Charité in vitro tests showed low wear and debris rates

Investigators reported the low rates in the first study to apply ASTM testing standards.

PHILADELPHIA — The Charité Artificial Disc showed low wear and debris generation rates in a series of in vitro tests, according to one researcher, who said the device’s design might be the reason for the low wear rates.

Researchers subjected the DePuy Spine device to loads and movements mimicking those of the lumbar spine, said Hassan Serhan, PhD, who reported on the test results at the North American Spine Society 20th Annual Meeting, here. Serhan is a DePuy research scientist.

“Although the long-term clinical history of this device has not raised concern over the wear debris, its characterization and fatigue performance was not done according to American Society for Testing and Materials standards until now,” Serhan said. “In summary, this study showed us that the device ... generates significantly low levels of ultra-high wear when compared to the total hips and total knees.”

The Charité disc has a biconvex floating polyethylene core and two concave cobalt-chrome alloy endplates. Physicians have clinically tested more than 10,000 devices over 18 years.

Serhan and his colleagues performed the study knowing that wear debris may cause osteolysis, component loosening and failure in some hip and knee prostheses. The Charité had no clinical evidence of osteolysis until recently, when two cases were reported, Serhan said. Both cases of osteolysis might be linked to gamma sterilization, which orthopedic physicians and researchers stopped using in 1997, he said.

Loads, rotation, bends and wear

The researchers studied nine Charité implants: three in a control group and six of the smallest devices in a test group. They subjected the six test group implants to 10 million cycles in a bovine serum at 37°C. They sampled the serum for wear debris and recorded core mass and height every 200,000 to 300,000 cycles, Serhan said.

Researchers cycled three devices in about 7.5° flexion-extension and about 1.5° of axial rotation. They cycled another three implants in about 7.5° left/right lateral bending and about 1.5º axial rotation. Full flexion yielded a load of 1850 N and hyperextension created a load of 900 N, Serhan said.

The mean generated wear was 1.1 mg, or 0.11 mg per 1 million cycles. The mean height loss was 0.20 mm. The cores seemed unaffected, except for slight scratches. The particles' mean diameter was 0.2 microns (range 0.08 to 16.3), Serhan said. Charité’s core slides in relation to both end plates, permitting anterior-posterior translation and rotation, and reducing internal shear stresses. The device shed polyethylene particles known to affect synovial tissues. However, there are no synovial tissues in the disk space, Serhan said.

For more information:
Serhan H., Dooris A., Ares P., Gabriel S. Wear characterization of the Charité Artificial Disc using ASTM guidelines. #143. Presented at the North American Spine Society 20th Annual Meeting. Sept. 27-Oct. 1, 2005. Philadelphia.