Technical ReviewMechanical testing of a novel hydrogel nucleus replacement implant
Section snippets
Background
Degenerative disc disease and low back pain are the leading cause of lost wages in the United States per year and account for the nearly 700,000 surgical procedures per year [1]. When surgical intervention is deemed necessary, the most common method of treatment is fusion of the adjacent vertebral bodies. Successful fusion can be a desirable short-term solution; however, the altered biomechanics of the motion segment often lead to future and increased problems in adjacent segments of the spine.
Purpose
The purpose of this investigation was to demonstrate preliminary efficacy of this nucleus replacement device. Bench-top mechanical tests relevant to satisfactory function in the lumbar spine were selected. Additionally, ability of the implant to resist extrusion from cadaveric spine segments was evaluated.
Methods
Prototypes of the NeuDisc implant were produced in two shapes (cylindrical and elliptical) for mechanical testing. The cylindrical implants had a circular cross-section or “footprint” with implant diameters of (nominally) 15 mm, 17.5 mm, 20 mm, 23 mm, or 25 mm and the elliptical implants had “footprint” dimensions of 25 mm×35 mm. Implants were manufactured to have a 15-mm (unrestricted) height upon full hydration in saline.
Radial deformability demonstration
The visual demonstration of the ability of the NeuDisc implant to resist radial deformation is shown pictorially in Figure 4. The NeuDisc implant showed considerably less “bulging” when compared with the identically sized implant produced from the same Aquacryl hydrogel but lacking the anisotropic expansion characteristics and the polyester reinforcement mesh layers.
Confined compression testing
Mean “apparent moduli” of the nucleus replacement implant are included in Table 1. Analysis of variance revealed significant
Discussion
A nucleus replacement may be indicated in patients suffering from degenerative disc disease. These individuals are experiencing back pain with or without leg pain and continue to experience severe back pain despite completing at least 6 months of conservative therapy, including injections and physical therapy. Although interbody fusion may be a suitable method of eliminating symptoms, it results in total loss of motion of the fused vertebral joint and is not without a significant complication
Conclusions
Based on the results of bench-top mechanical tests and the biomechanical test with cadaveric lumbar spine motion segments, the NeuDisc nucleus replacement device may prove to be a suitable nucleus substitute. Specifically, the novel implant avoided radial bulging as compared with an isotropic hydrogel lacking the reinforcment layers, provided suitable “lift” forces for use in the lumbar spine, and resisted compressive forces in 10 million cyles of fatigue testing. Also, the implant resisted
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2017, BiomaterialsCitation Excerpt :The approach that has the longest history of clinical use, and that serves as a key precedent in NP replacement, is a copolymeric hydrogel encased in a polyethylene fiber jacket [polyacrylonitrile and polyacrylamide (PDN™)]. This concept has also been key to the design of multiple implantable devices for NP replacement constructed from either semihydrated poly(vinyl) alcohol (PVA), a copolymer of (PVA) and poly(vinyl pyrrolidone) (PVP) [85], or modified poly(acrylonitrile) reinforced by a Dacron mesh [86]. The design concept is that these polymers will absorb water in the otherwise dehydrated, degenerated NP, and the degree of swelling will be restricted by the encasing jacket material.
FDA device/drug status: not approved for this indication (NeuDisc implant).
Author AP acknowledges a financial relationship (grant research support from Replication Medical, Inc.), which may indirectly relate to the subject of this manuscript.