Evaluation of Anterior Tibial Post Impingement during Gait in Posterior-stabilzed Total Knee Arthroplasty

Introduction: Posterior-stabilized total knee arthroplasty (TKA) has been widely used since its introduction nearly three decades ago, and long-term follow-up studies have reported satisfactory results. However, mechanical failure due to fracture of the tibial post recently has been described in several reports (1). The possible fracture mechanism was the repeated impingement between the intercondylar notch of the metal femoral component and the anterior aspect of the polyethylene tibial post (1, 2, 3). The main purpose of this study was to determine if anterior tibial post impingement occurs during gait activity after posterior-stabilized TKA using high resolution dynamic flat-panel detector images. Materials and Methods: The subjects who had undergone clinically successful arthroplasty gave informed consent to participate in this study as approved by the institutional review board. The study group consisted of 12 knees with a posterior-stabilized TKA (Nexgen LPS, Zimmer, Inc., Warsaw, IN). The average age at the time of knee arthroplasty was 71 ± 6 years, and the average follow-up after surgery was 19 ± 12 months. The average postoperative knee extension/flexion angle was -3 ± 4°/117 ± 21°. Continuous sagittal x-ray images of gait on a treadmill at 0.8 m/sec for each subject were taken using a flat panel detector (Hitachi, Clavis: 3 frames/sec, image area size 397(H)×298(V)mm, and 0.20×0.20mm/pixel resolution). For each subject, three x-ray images of single-leg stance and three x-ray images of swing phase respectively were captured from different gait cycles. A total of 72 images were used for the analysis: 36 images for the swing phase, 36 images for the stance phase. The implant flexion angles were measured using an image-matching method (4). Anterior post impingement was determined by the intersection of the 3-D model surfaces of the femoral component and tibial polyethylene insert (SolidWorks® 2001Plus SP3.0, SolidWorks Corporation). The skeletal knee flexion angle between the femoral shaft axis and the tibia shaft axis was measured at sagittal x-ray images using an angle scale. A radiological assessment of the femoral component flexion angle and the posterior tibial tilting angle at the lateral view was performed according to the Knee Society roentgenographic evaluation. Results: The average skeletal flexion angle at the stance/swing phase was 5.9 ± 8.6°/45.6 ± 13.1° (Fig. 1A). The average implant flexion angle at stance/swing phase was -6.1 ± 8.0°/33.6 ± 14.4° (Fig. 1A). At the stance phase, 12 of 36 x-ray images represented flexed position, and 7 images represented hyperextension more than 15° (Fig. 1B). Anterior post impingement was observed for all X-ray images in all knees when at the stance phase (Fig. 2). At the swing phase, all X-ray images displayed noncontact in the anterior or posterior aspects of the tibial post. The average postoperative alignment of femoral prosthesis on the lateral radiograph was 5.6 ± 2.7° of flexion relative to the distal half of the femoral shaft axis. The average postoperative alignment of the tibial prosthesis on the lateral radiograph was 5.9 ± 2.9° of posterior tilt relative to the proximal half of the tibial shaft axis. Discussion: Patients with NexGen posterior-stabilized TKA had a high incidence of anterior tibial post impingement during the stance phase of gait. There was no hyperextension and/or instability on clinical examination in any of the knees. There was also no evidence of component malpositioning observed by radiographic examinations in any of the knees. At the stance phase, the average skeletal alignment showed low flexion, but the average implant alignment showed approximately 6° hyperextension. Even in low implant knee flexion, the intercondylar notch of the femoral component impinged on the anterior aspect of the tibial post.