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Device Design

Biocompatibility Testing

Mechanical Testing

Endurance Testing

Range of Motion Testing

Expulsion Testing

Human Cadaver Fatigue Testing

Clinical Studies

Conclusion

Degenerative disk disease (DDD) and low back pain are the leading causes of lost wages in the United States with 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. In certain patients, fusion is successful in treating pain associated with DDD but may lead to undesirable consequences such as degenerative changes in adjacent segments due to the altered biomechanics and loss of flexibility.^2,3 Clearly, an unmet need exists for a nonfusion treatment of back pain that would restore function, maintain motion, and eliminate the pain associated with DDD. According to the new, defined-step algorithm for treatment of DDD, nucleus arthroplasty has its predefined role between simple disk degeneration and total disk replacement.^4,5

The concept of replacing the degenerating nucleus of the spine with a hydrogel or other synthetic material has been explored with varying degrees of success.^6–13 Early attempts at replacing the nucleus with materials ranging from metallic spheres¹⁴ to silicone gels have been plagued with several problems. Complications including extrusion from the disk space and subsidence into the vertebral end plate have been experienced in both animal models and human clinical testing.^6,7 Among the large number of materials explored, synthetic hydrogels hold considerable promise. Like the nucleus itself, these materials can absorb and desorb fluid in relation to applied load. Hydrogels can be prepared from several polymer sources, including polyvinyl alcohols, polyurethanes, and hydrolyzed polyacrylonitriles. In particular, the hydrolyzed polyacrylonitrile-based hydrogel holds considerable promise. In addition to the hydrogel chemistry, the form in which the hydrogel is introduced is particularly important. The implant must present with a small insertion geometry to minimize damage to the annulus created by the large incision. After insertion, the implant must swell or otherwise change shape such that it may fill the space previously occupied by the nucleus. Homogeneous, isotropic hydrogels will have a tendency to expand during swelling or deform in all directions or both under axial load. This is particularly true for the direction of least resistance, which may be in the direction of the defect created in the annulus fibrosus (AF) for implant insertion. There may also be portions of the AF that are weakened or otherwise damaged. The result may be either or both extrusion and reherniation.

New generations of nucleus arthroplasty devices like the NeuDisc™ device (Replication Medical, Inc., New Brunswick, NJ) have been designed to bypass these general problems. The NeuDisc comprises a proprietary hydrolyzed polyacrylonitrile hydrogel, Aquacryl. It is highly resistant to mechanical failure even at high water content. In the absence of mechanical restrictions, the hydrogel may contain 90% volume as a liquid. One of the properties of this novel implant is the capability to swell anisotropically (axial direction principally). When implanted in a dehydrated state, this hydrogel implant is substantially smaller than the volume of nucleotomy space and is easily placed through an incision in the annulus; however, it is substantially larger than the annular incision following hydration. The potential for extrusion is, therefore, greatly decreased over other designs. Furthermore, the implant has a “stacked” configuration, which includes layers of a medical-grade polyester fiber mesh within the hydrogel. These mesh layers act to restrict radial deformability (“bulging”) so that the device will not “creep” through a defect in the AF.

The device replaces the overall function of the native nucleus pulposus (NP), which has been compromised as a result of the disease process. The design of the NeuDisc takes into account the physical, mechanical, and physiological properties of the NP. Clinical considerations, including ease of insertion, fatigue life, wear characteristics, and device biocompatibility were also considered. The NeuDisc has been the subject of extensive preclinical in vivo and in vitro testing, biocompatibility testing, and rigorous mechanical characterizations to ensure its utility as a nucleus replacement.

Device Design

The NeuDisc is designed to simulate the essential properties of the native NP and has been developed to reproduce as closely as possible its elastic modulus, yield strength, and energy-absorptive properties. Mechanical studies suggest that compressive loads across the L3 disk of a 70 kg subject vary from 400 N to as much as 1400 N in different sitting postures and lifting activities.15 Based on the simplistic assumption that the load sharing between the nucleus and annulus is roughly proportional to the ratio of their respective cross-sectional areas,¹⁵ one might expect the nucleus to support maximal loads of ~200 to 700 N. Nucleus loads of this magnitude are in agreement with recent in vivo disk pressure measurements.¹⁶

The NeuDisc is also designed to resist extrusion during normal loading. The addition of the mesh layers to the device helps prevent radial deformability or “bulging.” The stiffness of the gel layers combined with the constraint added by the mesh layers prevents the NeuDisc from blebbing through defects in the AF.

The native NP has biological functions other than maintaining the mechanical properties of the intervertebral disk. The native NP continually absorbs and desorbs fluid during normal activity in a diurnal rhythm. This fluid action delivers the nutrients contained within the fluid to the cartilaginous end plates. The ability to absorb fluid and transport nutrients depends on the health of the end plates. It is arguable that a successful nucleus replacement must have the capability to perform this same biological function. The NeuDisc like the native NP is capable of absorbing and desorbing fluid under load. The NeuDisc is permeable to molecules as large as 60 to 100 kilodaltons (kDa) as the fluid component of the device reaches equilibrium with the surrounding in vivo intradiskal fluids. The osmotic gradient and fluid transfer ability allow the NeuDisc to perform the same function as the native NP.

The NeuDisc is designed to be inserted in a nearly dehydrated state, which allows a smaller incision to be used for insertion. The ability of the NeuDisc to hydrate anisotropically greatly decreases the chance of the device extruding from the relatively small insertion incision. The NeuDisc is similar in size to the native NP, which ranges between 25 and 50% of the cross-sectional area of the entire intervertebral disk. The hydrated volume of the NeuDisc must be closely similar to the volume of the native NP, which restores normal spinal function by transferring pressure to the surrounding annulus fibers, preloading these fibers physiologically while it strengthens the whole segment mechanically.

Biocompatibility Testing

Related posts:

Prosthetic Disk Nucleus Partial Disk Replacement: Pathobiological and Biomechanical Rationale for Design and Function Bryan Cervical Disc Device Spinal Kinetics Cervical Disc Biomechanical Testing Protocol for Evaluating Disk Arthroplasty DIAM (Device for Intervertebral Assisted Motion) Spinal Stabilization System Nonfusion Stabilization of the Degenerated Lumbar Spine with Cosmic

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