Restoration of compressive loading properties of lumbar discs with a nucleus implant-a finite element analysis study

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.

An in vitro comparison of three nucleus pulposus removal techniques for partial intervertebral disc replacement: An ultra-high resolution MRI study

Background

Nuclectomy, also known as nucleotomy, is a percutaneous surgical procedure performed to remove nucleus material from the center of the disc. Multiple techniques have been considered to perform a nuclectomy, however, the advantages and disadvantages of each are not well understood.

Aims

This in vitro biomechanical investigation on human cadaveric specimens aimed to quantitatively compare three nuclectomy techniques performed using an automated shaver, rongeurs, and laser.

Material & Methods

Comparisons were made in terms of mass, volume and location of material removal, changes in disc height, and stiffness. Fifteen vertebra-disc-vertebra lumbar specimens were acquired from six donors (40 ± 13 years) and split into three groups. Before and after nucleotomy axial mechanical tests were performed and T2-weighted 9.4T MRIs were acquired for each specimen.

Results

When using the automated shaver and rongeurs similar volumes of disc material were removed (2.51 ± 1.10% and 2.76 ± 1.39% of the total disc volume, respectively), while considerably less material was removed using the laser (0.12 ± 0.07%). Nuclectomy using the automated shaver and rongeurs significantly reduced the toe-region stiffness (p = 0.036), while the reduction in the linear region stiffness was significant only for the rongeurs group (p = 0.011). Post-nuclectomy, 60% of the rongeurs group specimens showed changes in the endplate profile while 40% from the laser group showed subchondral marrow changes.

Discussion

From the MRIs, homogeneous cavities were seen in the center of the disc when using the automated shaver. When using rongeurs, material was removed non-homogeneously both from the nucleus and annulus regions. Laser ablation formed small and localized cavities suggesting that the technique is not suitable to remove large volumes of material unless it is developed and optimized for this application.

Conclusion

The results demonstrate that both rongeurs and automated shavers can be used to remove large volumes of NP material but the reduced risk of collateral damage to surrounding tissues suggests that the automated shaver may be more suitable.

Phase 1 evaluation of an elastomeric nucleus pulposus device as an option to augment disc at microdiscectomy: Experimental results from biomechanical and biocompatibility testing and first in human

Objective

Whilst microdiscectomy is an excellent reliever of pain for recalcitrant lumbar disc herniation (LDH), it has a high failure rate over time due to the ensuing reduction in mechanical stabilization and support of the spine. One option is to clear the disc and replace it with a nonhygroscopic elastomer. Here, we present the evaluation of biomechanical and biological behavior of a novel elastomeric nucleus device (Kunovus disc device [KDD]), consisting of a silicone jacket and a two-part in situ curing silicone polymer filler.

Materials and Methods

ISO 10993 and American Society for Testing and Materials (ASTM) standards were used to evaluate the biocompatibility and mechanics of KDD. Sensitization, intracutaneous reactivity, acute systemic toxicity, genotoxicity, muscle implantation study, direct contact matrix toxicity assay, and cell growth inhibition assay were performed. Fatigue test, static compression creep testing, expulsion testing, swell testing, shock testing, and aged fatigue testing were conducted to characterize the mechanical and wear behavior of the device. Cadaveric studies to develop a surgical manual and evaluate feasibility were conducted. Finally, a first-in-human implantation was conducted to complete the proof of principle.

Results

The KDD demonstrated exceptional biocompatibility and biodurability. Mechanical tests showed no Barium-containing particles in fatigue test, no fracture of nucleus in static compression creep testing, no extrusion and swelling, and no material failure in shock and aged fatigue testing. Cadaver training sessions showed that KDD was deemed implantable during microdiscectomy procedures in a minimally invasive manner. Following IRB approval, the first implantation in a human showed no intraoperative vascular and neurological complications and demonstrated feasibility. This successfully completed Phase 1 development of the device.

Conclusion

The elastomeric nucleus device may mimic native disc behavior in mechanical tests, offering an effective way for treating LDH by way of Phase 2 and subsequent clinical trials or postmarket surveillance in the future.

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.

Finite Element Analysis of a Bionate Ring-Shaped Customized Lumbar Disc Nucleus Prosthesis

Study design: Biomechanical study of a nucleus replacement with a finite element model. Objective: To validate a Bionate 80A ring-shaped nucleus replacement. Methods: The ANSYS lumbar spine model made from lumbar spine X-rays and magnetic resonance images obtained from cadaveric spine specimens were used. All materials were assumed homogeneous, isotropic, and linearly elastic. We studied three options: intact spine, nucleotomy, and nucleus implant. Two loading conditions were evaluated at L₃-L₄, L₄-L₅, and L₅-S₁ discs: a 1000 N axial compression load and this load after the addition of 8 Nm flexion moment in the sagittal plane plus 8 Nm axial rotation torque. Results: Maximum nucleus implant axial compression stresses in the range of 16–34 MPa and tensile stress in the range of 5–16 MPa, below Bionate 80A resistance were obtained. Therefore, there is little risk of permanent implant deformation or severe damage under normal loading conditions. Nucleotomy increased segment mobility, zygapophyseal joint and end plate pressures, and annulus stresses and strains. All these parameters were restored satisfactorily by nucleus replacement but never reached the intact status. In addition, annulus stresses and strains were lower with the nucleus implant than in the intact spine under axial compression and higher under complex loading conditions. Conclusions: Under normal loading conditions, there is a negligible risk of nucleus replacement, permanent deformation or severe damage. Nucleotomy increased segmental mobility, zygapophyseal joint pressures, and annulus stresses and strains. Nucleus replacement restored segmental mobility and zygapophyseal joint pressures close to the intact spine. End plate pressures were similar for the intact and nucleus implant conditions under both loading modes. Manufacturing customized nucleus implants is considered feasible, as satisfactory biomechanical performance is confirmed.

Intervertebral Disc Mechanics With Nucleotomy: Differences Between Simple and Dual Loading

Painful herniated discs are treated surgically by removing extruded nucleus pulposus (NP) material (nucleotomy). NP removal through enzymatic digestion is also commonly performed to initiate degenerative changes to study potential biological repair strategies. Experimental and computational studies have shown a decrease in disc stiffness with nucleotomy under single loading modalities, such as compression-only or bending-only loading. However, studies that apply more physiologically relevant loading conditions, such as compression in combination with bending or torsion, have shown contradicting results. We used a previously validated bone-disc-bone finite element model (Control) to create a Nucleotomy model to evaluate the effect of dual loading conditions (compression with torsion or bending) on intradiscal deformations. While disc joint stiffness decreased with nucleotomy under single loading conditions, as commonly reported in the literature, dual loading resulted in an increase in bending stiffness. More specifically, dual loading resulted in a 40% increase in bending stiffness under flexion and extension and a 25% increase in stiffness under lateral bending. The increase in bending stiffness was due to an increase and shift in compressive stress, where peak stresses migrated from the NP-annulus interface to the outer annulus. In contrast, the decrease in torsional stiffness was due to greater fiber reorientation during compression. In general, large radial strains were observed with nucleotomy, suggesting an increased risk for delamination or degenerative remodeling. In conclusion, the effect of nucleotomy on disc mechanics depends on the type and complexity of applied loads.

Computational Challenges in Tissue Engineering for the Spine

This paper deals with a brief review of the recent developments in computational modelling applied to innovative treatments of spine diseases. Additionally, it provides a perspective on the research directions expected for the forthcoming years. The spine is composed of distinct and complex tissues that require specific modelling approaches. With the advent of additive manufacturing and increasing computational power, patient-specific treatments have moved from being a research trend to a reality in clinical practice, but there are many issues to be addressed before such approaches become universal. Here, it is identified that the major setback resides in validation of these computational techniques prior to approval by regulatory agencies. Nevertheless, there are very promising indicators in terms of optimised scaffold modelling for both disc arthroplasty and vertebroplasty, powered by a decisive contribution from imaging methods.

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.

Application of Finite Element Analysis for Investigation of Intervertebral Disc Degeneration: from Laboratory to Clinic

Due to the ethical concern and inability to detect inner stress distributions of intervertebral disc (IVD), traditional methods for investigation of intervertebral disc degeneration (IVDD) have significant limitations. Many researchers have demonstrated that finite element analysis (FEA) is an effective tool for the research of IVDD. However, the specific application of FEA for investigation of IVDD has not been systematically elucidated before. In the present review, we summarize the current finite element models (FEM) used for the investigation of IVDD, including the poroelastic nonlinear FEM, diffusive-reactive theory model and cell-activity coupled mechano-electrochemical theory model. We further elaborate the use of FEA for the research of IVDD pathogenesis especially for nutrition and biomechanics associated etiology, and the biological, biomechanical and clinical influences of IVDD. In addition, the application of FEA for evaluation and exploration of various treatments for IVDD is also elucidated. We conclude that FEA is an excellent technique for research of IVDD, which could be used to explore the etiology, biology and biomechanics of IVDD. In the future, FEA may help us to achieve the goal of individualized precision therapy.

Plasma processing of PDMS based spinal implants for covalent protein immobilization, cell attachment and spreading

PDMS is widely used for prosthetic device manufacture. Conventional ion implantation is not a suitable treatment to enhance the biocompatibility of poly dimethyl siloxane (PDMS) due to its propensity to generate a brittle silicon oxide surface layer which cracks and delaminates. To overcome this limitation, we have developed new plasma based processes to balance the etching of carbon with implantation of carbon from the plasma source. When this carbon was implanted from the plasma phase it resulted in a surface that was structurally similar and intermixed with the underlying PDMS material and not susceptible to delamination. The enrichment in surface carbon allowed the formation of carbon based radicals that are not present in conventional plasma ion immersion implantation (PIII) treated PDMS. This imparts the PDMS surfaces with covalent protein binding capacity that is not observed on PIII treated PDMS. The change in surface energy preserved the function of bound biomolecules and enhanced the attachment of MG63 osteosarcoma cells compared to the native surface. The attached cells, an osteoblast interaction model, showed increased spreading on the treated over untreated surfaces. The carbon-dependency for these beneficial covalent protein and cell linkage properties was tested by incorporating carbon from a different source. To this end, a second surface was produced where carbon etching was balanced against implantation from a thin carbon-based polymer coating. This had similar protein and cell-binding properties to the surfaces generated with carbon inclusion in the plasma phase, thus highlighting the importance of balancing carbon etching and deposition. Additionally, the two effects of protein linkage and bioactivity could be combined where the cell response was further enhanced by covalently tethering a biomolecule coating, as exemplified here with the cell adhesive protein tropoelastin. Providing a balanced carbon source in the plasma phase is applicable to prosthetic device fabrication as illustrated using a 3-dimensional PDMS balloon prosthesis for spinal implant applications. Consequently, this study lays the groundwork for effective treatments of PDMS to selectively recruit cells to implantable PDMS fabricated biodevices.

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.

Linear psoriasis following the typical distribution of the sciatic nerve

BACKGROUND

Some studies suggest that the nervous system plays a role in the onset of psoriasis and psoriasis flares including the symmetry of lesions, sparing of denervated skin and the role of stress in inducing lesions.

MAIN OBSERVATIONS

We describe an unusual case of psoriasis occurring in the same distribution as sciatic pain from a prolapsed intervertebral disc. The patient, a 45-year-old man with plaque psoriasis was treated with ustekinumab for 104 weeks, at a standard dose. During the eight month of therapy he developed an asymptomatic linear eruption on the left lower extremity along the distribution of the sciatic nerve. On examination, erythematous scaly plaques were noted. Histopathology confirmed the diagnosis of psoriasis. The treatment was continued and clobetasol proprionate 0.05% cream was added. At week 12 after the eruption, the patient reported a pain radiating through the buttock and posterior left leg during jogging. Magnetic resonance imaging showed lumbar disc herniation with compression of the L5-S1 spinal nerve roots. The patient stopped running and the psoriasis spontaneously receded, in a slow but complete fashion, without any local treatment.

CONCLUSION

There is substantial evidence that nerves play a key role in the pathogenesis of psoriasis. We hypothesized that local TNF-alpha, neuropeptides and nerve growth factor, which are produced by nerve root compression, played a critical role in this case of psoriasis onset in an area of pain from a bulging lumbar intervertebral disc. To our knowledge, a correlation of psoriasis and nerve root compression has not been described previously.

Trend of the incidence of lumbar disc herniation: decreasing with aging in the elderly

BACKGROUND

Compelling evidence has shown that the incidence of lumbar disc herniation (LDH) increases with age. In this study, retrospective clinical analysis of 601 cases of LDH has been conducted to investigate the role of age in the incidence of LDH in the elderly. The aim of the study is to investigate the relationship between the process of aging and the occurrence of LDH in old adults.

METHODS

Clinical cases (n = 601) of LDH were retrospectively analyzed.

RESULTS

The imaging examination with computed tomography and/or magnetic resonance imaging showed the occurrence of degeneration in LDH patients over 65 years of age. The most common site of LDH is toward the bottom of the spine at L4-L5 and/or L5-S1. The incidence of LDH drops with age in the elderly, especially after the age of 80 years. There is an obvious decrease in LDH in the elderly female.

CONCLUSION

A decreasing incidence of LDH with aging occurs in the elderly. This investigation indicates that aging is not a contributor to the performance of LDH in the elderly although the incidence of LDH is proportional to age.

Finite element study of human lumbar disc nucleus replacements

Currently, there are a number of nucleus replacements under development. The important concern is how well these implants duplicate the mechanical function of the native nucleus. This finite element model study aimed to investigate the influence of different nucleus replacements on the mechanical response of the disc. Models included partial, full, over-sized, partially saturated, elastic and poroelastic solid replacements. Over-sized nucleus replacements up to 25% yielded results that were comparable to those in the intact state. Differences were much greater in cases with under-sized nucleus replacements. The effect was most pronounced for the 75% under-sized replacement that resembled the condition with a full nucleotomy. Nucleus implants with elastic properties substantially altered load transmission when 10% under-sized and over-sized replacements were considered. Compared to intact, the under-sized implants should be avoided when using biphasic materials with properties similar to the native nucleus, whereas for elastic replacements both under- and over-sized implants should not be used.

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.