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Finding Hip Joint Implants That Last

Posted on: 11/11/2003
Getting a hip joint replacement? What type of implant is best: ceramic, plastic, or metal? How about half-and-half: part metal, part plastic, or ceramic on metal? There are quite a few choices, but researchers are trying to narrow down the best ones.

At the School of Mechanical Engineering in England, scientists have a simulation lab. They test joint replacements using different materials and under varying conditions. This study reports the results of testing a metal head (the round top at the upper end of the thigh bone) and a ceramic socket.

The implants were tested as if under low wear, high stress, and with different amounts of joint laxity. The simulator put the joints through a walking cycle equal to 12 months of wear.

Under low stress the metal head against the ceramic cup showed mild wear. The problem came when joint laxity(looseness) causes the head of the femur to hit the rim of the socket. This is called microseparation. It occurs just as the person puts the heel down to take a step.

The result is a tiny fracture in the ceramic where the femoral head hits the rim. Over time, a stripe of wear forms on the femoral head, with a matching wear pattern in the socket. The hip starts to squeak or click. The joint surface gets rough, and the fracture in the implant can worsen.

The authors of this study aren't in favor of using metal on ceramic in hip joint
implants. They say studies like this are needed for all kinds of joint replacements before they are used in humans.

Todd D. Stewart, PhD et al. Severe Wear and Fracture of Zirconia Heads Against Alumina Inserts in Hip Simulator Studies with Microseparation. In The Journal of Arthroplasty. September 2003. Vol. 18. No. 6. Pp. 726-734.

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