Abstract
Introduction
Proper rotational alignment of the tibial component is a critical factor affecting the outcome of TKA. Traditionally, the tibial component is oriented with respect to fixed landmarks on the tibia without reference to the plane of knee motion. In this study, we examined differences between rotational axes based on anatomic landmarks and the true axis of knee motion during a functional activity.
Materials and Methods
24 fresh-frozen lower limb specimens were mounted in a joint simulator which enable replication of lunging and squatting through application of muscle and body-weight forces. Kinematic data was collected using a 3D motion analysis system. Computer models of the femur and tibia were generated by CT reconstruction. The motion axis of each knee (TFA) was defined by the 3D path of the femur with respect to the tibia as the knee was flexed from 30 to 90 degrees. The orientation the TFA was compared to 5 different anatomic axes commonly proposed for alignment of the tibial component.
Results
The average alignment error of the 5 different anatomic axes ranged from 0.1° ER to 10.7°IR from the true direction of knee flexion. The most accurate indicator of the direction of motion was derived by projecting the trans-epicondylar axis of the femur onto the tibial plateau. On average, this axis was externally rotated by 0.1±6.9°. However, values varied over 21.6°. In comparison, an axis passing through the medial-third of the tibial tubercle from the center of the plateau was internally rotated by 0.3°±6.0° (range: 23.9°).
Conclusion
This study demonstrates that rotational axes derived from anatomic landmarks on the proximal tibia provide an estimate of the direction of movement of the femur that is highly variable. Constructions based on the epicondylar axis and the medial third of the tibial tubercle are accurate when averaged over large numbers of cases. However, these methods can lead to up to 10 degrees of internal rotation of the tibial component in individual cases.
