Effect of nucleus replacement device properties on lumbar spine mechanics

Injectable hydrogel induces regeneration of naturally degenerate human intervertebral discs in a loaded organ culture model

Low back pain resulting from disc degeneration is a leading cause of disability worldwide. However, to date few therapies target the cause and fail to repair the intervertebral disc (IVD). This study investigates the ability of an injectable hydrogel (NPgel), to inhibit catabolic protein expression and promote matrix expression in human nucleus pulposus (NP) cells within a tissue explant culture model isolated from degenerate discs. Furthermore, the injection capacity of NPgel into naturally degenerate whole human discs, effects on mechanical function, and resistance to extrusion during loading were investigated. Finally, the induction of potential regenerative effects in a physiologically loaded human organ culture system was investigated following injection of NPgel with or without bone marrow progenitor cells. Injection of NPgel into naturally degenerate human IVDs increased disc height and Young's modulus, and was retained during extrusion testing. Injection into cadaveric discs followed by culture under physiological loading increased MRI signal intensity, restored natural biomechanical properties and showed evidence of increased anabolism and decreased catabolism with tissue integration observed. These results provide essential proof of concept data supporting the use of NPgel as an injectable therapy for disc regeneration. STATEMENT OF SIGNIFICANCE: Low back pain resulting from disc degeneration is a leading cause of disability worldwide. However, to date few therapies target the cause and fail to repair the intervertebral disc. This study investigated the potential regenerative properties of an injectable hydrogel system (NPgel) within human tissue samples. To mimic the human in vivo conditions and the unique IVD niche, a dynamically loaded intact human disc culture system was utilised. NPgel improved the biomechanical properties, increased MRI intensity and decreased degree of degeneration. Furthermore, NPgel induced matrix production and decreased catabolic factors by the native cells of the disc. This manuscript provides evidence for the potential use of NPgel as a regenerative biomaterial for intervertebral disc degeneration.

Mechanical characterization and design of biomaterials for nucleus pulposus replacement and regeneration

Biomaterials for nucleus pulposus (NP) replacement and regeneration have great potential to restore normal biomechanics in degenerated intervertebral discs following nucleotomy. Mechanical characterizations are essential for assessing the efficacy of biomaterial implants for clinical applications. While traditional compression tests are crucial to quantify various modulus values, relaxation behaviors and fatigue resistance, rheological measurements should also be conducted to investigate the viscoelastic properties, injectability, and overall stability upon deformation. To recapitulate the physiological in vivo environment, the use of spinal models is necessary to evaluate the risk of implant extrusion and the restoration of biomechanics under different loading conditions. When designing devices for NP replacement, injectable materials are ideal to fully fill the nucleus cavity and prevent implant migration. In addition to achieving biocompatibility and desirable mechanical characteristics, biomaterial implants should be optimized to avoid implant extrusion or re-herniation post-operatively. This review discusses the most commonly used testing protocols for assessing mechanical properties of biomaterial implants and serves as reference material for enabling researchers to characterize NP implants through a unified approach whereby newly developed biomaterials may be compared and contrasted to existing devices.

Quantifying internal intervertebral disc strains to assess nucleus replacement device designs: a digital volume correlation and ultra-high-resolution MRI study

Introduction: Nucleus replacement has been proposed as a treatment to restore biomechanics and relieve pain in degenerate intervertebral discs (IVDs). Multiple nucleus replacement devices (NRDs) have been developed, however, none are currently used routinely in clinic. A better understanding of the interactions between NRDs and surrounding tissues may provide insight into the causes of implant failure and provide target properties for future NRD designs. The aim of this study was to non-invasively quantify 3D strains within the IVD through three stages of nucleus replacement surgery: intact, post-nuclectomy, and post-treatment. Methods: Digital volume correlation (DVC) combined with 9.4T MRI was used to measure strains in seven human cadaveric specimens (42 ± 18 years) when axially compressed to 1 kN. Nucleus material was removed from each specimen creating a cavity that was filled with a hydrogel-based NRD. Results: Nucleus removal led to loss of disc height (12.6 ± 4.4%, p = 0.004) which was restored post-treatment (within 5.3 ± 3.1% of the intact state, p > 0.05). Nuclectomy led to increased circumferential strains in the lateral annulus region compared to the intact state (-4.0 ± 3.4% vs. 1.7 ± 6.0%, p = 0.013), and increased maximum shear strains in the posterior annulus region (14.6 ± 1.7% vs. 19.4 ± 2.6%, p = 0.021). In both cases, the NRD was able to restore these strain values to their intact levels (p ≥ 0.192). Discussion: The ability of the NRD to restore IVD biomechanics and some strain types to intact state levels supports nucleus replacement surgery as a viable treatment option. The DVC-MRI method used in the present study could serve as a useful tool to assess future NRD designs to help improve performance in future clinical trials.

Bionate® nucleus disc replacement: bench testing comparing two different designs

BACKGROUND

Intervertebral disc nucleus degeneration initiates a degenerative cascade and can induce chronic low back pain. Nucleus replacement aims to replace the nucleus while the annulus is still intact. Over time, several designs have been introduced, but the definitive solution continues to be elusive. Therefore, we aimed to create a new nucleus replacement that replicates intact intervertebral disc biomechanics, and thus has the potential for clinical applications.

MATERIALS AND METHODS

Two implants with an outer ring and one (D2) with an additional midline strut were compared. Static and fatigue tests were performed with an INSTRON 8874 following the American Society for Testing and Materials F2267-04, F2346-05, 2077-03, D2990-01, and WK4863. Implant stiffness was analyzed at 0-300 N, 500-2000 N, and 2000-6000 N and implant compression at 300 N, 1000 N, 2000 N, and 6000 N. Wear tests were performed following ISO 18192-1:2008 and 18192-2:2010. GNU Octave software was used to calculate movement angles and parameters. The statistical analysis package R was used with the Deducer user interface. Statistically significant differences between the two designs were analyzed with ANOVA, followed by a post hoc analysis.

RESULTS

D1 had better behavior in unconfined compression tests, while D2 showed a "jump." D2 deformed 1 mm more than D1. Sterilized implants were more rigid and deformed less. Both designs showed similar behavior under confined compression and when adding shear. A silicone annulus minimized differences between the designs. Wear under compression fatigue was negligible for D1 but permanent for D2. D1 suffered permanent height deformation but kept its width. D2 suffered less height loss than D1 but underwent a permanent width deformation. Both designs showed excellent responses to compression fatigue with no breaks, cracks, or delamination. At 10 million cycles, D2 showed 3-times higher wear than D1. D1 had better and more homogeneous behavior, and its wear was relatively low. It showed good mechanical endurance under dynamic loading conditions, with excellent response to axial compression fatigue loading without functional failure after long-term testing.

CONCLUSION

D1 performed better than D2. Further studies in cadaveric specimens, and eventually in a clinical setting, are recommended. Level of evidence 2c.

Bionate Lumbar Disc Nucleus Prosthesis: Biomechanical Studies in Cadaveric Human Spines

DESIGN

cadaveric spine nucleus replacement study.

OBJECTIVE

determining Bionate 80A nucleus replacement biomechanics in cadaveric spines.

METHODS

in cold preserved spines, with ligaments and discs intact, and no muscles, L₃-L₄, L₄-L₅, and L₅-S₁ nucleus implantation was done. Differences between customized and overdimensioned implants were compared. Flexion, extension, lateral bending, and torsion were measured in the intact spine, nucleotomy, and nucleus implantation specimens. Increasing load or bending moment was applied four times at 2, 4, 6, and 8 Nm, twice in increasing mode and twice in decreasing mode. Spine motion was recorded using stereophotogrammetry. Expulsion tests: cyclic compression of 50-550 N for 50,000 cycles, increasing the load until there was extreme flexion, implant extrusion, or anatomical structure collapse. Subsidence tests were done by increasing the compression to 6000 N load.

RESULTS

nucleotomy increased the disc mobility, which remained unchanged for the adjacent upper level but increased for the lower adjacent one, particularly in lateral bending and torsion. Nucleus implantation, compared to nucleotomy, reduced disc mobility except in flexion-extension and torsion, but intact mobility was no longer recovered, with no effect on upper or lower adjacent segments. The overdimensioned implant, compared to the customized implant, provided equal or sometimes higher mobility. Lamina, facet joint, and annulus removal during nucleotomy caused more damaged than that restored by nucleus implantation. No implant extrusion was observed under compression loads of 925-1068 N as anatomical structures collapsed before. No subsidence or vertebral body fractures were observed under compression loads of 6697.8-6812.3 N.

CONCLUSIONS

nucleotomized disc and L₁-S₁ mobility increased moderately after cadaveric spine nucleus implantation compared to the intact status, partly due to operative anatomical damage. Our implant had shallow expulsion and subsidence risks.

Development of a finite element lumbar spine model to predict intervertebral disc herniation risk factors

The objective of the current study was to develop a lumbar motion segment FE model that predicts disc herniation risk. The posterolateral nucleus extrusion force and disc pressure increased as the amount of flexion and magnitude of compression was increased in all loading scenarios. The nucleus extrusion force and posterior stress in the annulus both increased when exposed to a combination of compression and flexion. Results of the current study confirmed the authors hypothesis that the model would accurately predict herniation risk when exposed to a biomechanical environment known to cause herniations.

Effect of Spiral Nucleus Implant Parameters on the Compressive Biomechanics of Lumbar Intervertebral Disc

OBJECTIVE

To determine the effect of spiral nucleus implant parameters on the biomechanical behavior of the lumbar intervertebral disc after nucleus replacement under compressive loading.

METHODS

A finite element (FE) model of nucleus replacement in the L4-5 intervertebral disc was constructed. The effects of a spiral implant parameters, such as elasticity, size, and friction property, on the biomechanical behavior of the disc under a compressive load were determined. The effect of an implant with a sharp edge on disc biomechanics was also examined. The stress distribution and contact pressure on the endplate and AF, axial stiffness of disc, and annular bulge of the nucleus replacement models were investigated.

RESULTS

Axial stiffness, annular bulge, and contact pressure were all insensitive to friction properties. Insertion of the spiral implant reversed the changes in the AF and endplates due to the removal of the nucleus. There was a positive correlation between axial stiffness and elasticity with implant size. Annular bulge was positively correlated with size but negatively correlated with elasticity. Compared with the base model, the implant with a sharp edge caused a decrease in disc axial stiffness but an increase in contact pressure on the AF in an annular bulge in the sagittal and coronal axis, respectively.

CONCLUSIONS

A spiral implant may provide similar biomechanical behavior as a normal disc during compressive loading, with an optimal modulus of approximately 7 MPa. The spiral implant should fully conform to the nucleus cavity during replacement for the best biomechanical results.

Finite Element Study of a Lumbar Intervertebral Disc Nucleus Replacement Device

Nucleus replacement technologies are a minimally invasive alternative to spinal fusion and total disc replacement that have the potential to reduce pain and restore motion for patients with degenerative disc disease. Finite element modeling can be used to determine the biomechanics associated with nucleus replacement technologies. The current study focuses on a new nucleus replacement device designed as a conforming silicone implant with an internal void. A validated finite element model of the human lumbar L3-L4 motion segment was developed and used to investigate the influence of the nucleus replacement device on spine biomechanics. In addition, the effect of device design changes on biomechanics was determined. A 3D, L3-L4 finite element model was constructed from medical imaging data. Models were created with the normal intact nucleus, the nucleus replacement device, and a solid silicone implant. Probabilistic analysis was performed on the normal model to provide quantitative validation metrics. Sensitivity analysis was performed on the silicone Shore A durometer of the device. Models were loaded under axial compression followed by flexion/extension, lateral bending, or axial rotation. Compressive displacement, endplate stresses, reaction moment, and annulus stresses were determined and compared between the different models. The novel nucleus replacement device resulted in similar compressive displacement, endplate stress, and annulus stress and slightly higher reaction moment compared with the normal nucleus. The solid implant resulted in decreased displacement, increased endplate stress, decreased annulus stress, and decreased reaction moment compared with the novel device. With increasing silicone durometer, compressive displacement decreased, endplate stress increased, reaction moment increased, and annulus stress decreased. Finite element analysis was used to show that the novel nucleus replacement device results in similar biomechanics compared with the normal intact nucleus.

Enhanced osteogenic potential of mesenchymal stem cells from cortical bone: a comparative analysis

Introduction

Mesenchymal stem cells (MSCs) hold great promise for regenerative therapies in the musculoskeletal system. Although MSCs from bone marrow (BM-MSCs) and adipose tissue (AD-MSCs) have been extensively characterized, there is still debate as to the ideal source of MSCs for tissue-engineering applications in bone repair.

Methods

MSCs were isolated from cortical bone fragments (CBF-MSCs) obtained from patients undergoing laminectomy, selected by fluorescence-activated cell sorting analysis, and tested for their potential to undergo mesodermic differentiation. CBF-MSCs were then compared with BM-MSCs and AD-MSCs for their colony-forming unit capability and osteogenic potential in both normoxia and hypoxia. After 2 and 4 weeks in inducing media, differentiation was assessed qualitatively and quantitatively by the evaluation of alkaline phosphatase (ALP) expression and mineral deposition (Von Kossa staining). Transcriptional activity of osteoblastogenesis-associated genes (Alp, RUNX2, Spp1, and Bglap) was also analyzed.

Results

The cortical fraction of the bone contains a subset of cells positive for MSC-associated markers and capable of tri-lineage differentiation. The hypoxic conditions were generally more effective in inducing osteogenesis for the three cell lines. However, at 2 and 4 weeks, greater calcium deposition and ALP expression were observed in both hypoxic and normoxic conditions in CBF-MSCs compared with AD- and BM-MSCs. These functional observations were further corroborated by gene expression analysis, which showed a significant upregulation of Bglap, Alp, and Spp1, with a 22.50 (±4.55)-, 46.56 (±7.4)-, 71.46 (±4.16)-fold increase compared with their uninduced counterparts.

Conclusions

This novel population of MSCs retains a greater biosynthetic activity in vitro, which was found increased in hypoxic conditions. The present study demonstrates that quantitative differences between MSCs retrieved from bone marrow, adipose, and the cortical portion of the bone with respect to their osteogenic potential exist and suggests the cortical bone as suitable candidate to use for orthopedic tissue engineering and regenerative medicine.

Translational challenges for the development of a novel nucleus pulposus substitute: Experimental results from biomechanical and in vivo studies

Nucleus pulposus replacement therapy could offer a less invasive alternative to restore the function of moderately degenerated intervertebral discs than current potentially destructive surgical procedures. Numerous nucleus pulposus substitutes have already been investigated, to assess their applicability for intradiscal use. Still, the current choice of testing methods often does not lead to efficient translation into clinical application. In this paper, we present the evaluation of a novel nucleus pulposus substitute, consisting of a hydromed core and an electrospun envelope. We performed three mechanical evaluations and an in vivo pilot experiment. Initially, the swelling pressure of the implant was assessed in confined compression. Next, we incorporated the implant into mechanically damaged caprine lumbar intervertebral discs to determine biomechanical segment behaviour in bending and torsion. Subsequently, segments were serially tested in native, damaged and repaired conditions under dynamic axial compressive loading regimes in a loaded disc culture system. Finally, nucleus pulposus substitutes were implanted in a live goat spine using a transpedicular approach. In confined compression, nucleus pulposus samples as well as implants showed some load-bearing capacity, but the implant exhibited a much lower absolute pressure. In bending and torsion, we found that the nucleus pulposus substitute could partly restore the mechanical response of the disc. During dynamic axial compression in the loaded disc culture system, on the other hand, the implant was not able to recover axial compressive behaviour towards the healthy situation. Moreover, the nucleus pulposus substitutes did not remain in place in the in vivo situation but migrated out of the disc area. From these results, we conclude that implants may mimic native disc behaviour in simple mechanical tests, yet fail in other, more realistic set-ups. Therefore, we recommend that biomaterials for nucleus pulposus replacement be tested in testing modalities of increasing complexity and in their relevant anatomical surroundings, for a more reliable prediction of clinical potential.

Human L3L4 intervertebral disc mean 3D shape, modes of variation, and their relationship to degeneration

Intervertebral disc mechanics are affected by both disc shape and disc degeneration, which in turn each affect the other; disc mechanics additionally have a role in the etiology of disc degeneration. Finite element analysis (FEA) is a favored tool to investigate these relationships, but limited data for intervertebral disc 3D shape has forced the use of simplified or single-subject geometries, with the effect of inter-individual shape variation investigated only in specialized studies. Similarly, most data on disc shape variation with degeneration is based on 2D mid-sagittal images, which incompletely define 3D shape changes. Therefore, the objective of this study was to quantify inter-individual disc shape variation in 3D, classify this variation into independently-occurring modes using a statistical shape model, and identify correlations between disc shape and degeneration. Three-dimensional disc shapes were obtained from MRI of 13 human male cadaver L3L4 discs. An average disc shape and four major modes of shape variation (representing 90% of the variance) were identified. The first mode represented disc axial area and was significantly correlated to degeneration (R(2)=0.44), indicating larger axial area in degenerate discs. Disc height variation occurred in three distinct modes, each also involving non-height variation. The statistical shape model provides an average L3L4 disc shape for FEA that is fully defined in 3D, and makes it convenient to generate a set of shapes with which to represent aggregate inter-individual variation. Degeneration grade-specific shapes can also be generated. To facilitate application, the model is included in this paper׳s supplemental content.

Elastic resistance of the spine: Why does motion preservation surgery almost fail?

Single metamere motility should not be interpreted merely as a movement on the 3 planes but also, and above all, as elastic resistance to dynamic stress on these 3 planes. In the light of this consideration, the aim of motion preservation is to neutralize excessive movements while preserving the physiological biomechanical properties of the metamere involved to interrupt the progression of degenerative processes and to prevent adjacent segment disease. Despite the fact that a myriad of devices have been developed with the purpose of achieving dynamic neutralization of the spine, there are now some doubts regarding the true efficacy of these devices.

Lumbar total disc replacement impingement sensitivity to disc height distraction, spinal sagittal orientation, implant position, and implant lordosis

STUDY DESIGN

A 3-dimensional finite element model of 2 lumbar motion segments (L4-L5 and L5-S1) was used to evaluate the sensitivity of lumbar total disc replacement (TDR) impingement to disc height distraction, spinal sagittal orientation, implant position, and implant lordosis. The models were implanted with a mobile-bearing TDR and exposed to simulated sagittally balanced erect posture.

OBJECTIVE

The objective of this study was to determine the sensitivity of TDR impingement to disc height distraction, implant lordotic angle, implant anterior-posterior position, and spinal orientation relative to the horizon.

SUMMARY OF BACKGROUND DATA

TDR has the potential to replace fusion as the "gold standard" for treatment of painful degenerative disc disease. However, complications after TDR have been associated with device impingement and accelerated polyethylene wear.

METHODS

A previously developed finite element model of the lumbar spine was altered to include implantation of a mobile-bearing TDR. A series of sensitivity analyses was performed to determine impingement risk. Specifically, spinal orientation, disc height distraction, footplate lordotic angle, and anterior-posterior position were evaluated.

RESULTS

Generally, TDR tended to result in an increase in extension rotation and facet contact force during simulated erect posture when compared with the intact models. Impingement risk was sensitive to all of the tested parameters.

CONCLUSION

The data from this study indicate that lumbar mobile-bearing TDR impingement is sensitive to disc height distraction, anterior-posterior position, implant lordosis, and spinal sagittal orientation. TDR impingement risk can be minimized by choosing an implant with an appropriate amount of lordosis, not overdistracting the disc space, and taking care not to place the implant too far anterior or posterior.

The effect of nucleotomy and the dependence of degeneration of human intervertebral disc strain in axial compression

STUDY DESIGN

Biomechanics of human intervertebral discs before and after nucleotomy.

OBJECTIVE

To noninvasively quantify the effect of nucleotomy on internal strains under axial compression in flexion, neutral, and extension positions, and to determine whether the change in strains depended on degeneration.

SUMMARY OF BACKGROUND DATA

Herniation and nucleotomy may accelerate the progression of disc degeneration. Removal of nucleus pulposus (NP) tissue has resulted in altered disc mechanics in vitro, including a decrease in internal pressure and an increase in the deformations at physiologically relevant strains. We recently presented a technique to quantify internal disc strains using magnetic resonance imaging (MRI).

METHODS

Degeneration was quantitatively assessed by the T1ρ relaxation time in the NP. Samples were prepared from human levels L3-L4 and/or L4-L5. A 1000-N compressive load was applied while in the magnetic resonance scanner. Nucleotomy was performed by removing 2 g of NP through the posterior-lateral annulus fibrosus (AF). The discs were rehydrated, reimaged, and retested. The analyzed parameters include axial deformation, AF radial bulge, and strains. RESULTS.: The axial deformation was more compressive after nucleotomy. In the neutral position, the axial deformation after nucleotomy correlated with degeneration (as quantified by T1ρ in the NP), with minimal alteration in nondegenerated discs. Nucleotomy altered the radial displacements and strains in the neutral position, such that the inner AF radial bulge decreased and the radial strains were more tensile in the lateral AF and less tensile in the posterior AF. In the bending loading positions the radial strains were not affected by nucleotomy.

CONCLUSION

Nucleotomy alters the internal radial and axial AF strains in the neutral position, which may leave the AF vulnerable to damage and microfractures. In bending, the effects of nucleotomy were minimal, likely due to more of the applied load being directed over the AF. Some of the nucleotomy effects are modulated by degeneration, where the mechanical effect of nucleotomy was magnified in degenerated discs and may further induce mechanical damage and degeneration.

Update on the pathophysiology of degenerative disc disease and new developments in treatment strategies

Degenerative disc disease (DDD) continues to be a prevalent condition that afflicts populations on a global scale. The economic impact and decreased quality of life primarily stem from back pain and neurological deficits associated with intervertebral disc degeneration. Although much effort has been invested into understanding the etiology of DDD and its relationship to the onset of back pain, this endeavor is a work in progress. The purpose of this review is to provide focused discussion on several areas in which recent advances have been made. Specifically, we have categorized these advances into early, middle, and late phases of age-related or degenerative changes in the disc and into promising minimally invasive treatments, which aim to restore mechanical and biological functions to the disc.

The restoration of lumbar intervertebral disc load distribution: a comparison of three nucleus replacement technologies

STUDY DESIGN

A validated L3-L4 nonlinear finite element model was used to evaluate strain and pressure in the surrounding structures for 4 nucleus replacement technologies.

OBJECTIVE

The objective of the current study was to compare subsidence and anular damage potential between 4 current nucleus replacement technologies. It was hypothesized that a fully conforming nucleus replacement would minimize the risk of both subsidence and anular damage.

SUMMARY OF BACKGROUND DATA

Nucleus pulposus replacements are emerging as a less invasive alternative to total disc replacement and fusion as a solution to degenerative intervertebral discs. Multiple technologies have been developed and are currently undergoing clinical investigation.

METHODS

The testing conditions were applied by excavating the nucleus of the intact model and virtually implanting models representing the various nucleus replacement technologies. The implants consisted of a conforming injectable polyurethane (E = 4 MPa), soft hydrogel (E = 4 MPa), stiff hydrogel (E = 20 MPa), and polyether-etherketone (PEEK) on PEEK articulating designs. The model was exercised in flexion, extension, lateral bending, axial rotation (7.5 Nm with 450 N preload), and compression (1000 N). Vertebral body strain, anular maximum shear strain, endplate contact pressure, anulus-implant contact pressure, and bone remodeling stimulus were reported.

RESULTS

The PEEK implant induced strain maxima in the vertebral bodies with associated endplate contact pressure concentrations. For the PEEK and hydrogel implants, areas of nonconformity with the endplate indicated adjacent bone resorption. Lack of conformity between the implant and inner anulus for the PEEK and hydrogel implants resulted in inward anular bulging with associated increased maximum shear strain. The conforming polyurethane implant maintained outward bulging of the inner anular wall and indicated no bone resorption or stress shielding adjacent to the implant.

CONCLUSION

A fully conforming nucleus replacement resulted in a decreased propensity for subsidence, anular bulging, and further degeneration of the anulus when compared with nonconforming implants.