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Mobile-Bearing Tibial Inserts Hold Up Better than Fixed-Bearing

Surgeons watch carefully to see what kind of wear patterns occur in the various parts of joint replacement implants. In this study, two designs of the tibial (knee) inserts are compared: mobile- and fixed-bearing.

The tibial implant rests on top of the tibial plateau. This is the flat shelf along the top of the lower leg bone. The shelf forms the bottom half of the knee joint.

The mobile-bearing insert rotates (turns) when the knee twists or rotates. This feature is designed to decrease uneven wear. The mobile-bearing knee implant decreases contact stress where pressure is applied to the bearing surface. It also decreases stress where the implant meets the bone.

Despite all these perceived advantages of the mobile-bearing implant, there's no evidence that this model performs better than the fixed-bearing design. The authors examined the location, amount, and type of wear on tibial implants that were taken out of 51 patients.

A special microscope and computer image system were used to analyze wear patterns. Some of the implants had come loose, broken, or failed because of uneven or excessive wear. The authors describe the various wear patterns in detail.

They found that low-grade wear was seen in the mobile-bearing knees. High-grade wear was more common in fixed-bearing knees. Low-grade refers to a burnished surface or abrasion seen on the surface of the implant. High-grade wear has scratches and pits in the surface. The protective covering can get worn off with high-grade wear.

Fixed-bearing inserts were more likely to be worn all the way through. Some of these implants also had metal debris from other parts of the prosthesis embedded in them. As suspected, the fixed insert had uneven wear.

Overall, the mobile-bearing insert held up much better than the fixed type. In fact, the authors report far fewer mobile-bearing inserts were even removed because of failure. The fixed-bearing were far more likely to fail requiring removal and revision.

References: Fang-Yuan Ho, MS, et al. Mobile-bearing Knees Reduce Rotational Asymmetric Wear. In Clinical Orthopaedic and Related Research. September 2007. Vol. 462. Pp. 143-149.

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