Elsevier

The Spine Journal

Volume 5, Issue 6, November–December 2005, Pages 672-681
The Spine Journal

Technical Review
Mechanical testing of a novel hydrogel nucleus replacement implant

https://doi.org/10.1016/j.spinee.2004.12.004Get rights and content

Abstract

Background context

Both total disc and nucleus replacement arthroplastic approaches are emerging as viable treatment modalities for discogenic pain resulting from degenerative disc disease. The results of the testing in this investigation show that this novel nucleus replacement implant exhibits physical and mechanical properties which make it uniquely suitable as a replacment to the degenerated nucleus pulposus.

Purpose

The purpose of this investigation was to evaluate key performance characteristics of a novel nucleus replacment device.

Study design

Mechanical tests were performed to evaluate essential properties of the implant. Of key importance was the ability of the implant to withstand fatigue testing and resist extrusion in cadaveric motion segments. Resistance of the implant to radial deformation is also relevant to the ability of the implant to resist extrusion and maintain disc height.

Methods

Resistance to radial bulging was demonstrated by applying axial compressive loads while examining the “footprint” of the implant through a clear plexiglass lower platen. A NeuDisc implant was compared with an isotropic implant produced from the same hydrolyzed polyacrylonitrile hydrogel. Confined compression tests were used to determine “lifting force” when implants were loaded within stainless steel cylinders submerged in Hanks' balanced salt solution. Fatigue tests involved loading for 10 million cycles. Implants were then examined for physical damage, retention of mechanical properties, and for the ability of the device to return to a complete level of hydration. The final evaluation was extrusion testing in a cadaveric model. Implanted specimens were either tested in compression, lateral bending, or flexion until catastrophic failure was apparent.

Results

The NeuDisc implant showed considerably less “bulging” when compared with the identically sized isotropic device. Confined compression testing revealed a lifting force of 400 N at 70% hydration for these hydrogel implants. After fatigue testing, all implants were physically intact and possessed similar swelling characteristics to control samples. In cadaver testing, mean force to failure in compression was 3581.3±1558.5 N. Mean moment to failure in lateral bending was 25.6±11.1 Nm. In flexion tests, the mean moment to failure was 52.2±18.3 Nm. End plate fracture and ligament failure were the most commonly observed failure modes.

Conclusions

These initial results suggest that this novel implant may prove to be a suitable nucleus pulposus substitute.

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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    Citation 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.

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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.

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