Published: Jan 2003
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Durability is critical for permanent spinal implants. Proposed test methods suggest that prosthetic intervertebral discs (PIDs) having fixation plates for vertebral attachment should be tested at a 45° angle to create combined compression-shear loads. Generally, a prosthetic nucleus (PN) does not have fixation plates and does not transmit shear loads as high as PIDs; thus, loading PNs through fixation plates would not mimic physiological conditions. Currently, most PNs are fatigue tested using compression only. A test method and fixture are proposed for testing a PN in a saline bath at 37 °C under combined compression-flexion-extension loading simulating physiological conditions. A multi-station test fixture, mounted in a servo-pneumatic test frame, was used to conduct the tests under load control. The fixture has a top platen to apply a controlled compression load and a bottom test bed that rotates on a horizontal axis at a set frequency creating a flexion/extension motion. Compression loads were monitored for each station, and the rotation and moment were monitored for the test bed. A software control algorithm coordinated the compression stroke of the test frame with the test bed rotation so that the maximum compression load occurred near the peak rotation angle. The PN compression load range was approximately 200 N–550 N applied at 1.5 Hz, and the test bed rotation was -3.6° to +2.3° to simulate lumbar flexion/extension motion during vigorous walking. The peak compression stress component was approximately 1 MPa, and the peak flexion/extension component was approximately 1 MPa giving a maximum combined stress on the implant edge of 2.0 MPa. Specimens were examined and photographed with magnification, and masses were monitored to assess implant wear. No cracks were observed on any of 6 implants tested to 12.7 million cycles, and the average total mass loss was approximately 6.9 mg (stdev = 4 mg). Testing continues.
prosthetic intervertebral disc nucleus, fatigue, wear, permanent set, in situ curable polyurethane, test fixture
Project Manager, Disc Dynamics, Inc., Minnetonka, MN
VP, R&D, Disc Dynamics, Inc., Minnetonka, MN