Mechanical testing of a novel hydrogel nucleus replacement implant

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 replacement to the degenerated nucleus pulposus.

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

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.

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.

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.

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