RADIOGRAPHIC WEAR ASSESSMENT IN A TOTAL KNEE PROSTHESIS - 5-YEAR TO 9-YEAR FOLLOW-UP-STUDY OF 158 KNEES

One hundred fifty-eight Porous-Coated Anatomic (Howmedica, Rutherford, NJ) primary total knee prostheses were evaluated clinically and radio graphically to measure the remaining thickness of the plastic insert. Anteroposterior radiographs were taken with the beam guided parallel t o the tibial plate by a fluoroscope. The knees were forced into varus and valgus, and the heights of the medial and lateral joint spaces, re spectively, were measured with a digitizing table. Plastic insert wear could be calculated after correction with a magnification error facto r, established by dividing the projected width of the tibial plate by the true size of the used component. After a mean follow-up period of 84 months (range, 58-116 months), wear was significantly higher for pa tients with osteoarthritis than rheumatoid arthritis and was 1.4 mm ve rsus 0.7 mm medially (P < .0001) and 0.7 mm versus 0.4 mm laterally (P = .01). Wear was not correlated to thickness of the plastic insert or length of follow-up period. Young age or varus alignment contributed slightly to the amount of wear.