Variable tibiofemoral articular contact stress in fixed-bearing total knee arthroplasties

Background: Rotational allowance at the tibiofemoral joint would be required during deep flexion. However, the amount of flexion and rotation has not been investigated in modern total knee arthroplasty (TKA) designs. The present study aimed to determine the contact stress in five posterior-stabilized fixed-bearing TKA designs. Hypothesis: We hypothesized that the contact area and stresses at the tibiofemoral articular surfaces vary according to the type of implant design and tested condition. Materials and methods: The contact area and mean and peak contact stresses at the tibiofemoral articular surfaces were determined when a compressive load of 1200 N was applied to a NexGen LPS Flex, Scorpio NRG, Genesis II, PFC Sigma, and Foundation implant. Measurements were performed at 0° and 45° flexion with 0° 5° 10° and 15° rotation, and at 90° and 135° flexion with 0, 5° 10° 15° and 20° rotation. Results: The LPS Flex showed that the femoral component could not achieve 20° rotation at 135° flexion. The Scorpio NRG showed less than 20 MPa of contact stress at all conditions. The Genesis II showed higher contact stress than 20 MPa at 135° flexion with 20° rotation. The PFC Sigma showed that the femoral component could not achieve > 10° rotation at any flexion angle. The Foundation showed more than 20 MPa of contact stress at 90° flexion with 20° rotation and at 135° flexion with 10° 15° and 20° rotation. Discussion: Surgeons should be more aware of the variable contact conditions of the tibiofemoral articular surfaces in individual TKA designs. Level of evidence: Level IV, basic science study.