Instability hastens polyethylene wear, however long-term usage of a knee prosthesis will eventually cause wear as well. Polyethylene wear is the most prevalent mechanism of total knee replacement failure.9 Wear occurs most frequently in the medial compartment of the polyethylene liner along biomechanical vectors of stress. Wear and debris initiates synovitis and foreign-body reaction, which can ultimately lead to severe bone loss. This is probably the most important factor that limits long-term use of knee replacements. Severe, progressive liner wear allows for full contact of the metal femoral and tibial components to produce metal wear.
Early radiographic detection of asymptomatic polyethylene wear is important. At this point, a relatively simple revision can be performed by exchanging the polyethylene liner component, avoiding the need for revision arthroplasty. Wear is demonstrated on radiographs as asymmetric joint space narrowing.
AP radiograph of both knees. The right knee has marked joint space narrowing worse in the medial compartment with metal-on-metal contact, consistent with polyethylene wear.
Polyethylene wear occurs in the form of pitting, delamination, fracture, burnishing, and abrasion. It is demonstrated on radiographs as asymmetric joint-space narrowing.
Photographs of various polyethylene spacers demonstrating wear.
Polyethylene Liner Dislocation
Rarely, liners may dislodge from joint arthroplasties and dislocate into the surrounding soft tissues. This occurs more frequently in the hip, but also may occur in the knee when the liner dislodges from the tibial tray. Polyethylene liners have low density on radiograph, and are conspicuous because they are surrounded by denser knee effusion and soft-tissue shadows.
Lateral knee radiograph of hinged total knee prosthesis with posteriority dislocated polyethylene liner (arrows).