Nucleus Pulposus Replacement Research Articles

Nucleus Implantation: The Biomechanics of Augmentation Versus Replacement With Varying Degrees of Nucleotomy

Nucleus pulposus replacement and augmentation has been proposed to restore disk mechanics in early stages of degeneration with the option of providing a minimally invasive procedure for pain relief to patients with an earlier stage of degeneration. The goal of this paper is to examine compressive stability of the intervertebral disk after either partial nucleus replacement or nuclear augmentation in the absence of denucleation. Thirteen human cadaver lumbar anterior column units were used to study the effects of denucleation and augmentation on the compressive mechanical behavior of the human intervertebral disk. Testing was performed in axial compression after incremental steps of partial denucleation and subsequent implantation of a synthetic hydrogel nucleus replacement. In a separate set of experiments, the disks were not denucleated but augmented with the same synthetic hydrogel nucleus replacement. Neutral zone, range of motion, and stiffness were measured. The results showed that compressive stabilization of the disk can be re-established with nucleus replacement even for partial denucleation. Augmentation of the disk resulted in an increase in disk height and intradiskal pressure that were linearly related to the volume of polymer implanted. Intervertebral disk instability, evidenced by increased neutral zone and ranges of motion, associated with degeneration can be restored by volume filling of the nucleus pulposus using the hydrogel device presented here.

Translation of an engineered nanofibrous disc-like angle-ply structure for intervertebral disc replacement in a small animal model

Intervertebral disc degeneration has been implicated in the etiology of low back pain; however, the current surgical strategies for treating symptomatic disc disease are limited. A variety of materials have been developed to replace disc components, including the nucleus pulposus (NP), the annulus fibrosus (AF) and their combination into disc-like engineered constructs. We have previously shown that layers of electrospun poly(ε-caprolactone) scaffold, mimicking the hierarchical organization of the native AF, can achieve functional parity with native tissue. Likewise, we have combined these structures with cell-seeded hydrogels (as an NP replacement) to form disc-like angle-ply structures (DAPS). The objective of this study was to develop a model for the evaluation of DAPS in vivo. Through a series of studies, we developed a surgical approach to replace the rat caudal disc with an acellular DAPS and then stabilized the motion segment via external fixation. We then optimized cell infiltration into DAPS by including sacrificial poly(ethylene oxide) layers interspersed throughout the angle-ply structure. Our findings illustrate that DAPS are stable in the caudal spine, are infiltrated by cells from the peri-implant space and that infiltration is expedited by providing additional routes for cell migration. These findings establish a new in vivo platform in which to evaluate and optimize the design of functional disc replacements.

Enhancing tissue repair in annulus fibrosus defects of the intervertebral disc: analysis of a bio‐integrative annulus implant in an in‐vivo ovine model

Annulus fibrosus repair techniques for the intervertebral disc (IVD) address the unsolved problem of reherniation after IVD herniation and might facilitate the development of nucleus pulposus replacement techniques for IVD diseases. This study investigates the suitability of a bio-integrative annulus implant.Standardized box defects were applied to the annulus L3/4 and L4/5 of 16 sheep, followed by randomized insertion of the textile polyglycolic acid/polyvinylidene fluoride annulus implant in one of the defects. Explantation was conducted after 2, 6 and 12 weeks, followed by provocative pressure testing and histological analysis. At 2 weeks' follow-up, all specimens of the control defect group demonstrated uncontained herniated nucleus pulposus tissue in the annulus defects. For the treated specimens, the annulus implant consistently provided an effective barrier for herniating nucleus pulposus tissue, with no implant dislocation at all time-points. After 2 weeks, a homogeneous cell infiltration of the annulus implant was observed, leading to a progressive directional matrix build-up. Repair tissue thickness was significantly stronger with the annulus implant at all follow-ups (p < 0.01). No pronounced foreign body reaction and no difference in the amount of supra-annular scar tissue over the defect sites were observed. The implantation procedure inflicted annulus damage adjacent to the defect. At later time-points, however, no difference in comparison with the control defect group was evident. The investigated biointegrative annulus implant showed promising results with regard to biointegration, enhancement of repair tissue and function as a mechanical barrier in an ovine model.

Lumbar motion-sparing surgery

Surgical treatment of the painful lumbar spine segment is a challenging endeavor. The ideal surgical intervention has a predictably good clinical effect, a low incidence of complications, and long-term durability. The gold standard for surgical intervention is a lumbar spine fusion, but the long-term durability of lumbar fusions is compromised with the development of adjacent level degeneration. Due to these concerns, motion-sparing surgical implants, including artificial discs, dynamic stabilization techniques, and nucleus pulposus replacements, have been developed to simultaneously treat the painful motion segment while at the same time preserving sufficient motion to decrease the chances of adjacent level degeneration.

Swelling behaviours and mechanical properties of silk fibroin–polyurethane composite hydrogels

Abstract Silk fibroin–polyurethane (SF–PU) hydrogels combining advantages of the natural/synthetic materials were fabricated. FTIR was applied to characterize the structure of hydrogels. It was found that cross-linking network formed through chemical reactions between NCO groups on PU chain and NH 2 , OH groups on SF chains, in addition to the hydrogen bonding interactions. SEM was taken to observe the morphology of hydrogels, which exhibited a porous structure. Hydrogels demonstrated impressive swelling–deswelling behaviours and good sensitivities to temperature, ionic strength and pH. The swelling ratio of hydrogel could be as high as 4.37 and it decreased with the increase of SF contents. Due to that the diffusion exponent ( n ) was about 0.45, the initial swelling stage was thought to be controlled by the Fickian diffusion. For the whole swelling process, experimental results well fitted into the Schott second-order kinetic equation. Mechanical tests showed that the elastic modulus of hydrogels were about 0.558 MPa. Properties of hydrogels could be finely controlled so as to fulfill these potential biomedical applications, such as in Nucleus Pulposus Replacement or controlled drug release.

Role of biomechanics in intervertebral disc degeneration and regenerative therapies: what needs repairing in the disc and what are promising biomaterials for its repair?

Background contextDegeneration and injuries of the intervertebral disc (IVD) result in large alterations in biomechanical behaviors. Repair strategies using biomaterials can be optimized based on the biomechanical and biological requirements of the IVD. PurposeTo review the present literature on the effects of degeneration, simulated degeneration, and injury on biomechanics of the IVD, with special attention paid to needle puncture injuries, which are a pathway for diagnostics and regenerative therapies and the promising biomaterials for disc repair with a focus on how those biomaterials may promote biomechanical repair. Study designA narrative review to evaluate the role of biomechanics on disc degeneration and regenerative therapies with a focus on what biomechanical properties need to be repaired and how to evaluate and accomplish such repairs using biomaterials. Model systems for the screening of such repair strategies are also briefly described. MethodsArticles were selected from two main PubMed searches using keywords: intervertebral AND biomechanics (1,823 articles) and intervertebral AND biomaterials (361 articles). Additional keywords (injury, needle puncture, nucleus pressurization, biomaterials, hydrogel, sealant, tissue engineering) were used to narrow the articles down to the topics most relevant to this review. ResultsDegeneration and acute disc injuries have the capacity to influence nucleus pulposus (NP) pressurization and annulus fibrosus (AF) integrity, which are necessary for an effective disc function and, therefore, require repair. Needle injection injuries are of particular clinical relevance with the potential to influence disc biomechanics, cellularity, and metabolism, yet these effects are localized or small and more research is required to evaluate and reduce the potential clinical morbidity using such techniques. NP replacement strategies, such as hydrogels, are required to restore the NP pressurization or the lost volume. AF repair strategies including cross-linked hydrogels, fibrous composites, and sealants offer promise for regenerative therapies to restore AF integrity. Tissue engineered IVD structures, as a single implantable construct, may promote greater tissue integration due to the improved repair capacity of the vertebral bone. ConclusionsIVD height, neutral zone characteristics, and torsional biomechanics are sensitive to specific alterations in the NP pressurization and AF integrity and must be addressed for an effective functional repair. Synthetic and natural biomaterials offer promise for NP replacement, AF repair, as an AF sealant, or whole disc replacement. Meeting mechanical and biological compatibilities are necessary for the efficacy and longevity of the repair.

Mechanical Properties of a Photopolymerizable Hydrogel for Intervertebral Disc Replacement

We report on the modelling and experimental validation of a photopolymerizable hydrogel for a Nucleus Pulposus replacement.

Biomechanical and rheological characterization of mild intervertebral disc degeneration in a large animal model

Biomechanical properties of healthy and degenerated nucleus pulposus (NP) are thought to be important for future regenerative strategies for intervertebral disc (IVD) repair. However, which properties are pivotal as design criteria when developing NP replacement materials is ill understood. Therefore, we determined and compared segmental biomechanics and NP viscoelastic properties in normal and mildly degenerated discs. In eight goats, three lumbar IVDs were chemically degenerated using chondroitinase ABC (CABC), confirmed with radiography and MRI after euthanasia 12 weeks post-operative. Neutral zone (NZ) stiffness and range of motion (ROM) were determined sagitally, laterally, and rotationally for each spinal motion segment (SMS) using a mechanical testing device. NPs were isolated for oscillatory shear experiments; elastic and viscous shear moduli followed from the ratio between shear stress and strain. Water content was quantified by weighing before and after freeze-drying. Disc height on radiographs and signal intensity on MRI decreased (6% and 22%, respectively, p < 0.01) after CABC treatment, confirming that chemical degeneration provides a good model of disc degeneration. Furthermore, CABC-injected IVDs had significantly lower NZ stiffness and larger ROM in lateral bending (LB) and axial rotation (AR) than controls. Rheometry consistently revealed significantly lower (10-12%) viscoelastic moduli after mild degeneration within goats, though the inter-animal differences were relatively large (complex modulus ∼12 to 41 kPa). Relative water content in the NP was unaffected by CABC, remaining at ∼75%. These observations suggest that viscoelastic properties have a marginal influence on mechanical behavior of the whole SMS. Therefore, when developing replacement materials the focus should be on other design criteria, such as biochemical cues and swelling pressure.

Regenerating Nucleus Pulposus of the Intervertebral Disc Using Biodegradable Nanofibrous Polymer Scaffolds

Low back pain is a leading health problem in the United States, which is most often resulted from nucleus pulposus (NP) degeneration. To date, the replacement of degenerated NP relies entirely on mechanical devices. However, a biological NP replacement implant is more desirable. Here, we report the regeneration of NP tissue using a biodegradable nanofibrous (NF) scaffold. Rabbit NP cells were seeded on the NF scaffolds to regenerate NP-like tissue both in vitro and in a subcutaneous implantation model. The NP cells on the NF scaffolds proliferated faster than those on control solid-walled (SW) scaffolds in vitro. Significantly more extracellular matrix (ECM) production (glycosaminoglycan and type II collagen) was found on the NF scaffolds than on the control SW scaffolds. The constructs were then implanted in the caudal spine of athymic rats for up to 12 weeks. The tissue-engineered NP could survive, produce functional ECM, remain in place, and maintain the disc height, which is similar to the native NP tissue.

EFFECT OF HYDROGEL INJECTION ON THE BIOMECHANICAL BEHAVIOUR OF HERNIATED DISCS

Nucleus pulposus replacements have been subjected to highly controversial discussions over the last 40 years. Their use has not yet resulted in a positive outcome to treat herniated disc or degenerated disc disease. The main reason is that not a single implant or tissue replacement was able to withstand the loads within an intervertebral disc. Here, we report on the development of a photo-polymerizable poly(ethylene glycol)dimethacrylate nano-fibrillated cellulose composite hydrogel which was tuned according to native tissue properties. Using a customized minimally-invasive medical device to inject and photopolymerize the hydrogel insitu, samples were implanted through an incision of 1 mm into an intervertebral disc of a bovine organ model to evaluate their long-term performance. When implanted into the bovine disc model, the composite hydrogel implant was able to significantly re-establish disc height after surgery (p < 0.0025). The height was maintained after 0.5 million loading cycles (p < 0.025). The mechanical resistance of the novel composite hydrogel material combined with the minimally invasive implantation procedure into a bovine disc resulted in a promising functional orthopedic implant for the replacement of the nucleus pulposus.

Thermo-Reversible Hydrogel for Nucleus Pulposus Replacement: Feasibility under Static Loading in a mild Papain-induced Disk Degeneration Model

Introduction Nucleus pulposus (NP) replacement by the application of injectable hydrogels seems a straightforward approach for tissue engineering. We investigate a thermo-reversible hydrogel (TH-RHG), based on a modified polysaccharide with a thermo-reversible polyamide (poly(N-isopropylacrylamide = pNIPAM). Using “click chemistry” and reversible addition fragmentation transfer (RAFT) polymerization,2,3 the gel is made to behave as a liquid at room temperature and hardens at &gt;32°C. To inject the hydrogel, a mild papain disk degeneration model (PDDM) is employed that creates a cavity in the intervertebral disk. Here, we investigate the performance of the TH-RHG in situ in bovine IVD with preconditioned human mesenchymal stem cells (hMSC) with rhGDF-5. Materials and Methods IVDs were harvested from six calf tails, aged 6 to 9 months, which were received from a local abattoir within 3 to 5 hour of slaughter. The bovine IVDs of intermediate size (max. height of 2 cm and diameter of 1.5 cm) were used included the bony endplates and cultured in vitro for 16 days. Utilizing our previously established PDDM was necessary to create a cavity in the NP. For the papain injection, a 25 G needle was used to inject centrally the enzyme into the bovine IVD. Simultaneously, primary hMSCs expanded from bone marrow obtained from five patients undergoing spine surgery (ethically approved) were seeded at 4M cells/mL in 3-D in the TH-RHG and preconditioned with 100 ng/mL rhGDF-5 for 7 days. Afterwards, the TH-RHG was reversed to a fluid and injected into the IVD organ culture and kept under static loading of 0.1 MPa for 7 days using custom-designed specimen chambers.1 Experimental designs are as follows (i) fresh d0 control PBS injection (ii) PDDM + PBS injection (iii) PDDM + TH-RHG + bovine NPCs (Cell control) (iv) PDDM + TH-RHG + hMSC.4 Magnetic resonance imaging (MRI) was employed to image the hydrogel-filled cavity before and after loading at day 9 and day 16, and RT-PCR comparison of the hMSCs at day 1, 8, and 15 investigated the changes in gene expression. On the same days of cell sampling, cell viability (trypan blue) was determined. Sulfated glycosaminoglycan synthesis (DMMB assay) and DNA content (Picogreen assay) from anterior and posterior annulus fibrosus (AF) segments of each disk at day 16 were performed. Results MRI images confirmed a central positioning of the TH-RHG into the PDDM. A considerable drop in volume across all groups in the NP region and consequently of disk height was observed between day 9 and day 16. The RT-PCR results of injected hMSCs showed significant differences between day 8 and day 15 for versican and Sox9 relative to day 1 (Fig. 1). The results of bovine AF gene expression relative to control AF showed significant differences between groups for ACAN, Col1, Col2, IL-b1, versican, ADAMTS-4, Casp-8, MMP13, and MMP3. Cell viability of injected cells dropped from around ∼100% down to ∼87% for bovine cells and ∼86% for hMSC after 7 days in 3D culture. It further dropped to ∼72% after organ culture. GAG/DNA ratio showed significant difference across groups irrespective of posterior or anterior location. Conclusion MRI imaging of TH-RHG seems a promising protocol for noninvasively observing the gel performance in vitro. 10% TH-RHG is a feasible scaffold for in vitro 3D preconditioning of hMSCs and can be used at the same time for direct injection into the IVD. However, the TH-RHG has been shown to be unable to bare static load leading to a drop of cell viability. Relative gene expression of the hMSC phenotype after “co-culture”, however, points towards “discogenic” differentiation.5 Acknowledgements This project is funded by HansJörg Wyss Start up Award 2011/Project no. 102646/AOSpine International. I confirm having declared any potential conflict of interest for all authors listed on this abstract No Disclosure of Interest None declared Chan, et al. Spine 2011. Mortisen, et al. Biomacromolecules 2010;11(5):1261–1272 Mortisen, et al. PCT2009, 3-3-2009 2009;141 Gantenbein-Ritter, et al. European Spine Journal 2011;20:962–971 Peroglio, et al. European Spine Journal 2011;epub

Thermo-Reversible Hydrogel for Nucleus Pulposus Replacement: Feasibility under Static Loading in a mild Papain-induced Disk Degeneration Model

Thermo-Reversible Hydrogel for Nucleus Pulposus Replacement: Feasibility under Static Loading in a mild Papain-induced Disk Degeneration Model

Tissue Engineering Construct Prevents Disk Degeneration after Nucleotomy in a Rat Model

IntroductionBack pain, a significant source of morbidity in our society, is directly linked to the pathology of the intervertebral disk (IVD). An IVD is a complex structure that is separated into the following three tissue types: the annulus fibrosus, the nucleus pulposus (NP), and the endplates. Recently, IVD engineering strategies have been increasingly focused on the regeneration or repair of the AF to increase the potential of NP engineering strategies and to mechanically assist NP replacement therapies.1,2 We have developed a novel scaffold-free three-dimensional tissue-engineered construct (TEC) derived from cultured synovial mesenchymal stem cells (MSCs) as a unique approach for cartilage repair.3,4 Previous in vitro studies reveal that growth factors and serum can modify the collagen expression and glycosaminoglycan (GAG) content of chondrogenic-differentiated MSCs.5 Therefore although there is a difference between hyaline cartilage and fibrocartilage, TEC derived from MSCs could have a potential to differentiate into the AF, a fibrocartilage in IVD. This study explores the use of TEC derived from synovial MSCs as a cell-based therapy for IVD regeneration.Materials and MethodsSynovial MSCs were isolated enzymatically from rat synovial membranes as previously reported.3,4 For development of the TEC, cells were cultured at the density of 4.0 × 105/cm2 in HG-DMEM in the presence of 0.2 mM ascorbic acid 2-phosphate for 2 weeks. The monolayer cultured cell-matrix complex was then detached from cell-substratum interface by addition of shear stress to convert to suspension culture. The cell-matrix complex immediately starts active contraction to form the three dimensional (3D) tissue-engineered construct (TEC, Fig. 1). To explore the use of TEC in degenerative spine, rat caudal IVD were denucleated and treated with TEC (controls were denucleation only). Nine male Sprague-Dawley rats (300 to 400 g) were used, and two most cranial tail disks were denucleated in each rat, giving three disks per group per time point. At 2, 8, and 12 weeks after implantation, the animals were euthanized and disks were evaluated for disk height based on micro-CT analysis, histology including Safranin O staining, and disk grade based on a scoring system.6ResultsAt the 2-week time point, there were no significant differences in disk height and disk grade between two groups (3.57 ± 0.28 vs 3.76 ± 0.46 points, p > 0.05) (462 ± 39 vs. 602 ± 49 um, p > 0.05). At the 8-week time point, the TEC-treated group showed significantly better results in disk grade (5 ± 0 vs. 3.14 ± 0.28 points, P<0.05). At the 12-week time point, TEC-treated group showed the higher disk height (234 ± 19 vs 584 ± 45 um, P<0.05) and better disk grade (5 ± 0 vs. 2.86 ± 0.46 points, p < 0.05). Untreated group experienced severe disruption of the AF and end plate, whereas such degenerative changes were alleviated with TEC implantation (Fig. 2).ConclusionThis study shows that TEC may prevent postnucleotomy disk degeneration in vivo. Larger animals and longer time points will be necessary to further judge potential clinical impact.I confirm having declared any potential conflict of interest for all authors listed on this abstractYesDisclosure of InterestNone declaredAndersson GB, et al. Directions for future research. Journal of Joint and Bone Surgery 2006Wilke HJ, et al. Is a collagen scaffold for a tissue engineered nucleus replacement capable of restoring disc height and stability in an animal model? European Spine Journal 2006W Ando, et al. Cartilage repair using an in vitro generated scaffold-free tissue-engineered construct derived from porcine synovial mesenchymal stem cells. Biomaterials 2007K Shimomura, et al. The influence of skeletal maturity on allogenic synovial mesenchymal stem cell-based repair of cartilage in a large animal model. Biomaterials 2010S Lee, et al. Effect of serum and growth factors on chondrogenic differentiation of synovium-derived stromal cells. Tissue Engineering: Part A 2009AA Allon, et al. Structured coculture of stem cells and disc cells prevent disc degeneration in a rat model. The Spine Journal 2010

Tissue Engineering Construct Prevents Disk Degeneration after Nucleotomy in a Rat Model

Introduction Back pain, a significant source of morbidity in our society, is directly linked to the pathology of the intervertebral disk (IVD). An IVD is a complex structure that is separated into the following three tissue types: the annulus fibrosus, the nucleus pulposus (NP), and the endplates. Recently, IVD engineering strategies have been increasingly focused on the regeneration or repair of the AF to increase the potential of NP engineering strategies and to mechanically assist NP replacement therapies.1,2 We have developed a novel scaffold-free three-dimensional tissue-engineered construct (TEC) derived from cultured synovial mesenchymal stem cells (MSCs) as a unique approach for cartilage repair.3,4 Previous in vitro studies reveal that growth factors and serum can modify the collagen expression and glycosaminoglycan (GAG) content of chondrogenic-differentiated MSCs.5 Therefore although there is a difference between hyaline cartilage and fibrocartilage, TEC derived from MSCs could have a potential to differentiate into the AF, a fibrocartilage in IVD. This study explores the use of TEC derived from synovial MSCs as a cell-based therapy for IVD regeneration. Materials and Methods Synovial MSCs were isolated enzymatically from rat synovial membranes as previously reported.3,4 For development of the TEC, cells were cultured at the density of 4.0 × 105/cm2 in HG-DMEM in the presence of 0.2 mM ascorbic acid 2-phosphate for 2 weeks. The monolayer cultured cell-matrix complex was then detached from cell-substratum interface by addition of shear stress to convert to suspension culture. The cell-matrix complex immediately starts active contraction to form the three dimensional (3D) tissue-engineered construct (TEC, Fig. 1). To explore the use of TEC in degenerative spine, rat caudal IVD were denucleated and treated with TEC (controls were denucleation only). Nine male Sprague-Dawley rats (300 to 400 g) were used, and two most cranial tail disks were denucleated in each rat, giving three disks per group per time point. At 2, 8, and 12 weeks after implantation, the animals were euthanized and disks were evaluated for disk height based on micro-CT analysis, histology including Safranin O staining, and disk grade based on a scoring system.6 Results At the 2-week time point, there were no significant differences in disk height and disk grade between two groups (3.57 ± 0.28 vs 3.76 ± 0.46 points, p &gt; 0.05) (462 ± 39 vs. 602 ± 49 um, p &gt; 0.05). At the 8-week time point, the TEC-treated group showed significantly better results in disk grade (5 ± 0 vs. 3.14 ± 0.28 points, P&lt;0.05). At the 12-week time point, TEC-treated group showed the higher disk height (234 ± 19 vs 584 ± 45 um, P&lt;0.05) and better disk grade (5 ± 0 vs. 2.86 ± 0.46 points, p &lt; 0.05). Untreated group experienced severe disruption of the AF and end plate, whereas such degenerative changes were alleviated with TEC implantation (Fig. 2). Conclusion This study shows that TEC may prevent postnucleotomy disk degeneration in vivo. Larger animals and longer time points will be necessary to further judge potential clinical impact. I confirm having declared any potential conflict of interest for all authors listed on this abstract Yes Disclosure of Interest None declared Andersson GB, et al. Directions for future research. Journal of Joint and Bone Surgery 2006 Wilke HJ, et al. Is a collagen scaffold for a tissue engineered nucleus replacement capable of restoring disc height and stability in an animal model? European Spine Journal 2006 W Ando, et al. Cartilage repair using an in vitro generated scaffold-free tissue-engineered construct derived from porcine synovial mesenchymal stem cells. Biomaterials 2007 K Shimomura, et al. The influence of skeletal maturity on allogenic synovial mesenchymal stem cell-based repair of cartilage in a large animal model. Biomaterials 2010 S Lee, et al. Effect of serum and growth factors on chondrogenic differentiation of synovium-derived stromal cells. Tissue Engineering: Part A 2009 AA Allon, et al. Structured coculture of stem cells and disc cells prevent disc degeneration in a rat model. The Spine Journal 2010

Intervertebral disc regeneration or repair with biomaterials and stem cell therapy - feasible or fiction?

The "gold standard" for treatment of intervertebral disc herniations and degenerated discs is still spinal fusion, corresponding to the saying "no disc - no pain". Mechanical prostheses, which are currently implanted, do only have medium outcome success and have relatively high re-operation rates. Here, we discuss some of the biological intervertebral disc replacement approaches, which can be subdivided into at least two classes in accordance to the two different tissue types, the nucleus pulposus (NP) and the annulus fibrosus (AF). On the side of NP replacement hydrogels have been extensively tested in vitro and in vivo. However, these gels are usually a trade-off between cell biocompatibility and load-bearing capacity, hydrogels which fulfill both are still lacking. On the side of AF repair much less is known and the question of the anchoring of implants is still to be addressed. New hope for cell therapy comes from developmental biology investigations on the existence of intervertebral disc progenitor cells, which would be an ideal cell source for cell therapy. Also notochordal cells (remnants of the embryonic notochord) have been recently pushed back into focus since these cells have regenerative potential and can activate disc cells. Growth factor treatment and molecular therapies could be less problematic. The biological solutions for NP and AF replacement are still more fiction than fact. However, tissue engineering just scratched the tip of the iceberg, more satisfying solutions are yet to be added to the biomedical pipeline.

Nucleus pulposus replacement and regeneration/repair technologies: Present status and future prospects

Degenerative disc disease is implicated in the pathogenesis of many painful conditions of the back, chief among which is low back pain. Acute and/or chronic low back pain (A/CLBP) afflicts a large number of people, thus making it a major healthcare issue with concomitant cost ramifications. When conservative treatments for A/CLBP, such as bed rest, anti-inflammatory medications, and physical therapy, prove to be ineffectual, surgical options are recommended. The most popular of these is discectomy followed by fusion. Although there are many reports of good to excellent outcomes with this method, there are concerns, such as long-term adverse biomechanical consequences to adjacent functional spinal unit(s). A surgical option that has been attracting much attention recently is replacement or regeneration/repair of the nucleus pulposus, an approach that holds the prospect of not compromising either mobility or function and causing no adjacent-level injury. There is a sizeable body of literature highlighting this option, comprising in vitro biomechanical studies, finite element analyses, animal-model studies, and limited clinical evaluations. This work is a review of this body of literature and is organized into four parts, with the focus being on replacement technologies, regeneration/repair technologies, and detailed expositions on 14 areas for future study. This review ends with a summary of the salient points made.

Injectable silk fibroin/polyurethane composite hydrogel for nucleus pulposus replacement

In degenerative disc disease, an injectable hydrogel can fill a degenerate area completely, reduce the risk of implant migration and subsequent loss of height of the intervertebral disc, and minimise surgical defects. Here, we propose a method of preparing an injectable silk fibroin/polyurethane (SF/PU) composite hydrogel by chemical cross-linking under physiological conditions. Mechanical testing was used to determine the mechanical strength of the hydrogel. The impact of hydrogel height on the biomechanical properties was discussed to estimate the working capacity of the hydrogel for further clinical application. Rheological properties were also examined to assess the practical ability of the hydrogel for clinical application. Hydrogel injection and cell assessment is also of interest for clinical application. An SF/PU composite hydrogel can be injected through a small incision. A cell proliferation assay using bone marrow stromal cells showed positive cell viability and increased proliferation over a seven-day period in culture. Importantly, the hydrogel can be monitored in real-time using X-ray fluoroscopy during and after surgery according to the results of X-ray fluoroscopy examination, and shows good visibility based on X-ray assays. In particular, the hydrogel offers the clinically important advantage of visibility in CT and T2-weighted magnetic resonance imaging. Based on the results of the current study, the SF/AU composite hydrogel may offer several advantages for future application in nucleus pulposus replacement.

Migration of intervertebral disc cells into dense collagen scaffolds intended for functional replacement

Invasion of cells from surrounding tissues is a crucial step for regeneration when using a-cellular scaffolds as a replacement of the nucleus pulposus (NP). The aim of current study was to assess whether NP and surrounding annulus fibrosus (AF) cells are capable of migrating into dense collagen scaffolds. We seeded freshly harvested caprine NP and AF cells onto scaffolds consisting of 1.5 and 3.0% type I collagen matrices, prepared by plastic compression, to assess cell invasion. The migration distance appeared both time and density dependent and was higher for NP (25%) compared to AF (10%) cells after 4 weeks. Migration distance was not enhanced by Hst-2, a peptide derived from saliva known to enhance fibroblast migration, and this was confirmed in a scratch assay. In conclusion, we revealed invasion of cells into dense collagen scaffolds and therewith encouraging first steps towards the use of a-cellular scaffolds for NP replacement.

Hyperelastic mechanical behavior of chitosan hydrogels for nucleus pulposus replacement—Experimental testing and constitutive modeling

Chitosan hydrogels (CHs) have been considered as a potential implant material for replacement and repair of the Nucleus Pulposus (NP) within the intervertebral disk. The nonlinear mechanical behavior of a CH material is investigated experimentally and computationally in this study. A series of confined and unconfined compression tests are designed and conducted for this hydrogel. Hyperelastic strain energy density functions (SEDFs) are calibrated using the experimental data. A hyperelastic constitutive model is selected to best fit the multi-axial behavior of the hydrogel. Its general prediction ability is verified using finite element (FE) simulations of hydrogel indentation experiments conducted using a spherical tip indentor. In addition, digital image correlation (DIC) technique is also used in the indentation test in order to process the full-field surface strains where the indentor contacts the hydrogel. The DIC test results in the form of top-surface strains compared well with those predicted by the FE model. Results show repeatability for the examined specimens under the applied tests. Confined and unconfined test results are found to be sufficient to calibrate the SEDFs. The Ogden model was selected to represent the nonlinear behavior of the CH material which can be used in future biomechanical simulations of the spine.

Evaluation of platelet-rich plasma and hydrostatic pressure regarding cell differentiation in nucleus pulposus tissue engineering

Generation of a biological nucleus pulposus (NP) replacement by tissue engineering appears to be a promising approach for the therapy of early stages of intervertebral disc degeneration. Thereby, autologous mesenchymal stem cells (MSCs) represent an attractive cell source compared to cells of the NP that are already altered in their phenotype due to degenerative processes. This study compares the influence of 3D pellet culture and alginate beads, as well as that of different media compositions, by the addition of human platelet-rich plasma (PRP) or transforming growth factor (TGF-β1 ) in interaction with hydrostatic pressure on chondrogenic differentiation of human MSCs compared to NP cells. We found that gene expression of the chondrogenic markers aggrecan, collagen type 2 and collagen type 1 and Sox9 was considerably lower in cells cultivated with PRP compared to TGF-β1 . Immunohistology confirmed this result at protein level in pellet culture. Additionally, the pellet culture system was found to be more suitable than alginate beads. A positive influence of hydrostatic pressure could only be shown for individual donors. In summary, in comparison to TGF-β1 , human PRP did not induce adequate chondrogenic differentiation for both culture systems and cell types used. The mixture of growth factors in PRP promoted proliferation rather than chondrogenic differentiation. Based on these results, an application of PRP in human NP tissue-engineering approaches cannot be recommended.