Posterior Tibial Slope in Computer-Navigated Total Knee Arthroplasty: The Transmalleolar Sagittal Axis Underestimates Slope Compared to Traditional Intramedullary Axis

Background: Tibial slope in total knee arthroplasty (TKA) impacts knee flexion, balance, and ligament strain. Implants were initially designed with tibial slope recommendations based on the intramedullary axis. However, technology-assisted TKA, such as robotics or navigation, determines slope from the ankle-knee axis connecting the center of the transmalleolar line to the proximal exit point of the tibial shaft axis. We sought to quantify the difference in tibial slope between the traditional intramedullary and transmalleolar sagittal tibial axes. Methods: We retrospectively identified 40 TKAs with preoperative computed tomography scans. We reconstructed the 3-dimensional geometry of the tibia and fibula and determined the intramedullary axis as the best fit cylinder to the tibial shaft. We defined the transmalleolar axis according to accepted industry standards. We measured the angular difference between both axes in the sagittal plane. Results: The transmalleolar axis was radiographically posterior to the intramedullary axis in 39 knees. Utilizing the transmalleolar axis to set posterior tibial slope would reduce the posterior tibial slope by a mean of 1.9 degrees +/- 1.3 degrees compared to the intramedullary axis. Furthermore, the posterior slope would be reduced between 0 degrees and 2 degrees in 24 knees (60%), between 2 degrees and 4 degrees in 10 knees (25%), and more than 4 degrees in 5 knees (13%). Conclusion: Tibial components implanted with technology assistance referencing the transmalleolar axis to set posterior slope will show an average of 1.9 degrees less posterior slope when measured in sagittal plain radiographs, potentially concerning for knee kinematics. (C) 2022 Elsevier Inc. All rights reserved.