Abstract
Objective: Not only the femoral rotational alignment but also the tibial rotational alignment of the total knee arthroplasty (TKA) is considered a significant factor in securing favorable knee joint function. However, for those patients with brittle bones, such as rheumatoid arthritis or elderly women, there are some cases in which we will give priority to bone coverage over rotational alignment. The purpose of this study was to examine the relationship between the tibial anteroposterior (AP) axis and the position of the tibial component that prioritizes bone coverage.
Methods: In total, 157 preoperative knees of 106 patients with varus osteoarthritis indicated for primary TKA were included in the study. Two knees in which it was not possible to identify the posterior cruciate ligament (PCL) insertion were excluded from the examination. Radiographic grading was performed by using the Kellgren-Lawrence (K-L) criteria (grade 3, n=59; grade 4, n=96). We used three-dimensional preoperative planning software (Athena, Soft Cube) to simulate the tibial component fixations in the assumed cases in which the bone coverage was maximized, and we compared them with Akagi’s tibial AP axis (a line connecting the center of the tibial PCL insertion and the medial border of the patella tendon). We defined the angle of rotating fixation as that between the component rotational axis, in cases in which the bone coverage was maximized, and the tibial AP axis.
Results: The simulated angle of rotating fixation between the component rotational axis and the tibial AP axis averaged 4.5°±4.2° (range, -5.1° to 16.2°). The average angle in K-L grade 4 was 4.9°±3.5°, significantly larger than the 3.5°±2.8° angle found in K-L grade 3 (p=0.04).
Conclusion: In most cases, the tibial components were fixed for external rotation relative to the tibial AP axis (138 knees, 89.0%). However, the variation among cases was not negligible, and the angle varied widely in the range -5.1° to 16.2°. Furthermore, we found a significant correlation between the angle of rotational fixation and the radiographic grade of osteoarthritis. In the present situation in which a consensus has not been established about the rotational position of the tibial component, we consider it is useful to use a mobile-type insert with a self-alignment mechanism.
