Sơ đồ của một xương đùi xa (tối line) và đường dẫn của bánh chè theo dõi từ phần mở rộng đầy đủ đến 90 ° bẻ gập lại (bên phải). Các trung tâm của vòng cung mô tả con đường bánh chè nằm ở một khoảng cách (khoảng 10 mm phía trước và 10 mm proximally) từ trục | 156 III Kinematics o Fig. 23-4. Schematic of a distal femur darkline and the path of the patellar tracking from full extension to 90 flexion right . The center of the arc describing the patellar path is located at a distance approximately 10 mm anteriorly and 10 mm proximally from the FE axis 3 . R Radius of the epicondylar circle r radius of the arc describing the patellar tracking clinical observation that external rotation of the femoral component aids patellar tracking in TKA. Patellar Component Forces Patellar tracking and patellar component failure are important aspects of total knee arthroplasty. While fluoroscopic in vivo studies provide data regarding tibiofemoral kinematics they do not show the effect of tibiofemoral kinematics on the patellofemoral articulation. In vitro studies can provide helpful information on patellar kinematics and patellar loading by including a patellar force transducer while measuring kinematics. In one study the anterior-posterior position of the femur on the tibia was examined and direct comparisons were made based on the status of the PCL 4 . The specific conditions that were o Fig. 23-5. The average patella shift in the frontal plane along the FE axis is mm. Open circles represent the patellar position at flexion angles between 0 and 15 solid circles represent the patellar position at flexion angles between 20 and 90 . From 5 studied included the intact PCL the loose PCL and a PCL-substituting design. Additionally the post position was varied from 2 mm more anterior to 4 mm more posterior. These implant conditions were shown to directly affect the anterior-posterior position of the femur on the tibia o Fig. 23-6a . Simultaneous measurement of patellofemoral contact forces demonstrated about 20 lower forces for the most posterior femoral position o Fig. 23-6b . Another in vitro study by D Lima et al. also supports these findings that a more posterior femoral position roll-back significantly decreases patellar compressive .
