Biomet metal on metal hip implant complexities

Medical device implants are complex topics best explained by your orthopedic surgeon or your lawyer who is well versed in hip implant medical device litigation.

Here's an overview on the Biomet hips:

A high percentage of first generation metal on metal hips occurred among a subset prostheses from a single source that had been manufactured with a very small clearance between the ball and cup, and with a cup wall thickness of only 2 mm.

Although a number of investigators have evaluated the effect of third-body abrasion on the wear of metal on polyethylene bearings and ceramic on polyethylene hip bearings metal on metal hips have not been widely examined.

Even if the wear per million cycles is known accurately for a metal on metal bearing; predicting how much wear will occur for a specific patient varies greatly among patients' activity levels.

Laboratory tests indicate most metal on metal hips operate under mixed lubrication. With a fluid-film, there is very low friction and possibly negligible wear. The thickness of the film and the percent of the bearing area that is separated by it, increase with increasing speed of oscillation, decreasing clearance (up to a point), increasing viscosity of the lubricant, and decreasing applied load. Because typical metal-metal bearings operate in the mixed lubrication mode, any change in one of the key parameters may shift the lubrication in favor of fluid film separation, substantially reducing the friction and wear, or in the direction of solid-solid contact, increasing friction and wear.

Because of the complex interaction of parameters, it's next to impossible, to calculate accurately the effect of ball diameter on wear but may be measured experimentally with a hip stimulator.

In laboratory tests using joint simulators, metal-on-metal bearings using high-carbon cobalt chrome alloy have consistently exhibited wear resistance superior to that of low-carbon against low-carbon. Consequently, the majority of metal-metal bearings now in use have one or both bearing surface of high-carbon alloy.

Source: Metal on metal Biomet hip testing

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