Single-unit artificial intervertebral disc.

Object: The authors describe a new type of artificial disc called a single-unit artificial disc (SUAD). It is a single-unit disc without components, and there is no fixation system with which to maintain it in the disc space. It is theorized that its shape, hardness, and surface consistency, together with the compressive force exerted by the body's axial load, should be adequate to maintain the position of the disc in the disc space. In this paper the authors present their results of the kinematic tests in which the stability and integrity of the SUADs was tested.

Methods: Panorobot was used for kinematic test fixture for fatigue testing for different types of SUAD. The test was performed after placing the disc between the C-5 and C-6 vertebral bodies (VBs) obtained from a cadaver. Eight pounds of weight was placed on the top of the container housing the C-5 VB to account for the weight of the head. The robot performed the following movements: 1) flexion-extension, 4.7 degrees each; 2) lateral left-right bending, +/- 2.1 degrees; and 3) coupled rotation, +/- 3.8 degrees. Two flat discs (FDs) of 85 durometer (D), four 30D FD, two recess-edged discs (REDs) of 85D, and four custom-designed discs (CDDs) (custom molded to the disc space) of 30D were tested. None of the discs showed cracks or breakage at the end of the study. After 1 million cycles of excursions all 85D and 30D FD, and both 85D REDs showed weight loss. The 30D CDDs showed minimal weight gain at 1 million cycles. One of the CDDs, tested up to 5 million cycles, did lose weight at 5 million cycles. One 85D FD was extruded after 0.2 million cycles and one 85D RED rotated within the disc space. All other discs maintained their position in the disc space. Dimensional changes were minimal. Scanning electron microscopy of particles collected from one 30D CDD sample after 1 million cycles showed rough irregular granular particles 1 to 600 microm in diameter.

Conclusions: These findings show that a 30D SUAD can maintain its position in the disc space without any anchoring device. Furthermore, at 1 million cycles of testing of 30 D SUAD, FDs did as well as the CDDs. This may be because softer discs mold to changes in dimension of the disc space. In addition the softer discs tend to wear less than the harder ones. Further fatigue study of 10 million cycles is needed to determine long-term efficacy, and the effects of wear on particles surrounding the joints need to be studied.