Patellofemoral contact pressures exceed the compressive yield strength of UHMWPE in total knee arthroplasties.

To address mechanisms involved in wear and permanent deformation of patellofemoral components in total knee arthroplasties, a previously reported knee joint loading model and pressure-sensitive film were used to measure patellofemoral contact areas and pressures in human cadaver knee joints after implantation with six different total knee joint designs. The joints were tested at three different Q angles (physiologic, -10 degrees, and +10 degrees) and four different flexion angles (30 degrees, 60 degrees, 90 degrees, and 120 degrees). Patellofemoral contact areas at normal Q angles ranged from 0.13 to 0.68 cm2 and increased with flexion angle up to 90 degrees. These contact areas differed significantly with flexion angle but not with Q angle. Variations in contact 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 increased 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 ultrahigh-molecular-weight polyethylene and at least three to four times greater than the recommended maximum compressive stress level of 10 MPa.