Summary

The anatomical tibial axis in kinematic alignment total knee arthroplasty was investigated from CT images. Although there is a great deal of individual variation, the average tibial axis is 2.9 degrees more internally rotated than Akagi line.

Abstract

Purpose

In total knee arthroplasty (TKA) using mechanical alignment, which has been the golden standard until now, the Akagi line was the typical axis of rotation of the tibia. Today, in kinematic alignment (KA), a rapidly expanding alignment technique, it is not known where the anatomic axis of rotation on the tibial side will be. The purpose of this study is to define the tibial axis in KA-TKA.

Methods

Fifty patients who underwent CT examination of the lower extremity at a single institution were included, using the same methods as in the study that validated the Akagi line. The posterior condylar axis (PCA) and surgical epicondylar axis (SEA) were identified in the CT axial view and projected onto the tibial slice. The respective perpendicular lines that pass through the attachment of the posterior cruciate ligament (PCL) were identified as the anatomic axis of rotation of the tibia relative to the PCA and SEA, and the position of each axis of rotation. Furthermore, the relationship of these perpendicular lines with the Akagi line were valuated.

Results

The axis of tibial rotation to the SEA was similar to that of the Akagi line; the axis of tibial rotation to the PCA was located approximately 2.9 degrees medial to the Akagi line, and when the origin of the tibial axis was set at the PCL attachment site, the intersection point of the tibial axis was approximately 2.5 mm medial to the medial border of the tibial tuberosity. The distribution of tibial axis had a wide range. This study was based on imaging indices, and it is unknown whether these indices can be adapted to actual patient knees. In particular, it should be noted that the main problem with tibial medial rotation in TKA is the high risk of patellofemoral dislocation, which was not evaluated in this study.

Conclusion

Although there is a large individual variation, the average tibial axis for KA-TKA is 2.9 degrees more internally rotated than Akagi line.