• Yukihide Minoda, Ryo Sugama, Yoichi Ohta, and Hiroaki Nakamura.
• Department of Orthopaedic Surgery, Osaka City University Graduate School of Medicine, 1-4-3 Asahi-machi Abeno-ku, Osaka, 545-8585, Japan. yminoda@msic.med.osaka-cu.ac.jp.
• Arch Orthop Trauma Surg. 2017 Jun 1; 137 (6): 861-865.

IntroductionPatella-post impingement (PPI), contact of the patellar component with the tibial post, occurs during deep knee flexion after posterior stabilized total knee arthroplasty (TKA). PPI can cause anterior knee pain, increased patellar pressure, wear of the polyethylene patellar component and tibial post, and poor range of motion. There have been no reports on the relationship between PPI and tibial post design. The aim of this study was to compare different tibial post designs with regard to PPI incidence.Materials And MethodsFive posterior stabilized total knee prostheses were implanted in full-length models of the femur and tibia (Sawbones) using a navigation system. The thickness of the tibial component was 10 mm in all prostheses. The patella was set on a transparent polypropylene sheet, which was fixed to the tibial tubercle. 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 varied from 18 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.ResultsThe PPI angle differed between tibial post designs (p < 0.05). Shorter TPCs resulted in smaller PPI angles and longer TPCs resulted in larger PPI angles (p < 0.05).ConclusionsThis is the first study to analyze the relationship between PPI and the design of tibial post. Our results clearly show that tibial post design and TPC affect PPI. To avoid patellar component and tibial post impingement in deep flexion, appropriate TPC must be achieved during surgery. Improvements in tibial post design, especially in constrained prostheses, are also desirable. This study highlights the potential risk of PPI during deep flexion after TKA.

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