The influence of isolated femur and tibia rotations on patella cartilage stress: a sensitivity analysis

Background: To determine the influence of femur and tibia rotations in the transverse and frontal planes on patella cartilage stress. Methods: Patella cartilage stress profiles of six healthy females were obtained during a squatting task using subject-specific finite element models of the patellofemoral joint (45 degrees of knee flexion). Input parameters for the finite element model included joint geometry, quadriceps muscle forces, and weight-bearing patellofemoral joint kinematics. The femur and tibia of each model were then rotated to 2 degrees, 4 degrees, 6 degrees, 8 degrees, and 10 degrees along their respective axes beyond that of the natural degree of rotation in weight-bearing. The process was repeated for internal rotation, external rotation, adduction, and abduction. Quasi-static loading simulations were performed to quantify average patella cartilage stress. Findings: Incremental femur internal rotation beyond that of the natural rotation resulted in progressively greater patella cartilage stress (41-77%), whereas incremental tibia internal rotation resulted in a decrease in patella cartilage stress (7-10%). Femur and tibia external rotation resulted in a mild increase in patella cartilage stress, but only at 10 degrees (9%). Incremental femur adduction resulted in an increase in patella cartilage stress, but only at 10 (43%). Femur abduction and frontal plane tibia rotation in either direction had no influence on patella cartilage stress. Interpretation: Femur internal rotation and adduction resulted in the greatest increases in patella cartilage stress. In contrast, tibia rotations in the transverse and frontal planes had minimal to no influence on patella cartilage stress. These results emphasize the need for clinicians to identify and correct faulty hip kinematics in persons with PFP.