PROGRESSIVE SUBLUXATION AND POLYETHYLENE WEAR IN TOTAL KNEE REPLACEMENTS WITH FLAT ARTICULAR SURFACES

One hundred eighty-six Microloc tricompartmental, cruciate-sparing, pr imary total knee replacements in 136 patients were performed between 1 983 and 1987. Femoral components were made of cobalt-chrome-molybdenum alloy. The tibial component baseplates were made of titanium-aluminum -vanadium alloy. The baseplates were made with three small porous-coat ed pegs for fixation with or without cement, or with a central stem fo r fixation with cement. The polyethylene was affixed to the baseplate by a mechanical locking mechanism that was nonmodular. The articular s urfaces of both components were flat mediolaterally and anteroposterio rly. The patellar components were dome shaped and metal backed with ti tanium-aluminum alloy. Seventy-three percent of the femoral components , 26% of the tibial components, and 48% of the patellar components wer e inserted without bone cement. The average time to maximum follow-up examination was 6.1 years (range, four to nine years). Of implants tha t had not been revised, 84.4% had good or excellent results (using the Hospital for Special Surgery scoring system) at final follow-up exami nation. The average range of motion of functioning implants at final f ollow-up examination was 108 degrees. Revisions for failure only of th e metal-backed patellar component were performed in 22 knees (11.8%). Forty knees (21.5%) required revision for failure of the femoral-tibia l articulation. Reasons for failure in this group included: polyethyle ne wear (57.5%); loosening (15.0%); tibial tray fracture (10.0%); seps is (12.5%); and dislocation or ligament laxity (5.0%). An additional 1 5 functioning knees (8.1%) have radiographic evidence of progressive f emoral-tibial subluxation. Future failure of these devices is consider ed likely. Polyethylene wear requiring revision was not statistically associated with patient age, gender, preoperative diagnosis, height, w eight, or component size. Polyethylene failure leading to revision was related to the radiographic appearance of femoral-tibial component su bluxation. This subluxation was most likely to occur after five years of component implantation. The development of progressive femoral-tibi al subluxation was statistically associated with postoperative extremi ty malalignment, excessive varus positioning of the tibial component, bone grafting of the tibial plateau, the use of the small-pegged tibia l component, and the use of this component without cement. Polyethylen e wear associated with progressive femoral-tibial subluxation occurred at the periphery of the tibial plateau and along the raised portion o f the tibial spine. The metal baseplate at the edge of the plateau was frequently exposed and worn. Bone lysis associated with the presence of polyethylene, metal, and cement debris was frequently observed.