Contact stress at the anterior aspect of the tibial post in posterior-stabilized total knee replacement

Background: Retrieval studies have demonstrated polyethylene wear and deformation of the anterior aspect of the tibial post of posterior-stabilized total knee replacements. We are not aware of any study that has analyzed the effects of the design of the femoral notch and the anterior aspect of the tibial post of posterior-stabilized total knee replacements on contact area, stress, and location. The purpose of the present study was to determine the levels of contact stress generated in three posterior-stabilized total knee replacement designs when the femoral intercondylar notch impinges on the anterior aspect of the tibial post. Methods: The contact area, mean and peak contact stresses, and contact location at the anterior aspect of the tibial post were determined when a posterior force of 100 N was applied to a NexGen LPS Flex, Genesis II, and Scorpio NRG total knee replacement. Measurements were performed at -15°, -10°, -5°, 0°, and 5° of flexion of the femoral component both in neutral and with 5° of rotation of the tibial component. Each measurement was sequentially performed five times, and the data were compared within and between the designs with use of analysis of variance and a post-hoc Scheffé F test. Results: The NexGen LPS Flex exhibited two contact areas on the medial and lateral corners of the anterior aspect of the post, whereas both the Genesis II and Scorpio NRG exhibited a single horizontal band. The NexGen LPS Flex showed the largest total contact area of the three designs at -15°, -10°, and 5° of flexion. The NexGen LPS showed the lowest mean contact stress at -15° and 5° but showed the highest peak contact stress at -5° and 0°. The Scorpio NRG showed the highest mean contact stress under all conditions and showed the highest peak contact stress at -15°, -10°, and 5°. With 5° of rotation of the tibial component, peak contact stress increased, relative to neutral, 1.2 to twofold (depending on the flexion angle) in the Genesis II design. Conclusions: The mean and peak contact stresses were variable for all three designs and the test conditions, approaching and in some cases exceeding the compressive yield stress for polyethylene. The geometry of the metal femoral notch and polyethylene tibial post in the axial and sagittal planes significantly affected contact area, mean and peak stresses, and contact location at the anterior aspect of the tibial post. Clinical Relevance: This study helps the surgeon to be more aware that high contact stresses under some conditions could lead to deformation and damage of the anterior aspect of the post in these three common designs of cruciate-sacrificing total knee replacements.