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Friday, March 12, 2004

Wear in polyethylene components not cause for metal component revision

By Mary Ann Porucznik

Mobile-bearing knee prostheses have several perceived advantages, but a recent report of a 47 percent incidence of osteolysis in an 8.5 year follow-up study of Low Contact Stress (LCS) mobile-bearing prostheses raised several concerns related to polyethylene wear in these prostheses. Investigators for Poster Exhibit P099 looked at polyethylene wear and the use of bearing exchange revision procedures for LCS prostheses.

The perceived advantages of mobile-bearing knee prostheses include reduced polyethylene wear associated with large tibial-femoral contact areas, and low shear stresses at the bone interface due to relatively unrestrained bearing motion. However, smaller particulate debris associated with the more conforming tibial-femoral articulation and abrasive polyethylene wear at the mobile "backside" articulation may contribute to osteolysis.

Researchers examined 29 retrieved meniscal-bearing inserts and 14 retrieved rotating-platform inserts from uncemented LCS prostheses. Patients had a mean age of 74 years old; the prosthesis had been in-situ for a mean of nine years. Twelve of the inserts were removed due to bearing wear; eight were removed for patella wear; five for pain or stiffness; three for instability; three for loosening; one for osteolysis and eight for other reasons. Three inserts were removed during autopsy.

The original femoral and tibial components were left in-situ in 85 percent of the knees at revision, and only the polyethylene articular bearings were exchanged. Thirty-nine of 52 meniscal bearings that were analyzed were delaminated, including 18 bearings with fractured articular surfaces. Six of the rotating platforms had delamination. Twenty of 25 patellas had delamination, including six with fractured articulations. Scratching was the dominant backside wear mode.

Despite severe wear on many of the polyethylene components, there was only a 5 percent incidence of osteolysis noted at revision. The need for complete revision of the metal tibial and femoral components was infrequent and usually occurred in early revisions for tibial loosening, pain, or lack of range of motion.

Researchers included: Melinda K. Harman, MS; Scott A. Banks, PhD; and William A. Hodge, MD, all of West Palm Beach, Fla., and George D. Markovich, MD, of Fort Myers, Fla.

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