Impingement of the patellar component against the tibial post depends on the design of the post-cam mechanism: Comparison between 12 posterior stabilized total knee prostheses.

BACKGROUND: Patella-post impingement (PPI), contact of the patellar component with the tibial post, occurs during deep knee flexion after posterior stabilized total knee arthroplasty (TKA). In a previous pilot study, only two product lines were investigated. The aim of this study was to compare PPI between 12 contemporary posterior stabilized knee prostheses.

METHODS: Twelve posterior stabilized knee prostheses were implanted in full-length sawbone models of the femur and tibia using a navigation system. The distance between the lower edge of the patellar component and the cut surface of the tibia was defined as the tibial-patellar clearance (TPC), which represents the length of the patellar tendon. The TPC was set from 20 to 40 mm and the knee was moved from full extension to deep flexion while the knee angle at which PPI occurred (PPI angle) was recorded.

RESULTS: The PPI angle differed between prostheses (P < 0.05). Shorter TPCs resulted in smaller PPI angles and longer TPCs resulted in larger PPI angles (P < 0.05). To achieve more than 130° of flexion without PPI, the TPC should be prepared at a minimum of 20 mm for the NexGen LPS-Flex, Persona PS, Legion PS, and Evolution PS, 22 mm for the Attune RP and Journey II, 24 mm for the Triathlon PS, PFC Sigma PS, and Attune PS, and 26 mm for the NRG PS, Vanguard PS, and Vanguard RP.

CONCLUSIONS: The design of the tibial post significantly affects the PPI angle. To avoid PPI during deep flexion, appropriate TPC should be prepared during surgery.