Biomechanical comparison of three stand-alone lumbar cages--a three-dimensional finite element analysis

The relative influence of model parameters on finite element analysis simulations of intervertebral body fusion device static compression performance

Non-clinical mechanical performance testing is a critical aspect of intervertebral body fusion device (IBFD) development and regulatory evaluation. Recently, stakeholders have begun leveraging computational modeling and simulations such as finite element analysis (FEA) in addition to traditional bench testing. FEA offers advantages such as reduced experiment time, lower costs associated with elimination of bench testing (e.g. specimen manufacture and test execution), and elucidating quantities of interest that traditional testing cannot provide (e.g. stress and strain distributions). However, best practices for FEA of IBFDs are not well defined, and modeler decision making can significantly influence simulation setup and results. Therefore, the goal of this study was to determine the relative influence of modeling parameters when using FEA to assess non-clinical mechanical performance of IBFDs. FEA was used to conduct a series of IBFD static uniaxial compression simulations. Several parameters relating to implant geometry, loading/boundary conditions, and material properties were carefully controlled to assess their relative influence on two output variables (IBFD stiffness and yield load). Results were most influenced by device geometry, while the effects of boundary conditions and material properties were more significant within IBFDs of identical or similar geometries. These results will aid stakeholders in the development of standardized best practices for using FEA to assess non-clinical mechanical performance of IBFDs.

Optimization of cervical cage and analysis of its base material: A finite element study

Nowadays, cervical disorders are common due to human lifestyles. Accordingly, the cage design should be optimized as an essential issue. For an optimal design, an objective function is utilized to calculate the proper geometrical parameters. Additionally, the base material of the cage plays a key role in its functionality and final cost. Novel materials are currently introduced with more compatibility with the bone in terms of mechanical and chemical properties. In this study, a cervical cage was modeled based on PEEK material with three types of tooth designs on its surface. The cervical cage is assumed to be implanted between C6 and C7 vertebrae. The geometric parameters of the cage were optimized to minimize the mass by determining allowable stress and subsidence. The effect of complete cortical removal was investigated as a surgical mistake. Finally, a new composition of PEEK/titanium was introduced as the base material of the cage. Ansys 18.2 was used for FEA. The cage with a straight tooth was chosen due to its lower stress and subsidence compared with other designs. Furthermore, the optimized structures of all three tooth designs were determined. The mass and volume of the optimal cages were reduced by 41.47% and 41.52% respectively. Besides, complete cortical resection should not be carried out during fusion surgery, since it may lead to higher subsidence. The composition of PEEK/titanium was chosen as an appropriate base material due to its better performance compared with PEEK or titanium alone.

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.

Predictors of inpatient admission in the setting of anterior lumbar interbody fusion: a Minimally Invasive Spine Study Group (MISSG) investigation

OBJECTIVE

While the anterior lumbar interbody fusion (ALIF) procedure may be amenable to ambulatory surgery, it has been hypothesized that limitations such as the risk of postoperative ileus and vascular complications have hindered transition of this procedure to the outpatient setting. Identification of independent risk factors predisposing patients to inpatient stays of ≥ 24 hours after ALIF may facilitate better postsurgical outcomes, target modifiable risk factors, and assist in the development of screening tools to transition appropriate patients to the ambulatory surgery center (ASC) setting for this procedure. The purpose of this study was to identify the most relevant risk factors that predispose patients to ≥ 24-hour admission following ALIF.

METHODS

A prospectively maintained surgical registry was reviewed for patients undergoing single ALIF between May 2006 and December 2019. Demographics, preoperative diagnosis, perioperative variables, and postoperative complications were evaluated according to their relative risk (RR) elevation for an inpatient stay of ≥ 24 hours. A Poisson regression model was used to evaluate predictors of inpatient stays of ≥ 24 hours. Risk factors for inpatient admission of ≥ 24 hours were identified with a stepwise backward regression model.

RESULTS

A total of 111 patients underwent single-level ALIF (50.9% female and 52.6% male, ≤ 50 years old). Eleven (9.5%) patients were discharged in < 24 hours and 116 remained admitted for ≥ 24 hours. The average inpatient stay was > 2 days (53.7 hours). The most common postoperative complications were fever (body temperature ≥ 100.4°F; n = 4, 3.5%) and blood transfusions (n = 4, 3.5%). Bivariate analysis revealed a preoperative diagnosis of retrolisthesis or lateral listhesis to elevate the RR for an inpatient stay of ≥ 24 hours (RR 1.11, p = 0.001, both diagnoses). Stepwise multivariate analysis demonstrated significant predictors for inpatient stays of ≥ 24 hours to be an operation on L4-5, coexisting degenerative disc disease (DDD) with foraminal stenosis, and herniated nucleus pulposus (RR 1.11, 95% CI 1.03-1.20, p = 0.009, all covariates).

CONCLUSIONS

This study provides data regarding the incidence of demographic and perioperative characteristics and postoperative complications as they pertain to patients undergoing single-level ALIF. This preliminary investigation identified the most relevant risk factors to be considered before appropriately transitioning ALIF procedures to the ASC. Further studies of preoperative characteristics are needed to elucidate ideal ASC ALIF patients.

Biomechanical effects of direction-changeable cage positions on lumbar spine: a finite element study

This finite element (FE) study of lumbar biomechanics aims to predict how the parameters like range of motion (ROM), intervertebral disc pressure (IDP), cage stress and screw stress are affected by different direction-changeable cage positions. Firstly, the three-dimensional FE model of L3-L5 segment was developed, and the model was adjusted to adapt different direction-changeable cage positions at the L4-L5 level though transforaminal lumbar interbody fusion (TLIF) with pedicle screws. The effects of Type A (the lateral region), Type B (the lateralcentral region) and Type C (the anteriocentral region) on ROM, IDP, cage stress and screw stress were examined. The results showed that after implantation of interbody cages at different positions, the ROM at surgical level L4-L5 decreased substantially in all motion modes. The maximal stress in cage decreased with Type A, B and C in all motion modes except flexion and extension. The maximal cage stress was observed in Type A with 720.5 MPa in left rotation, in Type B with 707 MPa in flexion, in Type C with 397.3 MPa in left rotation, respectively. The maximal IDP was similar in three types, with 1.6 MPa in left lateral bending in Type A, 1.5 MPa in flexion in Type B, and 1.4 MPa in flexion in Type C. The range of screw peak stress was 16.4 to 61.1 MPa in Type A, 15.9 to 50.9 MPa in Type B, and 14.6 to 46.1 MPa Type C. In conclusion, comparing the cages with different positions, anteriocentral position cage has more advantages like lower cage stress, ODL and screw stress.

Is there any advantage of using stand-alone cages? A numerical approach

Background

Segment fusion using interbody cages supplemented with pedicle screw fixation is the most common surgery for the treatment of low back pain. However, there is still much controversy regarding the use of cages in a stand-alone fashion. The goal of this work is to numerically compare the influence that each surgery has on lumbar biomechanics.

Methods

A non-linear FE model of the whole lumbar spine was developed to compare between two types of cages (OLYS and NEOLIF) with and without supplementary fixation. The motion of the whole spine was analysed and the biomechanical environment of the adjacent segments to the operated one was studied. Moreover, the risk of subsidence of the cages was qualitatively evaluated.

Results

A great ROM reduction occurred when supplementary fixation was used. This stiffening increased the stresses at the adjacent levels. It might be hypothesised that the overloading of these segments could be related with the clinically observed adjacent disc degeneration. Meanwhile, the stand-alone cages allowed for a wider movement, and therefore, the influence of the surgery on adjacent discs was much lower. Regarding the risk of subsidence, the contact pressure magnitude was similar for both intervertebral cage designs and near the value of the maximum tolerable pressure of the endplates.

Conclusions

A minimally invasive posterior insertion of an intervertebral cage (OLYS or NEOLIF) was compared using a stand-alone design or adding supplementary fixation. The outcomes of these two techniques were compared, and although stand-alone cage may diminish the risk of disease progression to the adjacent discs, the spinal movement in this case could compromise the vertebral fusion and might present a higher risk of cage subsidence.

Electronic supplementary material

The online version of this article (10.1186/s12938-019-0684-8) contains supplementary material, which is available to authorized users.

Anterior lumbar interbody fusion (ALIF): biometrical results and own experiences

Lumbar fusion is a mainstay in the treatment of low back pain resulting from degenerative disc disease. Anterior lumbar interbody fusion (ALIF) has become a reasonable treatment technique to achieve indirect foraminal decompression with high fusion rates. The aim of the study was to analyse the biometrical parameters of the lumbar spine and the clinical outcome. The medical records of 51 patients treated with ALIF between 2012 and 2016 were retrospectively reviewed. Anterior and posterior disc height (DH), lumbar lordosis (LL), local disc angle (LDA) and foraminal dimensions were obtained on pre- and postoperative plain radiographs and computed tomography scans using ImageJ and Surgimap software according to the pedicle–pedicle technique. To evaluate the interbody fusion status on the last follow-up CT scans, we used Bridwell criteria. Preoperative and 12 months postoperative Oswestry Disability Index (ODI) scores were determined for all patients. The average length of hospitalisation was 4 days. Most of the patients had degenerative disc disease with foraminal stenosis. Five patients had early complications like paresthesia of lower limbs, sympathetic dysfunction or wound infections, but there were no major complications. Statistically significant (P < .01) improvement was observed in foraminal dimensions (area = 49%, height = 33% and width = 19%), anterior DH (49%), posterior DH (69%), LDA (47%) and LL (17.5%). Posterior DH correlated significantly with foramen height improvement. Radiographic evidence of fusion according to the modified Bridwell criteria (grade I and grade II) was observed in 96% (49/51) of the patients in the last CT of the lumbar spine. We also observed significant improvement in functional recovery in 94% of patients. The mini-open ALIF approach is a reasonable alternative to the more extensive posterior approaches. ALIF significantly restores the height of the intervertebral disc, indirectly increases foraminal dimensions, increases lordosis angle with significant short and long-term pain relief and functional recovery.

Investigation into factors affecting the mechanical behaviours of a patient-specific vertebral body replacement

Most vertebral body implants that are currently designed and produced in batches have difficulty meeting the patient-specific demands. Moreover, several complications, including a low fusion rate, subsidence occurrence, and rod displacement, are associated with these implants. This study aims to investigate the effects of patient-specific geometric and clinical parameters on the biomechanics of a vertebral body replacement. A three-dimensional patient-specific vertebral body replacement model was established as the basic model for parametric studies, including the anatomic design of the endplates, tilting angle, thickness, and dislocation of the vertebral body implant. A finite element analysis was applied to determine the stress distribution of the vertebral body implant when under various loading conditions. The model with an anatomical interfacing design generates 75% less stress concentration compared to a flat design; the peak stress of the model with a tilted angle closely matching the replaced vertebra segment is decreased by 30%; and the thickness close to the cortical bone can offer better bone growth capability and long-term stability. Patient-specific geometrical parameters were found to significantly affect the biomechanics of a vertebral body replacement, and therefore, a design customized especially for the endplates is necessary for better stability and long-term longevity of the prostheses. Regardless of such progress, how to balance the stability of a vertebral body implant and the safety of the peripheral nervous system remains a clinical challenge.

Evaluating Outcomes of Stand-Alone Anterior Lumbar Interbody Fusion: A Systematic Review

BACKGROUND

Stand-alone anterior lumbar interbody fusion (ALIF) is an effective surgical approach for selected spinal pathologies. It avoids the morbidity and complications associated with instrumented ALIF, such as plate fixation and the traditionally used posterior approach. Despite improved disc space visualization and clearance, the associated posterior instability and increased risk of nonfusion present major challenges to this approach. The integral cage design aims to address these challenges by providing the necessary stabilization through intracorporeal screws. However, there is limited and controversial data available for stand-alone ALIF and integral cage fixation. To our knowledge, this is the first systematic review to evaluate recent findings on outcomes of stand-alone ALIF devices to explore areas of controversy and identify directions for future research.

METHODS

Two reviewers conducted independent, systematic literature searches for appropriate studies in 5 electronic databases as per Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Studies were filtered by the use of specified selection criteria, particularly exclusion of studies with supplementary fixation to ALIF and studies published before the year 2000. A total of 17 studies met the criteria, and their data were comprehensively extracted and analyzed.

RESULTS

The current literature is supportive of stand-alone ALIF due to acceptable clinical outcomes, promising fusion rates and disc height restoration. However, data and outcomes remain preliminary, and there are numerous areas of controversy.

CONCLUSIONS

There is evidence for the efficacy and safety of stand-alone ALIF. However, the extent of improvement based on specific indications for surgery remains unclear. Further investigation utilizing more methodologically rigorous studies of long-term outcomes is necessary to address these issues.

Can an Endplate-conformed Cervical Cage Provide a Better Biomechanical Environment than a Typical Non-conformed Cage?: A Finite Element Model and Cadaver Study

OBJECTIVES

To evaluate the biomechanical characteristics of endplate-conformed cervical cages by finite element method (FEM) analysis and cadaver study.

METHODS

Twelve specimens (C2 -C7 ) and a finite element model (C3 -C7 ) were subjected to biomechanical evaluations. In the cadaver study, specimens were randomly assigned to intact (I), endplate-conformed (C) and non-conformed (N) groups with C4-5 discs as the treated segments. The morphologies of the endplate-conformed cages were individualized according to CT images of group C and the cages fabricated with a 3-D printer. The non-conformed cages were wedge-shaped and similar to commercially available grafts. Axial pre-compression loads of 73.6 N and moment of 1.8 Nm were used to simulate flexion (FLE), extension (EXT), lateral bending (LB) and axial rotation (AR). Range of motion (ROM) at C4-5 of each specimen was recorded and film sensors fixed between the cages and C5 superior endplates were used to detect interface stress. A finite element model was built based on the CT data of a healthy male volunteer. The morphologies of the endplate-conformed and wedge-shaped, non-conformed cervical cages were both simulated by a reverse engineering technique and implanted at the segment of C4-5 in the finite element model for biomechanical evaluation. Force loading and grouping were similar to those applied in the cadaver study. ROM of C4-5 in group I were recorded to validate the finite element model. Additionally, maximum cage-endplate interface stresses, stress distribution contours on adjoining endplates, intra-disc stresses and facet loadings at adjacent segments were measured and compared between groups.

RESULTS

In the cadaver study, Group C showed a much lower interface stress in all directions of motion (all P < 0.05) and the ROM of C4-5 was smaller in FLE-EXT (P = 0.001) but larger in AR (P = 0.017). FEM analysis produced similar results: the model implanted with an endplate-conformed cage presented a lower interface stress with a more uniform stress distribution than that implanted with a non-conformed cage. Additionally, intra-disc stress and facet loading at the adjacent segments were obviously increased in both groups C and N, especially those at the supra-jacent segments. However, stress increase was milder in group C than in group N for all directions of motion.

CONCLUSIONS

Endplate-conformed cages can decrease cage-endplate interface stress in all directions of motion and increase cervical stability in FLE-EXT. Additionally, adjacent segments are possibly protected because intra-disc stress and facet loading are smaller after endplate-conformed cage implantation. However, axial stability was reduced in group C, indicating that endplate-conformed cage should not be used alone and an anterior plate system is still important in anterior cervical discectomy and fusion.

Anterior Lumbar Interbody Fusion Integrated Screw Cages: Intrinsic Load Generation, Subsidence, and Torsional Stability

OBJECTIVE

To perform a repeatable idealized in vitro model to evaluate the effects of key design features and integrated screw fixation on unloaded surface engagement, subsidence, and torsional stability.

METHODS

We evaluated four different stand-alone anterior lumbar interbody fusion (ALIF) cages with two, three, and four screw designs. Polyurethane (saw-bone) foam blocks were used to simulate the vertebral bone. Fuji Film was used to measure the contact footprint, average pressure, and load generated by fixating the cages with screws. Subsidence was tested by axially loading the constructs at 10 N/s to 400 N and torsional load was applied +/-1 Nm for 10 cycles to assess stability. Outcome measures included total subsidence and maximal torsional angle range.

RESULTS

Cages 1, 2, and 4 were symmetrical and produced similar results in terms of contact footprint, average pressure, and load. The addition of integrated screws into the cage-bone block construct demonstrated a clear trend towards decreased subsidence. Cage 2 with surface titanium angled ridges and a keel produced the greatest subsidence with and without screws, significantly more than all other cages ( P < 0.05). Angular rotation was not significantly affected by the addition of screws ( P < 0.066). A statistically significant correlation existed between subsidence and reduced angular rotation across all cage constructs ( P = 0.018).

CONCLUSION

Each stand-alone cage featured unique surface characteristics, which resulted in differing cage-foam interface engagement, influencing the subsidence and torsional angle. Increased subsidence significantly reduced the torsional angle across all cage constructs.

Clinical outcome of stand-alone ALIF compared to posterior instrumentation for degenerative disc disease: A pilot study and a literature review

The objective of the article was to: a) present results from a case cohort pilot study comparing stand-alone ALIF and TLIF and, b) review the literature on studies comparing the clinical outcome of stand-alone ALIF with posterior instrumentation including TLIF or PLIF, in patients with disabling low back pain resulting from degenerative disc disease. ALIF surgery has previously been linked with certain high risk complications and unfavorable long term fusion results. Newer studies suggest that stand-alone ALIF can possibly be advantageous compared to other types of posterior instrumented interbody fusion for a selected group of DDD patients. The methods and material consisted of a cohort pilot study of patients, with DDD treated with stand-alone ALIF or TLIF followed by a literature review conducted through a comprehensive PubMed database search of the English literature. Studies comparing stand-alone ALIF with posterior instrumented interbody fusion were selected and reviewed. Results from the pilot study, n = 21, showed a reduced perioperative blood loss, shorter operative time and a trend towards better pain reduction and decreased use of opioid analgesics in patients undergoing stand-alone ALIF compared to posterior instrumented fusion with TLIF. The literature review included three studies, n = 630. All three studies were retrospective cohort studies. The average patient follow-up was 2-years but with heterogeneous selected outcomes. Two of three articles documented significant advantages when using stand-alone ALIF on outcomes such as ODI, VAS, surgical time, blood loss and patient satisfaction. No study found stand-alone ALIF inferior in chosen outcomes including fusion. In conclusion the pilot study and the literature review, finds similar clinical outcomes and fusion rates after stand-alone ALIF and posterior interbody fusion. Stand-alone ALIF was associated with a shorter duration of surgery, less perioperative blood loss and a faster improvement post-operatively. Therefore stand-alone ALIF is a viable and important surgical option, which could be considered first choice as surgical treatment.