PATELLOFEMORAL CONTACT PRESSURES EXCEED THE COMPRESSIVE YIELD STRENGTH OF UHMWPE IN TOTAL KNEE ARTHROPLASTIES

To address mechanisms involved in wear and permanent deformation of pa tellofemoral components in total knee arthroplasties, a previously rep orted knee joint loading model and pressure-sensitive film were used t o measure patellofemoral contact areas and pressures in human cadaver knee joints after implantation with six different total knee joint des igns. The joints were tested at three different Q angles (physiologic, -10 degrees, and +10 degrees) and four different flexion angles (30 d egrees, 60 degrees, 90 degrees, and 120 degrees). Patellofemoral conta ct areas at normal Q angles ranged from 0.13 to 0.68 cm(2) and increas ed with flexion angle up to 90 degrees. These contact areas differed s ignificantly with flexion angle but not with Q angle. Variations in co ntact area with type of knee system were only marginally significant ( P < .04), and post hoc tests showed no significant differences between individual knee designs. Contact pressures at normal Q angle also inc reased with flexion angle and ranged from 10 MPa to more than 49 MPa. Contact pressures at flexion angles greater than 60 degrees were, for all systems, well in excess of the compressive yield strength of ultra high-molecular-weight polyethylene and at least three to four times gr eater than the recommended maximum compressive stress level of 10 MPa.