion segment stiffness, thus contributing to the stability of the lumbar spine after fusion. It can also cause less cage stress and endplate stress, which indicates its beneficial effect in avoiding cage injury or subsidence. However, the higher stress of the pedicle screws and rods indicates higher failure risk. PE-TLIF had better biomechanical performance than TLIF. Therefore, it is recommended that the surgeon implant the cage in the anterior district of the L5 vertebra's upper endplate in the traverse direction using the PE-TLIF technique.

The biomechanical effects of Ti versus PEEK used in the PLIF surgery on lumbar spine: a finite element analysis

Titanium (Ti) and polyetheretherketone (PEEK) are commonly used in posterior lumbar interbody fusion (PLIF). The study investigated biomechanical effects of Ti versus PEEK used as materials of cage and rods on the lumbar spine. Four different configurations of PLIF were constituted. Stiff Ti rods provided satisfactory initial stability but increased the stress on rods significantly under simulated physiological load conditions. Ti cage increased the stress on bone endplates significantly. Materials of cage and rods had insignificant effects on the nucleus pressure and facet joint force of non-instrumented segments. Further clinical studies and follow-up observations are essential for corroborating these findings.

Application of Spectroscopy in Additive Manufacturing

Additive manufacturing (AM) is a rapidly expanding material production technique that brings new opportunities in various fields as it enables fast and low-cost prototyping as well as easy customisation. However, it is still hindered by raw material selection, processing defects and final product assessment/adjustment in pre-, in- and post-processing stages. Spectroscopic techniques offer suitable inspection, diagnosis and product trouble-shooting at each stage of AM processing. This review outlines the limitations in AM processes and the prospective role of spectroscopy in addressing these challenges. An overview on the principles and applications of AM techniques is presented, followed by the principles of spectroscopic techniques involved in AM and their applications in assessing additively manufactured parts.

Load Sharing and Endplate Pressure Distribution in Anterior Interbody Fusion Influenced by Graft Choice

BACKGROUND

Cage subsidence is a known complication of spinal fusion. Various aspects of cage design have been investigated for their influence on cage subsidence, whereas the potential contribution of graft material to load sharing is often overlooked. We aimed to determine whether graft in the aperture affects endplate pressure distribution.

METHODS

The pressure distributions of a polyetheretherketone interbody cage with 3 different aperture graft conditions were evaluated: empty, demineralized bone matrix, and supercritical CO₂-treated allograft bone crunch (SCCO₂).

RESULTS

Graft materials contributed as much as half the load transmission for SCCO₂, whereas demineralized bone matrix contributed one third. Endplate areas in contact with the cage demonstrated decreased areas within the highest-pressure spectrum with SCCO₂ graft materials compared with empty cages.

CONCLUSIONS

Graft choice plays a role in reducing peak endplate pressures. This finding is relevant to implant subsidence, as well as graft loading and remodeling.

Porous fusion cage design via integrated global-local topology optimization and biomechanical analysis of performance

Porous fusion cage is considered as a satisfactory substitute for solid fusion cage in transforaminal lumbar interbody fusion (TLIF) surgery due to its interconnectivity for bone ingrowth and appropriate stiffness reducing the risk of cage subsidence and stress shielding. This study presents an integrated global-local topology optimization approach to obtain porous titanium (Ti) fusion cage with desired biomechanical properties. Local topology optimizations are first conducted to obtain unit cells, and the numerical homogenization method is used to quantified the mechanical properties of unit cells. The preferred porous structure is then fabricated using selective laser melting, and its mechanical property is further verified via compression tests and numerical simulation. Afterward, global topology optimization is used for the global layout. The porous fusion cage obtained by the Boolean intersection between global structural layout and the porous structure decreases the solid volume of the cage by 9% for packing more bone grafts while achieving the same stiffness to conventional porous fusion cage. To eliminate stress concentration in the thin-wall structure, framework structures are constructed on the porous fusion cage. Although the alleviation of cage subsidence and stress shielding is decelerated, peak stress on the cage is significantly decreased, and more even stress distribution is demonstrated in the reinforced porous fusion cage. It promises long-term integrity and functions of the fusion cage. Overall, the reinforced porous fusion cage achieves a favorable mechanical performance and is a promising candidate for fusion surgery. The proposed optimization approach is promising for fusion cage design and can be extended to other orthopedic implant designs.

Development and Validation of a Computationally Efficient Finite Element Model of the Human Lumbar Spine: Application to Disc Degeneration

INTRODUCTION

This study develops and validates an accurate, computationally efficient, 3-dimensional finite element model (FEM) of the human lumbar spine. Advantages of this simplified model are shown by its application to a disc degeneration study that we demonstrate is completed in one-sixth the time required when using more complicated computed tomography (CT) scan-based models.

METHODS

An osseoligamentous FEM of the L1-L5 spine is developed using simple shapes based on average anatomical dimensions of key features of the spine rather than CT scan images. Pure moments of 7.5 Nm and a compressive follower load of 1000 N are individually applied to the L1 vertebra. Validation is achieved by comparing rotations and intradiscal pressures to other widely accepted FEMs and in vitro studies. Then degenerative disc properties are modeled and rotations calculated. Required computation times are compared between the model presented in this paper and other models developed using CT scans.

RESULTS

For the validation study, parameter values for a healthy spine were used with the loading conditions described above. Total L1-L5 rotations for flexion, extension, lateral bending, and axial rotation under pure moment loading were calculated as 20.3°, 10.7°, 19.7°, and 10.3°, respectively, and under a compressive follower load, maximum intradiscal pressures were calculated as 0.68 MPa. These values compare favorably with the data used for validation. When studying the effects of disc degeneration, the affected segment is shown to experience decreases in rotations during flexion, extension, and lateral bending (24%-56%), while rotations are shown to increase during axial rotation (14%-40%). Adjacent levels realize relatively minor changes in rotation (1%-6%). This parametric study required 17.5 hours of computation time compared to more than 4 days required if utilizing typical published CT scan-based models, illustrating one of the primary advantages of the model presented in this article.

CONCLUSIONS

The FEM presented in this article produces a biomechanical response comparable to widely accepted, complex, CT scan-based models and in vitro studies while requiring much shorter computation times. This makes the model ideal for conducting parametric studies of spinal pathologies and spinal correction techniques.

Biomechanical Evaluation and the Assisted 3D Printed Model in the Patient-Specific Preoperative Planning for Thoracic Spinal Tuberculosis: A Finite Element Analysis

Posterior fixation is superior to anterior fixation in the correction of kyphosis and maintenance of spinal stability for the treatment of thoracic spinal tuberculosis. However, the process of selecting the appropriate spinal fixation method remains controversial, and preoperative biomechanical evaluation has not yet been investigated. In this study, we aimed to analyze the application of the assisted finite element analysis (FEA) and the three-dimensional (3D) printed model for the patient-specific preoperative planning of thoracic spinal tuberculosis. An adult patient with thoracic spinal tuberculosis was included. A finite element model of the T7−T11 thoracic spine segments was reconstructed to analyze the biomechanical effect of four different operative constructs. The von Mises stress values of the implants in the vertical axial load and flexion and extension conditions under a 400-N vertical axial pre-load and a 10-N⋅m moment were calculated and compared. A 3D printed model was used to describe and elucidate the patient’s condition and simulate the optimal surgical design. According to the biomechanical evaluation, the patient-specific preoperative surgical design was prepared for implementation. The anterior column, which was reconstructed with titanium alloy mesh and a bone graft with posterior fixation using seven pedicle screws (M+P) and performed at the T7–T11 level, decreased the von Mises stress placed on the right rod, T7 pedicle screw, and T11 pedicle. Moreover, the M+P evaded the left T9 screw without load bearing. The 3D printed model and preoperative surgical simulation enhanced the understanding of the patient’s condition and facilitated patient-specific preoperative planning. Good clinical results, including no complication of implants, negligible loss of the Cobb angle, and good bone fusion, were achieved using the M+P surgical design. In conclusion, M+P was recommended as the optimal method for preoperative planning since it enabled the preservation of the normal vertebra and prevented unnecessary internal fixation. Our study indicated that FEA and the assisted 3D printed model are tools that could be extremely useful and effective in the patient-specific preoperative planning for thoracic spinal tuberculosis, which can facilitate preoperative surgical simulation and biomechanical evaluation, as well as improve the understanding of the patient’s condition.

Finite element analysis and experimental evaluation on stress distribution and sensitivity of dental implants to assess optimum length and thread pitch

BACKGROUND AND OBJECTIVE

The dental implant is one of the long term proper remedies to recover a missed tooth as a different prosthetic rehabilitation way. The finite element (FE) method and photoelasticity test are employed to achieve stress distribution and sensitivity in dental implants in order to obtain optimum length and thread pitch.

METHODS

The finite element method and experimental test are developed to evaluate stress distribution and sensitivity around dental implants. Three dimensional FE models of implant-abutment, cortical bone and cancellous bone are created by considering a variation of 0.6 to -1 mm on threads pitch while the implant lengths range from 8.5 mm to 13 mm. Then, axial and oblique forces are applied to the models to obtain the resultant stress contours.

RESULTS

The results indicate that the resultant von Mises stresses in the implant-abutment, cortical bones, and cancellous bones are different. The optimized setting for length and pitch is suggested according to maximum von Mises stress and sensitivity analysis.

CONCLUSIONS

It is concluded that the present FE model accurately predicts stress distribution pattern in dental implants. The results indicate that sensitivity of length play a more significant role in comparison with thread pitch. The accuracy of FEM results in comparison with those of the photoelasticity test recommends applying computation methods in medical practice as great potential in terms of future studies.

Subject-specific finite element analysis of a lumbar cage produced by electron beam melting

The aim of this study was the analysis of the mechanical behaviour of a partially porous lumbar custom-made cage by means of a subject-specific finite element analysis (FEA). The cage, made of Ti6Al4V ELI alloy, was produced via electron beam melting (EBM) process and surgically implanted in a female subject, 50 years old. The novelty of this study was the customized design of the cage and of its internal structure, which is impossible to obtain with the traditional production techniques. The 3D model of the spine was obtained from the computed tomography (CT) of the patient. Moreover, high-resolution industrial CT was also used to reconstruct a 3D model of the cage, with its real (as-produced) features, such as superficial roughness, morphology of the bulk and of the porous structure. The workflow was divided in several steps: the main finite element analyses were non-linear and quasi-static regarding: the rhombic dodecahedron (RD) unit cell of the porous structure; the device; the whole L4-L5 motion segment with the implanted cage. Stress distribution was calculated under compression load for all models. For the RD unit cell, the maximum stress appeared at the connected cross nodes, where notch effect was present. For the cage subjected to a load of 1 kN, the porous structure did not present any functional failure. For the whole biomechanical system subjected to a physiological load of 360 N, the calculated stress in the bone was smaller than its yield strength value. On the axial view, a zone with higher compressive stresses was present on the L5 vertebral body. This was due to the contact stress between the cage and the vertebra. From the comparison between FE results and the CT images of the spine, bone remodelling was supposed, with the formation of new bone. Graphical abstract Workflow showing the phases of the research.

Finite element analysis of a ball-and-socket artificial disc design to suppress excessive loading on facet joints: A comparative study with ProDisc

Facet arthrosis at surgical level was identified as major complication after total disc replacement (TDR). One of the reasons for facet arthrosis after TDR has been speculated to be the hypermobility of artificial discs. Accordingly, the artificial disc that can constrain the hypermobility of ball-and-socket type artificial discs and reduce loading on facet joints is demanded. The proposed artificial disc, which is named as NewPro, was constructed based on the FDA-approved ProDisc but contained an interlocking system consisting of additional bars and grooves to control the range of motion (ROM) of lumbar spine in all anatomical planes. The three-dimensional finite element model of L1 to L5 was developed first, and the biomechanical effects were compared between ProDisc and NewPro. The ROM and facet contact force of NewPro were significantly decreased by 42.7% and 14% in bending and by 45.6% and 34.4% in torsion, respectively, compared with the values of ProDisc, thanks to the interlocking system. In addition, the ROM and facet contact force could be selectively constrained by modifying the location of the bars. The proposed artificial disc with the interlocking system was able to constrain the intersegmental rotation effectively and reduce excessive loading on facet joints, although wear and strength tests would be needed prior to clinical applications.

A comparison of the influence of three different lumbar interbody fusion approaches on stress in the pedicle screw fixation system: Finite element static and vibration analyses

This study aimed to examine breakage risk of the bilateral pedicle screw (BPS) fixation system under static and vibration loadings after three different types of lumbar interbody fusion surgery. A previously validated intact L1-sacrum finite element model was modified to simulate anterior, posterior, and transforaminal lumbar interbody fusion (ALIF, PLIF, and TLIF, respectively) with BPS fixation system (consisting of pedicle screws and rigid connecting rods) at L4-L5. As a risk parameter for breakage, the von Mises stresses in the pedicle screws and the rods for the ALIF, PLIF, and TLIF models under static loading (flexion, extension, lateral bending, and axial torsion moments) and vibration loading (sinusoidal vertical load) were calculated and compared. The calculated von Mises stresses were different in the ALIF, PLIF, and TLIF models, but these stresses for all the fusion models were found to be concentrated in neck of the pedicle screw and middle of the rod under both the static and vibration loadings. The results from static analyses showed that the maximum stress in the BPS fixation system was greater in the TLIF model than in the ALIF and PLIF models under all the applied static loadings. The results from transient dynamic analyses also showed that the TLIF generated greater dynamic responses of the stress in the BPS fixation system to the vertical vibration compared with the ALIF and PLIF. It implies that the TLIF procedure might incur a higher risk of breakage for the BPS fixation system than the ALIF and PLIF procedures.