Comparison of the effects of bilateral posterior dynamic and rigid fixation devices on the loads in the lumbar spine: a finite element analysis

Regional variations of mechanical responses of IVD to 7 different motions: An in vivo study combined with FEA and DFIS

The abnormal mechanical behaviour of a lumbar intervertebral disc (IVD) is commonly recognized as a direct indicator of intervertebral disc degeneration (IDD). However, current methods cannot evaluate the patient-specific mechanical performance of an IVD in vivo during movement. This study establishes a patient-specific (PS) model that combines the kinematics parameters of the lumbar spine obtained with a dual fluoroscopic imaging system (DFIS) and a finite-element (FE) method for the first time to reveal the mechanical behaviours of IVDs in vivo under seven motions. Three healthy participants were recruited for this study. CT images were obtained to create finite-element models of L3-L5 spine segments. Meanwhile, participants were required to take specific positions including upright standing, flexion, extension, left and right lateral bending, as well as left and right axial torsion in the DFIS. The in vivo kinematic parameters, calculated by registering the CT images with images obtained with DFIS, were considered as loading conditions in FE simulations. Significant differences of von Mises stresses and principal strains were found between PS model and GN model which employing a generalized moment as loading conditions, former resulting in up to 76.74 % lower strain than the GN model. Also, considerable differences were observed for five anatomical regions of the IVD (L3-L5). Under all motions, the stress in the centre region (nucleus pulposus) was the lowest, while the stress in the posterior region was the highest in extension motion. Therefore, activities such as stretching with an extension, should be avoided by patients with a herniated disc, in which the posterior region was the herniation site. The PS model combining in vivo kinematics and FE simulations shows the potential in the design and assessment of patient-specific implants.

Biomechanical investigation of a customized interspinous spacer system in the treatment of degenerative disc diseases: A finite element analysis

BACKGROUND

A novel interspinous fixation system based on anatomical parameters and incorporating transfacetopedicular screws, was developed to treat degenerative disc diseases. The biomechanical characteristics of the novel system were evaluated using finite element analysis in comparison to other classical interspinous spacers.

METHODS

The L1-S1 lumbar spine finite element models were surgically implanted with the novel system, Coflex and DIAM devices at the L4/L5 segment to assess the range of motion, the pression distribution of intervertebral disc, the peak stresses on the spinous process and implant during various motions.

FINDINGS

Range of motions of the L4/L5 surgical segment were reduced by 29.13%, 61.27%, 77.35%, 33.33%, and the peak stresses of intervertebral disc were decreased by 36.82%, 67.31%, 73.00%, 69.57% for the novel system in flexion, extension, lateral bending, and axial rotation when compared with the Coflex, and they were declined by 34.53%, 57.86%, 75.81%, 25.21%; 36.22%, 67.31%, 75.01%, 71.40% compared with DIAM. The maximum stresses of the spinous process were 29.93 MPa, 24.66 MPa, 14.45 MPa, 24.37 MPa in the novel system, and those of Coflex and DIAM were 165.3 MPa, 109 MPa, 84.79 MPa, 47.66 MPa and 52.59 MPa, 48.78 MPa, 50.27 MPa, 44.16 MPa during the same condition.

INTERPRETATION

Compared to other interspinous spacer devices, the novel interspinous fixation system demonstrated excellent stability, effectively distributing load on the intervertebral disc, and reducing the risk of spinous process fractures. The personalized design of the novel interspinous fixation system could be a viable option for treating degenerative disc diseases.

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 effects of iatrogenic muscle-ligaments complex damage on adjacent segments following posterior lumbar interbody fusion: A finite element analysis

OBJECTIVE

To analyze the biomechanical effects of proximal iatrogenic muscle-ligaments complex (MLC) damage on adjacent segments following posterior lumbar interbody fusion (PLIF) by finite element (FE) analysis.

METHODS

The multifidus muscle force was loaded in the validated intact lumbosacral finite element model. Based on whether undergoing PLIF or the proximal MLC damage, three models were established. Range of motion (ROM) and the maximum von Mises (VM) stress of adjacent segments were analyzed, as well as the average muscle force and work capacity in four loading directions.

RESULTS

PLIF results in significant changes in ROM and stress. ROM changed significantly in the upper adjacent segment, the PLIF model changed the most in extension, and the largest change in the lower adjacent segment occurred after MLC damage. The VM stress of the upper adjacent segment occurred in extension of the PLIF model, and that of the lower adjacent segment occurred in rotation after MLC damage. In flexion, ROM, and stress of the damaged MLC fusion model were significantly increased compared with the normal and PLIF models, there was a stepwise amplification. The average muscle force comparison of three models was 5.8530, 12.3185, and 13.4670 N, respectively. The total work capacity comparison was close to that of muscle force.

CONCLUSION

PLIF results in increased ROM and the VM stress of adjacent segments, the proximal MLC damage will aggravate this change. This may increase the risk of ASD and chronic low back pain. Preserving the proximal MLC reduces the biomechanical effects on adjacent segments.

Study of tractor side tilt operation on intervertebral disc injury between L4 and L5 in drivers

The tilting of the cab seat when the tractor is in deep ploughing operation changes the sitting position of the driver, which may accelerate lumbar spine injury. This paper adopts the musculoskeletal model and the finite element model of the lumbar L4-L5 segment to predict the maximum Von-Mises stress and maximum strain of the driver's lumbar L4-L5 segment intervertebral disc. In this study, we used 3D motion capture to obtain the driver's spine position spatial data when the tractor tilted at different angles. A tractor-driver musculoskeletal model and a finite element model of the lumbar spine L4-L5 segments were created in AnyBody™ and Abaqus, respectively. The tractor-driver musculoskeletal model was used to calculate the load of the driver's lumbar spine L4-L5 segment at different angles of tractor tilt, which was used as the load condition of the finite element model of the lumbar spine L4-L5 segment, and then the influence of tractor tilt angle and vibration on the driver's lumbar spine L4-L5 disc was studied. The results show that the maximum Von-Mises stress and maximum strain of the driver's lumbar L4-L5 intervertebral disc will increase due to the tilt. The maximum Von-Mises stress occurs in the annulus II, and the maximum strain occurs in the upper end plate of the intervertebral disc. With the occurrence of tilt, the position of the maximum Von-Mises stress changes, which can lead to disc injury to the driver, and vibration may exacerbate this injury.

Stability and Instrumentation Stresses Among Sacropelvic Fixation Techniques With Novel Porous Fusion/Fixation Implants: A Finite Element Study

BACKGROUND

Sacropelvic fixation is frequently combined with thoracolumbar instrumentation for correcting spinal deformities. This study aimed to characterize sacropelvic fixation techniques using novel porous fusion/fixation implants (PFFI).

METHODS

Three T10-pelvis finite element models were created: (1) pedicle screws and rods in T10-S1, PFFI bilaterally in S2 alar-iliac (S2AI) trajectory; (2) fixation in T10-S1, PFFI bilaterally in S2AI trajectory, triangular implants bilaterally above the PFFI in a sacro-alar-iliac trajectory (PFFI-IFSAI); and (3) fixation in T10-S1, PFFI bilaterally in S2AI trajectory, PFFI in sacro-alar-iliac trajectory stacked cephalad to those in S2AI position (2-PFFI). Models were loaded with pure moments of 7.5 Nm in flexion-extension, lateral bending, and axial rotation. Outputs were compared against 2 baseline models: (1) pedicle screws and rods in T10-S1 (PED), and (2) pedicle screws and rods in T10-S1, and S2AI screws.

RESULTS

PFFI and S2AI resulted in similar L5-S1 motion; adding another PFFI per side (2-PFFI) further reduced this motion. Sacroiliac joint (SIJ) motion was also similar between PFFI and S2AI; PFFI-IFSAI and 2-PFFI demonstrated a further reduction in SIJ motion. Additionally, PFFI reduced max stresses on S1 pedicle screws and on implants in the S2AI position.

CONCLUSION

The study shows that supplementing a long construct with PFFI increases the stability of the L5-S1 and SIJ and reduces stresses on the S1 pedicle screws and implants in the S2AI position.

CLINICAL RELEVANCE

The findings suggest a reduced risk of pseudarthrosis at L5-S1 and screw breakage. Clinical studies may be performed to demonstrate applicability to patient outcomes.

LEVEL OF EVIDENCE

Not applicable (basic science study).

Development, calibration and validation of a comprehensive customizable lumbar spine FE model for simulating fusion constructs

Instrumentation alters the biomechanics of the spine, and therefore prediction of all output quantities that have critical influence post-surgically is significant for engineering models to aid in clinical predictions. Geometrical morphological finite element models can bring down the development time and cost of custom intact and instrumented models and thus aids in the better inference of biomechanics of surgical instrumentation on patient-specific diseased spine segments. A comprehensive hexahedral morphological lumbosacral finite element model is developed in this work to predict the range of motions, disc pressures, and facet contact forces of the intact and instrumented spine. Facet contact forces are needed to predict the impact of fusion surgeries on adjacent facet contacts in bending, axial rotation, and extension motions. Extensive validation in major physiological loading regimes of the pure moment, pure compression, and combined loading is undertaken. In vitro, experimental corridor results from six different studies reported in the literature are compared and the generated model had statistically significant comparable values with these studies. Flexion, extension and bending moment rotation curves of all segments of the developed model were favourable and within two separately established experimental corridor windows as well as recent simulation results. Axial torque moment rotation curves were comparable to in vitro results for four out of five lumbar functional units. The facet contact force results also agreed with in vitro experimental results. The current model is also computationally efficient with respect to contemporary models since it uses significantly smaller number of elements without losing the accuracy in terms of response prediction. This model can further be used for predicting the impact of different instrumentation techniques on the lumbar vertebral column.

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 Sacropelvic Hardware on Axis and Center of Rotation of the Sacroiliac Joint: A Finite Element Study

BACKGROUND

The sacroiliac joint (SIJ) transfers the load of the upper body to the lower extremities while allowing a variable physiological movement among individuals. The axis of rotation (AoR) and center of rotation (CoR) of the SIJ can be evaluated to analyze the stability of the SIJ, including when the sacrum is fixed. The purpose of this study was to determine how load intensity affects the SIJ for the intact model and to characterize how sacropelvic fixation performed with different techniques affects this joint.

METHODS

Five T10-pelvis models were used: (1) intact model; (2) pedicle screws and rods in T10-S1; (3)pedicle screws and rods in T10-S1, and bilateral S2 alar-iliac screws (S2AI); (4) pedicle screws and rods in T10-S1, bilateral S2AI screws, and triangular implants inserted bilaterally in a sacral alar-iliac trajectory ; and (5) pedicle screws and rods in T10-S1, bilateral S2AI screws, and 2 bilateral triangular implants inserted in a lateral trajectory. Outputs of these models under flexion-extension were compared: AoR and CoR of the SIJ at incremental steps from 0 to 7.5 Nm for the intact model and AoR and CoR of the SIJ for the instrumented models at 7.5 Nm.

RESULTS

The intact model was validated against an in vivo study by comparing range of motion and displacement of the sacrum. Increasing the load intensity for the intact model led to an increase of the rotation of the sacrum but did not change the CoR. Comparison among the instrumented models showed that sacropelvic fixation techniques reduced the rotation of the sacrum and stabilized the SIJ, in particular with triangular implants.

CONCLUSION

The study outcomes suggest that increasing load intensity increases the rotation of the sacrum but does not influence the CoR, and use of sacropelvic fixation increases the stability of the SIJ, especially when triangular implants are employed.

CLINICAL RELEVANCE

The choice of the instrumentation strategy for sacropelvic fixation affects the stability of the construct in terms of both range of motion and axes of rotation, with direct consequences on the risk of failure and mobilization. Clinical studies should be performed to confirm these biomechanical findings.

Biomechanical comparison of anterior axis-atlanto-occipital transarticular fixation and anterior atlantoaxial transarticular fixation after odontoidectomy: A finite element analysis

Background: Anterior axis-atlanto-occipital transarticular fixation (AAOF) and anterior atlanto-axial transarticular fixation (AAF) are two common anterior screw fixation techniques after odontoidectomy, but the biomechanical discrepancies between them remain unknown. Objectives: To investigate the biomechanical properties of craniovertebral junction (CVJ) after odontoidectomy, with AAOF or AAF. Methods: A validated finite element model of the intact occipital-cervical spine (from occiput to T1) was modified to investigate biomechanical changes, resulting from odontoidectomy, odontoidectomy with AAOF, and odontoidectomy with AAF. Results: After odontoidectomy, the range of motion (ROM) at C1-C2 increased in all loading directions, and the ROM at the Occiput-C1 elevated by 66.2%, 57.5%, and 41.7% in extension, lateral bending, and torsion, respectively. For fixation models, the ROM at the C1-C2 junction was observably reduced after odontoidectomy with AAOF and odontoidectomy with AAF. In addition, at the Occiput-C1, the ROM of odontoidectomy with AAOF model was notably lower than the normal model in extension (94.9%), flexion (97.6%), lateral bending (91.8%), and torsion (96.4%). But compared with the normal model, in the odontoidectomy with AAF model, the ROM of the Occiput-C1 increased by 52.2%, -0.1%, 92.1%, and 34.2% in extension, lateral bending, and torsion, respectively. Moreover, there were no distinctive differences in the stress at the screw-bone interface or the C2-C3 intervertebral disc between the two fixation systems. Conclusion: AAOF can maintain CVJ stability at the Occiput-C1 after odontoidectomy, but AAF cannot. Thus, for patients with pre-existing atlanto-occipital joint instability, AAOF is more suitable than AAF in the choice of anterior fixation techniques.

[Research progress of effect of cage height on outcomes of lumbar interbody fusion surgery]

Objective

To summarize the effect of cage height on outcomes of lumbar interbody fusion surgery and the importance of the cage height selection.

Methods

The related literature was widely reviewed to summarize the research progress on the complications caused by inappropriate height of the cage and the methods of selecting cage height.

Results

Inappropriate height of the cage can lead to endplate injury, cage subsidence, internal fixation failure, adjacent segmental degeneration, over-distraction related pain, insufficient indirect decompression, instability of operation segment, poor interbody fusion, poor sequence of spine, and cage displacement. At present, the selection of the cage height is based on the results of the intraoperative model test, which is reliable but high requirements for surgical experience and hard to standardize.

Conclusion

The inappropriate height of the cage may have an adverse impact on the postoperative outcome of patients. It is important to develop a selection standard of the cage height by screening the related influential factors.

Cervical Lift-up Basket Laminoplasty after Resection of Spinal Intramedullary Tumors: A Finite Element Analysis and Clinical Image Evaluation

Although reconstructive laminoplasty is commonly performed after resection of spinal intramedullary tumors of the cervical spine, its biomechanical rigidity of laminoplasty framework remains unclear. The objective of this study was to examine the structural reliability of our unique method of cervical lift-up basket laminoplasty by using computed tomography (CT)-based finite element analysis (FEA) and clinical radiological evaluation. A finite element model of cervical laminoplasty was created based on CT images using FEA software. Cervical lift-up basket laminoplasty (Basket) was compared with the standard style of open-door basket laminoplasty (Open-door). Clinical subjects for radiological evaluation comprised 33 patients who underwent cervical lift-up basket laminoplasty after resection of spinal intramedullary tumors. An FEA-equivalent stress histogram showed that stress was moderately dispersed around the basket. Virtual displacement of the spinous process of the Basket model was equivalent to that of the Open-door model in any direction of posterior-to-anterior, right-to-left, or top-to-bottom force. In the clinical analysis, radiological data with a minimum postoperative period of 6 months were obtained in a total of 28 out of 33 patients. No patients underwent revision surgery because of implant-related complications. No significant differences in C2-C7 angle or cervical tilt angle were observed between pre- and postoperatively. The structural rigidity of cervical lift-up basket laminoplasty was equivalent to the open-door style on the FEA. Clinical radiological evaluation suggested that there were no serious adverse events associated with cervical laminoplasty, although the longer postoperative follow-up is mandatory.

Biomechanical Comparison between Isobar and Dynamic-Transitional Optima (DTO) Hybrid Lumbar Fixators: A Lumbosacral Finite Element and Intersegmental Motion Analysis

Biomechanical performance of longitudinal component in dynamic hybrid devices was evaluated to display the load-transfer effects of Dynesys cord spacer or Isobar damper-joint dynamic stabilizer on junctional problem based on various disc degenerations. The dynamic component was adapted at the mildly degenerative L3–L4 segment, and the static component was fixed at the moderately degenerative L4–L5 segment under a displacement-controlled mode for the finite element study. Furthermore, an intersegmental motion behavior was analyzed experimentally on the synthetic model under a load-controlled mode. Isobar or DTO hybrid fixator could reduce stress/motion at transition segment, but compensation was affected at the cephalic adjacent segment more than the caudal one. Within the trade-off region (as a motion-preserving balance between the transition and adjacent segments), the stiffness-related problem was reduced mostly in flexion by a flexible Dynesys cord. In contrast, Isobar damper afforded the effect of maximal allowable displacement (more than peak axial stiffness) to reduce stress within the pedicle and at facet joint. Pedicle-screw travel at transition level was related to the extent of disc degeneration in Isobar damper-joint (more than Dynesys cord spacer) attributing to the design effect of axial displacement and angular rotation under motion. In biomechanical characteristics relevant to clinical use, longitudinal cord/damper of dynamic hybrid lumbar fixators should be designed with less interface stress occurring at the screw-vertebral junction and facet joint to decrease pedicle screw loosening/breakage under various disc degenerations.

Novel force-displacement control passive finite element models of the spine to simulate intact and pathological conditions; comparisons with traditional passive and detailed musculoskeletal models

Passive finite element (FE) models of the spine are commonly used to simulate intact and various pre- and postoperative pathological conditions. Being devoid of muscles, these traditional models are driven by simplistic loading scenarios, e.g., a constant moment and compressive follower load (FL) that do not properly mimic the complex in vivo loading condition under muscle exertions. We aim to develop novel passive FE models that are driven by more realistic yet simple loading scenarios, i.e., in vivo vertebral rotations and pathological-condition dependent FLs (estimated based on detailed musculoskeletal finite element (MS-FE) models). In these novel force-displacement control FE models, unlike the traditional passive FE models, FLs vary not only at different spine segments (T12-S1) but between intact, pre- and postoperative conditions. Intact, preoperative degenerated, and postoperative fused conditions at the L4-L5 segment for five static in vivo activities in upright and flexed postures were simulated by the traditional passive FE, novel force-displacement control FE, and gold-standard detailed MS-FE spine models. Our findings indicate that, when compared to the MS-FE models, the force-displacement control passive FE models could accurately predict the magnitude of disc compression force, intradiscal pressure, annulus maximal von Mises stress, and vector sum of all ligament forces at adjacent segments (L3-L4 and L5-S1) but failed to predict disc shear and facet joint forces. In this regard, the force-displacement control passive FE models were much more accurate than the traditional passive FE models. Clinical recommendations made based on traditional passive FE models should, therefore, be interpreted with caution.

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).

A novel approach for tetrahedral-element-based finite element simulations of anisotropic hyperelastic intervertebral disc behavior

Intervertebral discs are microstructurally complex spinal tissues that add greatly to the flexibility and mechanical strength of the human spine. Attempting to provide an adjustable basis for capturing a wide range of mechanical characteristics and to better address known challenges of numerical modeling of the disc, we present a robust finite-element-based model formulation for spinal segments in a hyperelastic framework using tetrahedral elements. We evaluate the model stability and accuracy using numerical simulations, with particular attention to the degenerated intervertebral discs and their likely skewed and narrowed geometry. To this end, 1) annulus fibrosus is modeled as a fiber-reinforced Mooney-Rivlin type solid for numerical analysis. 2) An adaptive state-variable dependent explicit time step is proposed and utilized here as a computationally efficient alternative to theoretical estimates. 3) Tetrahedral-element-based FE models for spinal segments under various loading conditions are evaluated for their use in robust numerical simulations. For flexion, extension, lateral bending, and axial rotation load cases, numerical simulations reveal that a suitable framework based on tetrahedral elements can provide greater stability and flexibility concerning geometrical meshing over commonly employed hexahedral-element-based ones for representation and study of spinal segments in various stages of degeneration.

The Simulation of Muscles Forces Increases the Stresses in Lumbar Fixation Implants with Respect to Pure Moment Loading

Simplified loading conditions such as pure moments are frequently used to compare different instrumentation techniques to treat spine disorders. The purpose of this study was to determine if the use of realistic loading conditions such as muscle forces can alter the stresses in the implants with respect to pure moment loading. A musculoskeletal model and a finite element model sharing the same anatomy were built and validated against in vitro data, and coupled in order to drive the finite element model with muscle forces calculated by the musculoskeletal one for a prescribed motion. Intact conditions as well as a L1-L5 posterior fixation with pedicle screws and rods were simulated in flexion-extension and lateral bending. The hardware stresses calculated with the finite element model with instrumentation under simplified and realistic loading conditions were compared. The ROM under simplified loading conditions showed good agreement with in vitro data. As expected, the ROMs between the two types of loading conditions showed relatively small differences. Realistic loading conditions increased the stresses in the pedicle screws and in the posterior rods with respect to simplified loading conditions; an increase of hardware stresses up to 40 MPa in extension for the posterior rods and 57 MPa in flexion for the pedicle screws were observed with respect to simplified loading conditions. This conclusion can be critical for the literature since it means that previous models which used pure moments may have underestimated the stresses in the implants in flexion-extension and in lateral bending.

Influence of posterior pedicle screw fixation at L4-L5 level on biomechanics of the lumbar spine with and without fusion: a finite element method

Background

Posterior pedicle screw (PS) fixation, a common treatment method for widespread low-back pain problems, has many uncertain aspects including stress concentration levels, effects on adjacent segments, and relationships with physiological motions. A better understanding of how posterior PS fixation affects the biomechanics of the lumbar spine is needed. For this purpose, a finite element (FE) model of a lumbar spine with posterior PS fixation at the L4–L5 segment level was developed by partially removing facet joints (FJs) to imitate an actual surgical procedure. This FE study aimed to investigate the influence of the posterior PS fixation system on the biomechanics of the lumbar spine before and after fusion by determining which physiological motions have the most increase in posterior instrumentation (PI) stresses and FJ loading.

Results

It was determined that posterior PS fixation increased FJ loading by approximately 35% and 23% at the L3–L4 adjacent level with extension and lateral bending motion, respectively. This increase in FJ loading at the adjacent level could point to the possibility that adjacent segment disease has developed or progressed after posterior lumbar interbody fusion. Furthermore, analyses of peak von Mises stresses on PI showed that the maximum PI stresses of 272.1 MPa and 263.7 MPa occurred in lateral bending and flexion motion before fusion, respectively.

Conclusions

The effects of a posterior PS fixation system on the biomechanics of the lumbar spine before and after fusion were investigated for all physiological motions. This model could be used as a fundamental tool for further studies, providing a better understanding of the effects of posterior PS fixation by clearing up uncertain aspects.

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.

A comprehensive approach for the validation of lumbar spine finite element models investigating post-fusion adjacent segment effects

Spinal fusion surgery is usually followed by accelerated degenerative changes in the unfused segments above and below the treated segment(s), i.e., adjacent segment disease (ASD). While a number of risk factors for ASD have been suggested, its exact pathogenesis remains to be identified. Finite element (FE) models are indispensable tools to investigate mechanical effects of fusion surgeries on post-fusion changes in the adjacent segment kinematics and kinetics. Existing modeling studies validate only their intact FE model against in vitro data and subsequently simulate post-fusion in vivo conditions. The present study provides a novel approach for the comprehensive validation of a lumbar (T12-S1) FE model in post-fusion conditions. Sixteen simulated fusion surgeries, performed on cadaveric specimens using various testing and loading conditions, were modeled by this FE model. Predictions for adjacent segment range of motion (RoM) and intradiscal pressure (IDP) were compared with those obtained from the corresponding in vitro tests. Overall, 70% of the predicted adjacent segment RoMs were within the range of in vitro data for both intact and post-fusion conditions. Correlation (r) values between model and in vitro findings for the adjacent segment RoMs were positive and greater than 0.84. Most of the predicted IDPs were, however, out of the narrow range of in vitro IDPs at the adjacent segments but with great positive correlations (r ≥ 0.89). FE modeling studies investigating the effect of fusion surgery on in vivo adjacent segment biomechanics are encouraged to use post-surgery in vitro data to validate their FE model.

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.

A novel coupled musculoskeletal finite element model of the spine - Critical evaluation of trunk models in some tasks

Spine musculoskeletal (MS) models make simplifying assumptions on the intervertebral joint degrees-of-freedom (rotational and/or translational), representation (spherical or beam-like joints), and properties (linear or nonlinear). They also generally neglect the realistic structure of the joints with disc nuclei/annuli, facets, and ligaments. We aim to develop a novel MS model where trunk muscles are incorporated into a detailed finite element (FE) model of the ligamentous T12-S1 spine thus constructing a gold standard coupled MS-FE model. Model predictions are compared under some tasks with those of our earlier spherical joints, beam joints, and hybrid (uncoupled) MS-FE models. The coupled model predicted L4-L5 intradiscal pressures (R² ≅ 0.97, RMSE ≅ 0.27 MPa) and L1-S1 centers of rotation (CoRs) in agreement to in vivo data. Differences in model predictions grew at larger trunk flexion angles; at the peak (80°) flexion the coupled model predicted, compared to the hybrid model, much smaller global/local muscle forces (~38%), segmental (~44%) and disc (~22%) compression forces but larger segmental (~9%) and disc (~17%) shear loads, ligament forces at the lower lumbar levels (by up to 57%) and facet forces at all levels. The spherical/beam joints models predicted much greater muscle forces and segmental loads under larger flexion angles. Unlike the spherical joints model with fixed CoRs, the beam joints model predicted CoRs closer (RMSE = 2.3 mm in flexion tasks) to those of the coupled model. The coupled model offers a great potential for future studies towards improvement of surgical techniques, management of musculoskeletal injuries and subject-specific simulations.

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.

Development and validation of a timely and representative finite element human spine model for biomechanical simulations

Numerous spine Finite Element (FE) models have been developed to assess spinal tolerances, spinal loadings and low back pain-related issues. However, justified simplifications, in terms of tissue decomposition and inclusion, for such a complex system may overlook crucial information. Thus, the purpose of this research was to develop and validate a comprehensive and representative spine FE model inclusive of an accurate representation of all major torso elements. A comprehensive model comprised of 273 tissues was developed via a novel FE meshing method to enhance computational feasibility. A comprehensive set of indirect validation tests were carried out to validate every aspect of the model. Under an increasing angular displacement of 24°-41°, the lumbar spine recorded an increasing moment from 5.5 to 9.3 Nm with an increase in IVD pressures from 0.41 to 0.66 MPa. Under forward flexion, vertical vertebral displacements simulated a 6% and 13% maximum discrepancy for intra-abdominal and intramuscular pressure results, all closely resembling previously documented in silico measured values. The developed state-of-the-art model includes most physiological tissues known to contribute to spinal loadings. Given the simulation's accuracy, confirmed by its validation tests, the developed model may serve as a reliable spinal assessment tool.

Properties of PMMA end cap holders affect FE stiffness predictions of vertebral specimens

Bone cement is often used, in experimental biomechanics, as a potting agent for vertebral bodies (VB). As a consequence, it is usually included in finite element (FE) models to improve accuracy in boundary condition settings. However, bone cement material properties are typically assigned to these models based on literature data obtained from specimens created under conditions which often differ from those employed for cement end caps. These discrepancies can result in solids with different material properties from those reported. Therefore, this study aimed to analyse the effect of assigning different mechanical properties to bone cement in FE vertebral models. A porcine C2 vertebral body was potted in bone cement end caps, μCT scanned, and tested in compression. DIC was performed on the anterior surface of the specimen to monitor the displacement. Specimen stiffness was calculated from the load-displacement output of the materials testing machine and from the machine load output and average displacement measured by DIC. Fifteen bone cement cylinders with dimensions similar to the cement end caps were produced and subjected to the same compression protocol as the vertebral specimen and average stiffness and Young moduli were estimated. Two geometrically identical vertebral body FE models were created from the μCT images, the only difference residing in the values assigned to bone cement material properties: in one model these were obtained from the literature and in the other from the cylindrical cement samples previously tested. The average Youngs modulus of the bone cement cylindrical specimens was 1177 ± 3 MPa, considerably lower than the values reported in the literature. With this value, the FE model predicted a vertebral specimen stiffness 3% lower than that measured experimentally, while when using the value most commonly reported in similar studies, specimen stiffness was overestimated by 150%.

The use of triangular implants to enhance sacropelvic fixation: a finite element investigation

BACKGROUND CONTEXT

Long thoracolumbar fixation and fusion have become a consolidated treatment for severe spinal disorders. Concomitant sacropelvic fixation with S2 alar-iliac (S2AI) screws is frequently performed to limit instrumentation failure and pseudarthrosis at the lumbosacral junction.

PURPOSE

This study explored the use of triangular titanium implants in different configurations in which the implants supplemented standard sacropelvic fixation with S2AI screws in order to further increase the stability of S2AI fixation.

STUDY DESIGN

Finite element study.

METHODS

Four T10-pelvis instrumented models were built: pedicle screws and rods in T10-S1 (PED); pedicle screws and rods in T10-S1, and bilateral S2 alar-iliac screws (S2AI); pedicle screws and rods in T10-S1, bilateral S2AI screws, and triangular implants inserted bilaterally in a sacral alar-iliac trajectory (Tri-SAI); pedicle screws and rods in T10-S1, bilateral S2AI screws and two bilateral triangular titanium implants inserted in a lateral trajectory (Tri-Lat). The models were tested under pure moments of 7.5 Nm in flexion-extension, lateral bending and axial rotation.

RESULTS

SIJ motion was reduced by 50% to 66% after S2AI fixation; the addition of triangular titanium implants in either a SAI or a lateral trajectory further reduced it. S2AI, Tri-SAI, and Tri-Lat resulted in significantly lower stresses in S1 pedicle screws when compared to PED. Triangular implants had a protective effect on the maximal stresses in S2AI screws, especially when placed in the SAI trajectory. Sacropelvic fixation did not have any protective effect on the posterior rods.

CONCLUSIONS

Supplementing S2AI screws with triangular implants had a protective effect on the S2AI screws themselves, as well as the S1 pedicle screws, in the tested model.

CLINICAL SIGNIFICANCE

Triangular implants can substantially reduce the residual flexibility of the SIJ with respect to S2AI fixation alone, suggesting a possible role in patients needing reinforced fixation. In vivo investigation is needed to determine if these in vitro effects translate into clinically important differences.

Material property calibration is more important than element size and number of different materials on the finite element modelling of vertebral bodies: A Taguchi study

Finite element (FE) modelling of a vertebral body (VB) is considered challenging due to the many parameters involved such as element size and type, and material properties. Previous studies have reported how these parameters affect the mechanical behaviour of a VB model; however, most studies just compared results without any specific statistical tool to quantify their influence. The Taguchi Method (TM) has been successfully used in manufacturing and biomechanics to evaluate process parameters and to determine optimum set-up conditions. This study aimed to evaluate the influence of the main finite element modelling parameters on the mechanical behaviour of a VB model using the Taguchi Method. A FE model was developed based on a C2 juvenile porcine vertebral body and three of the most commonly used modelling parameters were evaluated using TM in terms of the change in the predicted stiffness in comparison to experimental values: element size, number of different material properties for VB (based on grey-scale bins) and calibration factor for grey-scale to density to Young's Modulus equation. The influence of the combined factors was also assessed. The Taguchi analysis showed that the three factors are independent. The calibration factor is the main contributor, accounting for 97% of the predicted stiffness, with the value of 0.03 most closely aligning the numerical and experimental results. Element size accounted for 2% of the predicted stiffness, with 0.75 mm being the optimal, while the number of grey-scale bins influenced the results by less than 1%. Our findings indicate that the calibration factor is the main modelling parameter, with the element size and number of bins accounting for less than 3% of the predicted stiffness. Therefore, calibration of material properties should be done based on a large number of samples to ensure reliable results.

Biomechanics of spinal implants-a review

Spinal instrumentations have been classified as rigid fixation, total disc replacement and dynamic stabilization system for treatment of various spinal disorders. The efficacy and biomechanical suitability of any spinal implant can be measured through in vitro, in vivo experiments and numerical techniques. With the advancement in technology finite element models are making an important contribution to understand the complex structure of spinal components along with allied functionality, designing and application of spinal instrumentations at preliminary design stage. This paper aimed to review the past and recent studies to describe the biomechanical aspects of various spinal implants. The literatures were grouped and reviewed in accordance to instrumentation category and their functionality in the spinal column at respective locations.

Quantitative T2 Mapping Shows Increased Degeneration in Adjacent Intervertebral Discs Following Kyphoplasty

OBJECTIVE

A minimally invasive treatment of osteoporotic and nonosteoporotic thoracic and lumbar spine fractures is cement augmentation (kyphoplasty). Little is known about the impact on adjacent intervertebral discs. A quantitative magnetic resonance imaging (MRI) approach in addition to morphological MRI is desirable to evaluate changes in the intervertebral disc. Our study aims to evaluate the feasibility of T2 mapping for the detection of subtle changes in the intervertebral discs in spines after kyphoplasty.

DESIGN

Intervertebral discs were assessed by quantitative MRI (3.0 T) using T2 relaxation time mapping. Region of interest (ROI; 6 per disc) analyses were performed. The ROIs at the anterior and posterior edges were interpreted as annulus fibrosus (AF). The 2 very inner zones were regarded as nucleus pulposus (NP) and the regions in between as intermediate transition zone. We compared T2 relaxation time values of intervertebral discs adjacent to the vertebrae after kyphoplasty with those nonadjacent to vertebrae after kyphoplasty, especially in the NP.

RESULTS

The analysis of the ROIs showed that the intervertebral discs of the adjacent vertebral segments are associated with reduced T2 values compared to those that are nonadjacent to the affected vertebrae.

CONCLUSION

This study is to our knowledge the first investigation of intervertebral discs after kyphoplasty by quantitative MRI. Quantitative T2 mapping shows increased degeneration in adjacent intervertebral discs following kyphoplasty. Besides its contribution to a broader knowledge of postoperative changes after kyphoplasty, our findings may help to improve differentiation between healthy and degenerated intervertebral discs using these techniques.

Posterior bone graft in lumbar spine surgery reduces the stress in the screw-rod system- A finite element study

PURPOSE

Analyze the biomechanical effect of postero-lateral instrumentation with and without posterior bone graft as well as effect of consolidation of the graft. Study objectives are (1) whether bone graft alone will provide enough additional strength to the weakened spine, (2) how the addition of posterior bone graft help in extending the life of the fusion construct, and (3) compare the effect of gradual consolidation of the bone-graft on the spine biomechanics.

METHODS

A lumbar spine finite element model was used to analyze the effects of bone-graft alone and varying grades of bone-graft consolidation with postero-lateral instrumentation on spine biomechanics. The spine stiffness and stresses in the posterior rods and screws were determined for moments applied in the three physiological directions in addition to pre-load.

RESULTS

Stiffness of a normal lumbar spine with a solid consolidated posterior bone graft was found to be 10 times that of an intact lumbar spine. Posterior instrumentation further increased the spine stiffness by 20 fold. A 50% solid consolidation of the graft reduced the screw-rod maximum von-Mises stress by 45% and a 65% reduction in screw-rod stress was calculated with completely fused graft.

CONCLUSION

A fused graft with posterior instrumentation provided a 200 fold increase in stiffness of an intact spine while producing stress shielding to the Ti rod-screw system. Considerable reduction of the maximum von-Mises stresses in the postero-lateral rod and screw fusion system (65%) will contribute to prevention of implant failure under repetitive loading highlighting the importance of consolidation of posterior bone-graft.

Cement augmentation of odontoid peg fractures: the effect of cement volume and distribution on construct stiffness

PURPOSE

The cement augmentation of a conventional anterior screw fixation in type II odontoid process fractures for elderly patients significantly increased stiffness and load to failure under anterior-posterior load in comparison with non-augmented fixation. The amount and quality of bone cement are usually taken ad hoc in clinical practise. In this study, we wanted to clarify the role of bone cement amount and its quality to the stiffness of odontoid and vertebrae body junction.

METHODS

Finite-element method was used to achieve different scenarios of cement augmentation. For all models, an initial stiffness was calculated. Model (1) the intact vertebrae were virtually potted into a polymethylmethacrylate base via the posterior vertebral arches. A V-shaped punch was used for loading the odontoid in an anterior-posterior direction. (2) The odontoid fracture type IIa (Anderson-D'Alonzo classification) was achieved by virtual transverse osteotomy. Anterior screw fixation was virtually performed by putting self-drilling titanium alloy 3.5 mm diameter anterior cannulated lag screw with a 12 mm thread into the inspected vertebrae. A V-shaped punch was used for loading the odontoid in an anterior-posterior direction. The vertebrae body was assumed to be non-cemented and cemented with different volume.

RESULTS

The mean cement volume was lowest for body base filling with 0.47 ± 0.03 ml. The standard body filling corresponds to 0.95 ± 0.15 ml. The largest volume corresponds to 1.62 ± 0.12 ml in the presence of cement leakage. The initial stiffness of the intact C2 vertebrae was taken as the reference value. The mean initial stiffness for non-porous cement (E = 3000 MPa) increased linearly (R2 = 0.98). The lowest stiffness (123.3 ± 5.8 N/mm) was measured in the intact C2 vertebrae. However, the highest stiffness (165.2 ± 5.2 N/mm) was measured when cement leakage out of the odontoid peg occurred. The mean initial stiffness of the base-only cemented group was 147.2 ± 8.4 N/mm compared with 157.9 ± 6.6 N/mm for the base and body cemented group. This difference was statistically significant (p < 0.0061). The mean initial stiffness for porous cement (E = 500 MPa) remains constant. Therefore, there is no difference between cemented and non-cemented junction. This difference was not statistically significant (p < 0.18).

CONCLUSION

The present study showed that the low porous cement was able to significantly influence the stiffness of the augmented odontoid screw fixation in vitro, although further in vivo clinical studies should be undertaken. Our results suggest that only a small amount of non-porous cement is needed to restore stiffness at least to its pre-fracture level and this can be achieved with the injection of 0.7-1.2 ml of cement. These slides can be retrieved under Electronic Supplementary Material.

Biomechanics of sacropelvic fixation: a comprehensive finite element comparison of three techniques

PURPOSE

Sacropelvic fixation is frequently used in combination with thoracolumbar instrumentation for complex deformity correction and is commonly associated with pseudoarthrosis, implant failure and loosening. This study compared pedicle screw fixation (PED) with three different sacropelvic fixation techniques, namely iliac screws (IL), S2 alar-iliac screws (S2AI) and laterally placed triangular titanium implants (SI), all in combination with lumbosacral instrumentation, accounting for implant micromotion.

METHODS

Existing finite element models of pelvis-L5 of three patients including lumbopelvic instrumentation were utilized. Moments of 7.5 Nm in the three directions combined with a 500 N compressive load were simulated. Measured metrics included flexibility, instrumentation stresses and bone-implant interface loads.

RESULTS

Fixation effectively reduced the sacroiliac flexibility. Compared to PED, IL and S2AI induced a reduction in peak stresses in the S1 pedicle screws. Rod stresses were mostly unaffected by S2AI and SI, but IL demonstrated a stress increase. In comparison with a previous work depicting full osteointegration, SI was found to have similar instrumentation stresses as those due to PED.

CONCLUSIONS

Fixation with triangular implants did not result in stress increase on the lumbosacral instrumentation, likely due to the lack of connection with the posterior rods. IL and S2AI had a mild protective effect on S1 pedicle screws in terms of stresses and bone-implant loads. IL resulted in an increase in the rod stresses. A comparison between this study and previous work incorporating full osteointegration demonstrates how these results may be applied clinically to better understand the effects of different treatments on patient outcomes. These slides can be retrieved under Electronic Supplementary Material.

Differential response to vibration of three forms of scoliosis during axial cyclic loading: a finite element study

Background

Scoliosis is a serious disease that can affect all segments of society. Few studies have investigated the response to vibration of differing sinusoidal axial cyclic loading frequencies for different forms of scoliosis in the lumbar spine.

Methods

In this study, four finite element models, comprising a healthy spine, Lenke-A, Lenke-B and Lenke-C scoliosis of the lumbar S1-L1 region were developed. Modal analysis extracted resonant frequencies of the FE models with an upper body mass of 40 kg and 400 N preload. A transient dynamic analysis was performed to obtain the response to vibration of models under a sinusoidal axial loading of ± 40N at frequencies of 3, 5, 7, 9, 11 and 13 Hz using an upper body mass of 40 kg and 400 N preload.

Results

The first-order resonant frequencies of healthy, Lenke-A, Lenke-B and Lenke-C spines were 9.2, 3.9, 4.6 and 5.7 Hz, respectively. A Lenke-A lumbar spine was more likely to deform at a lower vibration frequency and Lenke-C deformed more easily at a higher vibration frequency. Furthermore, the vibration amplitude in the Y-direction (left-right) was greatest and least in the Z-direction (top-bottom). The frequency of cyclic loading closest to the resonant frequency resulted in a maximum value of peak-to-peak vibrational displacement. Furthermore, the vibrational amplitudes in patients with scoliosis were larger than they were in healthy subjects. In addition, axial displacement of the vertebrae in the healthy spine changed steadily whereas fluctuations in the scoliotic vertebrae in scoliosis patients were greater than that of other vertebrae.

Conclusions

Different forms of scoliosis may have different vibrational characteristics, the scoliotic vertebrae being the weak link in scoliosis under loading condition of whole body vibration. Scoliosis was more sensitive to this form of vibration. Where the frequency of axial cyclic vibrational loading of the lumbar spine was closer to its resonant frequency, the vibrational amplitude was larger. These results suggest that vibration will exacerbate the degree of scoliosis and so such patients should reduce their exposure to vibration. Clinical treatment should pay attention to the scoliotic vertebrae and reduce their vibration. These findings may assist in the clinical prevention and treatment of scoliosis.

Finite Element Based-Analysis for Pre and Post Lumbar Fusion of Adult Degenerative Scoliosis Patients

STUDY DESIGN

Pre-post cohort finite elements (FE).

OBJECTIVES

To investigate the effect of adjacent load transfer pre and post fusion surgery of lumbar scoliotic spines using FE models.

SUMMARY OF BACKGROUND DATA

Adult degenerative scoliosis (ADS) results from age-related changes, leading to segmental instability, deformity, and stenosis. FE study is capable of capturing the biomechanical parameters internal to the bones and connective soft tissues of the spine, which is difficult to measure by experimental approaches. Literature that describes the underlying mechanisms responsible for spinal fusion in scoliosis patients is limited, and FE study with larger subject sample size should be conducted.

METHODS

Twenty three-dimensional nonlinear FE models of the lumbosacral spine were created from pre (Cobb angle: 28.1° ± 10.5°) and post scoliosis surgery in vivo CT scans. During surgery, pedicle screws and rods were implanted at lumbar and sacral levels. A compressive load and six different moments (flexion, extension, right lateral bending, left lateral bending, right axial rotation, left axial rotation) were applied to the top level of each model. Outcome measures were range of motion (RoM), intradiscal pressure (IDP), and facet joint forces (FJF). Spinal fusion did alter the mechanical function of the scoliotic spine.

RESULTS

Scoliotic spine presented abnormal and asymmetrical kinetic and kinematic behavior. RoM: At the adjacent level, spinal fusion surgery produced a statically significantly increased left and right later bending intersegmental rotation (p < .006) in comparison to presurgical scoliosis models. At the fused level, spinal fusion surgery produced a statically significantly reduced intersegmental rotation in all the loading conditions (p = .001) in comparison to presurgical scoliosis models. IDP: At the fused level, spinal fusion surgery produced a much lower IDP in all of the loading conditions (p = .001). FJF: At the adjacent level, spinal fusion surgery produced a considerably larger left lateral rotation FJF (p = .001) in comparison to presurgical scoliosis models. At the fused level, spinal fusion surgery produced considerably lower FJF in all the loading conditions (p = .001) in comparison to presurgical scoliosis models.

CONCLUSIONS

This study was the first to investigate the effect of adjacent load transfer before and after fusion surgery using in vivo CT scans of 10 scoliotic spines. A posterior fusion has only a minor effect on mechanical behavior and a large effect on pressure and forces at the adjacent level. As expected, a large effect in the kinematics and kinetics was found at the fused level.

LEVEL OF EVIDENCE

Level 3.

Adjacent Intervertebral Disk Height Decrease Phenomenon After Single-Level Transforaminal Lumbar Interbody Fusion of the Lumbar Spine

BACKGROUND

The increase of intradiskal pressure on the upper segment resulting from intervertebral distraction after lumbar intervertebral fusion decreases intervertebral height and aggravates degeneration. However, the incidence rate and risk factors of the adjacent intervertebral disk height decrease phenomenon have not been studied. The purpose of this study was to identify the incidence rate and risk factors of the adjacent intervertebral disk height decrease phenomenon after single-level transforaminal lumbar interbody fusion (TLIF) of the lumbar spine.

METHODS

A retrospection of 68 patients who underwent L4-5 TLIF. Patient age, sex, and body mass index were collected. Lumbar lordosis, facet sagittalization, Pfirrmann classification, L4-5 distraction height, and L3-4 reduction height were evaluated by radiologic image. The patients were divided into 2 groups based on whether their L3-4 intervertebral height decreased.

RESULTS

Forty of 68 patients (58.8%) had L3-4 intervertebral height decrease. The patients' mean age was 62.05 ± 10.90 years in the L3-4 intervertebral height decrease positive (IHDP) group, significantly higher than the 56.14 ± 12.06 years in the L3-4 intervertebral height decrease negative (IHDN) group (P = 0.039). The mean facet sagittalization angle in the IHDP group was 67.5° ± 20.36°, significantly larger than the 55.43° ± 14.97° in the IHDN group (P = 0.010). The preoperative lumbar lordosis was significantly higher in the IHDP group (P = 0.049). No significant effects of other factors on L3-4 height decrease were observed (P > 0.05).

CONCLUSIONS

Distraction of the L4-5 intervertebral space by cage insertion leads to a reduced height on the adjacent L3-4 segment in some patients. In addition, the decrease in L3-4 intervertebral height resulting from L4-5 distraction was correlated with age, preoperative lumbar lordosis, and facet joint sagittalization.

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.

Clinical experiences with a PEEK-based dynamic instrumentation device in lumbar spinal surgery: 2 years and no more

Background

Dynamic spine implants were developed to prevent adjacent segment degeneration (ASD) and adjacent segment disease (ASDi). Purpose of this study was to investigate the clinical and radiological outcomes of “topping off” devices following lumbar spinal fusion procedure using a PEEK-based dynamic rod system. Moreover, this study focused on the hypothesis that “topping off” devices can prevent ASD.

Methods

This prospective nonrandomized study included patients with indication for single-level lumbar fusion and radiological signs of ASD without instability. The exclusion criteria were previous lumbar spine surgery and no sign of disc degeneration in the adjacent segment according to magnetic resonance imaging. All patients were treated with single-level lumbar interbody fusion and dynamic stabilization of the cranial adjacent segment. Patients underwent a clinical examination and radiographs preoperatively and at 1 and 2 years after surgery. Analyses were performed on clinical data collected with the German Spine Registry using the core outcome measure index (COMI) and visual analogue scale (VAS) scores for back and leg pain.

Results

A total of 22 patients (6 male and 16 female) with an average age of 57.6 years were included in the study; 20 patients completed the follow-up (FU). The average COMI score was 9.0 preoperatively, 4.2 at the 1-year FU, and 4.7 at the 2-year FU. The average preoperative VAS scores for back and leg pain were 7.7 and 7.1, respectively. At the 1-year FU, the scores were 4.25 for back pain and 2.2 for leg pain, and at the 2-year FU, the scores were 4.7 for back pain and 2.3 for leg pain. At FU, failure of the dynamic topping off implant material was verified in four cases, and ASD of the segment cranial to the topping off was confirmed in three cases.

Conclusions

These results demonstrate significant improvements in clinical outcomes and pain reduction after lumbar spinal fusion with topping off at 2 years after surgery. However, the implant failed due to the high rate of implant failure and the development of ASD in the segment cranial to the dynamic stabilized segment.

FEBio finite element models of the human lumbar spine

Finite element analysis has proven to be a viable method for assessing many structure-function relationships in the human lumbar spine. Several validated models of the spine have been published, but they typically rely on commercial packages and are difficult to share between labs. The goal of this study is to present the development of the first open-access models of the human lumbar spine in FEBio. This modeling framework currently targets three deficient areas in the field of lumbar spine modeling: 1) open-access models, 2) accessibility for multiple meshing schemes, and 3) options to include advanced hyperelastic and biphasic constitutive models.

Biomechanical Analysis of a Long-Segment Fusion in a Lumbar Spine—A Finite Element Model Study

Examine the biomechanical effect of material properties, geometric variables, and anchoring arrangements in a segmental pedicle screw with connecting rods spanning the entire lumbar spine using finite element models (FEMs). The objectives of this study are (1) to understand how different variables associated with posterior instrumentation affect the lumbar spine kinematics and stresses in instrumentation, (2) to compare the multidirectional stability of the spinal instrumentation, and (3) to determine how these variables contribute to the rigidity of the long-segment fusion in a lumbar spine. A lumbar spine FEM was used to analyze the biomechanical effects of different materials used for spinal rods (TNTZ or Ti or CoCr), varying diameters of the screws and rods (5 mm and 6 mm), and different fixation techniques (multilevel or intermittent). The results based on the range of motion and stress distribution in the rods and screws revealed that differences in properties and variations in geometry of the screw-rod moderately affect the biomechanics of the spine. Further, the spinal screw-rod system was least stable under the lateral bending mode. Stress analyzes of the screws and rods revealed that the caudal section of the posterior spinal instrumentation was more susceptible to high stresses and hence possible failure. Although CoCr screws and rods provided the greatest spinal stabilization, these constructs were susceptible to fatigue failure. The findings of the present study suggest that a posterior instrumentation system with a 5-mm screw-rod diameter made of Ti or TNTZ is advantageous over CoCr instrumentation system.

Changes of Facet Joints After Dynamic Stabilization: Continuous Degeneration or Slow Fusion?

BACKGROUND

The nonfusion pedicle-screw system Dynesys stabilization (DS) for lumbar degenerative disease aims to better preserve range of motion (ROM) than fixation and fusion systems. However, decreased ROM and unexpected facet fusion at the index level were observed after DS was applied with unknown etiology. The aim of this study is using radiologic parameters to explain the phenomenon of facet arthrodesis.

METHODS

The patients who underwent surgery for L4-5 spinal stenosis were sorted retrospectively into 2 groups: DS and microdiskectomy (MicD). Radiologic parameters including facet degeneration, evaluated by computed tomography or magnetic resonance image, and ROM, evaluated by dynamic radiographs, were compared perioperatively. A linear regression model was fitted to data points to calculate correlation over time. Postoperative facet arthrodesis at the index level was detected by computed tomography. Functional outcomes were also compared between groups.

RESULTS

A total of 61 patients (DS-to-MicD = 38:23) were followed 36.9 ± 16.8 months postoperatively. After surgery, both groups of patients had significant clinical improvement without difference between the 2 groups (all P > 0.05). In the DS group, significantly decreased ROM was observed after 24-month follow-up (P < 0.05). The correlation coefficient of facet degeneration over time and the facet fusion rate in the DS group were both significantly higher than in the MicD group (both P < 0.05).

CONCLUSIONS

The patients who underwent DS for L4-5 grade 1 spondylolisthesis experienced significantly reduced ROM and a positive correlation of facet degeneration over time postoperatively. The limited ROM at the index level could be a potential risk of facet degeneration and cause unexpected arthrodesis.

Finite element simulation and clinical follow-up of lumbar spine biomechanics with dynamic fixations

Arthrodesis is a recommended treatment in advanced stages of degenerative disc disease. Despite dynamic fixations were designed to prevent abnormal motions with better physiological load transmission, improving lumbar pain and reducing stress on adjacent segments, contradictory results have been obtained. This study was designed to compare differences in the biomechanical behaviour between the healthy lumbar spine and the spine with DYNESYS and DIAM fixation, respectively, at L4-L5 level. Behaviour under flexion, extension, lateral bending and axial rotation are compared using healthy lumbar spine as reference. Three 3D finite element models of lumbar spine (healthy, DYNESYS and DIAM implemented, respectively) were developed, together a clinical follow-up of 58 patients operated on for degenerative disc disease. DYNESYS produced higher variations of motion with a maximum value for lateral bending, decreasing intradiscal pressure and facet joint forces at instrumented level, whereas screw insertion zones concentrated stress. DIAM increased movement during flexion, decreased it in another three movements, and produced stress concentration at the apophyses at instrumented level. Dynamic systems, used as single systems without vertebral fusion, could be a good alternative to degenerative disc disease for grade II and grade III of Pfirrmann.

The Effects of Physiological Biomechanical Loading on Intradiscal Pressure and Annulus Stress in Lumbar Spine: A Finite Element Analysis

The present study was conducted to examine the effects of body weight on intradiscal pressure (IDP) and annulus stress of intervertebral discs at lumbar spine. Three-dimensional finite element model of osseoligamentous lumbar spine was developed subjected to follower load of 500 N, 800 N, and 1200 N which represent the loads for individuals who are normal and overweight with the pure moments at 7.5 Nm in flexion and extension motions. It was observed that the maximum IDP was 1.26 MPa at L1-L2 vertebral segment. However, the highest increment of IDP was found at L4-L5 segment where the IDP was increased to 30% in flexion and it was more severe at extension motion reaching to 80%. Furthermore, the maximum annulus stress also occurred at the L1-L2 segment with 3.9 MPa in extension motion. However, the highest increment was also found at L4-L5 where the annulus stress increased to 17% in extension motion. Based on these results, the increase of physiological loading could be an important factor to the increment of intradiscal pressure and annulus fibrosis stress at all intervertebral discs at the lumbar spine which may lead to early intervertebral disc damage.

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.

Finite element method-based study for effect of adult degenerative scoliosis on the spinal vibration characteristics

Finite element analysis was used to investigate the responses of five healthy subjects and five adult degenerative scoliosis (ADS) subjects to cyclic vibration. The dynamic responses of the healthy and scoliotic spines to the sinusoidal cyclic vibrations have been investigated in previous studies by simulation or experimental approaches. However, no simulation or experimental results were available for the ADS subjects. The effect of the ADS on the vibrational characteristics of spines remained unknown. The objective of this study was to compare differences of the dynamic responses to the cyclic vibration input between the healthy subjects and subjects with ADS. Based on the simulations results in this study, the scoliotic spines are more sensitive to the cyclic vibrations than the healthy spines. More resonant frequencies were predicted in the scoliotic spines than the healthy spines. The scoliotic deformity in the spine was to make the vibrational response of the spine significantly more complex at the apical scoliotic region. This study suggested that ADS could severely increase spinal response to the cyclic vibrations, which could potentially lead to further scoliotic deformity in the spine.