Nuclear Replacement

Summary of Key Points

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Nucleus replacement and intradiscal electrothermal treatment evolved as options to treat discogenic back pain in patients with early nuclear degeneration with minimal or no collapse, no signs of instability, and no arthrosis of the motion segment. It is hoped that nucleus replacement will treat discogenic back pain and avoid fusion.
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Primary indication for nucleus replacement is discogenic pain caused by single-level disc degenerative disease with or without leg pain. Disc degeneration should be mild to moderate. Nucleus devices rely on intact annulus and the end plates for biomechanical restraint and load sharing.
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The Fernstrom ball never found mainstream acceptance as a nuclear replacement device; however, it did suggest that nuclear replacement can be a viable alternative to discectomy or fusion. Currently known devices can be classified as elastomers and nonelastomers, with several designs that have been clinically tested. Most clinical activity, if any, is occurring outside of the United States.
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Outcome data are limited, and a high incidence of implant expulsion and subsidence has led to limited current use of this technology. Initial excitement for different versions of the nucleus replacement devices is now tempered with sobering clinical data. It is hoped that the lessons learned from these efforts at nucleus replacement in this era, with continuing improvement in material selection, design properties, and surgical techniques, will provide a new direction for nucleus replacement technology in the future.

Spine surgeons contend with back pain on a daily basis. Back pain has acquired epidemic proportions in industrialized nations with a prevalence of 60% to 90%, which is second only to the common cold as a reason for a physician visit. With the total costs associated with back pain ranging from $100 to $200 billion annually, back pain is a major health and socioeconomic burden. Most episodes of back pain are short in duration. However, many have a recurrent course with further acute episodes affecting 20% to 44% of patients within 1 year in the working population and lifetime recurrences of up to 85%.

Lumbar disc degeneration is the most common cause of back pain. Disc degeneration results from reduced proteoglycan content in the nucleus and reduced nuclear hydration. Resulting biomechanical changes in the disc lead to loss of disc height and increasing biomechanical demand on the annulus with imbalance in the stress distribution across the disc space. When tension in the annulus is lost, anterior or posterior instability of the motion segment can ensue. Increasing loads on the annulus can lead to annular tears with or without disc herniations. Continued loss of disc height can lead to osteophyte formation, facet arthrosis, and stiffness of the motion segment. Pain from disc degenerative disease (DDD) can occur at any stage of this degenerative cascade from early disc degeneration to instability and deformity.

Traditional treatment modalities for symptoms resulting from disc degeneration are focused on decompression with or without fusion. These treatment modalities do not attempt to halt the degenerative cascade, and in many instances they can lead to further progression of degeneration. Although short-term outcomes after lumbar discectomy have been shown to be superior to conservative care long-term outcomes have been compromised by persistent back pain and a high risk of reoperations with a significant number of reherniations. Arthrodesis of the motion segment remains the gold standard for treatment of chronic disabling back pain of discogenic origin. However, it is difficult to predict the clinical response to arthrodesis, as it depends on multiple factors, such as the diagnosis, previous surgeries, prior fusion attempts, and the number of levels requiring fusion. Long-term studies have shown a fusion rate of 87% and clinical success rate of 76% for DDD. There are several disadvantages inherent to arthrodesis. Most important, arthrodesis can change the biomechanical loading of the adjacent segment leading to accelerated degeneration.

Treatment of discogenic pain is fraught with difficulty and unpredictable outcomes. Additionally, in patients with early disc degeneration with minimal or no loss of disc height, there are no reliable modalities for the diagnosis of origin of discogenic back pain. Discography has been used to aid in the diagnosis; however, its reliability has been questioned time and again.

Nucleus replacement and intradiscal electrothermal treatment (IDET) evolved as an option to treat discogenic back pain in patients with early nuclear degeneration with minimal or no collapse, no signs of instability, and arthrosis of the motion segment. Intradiscal electrothermal treatment can be used in patients with mild annular tears and early nuclear degeneration; however, it cannot be used in patients with more advanced disc degeneration and herniated nucleus pulposus. It is hoped that nucleus replacement will treat discogenic back pain in such patients and avoid fusion.

History of Nucleus Replacement

Early attempts at nuclear replacements consisted of injecting polymethylmethacrylate or self-curing silicone into the disc space after a nucleotomy. Clinical results were found to be similar to discectomy controls. The flow of the injected material and curing of the injected liquids were difficult to control. These variables led to abandonment of these procedures, but these initial attempts form the theoretic basis of present-day preformed nuclear replacement systems.

Fernstrom was the first to use stainless steel balls as nuclear replacement by inserting them into the central cavity created after a discectomy. He implanted these balls in 125 patients at 191 lumbar levels. In 1966, Fernstrom reported that the outcome after this kind of nuclear replacement was better than for discectomy alone and was similar to fusion. This procedure was largely abandoned owing to the subsidence of steel balls into the vertebral end plates. In 1995, McKenzie reported long-term outcomes of the Fernstrom ball. After 17 years of follow-up of 67 patients implanted with Fernstrom balls, McKenzie reported an 83% success rate in patients with one or more disc protrusions and a 75% success rate in patients with DDD. Although the Fernstrom ball never found mainstream acceptance as a nuclear replacement device, it did suggest that nuclear replacement could be a viable alternative to either a stand-alone discectomy or a fusion.

In 1981, Edeland suggested a design for an elastodynamic disc replacement device that would mimic the native disc biomechanically and biologically. Such a device would allow influx or efflux of water molecules in response to the load applied to it. Finding an elastodynamic material remained the most limiting factor. Attempts at finding this type of material in 1980s and 1990s focused either on silicone polymers or a hydrogel nucleus composed of polyvinyl alcohol (PVA) that was used to make soft contact lenses. Hou and colleagues designed a solid preformed silicone rubber horseshoe-shaped implant termed the lumbar intervertebral replacement prosthesis . Biomechanical cadaver studies and animal studies in monkeys showed restoration of the disc height and mechanical stability. Human studies with this implant have not been reported.

In 1991, Bao and Hingham patented a hydrogel nucleus made of PVA that imbibed and extruded water and could replicate the biomechanical viscoelastic properties of the native nucleus. An animal study by Allen and coworkers showed no evidence of systemic or local toxicity but found a high incidence of implant extrusion. Despite this setback, proved preclinical benefits with hydrogel acted as a springboard for other nucleus replacement technologies. With its first clinical use in 1996, the prosthetic disc nucleus (PDN) was the most well-known nucleus replacement device made of hydrogel. The device was made of hydrogel enclosed in a polyethylene terephthalate (PET) jacket to restrict expansion upon hydration. Multiple design and material changes have followed since its first use ( Fig. 185-1 ).

Current Nuclear Replacement Technology

Current nuclear replacement devices can be classified either as elastomeric or nonelastomeric. Elastomers can be subdivided into hydrogels and nonhydrogels that are either in situ cured or preformed. Preformed devices may be implanted in their final shapes or may take their final shape upon expanding by hydration. Nonelastomers can be subdivided into articulating or nonarticulating based on the presence or absence of articulation as part of the implant design.

Currently, there are several designs with active clinical or developmental activities. Most clinical activity, if any, is occurring outside of the United States. The majority of the devices are elastomers like HydraFlex, DASCOR, NuCore, NeuDisc, BioDisc, SINUX ANR, SINUX PNR, and Newcleus.

HydraFlex is the latest version of a PDN; it is inserted in a dehydrated form that rehydrates upon implantation ( Fig. 185-2 ). The hydrogel component is a block copolymer of polyacrylamide-polyacrylonitrile (Hypan) formed by solution casting. The jacket is formed of dense woven circular tube of PET, which is a thermoplastic polymer. The PET jacket is inelastic but flexible, which aids the hydrogel core to maintain its shape when subject to compressive forces. The hydrogel core absorbs water on implantation. Hypan80, which is currently being used, absorbs 80% of its weight by water, which makes it softer and flexible. HydraFlex takes approximately 32 hours to reach hydration equilibrium.

DASCOR is a two-part in situ curable polyurethane. It consists of a polyether polyurethane core encapsulated and adhered to a polycarbonate polyurethane balloon, which has cavity expansion and conforming capabilities. The device is fabricated by mixing a two-part, liquid, prepolyether polyurethane reactive system and injecting a liquid mixture in a polycarbonate polyurethane balloon delivery catheter placed within the nucleus cavity ( Fig. 185-3 ). A custom electromechanical injection system is used to apply a computer-controlled pressure profile designed to deliver the liquid polymer.

The NuCore Injectable Disc Nucleus is an in situ curing protein polymer hydrogel, which mimics the properties of the natural nucleus. The polymer chain is composed of silk and elastin components designed for both elasticity and toughness. The polymer (P27K) is mixed with a cross-linking agent at the time of implantation and is injected as a fluid through the annular defect where it adheres to the surrounding intradiscal tissue as it cures. No measurable temperature increase is produced during the cure process. NuCore has been studied as an adjunct to discectomy and is in the process of being studied for treatment of DDD ( Fig. 185-4 ).

The NeuDisc device is composed of a material that is a hydrolyzed polyacrylonitrile hydrogel (Aquacryl). It has a vertically layered structure that alternates soft hydrogel layers with a Dacron knitted mesh. The mesh layers restrict radial deformability so that the implant does not creep through any defects in the annulus fibrosus. The Dacron net also stabilizes the hydrogel implant and resists expulsion. In the absence of mechanical restrictions, the hydrogel imbibes 90% volume as a liquid. When implanted in a dehydrated state, this hydrogel implant is substantially smaller than the volume of resected nucleus and is easily placed though an incision in the annulus. After hydration, it expands anisotropically, in the axial direction principally, and becomes substantially larger than the incision. The axial expansion is not constrained by a jacket and results from its structure of polymer layers tied together with an internal substructure ( Figs. 185-1 and 185-5 ).