Design and Properties of Novel Self-Curing Acrylic Formulations for Application in Intervertebral Disks Restoration

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.

Evaluation of Electrospun Nanofiber-Anchored Silicone for the Degenerative Intervertebral Disc

The nucleus pulposus (NP) substitution by polymeric gel is one of the promising techniques for the repair of the degenerative intervertebral disc (IVD). Silicone gel is one of the potential candidates for a NP replacement material. Electrospun fiber anchorage to silicone disc, referred as ENAS disc, may not only improve the biomechanical performances of the gel but it can also improve restoration capability of the gel, which is unknown. This study successfully produced a novel process to anchor any size and shape of NP gel with electrospun fiber mesh. Viscoelastic properties of silicone and ENAS disc were measured using standard experimental techniques and compared with the native tissue properties. Ex vivo mechanical tests were conducted on ENAS disc-implanted rabbit tails to the compare the mechanical stability between intact and ENAS implanted spines. This study found that viscoelastic properties of ENAS disc are higher than silicone disc and comparable to the viscoelastic properties of human NP. The ex vivo studies found that the ENAS disc restore the mechanical functionality of rabbit tail spine, after discectomy of native NP and replacing the NP by ENAS disc. Therefore, the PCL ENF mesh anchoring technique to a NP implant can have clinical potential.

Treatment of the degenerated intervertebral disc; closure, repair and regeneration of the annulus fibrosus

Degeneration of the intervertebral disc (IVD) and disc herniation are two causes of low back pain. The aetiology of these disorders is unknown, but tissue weakening, which primarily occurs due to inherited genetic factors, ageing, nutritional compromise and loading history, is the basic factor causing disc degeneration. Symptomatic disc herniation mainly causes radicular pain. Current treatments of intervertebral disc degeneration and low back pain are based on alleviating the symptoms and comprise administration of painkillers or surgical methods such as spinal fusion. None of these methods is completely successful. Current research focuses on regeneration of the IVD and particularly on regeneration of the nucleus pulposus. Less attention has been directed to the repair or regeneration of the annulus fibrosus, although this is the key to successful nucleus pulposus, and therewith IVD, repair. This review focuses on the importance of restoring the function of the annulus fibrosus, as well as on the repair, replacement or regeneration of the annulus fibrosus in combination with restoration of the function of the nucleus pulposus, to treat low back pain.

Strategies towards injectable, load-bearing materials for the intervertebral disc: a review and outlook

Currently available treatments for the degenerated intervertebral disc present disadvantages, such as surgical invasiveness and inadequate load distribution results. Load-bearing, injectable materials may be interesting for future therapies, but have not been studied in depth. In this study, the existing literature was screened for studies on injectable materials for the intervertebral disc and a rationale for load-bearing, injectable materials was formulated. Requirements for such a material were discussed, partly based on the experience of materials used for similar applications. Important properties were discussed and found to include biocompatibility, bioactivity, porosity, handling, injectability, working time, setting time, radiopacity, containment and mechanical properties, where several of these properties are linked to one another. In conclusion, there is a need for consensus on the properties of new materials developed for use in minimally invasive procedures in the spine. A substantial amount of attention may need to be given to non-toxic setting reactions.

Feasibility of ceramic-polymer composite cryogels as scaffolds for bone tissue engineering

The purpose of the current study was to investigate whether the cryopolymerization technique is capable of producing suitable scaffolds for bone tissue engineering. Cryopolymers made of 2-hydroxyethyl methacrylate and acrylic acid with (W1 and W20) and without (W0) wollastonite particles were prepared. The elastic modulus of the specimens rose one order of magnitude from W1 to W20. Total porosity reached 56% for W0, 72% for W1 and 36% for W20, with pore sizes of up to 2 mm, large interconnection sizes of up to 1 mm and small interconnection sizes of 50-80 µm on dry specimens. Cryogels swell up to 224 ± 17% for W0, 315 ± 18% for W1 and 231 ± 27% for W20 specimens, while maintaining the integrity of the bodies. Pore sizes > 5 mm can be observed for swollen specimens. The biocompatibility of the samples was tested using human mesenchymal stem cells isolated from bone marrow and adipose tissues. Both types of cells attached and grew on the three tested substrates, colonized their inner regions and organized an extracellular cell matrix. Fibronectin and osteopontin levels decreased in the media from cells cultured on W20 samples, likely due to increased binding on the ECM deposited by cells. The osteoprotegerin-to-receptor activator of nuclear factor-κB ligand secretion ratios increased with increasing wollastonite content. Altogether, these results indicate that an appropriate balance of surface properties and structure that favours stromal cell colonization in the porous cryogels can be achieved by modulating the amount of wollastonite.

Rheological Characterization of Hyaluronic Acid Derivatives as Injectable Materials Toward Nucleus Pulposus Regeneration

Nucleus pulposus (NP) is the soft center of the intervertebral disc (IVD), able to resist compressive loads, while the annulus fibrosus withstands tension and gives mechanical strength.

NP function may be altered as consequence of several pathologies or injury and when a damaged IVD does not properly play its role. In the past years, a great effort has been devoted to the design of injectable systems as NP substitutes. The different synthetic- and natural hydrogel-based materials proposed, present many drawbacks and, in particular, they do not seem to mimic the required behavior. In the search for natural-based systems a dodecylamide of hyaluronic acid (HA), HYADD3®, has been proved as bioactive and suitable vehicle to carry cells for NP tissue engineering, while a crosslinked HA ester, HYAFF120® showed interesting results if used as injectable acellular material. Even though these derivatives showed appropriate biological behavior up to now, data on mechanical behavior of these derivatives are still missing. In this frame, the aim of this study was to provide a rheological characterization of these HA derivatives to asses their biomechanical compatibility with the NP tissue. To this, the rheological properties of these derivatives were studied through dynamic shear tests before and after injection through needles used in the current surgical procedure. Both HA derivatives showed a ‘gel-like’ rheological behavior similar to the native NP tissue and this behavior was not altered by injection.

New radiopaque acrylic bone cement. II. Acrylic bone cement with bromine-containing monomer

Bromine-containing methacrylate, 2-(2-bromopropionyloxy) ethyl methacrylate (BPEM), had been used in the formulation of acrylic radiopaque cements. The effect of this monomer incorporated into the liquid phase of acrylic bone cement, on the curing parameters, thermal properties, water absorption, density, compression tests and radiopacity was studied. A decrease of maximum temperature and an increase of the setting time were observed with the addition of the bromine-containing monomer in the radiolucent cement composition. Adding BPEM in radiolucent acrylic bone cements composition results in the decrease of glass transition temperature and increase of its thermal stability. Acrylic bone cements modified with bromine-containing comonomer are characterized by polymerization shrinkage lower than the radiolucent cement. Addition of bromine-containing comonomer in radiolucent acrylic bone cement composition determines the increase of compressive strength. Acrylic bone cements modified with bromine-containing comonomer proved to be radiopaque.

Self-curing acrylic formulations with applications in intervertebral disk restoration: drug release and biological behaviour

New injectable acrylic formulations have been prepared to be applied in restoration processes for intervertebral disks (IVDs). The solid phase of the formulations is composed of poly(methyl methacrylate) (PMMA), incorporating in some cases chondroitin sulphate (CS) as a regenerative bioactive molecule, whereas the liquid phase is constituted by an amphiphilic macromonomer (MT), 2-hydroxyethyl methacrylate (HEMA) and, in some formulations, acrylic acid (AA). The curing parameters and the mechanical properties of the IVD formulations make them excellent candidates for intervertebral application. In vitro and in vivo evaluation of the prepared IVD formulations is described in terms of CS release, surface analysis after immersion in SBF solutions, and biocompatibility studies based on MTT assay and Alamar blue test, as well as in vivo implantation in female Wistar rats, by injection of the IVD formulations followed by histological evaluations to assess tissue response.

Three-dimensional nanocomposite scaffolds with ordered cylindrical orthogonal pores

A silica reinforcement can improve the mechanical properties of hydrogels in the rubbery state. A method to prepare a scaffold with a well-ordered array of cylindrical pores is presented in this work, which yields a scaffold with a biphasic matrix of a hybrid nanocomposite: the hydrogel poly(2-hydroxyethyl acrylate) (PHEA) and a silica network obtained by an acid catalyzed sol-gel process of tetraethoxysilane (TEOS). As porogenic template of the scaffold stacked layers of commercial polyamide 6 fabrics were used, which were compressed and sintered. Porosity and dynamic mechanical response of the resulting scaffolds were measured and compared with the bulk properties. Removal of the organic polymer phase of the scaffold by pyrolysis revealed the overall continuity of the silica network; the residue maintained the original cylindrical pore structure of the scaffolds, though slightly shrunk. Atomic force microscopy topography measurements of these pyrolysed residues revealed a silica structure with particle aggregates having sizes around tens of nanometers. The silica distribution was assessed by X-ray microanalysis mapping, showing homogeneity at a micrometer scale.