Biomechanics of two-level Charité artificial disc placement in comparison to fusion plus single-level disc placement combination

Biomechanical study of oblique lumbar interbody fusion (OLIF) augmented with different types of instrumentation: a finite element analysis

Background

To explore the biomechanical differences in oblique lumbar interbody fusion (OLIF) augmented by different types of instrumentation.

Methods

A three-dimensional nonlinear finite element (FE) model of an intact L3-S1 lumbar spine was built and validated. The intact model was modified to develop five OLIF surgery models (Stand-alone OLIF; OLIF with lateral plate fixation [OLIF + LPF]; OLIF with unilateral pedicle screws fixation [OLIF + UPSF]; OLIF with bilateral pedicle screws fixation [OLIF + BPSF]; OLIF with translaminar facet joint fixation + unilateral pedicle screws fixation [OLIF + TFJF + UPSF]) in which the surgical segment was L4–L5. Under a follower load of 500 N, a 7.5-Nm moment was applied to all lumbar spine models to calculate the range of motion (ROM), equivalent stress peak of fixation instruments (ESPFI), equivalent stress peak of cage (ESPC), equivalent stress peak of cortical endplate (ESPCE), and equivalent stress average value of cancellous bone (ESAVCB).

Results

Compared with the intact model, the ROM of the L4–L5 segment in each OLIF surgery model decreased by > 80%. The ROM values of adjacent segments were not significantly different. The ESPFI, ESPC, and ESPCE values of the OLIF + BPSF model were smaller than those of the other OLIF surgery models. The ESAVCB value of the normal lumbar model was less than the ESAVCB values of all OLIF surgical models. In most postures, the ESPFI, ESPCE, and ESAVCB values of the OLIF + LPF model were the largest. The ESPC was higher in the Stand-alone OLIF model than in the other OLIF models. The stresses of several important components of the OLIF + UPSF and OLIF + TFJF + UPSF models were between those of the OLIF + LPF and OLIF + BPSF models.

Conclusions

Our biomechanical FE analysis indicated the greater ability of OLIF + BPSF to retain lumbar stability, resist cage subsidence, and maintain disc height. Therefore, in the augmentation of OLIF, bilateral pedicle screws fixation may be the best approach.

Biomechanical analysis of stand-alone lumbar interbody cages versus 360° constructs: an in vitro and finite element investigation

OBJECTIVE

Low fusion rates and cage subsidence are limitations of lumbar fixation with stand-alone interbody cages. Various approaches to interbody cage placement exist, yet the need for supplemental posterior fixation is not clear from clinical studies. Therefore, as prospective clinical studies are lacking, a comparison of segmental kinematics, cage properties, and load sharing on vertebral endplates is needed. This laboratory investigation evaluates the mechanical stability and biomechanical properties of various interbody fixation techniques by performing cadaveric and finite element (FE) modeling studies.

METHODS

An in vitro experiment using 7 fresh-frozen human cadavers was designed to test intact spines with 1) stand-alone lateral interbody cage constructs (lateral interbody fusion, LIF) and 2) LIF supplemented with posterior pedicle screw-rod fixation (360° constructs). FE and kinematic data were used to validate a ligamentous FE model of the lumbopelvic spine. The validated model was then used to evaluate the stability of stand-alone LIF, transforaminal lumbar interbody fusion (TLIF), and anterior lumbar interbody fusion (ALIF) cages with and without supplemental posterior fixation at the L4-5 level. The FE models of intact and instrumented cases were subjected to a 400-N compressive preload followed by an 8-Nm bending moment to simulate physiological flexion, extension, bending, and axial rotation. Segmental kinematics and load sharing at the inferior endplate were compared.

RESULTS

The FE kinematic predictions were consistent with cadaveric data. The range of motion (ROM) in LIF was significantly lower than intact spines for both stand-alone and 360° constructs. The calculated reduction in motion with respect to intact spines for stand-alone constructs ranged from 43% to 66% for TLIF, 67%-82% for LIF, and 69%-86% for ALIF in flexion, extension, lateral bending, and axial rotation. In flexion and extension, the maximum reduction in motion was 70% for ALIF versus 81% in LIF for stand-alone cases. When supplemented with posterior fixation, the corresponding reduction in ROM was 76%-87% for TLIF, 86%-91% for LIF, and 90%-92% for ALIF. The addition of posterior instrumentation resulted in a significant reduction in peak stress at the superior endplate of the inferior segment in all scenarios.

CONCLUSIONS

Stand-alone ALIF and LIF cages are most effective in providing stability in lateral bending and axial rotation and less so in flexion and extension. Supplemental posterior instrumentation improves stability for all interbody techniques. Comparative clinical data are needed to further define the indications for stand-alone cages in lumbar fusion surgery.

Finite element study on the influence of pore size and structure on stress shielding effect of additive manufactured spinal cage

The stress shielding effect occurs when the orthopedic implant reduces the load delivered to the bone, causing inefficient stress transfer to the host bone. The usage of porous additive manufactured (AM) cages reduces the stress shielding effect and promotes bone ingrowth also. The purpose of this work is to study the stress and deformation on porous hybrid spinal cages under different loading conditions using Finite Element Analysis (FEA). The spinal cages consisting of three porous structures with pore sizes ranging from 0.4 to 0.6 mm were investigated for stress shielding and fatigue strength. The results showed a significant reduction in stress shielding for the studied designs and conclude that the pore size has a greater significant effect on stress shielding than the porous structure in spinal cages.

A feasibility study of individual 3D-printed navigation template for the deep external fixator pin position on the iliac crest

Background

The aim of this study was to investigate the feasibility of an individual navigation template for the deep pin position on the iliac crest, based on digital design and 3D printing technology.

Methods

The preoperative CT images of 8 patients with pelvic fractures were collected. The data were reconstructed using a 3D imaging reconstruction workstation. An individual navigation template for the deep pin position on the iliac crest was designed on a virtual 3D model. The individual drill template and the solid pelvic model were produced using the 3D printing technology. The individual drill template was used for intraoperative deep pin position on the iliac crest after the preoperative simulation was completed.

Results

Thirty-two external fixator pins were inserted using the individual drill template. The average depth of pins was 84.82 mm. The trajectories were appropriate based on the postoperative X-ray and CT scan. No significant difference in the entry point, convergence angle, and caudal angle of the pins were noted before and after the operation (all P > 0.05). Finite element analysis indicated that the deep external fixator pin position could more reasonably distribute the stress in the cortical and spongy bones in the pelvis. All patients could perform partial weight-bearing activity 6 weeks postoperatively. No loosening and rupture of the pin, infection, and no damage of blood vessels and nervous tissue were found during the entire follow-up period.

Conclusions

The individual drill template technique is an improvement of the traditional technique, which could increase precision and the depth of pin position. In addition, good mechanical stability and low risk of pin-related complications occurred due to the individual drill template, which makes the external fixation technique a potential alternative.

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

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

Biomechanical evaluation of four surgical scenarios of lumbar fusion with hyperlordotic interbody cage: A finite element study

BACKGROUND

Lumbar spinal fusion in the interbody space is augmented with interbody fusion cages to provide structural support while arthrodesis occurs. Subsidence is a serious complication of interbody fusion. However, the biomechanical influence of anterior longitudinal ligament (ALL) and pedicle screws on subsidence has not been fully understood.

OBJECTIVE

To investigate biomechanical effects of the hyperlordotic cages in different surgical conditions using finite element analysis.

METHODS

Four surgical finite element (FE) models were constructed by inserting 15 degree lordosis cage at the L3-L4 disc space. The four surgical conditions were ALL intact (M1), ALL resected (M2), ALL intact and bilateral pedicle screws (M3), and ALL resected and bilateral pedicle screws (M4). Follow loads were applied at the L2 vertebral body while the inferior surface of L5 was fixed. FEA was implemented to simulate the four motion modes and biomechanical properties of four fusion scenarios with hyperlordotic interbody cage were compared.

RESULTS

The range of motion (ROM) and facet joint force (FJF) at L3-L4 decreased significantly after fusion during all the motion modes. The cage stress and endplate stress at L3-L4 increased significantly after fusion during all the motion modes. The cage stress and endplate stress at L3-L4 for M3 and M4 were smaller than that for M1 and M2 during all the motion modes. The FJF at L3-L4 for M3 and M4 were smaller than that for M1 and M2 during extension, bending, and rotation.

CONCLUSIONS

ALL has little effect on the biomechanics after lumbar fusion with hyperlordotic interbody cage. The bilateral pedicle screws significantly decreased the stress in cage, stress in endplate at L3-L4, and lowered facet contact force except for flexion mode. The implication is that the supplemental bilateral pedicle screws are recommended whether or not the ALL is resected.

Biomechanical Analysis of Lateral Lumbar Interbody Fusion Constructs with Various Fixation Options: Based on a Validated Finite Element Model

BACKGROUND

Lateral lumbar interbody fusion using cage supplemented with fixation has been used widely in the treatment of lumbar disease. A combined fixation (CF) of lateral plate and spinous process plate may provide multiplanar stability similar to that of bilateral pedicle screws (BPS) and may reduce morbidity. The biomechanical influence of the CF on cage subsidence and facet joint stress has not been well described. The aim of this study was to compare biomechanics of various fixation options and to verify biomechanical effects of the CF.

METHODS

The surgical finite element models with various fixation options were constructed based on computed tomography images. The lateral plate and posterior spinous process plate were applied (CF). The 6 motion modes were simulated. Range of motion (ROM), cage stress, endplate stress, and facet joint stress were compared.

RESULTS

For the CF model, ROM, cage stress, and endplate stress were the minimum in almost all motion modes. Compared with BPS, the CF reduced ROM, cage stress, and endplate stress in all motion modes. The ROM was reduced by more than 10% in all motion modes except for flexion; cage stress and endplate stress were reduced more than 10% in all motion modes except for rotation-left. After interbody fusion, facet joint stress was reduced substantially compared with the intact conditions in all motion modes except for flexion.

CONCLUSIONS

The combined plate fixation may offer an alternative to BPS fixation in lateral lumbar interbody fusion.

Biomechanical analysis of lumbar interbody fusion cages with various lordotic angles: a finite element study

Inappropriate lordotic angle of lumbar fusion cage could be associated with cage damage or subsidence. The biomechanical influence of cage lordotic angle on lumbar spine has not been fully investigated. Four surgical finite element models were constructed by inserting cages with various lordotic angles at L3-L4 disc space. The four motion modes were simulated. The range of motion (ROM) decreased with increased lordotic angle of cage in flexion, extension, and rotation, whereas it was not substantially changed in bending. The maximum stress in cage decreased with increased lordotic angle of cage in all motion modes. The maximum stress in endplate at surgical level increased with increased lordotic angle of cage in flexion and rotation, whereas it was not substantially changed in extension and bending. The facet joint force (FJF) was much smaller than that for the intact conditions in extension, bending, and rotation, while it was not substantially changed in flexion. In conclusion, the ROM, stresses in the cage and endplate at surgical level are sensitive to the lordotic angle of cage. The increased cage lordotic angle may provide better stability and reduce the risk of cage damage, whereas it may increase the risk of subsidence in flexion and rotation.

Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND

A porous additive manufactured (AM) cage may provide stability similar to that of traditional solid cages and may be beneficial to bone ingrowth. The biomechanical influence of various porous cages on stability, subsidence, stresses in cage, and facet contact force has not been fully described. The purpose of this study was to verify biomechanical effects of porous AM cages.

METHODS

The surgical finite element models with various cages were constructed. The partially porous titanium (PPT) cages and fully porous titanium (FPT) cages were applied. The mechanical parameters of porous materials were obtained by mechanical test. Then the porous AM cages were compared with solid titanium (TI) cage and solid polyetheretherketone (PEEK) cage. The 4 motion modes were simulated. Range of motion (ROM), cage stress, end plate stress, and facet joint force (FJF) were compared.

RESULTS

For all the surgical models, ROM decreased by >90%. Compared with TI and PPT cages, PEEK and FPT cages substantially reduced the maximum stresses in cage and end plate in all motion modes. Compared with PEEK cages, the stresses in cage and end plate for FPT cages decreased, whereas the ROM increased. Comparing FPT cages, the stresses in cage and end plate decreased with increasing porosity, whereas ROM increased with increasing porosity. After interbody fusion, FJF was substantially reduced in all motion modes except for flexion.

CONCLUSIONS

Fully porous cages may offer an alternative to solid PEEK cages in lateral lumbar interbody fusion. However, it may be prudent to further increase the porosity of the cage.

Improving the Process of Adjusting the Parameters of Finite Element Models of Healthy Human Intervertebral Discs by the Multi-Response Surface Method

The kinematic behavior of models that are based on the finite element method (FEM) for modeling the human body depends greatly on an accurate estimate of the parameters that define such models. This task is complex, and any small difference between the actual biomaterial model and the simulation model based on FEM can be amplified enormously in the presence of nonlinearities. The current paper attempts to demonstrate how a combination of the FEM and the MRS methods with desirability functions can be used to obtain the material parameters that are most appropriate for use in defining the behavior of Finite Element (FE) models of the healthy human lumbar intervertebral disc (IVD). The FE model parameters were adjusted on the basis of experimental data from selected standard tests (compression, flexion, extension, shear, lateral bending, and torsion) and were developed as follows: First, three-dimensional parameterized FE models were generated on the basis of the mentioned standard tests. Then, 11 parameters were selected to define the proposed parameterized FE models. For each of the standard tests, regression models were generated using MRS to model the six stiffness and nine bulges of the healthy IVD models that were created by changing the parameters of the FE models. The optimal combination of the 11 parameters was based on three different adjustment criteria. The latter, in turn, were based on the combination of stiffness and bulges that were obtained from the standard test FE simulations. The first adjustment criteria considered stiffness and bulges to be equally important in the adjustment of FE model parameters. The second adjustment criteria considered stiffness as most important, whereas the third considered the bulges to be most important. The proposed adjustment methods were applied to a medium-sized human IVD that corresponded to the L3–L4 lumbar level with standard dimensions of width = 50 mm, depth = 35 mm, and height = 10 mm. Agreement between the kinematic behavior that was obtained with the optimized parameters and that obtained from the literature demonstrated that the proposed method is a powerful tool with which to adjust healthy IVD FE models when there are many parameters, stiffnesses, and bulges to which the models must adjust.

Biomechanical Effects of the Geometry of Ball-and-Socket Artificial Disc on Lumbar Spine: A Finite Element Study

STUDY DESIGN

A three-dimensional finite element model of intact lumbar spine was constructed and four surgical finite element models implanted with ball-and-socket artificial discs with four different radii of curvature were compared.

OBJECTIVE

To investigate biomechanical effects of the curvature of ball-and-socket artificial disc using finite element analysis.

SUMMARY OF BACKGROUND DATA

Total disc replacement (TDR) has been accepted as an alternative treatment because of its advantages over spinal fusion methods in degenerative disc disease. However, the influence of the curvature of artificial ball-and-socket discs has not been fully understood.

METHODS

Four surgical finite element models with different radii of curvature of ball-and-socket artificial discs were constructed.

RESULTS

The range of motion (ROM) increased with decreasing radius of curvature in extension, flexion, and lateral bending, whereas it increased with increasing radius of curvature in axial torsion. The facet contact force was minimum with the largest radius of curvature in extension, flexion, and lateral bending, whereas it was maximum with the largest radius in axial torsion. It was also affected by the disc placement, more with posterior placement than anterior placement. The stress in L4 cancellous bone increased when the radius of curvature was too large or small.

CONCLUSION

The geometry of ball-and-socket artificial disc significantly affects the ROM, facet contact force, and stress in the cancellous bone at the surgical level. The implication is that in performing TDR, the ball-and-socket design may not be ideal, as ROM and facet contact force are sensitive to the disc design, which may be exaggerated by the individual difference of anatomical geometry.

LEVEL OF EVIDENCE

N/A.

A computational biomechanical investigation of posterior dynamic instrumentation: combination of dynamic rod and hinged (dynamic) screw

Currently, rigid fixation systems are the gold standard for degenerative disk disease treatment. Dynamic fixation systems have been proposed as alternatives for the treatment of a variety of spinal disorders. These systems address the main drawbacks of traditional rigid fixation systems, such as adjacent segment degeneration and instrumentation failure. Pedicle-screw-based dynamic stabilization (PDS) is one type of these alternative systems. The aim of this study was to simulate the biomechanical effect of a novel posterior dynamic stabilization system, which is comprised of dynamic (hinged) screws interconnected with a coiled, spring-based dynamic rod (DSDR), and compare it to semirigid (DSRR and RSRR) and rigid stabilization (RSRR) systems. A validated finite element (FE) model of L1-S1 was used to quantify the biomechanical parameters of the spine, such as range of motion, intradiskal pressure, stresses and facet loads after single-level instrumentation with different posterior stabilization systems. The results obtained from in vitro experimental intact and instrumented spines were used to validate the FE model, and the validated model was then used to compare the biomechanical effects of different fixation and stabilization constructs with intact under a hybrid loading protocol. The segmental motion at L4-L5 increased by 9.5% and 16.3% in flexion and left rotation, respectively, in DSDR with respect to the intact spine, whereas it was reduced by 6.4% and 10.9% in extension and left-bending loads, respectively. After instrumentation-induced intradiskal pressure at adjacent segments, L3-L4 and L5-S1 became less than the intact in dynamic rod constructs (DSDR and RSDR) except in the RSDR model in extension where the motion was higher than intact by 9.7% at L3-L4 and 11.3% at L5-S1. The facet loads were insignificant, not exceeding 12N in any of the instrumented cases in flexion. In extension, the facet load in DSDR case was similar to that in intact spine. The dynamic rod constructions (DSDR and RSDR) led to a lesser peak stress at screws compared with rigid rod constructions (DSRR and RSRR) in all loading cases. A dynamic construct consisting of a dynamic rod and a dynamic screw did protect the adjacent level from excessive motion.

Mid- to long-term results of total lumbar disc replacement: a prospective analysis with 5- to 10-year follow-up

BACKGROUND CONTEXT

The role of fusion of lumbar motion segments for the treatment of intractable low back pain (LBP) from degenerative disc disease (DDD) without deformities or instabilities remains controversially debated. Total lumbar disc replacement (TDR) has been used as an alternative in a highly selected patient cohort. However, the amount of long-term follow-up (FU) data on TDR is limited. In the United States, insurers have refused to reimburse surgeons for TDRs for fear of delayed complications, revisions, and unknown secondary costs, leading to a drastic decline in TDR numbers.

PURPOSE

To assess the mid- and long-term clinical efficacy as well as patient safety of TDR in terms of perioperative complication and reoperation rates.

STUDY DESIGN/SETTING

Prospective, single-center clinical investigation of TDR with ProDisc II (Synthes, Paoli, PA, USA) for the treatment of LBP from lumbar DDD that has proven unresponsive to conservative therapy.

PATIENT SAMPLE

Patients with a minimum of 5-year FU after TDR, performed for the treatment of intractable and predominant (≥80%) axial LBP resulting from DDD without any deformities or instabilities.

OUTCOME MEASURES

Visual analog scale (VAS), Oswestry Disability Index (ODI), and patient satisfaction rates (three-scale outcome rating); complication and reoperation rates as well as elapsed time until revision surgery; patient's professional activity/employment status.

METHODS

Clinical outcome scores were acquired within the framework of an ongoing prospective clinical trial. Patients were examined preoperatively, 3, 6, and 12 months postoperatively, annually from then onward. The data acquisition was performed by members of the clinic's spine unit including medical staff, research assistants, and research nurses who were not involved in the process of pre- or postoperative decision-making.

RESULTS

The initial cohort consisted of 201 patients; 181 patients were available for final FU, resembling a 90.0% FU rate after a mean FU of 7.4 years (range 5.0-10.8 years). The overall results revealed a highly significant improvement from baseline VAS and ODI levels at all postoperative FU stages (p<.0001). VAS scores demonstrated a slight (from VAS 2.6 to 3.3) but statistically significant deterioration from 48 months onward (p<.05). Patient satisfaction rates remained stable throughout the entire postoperative course, with 63.6% of patients reporting a highly satisfactory or a satisfactory (22.7%) outcome, whereas 13.7% of patients were not satisfied. The overall complication rate was 14.4% (N=26/181). The incidence of revision surgeries for general and/or device-related complications was 7.2% (N=13/181). Two-level TDRs demonstrated a significant improvement of VAS and ODI scores in comparison to baseline levels (p<.05). Nevertheless, the results were significantly inferior in comparison to one-level cases and were associated with higher complication (11.9% vs. 27.6%; p=.03) and inferior satisfaction rates (p<.003).

CONCLUSIONS

Despite the fact that the current data comprises the early experiences and learning curve associated with a new surgical technique, the results demonstrate satisfactory and maintained mid- to long-term clinical results after a mean FU of 7.4 years. Patient safety was proven with acceptable complication and reoperation rates. Fear of excessive late complications or reoperations following the primary TDR procedure cannot be substantiated with the present data. In carefully selected cases, TDR can be considered a viable treatment alternative to lumbar fusion for which spine communities around the world seem to have accepted mediocre clinical results as well as obvious and significant drawbacks.

Finite element analysis of 3 posterior fixation techniques in the lumbar spine

This study compared the biomechanics of 3 fixation techniques: bilateral pedicle screw (BPS) fixation, unilateral pedicle screw (UPS) fixation, and UPS supplemented with translaminar facet screw (UPS+TLFS) fixation. The study was conducted in an L3-L5 finite element model. Three different finite element models were created by adopting different fixation techniques after removal of the left L3-L4 and L4-L5 facet joints. A 500-N compressive preload combined with 8-NM moment were applied in 3 finite element models with 3 fixation techniques during different movements. Angular displacement and stress distribution were recorded. As described in this article, the UPS model had the most variation in angular displacement, the BPS model was intermediate, and the UPS+TLFS model had the least mobility. Most of the stress accumulated on the body and tail of the pedicle screws and the connecting rods in the UPS and BPS models, but stress accumulated on the rods and the part of the facet joint pierced by the TLFS in the UPS+TLFS model. The middle part of the pedicle screw endured little stress compared with the upper and lower parts. The maximum stress on the fixation devices was highest in the UPS model. The maximum stress in the UPS+TLFS model was the lowest among the 3 models. Biomechanically, UPS+TLFS fixation is superior to either UPS fixation or BPS fixation in improving stability and reducing stress. Bilateral pedicle screw fixation is intermediate, and UPS fixation is inferior.

Vertebroplasty: Patient and treatment variations studied through parametric computational models

BACKGROUND

Vertebroplasty is increasingly used in the treatment of vertebral compression fractures. However there are concerns that this intervention may lead to further fractures in the adjacent vertebral segments. This study was designed to parametrically assess the influence of both treatment factors (cement volume and number of augmentations), and patient factors (bone and disc quality) on the biomechanical effects of vertebroplasty.

METHODS

Specimen-specific finite element models of two experimentally-tested human three-vertebral-segments were developed from CT-scan data. Cement augmentation at one and two levels was represented in the respective models and good agreement in the predicted stiffness was found compared to the corresponding experimental specimens. Parametric variations of key variables associated with the procedure were then studied.

FINDINGS

The segmental stiffness increased with disc degeneration, with increasing bone quality and to a lesser extent with increasing cement volume. Cement modulus did not have a great influence on the overall segmental stiffness and on the change in the elemental stress in the adjoining vertebrae. However, following augmentation, the stress distribution in the adjacent vertebra changed, indicating possible load redistribution effects of vertebroplasty.

INTERPRETATION

This study demonstrates the importance of patient factors in the outcomes of vertebroplasty and suggests that these may be one reason for the variation in clinical results.

The biomechanics of a multilevel lumbar spine hybrid using nucleus replacement in conjunction with fusion

BACKGROUND CONTEXT

Degenerative disc disease is commonly a multilevel pathology with varying deterioration severity. The use of fusion on multiple levels can significantly affect functionality and has been linked to persistent adjacent disc degeneration. A hybrid approach of fusion and nucleus replacement (NR) has been suggested as a solution for mildly degenerated yet painful levels adjacent to fusion.

PURPOSE

To compare the biomechanical metrics of different hybrid implant constructs, hypothesizing that an NR+fusion hybrid would be similar to a single-level fusion and perform more naturally compared with a two-level fusion.

STUDY DESIGN

A cadaveric in vitro repeated-measures study was performed to evaluate a multilevel lumbar NR+fusion hybrid.

METHODS

Eight cadaveric spines (L3-S1) were tested in a Spine Kinetic Simulator (Instron, Norwood, MA, USA). Pure moments of 8 Nm were applied in flexion/extension, lateral bending, and axial rotation as well as compression loading. Specimens were tested intact; fused (using transforaminal lumbar interbody fusion instrumentation with posterior rods) at L5-S1; with a nuclectomy at L4-L5 including fusion at L5-S1; with NR at L4-L5 including fusion at L5-S1; and finally with a two-level fusion spanning L4-S1. Repeated-measures analysis of variance and corrected t tests were used to statistically compare outcomes.

RESULTS

The NR+fusion hybrid and single-level fusion exhibited no statistical differences for range of motion (ROM), stiffness, neutral zone, and intradiscal pressure in all loading directions. Compared with two-level fusion, the hybrid affords the construct 41.9% more ROM on average. Two-level fusion stiffness was statistically higher than all other constructs and resulted in significantly lower ROM in flexion, extension, and lateral bending. The hybrid construct produced approximately half of the L3-L4 adjacent-level pressures as the two-level fusion case while generating similar pressures to the single-level fusion case.

CONCLUSIONS

These data portend more natural functional outcomes and fewer adjacent disc complications for a multilevel NR+fusion hybrid compared with the classical two-level fusion.

Kinematic effects of a pedicle-lengthening osteotomy for the treatment of lumbar spinal stenosis

OBJECT

Lumbar spinal stenosis (LSS) may lead to disabling neurogenic symptoms and has traditionally been treated using open laminectomy. A new technique for correcting LSS involves lengthening the lumbar pedicles through bilateral percutaneous pedicle osteotomies. In this paper, the authors' goal was to evaluate the changes in spinal canal dimensions and kinematic behavior after pedicle-lengthening osteotomies.

METHODS

The kinematic behavior of 8 cadaveric lumbar segments was evaluated intact and after bilateral pedicle-lengthening osteotomies at the L-4, L-5, and L-4 and L-5 levels. Testing was conducted with and without a compressive preload using a custom kinematic apparatus that allowed for 3D tracking of each vertebra during flexion-extension, right-left bending, and right-left rotation. A validated finite element (FE) spine model was used to measure the changes in the cross-sectional area of the spinal canal and neural foramen after 2-, 3-, and 4.5-mm simulated pedicle-lengthening osteotomy procedures.

RESULTS

The overall and segmental kinematics were not significantly altered after the pedicle-lengthening osteotomy procedure at the L-4 and/or L-5 pedicles. The kinematic signatures of the intact and lengthened states were similar for all motion pairs. The FE spine model yielded kinematics predictions within or close to the 95% confidence interval for the cadaveric data. The FE spine demonstrated substantial, pedicle length-dependent enlargement of the cross-sectional areas of the spinal canal and neural foramen after simulated pedicle lengthening.

CONCLUSIONS

Bilateral pedicle-lengthening osteotomies produced substantial increases in the cross-sectional areas of the spinal canal and neural foramen without significantly altering normal spinal kinematics. This technique deserves further study as a less invasive treatment option for LSS.

Biomechanics of disc degeneration

Disc degeneration and associated disorders are among the most debated topics in the orthopedic literature over the past few decades. These may be attributed to interrelated mechanical, biochemical, and environmental factors. The treatment options vary from conservative approaches to surgery, depending on the severity of degeneration and response to conservative therapies. Spinal fusion is considered to be the "gold standard" in surgical methods till date. However, the association of adjacent level degeneration has led to the evolution of motion preservation technologies like spinal arthroplasty and posterior dynamic stabilization systems. These new technologies are aimed to address pain and preserve motion while maintaining a proper load sharing among various spinal elements. This paper provides an elaborative biomechanical review of the technologies aimed to address the disc degeneration and reiterates the point that biomechanical efficacy followed by long-term clinical success will allow these nonfusion technologies as alternatives to fusion, at least in certain patient population.

Effect of graded facetectomy on biomechanics of Dynesys dynamic stabilization system

STUDY DESIGN

Finite element (FE) method was used to compare the biomechanics of L3-S1 lumbar spine with graded facetectomy before and after placement of Dynesys.

OBJECTIVE

To evaluate the biomechanics of Dynesys as a function of graded bilateral facetectomies.

SUMMARY OF BACKGROUND DATA

Spinal fusion or posterior dynamic stabilization systems are used to restore stability after facetectomies.

METHODS

The intact FE spine was modified to simulate decompression at L4-L5 with 50% and 75% and total facetectomy with/without dynamic stabilization with Dynesys. Biomechanics of the implanted level was investigated under different physiological loadings.

RESULTS

Total facetectomy increased the motion in extension (8.7° vs. 2.7° for intact) and axial rotation (8.4° vs. 2.4° for intact). However the decrease in motion in the Dynesys model ranged from 65% in axial rotation to 80% in flexion for all facetectomies, except in the total facetectomy axial rotation case (motion higher than intact). The center of rotation of dynamic stabilized segment moved inferior/posterior in partial facetectomy and superior/posterior in total facetectomy with respect to the intact and destabilized cases. The Dynesys screws observed peak stresses up to 28% higher than those of a rigid fixation system in certain loadings, such as lateral bending and extension. The critical loosening torque applied to the screws in total facetectomy case was 6 times the partial facetectomy case in axial rotation.

CONCLUSION

Partial facetectomy had a minimal effect on range of motion on the Dynesys-implanted segment. However, in the case of total facetectomy the motion increased by almost 40% in flexion and by 200% in axial rotation. The higher stresses applied to the screws in Dynesys in specific loadings may lead to higher risk of screw failure in Dynesys than in a generic rigid fixation construct.

Kinematic evaluation of one- and two-level Maverick lumbar total disc replacement caudal to a long thoracolumbar spinal fusion

PURPOSE

Adjacent level degeneration that occurs above and/or below long fusion constructs is a documented clinical problem that is widely believed to be associated with the considerable change in stiffness caused by the fusion. Some researchers have suggested that early degeneration at spinal joints adjacent to a fusion could be treated by implanting total disc replacements at these levels. It is thought that further degeneration could be prevented through the disc replacement's design aims to reproduce normal disc heights, kinematics and tissue loading. For this reason, there is a clinical need to evaluate if a total disc replacement can maintain both the quantity of motion (i.e. range) and the quality of motion (i.e. center of rotation and coupling) at segments adjacent to a long spinal fusion. The purpose of this study was to experimentally evaluate range of motion (ROM-the intervertebral motion measured) and helical axis of motion (HAM) changes due to one- and two-level Maverick total disc replacement (TDR) adjacent to a long spinal fusion.

METHODS

Seven spine specimens (T8-S1) were used in this study (66 ± 19 years old, 3F/4 M). A continuous pure moment of ±5.0 Nm was applied to the specimen in flexion-extension (FE), lateral bending (LB) and axial rotation (AR), with a compressive follower preload of 400 N. The 5.0 Nm data were analyzed to evaluate the operated segment biomechanics at the level of the disc replacements. The data were also analyzed at lower moments using a modified version of Panjabi's proposed "hybrid" method to evaluate adjacent segment kinematics (intervertebral motion at the segments adjacent to the fusion) under identical overall (T8-S1) specimen rotations. The motion of each vertebra was monitored with an optoelectronic camera system. The biomechanical test was completed for (1) the intact condition and repeated after each surgical technique was applied to the specimen, (2) capsulotomy at L4-L5 and L5-S1, (3) T8-L4 fusion and capsulotomy at L4-L5 and L5-S1, (4) Maverick at L4-L5, and (5) Maverick at L5-S1. The capsulotomy was performed to allow measurement of facet joint loads in a companion study. Paired t tests were used to determine if differences in the kinematic parameters measured were significant. Holm-Sidak corrections for multiple comparisons were applied where appropriate.

RESULTS

Under the 5.0 Nm loads, L4-L5 ROMs tended to decrease in all directions following L4-L5 Maverick replacement (mean = 22 %, compared to the fused condition). Two-level Maverick implantation also tended to reduce L4-S1 ROM (mean 18, 7 and 31 % in FE, LB and AR, respectively, compared to the fused condition without TDR). Following TDR replacement, the HAM location tended to shift posteriorly in FE (at L5-S1), anteriorly in AR, and inferiorly in LB. However, although the above-mentioned trends were observed, neither one- nor two-level TDR replacement showed statistically significant ROM or HAM change in any of the three directions. At the identical T8-S1 posture identified by the modified hybrid analysis, the L4-L5 and L5-S1 levels underwent significant larger motions, relative to the overall specimen rotation, after fusion. In the hybrid analysis, there were no significant differences between the ROM after fusion with intact natural discs at L4-L5 and L5-S1 and the motions at those levels with one or two TDRs implanted.

CONCLUSIONS

The present results demonstrated that one or two Maverick discs implanted subjacent to a long thoracolumbar fusion preserved considerable and intact-like ranges of motion and maintained motion patterns similar to the intact specimen, in this ex vivo study with applied pure moments and compressive follower preload. The hybrid analysis demonstrated that, after fusion, the TDR-implanted levels are required to undergo large rotations, relative to those necessary before fusion, in order to achieve the same motion between T8 and S1. Additional clinical and biomechanical research is necessary to determine if such a kinematic demand would be made on these levels clinically and the biomechanical performance of these implants if it were.

Adjacent level effects of bi level disc replacement, bi level fusion and disc replacement plus fusion in cervical spine--a finite element based study

BACKGROUND

Studies delineating the adjacent level effect of single level disc replacement systems have been reported in literature. The aim of this study was to compare the adjacent level biomechanics of bi-level disc replacement, bi-level fusion and a construct having adjoining level disc replacement and fusion system.

METHODS

In total, biomechanics of four models- intact, bi level disc replacement, bi level fusion and fusion plus disc replacement at adjoining levels- was studied to gain insight into the effects of various instrumentation systems on cranial and caudal adjacent levels using finite element analysis (73.6N+varying moment).

FINDINGS

The bi-level fusion models are more than twice as stiff as compared to the intact model during flexion-extension, lateral bending and axial rotation. Bi-level disc replacement model required moments lower than intact model (1.5Nm). Fusion plus disc replacement model required moment 10-25% more than intact model, except in extension. Adjacent level motions, facet loads and endplate stresses increased substantially in the bi-level fusion model. On the other hand, adjacent level motions, facet loads and endplate stresses were similar to intact for the bi-level disc replacement model. For the fusion plus disc replacement model, adjacent level motions, facet loads and endplate stresses were closer to intact model rather than the bi-level fusion model, except in extension.

INTERPRETATION

Based on our finite element analysis, fusion plus disc replacement procedure has less severe biomechanical effects on adjacent levels when compared to bi-level fusion procedure. Bi-level disc replacement procedure did not have any adverse mechanical effects on adjacent levels.

In silico evaluation of a new composite disc substitute with a L3-L5 lumbar spine finite element model

When the intervertebral disc is removed to relieve chronic pain, subsequent segment stabilization should restore the functional mechanics of the native disc. Because of partially constrained motions and the lack of intrinsic rotational stiffness ball-on-socket implants present many disadvantages. Composite disc substitutes mimicking healthy disc structures should be able to assume the role expected for a disc substitute with fewer restrictions than ball-on-socket implants. A biomimetic composite disc prototype including artificial nucleus fibre-reinforced annulus and endplates was modelled as an L4-L5 disc substitute within a L3-L5 lumbar spine finite element model. Different device updates, i.e. changes of material properties fibre distributions and volume fractions and nucleus placements were proposed. Load- and displacement-controlled rotations were simulated with and without body weight applied. The original prototype reduced greatly the flexibility of the treated segment with significant adjacent level effects under displacement-controlled or hybrid rotations. Device updates allowed restoring large part of the global axial and sagittal rotational flexibility predicted with the intact model. Material properties played a major role, but some other updates were identified to potentially tune the device behaviour against specific motions. All device versions altered the coupled intersegmental shear deformations affecting facet joint contact through contact area displacements. Loads in the bony endplates adjacent to the implants increased as the implant stiffness decreased but did not appear to be a strong limitation for the implant biomechanical and mechanobiological functionality. In conclusion, numerical results given by biomimetic composite disc substitutes were encouraging with greater potential than that offered by ball-on-socket implants.

Spondylolysis originates in the ventral aspect of the pars interarticularis: a clinical and biomechanical study

Lumbar spondylolysis is a stress fracture of the pars interarticularis. We have evaluated the site of origin of the fracture clinically and biomechanically. Ten adolescents with incomplete stress fractures of the pars (four bilateral) were included in our study. There were seven boys and three girls aged between 11 and 17 years. The site of the fracture was confirmed by axial and sagittal reconstructed CT. The maximum principal tensile stresses and their locations in the L5 pars during lumbar movement were calculated using a three-dimensional finite-element model of the L3-S1 segment. In all ten patients the fracture line was seen only at the caudal-ventral aspect of the pars and did not spread completely to the craniodorsal aspect. According to the finite-element analysis, the higher stresses were found at the caudal-ventral aspect in all loading modes. In extension, the stress was twofold higher in the ventral than in the dorsal aspect. Our radiological and biomechanical results were in agreement with our clinical observations.

Effect of multilevel lumbar disc arthroplasty on spine kinematics and facet joint loads in flexion and extension: a finite element analysis

Total disc arthroplasty (TDA) has been successfully used for monosegmental treatment in the last few years. However, multi-level TDA led to controversial clinical results. We hypothesise that: (1) the more artificial discs are implanted, the stronger the increases in spinal mobility and facet joint forces in flexion and extension; (2) deviations from the optimal implant position lead to strong instabilities. A three-dimensional finite element model of the intact L1-L5 human lumbar spine was created. Additionally, models of the L1-L5 region implanted with multiple Charité discs ranging from two to four levels were created. The models took into account the possible misalignments in the antero-posterior direction of the artificial discs. All these models were exposed to an axial compression preload of 500 N and pure moments of 7.5 Nm in flexion and extension. For central implant positions and the loading case extension, a motion increase of 51% for two implants up to 91% for four implants and a facet force increase of 24% for two implants up to 38% for four implants compared to the intact spine were calculated. In flexion, a motion decrease of 5% for two implants up to 8% for four implants was predicted. Posteriorly placed implants led to a better representation of the intact spine motion. However, lift-off phenomena between the core and the implant endplates were observed in some extension simulations in which the artificial discs were anteriorly or posteriorly implanted. The more artificial discs are implanted, the stronger the motion increase in flexion and extension was predicted with respect to the intact condition. Deviations from the optimal implant position lead to unfavourable kinematics, to high facet joint forces and even to lift-off phenomena. Therefore, multilevel TDA should, if at all, only be performed in appropriate patients with good muscular conditions and by surgeons who can ensure optimal implant positions.

The effect of different design concepts in lumbar total disc arthroplasty on the range of motion, facet joint forces and instantaneous center of rotation of a L4-5 segment

Although both unconstrained and constrained core lumbar artificial disc designs are in clinical use, the effect of their design on the range of motion, center of rotations, and facet joint forces is not well understood. It is assumed that the constrained configuration causes a fixed center of rotation with high facet forces, while the unconstrained configuration leads to a moving center of rotation with lower loaded facets. The authors disagree with both assumptions and hypothesized that the two different designs do not lead to substantial differences in the results. For the different implant designs, a three-dimensional finite element model was created and subsequently inserted into a validated model of a L4-5 lumbar spinal segment. The unconstrained design was represented by two implants, the Charité disc and a newly developed disc prosthesis: Slide-Disc. The constrained design was obtained by a modification of the Slide-Disc whereby the inner core was rigidly connected to the lower metallic endplate. The models were exposed to an axial compression preload of 1,000 N. Pure unconstrained moments of 7.5 Nm were subsequently applied to the three anatomical main planes. Except for extension, the models predicted only small and moderate inter-implant differences. The calculated values were close to those of the intact segment. For extension, a large difference of about 45% was calculated between both Slide-Disc designs and the Charité disc. The models predicted higher facet forces for the implants with an unconstrained core compared to an implant with a constrained core. All implants caused a moving center of rotation. Except for axial rotation, the unconstrained and constrained configurations mimicked the intact situation. In axial rotation, only the Slide- Disc with mobile core reproduced the intact behavior. Results partially support our hypothesis and imply that different implant designs do not lead to strong differences in the range of motion and the location of center of rotations. In contrast, facet forces appeared to be strongly dependent on the implant design. However, due to the great variability in facet forces reported in the literature, together with our results, we could speculate that these forces may be more dependent on the individual spine geometry rather than a specific implant design.

Effect of lumbar total disc arthroplasty on the segmental motion and intradiscal pressure at the adjacent level: an in vitro biomechanical study

Object

The artificial disc has been proposed as an alternative to spinal fusion for degenerative disc disease. The primary aim of this biomechanical study was to compare motion and intradiscal pressure (IDP) in a ball-and-socket artificial disc–implanted cadaveric lumbar spine, at the operative and adjacent levels, using a displacement-controlled setup. A secondary comparison involved a “salvage” construct, consisting of pedicle screws (PSs) added in supplementation to the artificial disc construct.

Methods

Ten human cadaveric lumbosacral spines (L2–S1) were potted at L-2 and S-1. All measurements were initially made in the intact spine, followed by implantation of the artificial disc, and finally by the salvage PS condition. For the artificial disc condition, a Maverick ball-and-socket artificial disc was implanted at L4–5. For the PS condition, CD Horizon PSs were placed at L4–5, and the artificial disc was left in place. A displacement-controlled, custom-designed testing apparatus was used to impart motion in the sagittal and coronal planes. Motion at both the implanted level (L4–5) and immediately adjacent levels (L3–4 and L5–S1) was measured. Intradiscal pressure at the rostral adjacent level (L3–4) was also measured. The Tukey test was used for statistical analysis (p < 0.05).

Results

In flexion, no significant difference was noted between the artificial disc and the intact spine with regard to motion at the operative level, motion at adjacent levels, or IDP. In lateral bending, while the artificial disc significantly decreased operative-level motion (p < 0.05), no significant difference was noted in adjacent-level motion or IDP. With regard to extension, the artificial disc significantly increased operative level motion and decreased the rostral adjacent level (L3–4) motion and IDP (p < 0.05). Caudal adjacent-level (L5–S1) motion was not significantly different.

In flexion and lateral bending, the addition of PSs significantly decreased motion at the implanted level when compared with the intact spine and the artificial disc (p < 0.05). This decrease in motion at the index level was associated with a compensatory increase in motion at both adjacent levels in flexion only (p < 0.05), but not in lateral bending (p > 0.05). The IDP was significantly increased in lateral bending but not in flexion. With regard to extension, the significant decrease in IDP that was noted with the artificial disc persisted despite the addition of PSs (p < 0.05).

Conclusions

The artificial disc either maintains or reduces adjacent-level motion and pressure, compared with the intact spine. The addition of PSs to the artificial disc construct leads to significantly increased motion at adjacent levels in flexion and significantly increased IDP in lateral bending. At the operative level, the artificial disc is associated with hypermobility in extension, which is restored to the intact state after the addition of supplementary PSs.

Biomechanical analysis of a disc prosthesis distal to a scoliosis model

STUDY DESIGN

Biomechanical study of bovine spines.

OBJECTIVE

The purpose of this study was to perform a biomechanical test to analyze intervertebral deflections following placement of both 1 and 2 semiconstrained TDRs in the subjacent segments of a long fusion.

SUMMARY OF BACKGROUND DATA

Long-term sequela of long lumbar fusion for scoliosis include adjacent segment disease and flatback syndrome. Total disc replacement (TDR) is a viable option for the treatment of these conditions. Little data has been published regarding the placement of a TDR distal to a scoliosis fusion.

METHODS

Six thoracolumbar bovine spines (T12-S1) were instrumented from T12 to L5, with bilateral pedicle screw fixation at each level. L5-L6 and L6-S1 served as the test levels. One TDR (FlexiCore, Stryker Spine, Allendale, NJ) was initially performed adjacent to the fusion, followed by a subsequent TDR insertion at the last spinal segment. The applied load, total specimen deflection, and local transducer deflections were recorded before and after a TDR at both levels. The results were expressed as a percentage of the intact specimen. Flexion, extension, lateral bending, and torsional deflections were recorded.

RESULTS

There were no significant differences (P > 0.05) in sensor deflection observed at the L5-L6 and L6-S1 levels in the anterior and lateral transducers when compared to intact spines specimens. A similar effect was observed at the L5-L6 and L6-S1 levels in the anterior and lateral transducers when compared to intact or prior L5-L6 and intact L6-S1 constructs.

CONCLUSION

This study has shown that using the FlexiCore system at 1 and/or 2 intervertebral disc spaces caudal to a scoliosis fusion model did not significantly change the sensor deflection at the 2 segments adjacent to a scoliosis fusion construct. Future research will continue to define the clinical setting and patients best suited for management by TDR systems.

Lumbar fusion leads to increases in angular motion and stress across sacroiliac joint: a finite element study

STUDY DESIGN

The assessment of sacrum angular motions and stress across sacroiliac joint (SIJ) articular surfaces using finite element lumbar spine-pelvis model and simulated posterior fusion surgical procedures.

OBJECTIVE

To quantify the increase in sacrum angular motions and stress across SIJ as a function of fused lumbar spine using finite element lumbar spine-pelvis model.

SUMMARY OF BACKGROUND DATA

A review of the literature suggests that for 20% to 30% of spine surgery patients, failed back surgery syndrome as a possible complication. The SIJ might be a contributing factor in failed back surgery syndrome in 29% to 40% of cases. The exact pathomechanism which leads to SIJ pain generation is not well understood. We hypothesized that lumbar spine fusion leads to increased motion or stresses at the SIJ; this alone could be a trigger of the pain syndrome.

METHODS

A finite element model of the lumbar spine-pelvis was used to simulate the posterior fusion at L4-L5, L4-S1, and L5-S1 levels. The magnitude of the sacrum angular motion and average of stresses across SIJ articular surfaces were compared with intact model in flexion, extension, lateral bending, and axial rotation motions.

RESULTS

The computed sacrum angular motions in intact spine, after L4-L5, L5-S1, and L4-S1 fusion gradually increased with maximum value in L4-S1 fusion model. Also, the average stress on SIJ articular surfaces progressively increased from minimum in L4-L5 to maximum in L4-S1 fusion models.

CONCLUSION

The fusion at the lumbar spine level increased motion and stresses at the SIJ. This could be a probable reason for low back pain in patients after lumbar spine fusion procedures.

Influence of different artificial disc kinematics on spine biomechanics

BACKGROUND

There are several different artificial discs for the lumbar spine in clinical use. Though clinically established, little is known about the biomechanical advantages of different disc kinematics.

METHODS

A validated finite element model of the lumbosacral spine was used to compare the results of total disc arthroplasty at level L4/L5 performed by simulating the kinematics of three established artificial disc prostheses (Charité, ProDisc, Activ L). For flexion, extension, lateral bending, and axial torsion, the intervertebral rotations, the locations of the helical axes of rotation, the intradiscal pressures, and the facet joint forces were evaluated at the operated and adjacent levels.

FINDINGS

After insertion of an artificial disc, intervertebral rotation is reduced for flexion and increased for extension, lateral bending, and axial torsion for all studied discs at implant level. The positions of the helical axes are altered especially for lateral bending and axial torsion. Increased facet joint contact forces are predicted for the Charité disc during extension-- influenced by the existence of anterior scar tissue--and for the ProDisc and the Activ L during lateral bending and axial torsion. The studied artificial discs have only a minor effect on the adjacent levels.

INTERPRETATIONS

For some load cases, total disc arthroplasty leads to considerably altered kinematics and increased facet joint contact forces at implant level. The spinal kinematic alterations due to an artificial disc exceed by far the inter-implant differences, while facet joint contact force alterations are strongly implant and load case dependent. The importance of implant kinematics is often overestimated.

Load- and displacement-controlled finite element analyses on fusion and non-fusion spinal implants

This study used finite element (FE) analysis with the load-controlled method (LCM) and the displacement-controlled method (DCM) to examine motion differences at the implant level and adjacent levels between fusion and non-fusion implants. A validated three-dimensional intact (INT) L1—L5 FE model was used. At the L3—L4 level, the INT model was modified to surgery models, including the artificial disc replacement (ADR) of ProDisc II, and the anterior lumbar interbody fusion (ALIF) cage with pedicle screw fixation. The LCM imposed 10 N m moments of four physiological motions and a 150 N preload at the top of L1. The DCM process was in accordance with the hybrid testing protocol. The average percentage changes in the range of motion (ROM) for whole non-operated levels were used to predict adjacent level effects (ALE%). At the implant level, the ALIF model showed similar stability with both control methods. The ADR model using the LCM had a higher ROM than the model using the DCM, especially in extension and torsion. At the adjacent levels, the ALIF model increased ALE% (at least 17 per cent) using the DCM compared with the LCM. The ADR model had an ALE% close to that of the INT model, using the LCM (average within 6 per cent), while the ALE% decreased when using the DCM. The study suggests that both control methods can be adopted to predict the fusion model at the implant level, and similar stabilization characteristics can be found. The LCM will emphasize the effects of the non-fusion implants. The DCM was more clinically relevant in evaluating the fusion model at the adjacent levels. In conclusion, both the LCM and the DCM should be considered in numerical simulations to obtain more realistic data in spinal implant biomechanics.

Effect of an artificial disc on lumbar spine biomechanics: a probabilistic finite element study

The effects of different parameters on the mechanical behaviour of the lumbar spine were in most cases determined deterministically with only one uncertain parameter varied at a time while the others were kept fixed. Thus most parameter combinations were disregarded. The aim of the study was to determine in a probabilistic finite element study how intervertebral rotation, intradiscal pressure, and contact force in the facet joints are affected by the input parameters implant position, implant ball radius, presence of scar tissue, and gap size in the facet joints. An osseoligamentous finite element model of the lumbar spine ranging from L3 vertebra to L5/S1 intervertebral disc was used. An artificial disc with a fixed center of rotation was inserted at level L4/L5. The model was loaded with pure moments of 7.5 Nm to simulate flexion, extension, lateral bending, and axial torsion. In a probabilistic study the implant position in anterior-posterior (ap) and in lateral direction, the radius of the implant ball, and the gap size of the facet joint were varied. After implanting an artificial disc, scar tissue may develop, replacing the anterior longitudinal ligament. Thus presence and absence of scar tissue were also simulated. For each loading case studied, intervertebral rotations, intradiscal pressures and contact forces in the facet joints were calculated for 1,000 randomized input parameter combinations in order to determine the probable range of these output parameters. Intervertebral rotation at implant level varies strongly for different combinations of the input parameters. It is mainly affected by gap size, ap-position and implant ball radius for flexion, by scar tissue and implant ball radius for extension and lateral bending, and by gap size and implant ball radius for axial torsion. For extension, intervertebral rotation at implant level varied between 1.4 degrees and 7.5 degrees . Intradiscal pressure in the adjacent discs is only slightly affected by all input parameters. Contact forces in the facet joints at implant level vary strongly for the different combinations of the input parameters. For flexion, forces are 0 in 63% of the cases, but for small gap sizes and large implant ball radii they reach values of up to 533 N. Similar results are found for extension with a maximum predicted force of 560 N. Here the forces are mainly influenced by gap size, implant ball radius and scar tissue. The forces vary between 0 and 300 N for lateral bending and between 0 and 200 N for axial torsion. The parameters that have the greatest effect in both loading cases are the same as those for extension. Intervertebral rotation and contact force in the facet joints depend strongly on the input parameters studied. The probabilistic study shows a large variation of the results and likelihood of certain values. Clinical studies will be required to show whether or not there is a strong correlation of parameter combinations that cause high facet joint forces and low back pain after total disc replacement.

Finite element application in implant research for treatment of lumbar degenerative disc disease

Surgical treatment for disc degeneration can be roughly grouped as fusion, disc replacement and dynamic stabilization. The clinical efficacy and biomechanical features of the implants used for disc degenerations can be evaluated through short- or long-term follow up observation, in vitro and in vivo experiments and computational simulations. Finite element models are already making an important contribution to our understanding of the spine and its components. Models are being used to reveal the biomechanical function of the spine and its behavior when healthy, diseased or damaged. They are also providing support in the design and application of spinal instrumentation. The article reviewed the most recent studies in the application of FE models that address the issue of implant research for treatment of low back pain. The published studies were grouped and reviewed thoroughly based on the function of implants investigated. The considerations of the finite element analysis in these studies were further discussed.

Analysis of post-operative pain patterns following total lumbar disc replacement: results from fluoroscopically guided spine infiltrations

Although a variety of biomechanical laboratory investigations and radiological studies have highlighted the potential problems associated with total lumbar disc replacement (TDR), no previous study has performed a systematic clinical failure analysis. The aim of this study was to identify the post-operative pain sources, establish the incidence of post-operative pain patterns and investigate the effect on post-operative outcome with the help of fluoroscopically guided spine infiltrations in patients from an ongoing prospective study with ProDisc II. Patients who reported unsatisfactory results at any of the FU-examinations received fluoroscopically guided spine infiltrations as part of a semi-invasive diagnostic and conservative treatment program. Pain sources were identified in patients with reproducible (> or =2x) significant (50-75%) or highly significant (75-100%) pain relief. Results were correlated with outcome parameters visual analogue scale (VAS), Oswestry disability index (ODI) and the subjective patient satisfaction rate. From a total of 175 operated patients with a mean follow-up (FU) of 29.3 months (range 12.2-74.9 months), n = 342 infiltrations were performed in n = 58 patients (33.1%) overall. Facet joint pain, predominantly at the index level (86.4%), was identified in n = 22 patients (12.6%). The sacroiliac joint was a similarly frequent cause of post-operative pain (n = 21, 12.0%). Pain from both structures influenced all outcome parameters negatively (P < 0.05). Patients with an early onset of pain (< or =6 months) were 2-5x higher at risk of developing persisting complaints and unsatisfactory outcome at later FU-stages in comparison to the entire study cohort (P < 0.05). The level of TDR significantly influenced post-operative outcome. Best results were achieved for the TDRs above the lumbosacral junction at L4/5 (incidence of posterior joint pain 14.8%). Inferior outcome and a significantly higher incidence of posterior joint pain were observed for TDR at L5/S1 (21.6%) and bisegmental TDR at L4/5/S1 (33.3%), respectively. Lumbar facet and/or ISJ-pain are a frequent and currently underestimated source of post-operative pain and the most common reasons for unsatisfactory results following TDR. Further failure-analysis studies are required and adequate salvage treatment options need to be established with respect to the underlying pathology of post-operative pain. The question as to whether or not TDR will reduce the incidence of posterior joint pain, which has been previously attributed to lumbar fusion procedures, remains unanswered. Additional studies will have to investigate whether TDR compromises the index-segment in an attempt to avoid adjacent segment degeneration.

The effect of removing the lateral part of the pars interarticularis on stress distribution at the neural arch in lumbar foraminal microdecompression at L3-L4 and L4-L5: anatomic and finite element investigations

STUDY DESIGN

The assessment of L3 and L4 pars interarticularis thickness and finite element analysis of stress distribution across L3 and L4 pars interarticularis.

OBJECTIVE

To quantify the morphology of the region of the L3 and L4 pars interarticularis and to assess the stress increase as a function of access size using the finite element lumbar spine model.

SUMMARY OF BACKGROUND DATA

Inadequate decompression and traction of the nerve structures are several causes of the unsatisfactory outcomes in patients after foraminal stenosis decompression and far lateral disc herniation removal by extraforaminal exposure. Enlarging the access of the foraminal exposure by the removal of the lateral aspect of the pars interarticularis may be able to diminish the inadequate decompression and traction of the nerve structures; however, it may lead to increase stress and fracture of the neural arch.

METHODS

We used 15 human cadaver L3 and L4 lumbar vertebrae for measuring the thickness of the pars interarticularis. The ventral and dorsal surfaces were subdivided into 4 equal parts, and the thickness of each part was measured using a digital caliper. An experimentally validated 3-dimensional nonlinear finite element model of the intact L3-S1 segment was used to simulate the lateral removal of one fourth and one half of the L3 and L4 pars interarticularis.

RESULTS

The mean thicknesses of the pars interarticularis showed a gradual increase toward the lateral edge. Finite element model analyses predicted stresses increased to 35% and 40% after removal of one half of the lateral part of the L3 and L4 pars interarticularis, respectively, and were much closer to the intact spine after removal of one fourth of the lateral part of the pars interarticularis.

CONCLUSION

The removal of one fourth of the lateral aspect of the pars interarticularis has minimal influence on the stresses in the remaining L3 and L4 neural arches. The lateral half of the pars has the largest thickness, and its removal leads to considerable stress increases.

Finite element study of matched paired posterior disc implant and dynamic stabilizer (360° motion preservation system)

BACKGROUND

Anterior lumbar disc replacements are used to restore spinal alignment and kinematics of a degenerated segment. Compared to fusion of the segment, disc replacements may prevent adjacent segment degeneration. To resolve some of the deficiencies of anterior lumbar arthroplasty, such as the approach itself, difficulty of revision, and postoperative facet pain, 360° motion preservation systems based on posterior disc and posterior dynamic system (PDS) designs are being pursued. These systems are easier to revise and address all the pain generators in a motion segment, including the nerves, facets, and disc. However, biomechanics of the 360° posterior motion preservation system, including the contributions of the 2 subsystems (disc and PDS), are sparsely reported in the literature.nds.

METHODS

An experimentally validated 3-dimensional finite element model of the ligamentous L3-S1 segment was used to investigate the differences in biomechanical behavior of the lumbar spine. A single-level 360° posterior motion preservation system and its individual components in various orientations were simulated and compared with an intact model. Appropriate posterior surgical procedures were simulated. The PDS, a curved device with male and female components, was attached to the pedicle screws. The finite element models were subjected to 400 N of follower load plus 10Nm moment in extension and flexion.

RESULTS

The PDS restored flexion/extension motion to normal. The artificial disc led to increases in range of motion (ROM) compared with the intact model. ROM for the 360° system at the implanted and adjacent levels were similar to those of the respective intact levels. ROM was similar whether the discs were placed (a) both parallel to the midsagittal plane, (b) both angled 20° to the midsagittal plane, and (c) one at 20° and one parallel to the midsagittal plane. However, the stresses were slightly higher in the nonparallel disc configuration than in the parallel disc configuration, both in flexion and extension modes.

CONCLUSIONS

Posterior disc replacement with PDS restored the kinematics of the spine at all levels to near normal. In addition, placing the discs in a nonparallel configuration with respect to the midsagittal plane does not affect the functionality of the discs compared with parallel placement. Posterior disc replacement alone is not sufficient to restore the segment biomechanics to normal levels.

CLINICAL RELEVANCE

Finite element analysis results show that, unlike implants for fusion, PDS and posterior discs together (360° motion preservation system) are needed to preserve ROM. Such systems will prevent adjacent level degeneration and address pain from various spinal components, including facets.